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: 15 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 245〈/p〉 〈p〉Author(s): Melissa J. Murphy, Don Porcelli, Philip A.E. Pogge von Strandmann, Catherine A. Hirst, Liselott Kutscher, Joachim A. Katchinoff, Carl-Magnus Mörth, Trofim Maximov, Per S. Andersson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Increasing global temperatures are causing widespread changes in the Arctic, including permafrost thawing and altered freshwater inputs and trace metal and carbon fluxes into the ocean and atmosphere. Changes in the permafrost active layer thickness can affect subsurface water flow-paths and water-rock interaction times, and hence weathering processes. Riverine lithium isotope ratios (reported as δ〈sup〉7〈/sup〉Li) are tracers of silicate weathering that are unaffected by biological uptake, redox, carbonate weathering and primary lithology. Here we use Li isotopes to examine silicate weathering processes in one of the largest Russian Arctic rivers: the Lena River in eastern Siberia. The Lena River watershed is a large multi-lithological catchment, underlain by continuous permafrost. An extensive dataset of dissolved Li isotopic compositions of waters from the Lena River main channel, two main tributaries (the Aldan and Viliui Rivers) and a range of smaller sub-tributaries are presented from the post-spring flood/early-summer period at the onset of active layer development and enhanced water-rock interactions. The Lena River main channel (average δ〈sup〉7〈/sup〉Li〈sub〉diss〈/sub〉 ∼ 19‰) has a slightly lower isotopic composition than the mean global average of 23‰ (Huh et al., 1998a). The greatest range of [Li] and δ〈sup〉7〈/sup〉Li〈sub〉diss〈/sub〉 are observed in catchments draining the south-facing slopes of the Verkhoyansk Mountain Range. South-facing slopes in high-latitude, permafrost-dominated regions are typically characterised by increased summer insolation and higher daytime temperatures relative to other slope aspects. The increased solar radiation on south-facing catchments promotes repeated freeze-thaw cycles, and contributes to more rapid melting of snow cover, warmer soils, and increased active layer thaw depths. The greater variability in δ〈sup〉7〈/sup〉Li and [Li] in the south-facing rivers likely reflect the greater infiltration of melt water and enhanced water-rock interactions within the active layer.〈/p〉 〈p〉A similar magnitude of isotopic fractionation is observed between the low-lying regions of the Central Siberian Plateau (and catchments draining into the Viliui River), and catchments draining the Verkhoyansk Mountain Range into the Aldan River. This is in contrast to global rivers in non-permafrost terrains that drain high elevations or areas of rapid uplift, where high degrees of physical erosion promote dissolution of freshly exposed primary rock typically yielding low δ〈sup〉7〈/sup〉Li〈sub〉diss〈/sub〉, and low-lying regions exhibit high riverine δ〈sup〉7〈/sup〉Li values resulting from greater water-rock interaction and formation of secondary mineral that fractionates Li isotopes. Overall, the range of Li concentrations and δ〈sup〉7〈/sup〉Li〈sub〉diss〈/sub〉 observed within the Lena River catchment are comparable to global rivers located in temperate and tropical regions. This suggests that cryogenic weathering features specific to permafrost regions (such as the continual exposure of fresh primary minerals due to seasonal freeze-thaw cycles, frost shattering and salt weathering), and climate (temperature and runoff), are not a dominant control on δ〈sup〉7〈/sup〉Li variations. Despite vastly different climatic and weathering regimes, the same range of riverine δ〈sup〉7〈/sup〉Li values globally suggests that the same processes govern Li geochemistry – that is, the balance between primary silicate mineral dissolution and the formation (or exchange with) secondary minerals. This has implications for the use of δ〈sup〉7〈/sup〉Li as a palaeo-weathering tracer for interpreting changes in past weathering regimes.〈/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): Weifu Guo, Chen Zhou〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Triple oxygen isotope composition of carbonate minerals reflects the isotope composition of the water from which carbonates precipitate, and is emerging as a promising proxy for constraining past changes in Earth hydroclimate and surface environment. However, quantitative interpretation of this proxy is not straightforward when carbonate minerals do not form under isotope equilibrium with water. For these carbonates, isotope composition of dissolved inorganic carbon (DIC) in the precipitating solution usually exerts the dominant control on their isotope composition. Here we examine the systematics of triple oxygen isotope fractionation in the DIC-H〈sub〉2〈/sub〉O-CO〈sub〉2〈/sub〉 system by deriving the fundamental equilibrium and kinetic triple oxygen isotope fractionation factors in this system and simulating the evolution of triple oxygen isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H〈sub〉2〈/sub〉O isotope exchange, CO〈sub〉2〈/sub〉 degassing and CO〈sub〉2〈/sub〉 absorption.〈/p〉 〈p〉We show that under thermodynamic equilibrium dissolved HCO〈sub〉3〈/sub〉〈sup〉–〈/sup〉 and CO〈sub〉3〈/sub〉〈sup〉2–〈/sup〉 both exhibit similar 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi mathvariant="normal"〉Δ〈/mi〉〈/mrow〉〈mrow〉〈mo〉′〈/mo〉〈mn〉17〈/mn〉〈/mrow〉〈/msup〉〈mtext〉O〈/mtext〉〈/mrow〉〈/math〉 as calcite and aragonite (within 10 per meg at 25 °C), with temperature dependences around 0.61 per meg/°C. However, kinetic isotope fractionations associated with CO〈sub〉2〈/sub〉 hydration and hydroxylation and their reverse reactions can produce a range of disequilibrium triple oxygen isotope effects in DIC during all three processes we simulated. The magnitudes of these disequilibrium effects vary with both time and the physicochemical conditions of the solution, e.g., temperature, pH and initial composition. Particularly, we predict correlated enrichments in 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi mathvariant="normal"〉Δ〈/mi〉〈/mrow〉〈mrow〉〈mo〉′〈/mo〉〈mn〉17〈/mn〉〈/mrow〉〈/msup〉〈mtext〉O〈/mtext〉〈/mrow〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.gif" overflow="scroll"〉〈mrow〉〈msup〉〈mrow〉〈mi〉δ〈/mi〉〈/mrow〉〈mrow〉〈mn〉18〈/mn〉〈/mrow〉〈/msup〉〈mtext〉O〈/mtext〉〈/mrow〉〈/math〉 of DIC during CO〈sub〉2〈/sub〉 degassing but depletions during CO〈sub〉2〈/sub〉 absorption. The slopes of these correlations vary mainly as a function of solution pH but not temperature, DIC concentration or air pCO〈sub〉2〈/sub〉, yielding values of 8.8 and 12.0 per meg/‰ at pH = 8 and 9, respectively, at 25 °C. Such disequilibrium isotope effects in DIC are expected to be inherited by carbonate minerals that form from these solutions (e.g., speleothem, coral skeleton, and high pH travertine) and, if not accounted for, could lead to inaccurate estimates of the triple oxygen isotope compositions of the parent water and carbonate formation temperatures. Our numerical model provides a quantitative framework for interpreting triple oxygen isotope composition of DIC and for correcting disequilibrium triple oxygen isotope effects in natural carbonates.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 245〈/p〉 〈p〉Author(s): Gavin G. Kenny, Martin Schmieder, Martin J. Whitehouse, Alexander A. Nemchin, Luiz F.G. Morales, Elmar Buchner, Jeremy J. Bellucci, Joshua F. Snape〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Accurate and precise dating of terrestrial impact craters is a critical requirement for correlating impacts with events such as mass extinctions. A number of isotopic systems have been applied to impact chronology but it is important to understand what an age actually represents and, thus, if it accurately represents the ‘true’ impact age and is suitable for use in correlation. Here we report imaging, microstructural characterisation and high spatial resolution ion microprobe U-Pb analysis of shocked zircon from the approximately 23 km-in-diameter Lappajärvi impact structure, Finland, for which a well-established 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar framework exists. Microstructural analysis identified two distinct styles of shock recrystallisation: (i) granular zircon that displays multiple domains of similarly oriented neoblasts, some of which are interpreted to be the product of reversion from the high-pressure ZrSiO〈sub〉4〈/sub〉 polymorph, reidite, and (ii) granular zircon composed entirely of similarly oriented neoblasts. Only the former gave concordant U-Pb data interpreted to record the age of the impact. The U-Pb ‘concordia age’ reported here, 77.85 ± 0.78 Ma (1.0%; MSWD = 0.60; probability = 0.87; 〈em〉n〈/em〉 = 8; 2σ; full external uncertainty), is resolvable from the previously published ‘best estimate’ 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar age for impact melt rock (76.20 ± 0.29 Ma) and 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar K-feldspar ages as young as 75.11 ± 0.36 Ma, and is therefore interpreted to more accurately reflect the age of the impact event. The resolvable disparity between the zircon U-Pb and the 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar data indicates that even the oldest statistically robust 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar ages obtained at medium- and large-sized impact craters may not accurately record the timing of an impact event at a kyr level. The offset between the U-Pb and 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar data is interpreted to be, at least in part, a result of the zircon data recording a higher isotopic closure temperature, and the younger 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar ages recording the progressive cooling of different domains of the impact structure. The Lappajärvi impact structure is the first Phanerozoic impact structure dated by U-Pb analysis of shock-recrystallised zircon to better than, or equal to, 1.0% uncertainty. This further demonstrates that well-characterised granular zircon grains are likely to have wide utility in the accurate and precise dating of terrestrial impact events.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 245〈/p〉 〈p〉Author(s): Chuanyu Gao, Michael Sander, Svenja Agethen, Klaus-Holger Knorr〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Peatlands are a major source of atmospheric CH〈sub〉4〈/sub〉. The availability of terminal electron acceptors largely affects the ratio of CO〈sub〉2〈/sub〉 to CH〈sub〉4〈/sub〉 formation under water-logged anoxic conditions in these systems. Although the importance of peat organic matter as an electron acceptor is increasingly recognized, the actual budgets of electron accepting capacities of dissolved and particulate organic matter remain poorly characterized. To address this research need, we incubated three different peat materials and linked changes in the electron accepting capacities (EAC) of peat organic matter (OM), including dissolved and particulate organic matter, to the observed CO〈sub〉2〈/sub〉 and CH〈sub〉4〈/sub〉 formation. Under anaerobic conditions, EAC〈sub〉OM〈/sub〉 decreased inverse to non-methanogenic CO〈sub〉2〈/sub〉 formation. Only after utilizable EAC〈sub〉OM〈/sub〉 was depleted did strictly methanogenic conditions evolve with equimolar CH〈sub〉4〈/sub〉 and CO〈sub〉2〈/sub〉 formation rates, as theoretically expected. The reduction of OM and the resultant decrease in EAC〈sub〉OM〈/sub〉 explained between 26 and 56% of the non-methanogenic CO〈sub〉2〈/sub〉, which was between 5 and 39% of total CO〈sub〉2〈/sub〉 produced. Compared to EAC〈sub〉POM〈/sub〉, EAC〈sub〉DOM〈/sub〉 remained constant and may have served as a mediator in electron transfer to the POM. In summary, our study quantitatively demonstrated the important role of peat OM as terminal electron acceptor for anaerobic respiration in organic soils.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 15 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 245〈/p〉 〈p〉Author(s): Xiaoyi Guo, Bochao Xu, William C. Burnett, Zhigang Yu, Shouye Yang, Xiangtong Huang, Feifei Wang, Haiming Nan, Peng Yao, Fenglin Sun〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We have examined the possible use of Mn/Ca ratios in benthic foraminifera as a proxy for hypoxia, which has increasingly become a serious environmental issue in many coastal zones. We collected samples from the Yangtze River Estuary, one of the largest and most intense seasonally hypoxic zones in the world. In order to obtain high quality Mn/Ca data, unaffected by contamination, we examined two different cleaning protocols and instrument-based analysis methods. Our results showed that a relatively simple physical cleaning approach coupled to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provided excellent results on single specimens. Our observations were comparable to ICP analyses of solutions made up from dozens or even hundreds of specimens and involving complicated sample pretreatments. Using the LA methodology, we analyzed Mn/Ca ratios from living 〈em〉Florilus decorus〈/em〉 (〈em〉F. decorus〈/em〉) specimens from stations in the Yangtze River Estuary with varying bottom water dissolved oxygen (DO) concentrations. Our results showed that Mn/Ca ratios in penultimate chambers of living benthic foraminifera are sensitive to bottom water DO concentrations. We also observed significant variations of both Mn/Ca and Mg/Ca ratios from different chambers within foraminiferal shell specimens. These fluctuations were likely a response to changing ambient water DO and temperature during the specimen’s growth history. Combined use of foraminiferal Mn/Ca ratios together with other proxies could provide a powerful tool for historical reconstruction of low oxygen conditions in seasonal hypoxia areas.〈/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): Jörg Prietzel, Isabel Prater, Luis Carlos Colocho Hurtarte, Filip Hrbáček, Wantana Klysubun, Carsten W. Mueller〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Phosphorus (P) and sulfur (S) are essential elements for life, and peculiarities of Antarctic soils regarding their S and P status likely affect terrestrial and aquatic ecosystems in Antarctica. Current climate warming is accompanied by deglaciation, increased availability of water, and vegetation establishment in many regions of Antarctica. The effect of these changes on soil P and S availability, ecosystem succession, mineral weathering, and important processes of soil formation and geochemistry (〈em〉e.g.〈/em〉 organic carbon sequestration, formation of secondary minerals) are largely unknown. Detailed knowledge on different P and S pools in soils of Antarctica is required for sound predictions of climate warming effects on important (bio)geochemical processes. We quantified the P and S concentration and speciation for various Antarctic soils using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy and related the soil P and S status to important soil and site properties. The soils, which have mostly remained at an initial stage of pedogenesis, differed considerably with respect to their P and S status. The P stock of most soils was dominated by Ca-bound (apatite) P (60–70% of total P). Some soils showed a marked accumulation of either Al-bound P (up to 60% of total P), Fe-bound P (up to 57%), Na- and K-bound P (up to 37%), or organic P (up to 54%). The S speciation also differed considerably among soils. Similar to other terrestrial environments, we observed a transformation of lithogenic apatite P and sulfide S into Al- and Fe-bound phosphate and sulfate as well as organic S and P. The transformation proceeded much more slowly than in soils with temperate humid climate, reflecting the cold and arid climate as well as the absence of vascular plants. The different P and S status of the soils could be related to the following site factors: (i) 〈strong〉Parent material〈/strong〉 – soils formed from basalt had larger contents of total P as well as Al- and Fe-bound P compared to soils formed from mixture of basalt and sandstone. (ii) 〈strong〉Sea spray influence〈/strong〉 – An upland soil affected by eolian deposition of sea spray showed topsoil enrichment of alkali-bound P, Ca-bound P, and CaSO〈sub〉4〈/sub〉. (iii) 〈strong〉Inundation by seawater〈/strong〉 – Soils inundated by seawater had higher total P and S concentrations and higher concentrations of inorganic sulfide than upland soils. (iv) 〈strong〉Soil moisture〈/strong〉 – Soils with increased soil moisture or wet conditions showed increased concentrations of P bound to pedogenic Al and Fe minerals and smaller contributions of Ca-bound (apatite) P to total P than soils under permanently arid conditions. (v) 〈strong〉Colonization by mosses〈/strong〉 – Moss establishment resulted in topsoil P depletion and transformation of apatite P into Al- and Fe-bound P, but also in topsoil S accumulation and conversion of sulfide S into organic S forms. According to our study, the ongoing climate warming in Antarctica at many places will likely change soil P and S speciation as well as P and S availability to terrestrial and aquatic ecosystems, thus altering key biogeochemical processes like mineral weathering, secondary mineral formation, and soil C sequestration.〈/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): Haiyang Liu, He Sun, Yilin Xiao, Yangyang Wang, Lingsen Zeng, Wangye Li, Haihao Guo, Huimin Yu, Andreas Pack〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Sumdo eclogite, located in the central Lhasa Terrane, Tibet, has received much attention as it has important implications for the tectonic evolution of the Tibet orogenic belt. Aiming to constrain the Lithium (Li) isotope characteristics of rocks in subduction zones, we investigated whole-rock major and trace elements, Li isotopes as well as oxygen (O) and radiogenic (Sr, Nd, and Pb) isotopes on 20 Sumdo eclogite samples. The investigated samples display REE patterns similar to N-MORB, with relatively homogeneous bulk εNd〈sub〉(t=260Ma)〈/sub〉 values ranging from +4.0 to +7.8. These data suggest that protoliths of the Sumdo eclogite were N-MORB-type oceanic basalts. Yet, initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr and δ〈sup〉18〈/sup〉O are heterogeneous and range from 0.703703 to 0.706645 and from +4.9 to +8.9‰, respectively. A positive correlation between Sr and oxygen isotopic compositions suggests that the MORB protoliths had experienced low temperature seafloor alteration prior to subduction. The Li contents and δ〈sup〉7〈/sup〉Li values of Sumdo eclogite vary from 3.64 to 29.7 µg/g and −1.1 to +4.0‰, respectively, significantly lower than those of low T altered MORBs (AOC) that usually have high Li contents (up to 33 µg/g) and heavy Li isotopic compositions (δ〈sup〉7〈/sup〉Li up to +14‰). Modelling results suggests limited Li isotopic fractionation during dehydration. After eliminating the effect of diffusion, we suggest that a rehydration stage during exhumation may be responsible for the Li isotope systematics of the Sumdo eclogite. The fluids (〈5%) derived from continental-like sediments are the candidates for adding light Li isotopes to the residual slab. Our results indicate that Li may be redistributed within the slab among various lithologies, such that mantle heterogeneities formed from recycled materials may show a range of Li-Sr-Nd isotopic compositions that cannot be directly related to a specific protolith, such as sediments or altered oceanic crust.〈/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: 15 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta, Volume 247〈/p〉 〈p〉Author(s): Lin Ma, Anthony Dosseto, Jerome Gaillardet, Peter B. Sak, Susan Brantley〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To quantify chemical weathering processes, it is essential to develop and utilize new geochemical tools that can provide information about chemical weathering in the field. U-series isotopes have emerged as a useful chronometer to directly constrain the rates and duration of chemical weathering. However, the conventional solution-based MC-ICPMS method involves a long and expensive sample processing procedure that restricts the numbers of measurements of samples by U-series analysis that can be completed. Here, we report 〈em〉in situ〈/em〉 measurements of U-series disequilibria obtained with laser ablation (LA)-MC-ICPMS on weathering rinds collected from the tropical island of Basse-Terre in the archipelago of French Guadeloupe. We characterized two weathering rinds for U-series isotope compositions and elemental distributions with LA-MC-ICPMS and LA-Q-ICPMS. The 〈em〉in situ〈/em〉 measurements of U-series disequilibria were consistent with the previous bulk measurements obtained by conventional solution MC-ICPMS despite the larger analytical uncertainties. The LA technique allowed a greater number of measurements that accelerated sample throughput and improved spatial resolution of measurement. The rind formation age, weathering rates, and U-series mobility parameters modeled in this study are comparable to the results from previous studies conducted on the same clasts, and also reveal new insights on rind formation such as the impact of micro-fractures on weathering history and U-series ratios. The improved spatial resolution available with LA Q-ICPMS helps distinguish between linear and power law rind thickness-age relationships that were unresolvable using conventional solution-based MC-ICPMS. 〈em〉In situ〈/em〉 measurements with LA-Q-ICPMS in these weathering rinds also elucidates the sequences of mineral reactions during chemical weathering. The LA-Q-ICPMS maps of major and trace elements and elemental ratios reveal details about the rind formation processes at the weathering interfaces of clasts such as dissolution of primary phases, formation of new phases, development of porosity, and mobility behavior of U. This study demonstrates a new analytical method for determining weathering rates in rinds rapidly and accurately that can be used in a large number of rinds, providing key information at the clast scale.〈/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): Gisella Palma, Fernando Barra, Martin Reich, Victor Valencia, Adam C. Simon, Jeff Vervoort, Mathieu Leisen, Rurik Romero〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The ratio of the halogens F and Cl in apatite is highly sensitive to changes in the composition of an evolving silicate melt or aqueous fluid. For this reason apatite chemistry is widely used as a monitor of halogen behavior in magmatic-hydrothermal systems. Apatite is an ubiquitous mineral in iron oxide – apatite (IOA) mineral deposits, where P and volatiles such as F, Cl, H〈sub〉2〈/sub〉O and S play a major role in ore genesis. In this study, we present a combination of textural, micro-analytical and isotopic data for apatite from three Andean IOA deposits (Carmen, Fresia and Mariela) of Early Cretaceous age from the Coastal Cordillera of northern Chile.〈/p〉 〈p〉Apatite textures and compositions show evidence of post-crystallization alteration. Apatite is predominantly zoned with respect to Cl and F, showing a decoupled geochemical behavior between these two elements. Overall, four types of apatite or domains were identified in the analyzed grains based on the X〈sub〉Cl-apatite〈/sub〉/X〈sub〉F-apatite〈/sub〉 and X〈sub〉Cl-apatite〈/sub〉/X〈sub〉OH-apatite〈/sub〉 ratios determined using the atomic proportions of F, Cl and OH. F-apatite is characterized by a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉F-apatite〈/sub〉 〈 0.1 and a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉OH-apatite〈/sub〉 = 0.03–2; Cl-apatite displays a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉F-apatite〈/sub〉 〉 25 and a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉OH-apatite〈/sub〉 〉 4.5 and up to 3000; Cl-OH-F-apatite has a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉F-apatite〈/sub〉 = 0.15–8 and a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉OH-apatite〈/sub〉 = 0.25–5.45; and Cl-OH-apatite shows a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉F-apatite〈/sub〉 = 8–75 and a X〈sub〉Cl-apatite〈/sub〉/X〈sub〉OH-apatite〈/sub〉 = 0.5–3. Carmen apatite is mostly F-apatite, but shows Cl-OH compositions restricted to rims and fractures, whereas apatite from Mariela is dominantly Cl-apatite and Cl-OH-apatite. Apatite from Fresia have variable compositions between Cl-OH-F, Cl-OH- and Cl-apatite, where Cl- and Cl-OH-apatites are characterized by an enrichment of S, Na, Sr and Fe relative to F-apatite. Most notably, S and Na correlate with Cl. In Carmen and Fresia, Cl-OH-apatite is slightly depleted in LREEs, Th and U, a finding consistent with micro-textural evidence of metasomatic processes including coupled dissolution-reprecipitation and formation of secondary monazite and xenotime inclusions. In contrast, apatite from Mariela exhibits no depletion in LREEs and displays a homogeneous distribution of Th and U between the different apatite types, with no monazite and xenotime inclusions. The textural types of apatite identified in this study, coupled with the halogen and trace element composition of apatite, are consistent with modification of primary, F-apatite by interaction with hydrothermal fluids, which led to the formation of Cl-OH-apatite and Cl-apatite. In addition, the initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios and ε〈sub〉Nd〈/sub〉 values of apatite (0.7038 to 0.7050 and −0.3 to +6.5, respectively), calculated considering a 130 Ma age, are consistent with a magmatic origin for the primary F-apatite with minimal or no crustal contribution. Thus, textural, geochemical, and isotopic results of apatite support a magmatic-hydrothermal origin for these Andean IOA deposits with variable degrees of metasomatic overprint as evidenced by the formation of Cl-OH and Cl-apatite.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 17 December 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Geochimica et Cosmochimica Acta〈/p〉 〈p〉Author(s): Yan Li, Feifei Liu, Xiaoming Xu, Yuwei Liu, Yanzhang Li, Hongrui Ding, Ning Chen, Hui Yin, He Lin, Changqiu Wang, Anhuai Lu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Birnessite, primarily formed by biooxidation, is a common semiconducting Mn oxide in nature and a major controller of heavy metals cycling processes. In turn, the heavy metal sorption pathway alters its structural chemistry and thus the electronic structure. We synthesize three kinds of birnessite, namely biogenic (bio-birnessite), Cu coprecipitated (co-birnessite) and Cu adsorbed birnessite (ad-birnessite), to investigate the influence of Cu(II) occupancy on the structure, semiconducting property and photostability of birnessite. XRD show co-birnessite holds the same hexagonal symmetry as bio-birnessite, whereas ad-birnessite transforms to triclinic symmetry as reflected by the splitting reflections at 2.44 and 1.42 Å. The adsorption process is accompanied by a more intense releasing of Mn(II) (156.88 μM/L) from a bio-birnessite analog δ-MnO〈sub〉2〈/sub〉 in light than in dark (19.55 μM/L), suggesting the photoreductive releasing of Mn(II) promotes structure transformation. Conformably, both the Mn average oxidation state (AOS) (3.32) and mole ratio of Cu/Mn (0.08) in ad-birnessite is lower than those in co-birnessite (AOS: 3.47, Cu/Mn ratio: 0.17). EXAFS demonstrate different Cu complexing features in ad- and co-birnessite that the ratio of Cu incorporated (INC) into vacancies is 1.16 and 0.45 for ad- and co-birnessite, respectively. UV–vis diffuse reflection spectra (DRS) and photoelectron spectrometer (PS) exhibit the band gap (E〈sub〉g〈/sub〉) and valence band (VB) of bio-birnessite are 2.05 and −5.58 eV, while there is 0.1 and 0.05 eV decrease of E〈sub〉g〈/sub〉, and 0.08 and 0.16 eV lowering of VB in co- and ad-birnessite, respectively. The reduced E〈sub〉g〈/sub〉 guarantee them to generate photoelectron-hole pairs under mild indoor visible light, and the lower energy level of VB in ad-birnessite makes its VB holes more reactive to accept electrons from electron donors. Further density functional theory (DFT) calculations focus on interpreting the fine structures brought by different Cu sorption pathways on the electronic structure of birnessite. A Mn vacancy in a 2 × 2 × 1 hexagonal birnessite cell significantly reduces E〈sub〉g〈/sub〉 from 1.60 to 0.28 eV by hybridizing Mn 3d and O 2p states in VB. The doped Cu reduces E〈sub〉g〈/sub〉 mainly through incorporating Cu 3d orbital in VB. The INC Cu in hexagonal birnessite contributes more to reduce E〈sub〉g〈/sub〉 (1.60–0.34 eV) than TCS Cu (1.60–1.20 eV), while in triclinic birnessite cell, TCS Cu causes more obvious reduction of E〈sub〉g〈/sub〉 (1.68–0.28 eV) than INC Cu (1.68–1.55 eV). Thus, we conclude the vacancy imposes more effect on the electronic structure of hexagonal co-birnessite than doped Cu in TCS or INC sites; and the band structure of co-birnessite is more sensitive to INC Cu, in contrast to TCS Cu affecting more in ad-birnessite. Overall, the crystal structure differs according to Cu fixation pathway, which imposes a comprehensive effect on band structure and photostability contributed by vacancies, Cu occupancy sites and structural symmetry. Cu coprecipitating with birnessite is suggested as a preferred method in practical clean-up of metals such as Cu, due to the higher adsorption capacity for Cu, better stability of TCS Cu and better photostability influenced by Cu.〈/p〉〈/div〉 〈/div〉