ALBERT

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 260〈/p〉 〈p〉Author(s): Liqiang Zhao, Kotaro Shirai, Naoko Murakami-Sugihara, Tomihiko Higuchi, Kiyoshi Tanaka〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Stable carbon isotope ratios (δ〈sup〉13〈/sup〉C) of mussel soft tissues have been widely used to characterize baseline δ〈sup〉13〈/sup〉C isoscapes and identify carbon sources at the base of coastal food webs. Extending soft tissue δ〈sup〉13〈/sup〉C records back in time, however, is extremely challenging due to very limited sample availability. Here, we test if the stable carbon isotopic composition of periostracum (the outermost organic layer of the shell) in the Mediterranean mussel (〈em〉Mytilus galloprovincialis〈/em〉) can be used as an environmental archive, similar to soft tissue records. In general, spatial and seasonal variations of periostracum δ〈sup〉13〈/sup〉C values are comparable to those of soft tissues, but apparently the latter are more time-averaged and smoothed. Irrespective of such offset, there is a significant linear correlation between mussel periostracum and soft tissue δ〈sup〉13〈/sup〉C values (R〈sup〉2〈/sup〉 = 0.608, 〈em〉p〈/em〉 〈 0.001). Moreover, high-resolution δ〈sup〉13〈/sup〉C analysis of mussel periostracum indicates that it integrates much less time than soft tissue due to metabolically inert nature, consequently being able to record predictable events such as tidal changes and also unpredictable ephemeral events in coastal ecosystems. The present study demonstrates the potential of the periostracum as a viable alternative to the most widely used soft tissues in isotopic studies. Most promisingly, given ample collections over time scales up to hundreds of years and usually stored dry, this technique could be used to extend coastal organic carbon δ〈sup〉13〈/sup〉C records back in time.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 260〈/p〉 〈p〉Author(s): Scott A. Blumenthal, Thure E. Cerling, Tara M. Smiley, Catherine E. Badgley, Thomas W. Plummer〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We investigate how oxygen isotopes in equid teeth can be used as a record of seasonality. First, we use 〈em〉in situ〈/em〉 laser ablation and conventional microsampling techniques to understand time-averaging of environmental signals in intra-tooth isotope profiles in modern feral horse teeth (n = 5) from Mongolia, where there is a large seasonal gradient in the oxygen isotopic composition of precipitation. We demonstrate that laser ablation can be used to sample inner, middle, and outer enamel layers in large mammalian herbivore teeth. The inner enamel layer records less attenuated isotopic signals than other layers, as predicted by the mineralization patterns, but intra-tooth signal amplitude is similar for laser and conventional sampling methods. Second, we use modern zebra teeth (n = 21) collected in eastern Africa to evaluate how intra-tooth oxygen isotope variation relates to rainfall patterns in the tropics. We show that the intra-tooth isotopic range increases with intra-annual precipitation isotopic range, which in turn relates to aridity in the equatorial bimodal rainfall region but is influenced by other hydroclimate processes in the region as a whole. Finally, we address isotopic seasonality during the Early Pleistocene in eastern Africa using oxygen isotopes in fossil equid teeth from southwestern Kenya (n = 11) and northern Tanzania (n = 5). We find variable isotopic seasonality in the past, similar to present-day eastern Africa, consistent with the notion that hominins and other mammals were able to accommodate environmental variability on intra-annual scales in addition to well-documented orbital cycles.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 260〈/p〉 〈p〉Author(s): Dustin Trail, Paul S. Savage, Frédéric Moynier〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The silicon isotope composition of detrital quartz and zircon have the potential to inform us about secular changes to the silica cycle and weathering reactions on Earth. However, inferring source melt Si isotope composition from out-of-context minerals is hampered by the fact that, to-date, there is limited Si isotope equilibrium fractionation data for minerals. Here, we report experimental data to constrain Si isotope equilibrium fractionation between zircon and quartz, using two fundamentally different strategies, but with the same experimental design. First, zircon and quartz were hydrothermally synthesized from Zr(OH)〈sub〉4〈/sub〉 and SiO〈sub〉2〈/sub〉 at 1.5 GPa and temperatures of 725, 800, and 900 °C. The second experimental strategy utilized the three-isotope method; the starting materials consisted of natural zircon and isotopically-labelled SiO〈sub〉2〈/sub〉. Three sets of hydrothermal time-series experiments were conducted at the same pressure and temperatures as the direct synthesis experiments. For all experiments, quartz and zircon were separated and 〈sup〉30〈/sup〉Si/〈sup〉28〈/sup〉Si and 〈sup〉29〈/sup〉Si/〈sup〉28〈/sup〉Si ratios were measured by solution multi-collector inductively coupled plasma mass spectrometry. The three-isotope method, which provides the best indicator of equilibrium fractionations, yields the following relationship:〈/p〉 〈p〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msup〉〈mrow〉〈mi mathvariant="normal"〉Δ〈/mi〉〈/mrow〉〈mn〉30〈/mn〉〈/msup〉〈mi mathvariant="normal"〉S〈/mi〉〈mi mathvariant="normal"〉i〈/mi〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉q〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈mi mathvariant="normal"〉z〈/mi〉〈mtext〉-〈/mtext〉〈mi mathvariant="normal"〉z〈/mi〉〈mi mathvariant="normal"〉r〈/mi〉〈mi mathvariant="normal"〉c〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mo linebreak="goodbreak" linebreakstyle="after"〉=〈/mo〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mn〉0.53〈/mn〉〈mo〉±〈/mo〉〈mn〉0.14〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mn〉6〈/mn〉〈/msup〉〈mo stretchy="false"〉/〈/mo〉〈msup〉〈mrow〉〈mi mathvariant="normal"〉T〈/mi〉〈/mrow〉〈mn〉2〈/mn〉〈/msup〉〈/mrow〉〈/math〉〈/p〉 〈p〉where Δ〈sup〉30〈/sup〉Si(qtz-zrc) is the relative difference in 〈sup〉30〈/sup〉Si/〈sup〉28〈/sup〉Si between quartz and zircon in permil, 〈em〉T〈/em〉 is temperature in K, and the error is 2 s.e. This relationship can be used to calculate the fractionation between zircon and other phases, and to estimate the Si isotope composition of the melt from which a zircon crystallized. The results may be used to assess equilibrium-disequilibrium isotope fractionations between quartz and zircon and co-existing phases in igneous rocks. These data can also be applied to out-of-context zircon (and quartz) to estimate the isotope composition of the host rock. Zircons crystallizing from a melt derived from purely igneous sources – i.e., without the involvement of “weathered” material – are expected to display a δ〈sup〉30〈/sup〉Si〈sub〉NBS-28〈/sub〉 (permil deviation of the 〈sup〉30〈/sup〉Si/〈sup〉28〈/sup〉Si from the NBS-28 standard) range from −0.7 to −0.35‰. Deviations from this range indicate assimilation of non-igneous (i.e., sedimentary) material in the melt source.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 260〈/p〉 〈p〉Author(s): Mathieu Chassé, William L. Griffin, Suzanne Y. O’Reilly, Georges Calas〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Scandium is often considered as immobile during chemical weathering, based on its low solubility. In contrast to other conservative (〈em〉i.e.〈/em〉 relatively immobile) elements incorporated into accessory minerals resistant to weathering (〈em〉e.g.〈/em〉 zirconium, thorium or niobium), the scarcity of scandium minerals indicates that the processes accounting for scandium’s immobilisation are distinctive. However, the evolution of scandium speciation during weathering is unknown, limiting the understanding of the processes controlling its dynamics in the critical zone. Exceptional scandium concentrations in east Australian laterites provide the possibility of unravelling these mechanisms. We follow scandium speciation through thick lateritic profiles (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mo〉〉〈/mo〉〈/mrow〉〈/math〉30 m) using a multiscale mineralogical and spectroscopic approach involving electron microprobe, laser-ablation–inductively coupled plasma mass spectrometry, selective leaching and X-ray absorption near-edge structure spectroscopy, complemented by mass-transfer calculations. We show that the initial reservoir of scandium contained in the parent rock is preserved under reducing conditions occurring in the lowest horizons of the profiles. The dissolution of scandium-bearing clinopyroxene generates smectitic clays that immobilise and concentrate scandium. It is subsequently trapped in the lateritic duricrust by goethite. Scandium mobilisation appears in this horizon and increases upward as a result of the dissolution of goethite, possibly assisted by dissolved organic matter, and the precipitation of hematite. Molecular-scale analyses demonstrate that changes in speciation govern scandium dynamics, with substitution in smectitic clays and adsorption on iron oxyhydroxides playing a crucial role in scandium immobility in the saprolite and lower lateritic duricrust. The higher affinity of scandium for goethite relative to hematite drives scandium mobilisation in the upper lateritic duricrust, leading to its concentration downward in the lower lateritic duricrust. These successive mechanisms illustrate how the unique complexity of the critical zone leads to scandium concentrations that may form new types of world-class scandium deposits. Comparison with conservative elements and with rare-earth elements, expected to have similar geochemical properties, emphasizes the unique behaviour of scandium in the critical zone. While scandium remains immobile during the early stages of weathering, intense and long-term alteration processes, observed in lateritic contexts, lead to scandium mobilisation. This study highlights the dependence of scandium mobility on weathering conditions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 259〈/p〉 〈p〉Author(s): Alex J. McCoy-West, J. Godfrey Fitton, Marie-Laure Pons, Edward C. Inglis, Helen M. Williams〈/p〉

Description: 〈p〉Publication date: Available online 1 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Wang-Ye Li, Hui-Min Yu, Juan Xu, Ralf Halama, Keith Bell, Xiao-Yun Nan, Fang Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To investigate the behaviour of Ba isotopes during carbonatite petrogenesis and to explore the possibility of using carbonatites to constrain the Ba isotopic composition of the mantle, we report high-precision Ba isotopic analyses of: (1) carbonatites and associated silicate rocks from the only active carbonatite volcano, Oldoinyo Lengai, Tanzania, and (2) Archean to Cenozoic carbonatites from Canada, East Africa, Germany and Greenland. Carbonatites and associated phonolites and nephelinites from Oldoinyo Lengai have similar δ〈sup〉137/134〈/sup〉Ba values that range from +0.01 to +0.03‰, indicating that Ba isotope fractionation during carbonatite petrogenesis is negligible. The limited variation in δ〈sup〉137/134〈/sup〉Ba values from −0.03 to +0.09‰ for most carbonatite samples suggests that their mantle sources have a relatively homogeneous Ba isotopic composition. Based on the carbonatites investigated in this work, the average δ〈sup〉137/134〈/sup〉Ba value of their mantle sources is estimated to be +0.04 ± 0.06‰ (2SD, 〈em〉n〈/em〉 = 16), which is similar to the average value of +0.05 ± 0.06‰ for mid-ocean ridge basalts. The lower δ〈sup〉137/134〈/sup〉Ba value of −0.08‰ in a Canadian sample and higher δ〈sup〉137/134〈/sup〉Ba values of +0.14‰ and + 0.23‰ in two Greenland samples suggest local mantle isotopic heterogeneity that may reflect the incorporation of recycled crustal materials in their sources.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 9 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Logan M. Combs, Arya Udry, Geoffrey H. Howarth, Minako Righter, Thomas J. Lapen, Juliane Gross, Daniel K. Ross, Rachel R. Rahib, James M.D. Day〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The martian meteorite Northwest Africa (NWA) 10169 is classified as a member of the geochemically enriched poikilitic shergottites, based on mineral composition, Lu-Hf and Sm-Nd isotope systematics, and rare earth element (REE) concentrations. Similar to other enriched and intermediate poikilitic shergottites, NWA 10169 is a cumulate rock that exhibits a bimodal texture characterized by large pyroxene oikocrysts (poikilitic texture) surrounded by olivine-rich interstitial material (non-poikilitic texture). Olivine chadacrysts and pyroxene oikocrysts have higher Mg#s (molar Mg/Mg + Fe) than those in the interstitial areas, suggesting that the poikilitic texture represents early-stage crystallization and accumulation, as opposed to late-stage non-poikilitic (i.e., interstitial material) crystallization. Calculated oxygen fugacity values are more reduced (FMQ −2.3 ± 0.2) within the poikilitic regions, and more oxidized (FMQ −1.1 ± 0.1) within the interstitial areas, likely representing auto-oxidation and degassing during magma crystallization. Calculated parental melt compositions using olivine-hosted melt inclusions display a dichotomy between K-poor and K-rich melts, thus possibly indicating mixing of parental melt with K-rich melt. The 〈sup〉176〈/sup〉Lu-〈sup〉176〈/sup〉Hf crystallization age for NWA 10169 is 167 ± 31 Ma, consistent with the ages reported for other enriched shergottites. Based on the isochron initial 〈sup〉176〈/sup〉Hf/〈sup〉177〈/sup〉Hf value, the modeled source 〈sup〉176〈/sup〉Lu/〈sup〉177〈/sup〉Hf composition for NWA 10169 is 0.02748 ± 0.00037, identical within uncertainty to the source compositions of the enriched shergottites Shergotty, Zagami, LAR 06319, NWA 4468, and Roberts Massif (RBT) 04262, suggesting a shared, long-lived geochemical source, and distinct from the source of other enriched shergottites Los Angeles, NWA 856, and NWA 7320. This study reveals that at least two sources are responsible for the enriched shergottites, and that the martian mantle is more heterogeneous than previously thought. Additionally, the enriched shergottites, which share a source with NWA 10169, have consistent crystallization ages and magmatic histories, indicating that a common magmatic system on Mars is likely responsible for the formation of this group.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 261〈/p〉 〈p〉Author(s): Matthew R. Warke, Nicholas P. Edwards, Roy A. Wogelius, Phillip L. Manning, Uwe Bergmann, Victoria M. Egerton, Katalina C. Kimball, Russell J. Garwood, Nicolas J. Beukes, Stefan Schröder〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Ancient stromatolites can provide key insights into the early evolution of life on Earth. Neoarchean fenestrate stromatolites from the ∼2520 Ma Upper Nauga Formation (Transvaal Supergroup, South Africa) preserve cuspate morphologies. They possess clearly delineated support and drape structures interpreted as dolomitized microbial mat material. Petrographic observations show that the biogenic structures are composed of planar-s to non-planar ferroan dolomite, encased in ferroan calcite, including herringbone calcite textures. The cuspate stromatolites were analyzed using Synchrotron Rapid Scanning X-Ray Fluorescence (SRS-XRF) and more conventional techniques to determine: (i) whether element distributions could be distinguished in ancient stromatolites at both cm to dm scales, (ii) whether element distributions show variation between biogenic and abiogenic textures, and (iii) the sample’s paragenesis. The distributions of Ca, Fe, Mn, Pb, Cu, As, Br, Al, Si, P, and S directly correspond to dolomitized stromatolitic structures and show trace element distributions are principally controlled by calcite and dolomite occurrence. Dolomite formation was mainly driven by seawater-derived fluids given the high concentrations of Fe and retention of marine shale-normalized rare earth element and yttrium (REY〈sub〉SN〈/sub〉) patterns, however the spatial association of dolomite to stromatolite structures may reflect microbially-influenced mineral nucleation. Given the complexity of this sample’s paragenetic evolution, trace metal distributions cannot be conclusively tied to specific metabolic pathways, bioaccumulation or passive binding, however, the results show SRS-XRF can be used for quantifiable, spatial, 〈em〉in-situ〈/em〉 investigation of ancient microbialites.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): D.L. Schrader, T.J. Zega〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Sulfide minerals occur in many types of extraterrestrial samples and are sensitive indicators of the conditions under which they formed or were subsequently altered. Here we report that chemical and petrographic analyses of Fe,Ni sulfides can be used to determine the metamorphic type of the host LL chondrite, and constrain their alteration conditions. Our data show that the major- and minor-element compositions of the pyrrhotite-group sulfides (dominantly troilite) and pentlandite vary with degree of thermal metamorphism experienced by their host chondrite. We find that Fe,Ni sulfides in LL3 chondrites formed during chondrule cooling prior to accretion, whereas those in LL4 to LL6 chondrites formed during cooling after thermal metamorphism in the parent body, in agreement with previous work. High degrees of shock (i.e., ≥S5) caused distinct textural, structural, and compositional changes that can be used to identify highly shocked samples. Distinct pyrrhotite-pentlandite textures and minerals present in Appley Bridge (LL6) suggest that they cooled more slowly and therefore occurred at greater depth(s) in the host parent body than those of the other metamorphosed LL chondrites studied here. Sulfides in all LL chondrites studied formed under similar sulfur fugacities, and the metamorphosed LL chondrites formed under similar oxygen fugacities. The data reported here can be applied to the study of other LL chondrites and to sulfides in samples of asteroid Itokawa returned by the Hayabusa mission in order to learn more about the formation and alteration history of the LL chondrite parent body.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Manfred Vogt, Jens Hopp, Hans-Peter Gail, Ulrich Ott, Mario Trieloff〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Earth’s mantle contains Ne resembling the solar wind implanted Ne-B component in meteorites (〈sup〉20〈/sup〉Ne/〈sup〉22〈/sup〉Ne〈sub〉Ne-B〈/sub〉: ∼12.7). The atmosphere, instead, displays a “planetary” signature (〈sup〉20〈/sup〉Ne/〈sup〉22〈/sup〉Ne〈sub〉Atm〈/sub〉: 9.80). We explore the parameter space of a model that explains these isotopic variations by the contribution of late accreting volatile-rich material (e.g., carbonaceous chondrite-like) to Earth́s atmosphere, while Earth́s mantle incorporated solar wind type Ne that was previously implanted into part of the accreting material.〈/p〉 〈p〉Analyses of the present-day terrestrial influx mass distributions show two major peaks at large bodies 〉1km and small ∼200 µm dust particles. The latter dominate the influx of the surface implanted Ne-B component. Ne measurements of small particles define a maximum surface flux (neon reaching the terrestrial surface) peaking at 9 µm, while larger micrometeorites experience ablation losses and isotopic fractionation upon atmospheric entry. Using these data, we reconstruct the unfractionated Ne-B upper atmosphere flux which peaks at ∼75 µm. As the extraterrestrial influx mass distribution between larger bodies and debris dust is governed by equilibrium due to collisions and fragmentation, it is an approximation of the early solar system (after nebula dissipation), where the mass distribution was similar but total fluxes were higher.〈/p〉 〈p〉Contributions of Ne-B by small dust and planetary Ne-A from larger bodies strongly depend on formation region. Originating around the 1 AU region, early accretionary fluxes were dominated by Ne-B as large bodies likely contained only negligible amounts of Ne-A. Ne-B will be ultimately delivered to the earliest protoatmosphere by impact or thermal degassing and a significant fraction of Ne-B can enter the Earth́s interior via dissolution into a magma ocean before the Moon-forming impact. After the Moon-forming impact, Ne-B reenters the atmosphere by mantle degassing and a later meteoritic contribution modified the atmospheric composition. This meteoritic component was likely dominated by Ne-A, as the only remaining planetesimals at that time were in the asteroid belt or beyond, leading to preferential contributions of carbonaceous chondrite-type material.〈/p〉 〈p〉In our model we take into account possible variations of several parameters, e.g. the isotopic composition of the late accretion (i.e., 〈sup〉20〈/sup〉Ne/〈sup〉22〈/sup〉Ne: 5.2–9.2). For example, a 〈sup〉20〈/sup〉Ne/〈sup〉22〈/sup〉Ne ratio of 8.2 (Ne-A composition) would imply ∼2% mass increase of Earth from CC-type material after the Moon-forming impact, and would require that todaýs atmosphere (〈sup〉20〈/sup〉Ne/〈sup〉22〈/sup〉Ne=9.8) formed by roughly equal mixing of late accreted Ne-A and mantle Ne-B. The amount of Ne-B added from the mantle implies a certain degree of mantle degassing (in this case 82–96%, depending on todaýs mantle neon inventory) and constrains two further parameters: the fraction of solar wind irradiated material delivered to Earth before the Moon-forming impact and the magma ocean depth. The latter determines the fraction of Ne-B dissolved from a protoatmosphere. For example, magma ocean depths between 500 and 2900 km allow 4–15% dissolution of the protoatmospheric Ne-B inventory, and would require only less than 10% of irradiated accreting material. Only unreasonable magma ocean depths lower than 200 km require several ten percent of irradiated material.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 261〈/p〉 〈p〉Author(s): Christoph Burkhardt, Nicolas Dauphas, Ulrik Hans, Bernard Bourdon, Thorsten Kleine〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Isotope anomalies among planetary bodies provide key constraints on planetary genetics and the solar system's dynamical evolution. However, to unlock the full potential of these anomalies for constraining the processing, mixing, and transport of material in the disk it is essential to identify the main components responsible for producing planetary-scale isotope variations, and to investigate how they relate to the isotopic heterogeneity inherited from the solar system’s parental molecular cloud. To address these issues we measured the Ti and Sr isotopic compositions of Ca,Al-rich inclusions (CAIs) from the Allende CV3 chondrite, as well as acid leachates and an insoluble residue from the Murchison CM2 chondrite, and combine these results with literature data for presolar grains, hibonites, chondrules, and bulk meteorites. Our analysis reveals that the internal mineral-scale nebular isotopic heterogeneity as sampled by leachates and presolar grains is largely decoupled from the planetary-scale isotope anomalies as sampled by bulk meteorites. We show that variable admixing of CAI-like refractory material to an average inner solar nebula component can explain the planetary-scale Ti and Sr isotope anomalies and the elemental and isotopic difference between non-carbonaceous (NC) and carbonaceous (CC) nebular reservoirs for these elements.〈/p〉 〈p〉Combining isotope anomaly data for a large number of elements (Ti, Sr, Ca, Cr, Ni, Zr, Mo, Ru, Ba, Nd, Sm, Hf, W, and Os) reveals that the offset of the CC from the NC reservoir towards the composition of CAIs is a general trend and not limited to refractory elements. This implies that the CC reservoir is the product of mixing between NC material and a reservoir (called IC for Inclusion-like Chondritic component) whose isotopic composition is similar to that of CAIs, but whose chemical composition is similar to bulk chondrites. In our preferred model, the distinct isotopic compositions of these two nebular reservoirs reflect an inherited heterogeneity of the solar system's parental molecular cloud core, which therefore has never been fully homogenized during collapse. Planetary-scale isotopic anomalies are thus caused by variable mixing of isotopically distinct primordial disk reservoirs, the selective processing of these reservoirs in different nebular environments, and the heterogeneous distribution of the thereby forming nebular products.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 261〈/p〉 〈p〉Author(s): Florian Scholz, Mark Schmidt, Christian Hensen, Sümeyya Eroglu, Sonja Geilert, Marcus Gutjahr, Volker Liebetrau〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Enrichments of highly reactive iron (Fe) (sum of Fe (oxyhydr)oxide, carbonate and sulfide minerals) in marine sediments and sedimentary rocks are commonly interpreted as an indication of anoxic conditions in the bottom water at the time of deposition. The model system for this proxy rationale is the semi-restricted Black Sea, where sediments underneath the anoxic and sulfidic (i.e., euxinic) deep-water are enriched in reactive Fe, which was mobilized from the surrounding shelf areas. To test whether such a shelf-to-basin Fe shuttle can operate in semi-restricted basins without euxinic deep water, we investigated sedimentary Fe speciation and Fe isotope compositions in sediments of the Guaymas Basin, Gulf of California. Sediments on the slope underneath the eastern equatorial Pacific oxygen minimum zone and sediments within the oxic deep basin are both enriched in reactive Fe, with reactive Fe making up 45 ± 11% of the total Fe pool. The following mechanisms may contribute to these Fe enrichments: (1) Release of dissolved Fe from anoxic shelf and slope sediments followed by lateral transport of dissolved and/or particulate Fe in the water column; (2) preferential transport of fine-grained, terrigenous particles with a high reactive Fe content into the basin; (3) microbially mediated conversion of non-reactive silicate minerals to reactive Fe minerals during transport; (4) hydrothermal venting and lateral Fe transport within the deep water. The first process can explain reactive Fe enrichments in slope sediments, whereas all processes may contribute to sedimentary Fe enrichments in the deeper basin.〈/p〉 〈p〉The δ〈sup〉56〈/sup〉Fe value of sediments increases from shelf to slope and decreases from the slope into the basin. This lateral pattern of δ〈sup〉56〈/sup〉Fe, as well as the pattern of Fe enrichment, is similar to that observed in other marine systems with a Fe shuttle. However, the size of the Fe enrichment, and the range in δ〈sup〉56〈/sup〉Fe (−0.06 to +0.16‰) is smaller. This difference is due to higher terrigenous sedimentation rates in the Guaymas Basin and, therefore, more intense dilution of shuttle-derived reactive Fe. We argue that, depending on the extent of bathymetric restriction and terrigenous background sedimentation, reactive Fe enrichments can form under a broad range of redox conditions and in diverse sedimentary environments. The concepts applied in this study can be used to identify those circumstances in the paleo-record.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Zhen Tian, Heng Chen, Bruce Fegley, Katharina Lodders, Jean-Alix Barrat, James M.D. Day, Kun Wang (王昆)〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report new high-precision stable K isotope data for thirty achondrites, including three martian meteorites, one lunar meteorite, one ordinary chondrite, four terrestrial igneous United States Geological Survey (USGS) reference materials, and twenty howardite–eucrite–diogenite [HED] meteorites. The four martian samples define a relatively narrow δ〈sup〉41〈/sup〉K range with an average of −0.36 ± 0.12‰ (2 SD) that is slightly heavier than the Bulk Silicate Earth (BSE) K isotopic composition (−0.48 ± 0.03‰). Except for the four Northwest Africa samples which were terrestrially contaminated, all HED meteorites reveal substantial 〈sup〉41〈/sup〉K enrichment compared to BSE, lunar samples, martian meteorites, and chondrites. We propose that the average δ〈sup〉41〈/sup〉K (+0.36 ± 0.16‰) obtained from HED meteorites is representative of Bulk Silicate 4-Vesta. The coupled volatile depletion and heavy K isotope enrichment in 4-Vesta could be attributed to both nebula-scale processes and parent-body events. The asteroid 4-Vesta is likely to have accreted from planetary feedstocks that have been significantly volatile-depleted prior to the major phases of planetary accretion in the early Solar System, with secondary effects of K loss during accretionary growth and magma ocean degassing.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 261〈/p〉 〈p〉Author(s): Benjamin J.A. Moulton, Grant S. Henderson, Christine Martinet, Valérie Martinez, Camille Sonneville, Dominique de Ligny〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Silicate glasses show widely varying changes in their longitudinal sound velocities below 10 GPa. These changes are often attributed to structural changes in the glass (or liquid) network. This study reports both sound velocities and structural analysis of CaAl〈sub〉2〈/sub〉Si〈sub〉2〈/sub〉O〈sub〉8〈/sub〉 (anorthite) glass 〈em〉in situ〈/em〉 up to 20 GPa, based on Brillouin and Raman spectroscopy results. 〈em〉In situ〈/em〉 high-pressure Brillouin spectra of CaAl〈sub〉2〈/sub〉Si〈sub〉2〈/sub〉O〈sub〉8〈/sub〉 glass were taken during two compression-decompression cycles. The second compression-decompression cycle up to 12 GPa displayed a perfectly elastic behavior indicating that 8% faster sound velocity arose from permanent densification during the first cycle. The longitudinal sound velocity was calculated from previously reported refractive index data and displayed distinct changes in behavior at 2 and 5 GPa. Anorthite (CaAl〈sub〉2〈/sub〉Si〈sub〉2〈/sub〉O〈sub〉8〈/sub〉) glass displays an anomalous decrease in the longitudinal sound velocity up to ∼2 GPa. Above this pressure its longitudinal sound velocity is insensitive to pressure until 5 GPa and thereafter it displays a positive pressure dependence. The longitudinal sound velocity of CaAl〈sub〉2〈/sub〉Si〈sub〉2〈/sub〉O〈sub〉8〈/sub〉 glass is quite distinct from both polymerized (e.g. silica, albite) and depolymerized (e.g. diopside) silicate glasses.〈/p〉 〈p〉Raman spectroscopy reveals that below 2 GPa there is a rapid decrease in the inter-tetrahedral angle within the aluminosilicate network. In fact, the sigma parameter, indicative of the overall intertetrahedral angle, displays three distinct pressure regimes comparable to the longitudinal sound velocity. The lowest pressure regime, 〈2 GPa, involves a wrinkling of four-membered tetrahedral rings as well as the formation of fivefold coordinated aluminum. Between 2 and 5 GPa, the closure in the inter-tetrahedral angle becomes weak and the Raman bands associated with the three- and four-membered tetrahedral rings are lost. Above 5 GPa, new contributions to the spectra indicate the presence of either fivefold silicon or sixfold aluminum. Comparison with silica and sodium aluminosilicate glasses leads to the suggestion that the sharp changes at 5 GPa may be attributed to the formation of highly coordinated silicon because aluminum coordination changes are thought to be continuous. The loss of the tetrahedral rings and the formation of highly coordination tetrahedral cations, such as Si or Al, could be accomplished if the tetrahedral cations form edge-sharing geometries.〈/p〉 〈p〉In contrast to known polymerized (e.g. silica, albite) and depolymerized (e.g. diopside, enstatite) silicate glasses, CaAl〈sub〉2〈/sub〉Si〈sub〉2〈/sub〉O〈sub〉8〈/sub〉 glass displays a weak negative pressure dependence, as found in polymerized compositions, but a high overall longitudinal sound velocity, as found in depolymerized systems. These structure-property relationships suggest that fragility is a better measure of the high-pressure behavior of silicate glasses.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 261〈/p〉 〈p〉Author(s): C. Ercolani, D. Lemarchand, A. Dosseto〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Chemical weathering contributes to the regulation of the global carbon cycle and biogeochemical cycles. Accordingly, the identification of the parameters that control weathering reactions and transport of weathering signals at the catchment scale is essential. The use of boron (B) isotopes have been shown to be a useful proxy in tracing weathering reactions due to large isotope fractionation during weathering processes. However, our knowledge of how boron isotopes record the weathering regime at the catchment scale and how that weathering signal is transported from source areas to the depositional environment remains limited. Here we characterize B isotope and major element behavior during chemical weathering and transport by analyzing the B isotopic (δ〈sup〉11〈/sup〉B) and element compositions of riverine material (riverbank sands (〈63 µm), clay fractions (〈2 µm) extracted from sands, and dissolved load) along the course of the Murrumbidgee River (NSW, Australia), its upstream tributaries, and monolithologic subcatchments.〈/p〉 〈p〉In the Murrumbidgee, two distinct weathering regimes are present, one where mineral dissolution is associated with minimal neoformation at higher elevations and another where mineral neoformation dominates at lower elevations and in granitic lithologies. Significant B isotope difference between the clay fraction and the bedrock (Δ〈sup〉11〈/sup〉B〈sub〉clay-bedrock〈/sub〉) is observed in most monolithological catchments at high mean elevation (excluding granites), which correlates with a large B depletion. Smaller isotope difference between the clay fraction and bedrock is observed in monolithological catchments at lower elevations as well as in granitic catchments at all elevations and is associated with limited B removal. These results suggest that lithology and catchment topography influence B mobility during weathering and the isotopic composition of weathering products. By mass balance calculation, the B isotope and chemical composition of the clay and sand fractions in the Murrumbidgee River can be explained as a mixture of the clays and sands produced throughout the catchment delivered to the main channel by the tributaries. These results indicate that there is little or no chemical and isotopic modification of the river sediment during fluvial transport and that weathering signal produced in the sediment source areas is transferred to the depositional environment without significant modification. The boron content of the clay-sized fraction (∼40 ppm) is several orders of magnitude greater than that of the dissolved load while B isotope compositions of the clay-sized fraction are isotopically much lighter (up to 40‰). Because a maximum isotopic difference of 30‰ between the dissolved and solid phases is expected during adsorption processes, the observed isotope compositions in the dissolved load and the sediment clay fraction cannot be explained by pH-driven B partitioning. These observations suggest that clays are not directly precipitating from solutions compositionally similar to surface waters; deeper soil solutions are expected to play a significant role in clay formation.〈/p〉 〈p〉This research highlights the potential of B isotopes in river sediments to describe the present and past weathering regimes at the catchment scale, including possible paleoenvironment reconstruction as the B isotope signature of riverine material records the conditions of its formation.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Fabrizio Campanale, Enrico Mugnaioli, Luigi Folco, Mauro Gemmi, Martin R. Lee, Luke Daly, Billy P. Glass〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in impact rocks in the 1960s. Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs). Quartz and euhedral microcrystalline coesite are in direct contact, showing a recurrent pseudo iso-orientation, with the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mtext〉[1〈/mtext〉〈mover〉〈mtext〉1〈/mtext〉〈mo〉¯〈/mo〉〈/mover〉〈mtext〉1]〈/mtext〉〈/mrow〉〈/math〉* vector of quartz near parallel to the [010]* vector of coesite. Moreover, discontinuous planar features in coesite domains are in textural continuity with PDFs in adjacent quartz relicts. These observations indicate that quartz transforms to coesite after PDF formation and through a solid-state martensitic-like process involving a relative structural shift of 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mo stretchy="false"〉{〈/mo〉〈mover〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈mn〉011〈/mn〉〈mo stretchy="false"〉}〈/mo〉〈/mrow〉〈/math〉 quartz planes, which would eventually turn into coesite (010) planes. This process further explains the structural relation observed between the characteristic (010) twinning and disorder of impact-formed coesite, and the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.svg"〉〈mfenced open="{" close="}"〉〈mrow〉〈mn〉10〈/mn〉〈mover〉〈mn〉1〈/mn〉〈mo〉¯〈/mo〉〈/mover〉〈mn〉1〈/mn〉〈/mrow〉〈/mfenced〉〈/math〉 PDF family in quartz. If this mechanism is the main way in which coesite forms in impacts, a re-evaluation of peak shock pressure estimates in quartz-bearing target rocks is required because coesite has been previously considered to form by rapid crystallization from silica melt or diaplectic glass during shock unloading at 30-60 GPa.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 20 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Shan Liu, Hui Liu, Zhu Wang, Yanping Cui, Rong Chen, Zhaofeng Peng, Songhu Yuan, Liang Shi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The microbial reduction of Fe(III) (oxyhydr)oxide is widespread in subsurface and plays a critical role in both the biogeochemical cycle of iron and the fate of contaminants. Monocyclic aromatic compounds are ubiquitous constituents of organic matter in many geologic environments and contaminated subsurface. Benzene is a typical monocyclic aromatic compound and frequently occurs in the subsurface environment. Due to its carcinogenicity and cytotoxicity, benzene may be toxic to the coexisted Fe(III)-reducing bacteria and thereby inhibit the microbial Fe(III) reduction. However, there is limited knowledge about the impact of the coexisting monocyclic aromatic compounds on the microbial Fe(III) reduction. In this study, the reduction of ferrihydrite by the dissimilatory iron-reducing bacterium 〈em〉Shewanella oneidensis〈/em〉 MR-1 (MR-1) was investigated in the presence of benzene. Results showed that benzene had a negligible impact on the growth, cell morphology and integrity of MR-1, but it promoted the microbial Fe(III) reduction. The promotion of microbial Fe(III) reduction is maximum at benzene concentration of 3.8 μM. In the presence of 3.8 μM benzene, the produced Fe(II) from microbial Fe(III) reduction in 60 h doubled that in the absence of benzene, and the Fe(II)-O content of mineral surface after reduction experiment increased 4.73%. The promotion of microbial Fe(III) reduction was ascribed to the benzene induced increase of cell membrane permeability, which facilitated extracellular electron transfer and the secretion and release of flavin mononucleotide (FMN) as electron shuttle or cofactor. The impacts of benzene on the FMN secretion and microbial Fe(III) reduction have broad implications for both the cycling of iron and the biogeochemical transformation of redox-sensitive elements and contaminants in the benzene-containing subsurface environments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 10 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Geoffrey H. Howarth, Juliane Gross〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Olivine chemistry has been widely used to track the petrogenesis of mafic and ultramafic magmas from their mantle source to eruption at the surface. A major challenge in these studies is deciphering crystal growth versus diffusion controlled zoning. Here we report a multi-element approach using high-precision electron microprobe techniques to evaluate crystal growth versus diffusion in kimberlitic olivine from the Benfontein kimberlite, South Africa. These results have implications for both the petrogenesis of kimberlite magmas and the understanding of crystal growth and diffusion-based zoning in igneous olivine in general.〈/p〉 〈p〉The Benfontein olivine contain multiple phosphorous (P)-rich and P-poor zones. Core zones are characterized by homogenous low-P (〈78 ppm) concentrations, consistent with xenocrystic origins. Gradational changes in Fo, Ni, Cr and other minor/trace elements at core-margins are similarly characterized by constant low-P concentrations that are indistinguishable from the central regions of the core. Olivine P-maps effectively outline the original xenocryst core, whereas gradational margins are interpreted as diffusion controlled zones related to early-stage equilibration of xenocrystic olivine with proto-/kimberlite melt.〈/p〉 〈p〉Multiple P-poor (100–150 ppm) and P-rich (200–450 ppm) concentric, oscillatory zones with inclusions of kimberlitic oxide phases are observed surrounding the low-P xenocrystic cores. Oxide phases change from chromite in the inner zones to ilmenite in the intermediate zones to magnetite-rich spinel in the outer zones of the olivine. The P-zoning corresponds with changes in Fo content implying that stages of crystal growth was preserved by both fast and slow diffusing elements rather than diffusion processes. Elements compatible with olivine (±chromite) crystallization (i.e., Ni and Cr) display a constant decrease across all zones, suggesting that magma mixing is unlikely a controlling process for P-zoning. We interpret P-rich zones to result from stages of solute trapping related of rapid disequilibrium growth driven by extrinsic factors such as changes in pressure-temperature during kimberlite evolution. In contrast, P-poor zones represent stages of equilibrium crystal growth. The outer olivine zones are characterized by an increase in Fo contents up to Fo〈sub〉96〈/sub〉, and in conjunction with a change to more Fe〈sup〉3+〈/sup〉-rich oxides, suggest late stage increase in 〈em〉f〈/em〉O〈sub〉2〈/sub〉.〈/p〉 〈p〉Correlated Fo and P changes in the Benfontein olivine suggest that major element zonation represents an example where crystal growth-induced Fo zoning has been preserved in olivine. Furthermore, P-rich olivine zones preserve evidence for concentric growth rather than common dendritic structures seen in other occurrences. These results have implications for understanding the effect of magma dynamics and changes in pressure-temperature-〈em〉f〈/em〉O〈sub〉2〈/sub〉 conditions on olivine growth in igneous rocks.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 7 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Russell N. Drysdale, Giovanni Zanchetta, Ilaria Baneschi, Massimo Guidi, Ilaria Isola, Isabelle Couchoud, Leonardo Piccini, Alan Greig, Henri Wong, Jon D. Woodhead, Eleonora Regattieri, Ellen Corrick, Bence Paul, Christoph Spötl, Eleonor Denson, Jay Gordon, Stephane Jaillet, Florian Dux, John C. Hellstrom〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation. This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients (where the partitioning (〈em〉D〈/em〉) of element X (〈em〉D〈/em〉〈sub〉X〈/sub〉) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However, dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’ subaqueous calcite from a drill core taken through CD3 to derive 〈em〉D〈/em〉〈sub〉Mg〈/sub〉, 〈em〉D〈/em〉〈sub〉Sr〈/sub〉, 〈em〉D〈/em〉〈sub〉Ba〈/sub〉 and 〈em〉D〈/em〉〈sub〉U〈/sub〉 then compare these with published cave, cave-analogue and seawater-analogue studies. The 〈em〉D〈/em〉〈sub〉Mg〈/sub〉 for CD3 is anomalously high (0.042 ± 0.002) compared to previous estimates at similar temperatures (∼8°C). The 〈em〉D〈/em〉〈sub〉Sr〈/sub〉 (0.100 ± 0.007) is similar to previously reported values, but data from this study as well as those from Tremaine and Froelich, 2013, Day and Henderson, 2013 suggest that [Na/Sr] might play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. 〈em〉D〈/em〉〈sub〉Ba〈/sub〉 in CD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst 〈em〉D〈/em〉〈sub〉U〈/sub〉 (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates (〈1 μm a〈sup〉-1〈/sup〉), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis of the upper 7 μm of CD3, regarded as ‘modern’ for the purposes of this study, reveals considerable heterogeneity, particularly for Sr, Ba and U, which is potentially indicative of compositional zoning. This reinforces the need to conduct 2D mapping and/or multiple laser passes to capture the range of time-equivalent elemental variations prior to palaeoclimate interpretation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 2 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Heather D. Hanna, Xiao-Ming Liu, Young-Rok Park, Suzanne M. Kay, Roberta L. Rudnick〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report [Li] and δ〈sup〉7〈/sup〉Li values for a well-characterized suite of 52 geographically (165–184°W), compositionally (SiO〈sub〉2〈/sub〉 = 46–70 wt.%), and temporally (0–38 Ma) diverse lavas and intrusive samples. The δ〈sup〉7〈/sup〉Li in these rocks range from −0.7‰ to +14.2‰, with 32 of the 35 lavas and 12 of the 17 intrusive samples falling within the depleted mantle range (δ〈sup〉7〈/sup〉Li +1.6 to +5.6‰), as sampled by mid-ocean ridge basalts (MORB). The δ〈sup〉7〈/sup〉Li values of Aleutian lavas do not exhibit the spatial trends observed in other slab component tracers, nor do δ〈sup〉7〈/sup〉Li values correlate with any slab component indicators, such as radiogenic isotopes, oxygen isotopes, or trace element ratios such as Cs/La and Th/La. The δ〈sup〉7〈/sup〉Li values in Aleutian intrusions also do not exhibit temporal trends, however, an overall positive relationship exists between δ〈sup〉7〈/sup〉Li and Th/Nd. Mixing models for δ〈sup〉7〈/sup〉Li and 〈sup〉143〈/sup〉Nd/〈sup〉144〈/sup〉Nd values suggest that Aleutian samples within or above the MORB δ〈sup〉7〈/sup〉Li range can be explained by addition of 〈1–2% sediment-derived aqueous fluid and ≤3% sediment melt to depleted mantle; both are required to explain the range in δ〈sup〉7〈/sup〉Li that is observed. Sediment-derived fluid exerts a stronger control on Aleutian samples having higher δ〈sup〉7〈/sup〉Li values than the MORB range, while sediment melt skews the Li isotopic compositions of MORB-range samples to slightly lower values than if sediment fluid was the only slab influence. Our study demonstrates that a slab signature may be deciphered via modeling even in arcs where spatial trends in δ〈sup〉7〈/sup〉Li values and correlations with slab component indicators are lacking.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 2 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Qing-Feng Mei, Jin-Hui Yang, Ya-Fei Wang, Hao Wang, Peng Peng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The short-lived 〈sup〉182〈/sup〉Hf-〈sup〉182〈/sup〉W system has been used to constrain the early Earth’s accretion and differentiation. This paper reports high precision W isotopic compositions and trace element concentrations for Archean (4.0–3.0 Ga) intermediate to felsic rocks from the Slave, North China, and Kaapvaal cratons to discuss early Earth differentiation and mantle mixing processes. The 4.0–3.8 Ga diorite and trondhjemite samples from the Acasta Gneiss Complex (AGC) in the Slave Craton and Anshan Complex in the North China Craton have positive μ〈sup〉182〈/sup〉W values ranging from 8.3 ± 3.6 to 14.5 ± 4.0, whereas the 3.36–2.95 Ga TTG rocks from the North China Craton have a uniform modern mantle-like μ〈sup〉182〈/sup〉W value of 0.7 ± 3.9 (2 SD, n = 11); with one exception, the 3.32 Ga old sample F28-2, which has a positive μ〈sup〉182〈/sup〉W anomaly (7.3 ± 3.9 and 13.0 ± 3.2 for two individual analyses). The Earth’s oldest potassic granites, occurring as conglomerates in the Moodies Group, an intrusive trondhjemite, and an amphibolite with ages of ~3.55 Ga in the Kaapvaal Craton exhibit W isotopic compositions indistinguishable from what is proposed for the modern mantle. The positive 〈sup〉182〈/sup〉W anomalies in the 4.0–3.8 Ga rocks could possibly be the result of either early mantle differentiation that occurred within the lifetime of 〈sup〉182〈/sup〉Hf or a partial lack of late accreted material. The signature of 〈sup〉182〈/sup〉W excess in the 3.32 Ga sample (F28-2) is inherited from earlier crust, presumably similar to the 3.8 Ga TTGs, either by melting or crustal contamination. The transition in μ〈sup〉182〈/sup〉W values from positive to near-zero would indicates that a significant event occurred at ∼3.6 Ga which caused efficient mixing of early differentiated mantle and late accreted material, possibly indicating the transition of tectonic regimes from plume to plate tectonics on the Earth.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Cynthia McClain, Scott Fendorf, Shane Johnson, Alana Menendez, Kate Maher〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In agricultural regions of California where ultramafic sediments containing naturally occurring Cr(III) are present, correlations between Cr(VI) and nitrate in groundwater have been attributed to oxidation of Cr(III) in vadose sediments and mobilization by areal recharge, including irrigation return. However, the distribution of Cr and nitrate through the vadose zone have yet to be evaluated together to investigate the controls on geogenic Cr(VI) occurrence and resulting Cr(VI) production rates and export fluxes to groundwater. To develop a framework for evaluating geogenic Cr(VI) contamination, we analyze vadose zone sediment cores from the southwestern Sacramento Valley of California at high spatial resolution. In the sandy, oxic, ultramafic, Cr-rich Holocene alluvial sediment, Cr(III) is oxidized to Cr(VI), resulting in increasing Cr(VI) concentrations with depth up to 79 μg/kg. Oxidation is likely associated with μ-meter scale co-located Mn(IV)-oxides. Within the fine-grained Pleistocene sediments beneath the historic high water table (5-18 m), Cr(VI) concentrations decrease with depth to 〈 30 μg/kg due to subsequent reduction. Patterns in Cr(VI) concentration parallel nitrate due to the similar depth of production zones, oxidation-reduction potential and geochemical behavior. Field evidence in the shallow profile also supports Cr(VI) production by enhanced Cr(III) dissolution due to nitrification-induced acidification and subsequent oxidation by Mn-oxides. From Cr(VI) and nitrate concentration gradients with depth through the vadose zone (∼20 m), we calculate field-based net production and removal rates, quantify vadose zone storage (156 to 1168 kg Cr(VI)/km〈sup〉2〈/sup〉; 1 × 10〈sup〉5〈/sup〉 to 2.6 × 10〈sup〉5〈/sup〉 kg N/km〈sup〉2〈/sup〉), and estimate export fluxes to groundwater (40 to 1314 kg Cr(VI)/km〈sup〉2〈/sup〉/yr; 5 to 487 kg N/km〈sup〉2〈/sup〉/yr). The framework we present for evaluating vadose zone geogenic Cr(VI) contamination highlights the compounding effects that vadose zone lithology and hydrology can have on solute production, accumulation, development of redoxclines, and subsequent distribution of redox sensitive elements in alluvial sediment and groundwater.〈/p〉〈/div〉 〈/div〉 〈div xml:lang="en"〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0016703719304685-ga1.jpg" width="426" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 264〈/p〉 〈p〉Author(s): James McManus, C. Geoffrey Wheat, Wolfgang Bach〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We sampled low temperature (〈15 °C) hydrothermal fluids that discharge from the Dorado Outcrop on the eastern flank of the Cocos Ridge. Our sampling techniques included discrete sample collection using 〈em〉DSV Alvin〈/em〉 and autonomous time-series samplers deployed using 〈em〉RSV Jason II〈/em〉. The sampled fluids are enriched in dissolved inorganic carbon (DIC) by ∼0.10 mM and have a δ〈sup〉13〈/sup〉C〈sub〉DIC〈/sub〉 that is on average between 0.2 and 0.5‰ lower than the surrounding bottom seawater. Assuming that the measured DIC enrichment is representative of low temperature hydrothermal systems, the magnitude of the DIC source to the ocean would be 1 × 10〈sup〉12〈/sup〉 mol C/y, which is roughly the same magnitude as the high temperature hydrothermal source, but is more than a factor of three smaller than the estimated rate of carbon removal via carbonate precipitation within the ocean crust. Based on an isotope balance of the discharging fluids, which considers added sources of both basalt-derived inorganic and marine-derived organic carbon, the net DIC carbon isotope signature of vent fluids is most consistent with a primary carbon source from seawater (95.9%), plus a component from the weathering of basalt (3.4%) with a δ〈sup〉13〈/sup〉C value of −6‰, and a component from organic matter degradation (0.7%), with a δ〈sup〉13〈/sup〉C value of −22‰. This particular balance places the upper limit of organic carbon respiration at ∼0.3 × 10〈sup〉12〈/sup〉 mol C/y; however, if our DIC input estimate is too high, then the isotope balance requires a larger organic carbon component, which is not consistent with the dissolved oxygen and nitrate data. Although low temperature hydrothermal systems are often thought to be important locations for carbonate precipitation, there is little evidence for current carbonate precipitation at Dorado Outcrop. Similar trends in DIC are observed at North Pond, another low temperature (〈15 °C) ridge flank hydrothermal system. These data suggest that much of the current ridge flank discharge is a source of DIC to the ocean.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 262〈/p〉 〈p〉Author(s): Nicole Kowalski, Olaf Dellwig, Melanie Beck, Ulf Gräwe, Nadja Neubert, Thomas F. Nägler, Thomas H. Badewien, Hans-Jürgen Brumsack, Justus E.E. van Beusekom, Michael E. Böttcher〈/p〉

Description: 〈p〉Publication date: Available online 12 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): John F. Pernet-Fisher, Peter H. Barry, James M.D. Day, D. Graham Pearson, Sarah Woodland, Alexy M. Agashev, Lyudmila N. Pokhilenko, Nikolay P. Pokhilenko〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Siberian sub-continental lithospheric mantle (SCLM) is regionally heterogeneous due to the complex modification of ancient cratonic material by various metasomatic fluids and interaction with the Siberian plume at ∼250 Ma. Here, we assess the extent and origin of this heterogeneity by analysis of helium isotopes, rhenium-osmium isotopes, trace-element, and highly siderophile element [HSE: Os, Ir, Ru, Pt, Pd, Re] abundances in a suite of mantle-derived megacrystalline dunite xenoliths from the 360 Ma Udachnaya East kimberlite pipe, Siberia. This enables assessment of the style and extent of metasomatism acting to modify the volatile budget of the Siberian SCLM. The olivine megacrysts have 〈sup〉3〈/sup〉He/〈sup〉4〈/sup〉He values that range from 2.3 to 7.4 R〈sub〉A〈/sub〉 (where R〈sub〉A〈/sub〉 = the 〈sup〉3〈/sup〉He/〈sup〉4〈/sup〉He of air), outside the canonical range for the lithospheric mantle (6.1 ± 2.1 R〈sub〉A〈/sub〉; Day et al., 2015). High [He] and low U + Th concentrations in these olivine megacrysts indicate that the Udachnaya megacrystalline dunite xenolith suite has undergone minimal post-eruptive modification of He isotopes by 〈sup〉4〈/sup〉He recoil. We instead interpret He isotope variations to reflect pre- or syn-eruptive metasomatic signatures during kimberlite emplacement. The dunites can be divided into two groups, ultimately reflecting modification by two distinct metasomatic styles. Group 1 dunites are characterized by highly unradiogenic Os isotope compositions (γOs〈sub〉360Ma〈/sub〉 -16 to -13) and ancient melt depletion ages (∼3 Ga) typical of ancient cratonic lithospheric mantle; yet display a range of He isotope ratios (2.3 to 5.9 R〈sub〉A〈/sub〉). This indicates that group 1 dunites are modified by gas-rich metasomatic fluids acting to modify He abundances and isotope ratios, while preserving unradiogenic Os isotope and HSE abundance systematics. Group 2 dunites extend to significantly more radiogenic Os isotope signatures and higher He isotope values (γOs〈sub〉360Ma〈/sub〉 〉 -5 to +53; 〈sup〉3〈/sup〉He/〈sup〉4〈/sup〉He from 3.7 to 7.4 R〈sub〉A〈/sub〉). These two groups define a negative hyperbolic array between Os and He isotopes which we interpret to result from metasomatism by variable mixtures between: (1) a gas-rich and HSE-poor fluid (similar to the fluids acting to modify Group 1 dunites) with helium isotope compositions above the MORB range (〉 8 R〈sub〉A〈/sub〉) derived from the asthenosphere, and: (2) strongly radiogenic fluids (∼ 2 R〈sub〉A〈/sub〉; 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os 〉 0.25) likely sourced from within the SCLM. Thee Udachnaya megacrystalline dunite xenolith suite provides insight into how mantle-plume derived fluids interact with metasomatic fluids within the SCLM.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 6 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Weifu Guo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The carbonate clumped isotope thermometer, is a powerful tool in paleoclimate and paleoceanography research, because it constrains carbonate formation temperature based on the extent of 〈sup〉13〈/sup〉C and 〈sup〉18〈/sup〉O clumping within the carbonate lattice (Δ〈sub〉47〈/sub〉) and thus does not require knowledge of the isotopic composition of the water from which carbonates precipitate. However, there is growing evidence that kinetic processes in the precipitating solution, particularly the slow inter-conversion between CO〈sub〉2〈/sub〉 and HCO〈sub〉3〈/sub〉〈sup〉-〈/sup〉, can cause deviations of the clumped isotope composition of dissolved inorganic carbon (DIC) from their expected thermodynamic equilibrium values, leading to disequilibrium clumped isotope composition in the carbonate precipitates. Such disequilibrium effects impede the application of the carbonate clumped isotope thermometer to important climate archives (e.g., speleothems and corals), and hinder the realization of the full potential of this novel thermometer. Here, I systematically examine the patterns and controls of kinetic clumped isotope fractionations (i.e., Δ〈sub〉47〈/sub〉, Δ〈sub〉48〈/sub〉 and Δ〈sub〉49〈/sub〉) in the DIC-H〈sub〉2〈/sub〉O-CO〈sub〉2〈/sub〉 system through theoretical calculations and numerical modeling (IsoDIC), and explore their implications for the carbonate clumped isotope thermometry.〈/p〉 〈p〉I show that, in contrast to the large carbon and oxygen isotope effects, intrinsic clumped isotope fractionations associated with CO〈sub〉2〈/sub〉 hydration and hydroxylation and their reverse reactions (i.e., HCO〈sub〉3〈/sub〉〈sup〉-〈/sup〉 dehydration and dehydroxylation) will lead to only relatively small disequilibrium Δ〈sub〉47〈/sub〉 effects in the HCO〈sub〉3〈/sub〉〈sup〉-〈/sup〉 that are directly produced or consumed by these reactions. Instead, the disequilibrium Δ〈sub〉47〈/sub〉 effects observed in most natural and laboratory-synthesized carbonates arise in large part from the mixing or removal of DIC pools with similar Δ〈sub〉47〈/sub〉 compositions but distinct bulk carbon and oxygen isotope compositions. Further, my model simulations show characteristic evolutions of the clumped isotope composition of DIC during DIC-H〈sub〉2〈/sub〉O isotope exchange, CO〈sub〉2〈/sub〉 degassing, and CO〈sub〉2〈/sub〉 absorption, and predict correlated enrichments in δ〈sup〉13〈/sup〉C, δ〈sup〉18〈/sup〉O and Δ〈sub〉48〈/sub〉 but depletions in Δ〈sub〉47〈/sub〉 and Δ〈sub〉49〈/sub〉 of DIC during the early stage of CO〈sub〉2〈/sub〉 degassing and 〈em〉vice versa〈/em〉 during CO〈sub〉2〈/sub〉 absorption, yielding typical disequilibrium Δ〈sub〉47〈/sub〉/δ〈sup〉18〈/sup〉O, Δ〈sub〉48〈/sub〉/δ〈sup〉18〈/sup〉O and Δ〈sub〉47〈/sub〉/Δ〈sub〉48〈/sub〉 slopes of about -0.03, 0.03, and -1.0 for CO〈sub〉2〈/sub〉 degassing and about -0.02, 0.04, and -0.5 for CO〈sub〉2〈/sub〉 absorption. While both the physicochemical condition (e.g., T, pH, [DIC], pCO〈sub〉2〈/sub〉) and the isotopic composition (e.g., δ〈sup〉13〈/sup〉C, δ〈sup〉18〈/sup〉O, Δ〈sub〉47,〈/sub〉 Δ〈sub〉48〈/sub〉, Δ〈sub〉49〈/sub〉) of the aqueous solution and air CO〈sub〉2〈/sub〉 can affect the magnitudes of these disequilibrium isotope effects, the correlations among these disequilibrium effects are relatively insensitive to changes in most environmental parameters except the isotopic composition of the aqueous solution and air CO〈sub〉2〈/sub〉. The quantitative agreements between my model results and the existing observations from laboratory experiments and natural carbonates suggest that the model captures the key processes governing clumped isotope fractionations in the DIC-H〈sub〉2〈/sub〉O-CO〈sub〉2〈/sub〉-CaCO〈sub〉3〈/sub〉 system, and support the development of novel approaches to correct for disequilibrium clumped isotope effects in carbonate archives and derive accurate estimates of carbonate formation temperatures, e.g., based on the correlations between the disequilibrium Δ〈sub〉47〈/sub〉 and Δ〈sub〉48〈/sub〉 effects.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 263〈/p〉 〈p〉Author(s): Yang Bai, Ben-Xun Su, Yan Xiao, Chen Chen, Meng-Meng Cui, Xiao-Qing He, Li-Ping Qin, Bernard Charlier〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To investigate chromium diffusion kinetics in ultramafic cumulate minerals, we analyzed the Cr elemental and isotopic compositions of olivine, orthopyroxene, and chromite from the Stillwater layered intrusion. Core-to-rim compositional profiles reveal that Cr elemental concentrations decrease from 60 to 20 ppm in olivine and from 5000–4600 to 2700–2400 ppm in orthopyroxene. These zoned Cr distributions in olivine and orthopyroxene suggest that Cr was lost by diffusion into the melt. Olivine and orthopyroxene have δ〈sup〉53〈/sup〉Cr values ranging from –0.09 to 0.25‰ and from –0.11 to 0.07‰, respectively, higher than the values of coexisting chromite (–0.23 to –0.07‰). This isotopic disequilibrium can be explained by diffusion-driven kinetic fractionation during cooling. The preferential diffusion of light Cr isotopes from silicate minerals to the melt resulted in isotopically heavier olivine and orthopyroxene, but the kinetic diffusion between chromite and melt negligibly affected the isotopic compositions of chromite grains due to their high Cr concentrations. Modeling results based on the observed Cr contents and isotopic compositions of silicate minerals constrain the cooling time of the Peridotite Zone in the Stillwater magmatic system to have been 10–100 kyr.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 263〈/p〉 〈p〉Author(s): Zachary A. Torrano, Gregory A. Brennecka, Curtis D. Williams, Stephen J. Romaniello, Vinai K. Rai, Rebekah R. Hines, Meenakshi Wadhwa〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Calcium-aluminum-rich inclusions (CAIs) are the first solids to form in the early Solar System, and they exhibit nucleosynthetic anomalies in many isotope systems. The overwhelming majority of isotopic data for CAIs is limited to inclusions from the CV chondrite Allende and a few other CV, CO, CM, and ordinary chondrites. It is therefore important to ascertain whether previously reported values for CAIs are representative of the broader CAI-forming region and to make a more rigorous assessment of the extent and implications of isotopic heterogeneity in the early Solar System. Here, we report the mass-independent Ti isotopic compositions of a suite of 23 CAIs of diverse petrologic and geochemical types, including 11 from Allende and 12 from 7 other CV3 and CK3 chondrites; the data for CAIs from CK chondrites are the first reported measurements of Ti isotopic compositions of CAIs from this meteorite class. The resolved variation in the mass-independent Ti isotopic compositions of these CAIs indicates that the CAI-forming region of the early Solar System preserved isotopic variability. Nevertheless, the range of Ti isotopic compositions reported here for CAIs from CV and CK chondrites falls within the range observed in previously analyzed CAIs from CV, CO, CM, and ordinary chondrites. This implies that CAIs from CV, CK, CO, CM, and ordinary chondrites originated from a common nebular source reservoir characterized by mass-independent isotopic variability in Ti (and certain other elements). We further interpret these data to indicate that the Ti isotopic anomalies in CAIs represent the isotopic signatures of supernova components in presolar grains that were incorporated into the Solar System in an initially poorly mixed reservoir that was progressively homogenized over time. We conclude that the differing degrees of isotopic variability observed for different elements in normal CAIs are the result of distinct carrier phases and that these CAIs were likely formed towards the final stages of homogenization of the large-scale isotopic heterogeneity that initially existed in the solar nebula.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 262〈/p〉 〈p〉Author(s): Shintaro Kadoya, David C. Catling〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The partial pressure of atmospheric hydrogen (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si53.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉) on the early Earth is important because it has been proposed that high 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si54.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 warmed the planet or allowed prebiotic chemistry in the early atmosphere. However, such hypotheses lack observational constraints on 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si55.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉. Here, we use the existence of detrital magnetites in (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si56.svg"〉〈mrow〉〈mo〉∼〈/mo〉〈mn〉3.0〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mi mathvariant="normal"〉Ga〈/mi〉〈/mrow〉〈/math〉) Archean riverbeds to constrain 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si57.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉. Under the condition of high 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si58.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉, magnetite should disappear via reductive dissolution. We investigated the timescale for a magnetite particle in a river to dissolve, which depends on 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si59.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si60.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉CO〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉. Using published estimates of Archean 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si61.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉CO〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 and assuming the presence of Fe(III)-reducing microbes, the survival timescale is 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si62.svg"〉〈mrow〉〈mo〉∼〈/mo〉〈/mrow〉〈/math〉 1 kyr when 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si63.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 is 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si64.svg"〉〈mrow〉〈mo〉∼〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈mspace width="0.25em"〉〈/mspace〉〈mi mathvariant="normal"〉bar〈/mi〉〈/mrow〉〈/math〉, and decreases as 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si65.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 increases. Considering that the residence time of a particle in a short river (〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si66.svg"〉〈mrow〉〈mo〉〈〈/mo〉〈mn〉1000〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mi mathvariant="normal"〉km〈/mi〉〈/mrow〉〈/math〉) is 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si67.svg"〉〈mrow〉〈mo〉∼〈/mo〉〈mn〉1〈/mn〉〈mspace width="0.25em"〉〈/mspace〉〈mi mathvariant="normal"〉kyr〈/mi〉〈/mrow〉〈/math〉, the existence of detrital magnetite particles in Archean riverbeds likely indicates that 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si68.svg"〉〈mrow〉〈mi〉p〈/mi〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉H〈/mi〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 was below 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si69.svg"〉〈mrow〉〈mo〉∼〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉2〈/mn〉〈/mrow〉〈/msup〉〈mspace width="0.25em"〉〈/mspace〉〈mi mathvariant="normal"〉bar〈/mi〉〈/mrow〉〈/math〉. Such a level would preclude H〈sub〉2〈/sub〉 as a greenhouse gas or a strongly reducing Archean atmosphere. It is also consistent with limits imposed on H〈sub〉2〈/sub〉 by consumption by methanogens because conversion to CH〈sub〉4〈/sub〉 is thermodynamically favored.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 2 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): G. Jeffrey Taylor, Linda M.V. Martel, Paul G. Lucey, Jeffrey J. Gillis-Davis, David F. Blake, Philippe Sarrazin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We used X-Ray Diffraction (XRD) and Rietveld refinement to determine the modal mineralogy of 118 lunar regolith samples (〈150 µm size fraction) from all landed Apollo missions. Data were calibrated with mineral mixtures and compared to results based on an X-ray digital imaging procedure for six soils obtained by the Lunar Soil Characterization Consortium. Agreement between XRD and digital imaging for all minerals detectable in the six soils is excellent (R〈sup〉2〈/sup〉=0.953). XRD-based ternary plots (plagioclase-total pyroxene-olivine) vary from plagioclase-dominated (highlands as represented by Apollo 16 samples) to substantial mafic abundances at the mare sites. Olivine varies in relative abundance, with the Apollo 17 mare sites having the largest abundances. Olivine reaches 20 wt% at Apollo 17, but is a minor component at Apollo 14. The results agree with trends in mineral abundances obtained from reflectance spectroscopy for the Apollo sites. In a global context, however, the spectral data display a trend of increasing olivine at roughly constant pyroxene/plagioclase, reaching values of 40% olivine in the plagioclase-pyroxene-olivine ternary plot (e.g., Eratosthenian flows in Procellarum), indicating the presence of significant volumes of olivine-rich rock types on unsampled regions of the lunar surface.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 2 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Wei-Qiang Ji, Fu-Yuan Wu, Xiao-Chi Liu, Zhi-Chao Liu, Chang Zhang, Tong Liu, Jian-Gang Wang, Scott R. Paterson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The Oligocene to Miocene period is one of the most important stages during the formation and evolution of the Tibetan Plateau which generated the tectonic framework similar to the present. The rise of southern Tibet at this time was accompanied by widespread and intense tectonomagmatic activities. The Himalayan region were dominated by leucogranites considered as pure metasediments derived S-type granites, while the Lhasa terrane developed coeval thickened crust derived High Sr/Y calcalkaline rocks and metasomatized mantle derived ultrapotassic rocks. It is puzzling why the adjacent amalgamated regions show contrasting magmatic and dynamic process.〈/p〉 〈p〉This contribution presents a systematic study of newly found various dikes, including minette, aplite, granodiorite porphyry and granite porphyry, which are widespread from the southern Lhasa terrane, across the Yarlung Tsangpo suture zone (YTSZ) and into the Tethyan Himalaya. These dikes yield 16–11 Ma U-Th-Pb ages from zircon, titanite and monazite. The aplite and granodiorite porphyry dikes (high Sr/Y calcalkaline) and minette dikes (ultrapotassic) previal from the Lhasa terrane to the YTSZ. They are identical to the coeval high Sr/Y and/or ultrapotassic rocks from the Lhasa terrane, indicating a similar melting process of thickened lower crust and metasomatized lithospheric mantle. To the south of YTSZ, the middle Miocene granite porphyry in the Tethyan Himalaya exhibit high Sr/Y ratios (50–138), relatively low initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratio (0.7064–0.7098), high ε〈sub〉Nd〈/sub〉(t) value (−8.31 to −1.91) and positive ε〈sub〉Hf〈/sub〉(t) value (+1.55 to +4.33), in contrast to the high 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr and low ε〈sub〉Nd〈/sub〉(t) Himalayan leucogranites. They were likely generated by partial melting of thickened lower crust dominated by amphibolite with significant contributions from juvenile magma. The two-mica granite of Bendui pluton from the Tethyan Himalayan is similar to the granite porphyry in element composition, while the highly evolved muscovite granite there resembles the Himalayan leucogranite. Together, the granite porphyry, Bendui two-mica granite and leucogranite from the Tethyan Himalaya show a transition trend in elemental and Sr-Nd isotopic compositions, implying that the high Sr/Y granite porphyry dikes may represent the more primitive magma of Himalayan leucogranites. In this case, the Himalayan leucogranites, at least in part, represent highly evolved I-type granites with extensive assimilation of ancient crustal materials instead of pure metasediment-derived partial melts. Therefore, we suggest that the thickened lower crust in southern Tibet underwent pervasive partial melting during Oligocene to Miocene as a result of the removal of subducting Indian plate.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 1 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Ke Zhu, Paolo A. Sossi, Julien Siebert, Frédéric Moynier〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Although Solar System bodies exhibit large variations in their volatile element abundances, the mechanisms and conditions that lead to these variations remain ambiguous. The howardite-eucrite-diogenite (HED) meteorites that likely sample the asteroid 4 Vesta, provide evidence for extensive volatile depletion on their parent body. Isotopic variations in moderately volatile elements, such as Zn, have been used to track the origin of such volatile loss. Although not nominally volatile, Cr is useful because it has several oxidized gas species that render it volatile under the oxidizing conditions that characterize planetary accretion. As such, volatile loss of Cr has the potential to produce an isotopically light evaporation residue under an equilibrium regime. This contrasts with other moderately volatile elements that show heavy isotope enrichments in the residue following both kinetic or equilibrium fractionation. Here, we report the Cr stable isotope composition of 11 eucrites and four diogenites. The eucrites possess systematically lighter Cr isotope compositions than diogenites, which is onset by the accumulation of isotopically heavy Cr〈sup〉3+〈/sup〉-rich orthopyroxene and spinel in diogenites during their magmatic evolution. We estimate for the primary eucrite melt with Mg# ≈ 50, a δ〈sup〉53〈/sup〉Cr (〈sup〉53〈/sup〉Cr/〈sup〉52〈/sup〉Cr deviation relative to NIST SRM 979 in per mile) of −0.22 ± 0.03‰ (2SD), lighter than any chondritic meteorite group by ∼0.1‰. This deficit may result from either partial melting with residual Cr〈sup〉3+〈/sup〉-bearing phases (e.g. chromite) that retain heavy isotopes, or from vapor loss that occurred at equilibrium with a magma ocean on Vesta. Isotopic fractionation during partial melting would necessitate implausibly high Cr contents in the Vestan mantle, and oxygen fugacities high enough to stabilize chromite in the mantle source. Isotopic fractionation during evaporation would require an oxidized vapor and a reduced residue, as predicted by thermodynamic constraints on the composition of the vapor phase above a silicate magma ocean. Therefore, this Cr isotopic deficit between Vesta and chondrites may be caused by Cr loss at relatively high oxygen fugacity in a gas phase at equilibrium with the liquid from which it evolved. Temperatures of volatile loss are estimated to be lower than 2300 K, consistent with loss from a large-scale magma ocean model for formation of Vesta, which may be a common evolutionary stage in accreting planetesimals.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 31 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Karen H. Johannesson, Ningfang Yang, Alexandra S. Trahan, Katherine Telfeyan, T. Jade Mohajerin, Segun B. Adebayo, Omolola A. Akintomide, Darren A. Chevis, Saugata Datta, Christopher D. White〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Arsenic (As) concentrations and speciation were measured in groundwaters from the upper Chicot aquifer in southern Louisiana to: (1) ascertain the geochemical processes responsible for its mobilization into the groundwater; and (2) investigate the fate and transport of As in the aquifer following its mobilization into solution. Ancillary geochemical parameters, including dissolved iron (Fe) concentrations and speciation, major solutes, pH, and alkalinity were also quantified in the groundwaters, along with the mineralogy and geochemistry of the aquifer sediments. Arsenic concentrations as high as 644 nmol kg〈sup〉−1〈/sup〉 (48.2 μg kg〈sup〉−1〈/sup〉) were measured in the groundwaters, with the arsenite oxyanion (H〈sub〉3〈/sub〉As〈sup〉III〈/sup〉O〈sub〉3〈/sub〉〈sup〉0〈/sup〉) accounting for ca. 60%, on average, of the total dissolved As, and the remaining ca. 40% consisting of the arsenate oxyanion (i.e., H〈sub〉2〈/sub〉As〈sup〉V〈/sup〉O〈sub〉4〈/sub〉〈sup〉2−〈/sup〉). The groundwaters are Na – HCO〈sub〉3〈/sub〉 type waters of slightly alkaline pH (8.09 ≤ pH ≤ 8.34), moderately high mineralization (7.3 mmol kg〈sup〉−1〈/sup〉 ≤ 〈em〉I〈/em〉 ≤ 13.7 mmol kg〈sup〉−1〈/sup〉), and high Fe(II) concentrations (18 μmol kg〈sup〉−1〈/sup〉 ≤ Fe(II) ≤ 47 μmol kg〈sup〉−1〈/sup〉), and thus are compositionally similar to As-affected groundwaters from South and Southeast Asia (e.g., Bangladesh, West Bengal, India, and Vietnam). Groundwaters with the highest As concentrations also have the highest Fe(II) concentrations, which is consistent with reductive dissolution of Fe(III) oxides/oxyhydroxides releasing sorbed or co-precipitated As into the groundwaters. Biogeochemical reactive transport modeling that employs rates laws for microbial respiration indicates that dissimilatory reduction of Fe(III) oxides/oxyhydroxides coupled to organic matter oxidation can explain the high As concentrations along the mid-reaches of the studied flow path. Geochemical analysis of the aquifer sediments further demonstrates that the bulk of the environmentally mobile As in the aquifer is associated with Fe(III) oxides/oxyhydroxides and/or chemisorbed (i.e., inner-sphere surface complexed) onto aquifer mineral surfaces. Model calculations confirm that mobilization of 2–8% of this labile As could support the high As concentrations measured in these groundwaters. Reactive transport modeling coupled to the generalized double-layer surface complexation model predicts that As(III) could be transported ca. 10 km down gradient from the current location of the As “hot-spot” after 150 years, whereas As(V) is predicted to move between 2.4 km and 8.5 km depending on the composition of Fe(III) oxides/oxyhydroxides in the aquifer. Reactive transport modeling also illustrates how retardation and attenuation of As(III) and As(V) along flow paths in aquifer systems is strongly dependent on the type and content of Fe(III) oxides/oxyhydroxides present in the aquifer with higher contents of amorphous to poorly crystalline forms leading to greater retardation and attenuation of both As species. The data and modeling indicate that although As is readily mobilized by microbial reduction of Fe(III) oxides/oxyhydroxides, its tendency to form strong, inner sphere surface complexes on Fe(III) oxides/oxyhydroxides remaining in the aquifer will control the fate and transport of As once released into solution. We suggest that these observations may help explain the “patchy” spatial distributions of As concentrations commonly observed in As-affected aquifers, such as those of South and Southeast Asia.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 31 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Joshua F. Snape, Alexander A. Nemchin, Martin J. Whitehouse, Renaud E. Merle, Thomas Hopkinson, Mahesh Anand〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 264〈/p〉 〈p〉Author(s): H.G.M Eggenkamp, P. Louvat, P. Agrinier, M. Bonifacie, A. Bekker, V. Krupenik, J. Griffioen, J. Horita, J.J. Brocks, R. Bagheri〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We determined the chlorine and bromine isotope compositions of 83 halite samples from nine different geological periods between the Orosirian and the present in order to study the secular Cl and Br isotope variations in the ocean during the last 2 billion years. Relatively large Cl (−0.24 to +0.51‰ vs. SMOC) and Br (−0.24 to +1.08‰ vs. SMOB) isotope variations are found in these halite samples. Two different methods, one in which the isotope fractionation between the brine and the salt is used, and a second in which the relationship between the isotope compositions and the Br/Cl ratios in the halite samples is used were applied to establish the original Cl and Br isotope compositions of the ocean. Both approaches showed that the Cl and Br isotope compositions of the ocean have always been close to the modern value (which is by definition 0‰ for both isotope systems) and that at most very small variations in Br and Cl isotope composition of seawater have occurred during the last 2 billion years. This indicates that, unlike in other isotope systems that often show significant isotope variations over geologic time, Cl and Br isotope compositions can be used directly to determine processes that occurred in the deposits of interest without need for correction for secular variations.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 263〈/p〉 〈p〉Author(s): Marco Griepentrog, Lien De Wispelaere, Marijn Bauters, Samuel Bodé, Andreas Hemp, Dirk Verschuren, Pascal Boeckx〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Leaf-wax 〈em〉n〈/em〉-alkanes are produced by terrestrial plants, and through long-term preservation in sediments their stable hydrogen-isotopic signature (δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉) provides useful information on past hydrological variation for paleoclimate reconstructions. However, gaps remain in our understanding of the relationships between the isotopic signatures of leaf waxes and the plants’ source water. In this study, we investigated the influence of plant growth form, habitat and season on the distribution patterns and δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 values of 14 plant species (among which are two grasses, five trees and seven shrubs) sampled during four successive dry and wet seasons in three distinct habitats around Lake Chala in equatorial East Africa. Variation in δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 was analyzed with linear mixed-effect models and compared with the associated values of xylem water (δ〈sup〉2〈/sup〉H〈sub〉xylem〈/sub〉), leaf water (δ〈sup〉2〈/sup〉H〈sub〉leaf〈/sub〉) and biosynthetic hydrogen fractionation (ε〈sub〉bio〈/sub〉). Our results show that plant growth form was the most important driver of modern-day δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 variability in the study area, and that differences in δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 among habitats to a large extent reflect how each major growth forms is represented in those habitats. Individual plant species appear to express substantial species-specific isotopic fractionation that cannot be attributed to the tested external factors but rather seem to depend on intrinsic (e.g., plant phenological and biosynthesis-related) factors. For the purpose of calibrating δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 signatures against vegetation types, it is thus crucial to analyze representative samples of the plant communities present in the study area. Our results further indicate that paleohydrological studies in regions receiving rain from multiple moisture sources must take into account possible seasonal bias in the δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 signature relative to annual rainfall, due to unequal use of those moisture sources by the plants. Finally, the strong influence of plant growth form on δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 values argues for δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉 variation in paleo-records being evaluated in conjunction with independent proxy data on changes in vegetation composition. Differences in 〈em〉n〈/em〉-alkane distribution patterns among trees, shrubs and grasses (e.g., average chain length, carbon preference index and C〈sub〉31〈/sub〉/(C〈sub〉29〈/sub〉 + C〈sub〉31〈/sub〉) ratio) may provide such proxies, and can be produced from the same leaf-wax 〈em〉n〈/em〉-alkane dataset used to determine δ〈sup〉2〈/sup〉H〈sub〉wax〈/sub〉.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 263〈/p〉 〈p〉Author(s): Michael T. Thorpe, Joel A. Hurowitz, Erwin Dehouck〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The weathering of basalt has implications for the global carbon cycle on Earth as well as for understanding sedimentary processes on other terrestrial bodies dominated by a basaltic crust (e.g., Mars), but yet studies in mafic terrains are limited in comparison to their felsic counterpart. Our work details the compositional transformations resulting from the sedimentation process of first-cycle sediments generated in a basalt dominated watershed of southwest Iceland. By sampling multiple sites along the Hvítá S river transport pathway and analyzing the sedimentology, geochemistry, and mineralogy, this work provides a geologically integrated approach to understanding a fluvial source-to-sink system in mafic terrains. Environmental conditions such as climate, hydrology, and transport distance all influence the overall sediment composition, and in response, chemical weathering, physical abrasion, sorting, transport, and mixing are all sedimentary processes unraveled in the geochemical and mineralogical relationships of individual grain size bins. Chemical weathering initiates in the upper reaches of the watershed with the alteration of primary mafic phases to form secondary weathering products. As sediment continues to move downstream, the entire sediment suite becomes more altered, and the extent of chemical weathering is intensified, as evidenced by a higher abundance of clay minerals and glassy material. Fluvial sorting separates detritus by particle size and differentiates them by compositional gradients in geochemistry and mineralogy. As grain size decreases, mafic minerals become less abundant and the finer grain sizes are preferentially enriched in immature weathering minerals and mineraloids (e.g., smectite clays and X-ray amorphous phases). This grain size trend is also correlated with elemental fractionation, illustrated most notably by the clay size fraction (〈2 μm) hosting the most altered material in river sediments. However, we note that the environmental conditions of Iceland (e.g., a cold climate and glacial reworking) result in limited element mobility when compared to more temperate climates around the world. Therefore, we suggest that the clay size fraction of river sediments provide a valuable target for understanding the intensity of weathering in these systems, particularly in a colder climate where mineralogical transformations are not always accompanied by a high degree of elemental loss. In addition to weathering and sorting, trace element abundances suggest the mixing of sediments from varying provenances. While the Hvítá S watershed is dominated by basalt, even minor amounts of evolved volcanics (e.g., andesites) contribute to the overall sediment composition. Overall, this work fills a knowledge gap in sedimentation processes in basaltic terrains on Earth, while additionally providing a valuable terrestrial analog for ancient fluvial-deltaic environments preserved in the sedimentary rock record of Mars (e.g., Gale Crater and Jezero Crater).〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 8 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Claudie Hulin, Lionel Mercury〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉In porous media subject to drying conditions such as arid regions and excavation zones (deep gas injection or nuclear waste disposal), capillarity is involved in weathering processes because it modifies the geochemical and poromechanical balances within the porous network. Heterogeneous porous media like sedimentary rocks can host significant volumes of tensile capillary water in large pore bodies, and the negative pressure within is controlled by capillary forces exerted at nanometric pore throats.〈/p〉 〈p〉We have developed experiments using synthetic bimodal pore systems conducive to capillary tension. In microtubes, salts precipitated in an evaporating solution to build a dual-porosity system. A large volume (ø 200 µm) became trapped behind nanometric pores, where high capillary tension was applied. We investigated the gas-water interactions there, especially how gas nucleated in the trapped liquid and how it subsequently changed size. After gas nucleation, the decreasing of gas volume that we observed has been attributed to two complementary geochemical effects. On the one hand, the water’s tensile state increases gas solubility, as predicted by thermodynamics: capillarity is a “gas-in” process. On the other hand, while the total volume of the gas-water assemblage remains constant, the water’s molar volume increases by capillary forces. Consequently, capillary forces exerted at the nano-throats can (re)induce a superheated monophasic liquid state from a biphasic liquid-gas assemblage even after gas nucleation. Tensions required for gas shrinkage have been estimated at 7 ± 3 MPa and 53 ± 15 MPa. This regeneration process offers opportunities for water to regularly return to a capillary state, making the capillary lifetime less limited than expected.〈/p〉 〈p〉This shows that pore heterogeneity in rocks submitted to drying processes results in tension for water in pores that is long-lived. As a consequence, capillarity may significantly impact the long-term geochemical budget through its effects on gas and solid solubility and/or poromechanics (compaction, tensile stress, fracturing, etc.), so that it may play an important role in the weathering of drying porous materials.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 7 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): A. Stephant, M. Anand, H.O. Ashcroft, X. Zhao, S. Hu, R.L. Korotev, S. Strekopytov, R.C. Greenwood, E. Humphreys-Williams, Y. Liu, G. Tang, Q. Li, I.A. Franchi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Northwest Africa (NWA) 10989 is a recently found lunar meteorite, we used to elucidate the history of volatiles (H and Cl) in the Moon through analysis of its phosphates. The petrology, bulk geochemistry and mineralogy of NWA 10989 are consistent with it being a lunar meteorite with intermediate-iron bulk composition, composed of 40% of mare basaltic material and ∼60% non-mare material, but with no obvious KREEP-rich basaltic components. It is probable that the source region for this meteorite resides near a mare–highlands boundary, possibly on the farside of the Moon. Analyses of chlorine and hydrogen abundances and isotopic composition in apatite and merrillite grains from NWA 10989 indicate sampling of at least two distinct reservoirs of volatiles, one being similar to those for known mare basalts from the Apollo collections, while the other potentially represents a yet unrecognized reservoir. In situ Th-U-Pb dating of phosphates reveal two distinct age clusters with one ranging from 3.98±0.04 to 4.20±0.02 Ga, similar to the ages of cryptomare material, and the other ranging from 3.32±0.01 to 3.96±0.03 Ga, closer to the ages of mare basalts known from the Apollo collections. This lunar breccia features mixing of material, among which a basaltic D-poor volatile reservoir which doesn’t appear to have been recorded by Apollo samples.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 263〈/p〉 〈p〉Author(s): Luanjing Guo, Scott L. Painter, Scott C. Brooks, Jerry M. Parks, Jeremy C. Smith〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Speciation plays an important role in determining the fate and transport of metals in terrestrial surface and subsurface systems. Equilibrium speciation modeling in aqueous systems relies on thermodynamic constants (log 〈em〉K〈/em〉 values) of complexes, which are subject to uncertainties. Here, using Monte Carlo (MC) simulations with Latin hypercube sampling (LHS) we systematically analyze the propagation of thermodynamic constant uncertainty through speciation modeling of an inorganic mercury-sulfide-chloride-water system. We find that seemingly small variances of the input log 〈em〉K〈/em〉 normal distributions can lead to output species concentrations spanning multiple orders of magnitude, with highly skewed probability distributions. When equilibrium with mineral metacinnabar (β-HgS(s)) is neglected, the relative uncertainty of each output species is strongly positively correlated with the skewness of its concentration probability distribution, i.e., the lowest uncertainty occurs when the species concentration probability distribution is the most negatively skewed as its concentration approaches the total element concentration limit. The highest uncertainty in the identity of dominant species is located around species equivalence points. For cases where the mineral equilibrium is included, we derive analytical probability density functions for the log concentrations of all major species in the system. The mineral log 〈em〉K〈/em〉 uncertainty is found to be an important contributor to all output concentration uncertainties. The analysis of combined effects of both pH and total sulfide concentration on output concentration uncertainties shows that high concentration uncertainties occur under highly sulfidic alkaline conditions and low uncertainties at low pH and sulfide concentrations. An analysis as presented here can distinguish between conditions that require a full uncertainty analysis and those for which the classical deterministic speciation modeling suffices.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 5 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): A. Füger, S. Bruggmann, R. Frei, A. Leis, M. Dietzel, V. Mavromatis〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The Cr(VI) incorporation and chromium stable isotope composition in calcite has been studied in experiments performed in the pH range between 8.0 and 10.6 at constant temperature (T = 25 ± 1 °C), precipitation rate (r〈sub〉p〈/sub〉 = 10〈sup〉-7. 7 ± 0.2〈/sup〉 mol m〈sup〉-2〈/sup〉 s〈sup〉-1〈/sup〉) and total aqueous Cr(VI) concentration (Cr(VI) = 49.6 ± 1.3 mM). The obtained results indicate that Cr(VI) incorporation in calcite is pH-dependent and it is likely significantly affected by the formation of the aqueous CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉 species. The experimental findings suggest that during calcite growth at pH 〈 9.4 Cr(VI) uptake in the solid phase is likely controlled by the initial adsorption and subsequent incorporation of CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉 complexes, whereas this process is balanced by the additional uptake of aqueous CrO〈sub〉4〈/sub〉〈sup〉2-〈/sup〉 species when calcite forms at pH ≥ 9.4. This distinct mechanism of Cr(VI) incorporation into calcite is further confirmed by the Cr(VI) isotope fractionation between calcite and the precipitating solution. Owing to the longer Cr-O bond lengths in aqueous CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉 compared to CrO〈sub〉4〈/sub〉〈sup〉2-〈/sup〉 species the lighter 〈sup〉52〈/sup〉Cr(VI) isotope is preferentially abundant in the aqueous CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉. The preferential uptake of the isotopically lighter CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉 in the growing calcite results in Cr(VI) isotope fractionation, Δ〈sup〉53〈/sup〉Cr〈sub〉calcite-solution〈/sub〉 = δ〈sup〉53〈/sup〉Cr 〈sub〉calcite〈/sub〉 – δ〈sup〉53〈/sup〉Cr〈sub〉solution〈/sub〉, as low as -0.7 ‰ at pH 8. In contrast, at pH 〉 9.4 the smaller contributions of CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉 in the total concentration of Cr(VI) in calcite yields in a Δ〈sup〉53〈/sup〉Cr〈sub〉calcite-solution〈/sub〉 value close to 0 ‰.〈/p〉 〈p〉Our results imply that the chromium isotope tracer system applied to calcite, as an environmental proxy for the reconstruction of ocean redox conditions, is not solely a mirror of redox effects in the aqueous fluid from which the carbonates precipitate, but additionally is controlled by the pH of the forming fluid and consequently by the relative stability and the distribution of aquo-complexes. Speciation calculations that include the presence of CaCrO〈sub〉4〈/sub〉〈sup〉0〈/sup〉 for calcite precipitated from seawater predict isotope fractionation values that lay within -0.67 ‰ 〈 Δ〈sup〉53〈/sup〉Cr〈sub〉calcite-solution〈/sub〉 〈 -0.43 ‰, and come in excellent agreement with the experimental results of this study at similar pH conditions. This theoretical model predicts that calcite formation under pH conditions below 8.5 results in depletion of 〈sup〉53〈/sup〉Cr(VI) in the growing calcite crystal. In contrast, the Cr(VI) isotopic composition of precipitating calcite in alkaline solutions can be reasonably ascribed to directly depict the Cr(VI) isotopic signature of the aqueous solution from which calcite forms, at least within the range of experimental conditions.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Oded Elazar, Dan Frost, Oded Navon, Ronit Kessel〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Eclogites play a significant role in geodynamic processes, transferring large amounts of basaltic material and volatiles (chiefly CO〈sub〉2〈/sub〉 and H〈sub〉2〈/sub〉O species) into the earth's mantle via subduction. Previous studies of eclogite melting focused on two end member systems: either carbonated or hydrous eclogites. Here we focus on the hydrous carbonated eclogitic system in order to define the position of its solidus and determine the near solidus fluid and melt compositions at 4-6 GPa and 900-1200°C. Experiments were performed on a rocking multi-anvil press. The total dissolved solids in the equilibrated fluids were analyzed following the cryogenic technique using a LA-ICP-MS. H〈sub〉2〈/sub〉O and CO〈sub〉2〈/sub〉 content were determined by mass balance calculations. Solid phases were chemically characterized using an EPMA. Garnet and clinopyroxene are present in all experiments, assembling the eclogitic rock. A carbonate phase was detected at all temperatures at 4 GPa and at temperatures below 1200°C at 5 and 6 GPa. Coesite was observed at all pressures below 1200°C. The solidus was crossed between 1000 and 1100°C at 4 and 5 GPa. At 6 GPa we observed a relatively smooth decrease in the H〈sub〉2〈/sub〉O and CO〈sub〉2〈/sub〉 content of the fluid phase with rising temperature, suggesting the presence of a supercritical fluid. The second critical endpoint is thus defined in this system at ∼5.5 GPa and 1050°C. The composition of fluids and melts reported in this study indicates that the hydrous carbonated eclogite system is a plausible source-rock for high density fluids (HDFs) found in microinclusions in diamonds, specifically for the intermediate compositions along the array spanned between low-Mg carbonatitic HDFs and hydrous-silicic ones. Our results suggest that the whole array reflects melting in a heterogeneous mantle. Melting of water-rich eclogite produces silicic HDFs, carbonate-rich zones will produce carbonatitc HDFs, while source-rocks with varying H〈sub〉2〈/sub〉O/CO〈sub〉2〈/sub〉 ratios produce intermediate compositions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 26 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Hua-Wen Qi, Rui-Zhong Hu, Ke Jiang, Ting Zhou, Yue-Fu Liu, Yan-Wen Xiong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Chemical weathering of silicate rocks controls the fluvial input of dissolved Ge and Si into the ocean, and has substantial influence on the global Ge and Si geochemical cycles. The heavier dissolved Ge isotope compositions in the rivers (relative to the bulk silicate earth) suggest preferential incorporation of light isotopes into secondary weathering products during rock weathering (Baronas et al., 2017a). In this paper, we present the Ge isotope and Ge/Si ratio variations in the solid weathering products (soil and saprolite) from a well-developed basalt weathering profile (〉15m thick, including soil, saprolite, semi-weathered rock and fresh basalt) on the tropical island of Hainan (South China). We discussed the elemental/isotopic fractionation mechanism and the possible influence of major oxide composition on Ge isotope fractionation during extreme weathering of basalts in tropical climate. The Ge content ([Ge] = 2.19 − 4.12 ppm, 2.93 ppm on average, n = 52) and Ge/Si ratios (5.55 − 13.7 μmol/mol, 7.42 μmol/mol on average, n = 52) of solid weathering products are distinctly higher than those of the fresh basalts (avg. [Ge] = 1.64 ppm, Ge/Si = 2.66 μmol/mol, n = 5). The δ〈sup〉74〈/sup〉Ge values of solid weathering products range from −0.02 ± 0.10‰ (2σ) to 0.63 ± 0.10‰ (2σ), and exhibits complex stratigraphic variation across the weathering profile. There are no distinct correlations between the concentrations of Ge (or the Ge/Si ratio and δ〈sup〉74〈/sup〉Ge values) and SiO〈sub〉2〈/sub〉, Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉, Fe〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 and MnO for most of the soil and saprolite samples. The distinct positive δ〈sup〉74〈/sup〉Ge vs. Ge (or 〈em〉τ〈sub〉Ge〈/sub〉〈/em〉 value and Ge/Si ratio) correlations for most soil and saprolite samples indicate that the enrichment (or depletion) of Ge content, Ge isotopes and Ge/Si fractionation are controlled by a common sorption process of solid weathering products. The predicted opposite relationships between Ge/Si ratios and δ〈sup〉74〈/sup〉Ge values for pore water and that for solid weathering products can be evidenced by the positive Ge/Si vs. δ〈sup〉74〈/sup〉Ge correlation in the Wenchang basalt weathering profile and a negative one in the river water (Baronas et al., 2017a). Moreover, the estimated negative Δ〈sup〉74〈/sup〉Ge 〈sub〉solid-dissolved〈/sub〉 (1000*Ln(α)) value (−1.38 ± 0.28‰ (2σ)) indicates that solid weathering products are a sink for light Ge isotopes, possibly balancing the isotopic budget with heavy Ge isotope compositions in global rivers and oceans.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 29 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Quinten H.A. van der Meer, James M. Scott, Simon H. Serre, Martin J. Whitehouse, Magnus Kristoffersen, Petrus J. Le Roux, Emily C. Pope〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Megacrystic zircon grains from alkaline basaltic fields are rare but can provide fundamental insights into mantle metasomatic processes. Here, we report in-situ U-Pb ages, trace element concentrations and hafnium and oxygen isotopes for fourteen zircon megacrysts from two intraplate alkaline basalt locations in New Zealand. U-Pb ages indicate the zircons crystallised between 12.1 and 19.8 Ma. Zircon oxygen isotopic compositions range from low to mantle-like compositions (δ〈sup〉18〈/sup〉O = 3.8-5.1‰). Hafnium isotopes (εHf〈sub〉(t)〈/sub〉 = +3.3 to +10.4) mostly overlap with intraplate mafic rocks and clinopyroxene in metasomatized peridotitic mantle xenoliths but show no correlation with most trace element parameters or oxygen isotopes. The zircons are interpreted to have formed by the reaction between low-degree melts derived from pre-existing mantle metasomes and the depleted mantle lithosphere prior to eruption and transport to the surface. The low Hf concentration, an absence of Eu anomalies, and elevated U/Yb compared to Nb/Yb in the megacrystic zircons are interpreted to show that the source metasomes comprised subduction- and carbonatite-metasomatised lithospheric mantle. As these trace element characteristics are common for megacrystic zircon in intra-plate basaltic fields globally, they suggest the prevalence of subduction- and carbonatite-metsasomatised mantle under these intraplate volcanic regions. The unusually low δ〈sup〉18〈/sup〉O was likely present prior to metasomatic enrichment and may have resulted from high-temperature hydrothermal alteration during initial mantle lithosphere formation at a mid ocean ridge or, possibly, during subduction-related processes associated with continent formation. The combination of proportionally varied contributions from carbonatite- and subduction-metasomatised lithospheric melts with asthenospheric melts may explain the variety of primitive intraplate basalt compositions, including low δ〈sup〉18〈/sup〉O reported for some local intraplate lavas.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Geoffrey H. Howarth, Andy E. Moore, Chris Harris, Quinten H.A. van der Meer, Petrus le Roux〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Carbonate-bearing assemblages in the mantle have been interpreted to be the source for Si-undersaturated, CO〈sub〉2〈/sub〉-rich magmas, including kimberlites. However, direct evidence for carbonate in the mantle is rare in the contemporary literature. Here we present petrography, trace element, and C-O-Sr-Nd-Pb isotope composition for a suite of carbonate xenoliths from the Kimberley region kimberlites to ascertain their mantle or crustal origin and gain insight to the potential for the occurrence of carbonate in the mantle. Carbonate xenoliths were found in large kimberlite blocks from the Bultfontein kimberlite and Big Hole region. The xenoliths are characterised by pale green alteration margins made of fine-grained microlites of an unknown mineral as well as spherules surrounded by glassy material. They are generally 1-4 cm in size, coarse-grained (1-2 mm), and comprised entirely of calcite.〈/p〉 〈p〉Carbonate xenoliths from the Bultfontein kimberlite have low total REE concentrations (0.2-4.9 ppm), constant initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉i〈/sub〉 (0.7047-0.7049) combined with variable ƐNd〈sub〉i〈/sub〉 (-0.1 to -26.2) and 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉i〈/sub〉, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉i〈/sub〉, and 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉i〈/sub〉 of 16.7-18.8, 15.3-15.6, 36.5-38.4, respectively. Xenoliths from the Big Hole sample have higher initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉i〈/sub〉 (0.7088-0.7095), lower ƐNd〈sub〉i〈/sub〉 (-24.5 to -33.8), and 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉i〈/sub〉, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉i〈/sub〉, and 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉i〈/sub〉 of 18.9-19.9, 15.7-15.8, 38.4-38.8, respectively. The δ〈sup〉13〈/sup〉C values for both Bultfontein (-5.7 to -6.6 ‰) and Big Hole (-4.7 to -5.4 ‰) carbonates are within the typical range expected for mantle-derived carbonate. The δ〈sup〉18〈/sup〉O values (15.5 – 17.5 ‰) are higher than those of mantle silicate rocks, indicative of late-stage low-temperature interaction with fluids; a common feature of groundmass calcite in the Kimberley kimberlites.〈/p〉 〈p〉The Sr and C isotope composition of the Bultfontein xenoliths indicates a mantle origin whereas the Big Hole xenolith Sr- and C-isotopes are more ambiguous. Isotope mixing models are inconsistent with interaction between the host kimberlite and carbonate xenoliths. Correlation between ƐNd〈sub〉i〈/sub〉 and δ〈sup〉18〈/sup〉O values for the Bultfontein xenoliths indicates late-stage interaction with low-temperature fluids, which may also be responsible for the large range in ƐNd〈sub〉i.〈/sub〉 This in turn indicates that the highest ƐNd〈sub〉i〈/sub〉 of -0.1 represents the primary carbonate xenolith signature, and this value overlaps typical Group I kimberlites.〈/p〉 〈p〉There are two possible mantle origin for the carbonate xenoliths. 1) Carbonate xenoliths from the sub-continental lithospheric mantle (SCLM), where quenched margins and the large range of ƐNd〈sub〉i〈/sub〉 are related to formation in the mantle. 2) Carbonate xenoliths from an earlier phase of carbonatite magmatism. The similarity of isotope signatures of the Bultfontein carbonates to Group I kimberlite may further suggest a link between kimberlite and carbonatite volcanism such as observed elsewhere in the world.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 30 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Anne Perez, Damien Daval, Maxime Fournier, Mélanie Vital, Jean-Marc Delaye, Stéphane Gin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To evaluate the impact of atomic short- and long-range orders on silicate dissolution kinetics, the dissolution of amorphous and crystalline oligoclase was investigated at pH 1.5 and 10 at 90°C. Experiments in solution saturated with respect to SiO〈sub〉2 am〈/sub〉 were additionally performed to constrain the effect of Si-rich surface layer formation on dissolution rates. The face-specific dissolution rates of the crystalline oligoclase and of the oligoclase glass were determined from element budget in solution and surface retreat measured by vertical scanning interferometry. The results show that atomic ordering primarily impacts solid reactivity, irrespective to the pH of the solution. A strong relation between the crystal surface orientation, the evolution of its topography and its dissolution rate was observed. The (001), (010) and (10-1) flat faces containing the strongest bonds dissolved the most slowly and their dissolution rates remained constant throughout the experiments. In contrast, the stepped (1-11) face was characterized by the highest initial dissolution rate, but progressively decreased, suggesting that the preferential dissolution of stepped sites expose afterwards more stable planes. The differences in terms of etch pit density from one surface to another also explained the difference in dissolution rates for the (001) and (010) faces. The fluid chemistry suggested the formation of very thin (100-200 nm) Si-rich surface layers in acidic conditions, which weakly affected the dissolution rate of the pristine crystal. At pH 1.5, oligoclase glass dissolves at a rate similar to that of the fastest studied faces of the crystal, suggesting the absence of structural effect on oligoclase dissolution. Whereas Si-rich surface layers likely formed by interfacial dissolution-reprecipitation for oligoclase crystal, molecular dynamic calculations suggest that the slightly more open structure of the glass could also allow ion-exchange following water diffusion into the solid. This mechanism could explain why the surface layer of the glass is characterized by a different chemical composition. Results at pH 10 are strikingly different, as the oligoclase glass dissolves up to 50 times faster than its crystalline equivalent. This non-linear response of the material upon pH was linked to the density of critical bonds in oligoclase that is indeed pH-dependent. In acidic pH, the preferential dissolution of Al leaves a highly polymerized and relaxed Si-rich surface, whereas in basic pH the preferential dissolution of Si leads to a complete de-structuration of the network because of the lack of Si-O-Al bonds.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Elisa A. Bonnin, Zihua Zhu, Jennifer S. Fehrenbacher, Ann D. Russell, Bärbel Hönisch, Howard J. Spero, Alexander C. Gagnon〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The calcite shells, or tests, of foraminifera provide a window into Earth history because they are archived in most marine sediments and contain useful geochemical proxies for paleoceanography. Previous observations of diurnal heterogeneity in proxies like Mg/Ca demonstrate a complex relationship between environmental conditions and test composition. The causes for this diurnal banding and the potential impact for proxy interpretation in systems other than Mg/Ca have yet to be determined. Recently, Mg and Na in shells of the planktic foraminifer species 〈em〉Orbulina universa〈/em〉 have been observed to be high at the location of the primary organic sheet (POS), i.e. the organic template upon which the calcite test is formed. Here we use time-of-flight secondary ion mass spectrometry (ToF-SIMS), a chemical and isotope mapping technique with a spatial resolution of 300 nm, to show that Na banding is a consistent feature in the tests of 45 individual cultured 〈em〉O. universa〈/em〉. This banding occurs in two distinct forms: (1) sharp Na bands associated with organic sheets that are embedded in the calcite test after chamber formation; and (2) regular, thicker, but lower-amplitude Na bands that are found throughout the test. We use the pattern of the first type of banding to indicate the extent and sequence of calcite growth during chamber formation. Specifically, we show that new chamber formation involves growth over the previous chamber in 〈em〉Orbulina bilobata〈/em〉, a morphotype of 〈em〉O. universa〈/em〉 that develops a second partial spherical chamber attached to the primary sphere. This is consistent with a bilamellar model of foraminiferal growth. However, a SIMS mapping survey of the morphologically more complex 〈em〉Globigerina bulloides〈/em〉 and 〈em〉Neogloboquadrina dutertrei〈/em〉 suggests that the pattern of growth during chamber formation and the prevalence of different types of Na bands may be species-specific. The wide, repeating Na bands that occur throughout the test of 〈em〉O. universa〈/em〉 generally occur in an inverse pattern with respect to Mg banding for the first few days of the foraminifer's life, but this pattern changes as the organism ages. We use the magnitude, timing, and coherency between Na and Mg bands to put constraints on various proposed mechanisms for banding, including antiport Mg〈sup〉2+〈/sup〉-2Na〈sup〉+〈/sup〉 exchange and kinetic growth rate effects.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Nina M. Whitney, Beverly J. Johnson, Philip T. Dostie, Katherine Luzier, Alan D. Wanamaker〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Developing high resolution, well-dated marine proxies of environmental, climatic, and oceanographic conditions is critical in order to advance our understanding of the ocean’s role in the global climate system. While some work has investigated bulk and compound specific stable nitrogen isotopes (δ〈sup〉15〈/sup〉N) in bivalve shells as proxies for environmental variability, the small concentrations of nitrogen found in the organic matrix of the shell calcium carbonate (CaCO〈sub〉3〈/sub〉) makes developing high resolution records challenging. This study investigates the potential of using the bulk and amino acid δ〈sup〉15〈/sup〉N of bivalve periostracum, the protein layer on the outside of the shell, as a proxy archive of nitrogen cycling processes and water source variability.〈/p〉 〈p〉Bulk δ〈sup〉15〈/sup〉N values were measured on the periostracum, aragonitic CaCO〈sub〉3〈/sub〉, and adductor muscle of 〈em〉Arctica islandica〈/em〉 shells collected in the Gulf of Maine. Increased variability of isotopic values across growth lines compared to along growth lines support mechanistic reasoning based on growth processes that periostracum is recording changes in δ〈sup〉15〈/sup〉N over the course of the clam’s lifetime (up to 500 years). In addition, the statistically significant relationship between periostracum δ〈sup〉15〈/sup〉N and contemporaneous carbonate δ〈sup〉15〈/sup〉N of the same shell (r= 0.82, p〈.0001, n=40) suggests that periostracum preserves a similar δ〈sup〉15〈/sup〉N signal to that preserved in the carbonate. This finding, coupled with the fact that source amino acid δ〈sup〉15〈/sup〉N values of periostracum are similar to that of the adductor muscle and the particulate organic matter (POM) consumed by the clam, suggests that periostracum bulk δ〈sup〉15〈/sup〉N reflect the δ〈sup〉15〈/sup〉N of the clam’s food source. The isotopic offsets between periostracum, carbonate, and adductor muscle δ〈sup〉15〈/sup〉N values are primarily caused by differences in amino acid composition of the different tissue types, as evidenced by isotope mass balance calculations, although may also be related to differences in δ〈sup〉15〈/sup〉N values of the individual amino acids of the different tissue types, especially the trophic amino acids.〈/p〉 〈p〉Compound specific δ〈sup〉15〈/sup〉N analyses of the periostracum of 〈em〉A. islandica〈/em〉 shells were used to determine that the calculated trophic position of the clams in this study (1.4±0.4) did not change significantly between 1783 and 1997. Phenylalanine δ〈sup〉15〈/sup〉N values over the last 70 years show similar trends to that of the bulk record, suggesting that changes in bulk δ〈sup〉15〈/sup〉N of that time period are related to changes in baseline δ〈sup〉15〈/sup〉N. Periostracum δ〈sup〉15〈/sup〉N values from shells collected in the western Gulf of Maine have decreased by ∼1‰ since the mid-1920s. This trend (-0.008‰/year) is not statistically different from the trend of previously published δ〈sup〉15〈/sup〉N values of deep-sea corals from the entrance to the Gulf of Maine over the same time period. This coral record has been shown to indicate a shift in water mass source to the region and therefore the similarity between the two records suggest that changes in periostracum δ〈sup〉15〈/sup〉N values are reflecting broader North Atlantic hydrographic changes. Our study introduces a new, high-resolution and absolutely dated paleoceanographic proxy of baseline δ〈sup〉15〈/sup〉N, presenting the opportunity for future reconstructions of aspects of nitrogen cycling and water source changes in the global oceans.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Boris R. Tagirov, Olga N. Filimonova, Alexander L. Trigub, Nikolay N. Akinfiev, Maximilian S. Nickolsky, Kristina O. Kvashnina, Dmitriy A. Chareev, Alexander V. Zotov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hydrothermal chloride-rich fluids are identified at the late stages of the formation of platinum group element (PGE) deposits in giant layered intrusions, and are considered as the PGEs transport media in Cu(-Mo,Au) porphyry systems. In order to quantify the hydrothermal mobility of Pt we performed an investigation of the speciation of Pt in hydrothermal chloride-bearing fluids and dry melt by means of X-ray absorption spectroscopy (XAS). The experiments consisted in recording the Pt 〈em〉L〈/em〉〈sub〉3〈/sub〉-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS) spectra of Pt-bearing fluids obtained by dissolution of Pt metal in KCl/HCl and CsCl/HCl fluids in the temperature range from 450 to 575 °C at pressures from 0.5 to 5 kbar. A spectrum of Pt dissolved in dry CsCl/NaCl/KCl + K〈sub〉2〈/sub〉S〈sub〉2〈/sub〉O〈sub〉8〈/sub〉 melt was recorded at 650 °C. The capillary method, when the experimental solution together with Pt〈sub〉(cr)〈/sub〉 is sealed inside a silica glass capillary, was used. As was determined from the XANES spectra, in all the experimental systems Pt existed in the +2 oxidation state. Analysis of EXAFS spectra showed that Pt is coordinated by four Cl atoms with 〈em〉R〈/em〉〈sub〉Pt-Cl〈/sub〉 = 2.31±0.01 Å independently of the 〈em〉T-P-〈/em〉compositional parameters. No evidence of the formation of complex with alkali metal cations in the second coordination sphere of Pt was found by the analysis of the EXAFS spectra of concentrated CsCl brines and melt. Our results imply that PtCl〈sub〉4〈/sub〉〈sup〉2-〈/sup〉 is the main Pt-Cl complex which predominates in hydrothermal fluids at 〈em〉t〈/em〉 〉 400 °C and fluid density 〈em〉d〈/em〉 〉 0.3 g⋅cm〈sup〉-3〈/sup〉. Experimental data obtained for dry melt of alkali metal chlorides suggest that Pt-Cl complexes can dominate Pt speciation in chloride-bearing aluminosilicate melts where Cl exhibits a salt-like atomic arrangement and ionic bonding. The literature data on the Pt solubility constant, 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mrow〉〈mi mathvariant="normal"〉P〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈/mrow〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉c〈/mi〉〈mi mathvariant="normal"〉r〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/msub〉〈mo〉+〈/mo〉〈msubsup〉〈mrow〉〈mn〉2〈/mn〉〈mi mathvariant="normal"〉H〈/mi〉〈mi mathvariant="normal"〉C〈/mi〉〈mi mathvariant="normal"〉l〈/mi〉〈/mrow〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉a〈/mi〉〈mi mathvariant="normal"〉q〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mi mathvariant="normal"〉o〈/mi〉〈/msubsup〉〈mo〉+〈/mo〉〈msup〉〈mrow〉〈mn〉2〈/mn〉〈mi mathvariant="normal"〉C〈/mi〉〈mi mathvariant="normal"〉l〈/mi〉〈/mrow〉〈mo〉-〈/mo〉〈/msup〉〈mo〉=〈/mo〉〈msubsup〉〈mrow〉〈mi mathvariant="normal"〉P〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈mi mathvariant="normal"〉C〈/mi〉〈mi mathvariant="normal"〉l〈/mi〉〈/mrow〉〈mrow〉〈mn〉4〈/mn〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈mo〉-〈/mo〉〈/mrow〉〈/msubsup〉〈mo〉+〈/mo〉〈msub〉〈mi mathvariant="normal"〉H〈/mi〉〈msub〉〈mn〉2〈/mn〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉a〈/mi〉〈mi mathvariant="normal"〉q〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/msub〉〈/msub〉〈/mrow〉〈/math〉, are compiled and fitted to the simple density model equation 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mi mathvariant="normal"〉log〈/mi〉〈msubsup〉〈mi〉K〈/mi〉〈mrow〉〈mi〉s〈/mi〉〈/mrow〉〈mi mathvariant="normal"〉o〈/mi〉〈/msubsup〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈msubsup〉〈mrow〉〈mi mathvariant="normal"〉P〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈mi mathvariant="normal"〉C〈/mi〉〈mi mathvariant="normal"〉l〈/mi〉〈/mrow〉〈mrow〉〈mn〉4〈/mn〉〈/mrow〉〈mrow〉〈mn〉2〈/mn〉〈mo〉-〈/mo〉〈/mrow〉〈/msubsup〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mo〉=〈/mo〉〈mn〉0.973〈/mn〉〈mo〉-〈/mo〉〈mn〉8202〈/mn〉〈msup〉〈mrow〉〈mi〉T〈/mi〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉K〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/msup〉〈mo〉-〈/mo〉〈mn〉5.505〈/mn〉〈mi mathvariant="normal"〉log〈/mi〉〈mi〉d〈/mi〉〈mfenced open="(" close=")"〉〈mrow〉〈mi mathvariant="normal"〉w〈/mi〉〈/mrow〉〈/mfenced〉〈mo〉+〈/mo〉〈mn〉2223〈/mn〉〈mi mathvariant="normal"〉log〈/mi〉〈mi〉d〈/mi〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉w〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈msup〉〈mrow〉〈mi〉T〈/mi〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉K〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉, where 〈em〉d〈/em〉(w) is the pure water density in g⋅cm〈sup〉-3〈/sup〉. The equation, combined with the extended Debye-Hückel equation for activity coefficients, can be used to calculate the solubility of Pt up to 1000 °C/5 kbar. It accurately predicts the solubility of Pt in concentrated chloride brine (up to 50 wt% NaCl) at parameters of magmatic-hydrothermal transition (800 °C/1.4 kbar). At fluid/vapor density below 0.3 g⋅cm〈sup〉-3〈/sup〉 a neutral complex PtCl〈sub〉2〈/sub〉°〈sub〉(aq)〈/sub〉 is suggested as the dominant Pt species. Our data demonstrate that Pt is highly mobile in high-temperature oxidized chloride-rich hydrothermal fluids. For example, at 800 °C/2 kbar the concentration of Pt can reach a few wt.% in the 1 wt% HCl/50 wt% NaCl fluid which is in equilibrium with magnetite-hematite buffer. Once a Cl-reach fluid exsolves from alumuinosilicate melt, Pt follows Cl and enriches the fluid phase where it exists mostly in the form of PtCl〈sub〉4〈/sub〉〈sup〉2-〈/sup〉. Decrease of temperature, acidity, and fluid chlorinity results in precipitation of Pt from the fluid phase.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 246〈/p〉 〈p〉Author(s): Zvi Steiner, Boaz Lazar, Clare E. Reimers, Jonathan Erez〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Whether CaCO〈sub〉3〈/sub〉 dissolves within the top centimeters of marine sediments overlaid by deep, supersaturated bottom waters remains an area of debate in geochemistry. This uncertainty stems from the fact that different methods used to assess CaCO〈sub〉3〈/sub〉 dissolution rates often provide what appear to be profoundly different results. Here we combine microelectrode and porewater chemistry profiles, core incubation experiments, mineral characterizations and observations of the state of preservation of coccolithophorid exoskeletons for a holistic view of carbonate reactions within the top 30 centimeters of hemipelagic sediments from the Gulf of Aqaba, Red Sea. Calculations based on pH and O〈sub〉2〈/sub〉 microelectrode data suggest that rapid metabolic dissolution of carbonate minerals occurs in these sediments within the top two millimeters. Porewater chemistry supports these calculations. The porewater-based observations are further supported by sedimentological characteristics such as aragonite content, and dissolution pitting and fragmentation of coccoliths in sediment layers deposited over the last 200 y. Dissolution appears to be occurring today within surface sediments despite the bulk porewater solution being supersaturated with respect to aragonite and Mg-calcite. In spite of intense dissolution within the sediments, there is no evidence for significant alkalinity and/or calcium fluxes (transport) into bottom waters. It appears that the supersaturated bottom water promotes the removal of all excess alkalinity and calcium produced within the sediment, by CaCO〈sub〉3〈/sub〉 precipitation at or above the sediment/bottom water interface. The precipitation mechanism may be by either benthic organisms (biogenic precipitation) or inorganically (direct precipitation on settling CaCO〈sub〉3〈/sub〉 grains). We suggest that authigenic precipitation of (Ca,Mn)CO〈sub〉3〈/sub〉 as it becomes supersaturated below 3 cm in the sediments can reconcile the evidence for carbonate dissolution in what appears to be supersaturated conditions. This means that MnCO〈sub〉3〈/sub〉 replaces CaCO〈sub〉3〈/sub〉 within the nanofossils below ∼3 cm, and that part of the manganese rich CaCO〈sub〉3〈/sub〉 is bioturbated upwards into undersaturated conditions, facilitating dissolution of these fossils. Diminished calcite and aragonite concentrations in sediments deposited in recent decades are proposed to be a result of increased manganese cycling rates and greater rates of coupled dissolution within the interfacial sediments, possibly combined with diminished calcareous plankton productivity, in response to increased surface water primary productivity.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 246〈/p〉 〈p〉Author(s): Peter E. Carlson, Jay L. Banner, Kathleen R. Johnson, Richard C. Casteel, Daniel O. Breecker〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉From 1955 to 1963, atmospheric testing of nuclear weapons caused a significant rise in atmospheric radiocarbon activity. This “bomb peak” has been used to calculate turnover rates of organic carbon in soils and other recent sedimentary deposits. Some speleothems contain precise and independently dated records of radiocarbon activity. These records can be used to understand, through inverse modeling, the processes and rates of turnover of subsurface organic carbon in karst regions. This approach is complicated, however, by the contribution of radioactively “dead” carbon to the stalagmite by the dissolution of host-rock limestone and/or by the respiration of relatively old soil organic matter. Previously published inverse models of the radiocarbon bomb peak in speleothems constrain the dead carbon proportion (DCP, in percent) by comparing measurements of speleothem radiocarbon activity from before the onset of the bomb peak to measurements of coeval atmospheric radiocarbon. This approach precludes modeling of speleothems that began growing after the onset of atmospheric nuclear weapons testing in 1955. Here, we advance the inverse modeling framework to calculate DCP using the entire length of the speleothem record, allowing for the modeling of speleothems that began growing after the initiation of atmospheric nuclear weapons testing. We test the sensitivity and resolution of this model and find that it can precisely resolve the turnover times and relative contributions of subsurface organic matter pools with residence times of less than a decade. The model fails to resolve turnover times or relative contributions of organic matter pools on millennial or greater timescales. These results also hold for the previously published models from which the current model is derived. We find that imprecise estimates of slow-turnover carbon add significant uncertainty to the calculated average age of respired carbon, which is a common metric of subsurface carbon cycling. The high precision and resolution attainable between sub-decadal carbon pools will allow researchers to differentiate the (sub-)annual pool, which is likely dominated by root/rhizosphere respiration, from the 2- to 10-year pools, which are likely dominated by microbial decomposition of labile organic carbon. The high precision attainable in fast-turnover pools also suggests that when there are multiple viable chronological interpretations for the same speleothem, bomb peak models could be used to help select which chronology is most likely to be accurate. This is important for high-resolution (sub-annual) speleothem climate records, where even single-year chronological offsets can result in misleading calibrations to the instrumental record.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 248〈/p〉 〈p〉Author(s): Xiao-Ming Liu, Dalton S. Hardisty, Timothy W. Lyons, Peter K. Swart〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Inferring redox conditions for ancient marine environments is critical to our understanding of biogeochemical cycles over Earth history. Because of the redox sensitivity of cerium (Ce) relative to other rare earth elements (REEs) and its uptake in marine carbonates, the Ce anomaly (Ce/Ce*) is widely applied to ancient carbonates as a proxy for local redox conditions in the water column. However, carbonate sediments and rocks are particularly vulnerable to multiple stages and styles of post-depositional diagenetic alteration where the diagenetic redox conditions and fluid compositions can vary widely from overlying seawater. Evaluations of the effects of this post-depositional alteration for the Ce anomaly have mostly been limited to ancient carbonate rocks rather than recent, well-characterized analog facies. Here, we report on analyses of REE plus yttrium concentrations (REY) and Ce anomalies in bulk carbonate samples from drill cores collected in the Bahamas (Clino and Unda) that allow us to track loss or retention of primary signals of initial oxic deposition through a range of subsequent alteration scenarios mostly under anoxic conditions. Specifically, these materials have experienced well-constrained overprints linked to meteoric processes and marine burial diagenesis, including dolomitization. Our results show that, regardless of mineralogy, diagenetic fluid composition, and redox state, the REY patterns in these carbonates, including the Ce anomaly, are similar to those of modern oxic seawater, indicating that they likely record the seawater signatures of primary deposition. As such, the Ce anomaly in shallow marine carbonates has the potential to preserve records of primary deposition even when subject to multiple stages and styles of diagenetic alteration, confirming its utility in studies of ancient marine redox.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): Yi Liu, Xiaohua Li, Zhen Zeng, Hui-Min Yu, Fang Huang, Thomas Felis, Chuan-Chou Shen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Barium/calcium ratios in the skeletons of scleractinian shallow-water corals have been used as proxies for coastal and oceanic processes such as river discharge, oceanic upwelling and surface ocean productivity. However, the variations in Ba/Ca ratios in aragonitic coral skeletons remain difficult to interpret as an environmental proxy. This difficulty is mainly due to the influence of internal (biomineralization) and multiple external (environmental) processes on Ba incorporation into coral skeletons, and these processes are hard to constrain with Ba/Ca alone. Here we present the first annually-resolved records of the Ba isotopic compositions (δ〈sup〉138/134〈/sup〉Ba) in shallow-water corals (〈em〉Porites〈/em〉) collected alive in the field, supplemented by the analysis of Ba/Ca ratios. Seven coral cores were recovered at different oceanic settings in the South China Sea, extending from the northern inner shelf to the central and southern deep basin. The annual δ〈sup〉138/134〈/sup〉Ba records of six corals fell within a narrow range from 0.24 ± 0.03‰ to 0.38 ± 0. 03‰ (2SD), with a mean value of 0.33 ± 0.08‰ (2SD, N = 21). One single inner-shelf coral revealed low δ〈sup〉138/134〈/sup〉Ba values (0.10 ∼ 0.11 ± 0.03‰), which might reflect the influence of terrestrial water/sediment. In contrast, the coral Ba/Ca ratios showed a wide range of intercolony differences, from 2 to 14 μmol/mol. This variation is too large to be ascribed to the changes in the Ba concentrations of seawater or other environmental parameters. Rayleigh fractionation between corals and seawater during biomineralization was proposed to explain the anomalous variations in the Ba/Ca ratios observed in coral skeletons. However, this result is incompatible with the relatively constant Ba isotopic compositions in coral. Instead, we suggest that the probable precipitation of witherite (BaCO〈sub〉3〈/sub〉) within the domains of aragonite under oversaturated calcifying fluid could explain the large variability in the coral Ba/Ca ratios. The coral δ〈sup〉138/134〈/sup〉Ba records from diverse oceanic settings were largely unaffected by biomineralization processes and temperature and displayed a relatively constant negative offset from typical surface seawaters. Our results suggest that Ba isotopes in 〈em〉Porites〈/em〉 could be a proxy for reconstructing the δ〈sup〉138/134〈/sup〉Ba of seawater and hence provide new insights into Ba cycling in the upper oceans in the past.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): N.C. Hurtig, S.V. Georgiev, H.J. Stein, J.L. Hannah〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Re-Os geochronometer can be applied to date petroleum, which is the end product of complex natural processes. Improved interpretation of natural Re-Os datasets requires identification of key controlling factors for Re-Os systematics and their isotopic imprint in oils. Here, we focus on the role of water in (re)setting the Re-Os geochronometer in oils, as formation waters and oils often coexist in basinal systems. To quantify Re-Os systematics during water-oil interaction, we performed a series of equilibration experiments between different oils and dilute aqueous solutions doped with Re and Os. The investigated parameters include the oil composition, water to oil ratio, interaction time and the initial Re-Os content in the aqueous solution. In experiments performed across a high concentration gradient, the oils fully homogenized with respect to their Os isotopic composition but had variable 〈sup〉187〈/sup〉Re/〈sup〉188〈/sup〉Os ratios. This indicates that the Re-Os geochronometer has been reset during water-oil interaction. However, in experiments investigating partial isotopic overprint, the crude oil and its maltene and asphaltene fractions preserved their isochron-derived age. Thus, the Re-Os radiometric clock of a single crude oil sample remains intact, even during partial equilibration of the 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os ratio. These experimental findings provide a new foundation for interpretation of Re-Os systematics in natural water-hydrocarbon systems.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 246〈/p〉 〈p〉Author(s): Yann-Aurélien Brugier, Jean-Alix Barrat, Bleuenn Gueguen, Arnaud Agranier, Akira Yamaguchi, Addi Bischoff〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Ureilite Parent Body (UPB) was a C-rich planetary embryo disrupted by impact. Ureilites are fragments of the UPB mantle and among the most numerous achondrites. Zinc isotopic data are presented for 26 unbrecciated ureilites and a trachyandesite (ALM-A) from the same parent body. The δ〈sup〉66〈/sup〉Zn values of ureilites range from 0.40 to 2.71‰ including literature results. Zinc isotopic compositions do not correlate with the compositions of olivine cores, with C and O isotopic compositions, with Zn abundances, nor with shock grades. The wide range of δ〈sup〉66〈/sup〉Zn displayed by the ureilites is chiefly explained by evaporation processes that took place during the catastrophic breakup of the UPB. During breakup, the high temperatures of the UPB mantle allowed Zn to evaporate, regardless of the intensity the shock suffered by the ureilitic debris. For the most shocked of them, post-shock heating permitted greater evaporation, and heavier Zn isotopic compositions. The surface of the UPB was certainly much colder than the mantle before the breakup. Therefore, crustal rocks were probably less prone to Zn evaporation. ALM-A, the sole crustal rock analyzed at present, has a δ〈sup〉66〈/sup〉Zn value (0.67‰) significantly higher than those of regular chondrites. This result indicates that its mantle source displayed already non-chondritic Zn isotopic compositions before the breakup of the UPB.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): Jeff R. Havig, Trinity L. Hamilton〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Earth has experienced periodic local to global glaciation for nearly 3 billion years, providing supra- and subglacial environments for colonization by microbial communities. A number of studies have reported on the role of microbial communities in glacial ecosystems including their influence on element cycling and weathering, but there is a paucity data on volcanic rock-hosted glacial ecosystems. Glaciers on stratovolcanoes in the Pacific Northwest override silica-rich rocks which represent analogues to an early Martian cryosphere. On these glaciers, blooms of photosynthetic snow algae support supraglacial microbial communities as has been observed on snowfields, glaciers, and ice sheets. In subglacial environments of volcanic rock-hosted glacial systems, weathering is driven, at least in part, by carbonic acid, suggesting a link between supraglacial carbon sources and subglacial heterotrophic microbial communities. Here, we report inorganic carbon assimilation and microbial community composition on glaciers across three stratovolcanoes ranging in composition from dacitic to mafic in the Pacific Northwest of the United States to begin to constrain the role of supraglacial primary productivity in subglacial weather processes. These data, coupled to contextual carbon and nitrogen isotope analyses of biomass and aqueous geochemistry, indicate snow algae drive light dependent carbon uptake across supraglacial and periglacial environments. Furthermore, snow algae microbial communities are supported by fixed nitrogen predominantly from deposition via precipitation. Our data highlight intense cycling of carbon and nitrogen driven by supraglacial microbial communities that feeds subglacial microbial communities which in turn may drive weathering processes. These results further underscore the role of glacial ecosystems in global biogeochemical cycling, especially during past global glaciations. Finally, these results lend support for glaciers as refugia for biodiversity on Earth and potentially on other bodies such as Mars where evidence exists for widespread and long-lived cryosphere including glaciers and ice sheets.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): Shuai Lan, Xiaoming Wang, Peng Yang, Zhangjie Qin, Mengqiang Zhu, Jing Zhang, Fan Liu, Wenfeng Tan, Qiaoyun Huang, Xionghan Feng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Birnessite (δ-MnO〈sub〉2〈/sub〉) is the most common manganese (Mn) oxide mineral in soils, sediments, and ocean manganese nodules, and it significantly affects the speciation and mobility of trace metals and organic pollutants. Abiotic oxidation of Mn(II) by dissolved O〈sub〉2〈/sub〉 is an important birnessite formation pathway, however, it has rarely been reported at neutral pH due to its very slow oxidation kinetics. Anthraquinone-2, 6-disulfonate (AQDS) is an important electron shuttle in biotic systems and might induce birnessite formation by promoting abiotic Mn(II) oxidation. Herein, the effects of AQDS concentration and types of mineral surfaces on 24 mM Mn(II) oxidation were explored at pH 7.0 using macroscopic and spectroscopic analyses. In the absence of AQDS, birnessite cannot form through the abiotic oxidation of 24 mM Mn(II) unless pH ≥ 8.5. In contrast, birnessite rapidly forms at pH 7.0 in the presence of AQDS, which acts as a catalyst. The catalytic effect of AQDS first increases and then decreases with increasing concentration, and as a “shuttle”, the concentration almost remains constant during Mn(II) oxidation. Additionally, in the absence of ferrihydrite or in the presence of montmorillonite, which is an analogous insulating mineral, AQDS shows a weak catalytic effect on Mn(II) oxidation; thus, no birnessite forms. The mechanisms of Mn(II) oxidation promoted by AQDS and ferrihydrite can be described as AQDS acting as an electronic carrier with semiconductor ferrihydrite as a specific channel facilitating electron transfer between Mn(II) and O〈sub〉2〈/sub〉 on its surface. This study provides new evidence that AQDS can transfer electrons in abiotic systems as in biotic systems, leading to efficient Mn(II) oxidation and birnessite formation through an abiotic pathway at circumneutral pH in various geological settings.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): François Fripiat, Alfredo Martínez-García, Sarah E. Fawcett, Preston C. Kemeny, Anja S. Studer, Sandi M. Smart, Florian Rubach, Sergey Oleynik, Daniel M. Sigman, Gerald H. Haug〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Both the nitrogen (N) isotopic composition (δ〈sup〉15〈/sup〉N) of the nitrate source and the magnitude of isotope discrimination associated with nitrate assimilation are required to estimate the degree of past nitrate consumption from the δ〈sup〉15〈/sup〉N of organic matter in Southern Ocean sediments (e.g., preserved within diatom microfossils). It has been suggested that the amplitude of isotope discrimination (i.e. the isotope effect) correlates with mixed layer depth, driven by a physiological response of phytoplankton to light availability, which introduces complexity to the interpretation of sedimentary records. However, most of the isotope effect estimates that underpin this hypothesis derive from acid-preserved water samples, from which nitrite would have been volatilized and lost during storage. Nitrite δ〈sup〉15〈/sup〉N in Antarctic Zone surface waters is extremely low (−61 ± 20‰), consistent with the expression of an equilibrium isotope effect associated with nitrate–nitrite interconversion. Its loss from the combined nitrate + nitrite pool would act to raise the δ〈sup〉15〈/sup〉N of nitrate, potentially yielding overestimation of the isotope effect. Here, we revisit the nitrate assimilation isotope effect in the Antarctic Zone with measurements of the δ〈sup〉15〈/sup〉N and concentration of nitrate with and without nitrite, using frozen sea water samples from 5 different cruises that collectively cover all sectors of the Southern Ocean. The N isotope effect estimated using nitrate + nitrite δ〈sup〉15〈/sup〉N is relatively constant (5.5 ± 0.6‰) across the Antarctic Zone, shows no relationship with mixed layer depth, and is in agreement with sediment trap δ〈sup〉15〈/sup〉N measurements. Estimates of the N isotope effect derived from nitrate-only δ〈sup〉15〈/sup〉N are higher and more variable (7.9 ± 1.5‰), consistent with an artifact from nitrate-nitrite isotope exchange. In the case of the Southern Ocean, we conclude that the δ〈sup〉15〈/sup〉N of nitrate + nitrite better reflects the isotope effect of nitrate assimilation. The stability of this isotope effect across the Antarctic Zone simplifies the effort to reconstruct the past degree of nitrate consumption.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): Khoren Avetisyan, Tamir Buchshtav, Alexey Kamyshny〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A detailed study of kinetics of reaction between hydrogen sulfide and orthorhombic cyclooctasulfur at environmentally relevant conditions, which results in formation of inorganic polysulfides, was performed. Rates of reaction were measured as a function of pH, temperature and concentrations of S〈sup〉2−〈/sup〉 and S〈sup〉0〈/sup〉 in airtight stirred batch reactor. Reaction was carried out at [S〈sup〉0〈/sup〉]/[S〈sup〉2−〈/sup〉] 〈 0.1 in order to minimize a contribution of interfering reaction between orthorhombic cyclooctasulfur and polysulfides, which are stronger nucleophiles than sulfanide (HS〈sup〉−〈/sup〉). Reaction was found to follow the 0.28 order with respect to activity of hydroxyl anion and first order with respect to concentrations of both hydrogen sulfide and orthorhombic cyclooctasulfur. The reaction activation energy was found to be 69 kJ mol〈sup〉−1〈/sup〉. At conditions relevant for sulfidic marine sediments, the characteristic time of the reaction is c.a. 1 year. Relatively high activation energy of the reaction and distribution of polysulfide species testify to formation of polysulfides by a reaction between sulfanide and dissolved cyclooctasulfur rather than by direct nucleophilic dissolution of orthorhombic cyclooctasulfur.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 December 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Jessica J. Barnes, Ian A. Franchi, Francis M. McCubbin, Mahesh Anand〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The isotopes of chlorine (〈sup〉37〈/sup〉Cl and 〈sup〉35〈/sup〉Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ〈sup〉37〈/sup〉Cl (‰) = [(〈sup〉37〈/sup〉Cl/〈sup〉35〈/sup〉Cl〈sub〉sample〈/sub〉/〈sup〉37〈/sup〉Cl/〈sup〉35〈/sup〉Cl〈sub〉SMOC〈/sub〉) − 1] × 1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14‰ (Apollo 15), +10 to +19‰ (Apollo 12), +9 to +15‰ (70017), +4 to +8‰ (MIL 05035), and +15 to +22‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al. (2015) and Barnes et al. (2016), as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): Brice Lacroix, Nathan A. Niemi〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The Δ〈sub〉47〈/sub〉 clumped isotope thermometer has been applied to multiple studies aimed at reconstructing the paleoelevation and paleoclimate of sedimentary basins and paleosol sequences. Ideally, this technique directly preserves the temperature of carbonate formation, avoiding any speculation on the composition of surface water from which the carbonate precipitated. Recently, however, concerns about post-depositional alteration of the Δ〈sub〉47〈/sub〉 isotope signature from the effects of burial and/or diagenetic alteration have arisen, potentially complicating the application of the Δ〈sub〉47〈/sub〉 clumped isotope thermometer for determining paleo-surface temperatures.〈/p〉 〈p〉Here we investigate the effect of burial history on mass-47 clumped isotope. To this purpose we collected samples, from the surface and from drill cores, in two different areas of the Greater Green River basin: the Washakie Basin near Rock Springs, Wyoming and the Green River Basin near Pinedale, Wyoming. Both basins are filled with a thick sequence of Eocene lacustrine strata and the thermal histories of both basins are well documented from petroleum prospecting studies. Clumped isotope Δ〈sub〉47〈/sub〉 compositions were measured from lacustrine micritic limestones with a range of peak burial depths from 1 to 6.5 km. For samples from the Washakie Basin that did not experience burial depths exceeding ∼2000 meters, the measured Δ〈sub〉47〈/sub〉 values vary from 0.623‰ to 0.684‰, yielding carbonate formation temperatures consistent with previously hypothesized Paleocene-Eocene surface temperatures. In contrast, samples from the Green River Basin, collected from greater burial depths, demonstrate significantly lower Δ〈sub〉47〈/sub〉 values (higher temperatures). The consistency of δ〈sup〉18〈/sup〉O values and lack of CL evidence for recrystallization suggest closed-system resetting of the clumped isotope thermometer by either partial solid-state re-ordering or carbonate reprecipitation during burial. The Δ〈sub〉47〈/sub〉 values of the deeply buried samples have been compared to values derived from recrystallization and temperature history reordering models (THRMs). Both recrystallization and reordering models are successful at predicting Δ〈sub〉47〈/sub〉 reordering at burial temperatures 〉100 °C. These observations, from samples collected from a basin with a remarkably well-constrained thermal history and geothermal gradient, highlight the challenges in elucidating burial histories or diagenetic processes solely from clumped isotopic compositions.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Naman Deep Singh, Venkatesh Chinni, Sunil Kumar Singh〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Dissolved aluminium (hereafter, dAl) concentrations have been determined on 34 full vertical water column profiles, sampled along the two separate GEOTRACES-India transects (GI-01 and GI-06) to understand the biogeochemical controls on dAl distribution in the northern (the Bay of Bengal, the Andaman Sea and the Arabian Sea), equatorial and subtropical gyre region of the Indian Ocean. Al released due to the partial dissolution of and/or Al desorption from the suspended lithogenic sediments, supplied in a huge amount to the Bay of Bengal (BoB) waters by the Ganga-Brahmaputra (G-B) river system, Indian peninsular rivers and sediment resuspension from the continental shelf and slope, predominantly controls the dAl distribution in the BoB and results in an overall increase in the dAl concentrations throughout the water column towards the northern BoB and the eastern coast of India. Considering the steady-state balance between the dAl input from the lithogenic sediment flux in the upper water column and first-order scavenging removal of dAl, the fractional solubility of Al from the lithogenic sediments in the BoB surface waters is estimated to be in the range of 1.1-4.7 %. Advection of Indonesian Sea surface waters through the Strait of Malacca and dAl input from the partial dissolution of resuspended terrigenous sediments, sourced from the insular shelf of the Nicobar Islands, set major controls on the dAl distribution in the surface and upper thermocline waters of the southern Andaman Sea and the southern BoB region near the Nicobar Islands. Rapid renewal of and vertical mixing in the deeper waters (〉1000 m) relative to scavenging removal of dAl results in near-uniform dAl concentrations (∼ 3.7 nM) in the deeper waters at the center of the southern Andaman Sea. In the south-eastern Arabian Sea, the continental outflow of mineral dust and advection of dAl enriched Bay of Bengal surface waters control the dAl distribution in the surface water layer. dAl in the surface waters of the equatorial Indian Ocean, during the late winter sampling period of the GI-06 cruise, is predominantly determined by the mixing between dAl-rich surface waters of the southern BoB and relatively, dAl-depleted surface waters of the southern Arabian Sea under the influence of the Northeast Monsoon Current. The scavenging residence time of dAl in the deep waters of the equatorial Indian Ocean is estimated using a 1-D scavenging-advection-diffusion model and found to be in the range of 92-141 years. Deposition of Australian dust and advection of Indonesian Throughflow Water translates to the dAl enrichment in the upper water column (〈500 m) at the northern end of the Indian Subtropical Gyre. Sediment resuspension near the Central Indian Ridge probably enrich the dAl in the ambient deep water depth (2000–3500 m) and this signal dampens as the deep waters progress north-westward in the Central Indian Ocean Basin. The bottom water (〉3500 m) advects across the Ninety East Ridge from the Western Australian Basin to bring the dAl-rich waters to the northern end of the Central Indian Ocean Basin.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 26 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Jemma Davidson, Devin L. Schrader, Conel M.O'D. Alexander, Larry R. Nittler, Roxane Bowden〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉We re-examine the Renazzo-like (CR) chondrite metamorphic trend based on Cr〈sub〉2〈/sub〉O〈sub〉3〈/sub〉 contents of FeO-rich olivine, indicating that it is only appropriate to use such analyses to identify endmembers (i.e., those that have experienced either no detectable heating or significant heating). As such Miller Range (MIL) 090657 appears to have experienced very minimal (if any) thermal processing and is one of the most pristine CR chondrites analyzed to date, while Graves Nunataks 06100 is the most shock-heated CR chondrite studied.〈/p〉 〈p〉On the basis of bulk H-C-N isotopic compositions, MIL 090657 appears to be of petrological type 2.7. We also report the H-C-N isotopic compositions of extracted insoluble organic matter, 〈em〉in situ〈/em〉 chemical compositional data, presolar grain abundances, and a petrologic description of MIL 090657. As a minimally altered CR chondrite of relatively high mass (133.1 g), MIL 090657 provides an invaluable opportunity to perform coordinated, often destructive, analyses on pristine CR chondrite material.〈/p〉 〈p〉By combining a number of petrographic characteristics (Cr〈sub〉2〈/sub〉O〈sub〉3〈/sub〉-content of ferroan olivine, Co/Ni ratios of Fe,Ni metal, ratios of Fe# in chondrule olivine and low-Ca pyroxene, and the presence of excess silica in chondrule plagioclase) with bulk isotopic compositions, we demonstrate their utility as indicators for determining the relative pristinity/heating of low petrographic (type 1 to 3) chondrites.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 265〈/p〉 〈p〉Author(s): Fei Wu, Jeremy D. Owens, Limei Tang, Yanhui Dong, Fang Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Vanadium (V) isotopes can be significantly fractionated during the delivery, cycling, and burial to the ocean. The utilization of V isotopes might provide a useful geochemical tracer for the evolution of the oceans. This study provides the first detailed investigation of V isotopes in ferromanganese (Fe-Mn) crusts and nodules, which are widely distributed in the modern oxic ocean. Our results show a large variability of V isotope compositions in Fe-Mn crusts and nodules with a δ〈sup〉51〈/sup〉V range from −0.89 to −1.65‰, much greater than current analytical uncertainty (±0.10‰, 2 SD). Therein, the most recent layers of hydrogenetic Fe-Mn crust and hydrogenous nodules have a narrower V isotope range (−0.89‰ to −1.25‰), with no correlation to their collection location or water depth. Thus, our results suggest a relatively homogeneous V isotope composition of marine hydrogenetic Fe-Mn crusts and nodules, with an average δ〈sup〉51〈/sup〉V of −1.05 ± 0.16‰ (2 SD). The hydrogenetic Fe-Mn crusts and nodules are depleted in 〈sup〉51〈/sup〉V compared with the recently reported seawater value (0.2 ± 0.15‰) by ∼1.2 ± 0.2‰ (2 SD). This can be explained by isotope fractionation during the adsorption of V onto Fe-Mn oxyhydroxides. The various δ〈sup〉51〈/sup〉V in Fe-Mn nodules (−0.98‰ to −1.65‰) might be caused by diagenetic precipitation of V from pore fluids with lighter isotope composition compared to seawater, thus driving a negative V isotope shift recorded in Fe-Mn nodules with a diagenetic imprint. Additionally, a depth profiles across an Fe-Mn crusts reveals systematic changes in δ〈sup〉51〈/sup〉V from −1.04 ± 0.13‰ (2 SD) in upper layers to 1.32 ± 0.06‰ (2 SD) in lower layers representing the relatively older deposition. The observed δ〈sup〉51〈/sup〉V depth profiles might record the changes of V isotope composition of seawater. The proposed temporal variations of δ〈sup〉51〈/sup〉V in seawater could be controlled by the isotope fractionation and fluxes of various V sources and sinks to the ocean that is likely related to the global redox state of the oceans. Another potential interpretation of this isotopic shift in the crusts could be related to the modification of the primary V isotope signature due to diagenetic remobilization and reorganization. However, there is no obvious evidence that definitively documents a diagenetic control or signal. This study highlights the burial of V with Fe-Mn oxyhydroxide as an important control on the V isotope composition of seawater, and the potential application of Fe-Mn crusts to track the temporal V isotope variations of seawater.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 265〈/p〉 〈p〉Author(s): N.J. Ingrao, T. Hammouda, M. Boyet, M. Gaborieau, B.N. Moine, I. Vlastelic, M.A. Bouhifd, J.-L. Devidal, O. Mathon, D. Testemale, J.-L. Hazemann, O. Proux〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We present the first complete dataset of partition coefficients of Rare Earth Elements (REE) between oldhamites or molten FeS and silicate melts. Values have been determined at 1300 and 1400 °C from experiments on mixtures of a natural enstatite chondrite and sulphides powders (FeS or CaS) performed in evacuated silica tubes for different 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msub〉〈mi〉f〈/mi〉〈msub〉〈mi〉O〈/mi〉〈mn〉2〈/mn〉〈/msub〉〈/msub〉〈/mrow〉〈/math〉 conditions (from IW-6.9 to IW-4.1). Obtained REE partitioning values are between 0.5 and 5 for oldhamites and between 0.001 and 1 for FeS. In both sulphides, Eu and Yb are preferentially incorporated compared to neighbouring REE. X-ray Absorption Near Edge Structure measurements on Yb and Sm demonstrate the partial reduction to 2+ valence state for both elements, Yb reduction being more pronounced. Therefore, the Yb anomaly in the sulphides is interpreted to be an effect of the presence of Yb〈sup〉2+〈/sup〉 in the system and the amplitude of the anomaly increases with decreasing oxygen fugacity. The obtained oldhamite/silicate melt partition coefficients patterns are unlike any of the observed data in natural oldhamites from enstatite chondrites and achondrites. In particular, the low values do not explain the observed enrichments in oldhamite crystals. However, positive Eu and Yb anomalies are observed in some oldhamites from EH chondrites and aubrites. We attribute these anomalies found in meteorites to the sole oldhamite control on REE budget. We conclude that the presence of positive Eu and Yb anomalies in oldhamites is a good indicator of their primordial character and that these oldhamites carry a condensation signature from a highly reduced nebular gas.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 265〈/p〉 〈p〉Author(s): Xiao-Ying Gao, Ling Wang, Yi-Xiang Chen, Yong-Fei Zheng, Ren-Xu Chen, Fang Huang, Qiang-Qiang Zhang, Min Ji, Zi-Yue Meng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Metamorphic fluids produced by dehydration of subducting crust transport mass and energy at the slab-mantle interface in subduction channels. It is commonly assumed that fluid flow is significant in oceanic subduction channels but insignificant in continental subduction channels. This assumption is challenged by a combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements in coesite-bearing jadeite quartzites from the Dabie orogen, China. Although the target samples were collected from different outcrops in an area of ∼50 km〈sup〉2〈/sup〉, zircon U-Pb dating yields similar discordia lines with not only consistent upper intercept ages of 1.9–2.0 Ga but also consistent lower intercept ages of 224–235 Ma. This indicates the same Paleoproterozoic protolith and the same Triassic metamorphism for these jadeite quartzites. The O isotope analysis of mineral separates and whole-rock yields variable δ〈sup〉18〈/sup〉O values from 6.3‰ to 9.4‰, indicating involvement of supracrustal components. Except for one outlier at −0.43‰, all the other rocks give variable δ〈sup〉26〈/sup〉Mg values from −0.16‰ to 0.61‰, much higher than normal mantle values. The whole-rock Mg isotopes show significant positive correlations not only with MgO contents but also with Rb/La, Rb/Gd and Rb/Nb ratios, but a negative correlation with Na〈sub〉2〈/sub〉O contents. These observations indicate that the middle Paleoproterozoic protolith of jadeite quartzites was weathered to produce a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ〈sup〉26〈/sup〉Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks during their subduction to subarc depths for ultrahigh-pressure metamorphism. They would have acquired their geochemical compositions not only from the biotite breakdown but also through leaching reaction with the TTG provenance. In view of the spatial occurrences of the target samples, the metamorphic fluids would have flowed inside the continental subduction channel on a large scale. This is the first report of the large-scale fluid flow in the continental subduction zone and therefore demonstrates that fluid flow can be significant in continental subduction channels.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 12 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Penny E. Wieser, Stephen J. Turner, Tamsin A. Mather, David M. Pyle, Ivan P. Savov, Gabriel Orozco〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Magmas from continental arcs built on thick crust have elevated incompatible element abundances and “enriched” radiogenic isotope ratios compared to magmas erupted in island and continental arcs overlying thinner crust. The relative influence of the slab, mantle, and upper plate on this variability is heavily debated. The Andean Southern Volcanic Zone (SVZ; 33-46° S) is an ideal setting to investigate the production of enriched continental arc compositions, because both crustal thickness and magma chemistry vary coherently along strike. However, the scarcity of primitive magmas in the thick-crusted northern SVZ has hindered previous regional studies. To better address the origin of enriched continental compositions, we investigate the geochemistry (major and trace element abundances, 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr and 〈sup〉143〈/sup〉Nd/〈sup〉144〈/sup〉Nd ratios) of new mafic samples from Don Casimiro and Maipo volcanoes in Diamante-Maipo Caldera Complex of the northern SVZ. While evolved Diamante-Maipo samples show evidence for crustal assimilation, the trace element and isotopic enrichment of the most mafic samples cannot result from crustal processing, as no known regional or global basement lithologies are enriched in all of the necessary incompatible trace elements. Subduction erosion models similarly fail to account for the enriched isotopic and trace element signature of these samples. Instead, we suggest that the enrichment of northern SVZ magmas is derived from an enriched ambient mantle component (similar to EM1-type ocean island basalts), superimposed on a northward decline in melt extent. A substantial, but nearly uniform contribution of melts from subducting sediment and altered oceanic crust are required at all latitudes. The EM1-like enrichment may arise from recycling of metasomatized subcontinental lithospheric mantle (M-SCLM), as the isotopic trajectory of primitive rear-arc monogenetic cones trend towards the compositions of SCLM melts sampled across South America. Isotopic data from spatially distributed rear-arc centres demonstrate that the arc-parallel variations in the degree of EM1-type enrichment observed in arc-front samples are also present up to 600 km behind the trench in the rear-arc. Rear-arc trace element systematics require significant but variable quantities of slab melts to be transported to the mantle wedge at these large trench distances. Overall, we show that a unified model incorporating variable mantle enrichment, slab additions, and melt extents can account for along and across-arc trends within the SVZ. The recognition that mantle enrichment plays a key role in the production of enriched continental compositions in the SVZ has important implications for our understanding of the chemical evolution of the Earth. If ambient mantle enrichment is not taken into account, petrogenetic models of evolved lavas may overestimate the role of crustal assimilation, which, in turn, may lead models of continental crust growth to overestimate the amount of continental material that has been recycled back into the mantle.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 November 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 265〈/p〉 〈p〉Author(s): B.L.A. Charlier, F.L.H. Tissot, N. Dauphas, C.J.N. Wilson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We present new nucleosynthetic, radiogenic and stable Sr isotopic data from fifteen previously studied CAIs from the Allende CV3 meteorite, including the highly altered 〈em〉Curious Marie〈/em〉 inclusion. We use double-spike TIMS techniques to determine the degrees of isotopic mass fractionation, and also present internally normalised data for the same sample digestions to permit comparisons with previous studies and couple these isotopic data with Rb, Sr, Eu and Th abundance data to consider the origins and relationships of the isotopic variations documented here. Analysed CAIs display elevated μ〈sup〉84〈/sup〉Sr anomalies of +58 ppm to +287 ppm, with variability far outside of analytical uncertainties (13 ppm 2 s.d.). We cannot tell at present whether these variations arise from heterogeneities in 〈em〉p〈/em〉-process 〈sup〉84〈/sup〉Sr or in the other non-radiogenic isotopes of Sr (〈sup〉86〈/sup〉Sr, 〈sup〉88〈/sup〉Sr) that are produced by the main 〈em〉s〈/em〉-process, weak 〈em〉s〈/em〉-process, and 〈em〉r〈/em〉-process. All inclusions fall on an offset mass-dependent fractionation line in three-isotope space (δ〈sup〉88/86〈/sup〉Sr vs δ〈sup〉84/86〈/sup〉Sr) identical within error to that previously defined by bulk undifferentiated meteorites, and have a total range of δ〈sup〉88/86〈/sup〉Sr of ∼5.3 ‰ (+1.67 ‰ to −3.67 ‰), reflecting kinetic isotope effects during partial condensation/evaporation and/or low-temperature alteration processes. CI-normalized Sr/Th ratios in our CAIs correlate with normalized Eu/Th ratios with a ∼ 1:1 relationship, regardless of texture or Sr-isotopic values. This indicates that Sr and Eu had similar condensation behaviors with Eu condensing as Eu〈sup〉2+〈/sup〉 and having the same chemical behavior in minerals as Sr〈sup〉2+〈/sup〉 under conditions relevant to CAI formation in the solar nebula. Rb/Th ratios are highly variable: fine-grained CAIs display elevated Rb/Th ratios, consistent with the introduction of Rb into the CAIs by alkali-rich secondary alteration fluids. The μ〈sup〉84〈/sup〉Sr anomalies measured in our CAIs are similar (in magnitude) to those found in carbonaceous chondrites that formed in the outer part of the solar system. A way to reconcile this observation with the formation of CAIs near the Sun would be if the inventories of Sr and other refractory elements in carbonaceous chondrites are dominated by a cryptic refractory dust component (CRD) that was formed early and near the Sun, and was subsequently transported outwards to the carbonaceous chondrite-forming region.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 268〈/p〉 〈p〉Author(s): Bernard P. Boudreau, Olivier Sulpis, Alfonso Mucci〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Prediction of the neutralization of anthropogenic CO〈sub〉2〈/sub〉 in the oceans and the interpretation of the calcite record preserved in deep-sea sediments requires the use of the correct kinetics for calcite dissolution. Dissolution rate information from suspended calcite-grain experiments consistently indicates a high-order nonlinear dependence on undersaturation, with a well-defined rate constant. Conversely, stirred-chamber and rotating-disc dissolution experiments consistently indicate linear kinetics of dissolution and a strong dependence on the fluid flow velocity. Here, we resolve these seeming incongruities and establish reliably the kinetic controls on deep-sea calcite dissolution. By equating the carbonate-ion flux from a dissolving calcite bed, governed by laboratory-based nonlinear kinetics, to the flux across typical diffusive boundary layers (DBL) at the seafloor, we show that the net flux is influenced both by boundary layer and bed processes, but that flux is strongly dominated by the rate of diffusion through the DBL. Furthermore, coupling that calculation to an equation for the calcite content of the seafloor, we show that a DBL-transport dominated model predicts lysoclines adeptly, i.e., CaCO〈sub〉3〈/sub〉 vs ocean depth profiles, observed across the oceans. Conversely, a model with only sediment-side processes fails to predict lysoclines in all tested regions. Consequently, the past practice of arbitrarily altering the calcite-dissolution rate constant to allow sediment-side only models to fit calcite profiles constitutes confirmation bias. From these results, we hypothesize that the reason suspended-grain experiments and bed experiments yield different reaction orders is that dissolution rates of individual grains in a bed are fast enough to maintain porewaters at or close to saturation, so that the exact reaction order cannot be measured accurately and dissolution appears to be linear. Finally, we provide a further test of DBL-transport dominated calcite dissolution by successfully predicting, not fitting, the in-situ pH profiles observed at four stations reported in the literature.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 268〈/p〉 〈p〉Author(s): Emily Dearing Crampton-Flood, Jessica E. Tierney, Francien Peterse, Frédérique M.S.A. Kirkels, Jaap S. Sinninghe Damsté〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Accurate temperature records for the deep geological past are a vital component of paleoclimate research. Distributional changes of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids in geological archives including paleosoils are a promising indicators to infer past continental air temperatures. However, the ‘orphan’ status of the brGDGTs, the potential effect of temperature-independent parameters on their relative distribution, and the uneven geographical distribution of the soils used for calibration contribute to the high uncertainty of brGDGT-based transfer functions (root mean squared error, RMSE: ±5 °C). Here, we expand the soil dataset from the previous calibration(s) with new and published soil data. We use Bayesian statistics to calibrate the relationship of the 5-methyl brGDGTs (MBT′〈sub〉5Me〈/sub〉) and mean annual air temperature (MAAT). The addition of soils from warm (〉28 °C) environments from India substantially increases the upper limit of the Bayesian calibration (BayMBT) from 25 to 29 °C, aiding in the generation of temperature records for past greenhouse climates, such as the Eocene. The BayMBT model also effectively minimizes the structured MAAT residuals prevalent in previous calibrations, therefore giving the opportunity to explore confounding factors on the calibration. We formulate a set of alternative calibration models to test the effect of specific environmental parameters and show that soils at mid-latitudes with temperature seasonalities 〉20 °C are not well described by the BayMBT model. We find that the MBT′〈sub〉5Me〈/sub〉 index is best correlated to the average temperature of all months 〉0 °C, called the BayMBT〈sub〉0〈/sub〉 model. This finding supports the hypothesis that brGDGT production ceases or slows down in the winter months. However, a persistent feature of the BayMBT model and previous calibrations is the significant scatter at mid-latitudes, which is speculatively linked with a possible increase in diversity of microbial brGDGT-producing communities in these locations.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 268〈/p〉 〈p〉Author(s): Naomi S. Wells, Damien Maher, Peisheng Huang, Dirk V. Erler, Paul Maxwell, Matthew R. Hipsey, Bradley D. Eyre〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Combined pressures from inland agricultural intensification and coastal development are dramatically altering estuaries’ structure and function. Despite the established global significance of estuarine carbon (C) cycling, the impact of growing anthropogenic stress on coastal C inputs and exports is unclear. To address this gap, we evaluated the magnitude and drivers of estuary C fluxes in eight sub-tropical estuaries at Low (〈em〉n〈/em〉 = 3), Moderate (〈em〉n〈/em〉 = 2), and High (〈em〉n〈/em〉 = 3) levels of nutrient enrichment. We measured changes in the concentration and isotopic composition (δ〈sup〉13〈/sup〉C) of the major C pools (organic and inorganic) and gaseous product of C turnover (CO〈sub〉2〈/sub〉) over wet and dry seasons. Over both sampling periods estuaries classified Moderate and High emitted far more CO〈sub〉2〈/sub〉 (37 ± 10 mmol m〈sup〉−2〈/sup〉 d〈sup〉−1〈/sup〉) than those classified Low (6.3 ± 4 mmol m〈sup〉−2〈/sup〉 d〈sup〉−1〈/sup〉). However, estuaries with both high nutrients and high turbidity produced less CO〈sub〉2〈/sub〉, and thus exported more DIC, than expected from hydrodynamics (freshwater flushing time). Differences in estuary phytoplankton biomass (Chl〈em〉a〈/em〉 concentrations) corresponded with differences in the biological CO〈sub〉2〈/sub〉 production (respiration) rates estimated from δ〈sup〉13〈/sup〉C-DIC variations, although respiration rates were higher than predicted based on hydrodynamics (surface area/discharge) in high nutrient, low turbidity systems. Together these findings demonstrate that land-use intensification can alter both the source and the production of estuary CO〈sub〉2〈/sub〉, and suggest that the direction of this shift can depend on ancillary factors like turbidity as well as nutrient enrichment. Evidence that human alterations to coastal ecosystems can shift the balance between DIC downstream export and CO〈sub〉2〈/sub〉 emissions outside of the range predicted by hydrodynamic factors like residence time, surface area, and discharge has implications for global C models.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 8 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Xin Gu, Daniella M. Rempe, William E. Dietrich, A. Joshua West, Teng-Chiu Lin, Lixin Jin, Susan L. Brantley〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The rate of chemical weathering has been observed to increase with the rate of physical erosion in published comparisons of many catchments, but the mechanisms that couple these processes are not well understood. We investigated this question by examining the chemical weathering and porosity profiles from catchments developed on marine shale located in Pennsylvania, USA (Susquehanna Shale Hills Critical Zone Observatory, SSHCZO); California, USA (Eel River Critical Zone Observatory, ERCZO); and Taiwan (Fushan Experimental Forest). The protolith compositions, protolith porosities, and the depths of regolith at these sites are roughly similar while the catchments are characterized by large differences in erosion rate (1-3 mm yr〈sup〉-1〈/sup〉 in Fushan 〉〉 0.2-0.4 mm yr〈sup〉-1〈/sup〉 in ERCZO 〉〉 0.01-0.025 mm yr〈sup〉-1〈/sup〉 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr〈sup〉-1〈/sup〉 in Fushan 〉 1.9 m yr〈sup〉-1〈/sup〉 in ERCZO 〉 1.1 m yr〈sup〉-1〈/sup〉 in SSHCZO), so the fastest eroding site experiences nearly twice the mean annual temperature of the other two.〈/p〉 〈p〉Even though erosion rates varied by about 100×, the depth of pyrite and carbonate depletion (defined here as regolith thickness) is roughly the same, consistent with chemical weathering of those minerals keeping up with erosion at the three sites. These minerals were always observed to be the deepest to react, and they reacted until 100% depletion. In two of three of the catchments where borehole observations were available for ridges, these minerals weathered across narrow reaction fronts. On the other hand, for the rock-forming clay mineral chlorite, the depth interval of weathering was wide and the extent of depletion observed at the land surface decreased with increasing erosion/precipitation. Thus, chemical weathering of the clay did not keep pace with erosion rate. But perhaps the biggest difference among the shales is that in the fast-eroding sites, microfractures account for 30-60% of the total porosity while in the slow-eroding shale, dissolution could be directly related to secondary porosity. We argue that the microfractures increase the influx of oxygen at depth and decrease the size of diffusion-limited internal domains of matrix, accelerating weathering of pyrite and carbonate under high erosion-rate conditions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 269〈/p〉 〈p〉Author(s): Jia-Xin She, Tianhua Wang, Hengdi Liang, M.N. Muhtar, Weiqiang Li, Xiandong Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Volatilization is an important pathway of element transport in nature, and this process may be associated with stable isotope fractionation, which could be used to understand the elemental volatilization mechanisms. In this study, we report that evaporation of Sn(IV) chloride solution under experimental conditions (96 °C, 1 atmospheric pressure) results in significant loss of aqueous Sn(IV) and Sn stable isotope fractionation. The δ〈sup〉122/116〈/sup〉Sn of the residue solution can increase by up to 0.50‰ (or a 0.33‰ increase in δ〈sup〉122/118〈/sup〉Sn) after repeated evaporation, indicating preferential partitioning of isotopically light Sn species into the vapor phase during evaporation. The observed Sn loss and associated Sn isotope fractionation during the evaporation experiments can be described using a Rayleigh fractionation function, with a best-fitting isotope fractionation factor of −0.36‰ in Δ〈sup〉122/116〈/sup〉Sn〈sub〉vapor-aq〈/sub〉 (or −0.24‰ in Δ〈sup〉122/118〈/sup〉Sn〈sub〉vapor-aq〈/sub〉). We also performed quantum mechanical calculations to assess the stability of different potential Sn(IV) species in aqueous and vapor phases, and derived the equilibrium Sn isotope fractionation factors between these Sn(IV) species. The calculation results suggest that the dominant gaseous species of Sn(IV), SnCl〈sub〉4〈/sub〉, is isotopically heavier than the aqueous Sn(IV) species by 1.24‰–0.35‰ in δ〈sup〉122/116〈/sup〉Sn (or 0.82‰–0.19‰ in δ〈sup〉122/118〈/sup〉Sn) under equilibrium at 96 °C, which is opposite to the experimental results. Such contrast in the direction of Sn isotope fractionation implies that kinetic isotope fractionation, rather than thermodynamic equilibrium isotope fractionation, took place for SnCl〈sub〉4〈/sub〉 in the evaporation experiments at 96 °C. The observed experimental data can be explained by a kinetic isotope fractionation model involving backward reaction of SnCl〈sub〉4〈/sub〉 vaporization at the solution-vapor boundary. This study, in combination with a recent report of positive Δ〈sup〉122/116〈/sup〉Sn〈sub〉vapor-aq〈/sub〉 (or Δ〈sup〉122/118〈/sup〉Sn〈sub〉vapor-aq〈/sub〉) factor during evaporation of SnCl〈sub〉4〈/sub〉 at 150 °C (Wang et al., 2019a), suggests that Sn(IV) volatilization mechanisms may be different with and without fluid boiling. Combined laboratory experiments and quantum mechanical calculations on the isotopic effects of Sn(IV) chloride solution evaporation provide important constraints for understanding the rapidly accumulating Sn isotopes data from studies on Sn ore-forming processes, bronze metallurgy and archeology, and volatile elements in planetary processes.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 269〈/p〉 〈p〉Author(s): Yuyang He, Huiming Bao, Yun Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A predicted equilibrium intramolecular isotope distribution (Intra-ID) serves as a reference for measured position-specific (PS) isotope composition variation in an organic molecule. Equilibrium Intra-ID can be estimated from calculated reduced partition function ratios (RPFR or β factor), which are largely absent to date. For relatively small molecules, the PS β factor can be calculated directly. However, estimating the PS β factor considering an entire, large organic molecule is computationally prohibitive. The isotope effect is local in that the vibrational frequencies of an atom are only affected by its proximal bonding environment. Therefore, the cutoff calculation, which simplifies the calculation of an entire molecule to a local area, was previously proposed for large organic molecules. However, the cutoff size was not validated, which has hindered the application of the cutoff calculation. Here, we calculated a series of small organic molecules with 2–18 carbon atoms to test the influence of cutoff size on the 〈sup〉13〈/sup〉β value estimation of a target carbon position in a carbon chain or a carbon ring. We calculated nineteen small molecules that have a methyl carbon and a functional group that is at least three bonds away from the target methyl position. The result showed that the equilibrium 〈sup〉13〈/sup〉C enrichment of the methyl group relative to CO〈sub〉2〈/sub〉 at 25 °C (ln〈sup〉13〈/sup〉〈em〉α〈sub〉(eq)〈/sub〉〈/em〉) for the nineteen molecules varied in a small range, with a standard deviation of 0.2‰. Fourteen aromatic hydrocarbons with a benzene and one adjacent functional group were calculated to test the influence of different adjacent functional groups on similar carbon positions in benzene. The results showed that different adjacent functional groups had significant influence only on the predicted ln〈sup〉13〈/sup〉〈em〉α〈sub〉(eq)〈/sub〉〈/em〉 value of the carbon position directly connected to them (standard deviation = 1.0‰, n = 14), with a negligible influence on the predicted ln〈sup〉13〈/sup〉〈em〉α〈sub〉(eq)〈/sub〉〈/em〉 value of the remaining carbons in benzene (standard deviation = 0.2‰, n = 14). The PS 〈sup〉13〈/sup〉β value of a specific carbon position in CoA calculated by the cutoff calculation differed from that of the entire-molecule calculation by 0.0–0.3‰. We concluded that the cutoff calculation simplified the calculation of a target position from an entire molecule to a cluster of three proximal bonds in a chain and/ or an adjacent ring, providing PS 〈sup〉13〈/sup〉β values of sufficient accuracy for large organic molecules.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 269〈/p〉 〈p〉Author(s): David M. Singer, Elizabeth Herndon, Kortney Cole, Michael Burkey, Sarah Morisson, Michael Cahill, Matthew A. Bartucci〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Coal mine spoil is a long-lasting legacy of historic coal mining operations that continues to impact water quality across Appalachia. Metal(loid) release from abandoned coal mine spoil, which can be a significant source of acid mine drainage (AMD), is dependent on speciation and distribution within the parent coal shale. This work aimed to determine how the micron- and sub-micron scale mineralogy, morphology, and texture of metal(loid)-bearing phases in a coal-shale control the rate and release of metal(loid)s during subsequent weathering through two primary objectives: (1) determine the microscale speciation and distribution of Fe and other metal(loid)s in a parent coal shale, and (2) determine the amount of metal(loid)s released during simulated weathering of shale physically crushed into silt- to sand-sized particles. This work was accomplished through a combination of electron microscopy and synchrotron-based X-ray microprobe analyses. This suite of techniques also provides insight into the geochemical history of the coal shale that could not be determined by bulk techniques. Trace elements were associated with either Fe-sulfides or with other phases that included metal(loid)-sulfides, aluminosilicates, and organic matter. Three distinct pools of Fe-sulfides were present: (i) larger mm-scale grains, with minimal internal fractures; (ii) μm- to mm-scale aggregates of μm-scale crystals forming secondary coatings on the larger mm-scale grains, and (iii) μm- to mm-scale aggregates forming framboidal grains. Fe-sulfide mineralogy was dominated by pyrite with minor contributions of marcasite and a S(-II)-bearing phase. Larger pyrite grains and their secondary coatings contained homogeneous distributions of Fe and Mn as well as other trace metal(loids) including V, Ti, Cr, As, Se, Cu, and Zn. The fine-grained pyrite aggregates were strongly enriched in As and Se and contained discrete Cu- and Zn-bearing particles. Clays and organic matter surrounding the sulfide minerals were identified by micro-XRD and FT-IR spectroscopy. Simulated batch weathering of the physically crushed shale resulted in metal(loid) release that varied as function of size fraction and time. Metal(loid) release increased with decreasing particle size from sand-sized to silt-sized fractions. Although small amounts of Fe were released into solution after two days, the bulk of Fe release occurred after 10 days and continued for six months, after which Fe was gradually removed from solution. The weathering trends for Mn, Cu, Zn, and Ni were similar to Fe. In contrast, As and Se were characterized by rapid release into solution followed by removal prior to 10 days. These results indicate that micrometer-scale metal(loid) distribution within Fe sulfides controlled metal(loid) release into solution during simulated weathering of a coal shale. Specifically, elements concentrated in mineral coatings (As, Se) were rapidly mobilized in a short-lived pulse whereas elements associated with the larger grains (Fe, Mn, Cu, Zn, and Ni) exhibited a delayed but prolonged release into solution. The non-concomitant contaminant release indicates that an understanding of the textural relationships in the source material is required to understand weathering trends. This work highlights the importance of non-point sources of AMD, and addressing these sources is a critical step in improving water quality in the region.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 7 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Yuefei Zhou, Yang Gao, Qiaoqin Xie, Jin Wang, Zhengbo Yue, Lin Wei, Yang Yang, Ling Li, Tianhu Chen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Magnetite and maghemite are important components of iron oxides that determine the magnetic properties of rocks, soils, and sediments, and are also materials with broad industrial applications. We investigated the reduction and transformation of both phases with a strain of sulfate-reducing bacteria (SRB). SRB growth resulted in 28.1% and 7.1% sulfate to acid volatile sulfur conversion in magnetite and maghemite, respectively. Transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) analyses indicate that monosulfides (mackinawite and greigite) and polysulfides are the main secondary sulfides in the magnetite experiment, while the maghemite experiment also contained a high proportion of pyrite. XPS analyses indicate the reduction of Fe(III) to Fe(II) on the surface of magnetite and maghemite both by dissolved sulfides and SRB. Mössbauer spectroscopy measurements reveal the formation of superparamagnetic phases in microbial experiments, which indicates the dissolution and particle size decrease of the two minerals both by dissolved sulfides and SRB. X-ray diffraction and Mössbauer spectroscopy analyses suggest a complete transformation of nanomaghemite to nanomagnetite under the mediation of SRB through solid phase Fe(III) reduction. This transformation controls the changing and different patterns of both magnetic susceptibility and magnetic hysteresis for the two minerals. It is suggested that the structural similarity between magnetite and maghemite, and the conductivity of magnetite, constrain the unique solid phase transformation. Our findings indicate that the maghemite–magnetite solid solution is a potential natural battery for the growth of anaerobic microbes in sulfidic environments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 13 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): James Z. Sippo, Damien T. Maher, Kai G. Schulz, Christian J. Sanders, Ashly McMahon, James Tucker, Isaac R. Santos〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mangrove soil carbon stocks are known to decrease following forest loss due to respiration and enhanced soil CO〈sub〉2〈/sub〉 emissions. However, changes in carbon outwelling to the coastal ocean due to mangrove forest disturbance have not been considered. In December 2015, an extremely large mangrove dieback event (∼7000 hectares, spanning 1000 km of coastline) occurred in the Gulf of Carpentaria, Australia. To assess the effect this dieback event had on carbon outwelling, we used radium isotopes and dissolved carbon measurements (dissolved organic carbon, DOC, dissolved inorganic carbon, DIC, and total alkalinity, TAlk) to estimate cross-shelf carbon transport from living and dead mangrove areas and to calculate the carbon losses from living and dead forest soils via SGD. Radium distributions imply cross shelf eddy diffusivity of 107.5 ± 26.9 and 104.6 ± 23.9 m〈sup〉-2〈/sup〉 s〈sup〉-1〈/sup〉 from dead and living areas and radium water ages reveal that mangrove carbon reaches 10 km offshore within 7 days. Outwelling rates from living and dead areas were explained by soil carbon losses via SGD. This study suggests a decrease in carbon outwelling to the ocean from dead forest areas compares to living areas by 0-12% for DOC, 50-52% for DIC and by 37-51% for TAlk ∼8 months after the dieback event occurred. Changes to oceanic carbon outwelling rates following mangrove loss are likely driven by a gradual depletion of carbon stocks from the sediment profile.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 253〈/p〉 〈p〉Author(s): N. Löffler, J. Fiebig, A. Mulch, T. Tütken, B.C. Schmidt, D. Bajnai, A.C. Conrad, U. Wacker, M.E. Böttcher〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Clumped isotope data from carbonated apatite from 〈em〉in vivo〈/em〉 and 〈em〉in vitro〈/em〉 samples are presented to refine the relationship between mineral growth temperature and carbonate clumped isotopic composition (Δ〈sub〉47〈/sub〉). Δ〈sub〉47〈/sub〉, δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C data were obtained from phosphoric acid digestion (T = 110 °C) of chemically untreated teeth from an African elephant, Greenland sharks, sand tiger sharks and synthetic apatites. These data cover a temperature range between 1 °C and 80 °C and enlarge the calibration dataset presented in Wacker et al. (2016) by a factor of five. Taxon-specific analyses of tooth enamel(oid) and dentine reveal that both tissues show identical Δ〈sub〉47〈/sub〉 values even though the content of organic matter differs by an order of magnitude. The following Δ〈sub〉47〈/sub〉 temperature calibration for (bio)apatite is derived (R〈sup〉2〈/sup〉 = 0.9924, 〈em〉p〈/em〉-value 〈 0.0001, n = 122; 8 samples):〈/p〉 〈p〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msub〉〈mi mathvariant="normal"〉Δ〈/mi〉〈mrow〉〈mn〉47〈/mn〉〈mspace width="2.5pt"〉〈/mspace〉〈mi mathvariant="normal"〉C〈/mi〉〈mi mathvariant="normal"〉D〈/mi〉〈mi mathvariant="normal"〉E〈/mi〉〈mi mathvariant="normal"〉S〈/mi〉〈mspace width="2.5pt"〉〈/mspace〉〈mn〉110〈/mn〉〈/mrow〉〈/msub〉〈mo〉=〈/mo〉〈mn〉0.0325〈/mn〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mo〉±〈/mo〉〈mn〉0.0012〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mn〉6〈/mn〉〈/msup〉〈mo stretchy="false"〉/〈/mo〉〈msup〉〈mrow〉〈mi mathvariant="normal"〉T〈/mi〉〈/mrow〉〈mn〉2〈/mn〉〈/msup〉〈mo〉+〈/mo〉〈mn〉0.2137〈/mn〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mo〉±〈/mo〉〈mn〉0.0124〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mspace width="2.5pt"〉〈/mspace〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉w〈/mi〉〈mi mathvariant="normal"〉i〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈mi mathvariant="normal"〉h〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉T〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉i〈/mi〉〈mi mathvariant="normal"〉n〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉K〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉a〈/mi〉〈mi mathvariant="normal"〉n〈/mi〉〈mi mathvariant="normal"〉d〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈msub〉〈mi mathvariant="normal"〉Δ〈/mi〉〈mn〉47〈/mn〉〈/msub〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉i〈/mi〉〈mi mathvariant="normal"〉n〈/mi〉〈mspace width="2.5pt"〉〈/mspace〉〈mi〉‰〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉〈/p〉 〈p〉This calibration becomes indistinguishable from a reprocessed empirical calibration of calcite made in the same laboratory if a difference of the acid fractionation factors (AFF) of 0.110‰ between 25 °C and 110 °C is considered. The measured AFF for bioapatite matches the one that is extrapolated from experimental data on calcite and aragonite. The oxygen isotope fractionation between structural carbonate in the synthesized carbonated hydroxylapatites (CHAP) and water between 7 °C and 80 °C closely follows the temperature dependence for the calcite-water system. It is described by the following (CHAP-water) equation (R〈sup〉2〈/sup〉 = 0.997, 〈em〉p〈/em〉-value 〈 0.04, n = 17; 3 samples):〈/p〉 〈p〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.gif" overflow="scroll"〉〈mrow〉〈mn〉1000〈/mn〉〈mi mathvariant="normal"〉l〈/mi〉〈mi mathvariant="normal"〉n〈/mi〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈msub〉〈mi〉α〈/mi〉〈mrow〉〈mi mathvariant="normal"〉C〈/mi〉〈mi mathvariant="normal"〉H〈/mi〉〈mi mathvariant="normal"〉A〈/mi〉〈mi mathvariant="normal"〉P〈/mi〉〈mo〉-〈/mo〉〈mi mathvariant="normal"〉w〈/mi〉〈mi mathvariant="normal"〉a〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈mi mathvariant="normal"〉e〈/mi〉〈mi mathvariant="normal"〉r〈/mi〉〈/mrow〉〈/msub〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mo〉=〈/mo〉〈mn〉17.23〈/mn〉〈mrow〉〈mfenced open="(" close=")"〉〈mrow〉〈mo〉±〈/mo〉〈mn〉0.59〈/mn〉〈/mrow〉〈/mfenced〉〈/mrow〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mn〉10〈/mn〉〈/mrow〉〈mn〉3〈/mn〉〈/msup〉〈mo〉×〈/mo〉〈msup〉〈mrow〉〈mi mathvariant="normal"〉T〈/mi〉〈/mrow〉〈mrow〉〈mo〉-〈/mo〉〈mn〉1〈/mn〉〈/mrow〉〈/msup〉〈mo〉-〈/mo〉〈mn〉27.28〈/mn〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mo〉±〈/mo〉〈mn〉1.73〈/mn〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈mspace width="5.0pt"〉〈/mspace〉〈mrow〉〈mo stretchy="false"〉(〈/mo〉〈mi mathvariant="normal"〉w〈/mi〉〈mi mathvariant="normal"〉i〈/mi〉〈mi mathvariant="normal"〉t〈/mi〉〈mi mathvariant="normal"〉h〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉T〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉i〈/mi〉〈mi mathvariant="normal"〉n〈/mi〉〈mspace width="5.0pt"〉〈/mspace〉〈mi mathvariant="normal"〉K〈/mi〉〈mo stretchy="false"〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉〈/p〉 〈p〉Both calibrations are applied to shark teeth from a modern Greenland shark and a fossil megatooth shark (〈em〉Carcharodon megalodon〈/em〉) specimen to reconstruct the apparent Δ〈sub〉47〈/sub〉-based habitat temperature of 〈em〉C. megalodon〈/em〉 (19 ± 4 °C) and the oxygen isotopic compositions of seawater.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 252〈/p〉 〈p〉Author(s): Lea Scholten, Christian Schmidt, Pilar Lecumberri-Sanchez, Matthew Newville, Antonio Lanzirotti, Mona-Liza C. Sirbescu, Matthew Steele-MacInnis〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Iron is among the most abundant elements in Earth's crust and is also a major aqueous solute in a variety of hydrothermal settings, yet major questions remain regarding the solubility and speciation of iron at hydrothermal conditions. Here, we conducted hydrothermal diamond-anvil cell experiments using synchrotron-radiation micro-XRF and XANES analyses as well as Raman spectroscopy, in situ at hydrothermal temperatures and pressures, to characterize the solubility and speciation of iron in fluids buffered by a variety of mineral assemblages. Our experiments included the assemblages hematite-magnetite (HM), fayalite-magnetite-quartz (FMQ), and magnetite-pyrite-pyrrhotite (MPP). Our results indicate highest solubilities of HM in HCl solutions. In sodium chloride solutions of similar molalities, FMQ shows higher Fe solubility than MPP. XANES data are interpreted as preponderance of ferrous iron in all experiments. Comparison of XANES spectra of these solutions with calculated XANES spectra from the literature indicates octahedral FeCl〈sub〉x〈/sub〉(H〈sub〉2〈/sub〉O)〈sub〉6−x〈/sub〉〈sup〉2−x〈/sup〉 (x = 0–3) as predominant Fe(II) species at lower Cl-Fe ratios, and suggests additional tetrahedral FeCl〈sub〉4〈/sub〉〈sup〉2−〈/sup〉 or FeCl〈sub〉3〈/sub〉(H〈sub〉2〈/sub〉O)〈sup〉−〈/sup〉 at high Cl-Fe ratios. Raman spectra to 600 °C show that the predominant species in ferric iron solutions is FeCl〈sub〉2〈/sub〉(H〈sub〉2〈/sub〉O)〈sub〉4〈/sub〉〈sup〉+〈/sup〉 at temperatures less than 100 °C, which transitions to FeCl〈sub〉4〈/sub〉〈sup〉−〈/sup〉 between 100 and 200 °C. An additional Raman band that occurred in some spectra of a H〈sub〉2〈/sub〉O + HCl fluid equilibrated with hematite ± magnetite at temperatures greater than 300 °C may originate from an FeCl〈sub〉3〈/sub〉(H〈sub〉2〈/sub〉O)〈sub〉x〈/sub〉 (x = 0–3) species. All Raman spectra of ferrous iron solutions show a fairly broad band at about 280 cm〈sup〉−1〈/sup〉, which can be interpreted to stem from Fe(II)–Cl vibrations of FeCl〈sub〉3〈/sub〉(H〈sub〉2〈/sub〉O)〈sub〉x〈/sub〉〈sup〉−〈/sup〉 (x = 0–3) (point group D〈sub〉3h〈/sub〉) species or, at low Cl-Fe ratios of about two or less, from octahedral FeCl〈sub〉x〈/sub〉(H〈sub〉2〈/sub〉O)〈sub〉6−x〈/sub〉〈sup〉2−x〈/sup〉 (x = 0–3) species. These results provide important constraints on the hydrothermal mobilization of iron and fluid-rock reactions involving iron- and chloride-bearing fluids.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 9 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Richard E. Zeebe, Toby Tyrrell〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In an earlier contribution to this journal, we provided a reconstruction of seawater carbonate ion concentration over the last 100 million years (Tyrrell and Zeebe, 2004; TZ04 hereafter). Since then, multiple new and more robust data sets on past ocean carbonate chemistry, atmospheric CO〈sub〉2〈/sub〉, and major ion seawater composition have emerged, which prompt new CO〈sub〉2〈/sub〉 system reconstructions. In addition, we have gained new insight into the effects of past major ion seawater composition on equilibrium constants affecting CO〈sub〉2〈/sub〉 system calculations — most notably due to sulfate. Here we present new reconstructions of past ocean carbonate chemistry and atmospheric CO〈sub〉2〈/sub〉 based on new data and revised calculations, including error analysis. We also provide simple corrections for past equilibrium constants, supported by experimental data and well-suited for numerical models and observational studies on multi-million year time scales. Our updated result for just the seawater carbonate ion concentration (∼2.3 to 4-fold lower 100 Myr ago) is similar to TZ04, indicating that our core approach is robust. However, all revised reconstructions using new alkenone and boron data now suggest that long-term ocean inventories of total dissolved inorganic carbon (DIC) and total alkalinity (TA) were similar to modern over the Cenozoic. This result contrasts strongly with one of TZ04’s scenarios, which featured high Paleocene-Eocene DIC/TA inventories and was based on boron-derived 〈em〉p〈/em〉H values that have recently been revised. Because the carbonate system has two degrees of freedom, consistency checks can be made when three or more parameters are determined. Overall, our estimated long-term trends in CO〈sub〉2〈/sub〉 system parameters across the Cenozoic appear consistent, regardless of whether we combine our carbonate ion concentration with alkenone-derived 〈em〉p〈/em〉CO〈sub〉2〈/sub〉 or boron-derived 〈em〉p〈/em〉H. Our results suggest convergence towards a consistent picture of Cenozoic atmospheric CO〈sub〉2〈/sub〉 and seawater chemistry. Finally, we identify changes in past seawater sulfate as a conceptual and practical problem for seawater 〈em〉p〈/em〉H reconstructions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 253〈/p〉 〈p〉Author(s): Muammar Mansor, Matthew S. Fantle〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The variation in the iron isotopic composition (δ〈sup〉56〈/sup〉Fe) of sedimentary pyrite is often interpreted to reflect the degree of Fe redox cycling in modern and ancient environments. However, the degree to which precipitation pathways, isotopic exchange, and precipitation rates can affect the isotopic fractionation associated with pyrite precipitation from aqueous Fe(II) (Fe(II)〈sub〉aq〈/sub〉) is poorly understood. In this study, pyrite is precipitated at 80 °C in batch reactors through the H〈sub〉2〈/sub〉S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe〈sub〉3〈/sub〉S〈sub〉4〈/sub〉) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ〈sup〉56〈/sup〉Fe〈sub〉pyrite〈/sub〉 - δ〈sup〉56〈/sup〉Fe〈sub〉FeSx〈/sub〉, where FeS〈sub〉x〈/sub〉 includes FeS, Fe(II)〈sub〉aq〈/sub〉, and greigite) of −0.51 ± 0.22‰ throughout the experiments irrespective of precipitation pathways and greigite formation. Early-stage precipitation is associated with ∼0.3‰ larger isotopic fractionation than late-stage precipitation, possibly indicating either a rate-dependent kinetic isotope effect (KIE) or a different isotopic fractionation factor for early-stage pyrite nucleation compared to later-stage growth. Overall, the magnitude of the apparent isotopic fractionation is significantly smaller than the 〈−2‰ isotopic fractionation determined in previous experiments (Guilbaud et al., 2011b). Numerical models indicate that isotopic exchange between pyrite and Fe(II)〈sub〉aq〈/sub〉 is necessary to explain the experimental data. The inferred rate of isotopic exchange decreases with time in our experiments, likely as a function of particle size, but shows no clear correlation with temperature across different studies. In the presence of isotopic exchange, modeling results indicate that pyrite precipitated from Fe(II)〈sub〉aq〈/sub〉 may theoretically have δ〈sup〉56〈/sup〉Fe values ranging from −3 to + 4‰, which spans nearly the whole δ〈sup〉56〈/sup〉Fe range observed in nature. Negative values reflect the expression of the KIE when isotopic exchange is slow (relative to net precipitation rate) while positive values reflect the expression of the equilibrium isotope effect (EIE) when isotopic exchange is relatively fast. We therefore propose that the variation in sedimentary pyrite δ〈sup〉56〈/sup〉Fe can be explained in terms of varying expression of the KIE and the EIE, either during different stages of precipitation or as controlled by the availability of Fe(II), sulfide, and oxidants throughout Earth’s history. The predominantly negative (but highly variable) pyrite δ〈sup〉56〈/sup〉Fe values in modern marine sediments suggest a higher expression of the KIE in low temperature systems, but do not rule out the importance of isotopic exchange. The isotopic exchange rate is currently underconstrained in low temperature systems with an uncertainty range that spans 8 orders of magnitude. Our work suggests that isotopic exchange has the potential to affect sedimentary pyrite δ〈sup〉56〈/sup〉Fe unless the current upper limit for isotopic exchange rate is overestimated by 5 orders of magnitude.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 19 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Shun Guo, Kuidong Zhao, Timm John, Pan Tang, Yi Chen, Bin Su〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Tourmaline, one of the most important hosts of boron (B) in crustal rocks, has rarely been found in natural high-pressure (HP) and ultra-HP (UHP) eclogites. Here, we report the first finding of tourmaline-bearing UHP eclogites and veins in the Dabie terrane, China. These distinctive samples occur exclusively in impure UHP marbles. Investigations on the eclogite-vein-marble system provide important insights into the origin, flow, and metasomatic effects of eclogite-facies, 〈sup〉11〈/sup〉B-rich fluids in continental subduction zones. Petrologic and geochemical evidence indicates that tourmaline in the eclogites (Tur-E) formed by metasomatism due to the infiltration of a B-rich fluid under conditions of 〈em〉ca.〈/em〉 2.2–2.6 GPa and 610–660 °C. The HP veins, containing euhedral tourmaline (Tur-V) and occurring at the eclogite-marble contacts or in the interiors of eclogite lenses, represent the crystallized products of the infiltrating fluid. Systematic compositional variations along the profile from the interiors of the eclogite lenses to their margins and mass-balance calculations indicate large inputs of B, carbon, large ion lithophile elements, and light rare earth elements to the eclogites during fluid infiltration.〈/p〉 〈p〉All types of tourmaline, including the Tur-E and Tur-V as well as minor amounts of tourmaline in the marbles (Tur-M), have similar compositions (dravitic) with X〈sub〉Mg〈/sub〉 values [= Mg/(Mg+Fe)] of 0.7–0.8. 〈em〉In situ〈/em〉 analyses using laser ablation multicollector inductively coupled plasma mass spectrometry show that all of tourmaline has high δ〈sup〉11〈/sup〉B values (ranging from +6 to +15‰), which suggest a 〈sup〉11〈/sup〉B-rich isotope signature for the infiltrating fluid. The tourmaline B isotope data, together with the detailed field observations and whole-rock Sr-Nd isotopes, reveal that the infiltrating fluid was derived from the impure marbles. Influx of such fluid was highly channelized and was mainly achieved along lithologic boundaries between UHP marbles and eclogites or along fractures in the eclogites. This study indicates that impure marbles are an important reservoir of isotopically heavy B in deeply subducted continental slabs. This point may be of particular importance because most B reservoirs in subducted continental crust typically have light B isotopic compositions. Our results highlight that subducted metacarbonate rocks could liberate eclogite-facies, B-rich, high-δ〈sup〉11〈/sup〉B fluids and thus might exert important effects on the cycle of B and its isotopes in continental subduction zones.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 14 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): V. Assis Fernandes, J. Hopp, W.H. Schwarz, J.P. Fritz, M. Trieloff, H. Povenmire〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉This study presents 〈sup〉40〈/sup〉Ar-〈sup〉39〈/sup〉Ar step heating ages of four North American tektites (three bediasites and one georgiaite) and of two groundmass samples extracted at different depths from clast-rich impact melt rocks (CB-W61 and CB-W84) recovered by the USGS-ICDP Eyreville B drill-core about 9 km from the centre of the Chesapeake Bay impact structure. Radiometric age determination on both North American tektites and impact melt rocks from within Chesapeake Bay crater offers the first possibility to confirm the origin of these tektites. For this aim, argon data from 13 samples/aliquots of tektite rims, cores and bulk, and 4 samples/aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions. Age spectra of all tektite samples show plateaux comprising 62–98% of the 〈sup〉39〈/sup〉Ar release over consecutive intermediate and high temperature heating steps. Few low temperature extractions indicate excess 〈sup〉40〈/sup〉Ar. Inverse isochron 〈sup〉40〈/sup〉Ar/〈sup〉36〈/sup〉Ar intercepts of tektite samples are indistinguishable from air (295.5). However, impact melt rock spectra presented complex Ar-release affecting primarily the low temperature heating-steps. Inverse isochrones indicate excess argon from which the 〈sup〉40〈/sup〉Ar/〈sup〉36〈/sup〉Ar intercept was used to correct the age calculation. CB-W61 and CB-W61-2 〈sup〉40〈/sup〉Ar/〈sup〉36〈/sup〉Ar intercepts are 354.5 ± 2.5 and 327.2 ± 6.3, respectively, and those for CB-W84 and CB-W84-2 are 332.0 ± 7.3 and 329.6 ± 5.6, respectively. The inverse isochron weighted mean age (according to currently suggested K-decay constants revisions by Schwarz et al. (2011) and Renne et al. (2011)) for all four tektites is 34.86 ± 0.25 Ma (MSWD = 0.96, P = 0.41; n = 4) and for the two impact melt rocks is 37.16 ± 3.65 Ma (MSWD = 0.83, P = 0.36). The combined tektite and impact melt rocks isochron mean age of 34.86 ± 0.23 (0.32) Ma (MSWD = 0.87, P = 0.43) is slightly – though not significantly – higher than the plateau mean age of 34.55 ± 0.27 (0.36) Ma (MSWD = 0.66, P = 0.62). Placing this age in the Global Stratotype Section and Point (GSSP) marine section exposed at Massignano, Italy, it falls below the Eocene/Oligocene (E/O) boundary overlapping with the 10.28 m Ir-anomaly. These results agree within errors with previously reported ages of 35.20 ± 0.54 Ma, especially those derived from K-Ar and Ar-Ar total fusion analysis. An age of 34.86 ± 0.32 Ma sets the Chesapeake Bay impact event close to the youngest of the three Ir anomalies at ∼35.0 Ma in the case the impactor was Ir-rich (e.g, a chondrite, primitive achondrite, stony-iron or iron meteorite). The concordance with the E/O boundary at ∼33.9 Ma seems only marginally possible, and only if the Ir contribution from the ejecta were, potentially, due either to its small amount becoming diluted in the geologic record or the impactor being Ir poor, e.g., of differentiated achondritic composition. This study also brings to front the need to re-establish the stratigraphic and palaeo-magnetic correlations across the globe for the Ir-anomalies and the magneto-stratigraphy during the mid- to late-Eocene and early-Oligocene, and the need to re-evaluate the markers for the Eocene-Oligocene boundary.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 13 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Wolfgang Müller, Alessia Nava, David Evans, Paola F. Rossi, Kurt W. Alt, Luca Bondioli〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mammalian dental enamel is a key archive for the reconstruction of past environments. Sequentially mineralizing enamel provides continuous, several year-long records, which spatially-resolved sampling can ‘read’ at seasonal or higher time resolution. Yet it remains underexplored how much an initially incorporated compositional signal is overprinted by the two-stage enamel mineralization process, which affects the finally achievable time-resolution. We report results of a systematic investigation into histologically-defined compositional profiles from human enamel obtained by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS). Sr/Ca, Zn/Ca, Ba/Ca and Pb/Ca are investigated as commonly-utilized proxies of (palaeo)diet, mineralization and/or pollution in modern and archaeological samples. Our modern human teeth are from unequivocally breast-fed and formula-fed individuals, respectively. By focusing on two profiles that are time-equivalent yet topographically different, namely the enamel-dentine junction (EDJ) vs. enamel prisms (P), we evaluate the compositional effect of enamel secretion vs. maturation on elemental proxies throughout enamel thickness. These two broadly orthogonal orientations are compared with the connecting Neonatal/Retzius (NNL/R) profiles that represent nominal isogrowth events during enamel secretion, which – if later maturation had no effect – should show invariant compositional signals across enamel. However, we find that all NNL/R profiles record systematically varying compositions across enamel, with Sr/Ca always decreasing by 30-80% and Zn/Ca increasing near-exponentially 20-35 times towards outer enamel. As such they can be used to benchmark the extent of enamel maturation. Since all prism profiles reveal signals similar to NNL/R for Sr, Zn and in part Ba, this unequivocally demonstrates that P-orientations are equally overprinted by maturation, in all cases with increasing intensity towards outer enamel. EDJ and P profiles do not match one-another despite representing coeval secretion time, and we infer that the highest degree of initial signal variability can be retrieved along the EDJ (approx. ≤100 μm) where maturation had the least effect. Using a simple two-component mixing model we show that during maturation only a moderate increase (1.3-1.9×) in discrimination against Sr is required to explain the Sr/Ca patterns. The Zn/Ca increase on the other hand is interpreted to reflect the Zn-binding motifs of the enzymes MMP-20 (matrix metalloproteinase-20) and KLK-4 (kallikrein-4) active during enamel secretion and maturation, which may preferentially imprint a Zn compositional signal in maturing enamel. In the case of Ba/Ca we find no systematic patterns analogous to that of Sr and Zn in modern samples. Moreover, cryptic diagenesis affecting Ba in otherwise well-preserved fossil teeth argues for caution when using Ba/Ca to infer nutritional signals without additional elemental/isotopic corroboration. Unlike all other investigated elements, Pb/Ca shows broadly similar EDJ-P profiles, and nearly invariant R-signals across enamel, which may reflect the affinity of Pb to both organic molecules and inorganic apatite during enamel mineralization. Overall our results reveal the element-specific behaviour of Sr, Zn, Ba and Pb during enamel mineralization.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 252〈/p〉 〈p〉Author(s): Lukáš Ackerman, Roman Skála, Šárka Křížová, Karel Žák, Tomáš Magna〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of 〉〉90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 13 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Jing Yang, Chi Zhang, Masaaki Miyahara, Xu Tang, Lixin Gu, Yangting Lin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The ungrouped achondrite NWA 7325 is a cumulate olivine gabbro (Irving et al., 2013). It contains abundant and unique micro-inclusions of Ca-pyroxene (Bischoff et al., 2013) and spinel-like Al-Mg oxide (Goodrich et al., 2017) in plagioclase, indicating a remelting event induced either by impact (Goodrich et al., 2017) or by magma intrusion (Bischoff et al., 2013; Weber et al., 2016). In this work, a combined FIB-TEM study has been conducted on these micro-inclusions to address their petrogenesis and the related history of NWA 7325. TEM study revealed that micro-inclusions in the interiors of large plagioclase grains are Al-enriched spinel (Mg/Al atomic ratio: 0.03–0.4) with minor needle-like α-corundum, whereas those in the margins are predominantly Al-rich diopside (En〈sub〉44.5-46.6〈/sub〉Fs〈sub〉1.2-1.5〈/sub〉Wo〈sub〉31.2-36.7〈/sub〉CaTs〈sub〉17.6-22.4〈/sub〉) with minor forsterite (Fo〈sub〉94.6-94.7〈/sub〉). The Mg/Al atomic ratios of the spinel micro-inclusions are negatively correlated with the distance away from the interface of plagioclase-pyroxene. Large plagioclase grains also exhibit a decrease in the Mg/Al atomic ratio from the rims to the cores. Based on the reaction texture at the interfaces of plagioclase-pyroxene, we infer that the Mg concentration gradient in large plagioclase grains could have resulted from Mg diffusion from the remelted rims of pyroxene into plagioclase. In addition, TEM observations showed that large plagioclase grains are not single crystals, but assemblages of submicron to micron-sized crystals. The preservation of Mg concentration gradients, submicron-sized polycrystalline plagioclases, and the consistent presence of micro-inclusions within large plagioclase grains likely indicate complete remelting of plagioclase and partial remelting of pyroxene (only rims of pyroxene with plagioclase) followed by fast cooling. We propose that micro-inclusions of diopside, forsterite, Al-rich spinel and corundum crystallized from the melt, which developed a Mg concentration gradient during the remelting of NWA 7325.〈/p〉 〈p〉The heating temperatures of pyroxene and plagioclase in the remelting event were estimated to be 1274–1327 °C and ≥1530 °C, respectively. A subsequent cooling rate of ∼500–650 °C/h at 1300 °C was found by fitting the measured Mg concentration gradient in large plagioclase grains with a Fick’s second law model that incorporated the diffusion coefficients of Mg in plagioclase-melt. These results are better explained by a shock event; a magmatic intrusion process is ruled out. To achieve the coexistence of shock-induced high temperature (≥1274 °C) 〈em〉in-situ〈/em〉 melting and only undulatory extinction of forsterite grains, an ambient temperature of 1000–1100 °C in the surrounding, parent rock of NWA 7325 was required prior to impact. This work suggests a very early shock event when NWA 7325 was hot and buried in the crust of its parental planetesimal, which is a scenario consistent with its magma crystallization age (∼4.3 Ma after CAIs, e.g., Koefoed et al., 2016). This work also implies that impacts are a potential heat source for melting hot planetesimals in the early Solar System.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 252〈/p〉 〈p〉Author(s): Kalin T. McDannell, Dale R. Issler, Paul B. O'Sullivan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Apatite fission track (AFT) analyses for granitoid and metamorphic bedrock samples from the Western Superior Province (Ontario), the Churchill-Rae Province (Melville Peninsula and Southampton Island, Nunavut), and the Slave craton region (Northwest Territories) show a broad range of single grain effective uranium concentrations (eU) (〈1 to ∼300 ppm) and some of the oldest reported AFT ages in North America. Although most of our samples are characterized by near-endmember fluorapatite composition with implied low track retentivity (〈0.1 apfu Cl, r〈sub〉mr0〈/sub〉 ∼0.85–0.82), single-grain AFT ages are statistically overdispersed and ages decrease with increasing eU content. This eU-age relationship is resonant of the Hendriks and Redfield (2005) argument for α-radiation enhanced fission track annealing (REA) and is analogous to the negative age-eU correlations observed in published zircon and titanite (U-Th)/He data from slowly-cooled cratonic rocks. In all cases, the samples fail the canonical χ〈sup〉2〈/sup〉 test (〈5%), generally considered to indicate that the ages are unlikely to be drawn from a single Poissonian distribution with a discrete mean value and may represent multiple populations. The high intra-sample age variability for low-Cl bedrock apatites with protracted histories (〉200–500 m.y.) at 〈100 °C since the Precambrian suggests strong REA control on AFT ages. Conversely, some low Cl AFT samples with a narrower eU range show less age dispersion and a weak apparent age-eU correlation. A complex trade-off between radiation damage, chemical composition (e.g. low Cl and REE enrichment), and thermal history is implied when eU and r〈sub〉mr0〈/sub〉 are positively correlated. Previous assessments of the influence of REA on AFT age were based on evaluating central age and mean track length, which potentially mask high single-grain age scatter and REA effects due to the modal nature of central age determination. REA is also supported by and compatible with materials science and nuclear waste studies of radiation damage in different apatite groups, therefore it is crucial that bedrock samples exhibiting high age scatter are evaluated in terms of intra-sample compositional heterogeneity. Samples with relatively low Cl concentrations are especially prone to greater REA effects on AFT grain ages and this underscores the need for routine acquisition of compositional data for AFT datasets. Our broad range in single-grain AFT ages (with no other clear, strong compositional controls) supports the notion that radiation damage affects both the AFT and (U-Th)/He thermochronometers in slowly-cooled settings and must be accounted for during thermal history modeling and interpretation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 18 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Özlem Ersoy, Igor K. Nikogosian, Manfred J. van Bergen, Paul R.D. Mason〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Olivine phenocrysts in basaltic rocks carry valuable mineral-chemical information on early evolution processes in mafic magmatic systems. Fast intra-crystalline diffusion and re-equilibration weakens this potential for major cation constituents of olivine, but the relative immobility of phosphorous makes this element a promising tracer of early crystallization histories. Although phosphorous (P) zoning patterns and underlying kinetic controls have been studied in recent years, little is known about compositional controls on phosphorous incorporation into igneous olivines. We have analysed olivine phenocrysts, hosting Mg-rich melt inclusions, from a range of mafic potassium-rich lavas from Quaternary volcanic centres in Italy for phosphorous and associated trace elements by laser ablation inductively coupled mass spectrometry (LA-ICP-MS), and intra-crystal zoning by electron probe micro-analysis (EPMA) using Kα X-ray elemental maps and quantitative traverses. The studied olivines are marked by low and variable phosphorous concentrations (generally ≤200 ppm, but up to 435 ppm in enriched zones). In most cases, phosphorous zoning is decoupled from zoning in any other element or forsterite content. From a comprehensive database of melt inclusions and host phenocrysts, we infer that the composition of the host melt (silica and phosphorous activities) and olivine crystallization dynamics (interplay between diffusion rate of cation constituents in the melt and crystal growth rate) largely regulate phosphorous incorporation in olivines. Melt composition is likely the most important control under near-equilibrium crystallization conditions, as apparent phosphorous partition coefficients tend to increase with increasing silica activity. A negative relationship between apparent partition coefficients and 〈em〉X〈/em〉〈sub〉PO2.5〈/sub〉 indicates that phosphorous partitioning into olivine may deviate from Henry’s law behaviour. Melt inclusions are virtually always surrounded by phosphorous-poor zones that are also depleted in Cr and enriched in Al and Ti, suggesting that supply-limited slow growth and coupled-substitution mechanisms largely govern phosphorous uptake here. Our results demonstrate the potential versatility of phosphorous as sensitive indicator of crystal-growth histories and magmatic evolution processes in mafic systems.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 14 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Jan Render, Samuel Ebert, Christoph Burkhardt, Thorsten Kleine, Gregory A. Brennecka〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Insights into the earliest stages of our Solar System can be derived from its oldest dated solids, calcium-aluminum-rich inclusions (CAIs). In particular, investigating isotopic anomalies of nucleosynthetic origin in CAIs offers potential clues to the genetic heritage of refractory inclusions and the reservoir(s) involved in their formation. To this point, however, nucleosynthetic anomalies in refractory inclusions have almost exclusively been recognized in (1) relatively large CAIs from CV3 chondrites, employing chemical purification and high-precision mass spectrometry, or (2) from sub-mm-sized hibonite-rich objects (〈em〉e.g.〈/em〉, PLACs, SHIBs) from the Murchison CM2 chondrite using much less precise 〈em〉in-situ〈/em〉 techniques. Whereas the latter have been shown to be highly anomalous in their isotopic compositions, their genetic connection to ‘regular’ CAIs from carbonaceous chondrites remains poorly understood.〈/p〉 〈p〉Here, we aim to address this issue by taking advantage of a new technique that allows for high-precision analysis of sub-mm-sized inclusions. Using this method, we report Ti isotope anomalies in a suite of twelve CAIs from five different CO carbonaceous chondrites, as well as ten refractory inclusions from the CM2 chondrite Jbilet Winselwan using MC-ICPMS. We find that these CO and CM CAIs exhibit Ti isotopic compositions very similar to those of previously investigated CV3 (and of two CK3) CAIs, suggesting a fundamental genetic relationship of CAIs found within these chondrite groups. As such, our data indicates that CAIs from various groups of carbonaceous chondrites formed from similar matter and in a single region of the solar nebula (〈em〉i.e.〈/em〉, derived from a single common CAI-formation region). Collectively, these data show evidence of large-scale transport of CAIs over a significant range of heliocentric distances, covering at least the accretion areas of the CV, CK, CO, and CM chondrites. In addition, we report two inclusions consisting of hibonite-rich crystal aggregates from Jbilet Winselwan that exhibit highly irregular nucleosynthetic Ti signatures, implying a distinct origin from the aforementioned CAIs. These inclusions may represent an earlier generation of refractory material, perhaps more akin to the previously mentioned PLACs and/or SHIBs.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Trudi Kennedy, Fred Jourdan, Ela Eroglu, Celia Mayers〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The thermal/impact histories of sixteen eucrite meteorites were investigated: three monomict eucrites (NWA 999, A-87272,87, and Stannern), five polymict eucrites (NWA 1000, NWA 1666, NWA 5601, Y-980066,100, and Y-980255,100), three quench-textured, eucrite melt rocks (Y-981646,21, Y-981651,105, and MIL 0766214), one eucrite dominantly comprised of quench-textured clasts (QUE 99005,11), three unclassified eucrite breccias (LAP 031316,9, LAR 06870,5, QUE 99799,4) and one unbrecciated eucrite (EET 92004,17), included here due to its shock features. We have measured fifteen high-precision new 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar plateau ages on plagioclase and matrix for ten of these meteorites with a tight cluster of nine ages obtained from three different polymict breccias. These ages range from 4534 ± 56 Ma to 4491 ± 16 Ma resulting in a concordant age population (〈em〉P〈/em〉 = 0.16). The fact that such a cluster of ages is recorded in unrelated breccias which are made of a priori unrelated components, leads us to propose that those ages recorded a single heating event on a large scale, and is interpreted here as a high-energy impact event, early in the history of Vesta at 4500 ± 4 Ma. We propose that the debris was ejected and isolated from subsequent large impacts in a secondary rubble pile asteroid where the energy of the outgoing shock wave from an impact is significantly reduced as it compacts the target material (Holsapple et al., 2002, and references therein) .〈/p〉 〈p〉The other analyses define a spread of five plateau ages ranging from 3851± 21 Ma to 3469 ± 35 Ma, over ∼380 Ma. An additional apparent plateau age of 4288 ± 38 Ma, but with a diffusion profile of cumulative 〈sup〉39〈/sup〉Ar release, along with published U-Pb apatite age of ∼4.14 Ga, suggests that the data might either define a true continuum (normal background bombardment) from 4.5 Ga to 3.47 Ga or cluster between ∼3.85 Ga and ∼3.47 Ga (excavation of a fresh surface at 3.85 Ga continuously bombarded until 3.47 Ga). Both scenarios are compatible with a final ejection age of 3.47 Ga when a major impact liberates the bulk of the brecciated meteorites into another secondary rubble pile asteroid, where the brecciated eucrites stayed relatively well protected from subsequent 〈em〉major〈/em〉 impacts. Based on these results and recent crater counting measurements, we propose that the excavation and bulk ejection were caused by the Rheasilvia (ca. 3.47 Ga) basin-forming impact.〈/p〉 〈p〉Diffusion models on plagioclase crystals with different Ar age spectrum signatures, from a single breccia (NWA1666; 4501 ± 7 Ma), suggest that either: (1) the formation of the breccia is very young, or (2) different plagioclase crystals have different diffusion characteristics, and/or (3) the porosity caused heterogeneous temperatures during an impact heating event, particularly likely if the 4.5 Ga brecciated eucrites were stored in a rubble pile asteroid. Many or possibly most large asteroids being re-accumulated rubble piles with potential large porosity (Holsapple et al., 2002). Scenarios (2) and (3) preclude the usage of multi-grain aliquots to decipher the time-temperature history of most impact breccia using 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar thermochronology.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 17 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): M. Telus, C.M.O'D. Alexander, E.H. Hauri, J. Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To constrain the conditions of aqueous alteration in early planetesimals, we carried out 〈em〉in situ〈/em〉 C and O isotope analyses of calcite and dolomite and O isotope analyses of magnetite from the highly altered CM chondrites ALH 83100, ALH 84034, and MET 01070. Petrographic and isotopic analyses of these samples support previous findings of multiple generations of carbonate growth. We observe wide ranges in the C and O isotope compositions of carbonates of up to 80‰ and 30‰, respectively, that span the full range of previously reported bulk carbonate values for CM chondrites. Variations in the Δ〈sup〉17〈/sup〉O values indicate that fluid evolution varied for each chondrite. ALH 83100 dolomite-magnetite δ〈sup〉18〈/sup〉O fractionation of 23‰ ± 7‰ (2SD) corresponds to dolomite formation temperature of 125°C ± 60°C. δ〈sup〉13〈/sup〉C vs δ〈sup〉18〈/sup〉O values fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. The positive correlation between δ〈sup〉13〈/sup〉C and δ〈sup〉18〈/sup〉O for primary calcite is consistent with the precipitation of calcite in equilibrium with a gas mixture of CO (or CH〈sub〉4〈/sub〉) and CO〈sub〉2〈/sub〉. The isotopic composition of calcite in CM1s and CM2s overlap significantly; however, many CM1 calcite grains are more depleted in δ〈sup〉18〈/sup〉O compared to CM2s. Altogether, the data indicate that the fluid composition during calcite formation was initially the same for both CM1s and CM2s. CM1s experienced more episodes of carbonate dissolution and reprecipitation where some fraction of the carbonate grains survive each episode resulting in a highly disequilibrium assemblage of carbonates on the thin-section scale.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 5 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Hailiang Dong, Xiancai Lu, Evert Elzinga〈/p〉

Description: 〈p〉Publication date: Available online 5 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Stephen E. Haggerty〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The origin of micro-diamonds is controversial and although the application to determine the grade and value of macro-diamonds in kimberlite/lamproite bodies continues to receive widespread usage there are several outstanding factors generally not considered, the most important of which is genesis. The issue is addressed in this study in the context that two classes of small diamonds (generally 〈0.5 mm and rarely 〈1 mm) are recognized. Micro-diamonds 〈em〉sensu-stricto〈/em〉 (MDS) are typically sharp-edged octahedra, free of mineral inclusions and surface etching or corrosion, increase exponentially with decreasing size and are in overwhelming larger concentrations, by orders of magnitude, relative to macro-diamonds (〉0.5 mm). The second class of small diamonds (SD 〈0.5 mm), used in industrial applications, may have modified solution-growth morphologies (e.g. dodecahedra, tetrahexahedra and related forms), and include loosely bonded polycrystalline diamonds (framesite), boart, fibrous cubes and broken fragments. There are large differences in volume to surface-area ratios between MDS and SD, demonstrating unequivocally that pristine and solution-modified forms could not have co-existed in equilibrium under the same P-T-t-〈em〉f〈/em〉O〈sub〉2〈/sub〉 conditions in the mantle. From detailed studies of N and C in diamond, and experimental results on the redox-partitioning of N in the presence of metallic Fe, it is concluded that MDS are plume-related from the D″ core-mantle boundary, and are melt-derived in lower mantle proto-kimberlite. The lower mantle is expectedly saturated in metallic Fe, and is highly depleted in N which is siderophile under very low 〈em〉f〈/em〉 O〈sub〉2〈/sub〉 conditions, a setting in which excessively large (∼100 to 3000 ct), but rare Type II mega-diamonds (but also MDS) are inferred to have originated. These diamonds (Type II, Ib, IaA) are distinct from the majority of N-rich Type Ia upper mantle macro-diamonds that grew slowly by metasomatic processes and annealed over long periods. Two crystal growth laws are possibly applicable to the size-distribution of diamonds encountered in kimberlites/lamproites. Gibrat’s Law of proportionate, short-term crystal growth in open systems by advection is applicable to magmatic MDS, whereas macro-diamonds bear some relation to McCabe’s Law of long-term, relatively constant crystal growth, by diffusion metasomatism. The range from small to large diamonds (SFD size-frequency-distribution) is lognormal but is composed of two segments: the smaller size (〈0.5 mm) fraction has an overall linear distribution, whereas macro-diamonds (〉0.5 mm) are quadratic. The two distributions meet or overlap in a marked discontinuity, implying but not proving distinct origins. The power law governing SFD lognormal distributions is fundamental and is widespread across an enormous number of disciplines (from biology to economics), and may be universal (e.g. it is applicable to planetary scale meteorite impact craters, and to the SFD of cosmic-diamonds from supernovae explosions). Industry applications in resource predictions are from mixtures of diamonds (MDS and SD), and extrapolation to larger stones is valid because the fundamental law is independent of origins.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 254〈/p〉 〈p〉Author(s): Narottam Saha, Gregory E. Webb, Jian-Xin Zhao, Ai Duc Nguyen, Stephen E. Lewis, Janice M. Lough〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Modern water quality gradients on the inner-shelf of the Great Barrier Reef (GBR) are influenced by catchment modification and increased terrigenous suspended sediment delivery. Proxy-based reconstructions of terrestrial sediment discharge are critical to trace the environmental drivers that modulate inshore water quality and to quantify the magnitude and timing of major changes. In this study we present high-resolution 〈em〉Porites〈/em〉 coral rare earth element (REE) data over the period 1987–2012 from inshore Magnetic Island, GBR. Shale-normalized REE distributions in coral samples effectively captured the major features of seawater. Terrigenous sediment derived characteristics of shale-normalised REE and Y (e.g., higher ΣREE, reduced LREE depletion, and lower Y/Ho ratios) show linear relationships with each other on annual to multi-annual timescales thus providing a specific fingerprint for terrestrially derived turbidity. Temporal variation of the proxies generally shows spikes associated with increased water turbidity during strong summer floods. However, anomalous spikes coinciding with negligible river discharge during dry periods were also observed and highlight that data from non-flood year summers and winters show less predictable relationships possibly related to wind driven resuspension events, channel dredging and other as yet undetermined factors. Regardless, temporal variability of the proxies show synchronicity with tested environmental drivers (e.g., river discharge and rainfall) on multi-annual timescales with greater terrigenous influence during wet periods from the late-1980s to early-1990s, late-1990s to early 2000s and late 2000s to early-2010s, consistent with increased delivery of terrestrial sediment at those times. Cerium (Ce) anomalies showed complex behaviour during lower discharge seasons but have enhanced negative anomalies during strong flood-year summers when large flood plumes enriched in terrigenous nutrients were likely to reach waters around Magnetic Island. We speculate that higher abundance of Ce-oxidizing bacteria during strong flood events modulate local nearshore marine Ce anomalies. Based on these findings, we demonstrate that robust, time-resolved shale-normalized REE distributions in coral skeletons, not just elemental concentrations, provide very useful proxies for monitoring terrigenous sediment discharge related changes in inshore water quality and for tracing coastal biological activities, particularly at annual and multi-annual timescales.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 1 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 254〈/p〉 〈p〉Author(s): Jonathan M. Tucker, Erik H. Hauri, Aaron J. Pietruszka, Michael O. Garcia, Jared P. Marske, Frank A. Trusdell〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The deep mantle carbon content and flux are fundamental quantities in understanding global volatile cycles and distributions. Here, we present CO〈sub〉2〈/sub〉 concentrations measured in 407 olivine-hosted melt inclusions from Hualalai, Kilauea, Koolau, Loihi, and Mauna Loa to constrain the Hawaiian mantle CO〈sub〉2〈/sub〉 content and flux. Quantification of melt inclusion CO〈sub〉2〈/sub〉 is complicated by the ubiquitous presence of vapor or “shrinkage” bubbles. The contribution from exsolved shrinkage bubble CO〈sub〉2〈/sub〉 was determined from the measured bubble size and a CO〈sub〉2〈/sub〉 equation of state, and added to the dissolved CO〈sub〉2〈/sub〉 to reconstruct total melt inclusion CO〈sub〉2〈/sub〉 concentrations. Bubbles typically contain ∼90% of melt inclusion C, much of which may be sequestered in precipitated phases on bubble walls, and thus not amenable to measurement by Raman spectroscopy. Based on our dataset of total (dissolved + bubble) CO〈sub〉2〈/sub〉 concentrations, we estimate that parental melts from the five Hawaiian volcanoes have CO〈sub〉2〈/sub〉 concentrations ranging from 3900 to 10,000 ppm CO〈sub〉2〈/sub〉. Among the active volcanoes, CO〈sub〉2〈/sub〉 concentrations decrease to the northwest, likely reflecting mantle source heterogeneity, although differences in CO〈sub〉2〈/sub〉 degassing related to the relative depths of the magma chambers may also play a role. Mantle sources of the Hawaiian volcanoes range from 380 to 480 ppm CO〈sub〉2〈/sub〉 suggesting that the Hawaiian plume is at least a factor of ∼4 more C-rich than the upper mantle sampled by mid-ocean ridge basalts. This enrichment is likely due to the presence of recycled surficial C and/or C-rich primitive material in the Hawaiian mantle.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 6 April 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Ming Tang, Cin-Ty A. Lee, Roberta L. Rudnick, Kent C. Condie〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere (Jordan, 1988, Boyd, 1989; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb– and MnO/FeO〈sub〉T〈/sub〉–SiO〈sub〉2〈/sub〉 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60–70 km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10〈sup〉−13〈/sup〉–10〈sup〉−12〈/sup〉 s〈sup〉−1〈/sup〉, requiring mantle flow rates associated with late Archean crust formation to be 10–100 times faster than today.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 25 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): J.D. Toner, D.C. Catling〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Cyanide plays a critical role in origin of life hypotheses that have received strong experimental support from cyanide-driven synthesis of amino acids, nucleotides, and lipids. However, relatively high cyanide concentrations are needed. Such cyanide could have been supplied by reaction networks in which hydrogen cyanide in the early Earth’s atmosphere reacted with iron to form ferrocyanide salts, followed by thermal decomposition of ferrocyanide salts to cyanide. Using an aqueous model supported by new experimental data, we show that sodium ferrocyanide salts precipitate in closed-basin, alkaline lakes over a wide range of plausible early Earth conditions. Such lakes were likely common on the early Earth because of chemical weathering of mafic or ultramafic rocks and evaporative concentration. Subsequent thermal decomposition of sedimentary sodium ferrocyanide yields sodium cyanide (NaCN), which dissolves in water to form NaCN-rich solutions. Thus, geochemical considerations newly identify a particular geological setting and NaCN feedstock nucleophile for prebiotic chemistry.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 24 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Chenghuan Guo, Youxue Zhang〈/p〉

Description: 〈p〉Publication date: Available online 22 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Zhuqing Xue, Long Xiao, Clive R. Neal, Yigang Xu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉We conducted a thorough analysis of the feldspathic breccia meteorite Dhofar 1428 with the aim of better understanding the composition and evolution of lunar crust. This sample comprises a heterogeneous array of lithic fragments including magnesian and ferroan anorthositic granulites, mafic granulites/granulitic breccia, basalts, and different kinds of impact melt rocks. In which, a high-Ti basalt clast comprising large zoned pyroxene was observed. Based on equilibrium melt calculations of mineral zonations from this basalt, Mg-pyroxene cores were interpreted to be formed from a light rare earth element (LREE) enriched liquid, whereas the Fe-pyroxene rims grew from an LREE-depleted magma. We propose that LREE-depleted signature of Fe-pyroxene results from co-crystallization with apatite. The Mg-pyroxenes suggest that enriched liquids with higher REE contents and different REE patterns relative to KREEP existed within lunar interior. Oscillating Ti/Al ratios across pyroxene in this basalt may indicate several magma recharge events or crystal movement within a zoned magma chamber. This feature illustrates that magmas were derived from a variety of sources around the time of formation of this basalt. In situ U-Pb dating was conducted on apatite grains within this basalt, the excellent consistence between the U-Pb Concordia age (3941±24 Ma, 2σ) and 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb isochron age (3934±24Ma, 2σ) indicates the most likely crystallization age of this high-Ti basalt at ∼3940 Myr, making it one of the oldest high-Ti basalts formed on the Moon.〈/p〉 〈p〉Magnesian anorthositic granulites are mineralogically and geochemically similar to those trace element-poor magnesian anorthositic granulites in many lunar meteorites. These magnesian granulites cannot form from simple mixing of pristine Ferroan Anorthosite and lithologies from the Mg-Suite, and do not have any affinities with KREEP or the Procellarum KREEP Terrane, and they could be important components of farside highlands.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 22 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Paolo A. Sossi, Stephan Klemme, Hugh St.C. O'Neill, Jasper Berndt, Frédéric Moynier〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Moderately volatile elements (MVEs) are sensitive tracers of vaporisation in geological and cosmochemical processes owing to their balanced partitioning between vapour and condensed phases. Differences in their volatilities allows the thermodynamic conditions, particularly temperature and oxygen fugacity (〈em〉f〈/em〉O〈sub〉2〈/sub〉), at which vaporisation occurred to be quantified. However, this exercise is hindered by a lack of experimental data relevant to the evaporation of MVEs from silicate melts. We report a series of experiments in which silicate liquids are evaporated in one-atmosphere (1-atm) gas-mixing furnaces under controlled 〈em〉f〈/em〉O〈sub〉2〈/sub〉s, from the Fe-“FeO” buffer (iron-wüstite, IW) to air (10〈sup〉-0.68〈/sup〉 bars), bracketing the range of most magmatic rocks. Time- (〈em〉t〈/em〉) and temperature (〈em〉T)〈/em〉 series were conducted from 15 to 930 minutes and 1300-1550°C, at or above the liquidus for a synthetic ferrobasalt, to which 20 elements, each at 1000 ppm, were added. Refractory elements (〈em〉e.g.〈/em〉, Ca, Sc, V, Zr, REE) are quantitatively retained in the melt under all conditions. The MVEs show highly redox-dependent volatilities, where the extent of element loss as a function of 〈em〉f〈/em〉O〈sub〉2〈/sub〉 depends on the stoichiometry of the evaporation reaction(s), each of which has the general form M〈sup〉x+〈/sup〉〈em〉〈sup〉n〈/sup〉〈/em〉O〈sub〉(x+〈/sub〉〈em〉〈sub〉n〈/sub〉〈/em〉〈sub〉)/2〈/sub〉 = M〈sup〉x〈/sup〉O〈sub〉x/2〈/sub〉 + 〈em〉n〈/em〉/4O〈sub〉2〈/sub〉. Where 〈em〉n〈/em〉 is positive (as in most cases), the oxidation state of the element in the gas is more reduced than in the liquid, meaning lower oxygen fugacity promotes evaporation. We develop a general framework, by integrating element vaporisation stoichiometries with Hertz-Knudsen-Langmuir (HKL) theory, to quantify evaporative loss as a function of 〈em〉t〈/em〉, 〈em〉T〈/em〉 and 〈em〉f〈/em〉O〈sub〉2〈/sub〉. Element volatilities from silicate melts differ from those during solar nebular condensation, and can thus constrain the conditions of volatile loss in post-nebular processes. Evaporation in a single event strongly discriminates between MVEs, producing a step-like abundance pattern in the residuum, similar to that observed in the Moon or Vesta. Contrastingly, the gradual depletion of MVEs according to their volatility in the Earth is inconsistent with their loss in a single evaporation event, and instead likely reflects accretion from many smaller bodies that had each experienced different degrees of volatilisation.〈/p〉〈/div〉 〈/div〉