ALBERT

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Mengxue Li, Haibo Liu, Tianhu Chen, Lin Wei, Can Wang, Wei Hu, Hanlin Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The transformation of Al-substituted goethite in oxidative and reductive atmosphere, which models natural fire, was investigated in detail. Various characterization results indicated that Al-substituted goethite transformed into hematite in an oxidative atmosphere and magnetite followed by zero-valent iron in a reductive atmosphere. Interestingly, the substitution of Al for Fe not only changed the crystal morphology affecting crystal surface reactivity, but also restrained the transformation of goethite into hematite, magnetite and zero-valent iron. In addition, the Al in the goethite was embedded into the crystal structure of thermally formed hematite and magnetite, considerably influencing their surface reactivity. The macroscopic adsorption results indicated that the substitution of Al for Fe increased the adsorption capacity of goethite and the corresponding derivatives except zero-valent iron. For the same Al amount, the adsorption capacity followed an order of goethite 〉 hematite 〉 magnetite, implying a loss of phosphate in goethite-rich soil after experiencing natural fire. However, with increasing in temperature in the reductive atmosphere, Al-magnetite transformed into a mixture of Al-magnetite and zero-valent iron which displayed excellent phosphate adsorption capacities increasing to 1.21–5.96 mg/g. The phosphate adsorption behaviors to thermally formed products were fitted well by surface complexation modeling with five complexation sites, which were more obvious for goethite than for hematite and magnetite. These findings presented in this study represent significant progress toward an understanding of the migration, enrichment and transformation of phosphate in Al-substituted goethite-rich soil in cases experiencing natural fire.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Xue-Feng Hu, Jing-Long Zhao, Pei-Feng Zhang, Yong Xue, Bo-Nian An, Fang Huang, Hui-Min Yu, Gan-Lin Zhang, Xiang-Jun Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To probe Fe isotopic signatures of the Quaternary Red Clay (QRC) in subtropical southeastern China, two QRC profiles located in southern Anhui Province, Southeast China, were studied. The whole profiles were generally in a narrow range in δ〈sup〉56〈/sup〉Fe of −0.040 ± 0.061‰—0.117 ± 0.048‰. The Yellow-brown Earth (YBE) and Uniform Red Clay (URC) of the upper profiles were similar in δ〈sup〉56〈/sup〉Fe to the Xiashu Loess along the Yangtze River, Southeast China, and the loess, paleosols and Tertiary red clay in the Chinese Loess Plateaus, Northwest China, as well as the baseline of terrestrial igneous rocks. The slight variation of δ〈sup〉56〈/sup〉Fe from the YBE to the URC suggests that the chemical weathering and rubification on the aerobic surface did not fractionate Fe isotopes significantly. The Reticulate Red Clay (RRC) underlying the YBE and URC fluctuated in δ〈sup〉56〈/sup〉Fe to some extent. The white veins separated from the lower RRC were depleted of Fe oxides and heavier in Fe isotopes, and the red veins, on the contrary, were isotopically lighter in Fe. δ〈sup〉56〈/sup〉Fe of the white veins, 0.345 ± 0.053‰ on average, was significantly higher than the bulk samples, 0.033 ± 0.044‰ on average, and the red veins, −0.021 ± 0.046‰ on average. This suggests that the reticulate textures of the RRC were formed by the reductive removal/re-precipitation of Fe due to the cyclical changes of redoxic conditions. Fe isotopic composition of the QRC was dominantly controlled by the redoxic mobility of Fe, rather than the degree of weathering. δ〈sup〉56〈/sup〉Fe of the profiles was not correlated with the paleoclimatic events, and thus cannot be used as a paleoclimatic proxy. The fractionation of Fe isotopes in the lower RRC, however, indicated the occurrence of over-humid paleoclimate with alternations between drought and wetness in Southern China during the Pleistocene.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 4 July 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): J.K. Gorman, S.C. Penniston-Dorland, H.R. Marschall, R.J. Walker〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Traverses across amphibole-chlorite schist metamorphic reaction zones at contacts between meta-igneous and ultramafic rocks in high-pressure mélange zones were analyzed for highly siderophile element (HSE) concentrations and 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os in two mélange zones (Catalina Schist, Santa Catalina Island, CA, and Attic-Cycladic Complex, Syros, Greece) in order to investigate the relative effects of mechanical mixing and diffusive and advective transport by fluids in subduction-related reaction zones. Results from Catalina Schist traverses show co-varying elevated concentrations of Os, Ir, Ru, Cr, and MgO and inversely correlated initial 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os that display irregular patterns across the chlorite-actinolite schist reaction zones. These elements are strongly enriched in peridotite relative to mafic crust, and are relatively fluid immobile. The observations suggest that these components were added via mechanical mixing of mantle peridotitic material with material derived from mafic blocks. In a traverse across a Syros mafic eclogite block with a narrow chlorite schist reaction zone and broader glaucophane-rich reaction zone, the Os, Ir, Ru, and Cr concentrations in the chlorite schist are just barely higher than those in the eclogite block, while the initial 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os ratio falls between the eclogite core and adjacent serpentinite. These compositions show a relatively minor metasomatic exchange between eclogite block and serpentinite. A second sample set from Syros was chosen across a contact zone between a meta-tuffite and a serpentinite because previous studies suggest it was not affected by mechanical mixing. This traverse contains a ~1 m wide complex metasomatic reaction zone that is used to study the relative contributions of diffusive and advective transport, respectively. The traverse reveals strong mobilization of Os, Ir, Ru and Pt beyond the scale of the outcrop, and minor advective enrichment of Re and Pd. This indicates that all HSE may be mobilized by fluid advection beyond the local scale in some subduction zone mélange zones.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Rasesh Pokharel, Ruben Gerrits, Jan A. Schuessler, Friedhelm von Blanckenburg〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉To unravel the dissolution mechanisms of olivine by a rock-inhabiting fungus we determined the stable isotope ratios of Mg on solutions released in a laboratory experiment. We found that in the presence of the fungus 〈em〉Knufia petricola〈/em〉 the olivine dissolution rates were about seven-fold higher (1.04 × 10〈sup〉−〈/sup〉〈sup〉15〈/sup〉 mol cm〈sup〉−〈/sup〉〈sup〉2〈/sup〉 s〈sup〉−〈/sup〉〈sup〉1〈/sup〉) than those in the abiotic experiments (1.43 × 10〈sup〉−〈/sup〉〈sup〉16〈/sup〉 mol cm〈sup〉−〈/sup〉〈sup〉2〈/sup〉 s〈sup〉−〈/sup〉〈sup〉1〈/sup〉) conducted under the same experimental condition (pH 6, 25 °C, 94 days). Measured element concentrations and Mg isotope ratios in the supernatant solutions in both the biotic and the abiotic experiment followed a dissolution trend in the initial phase of the experiment, characterized by non-stoichiometric release of Mg and Si and preferential release of 〈sup〉24〈/sup〉Mg over 〈sup〉26〈/sup〉Mg. In a later phase, the data indicates stoichiometric release of Mg and Si, as well as isotopically congruent Mg release. We attribute the initial non-stoichiometric phase to the rapid replacement of Mg〈sup〉2+〈/sup〉 in the olivine with H〈sup〉+〈/sup〉 along with simultaneous polymerization of Si tetrahedra, resulting in high dissolution rates, and the stoichiometric phase to be influenced by the accumulation of a Si-rich amorphous layer that slowed olivine dissolution. We attribute the accelerated dissolution of olivine during the biotic experiment to physical attachment of 〈em〉K. petricola〈/em〉 to the Si-rich amorphous layer of olivine which potentially results in its direct exposure to protons released by the fungal cells. These additional protons can diffuse through the Si-rich amorphous layer into the crystalline olivine. Our results also indicate the ability of 〈em〉K. petricola〈/em〉 to dissolve Fe precipitates in the Si-rich amorphous layer either by protonation, or by Fe(III) chelation with siderophores. Such dissolution of Fe precipitates increases the porosity of the Si-rich amorphous layer and hence enhances olivine dissolution. The acceleration of mineral dissolution in the presence of a rock-dissolving fungus further suggests that its presence in surficial CO〈sub〉2〈/sub〉 sequestration plants may aid to accelerate CO〈sub〉2〈/sub〉 binding.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 30 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 523〈/p〉 〈p〉Author(s): Xiaoping Feng, William J. D'Andrea, Cheng Zhao, Shouzhen Xin, Can Zhang, Weiguo Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The orogenic history of the Tibetan Plateau (TP) and surrounding mountain ranges continues to be a major source of disagreement among geologists, particularly concerning the uplift models for the Cenozoic evolution of the TP and estimates for when the highest and largest plateau on Earth reached its current elevation. Quantitative reconstructions of past elevation from geologic samples are necessary to document the uplift history of TP and examine the interactions between tectonic-relief and climate over geological time-scales. Several studies establishing lipid biomarker-based paleoaltimetry based on leaf wax δD values and brGDGTs have been reported in recent years for the TP and surrounding regions, but have yet to be synthesized into a regional framework for paleoelevation determination and uncertainty analysis. Here we report new leaf wax δD and brGDGTs data developed from surface soil samples along an elevation transect spanning ~1250–3900 m.a.s.l in the Hengduan Mountains on the southeastern edge of the TP. We find that the abundance-weighted mean leaf wax δD (〈em〉n〈/em〉-C〈sub〉27〈/sub〉, 〈em〉n〈/em〉-C〈sub〉29〈/sub〉 and 〈em〉n〈/em〉-C〈sub〉31〈/sub〉) values (〈em〉δ〈/em〉D〈sub〉wax〈/sub〉) lapse rates determined for the Hengduan Mountains and for five other nearby study locations are statistically indistinguishable, and can be combined to provide a regional 〈em〉δ〈/em〉D〈sub〉wax〈/sub〉 lapse rate of −1.97 ± 0.04‰ (1〈em〉σ〈/em〉)/100 m for use in regional paleoelevation studies across the southeastern TP. We also find a strong correlation (R〈sup〉2〈/sup〉 = 0.71) between brGDGTs and elevation-dependent mean annual air temperature, which contributes to a number of studies in the region that support the use of fossil brGDGTs as a paleoelevation proxy. Our results reveal that 〈em〉δ〈/em〉D〈sub〉wax〈/sub〉 and brGDGTs for the Hengduan Mountains provide similar empirical uncertainty in paleoelevation reconstruction, with standard errors of elevation estimation (〈em〉SE〈/em〉) of ±483 m (±1〈em〉σ〈/em〉) and ± 394 m (±1〈em〉σ〈/em〉), respectively. We propose a paleoaltimetric approach that combines 〈em〉δ〈/em〉D〈sub〉wax〈/sub〉 and brGDGT data, in order to derive paleoelevation estimates with lower uncertainties. In the Hengduan Mountains data set, the approach yields a 〈em〉SE〈/em〉 (±286 m; ±1〈em〉σ〈/em〉) that is 27–40% lower than when 〈em〉δ〈/em〉D〈sub〉wax〈/sub〉 values and brGDGTs are applied separately.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 30 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 523〈/p〉 〈p〉Author(s): Fanqi Qin, Lauren E. Beckingham〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Imaging has emerged as a valuable means of geological sample characterization and parameterizing reactive transport simulations where image analysis can provide porosity, mineral composition and mineral accessible surface area data, for example. Images can be collected using a variety of techniques and at a range of resolutions, yet the impact of image resolution on measured properties is largely unknown. In this work, the impact of 2D image resolution on the calculated mineral abundances, accessibilities and effective surface areas are examined for a sample from the Paluxy formation, Kemper County, Mississippi. Scanning electron microscopy (SEM) backscatter electron (BSE) images of thin sections were captured at resolutions ranging from 0.3 μm to 6 μm. Images were segmented into pores and discrete mineral phases using ImageJ and algorithms developed in MATLAB. Porosity, mineral abundances and mineral accessibilities were calculated by counting pore and mineral pixels in the segmented image where accessible minerals were deemed as those adjacent to the pore space. A 3D X-ray computed tomography (CT) image of a core sample was collected, segmented, and analyzed to evaluate the 3D connected porosity. Cuboids with the same total area as the 2D image were randomly sampled and used to calculate the 3D connected surface area. This was then multiplied by mineral accessibility to calculate accessible mineral surface areas for non-clay minerals. Minimum variations were observed for mineral abundances calculated from images with varying resolutions. For high resolution images, 0.3 μm to 1 μm, mineral accessibilities agreed relatively well. For images with resolutions from 1 μm to 6 μm, the calculated accessibility of smectite/illite decreased with decreasing resolution while quartz accessibility increased. This in turn resulted in higher effective surface areas for quartz with decreasing resolution. No significant variations were observed for calcite, siderite and K-feldspar.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Taiyu Huang, Daizhao Chen, Yong Fu, Rumana Yeasmin, Chuan Guo〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes. To unravel the oceanic redox conditions, high-resolution analyses of iron speciation and redox-sensitive trace elements (Mo and U) within the framework of sequence stratigraphy were carried out on the Niutitang Formation (~528–521 Ma). The two examined sections, the Daotuo and Bahuang sections, were respectively located in a mid-upper slope setting and a lower slope to basin settings behind a seaward submerged sill on the middle Yangtze Block, South China. At Daotuo, the Fe〈sub〉py〈/sub〉/Fe〈sub〉HR〈/sub〉 values display moderate positive covariances with the total organic carbon contents (TOC) and the Mo/TOC ratios, notably in the basal part of the Niutitang Formation. Therefore, at this locality the euxinic water mass wedge was developed in association with a high primary organic productivity/burial rate, likely within a high-productivity zone. In contrast, at Bahuang, the Fe〈sub〉py〈/sub〉/Fe〈sub〉HR〈/sub〉 values show weak to negative covariances with the Mo/TOC ratios and TOC contents in the basal part, indicating that the biogeochemical cycles of Fe, Mo with C were decoupled in a lower-productivity, ferruginous deeper basinal setting. These spatial changes in marine redox structures and biogeochemical cycles can be reasonably explained by the existence of oceanic upwelling in the presence of strong offshore currents and a seaward submarine sill, reconciling the oxygen minimum zone (OMZ) in modern oceanic margins to some extent. This spatial redox pattern also accounts well for previous data based on which the euxinic state intermittently invaded upward onto the shelf margin and evolved into a ferruginous-dominant anoxia in the inner shelf subbasins. In addition, the euxinic wedge dynamically fluctuated upslope and downslope along the transect from the ferruginous outer shelf slope to the basin in response to rise and fall of sea-level. Consequently, the temporal evolution of the redox conditions was driven in part by eustatic variations. Moreover, the paleogeographic position of the Yangtze Block in the mid-lower latitudes suggests that the block was associated with relatively strong offshore currents induced by trade winds, which notably enhanced the oceanic upwelling near the transgression maximum. On the other hand, the dominance of deep ferruginous waters with the local occurrence of a euxinic wedge on the outer shelf slope indicates relatively low concentrations of seawater sulfate and atmospheric oxygen during the deposition of the Niutitang black shales. This redox model thus highlights the important roles of paleogeographic, paleoclimatic and eustatic sea-level changes in controlling the spatiotemporal fluctuations in marine redox conditions and biogeochemical cycling in the early Cambrian ocean.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Christian Grimm, Raul E. Martinez, Oleg S. Pokrovsky, Liane G. Benning, Eric H. Oelkers〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Particulate material plays a major role in the transport of sparingly soluble nutrients such as P and Fe in natural surface waters. Microbes might gain access to these nutrients either indirectly through particulate dissolution or directly through microbial attack. As such, it seems reasonable to expect a link between the particulate material concentration and bacterial growth in natural surface waters. To explore this link, a series of microcosm growth experiments were performed in the presence of a typical freshwater cyanobacteria, 〈em〉Synechococcus sp.〈/em〉, grown in dilute BG-11 culture media in the presence and absence of basaltic and continental riverine particulate material. Results demonstrate that riverine particulates can increase bacterial biomass by 1) triggering bacterial growth in otherwise unfavourable conditions, 2) increasing total maximum biomass concentration, and 3) inducing bacteria growth during the post-exponential phase. These effects are found to be enhanced by increasing particulate concentration. Results also indicate a positive feedback between the nutrient release from the particulates and growing bacteria, where dissolving particulates enhance bacterial growth, which further promotes particulate dissolution by altering fluid pH. Microscopic analysis showed direct physical contact between particulates and cyanobacteria, suggesting that bacteria attach directly on mineral surfaces to gain required nutrients. Furthermore, frequent bacteria clusters were observed associated with particulates, indicating an increasing aggregation of bacteria in the presence of particulate material, which may facilitate a higher burial efficiency of organic carbon.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): P.L. Sullivan, G.L. Macpherson, J.B. Martin, R.M. Price〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉〈em〉Carbonate terrains〈/em〉 (CT) underlie one-fifth of terrestrial, ice-free land and are an important supply of potable water to the world's population, and yet processes endemic to CT critical zones (CZ) and responses of these processes to climatic and anthropogenic pressures are not well understood. Given the rapid dissolution rates and ability to generate well-developed networks of secondary porosity these landscapes can be highly sensitive to impacts from climate change (e.g., modifications of temperature, precipitation, sea level) and human disturbance (e.g., water withdrawal/diversions, changes in land use/land cover). This special issue includes 16 papers focused on CT-CZ processes and potential responses to climatic and human perturbations. Five major themes emerge from these papers, namely: (1) anthropogenic climate and land use changes alter CT-CZ weathering rate and diagenesis, (2) metal and carbon fluxes in CT-CZ will respond to increasing hydrologic variance caused by climate change, (3) endogenous and exogenous processes operating over short time periods (〈10,000 yrs) form landscape patterns in carbonate terrains, (4) rates of carbonate mineral dissolution depend on vadose zone and soil thickness, and (5) open systems may not always promote greater carbonate weathering rates in CT-CZ. These findings reflect the importance of carbonate minerals in Earth's CZ, both in terms of processes unique to carbonate minerals, as well as a predictor of future responses to anthropogenic and environmental changes.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Beichen Wang, Jian Yang, Hongchen Jiang, Guojing Zhang, Hailiang Dong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉〈em〉N〈/em〉-alkane-based proxies are widely employed to reconstruct paleoclimate and paleoenvironment in lacustrine environments. However, little is known about the influence of microbially mediated alkane-degradation on 〈em〉n〈/em〉-alkane-derived proxies. In this study, the chemical composition of 〈em〉n〈/em〉-alkanes and microbially mediated 〈em〉n〈/em〉-alkane degradation potential were investigated in the surface sediment samples collected from seven lakes with a range of salinity from freshwater to salt saturation on the northern Qinghai-Tibetan Plateau (QTP). The results showed that the chemical composition of 〈em〉n〈/em〉-alkanes differed among the studied QTP lakes. Significant correlations were observed between salinity and some 〈em〉n〈/em〉-alkane-based paleoclimate and paleoenvironment proxies, such as ratio of C〈sub〉21〈/sub〉〈sup〉−〈/sup〉/C〈sub〉22〈/sub〉〈sup〉+〈/sup〉, average chain length (ACL) and carbon preference index (CPI). This suggested that salinity may affect the validity of some 〈em〉n〈/em〉-alkane-based paleoclimate and paleoenvironment proxies. Alkane-degrading bacteria were abundant and widespread in the studied freshwater and saline/hypersaline lakes but were minor or absent in salt-saturation lakes. The obtained alkane-degrading bacterial strains showed active ability to degrade 〈em〉n〈/em〉-hexadecane. This suggested that the salinity influence on the 〈em〉n〈/em〉-alkane distribution may be partially related to microbial degradation, which awaits further in-situ investigation. So salinity variation should be taken into account when using 〈em〉n〈/em〉-alkane-based proxies for reconstructing paleoclimate and paleoenvironment in lakes.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 30 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 523〈/p〉 〈p〉Author(s): Pavel Pracný, Jiří Faimon, Dalibor Všianský, Antonín Přichystal〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Magnesium (Mg) and strontium (Sr) preserved in speleothems are among the terrestrial climatic proxies. The mechanism of their release from impurities in limestone bedrock is usually neglected in paleoclimatic studies mainly because it is not very well understood; most cation release studies focus on pure minerals instead of limestone. Therefore, the Mg and Sr released from six types of limestone from the Moravian Karst (Czech Republic) were studied experimentally and compared with the dripwater data coming from the Punkva Caves (Moravian Karst) formed in two of the studied limestone types. The dissolution data were used to calibrate a kinetic dissolution model. The results show an incongruent dissolution of Mg-calcite and dolomite, pronounced especially in advanced stages of the dissolution. According to the results, the trace element ratios in solution were dependent mainly on (1) the limestone composition (i.e., the Mg-calcite and dolomite availability and the stoichiometry of individual minerals) and (2) on the extent of limestone-solution interaction (controlled by dissolution dynamics). Modeling indicated that the dynamics depend on the ratios of the area of rock–water and air–water interfaces to the water volume. Moreover, modeling suggests that the dynamics and subsequently also the trace element ratios are influenced by (a) the fresh surface dissolution during the initial stages of interaction and by (b) the preferential release of trace elements from mineral surfaces (nonstoichiometric dissolution). The effects of disturbed surfaces and preferential leaching are limited by available fresh surfaces. Fresh surfaces are formed through rock crushing, e.g., during physical weathering (in the field) or sample preparation (in a laboratory). The dripwaters from Punkva Caves had trace element ratios with values between the ratios resulting from the experimental dissolution of the limestones present on the site, indicating a naturally mixed composition of waters.〈/p〉 〈p〉This study has shown that Mg/Ca and Sr/Ca ratios in dripwaters might be controlled by various climate-independent variables, which raises further questions about utilizing the ratios as paleoclimatic proxies.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Jennifer Thompson, Simon W. Poulton, Romain Guilbaud, Katherine A. Doyle, Stephen Reid, Michael D. Krom〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We report the development of a modified method for evaluating different reservoirs of sedimentary phosphorus (P) in ancient marine sedimentary rocks and in modern Fe-rich sediments. Utilising the existing SEDEX scheme for P partitioning in modern sediments, we initially demonstrate limitations in the application of the original scheme to sediments and rocks containing crystalline hematite and magnetite. We tested additional extractions for these crystalline Fe phases, using both synthetic minerals, and modern and ancient sediments. The addition of 6 h oxalate and 6 h citrate-dithionate-acetate extractions considerably enhanced the total recovery of synthetic magnetite and hematite to 88.7 ± 1.1% and 76.9 ± 3.8%, respectively. In addition, application of the 6 h oxalate extraction to synthetic P-containing magnetite recovered 93.9 ± 1.7% of the Fe present and 88.2 ± 12.8% of the co-precipitated P. Based upon these results we developed a modified SEDEX extraction scheme. The modified scheme was applied to modern Fe-rich sediments from Golfo Dulce, Costa Rica, which resulted in 16% higher Fe-bound P recovery. Application of the scheme to a variety of ancient marine rocks increased the recovery of Fe-bound P by up to 22%. We also highlight the potential for authigenic carbonate fluorapatite to convert to more crystalline apatite in ancient rocks during deep burial and metamorphism. We suggest that in such systems minimum and maximum estimates of the total reactive P pool may be calculated with and without the inclusion of crystalline P. It is noted that the application of the revised method may have important implications for understanding the cycling of P in ancient marine environments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): L. Millière, N. Gussone, T. Moritz, S. Bindschedler, E.P. Verrecchia〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Strontium (Sr) and calcium (Ca) isotopic compositions and Sr/Ca ratios have been analysed in different fractions of pedogenic needle fibre calcite (NFC) from the Swiss Jura Mountains (Villiers, Neuchâtel, Switzerland) in order to determine their Ca sources and to investigate the potential role of fungi in the origin of NFC. 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios have been measured to trace the Ca trapped within different pedogenic carbonate. This ratio was compared for previously identified microscopic morphological groups of NFC, the bulk soil fine earth (〈2 mm), the carbonate host rock (CHR), a late calcitic cement (LCC, another pedogenic carbonate precipitated physicochemically at the same depth as NFC in soil pores), the soil solution, and the throughfall. The 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr indicated a mixing of weathered allochthonous minerals (an atmospheric component) and the dissolved CHR (the local geogenic component). Moreover, the comparison of 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr ratios of the different types of NFC with LCC suggests that the contribution of Ca from the main Ca sources slightly differs between NFC morphotypes and LCC. In addition, the three NFC morphotypes displayed Sr isotopic compositions different from each other, emphasizing a direct relationship between the microscopic shapes of NFC and the processes involved in their respective formation, including the origin of the Ca. Strontium concentrations and δ〈sup〉44/40〈/sup〉Ca values of NFC and LCC crystals were used to determine possible differences in their growth rate and/or micro-environmental conditions during their formation. Crystals described micromorphologically as simple needles (SN) were enriched in 〈sup〉44〈/sup〉Ca and depleted in Sr compared to LCC, suggesting that the elongated shape of the SN crystal cannot be related to a rapid precipitation rate, but rather to a slow precipitation. Calculations applied to the soil solution evolution during calcite precipitation demonstrates the utilisation of large portions of the Ca pool for NFC formation within a closed system, e.g. inside an organic mould, such as fungal hyphae. The differences between the three microscopic groups can be explained by a mixing of the SN with a LCC-like compound originating from the soil solution.〈/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-S0009254119303171-ga1.jpg" width="260" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 14 June 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Kenji Shimizu, Motoo Ito, Qing Chang, Takashi Miyazaki, Kenta Ueki, Chiaki Toyama, Ryoko Senda, Bogdan S. Vaglarov, Tsuyoshi Ishikawa, Jun-Ichi Kimura〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The lithophile elements and isotopic compositions of oceanic basalts suggest they derive from different mantle components. We present new analytical results of un-degassed deep-marine basaltic glasses from various regions, and we find strong linear correlations of R〈sup〉2〈/sup〉 ≥ 0.997 between H〈sub〉2〈/sub〉O and F as well as H〈sub〉2〈/sub〉O and Ce, for which we propose mantle trends. The mantle trends represent global variations of mantle components, ranging from a depleted dry peridotite of mid-ocean ridge basalt (MORB) (DMM, depleted MORB source mantle: H〈sub〉2〈/sub〉O = 100 ppm; H〈sub〉2〈/sub〉O/Ce = 200; H〈sub〉2〈/sub〉O/F = 10) to a hydrous peridotite of Hawaiian ocean island basalt (OIB) of deep mantle origin (FOZO, focal zone: H〈sub〉2〈/sub〉O = 750 ppm; H〈sub〉2〈/sub〉O/Ce = 200; H〈sub〉2〈/sub〉O/F = 18.5). Accordingly, we defined the correlation as a volatile DMM–FOZO trend. Based on our findings, we report novel H〈sub〉2〈/sub〉O–F–Ce systematics to discriminate the degree of water depletion in the source mantle and the rehydration of the mantle with recycled surface water through oceanic plate subduction. Using this method, most OIBs are distinguished clearly from the DMM–FOZO trend, and we find that the water in their sources originates from recycled water derived from the hydrated oceanic crust and sediment after various degrees of dehydration (75–95%) in subduction zones.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Luis Monasterio-Guillot, Fulvio Di Lorenzo, Encarnacion Ruiz-Agudo, Carlos Rodriguez-Navarro〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The kinetics of silicate carbonation in aqueous solutions are typically sluggish, especially at neutral to alkaline conditions. This hampers the complete understanding of the mechanisms and parameters that control mineral carbonation during carbon capture and storage (CCS). Here we study the hydrothermal dissolution and carbonation of pseudowollastonite (psw; 〈em〉α〈/em〉-CaSiO〈sub〉3〈/sub〉), one of the most reactive silicates known, under a range of geochemical conditions ranging from acidic to strongly alkaline pH, presence/absence of different background alkali metal ions and carbonate sources (K〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉 and Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉, pH ~13, or NaHCO〈sub〉3〈/sub〉 and KHCO〈sub〉3〈/sub〉, pH ~9). We show that in addition to amorphous silica precipitation, the formation of secondary Na + Ca- or K + Ca-silicates in the presence of Na〈sup〉+〈/sup〉 and K〈sup〉+〈/sup〉 background ions, respectively, fosters the progress of psw carbonation. However, the formation of Ca-containing secondary crystalline silicates and Ca-containing amorphous silica is shown to be a strong handicap for a fully effective carbonation. In all cases a higher conversion into CaCO〈sub〉3〈/sub〉 (up to ~70 mol%) is achieved when using bicarbonate salts (i.e., lower initial pH). By using a reactor with a pressurized CO〈sub〉2〈/sub〉-solution, with and without Na〈sup〉+〈/sup〉 or K〈sup〉+〈/sup〉 background ions, rapid and nearly complete conversion of psw with a CaCO〈sub〉3〈/sub〉 yield ~92 mol% is achieved because, in addition to the initial low pH (~3.7) that favored 〈em〉α〈/em〉-CaSiO〈sub〉3〈/sub〉 dissolution, abundant Ca-free non-passivating amorphous silica formed along with calcite. These results imply that the presence (e.g., use of sea water during CO〈sub〉2〈/sub〉 injection or mixing with saline formation solutions) or the release of different alkali metal ions (e.g., after feldspar and/or basaltic glass dissolution) in combination with a reaction-induced pH increase during in situ CCS scenarios may strongly limit carbonation due to the capture of alkaline-earth metals in secondary silicates and a reduction in reaction rates. In turn, our results show that the high conversion achieved in pure CO〈sub〉2〈/sub〉-aqueous systems, while relevant for ex situ CCS, may not reflect the actual conversion in multicomponent natural systems following reactive transport during in situ CCS. Moreover, the precipitation of secondary silicate and calcium carbonate phases have a direct cementing effect, which could be detrimental for in situ CCS, as it would likely reduce host rock permeability, but would be relevant and beneficial for the setting of novel CaSiO〈sub〉3〈/sub〉-based non-hydraulic cements with reduced CO〈sub〉2〈/sub〉 footprint.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 524〈/p〉 〈p〉Author(s): Hong-Kun Dai, Jian-Ping Zheng, Suzanne Y. O'Reilly, William L. Griffin, Qing Xiong, Rong Xu, Yu-Ping Su, Xian-Quan Ping, Fu-Kun Chen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Crustal recycling is an important cause of mantle heterogeneity and can have significant control on basalt compositions. Recycled components from the subducted (Paleo-) Pacific slab have frequently been recognized in Cenozoic basalts from the eastern North China Craton (NCC). However, it still remains unclear if the subducted Paleo-Asian oceanic slab contributed to intraplate basalts in this Craton.〈/p〉 〈p〉In a search of evidence for the recycled components from this slab, we have studied the Ar〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Ar age and elemental and Sr-Nd-Pb isotope compositions of newly-discovered basalts from the Langshan area and compiled a regional synthesis of Cenozoic alkali basalts from the northwest NCC. This region is far from the Pacific domain but near the suture zone of the Paleo-Asian Ocean. With a Late Cretaceous eruption age (~89 Ma), Langshan basalts have low silica and high FeO, MgO and alkali contents, high incompatible elemental concentrations, positive Sr, Eu, Ba, Nb, Ta and negative Pb and Ti anomalies with high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios, superchondritic Zr/Hf ratios, and uniform radiogenic isotopes (ɛ〈sub〉Nd(t)〈/sub〉 = 2.20–2.51, 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉〈em〉i〈/em〉〈/sub〉 = 0.703807–0.704348, 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉〈em〉i〈/em〉〈/sub〉 = 18.200–18.384, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉〈em〉i〈/em〉〈/sub〉 = 15.470–15.515, 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb〈sub〉〈em〉i〈/em〉〈/sub〉 = 38.007–39.430). These compositional characteristics are shared by the compiled Cenozoic alkali basalts from northwest NCC.〈/p〉 〈p〉The low silica and high MgO, FeO and alkali contents together with the positive Nb, Ta and negative Ti anomalies were probably controlled by silica-deficient garnet pyroxenite in the mantle source. The partial melting is estimated to have occurred in the asthenosphere under an average mantle potential temperature of 1300–1450 °C and a pressure of ~2.5 GPa. The positive Sr, Eu, Ba, Nb, Ta anomalies and the canonical indices (high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios) indicate the involvement of a subducted oceanic igneous slab. Considering 1) the particular tectonic locality of the study region, 2) the inferred northward increase of silica-deficient pyroxenite in the mantle source of Cenozoic alkali basalts, 3) the evidence for strong lithospheric modification beneath the northwest NCC induced by slab-derived components, and 4) the longevity of subducted slabs in convecting mantle, the subducted Paleo-Asian oceanic materials could have introduced ubiquitous mantle heterogeneity beneath the northwest NCC and played a significant role in the generation of the Late Cretaceous to Cenozoic intraplate alkali basalts there.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 10 May 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Jérôme Gaillardet, Damien Calmels, Gibran Romero-Mujalli, Elena Zakharova, Jens Hartmann〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Carbonate rocks are a peculiarity of the Earth relative to other planets in the solar system. Large terrestrial areas are covered by carbonate lithology, which actively reacts with atmospheric/biospheric CO〈sub〉2〈/sub〉. Although carbonate rocks represent a major component of the global carbon cycle, their intensity and rates of chemical weathering have been overlooked. In this study, we examine three global databases of rivers and springs draining carbonate regions under various climate conditions (from −15 °C to +30 °C). Using Ca〈sup〉2+〈/sup〉 + Mg〈sup〉2+〈/sup〉 concentrations as a proxy, we show that carbonate weathering intensity depends upon land temperature according to a boomerang-type relationship, with maximum dissolution between 10 and 15 °C. We show that this pattern is primarily controlled by thermodynamics if we assume that the partial pressure of CO〈sub〉2〈/sub〉 in soil (〈em〉pCO〈/em〉〈sub〉2〈/sub〉) increases from atmospheric-like levels under cold climate up to 100 times the present day atmospheric concentration under hot climate. The link between soil 〈em〉pCO〈/em〉〈sub〉2〈/sub〉 and land temperature is still not very well known, but by using three different published predictive soil 〈em〉pCO〈/em〉〈sub〉2〈/sub〉 vs. T curves, we show that the boomerang shape can be, at least qualitatively, reproduced.〈/p〉 〈p〉This study shows that more data on carbonate weathering in various environments are needed to predict with more accuracy the role that carbonate lithologies and overlying ecosystems could play in the Anthropocene.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 23 May 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Catherine A. Chamberlin, Thomas S. Bianchi, Amy L. Brown, Matthew J. Cohen, Xiaoli Dong, Madison K. Flint, Jonathan B. Martin, Daniel L. McLaughlin, A. Brad Murray, Andrea Pain, Carlos J. Quintero, Nicholas D. Ward, Xiaowen Zhang, James B. Heffernan〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We constructed mass balances of both calcium and phosphorus for two watersheds in Big Cypress National Preserve in southwest Florida (USA) to evaluate the time scales over which its striking landscape pattern developed. This low-relief carbonate landscape is dotted with evenly spaced, evenly sized, shallow surface depressions that annually fill with surface water and thus support wetland ecosystems (e.g. cypress domes) embedded in a pine-dominated upland matrix with exposed bedrock. Local and landscape scale feedbacks between hydrology, ecological dynamics and limestone dissolution are hypothesized to explain this karst dissolution patterning. This hypothesis requires the region to be wet enough to initiate surface water storage, which constrains landscape formation to interglacial periods. The time scale therefore would be relatively recent if creation of the observed pattern occurred in the current interglacial period (i.e. Holocene), and older time scales could reflect inherited patterns from previous inter-glacial periods, or from other processes of abiotic karstification. We determined phosphorus stocks across four landscape compartments and estimated the limestone void space (i.e., wetland depression volume) across the landscape to represent cumulative calcium export. We calculated fluxes in (e.g., atmospheric deposition) and out (i.e., solute export) of the landscape to determine landscape denudation rates through mass balance. Comparing stocks and annual fluxes yielded independent estimates of landscape age from the calcium and phosphorus budgets. Our mass balance results indicate that the landscape began to develop in the early-mid Holocene (12,000–5000 ybp). Radiocarbon dating estimates implied similar rates of dissolution (~1 m per 3000–3500 years), and were in agreement with Holocene origin. This supports the hypothesis that ecohydrologic feedbacks between hydrology and vegetation occurring during the present interglacial period are sufficient to shape this landscape into the patterns we see today, and more broadly suggests the potential importance of biota in the development of macro-scale karst features.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 12 February 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Alan D. Wanamaker, David P. Gillikin〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In order to constrain spatial and temporal temperatures and environmental conditions in the North Atlantic Ocean during the Holocene, high-resolution (seasonal to annual) marine proxies with excellent chronological constraints are needed. The long-lived ocean quahog, 〈em〉Arctica islandica〈/em〉, which has the potential to provide a precise annually-dated record, via crossdating techniques, is a fairly well-developed and tested marine proxy archive. In particular, oxygen isotopes derived from 〈em〉A. islandica〈/em〉 shell carbonate have provided a wealth of information on marine climate and ocean circulation dynamics, however, shell-derived oxygen isotopes are influenced by both the isotopic source water signature (covarying with salinity) and seawater temperature. If seawater isotopic signature is not known, temperature reconstructions become challenging. Thus, an independent technique to estimate past seawater temperatures is highly desired, however based on previous studies on adult and juvenile clams, the utility of elemental ratios in 〈em〉A. islandica〈/em〉 shell material as environmental proxies remains questionable. To further evaluate the influence of seawater temperature on elemental and isotopic incorporation during biomineralization, 〈em〉A. islandica〈/em〉 shells were grown at constant temperatures under two regimes during a 16-week period from March 27 to July 21, 2011 at the Darling Marine Center in Walpole, Maine. Individual juvenile clams were stained with calcein and cultured at 10.30 ± 0.30 °C for six weeks. After this, the clams were again stained with calcein and cultured at 15.00 ± 0.40 °C for an additional 9.5 weeks. Average salinity values were 30.20 ± 0.70 and 30.70 ± 0.70 in the first and second phases of the experiment, respectively. Continuous sampling within and across the temperature conditions (from 10.30 °C to 15.00 °C) coupled with the calcein markings provided the ability to place each sample into a precise temporal framework and to establish exact average growth rates for the shells sampled. After accounting for changes in the isotopic composition of seawater, oxygen isotopes from one sampled shell effectively recorded seawater temperatures during the study and also gave confidence to the temporal fit of the data. Elemental ratios (Sr/Ca, Mg/Ca, Ba/Ca) from five aragonitic shells were determined via laser ablation inductively coupled plasma mass spectrometry. Sr/Ca and Mg/Ca data showed little coherence with temperature during the culture experiment, including the rapid 5 °C increase in seawater temperature. However, Ba/Ca ratios showed an inverse relationship with seawater temperatures although this relationship was noisy. Additionally, salinity interactions were present during the 15.00 °C treatment, further highlighting complex incorporation of elements during biomineralization. Incorporation of Sr, Mg, and Ba were strongly and variably impacted by growth rates. Combined, the results from these culture experiments demonstrate that Sr/Ca, Mg/Ca, Ba/Ca ratios in juvenile 〈em〉A. islandica〈/em〉 shell material are dominated by physiological processes and thus not reliable as environmental proxies.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 9 March 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Matthew D. Covington, Kiefer A. Vaughn〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Within numerical models of karst development, geochemical and hydrological boundary conditions are typically assumed to be constant. However, rates of calcite dissolution in natural karst systems can vary substantially in time. In particular, variation in carbon dioxide (CO〈sub〉2〈/sub〉) concentrations has been shown to produce time variability in dissolution rates in karst streams, but controls on CO〈sub〉2〈/sub〉 variation within karst systems are relatively poorly quantified. Here we analyze hourly in-situ measurements of dissolved CO〈sub〉2〈/sub〉, discharge, and specific conductance at a pair of karst underflow-overflow springs and examine potential drivers of variability in CO〈sub〉2〈/sub〉 and dissolution rates. The springs display strong seasonal variability in CO〈sub〉2〈/sub〉 and saturation state as well as moderate variation during storm events. Though both springs have elevated CO〈sub〉2〈/sub〉 concentrations in the summer season, the overflow spring experiences a substantial decrease in dissolved CO〈sub〉2〈/sub〉 below a critical discharge threshold. We hypothesize that this decrease results from ventilation within the overflow portion of the system as segments of the flow path transition from full pipe to open channel flow. The overflow spring experiences substantially lower average dissolution rates than the underflow spring, despite larger discharge and chemical variability during high flow events at the overflow spring. Though open systems are frequently presumed to have higher dissolution rates, because of their ability to replenish CO〈sub〉2〈/sub〉 that is consumed in the dissolution process, these data suggest that dissolution rates within the closed portion of the system may in some cases be higher due to the inability of closed flow paths to ventilate excess CO〈sub〉2〈/sub〉 to the atmosphere. Such conditions are likely to occur in conduits where CO〈sub〉2〈/sub〉 concentrations are elevated above atmospheric levels and average flow through times are short compared to the time scale over which water reaches equilibrium with respect to calcite.〈/p〉〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Brendan A. Bishop, Shannon L. Flynn, Tyler J. Warchola, Md. Samrat Alam, Leslie J. Robbins, Yuxia Liu, George W. Owttrim, Daniel S. Alessi, Kurt O. Konhauser〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Marine bacterial plankton play a key role in elemental cycling through their ability to bind, assimilate, metabolize, and modify the redox state of trace metals in seawater. Of those processes, arguably the least studied are the mechanisms underpinning trace metal adsorption to planktonic marine bacteria, despite a plethora of literature pertaining to terrestrial species. Recently, Liu et al. (2015) demonstrated that the marine cyanobacterium 〈em〉Synechococcus〈/em〉 sp. PCC 7002 has the capacity to remove appreciable amounts of Cd〈sup〉2+〈/sup〉, a proxy for other divalent cations, from seawater by adsorption. In this study, we build on that work and employ a surface complexation modelling (SCM) approach using titration and pH adsorption edge experiments to calculate the thermodynamic binding constants of four bioessential transition metals (Co, Ni, Cu, Zn) to 〈em〉Synechococcus〈/em〉 in simulated seawater. Based on the titration results, the major functional groups involved in metal binding were carboxyl groups with a pK〈sub〉a〈/sub〉 of 5.59 and phosphoryl groups with a pK〈sub〉a〈/sub〉 of 7.61. Metal adsorption experiments indicate that 〈em〉Synechococcus〈/em〉 can bind considerable concentrations of Zn, Cu, Ni, and Co at pH 8. When all four metals are simultaneously added to solution, the same adsorption pattern of Zn 〉 Cu 〉 Ni 〉 Co is maintained, and accurately predicted by the SCM. Based on average marine cell densities and turnover rates of 〈em〉Synechococcus〈/em〉 cells in the photic zone, we calculate that 〈em〉Synechococcus〈/em〉, in the absence of competing ligands such as dissolved organic matter (DOM), has the theoretical capacity to remove nearly all of the free metal cations from seawater. These observations highlight the surface reactivity of marine cyanobacteria as a potentially important vector for the transfer of dissolved metals from the photic zone to deeper waters or the seafloor in modernoceans, but they also have implications for the Precambrian oceans as sinking cyanobacteria could have acted as an exit channel for trace elements into ancient sediments including banded iron formations (BIF).〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 22 September 2017〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Melita Peharda, Julien Thébault, Krešimir Markulin, Bernd R. Schöne, Ivica Janeković, Laurent Chauvaud〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉High-resolution stable-isotope ratio data (δ〈sup〉18〈/sup〉O, δ〈sup〉13〈/sup〉C) were used to study growth strategies of two bivalve species, 〈em〉Pecten jacobaeus〈/em〉 (calcitic shell) and 〈em〉Glycymeris pilosa〈/em〉 (aragonitic shell) from the North Adriatic Sea. The principal objectives of this study were to identify the period of the year when the growth line is formed in the shell of two target species, to identify the main growing season of these two species, to identify the environmental drivers of shell growth, and to evaluate the potential applicability of δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C values for the reconstruction of environmental variability. Samples were collected from the North Adriatic Sea by commercial bean trawl (〈em〉P. jacobaeus〈/em〉, December 2013 and January 2014, N = 4) and SCUBA diver (〈em〉Glycymeris pilosa〈/em〉, March 2016, N = 3). Samples for the oxygen (δ〈sup〉18〈/sup〉O) and carbon (δ〈sup〉13〈/sup〉C) isotope composition of the calcium carbonate were collected by drilling the outer shell layer across several annual cycles. Temporal and spatial temperature and salinity values inside the investigated area were simulated using the 3D numerical ocean model - ROMS. The δ〈sup〉18〈/sup〉O cycles corresponded to the number of seasonal growth marks observed on the external shell surface of both target species, thereby confirming the annual periodicity of these growth patterns. In February 2012, extreme cooling of the water column accompanied by dense water formation occurred in the Adriatic Sea - an event recorded by 〈em〉P. jacobaeus〈/em〉 shells. This study indicates that 〈em〉P. jacobaeus〈/em〉 and 〈em〉G. pilosa〈/em〉 have contrasting shell growth strategies. 〈em〉Pecten jacobaeus〈/em〉 grows during winter and slows shell growth during the warmest part of the year, and thereby may be an interesting archive for winter conditions. Due to its longevity and continuous growth during the warmest part of the year, 〈em〉G. pilosa〈/em〉 is a promising archive for the reconstruction of summer seawater temperatures.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 8 January 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Thomas J. Geeza, David P. Gillikin, David H. Goodwin, Scott D. Evans, Thomas Watters, Nathaniel R. Warner〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Freshwater bivalve shells may record inter-annual fluctuations in water chemistry, which in turn may archive variations in solute load due to pollution or discharge events. Here, using weekly surface water chemistry collected at two locations and shells of 〈em〉Lampsilis cardium〈/em〉 grown in the surface water we investigate if the Mg/Ca, Mn/Ca, Sr/Ca, and Ba/Ca ratios measured at high resolution in the shells on an intra-annual time scale consistently reflect the chemistry of the surface water. Two relationships appear highly correlated, Sr/Ca〈sub〉carb〈/sub〉 and Sr/Ca〈sub〉water〈/sub〉, and Mn/Ca〈sub〉carb〈/sub〉 and temperature. Ba/Ca〈sub〉carb〈/sub〉 correlated strongly with Ba/Ca〈sub〉water〈/sub〉 in one shell but temperature in another shell yielding inconclusive results. Mn/Ca〈sub〉carb〈/sub〉 and Mg/Ca〈sub〉carb〈/sub〉 were not strongly correlated with elemental ratios in the water, but Mn/Ca〈sub〉carb〈/sub〉 did show variable, weak to moderate correlation with pH〈sub〉.〈/sub〉 We found consistent partition coefficient (D〈sub〉Me〈/sub〉) values between individual shell samples grown in the same water with mean D〈sub〉Mg〈/sub〉 = 0.0006, D〈sub〉Mn〈/sub〉 = 17, D〈sub〉Ba〈/sub〉 = 0.13, and D〈sub〉Sr〈/sub〉 = 0.176. Sr/Ca〈sub〉carb〈/sub〉 and Sr/Ca〈sub〉water〈/sub〉 exhibited the strongest, most consistent correlation with partitioning coefficients that were consistent with published values, suggesting Sr/Ca〈sub〉carb〈/sub〉 can be used as a proxy for Sr/Ca〈sub〉water〈/sub〉 in freshwater mussels.〈/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-S000925411830007X-fx1.jpg" width="211" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Sophie M. Green, Jennifer A.J. Dungait, Chenglong Tu, Heather L. Buss, Nicole Sanderson, Simon J. Hawkes, Kaixiong Xing, Fujun Yue, Victoria L. Hussey, Jian Peng, Penny Johnes, Tim Barrows, Iain P. Hartley, Xianwei Song, Zihan Jiang, Jeroen Meersmans, Xinyu Zhang, Jing Tian, Xiuchen Wu, Hongyan Liu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Covering extensive parts of China, karst is a critically important landscape that has experienced rapid and intensive land use change and associated ecosystem degradation within only the last 50 years. In the natural state, key ecosystem services delivered by these landscapes include regulation of the hydrological cycle, nutrient cycling and supply, carbon storage in soils and biomass, biodiversity and food production. Intensification of agriculture since the late-20th century has led to a rapid deterioration in Critical Zone (CZ) state, evidenced by reduced crop production and rapid loss of soil. In many areas, an ecological ‘tipping point’ appears to have been passed as basement rock is exposed and ‘rocky desertification’ dominates. This paper reviews contemporary research of soil processes and ecosystems service delivery in Chinese karst ecosystems, with an emphasis on soil degradation and the potential for ecosystem recovery through sustainable management. It is clear that currently there is limited understanding of the geological, hydrological and ecological processes that control soil functions in these landscapes, which is critical for developing management strategies to optimise ecosystem service delivery. This knowledge gap presents a classic CZ scientific challenge because an integrated multi-disciplinary approach is essential to quantify the responses of soils in the Chinese karst CZ to extreme anthropogenic perturbation, to develop a mechanistic understanding of their resilience to environmental stressors, and thereby to inform strategies to recover and maintain sustainable soil function.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 April 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Zita Kelemen, David P. Gillikin, Steven Bouillon〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Both climate change and land-use changes represent serious environmental issues throughout Africa, and are expected to lead to significant changes in river hydrology and geochemistry. Freshwater bivalve shell geochemistry may provide useful information about past river characteristics (discharge, geochemistry) providing a baseline to help understand modern environmental changes. Shells of two species of freshwater mussels (〈em〉Chambardia wissmanni〈/em〉 and 〈em〉Aspatharia dahomeyensis〈/em〉) were collected from two large African rivers (Oubangui and Niger Rivers) and were analyzed for Sr/Ca, Ba/Ca, Mg/Ca and Mn/Ca ratios along their maximal growth axis. Shell data were compared with geochemical data collected in these rivers every two weeks over a two-year period. To verify whether element ratios in shells were related to the host water, the elemental concentrations measured in the shells were set in a temporal context based on δ〈sup〉18〈/sup〉O〈sub〉shell〈/sub〉 cycles. Shells from the Oubangui River to some extent recorded water element ratios, with variations in water Sr/Ca ratios being reflected in all three bivalve specimens analyzed, while only the youngest specimen exhibited complete coverage. The two older specimens recorded only segments of the water data, which suggests an influence of growth rate. This relationship was not observed in 〈em〉A. dahomeyensis〈/em〉 shells from the Niger, suggesting a species-specific relationship between freshwater shells and water. The Ba/Ca and Mg/Ca record showed no similarity between the water elements and analyzed shells, but an ontogenetic decrease was observed in older specimens. Shell Mn/Ca ratios tracked pH in shells from the Oubangui, with a time-lag, which might be due to bivalves taking up the increased Mn via phytoplankton which are expected to show a similar time-lag. This relationship was not observed in shells from the Niger, where due to the high turbidity throughout the year we expect very low primary production rates. In conclusion, no consistent relationships between water chemistry and shell chemistry were observed. While Sr/Ca and Mn/Ca ratios in shells of 〈em〉C. wissmanni〈/em〉 from the Oubangui were most promising as proxies of water chemistry, the utility of element ratios as tracers is compromised by species-specific effects or by site-specific phenomena.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 29 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Florian Bulle, Daniela Rubatto, Giovanni Ruggieri, Cindy Luisier, Igor M. Villa, Lukas Baumgartner〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Microscale oxygen isotope analysis (〈sup〉18〈/sup〉O/〈sup〉16〈/sup〉O) of minerals can identify distinct events of fluid-rock interaction. This method is, however, still limited to a few major and accessory minerals of which most are anhydrous minerals. We present the systematic study of oxygen isotope distribution in white mica by Secondary Ion Mass Spectrometry (SIMS). Texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the Larderello-Travale geothermal field (LTGF), Italy, record stages of magmatic crystallization, metamorphic-hydrothermal replacement and fluid-rock interaction. The large range in intra- and inter-grain white mica δ〈sup〉18〈/sup〉O values between 1 and 14‰ reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180–450 °C present-day temperatures; p.d.T.). This variability reflects the large-scale circulation of both (1) magmatic, 〈em〉syn〈/em〉-intrusive to early contact metamorphic hydrothermal fluids with high-δ〈sup〉18〈/sup〉O values, and (2) meteoric fluids with δ〈sup〉18〈/sup〉O values of −7‰ during a post-intrusive, late hydrothermal stage.〈/p〉 〈p〉Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (μm-scale), including detrital grains (δ〈sup〉18〈/sup〉O = 12–14‰; high Na, low Mg), partially altered white mica (δ〈sup〉18〈/sup〉O = 8–9‰) and late hydrothermal white mica (1–6‰; low Na, mid Mg). The late hydrothermal white mica has similar δ〈sup〉18〈/sup〉O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a δ〈sup〉18〈/sup〉O of −6 to 0.5‰, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous δ〈sup〉18〈/sup〉O values of ca. 9‰ likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 °C), or (2) a large spread in δ〈sup〉18〈/sup〉O from 2 to 12‰ within and across grains of variable texture and chemistry in the host rock and a cross-cutting quartz-white mica vein (ca. 300 °C, present day temperature; hereafter p.d.T.). This contrasting δ〈sup〉18〈/sup〉O signature of white mica is also recorded in granite cored at up to 4.6 km depth. The Carboli granite contains white mica with a homogeneous magmatic δ〈sup〉18〈/sup〉O of 10 ± 0.6‰, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in δ〈sup〉18〈/sup〉O from 4 to 10‰. A pronounced intra-grain δ〈sup〉18〈/sup〉O variability of up to 6‰ occurs in white mica domains with higher Fe-Mg-Ti halos around inclusions of chloritized biotite, as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4.6 km. In the Porto Azzurro granite on Elba, Italy, altered white mica has δ〈sup〉18〈/sup〉O values of 2.6‰ down from 10‰ in unaltered grains.〈/p〉 〈p〉The distribution of oxygen isotope ratios in white mica (“〈sup〉18〈/sup〉O/〈sup〉16〈/sup〉O retentivity”) is thus firstly a result of pervasive versus selective fluid alteration (at depth, sample and grain scale). Secondly, the actual preservation of these μm-scale variabilities indicates that volume diffusion is not detectable at microscale at p.d.T at or below 350 °C where most of the heterogeneous white mica is found. Selective, sample- and grain-scale fluid penetration occurs episodically and anisotropically, on micro- and megascale, along faults, fractures and cleavages, producing lower δ〈sup〉18〈/sup〉O white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 23 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Wenjing Wang, Marie-Camille Caumon, Alexandre Tarantola, Jacques Pironon, Wanjun Lu, Yahao Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Density of fluid inclusions is one of the most important parameters to reconstruct the 〈em〉P〈/em〉-〈em〉T〈/em〉 conditions of entrapment of geo-fluids. Co-existing with microthermometry methods, Raman spectroscopy also became a useful tool to determine the density of CO〈sub〉2〈/sub〉-bearing inclusions via Fermi diad splitting (Δ). In this study, a new CO〈sub〉2〈/sub〉 densimeter which takes into account the effects of temperature, pressure and H〈sub〉2〈/sub〉O (±NaCl) on Fermi diad splitting was built from experiments using a High Pressure Optical Cell. Raman spectra of CO〈sub〉2〈/sub〉 were collected in the range −20 to 360 °C and 0.51 to 50 MPa, i.e. a density range from 0.0057 to 1.1600 g·cm〈sup〉−3〈/sup〉. Our results show that temperature and pressure have significant impact on Δ-density relationship, with Δ lower by up to about 0.5 cm〈sup〉−1〈/sup〉 with increasing pressure and temperature at a given density. On the contrary, dissolved H〈sub〉2〈/sub〉O (±NaCl) had no effect on the Δ-density relationship, so the function defined for pure CO〈sub〉2〈/sub〉 system can also be applied to calculate the density of CO〈sub〉2〈/sub〉-H〈sub〉2〈/sub〉O-NaCl system (up to 360 °C, 50 MPa and 20.35 wt% NaCl). The new densimeters presented in this study provide a useful tool to determine the pressure of CO〈sub〉2〈/sub〉-rich fluid inclusions all along the isochoric path for any water concentration.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 29 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Kohen W. Bauer, Devon B. Cole, Dan Asael, Roger Francois, Stephen E. Calvert, Simon W. Pouton, Noah J. Planavsky, Sean A. Crowe〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hydrothermal chromium (Cr) cycling contributes to marine Cr inventories and their Cr isotopic composition, yet Cr isotope effects associated with this cycling remain poorly documented. Here we determine the distribution, isotopic composition, and diagenetic mobility of Cr in hydrothermal sediments from the distal flank of the South East Pacific Rise (SEPR, DSDP-site 598). We find that Cr is primarily associated with the metalliferous iron (oxyhydr) oxide and detrital components of the sediment (0.4–3.6 mg kg〈sup〉−1〈/sup〉), whereas Cr concentrations are much lower in the dominant carbonate phase (〈0.03 ± 0.2 mg kg〈sup〉−1〈/sup〉). The Cr:Fe ratio of the metalliferous component, however, decreases with increasing depth below the sediment water interface, with an apparent loss of 〉80% Cr from the sediment relative to Fe. We propose this loss is tied to oxidation of authigenic Cr(III) to Cr(VI) followed by diagenetic remobilization and efflux from the sediment pile. The bulk δ〈sup〉53〈/sup〉Cr composition of the SEPR sediments is isotopically light (−0.24 to −0.57 ± 0.05‰) and the authigenic δ〈sup〉53〈/sup〉Cr is as light as −1.2 ± 0.2‰, and we argue that this light Cr isotopic composition results from the partial reduction of oxic seawater-bearing Cr(VI) by reduced hydrothermal vent fluids enriched in Fe(II)〈sub〉aq〈/sub〉. Diagenetic oxidation of the reactive Cr pool by Mn-oxides and loss of Cr(VI) from the sediment may further deplete the sediment in 〈sup〉53〈/sup〉Cr during diagenesis. The δ〈sup〉53〈/sup〉Cr composition of the detrital Cr fraction of the sediment (average δ〈sup〉53〈/sup〉Cr composition = −0.05 ± 0.04‰) falls within the igneous silicate earth (ISE) range, revealing that detrital Cr delivered to this region of the Pacific ocean is unfractionated, and has carried a relatively constant δ〈sup〉53〈/sup〉Cr composition over the last 5.7 million years. Together our results show that light δ〈sup〉53〈/sup〉Cr compositions in hydrothermal sediments are imparted through a combination of processes previously overlooked in the marine Cr biogeochemical cycle, and that the δ〈sup〉53〈/sup〉Cr composition of such sediments may provide a rich source of information on paleo-marine redox conditions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 20 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Wei Sun, Shijin Zhao, Hongye Pei, Huan Yang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The southeast Qaidam Basin (SQB) lies near the boundary between the modern-day Asian summer monsoon and mid-latitude westerly jet, and the paleoclimate variation in this region can be affected by these two atmospheric circulation systems. Reconstructions of paleotemperature and palaeohydrology are therefore critical to constraining the driving forces of climate in this region, where the ecological environment is fragile. Here, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs), microbial membrane lipids occurring ubiquitously in aquatic and terrestrial environments across the globe, in an aeolian sediment profile over the last 7000 years from Xiangride Town (XRD) of southeast Qaidam Basin (SQB), China. The temperature record was generated using the global calibration of MAT〈sub〉mr〈/sub〉 based on branched GDGTs (brGDGTs), whereas the palaeohydrological condition was reconstructed from the relative abundance of isoprenoid GDGTs vs. brGDGTs (R〈sub〉i/b〈/sub〉), the cyclisation index of brGDGTs (CBT), and the brGDGT-based pH indices. The results show that the paleoclimate during the mid-Holocene was relatively warmer and wetter in the SQB. Afterwards it was a trend to a cold and dry climate. Temperature variation was highly coupled with the moisture change during the mid- to late Holocene, as opposed to some previous studies showing a warm-dry and cold-wet climate pattern in the northern Qaidam basin. The palaeohydrological evolution agrees with the precipitation pattern of the Chinese loess plateau but opposes to that in Northeastern China, the middle reaches of Yangtze river and Arid Central Asia (ACA), implying that the Asian summer monsoon strength is the driving force of precipitation evolution during the mid- to late Holocene in the SQB and the spatial heterogeneity of the mid- to late Holocene precipitation pattern across China is remarkable. A rapid cold and drought event at around 4 ka before present (BP) was identified, which might be a key factor that caused the decline in the agricultural civilization during the late Neolithic Age in northwestern China.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 21 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Honghua He, Chenbin Fan, Qi Peng, Miaomiao Wu, Jiyong Zheng, Gao-Lin Wu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Coal fly ash (CFA) is often used as a soil amendment to improve the physicochemical properties of the soil. However, CFA contains abundant rare earth elements (REEs), which may negatively affect plant growth and physiology and cause environmental hazards. We performed pot experiments to grow alfalfa (〈em〉Medicago sativa〈/em〉 L.) and erect milkvetch (〈em〉Astragalus adsurgens〈/em〉 Pall.) in soils treated with different rates of CFA, in order to investigate the impacts of CFA on the accumulation and translocation of REEs such as lanthanum (La), cerium (Ce), scandium (Sc) and yttrium (Y). The results showed that La, Ce, Sc and Y concentrations were considerably higher in the CFA than in soils, but adding CFA to soils did not always significantly increase their concentrations in plants. The element abundance in soils and CFA always followed the order of Ce 〉 La 〉 Y 〉 Sc, but the element bioaccumulation factor was generally in the order of Sc 〉 Y 〉 La 〉 Ce. In plants, La, Ce, Sc and Y concentrations generally followed the order of root 〉 shoot and leaf 〉 stem, with Ce concentrations being always higher than La and Y concentrations. Most of these elements absorbed were sequestered in roots without being further transported to the aerial parts. The root-to-shoot translocation factor of Sc was the highest, but the stem-to-leaf translocation factor of Sc was the lowest among the elements studied. The biogeochemical behaviours of Sc were quite different from those of La, Ce and Y. Adding CFA to the soils often considerably increased the amounts of La, Ce, Sc and Y accumulated in shoots due to significant biomass increase. Furthermore, REE speciations in the topsoil can be changed by both CFA addition and plant recycling. Therefore, land application of CFA for revegetation and soil reclamation proposes can affect the biogeochemical behaviours of La, Ce, Sc and Y.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 19 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Anaïs Pagès, Steve Barnes, Susanne Schmid, Chris Ryan, David Paterson, Colin MacRae, Jamie Laird, Margaux Le Vaillant, Haifeng Fan, Hanjie Wen〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Early Cambrian black shales of the Niutitang Formation are found across the Yangtze Platform in a 1600-km belt extending across south China. A thin organic matter-rich layer in the lowermost part of this formation contains exceptional concentrations in Mo, Ni, Se, Re, Os, As, Hg, Sb, Ag, Au, Pt, Pd, and Ag. Due to their extreme metal enrichment, these black shales provide a rare opportunity to study the interface between semi-metals, metals and biogenic material. We report the first detailed μm-scale investigation of metal distributions and associations in samples from two sites, Zunyi and Sancha, located hundreds of km apart, using a combination of analytical techniques including X-ray fluorescence (XRF) mapping, scanning electron microscopy (SEM), Synchrotron-based XRF mapping, electron probe micro-analyser (EPMA), particle induced x-ray emission (PIXE) probe and x-ray absorption near-edge structures (XANES) imaging. Strong μm-scale variations in metal and semi-metal distributions were highlighted by Synchrotron-based XRF mapping of samples from both sites. At both locations, U is present within phosphorite nodules. Arsenic, Mo and Se are particularly abundant in the organic-rich matrix, showing strong associations with organic matter. Nickel, however, shows different distributions between the two sites. It is mainly present in association with the organic matter at the Sancha site while it is found in abundance in millerite at the Zunyi site, suggesting slightly different local conditions at Zunyi favouring Ni-S associations over Ni-organic matter. At both sites, biogenic structures were re-mineralised with laminations dominated by different metals, indicating a likely control of organic matter over metal distributions. In addition, the XANES imaging highlighted different redox states of As over μm-scale areas. While As〈sup〉−1〈/sup〉 appears to be mainly present in pyrite, As〈sup〉+3〈/sup〉 was mainly detected in association within the carbon and MoS〈sub〉2〈/sub〉 mixed layer phase (MoSC). Overall, the present results emphasize the complexity of metal associations in this mineralised layer, the significant role of organic matter in the accumulation and precipitation of metals and semi-metals in these metalliferous shales and highlight how biogenic activity can induce μm-scale variations in redox conditions in sediments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 528〈/p〉 〈p〉Author(s): A. Villaros, M. Pichavant〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉We constrain the genetic relation between granite and pegmatite parental melts from the Variscan Saint Sylvestre leucogranite complex and associated pegmatite bodies (Massif Central, France) through compositions of micas. Using mica trace element concentrations and available partition coefficients for Li, Rb, Ba, Cs and F, we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas. Biotite in leucogranites and pegmatites ranges mostly from Fe-biotite to siderophyllite. More evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite. White micas have homogeneous compositions, from muscovite to Li-phengite. In granites, biotite and muscovite trace element distributions are clustered. In pegmatites, mica trace element contents globally follow differentiation from the least to the most evolved body. The reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in Li, Rb, Cs and depletion in Ba that suggest a similar origin. However, micas are shown to have selectively equilibrated with the last melt (or fluid) in contact and so trace element concentrations of early magmatic liquids are rarely preserved. Inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization. Leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent-daughter genetic relation. The data and the modelling suggest that they represent non-cogenetic melts generated by discrete episodes of partial melting. Differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt/magma. We suggest that both the leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 17 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Xudong Wang, Germain Bayon, Jung-Hyun Kim, Dong-Hun Lee, Dahae Kim, Bleuenn Guéguen, Marie-Laure Rouget, Jean-Alix Barrat, Laurent Toffin, Dong Feng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Seeping of methane-rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates. Some trace elements play an important role in the biogeochemical processes operating at cold seeps, especially as specific enzymatic co-factors related to methanogenesis and the anaerobic oxidation of methane (AOM). However, it is unclear whether microbial trace metal utilization can be traced by the geochemical composition of seep carbonates. In this study, we analyzed a series of authigenic carbonate samples recovered from various seep settings worldwide and report for the first time trace element concentrations for total lipid fractions, combined with biomarker analyses and determination of elemental abundances in associated inorganic mineral phases (carbonate phases, sulfides, organic compounds and detrital fractions). Our results indicate marked enrichments of Co, Ni, Cu, Mo and W in the archaeal and bacterial lipids associated with authigenic carbonates, which can all be ascribed to previously identified enzymatic pathways. In addition to the microbial communities involved in AOM, which most likely control specific lipid-bound enrichments of Co, Ni, Mo and W in seep carbonates, Cu was found to display higher concentrations in the lipid fractions extracted from a few authigenic carbonate samples formed closer to the sediment-water interface, hence possibly related to the presence of aerobic methane-oxidizing bacterial assemblages in the near seafloor environment. While the above mentioned trace metals are relatively enriched in all studied inorganic and organic fractions, the very low W concentrations measured in carbonate phases, combined with their pronounced enrichment in associated lipid fractions and inferred microbial requirement, suggest that tungsten depletion in pore waters could possibly act as a limiting factor on AOM at cold seeps. Finally, two other trace elements (Li and Ti) also displayed particular enrichments in studied lipid fractions, which, despite no reported evidence, could possibly indicate that they are also involved as metalloenzymes in microbial methane oxidation processes at cold seeps.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 528〈/p〉 〈p〉Author(s): John B. Creech, Frédéric Moynier, Christian Koeberl〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven tektites, representing three of the four known tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 〈sup〉122〈/sup〉Sn〈sup〉/118〈/sup〉Sn ratio) in tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten tektite droplets. An Australasian Muong Nong-type tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 528〈/p〉 〈p〉Author(s): Steven J. Jaret, R. Scott Harris, E. Troy Rasbury〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Impact ejecta deposits have the potential for being ideal stratigraphic markers. Identifying impact horizons, however, is done by the identification of shock-metamorphic minerals. However, such work is time consuming, often requiring searching through hundreds of grains, and is therefore not feasible when searching through extensive stratigraphic sections. Alternatively, impact horizons are frequently identified by the presence of Ir anomalies. This is typically done through either Ni Fire Assay and column chemistry or by Instrumental Neutron Activation Analyses. Both of which are time consuming and/or require specialized training. Here we present results of our new technique for measuring Ir without Fire Assay or INAA. ~90 mg of sample is dissolved in HF + Nitric followed by a mixture of HCl and Nitric. Analyses are conducted on a quadrupole mass spectrometer, using Indium to correct for instrumental drift and standard addition to avoid matrix complication. We have tested this technique with a mix of both Cody Shale doped with known amounts of Ir and with naturally Ir-rich samples. This technique is capable of identifying anomalies as low as ~1 ppb. While other geochemical techniques can provide more precise concentrations, particularly at lower concentration, the ease of this technique is well suited as a first-pass screening tool for identifying impact horizons in large stratigraphic sections.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Lei Yu, Leyla M. Daniels, Josephina J.P.A. Mulders, Giuseppe D. Saldi, Anna L. Harrison, Li Liu, Eric H. Oelkers〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Coupled gypsum dissolution-calcium carbonate precipitation experiments were performed in closed system reactors and in the presence of either aqueous 0.1 M Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉, 0.1 M NaHCO〈sub〉3〈/sub〉 or 0.1 M Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉 + 0.2 M NaOH solutions. Gypsum dissolved immediately at the start of each experiment provoking the precipitation sequentially of vaterite, calcite and trace aragonite. Fine-grained amorphous carbon carbonate may also be present shortly after each experiment began. Each experiment approached equilibrium within 119 h leading to the maximum possible transformation of gypsum to calcite over this time frame. The rapid transformation of gypsum to calcite in these experiments suggests a similar rapid transformation of gypsum or anhydrite into calcite could occur during subsurface carbon storage efforts where evaporites are present. Evaporite deposits could thus potentially be used for carbonation if sufficient alkalinity is available to neutralize the acid liberated by the gypsum carbonation reaction. Due to a negative volume of the gypsum or anhydrite to calcite transformation, however, the carbonation of these minerals will potentially damage the integrity of evaporite caprocks.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 528〈/p〉 〈p〉Author(s): Jenna Poonoosamy, Christoph Westerwalbesloh, Guido Deissmann, Mohamed Mahrous, Enzo Curti, Sergey V. Churakov, Martina Klinkenberg, Dietrich Kohlheyer, Eric von Lieres, Dirk Bosbach, Nikolaos I. Prasianakis〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Dissolution and precipitation processes control fundamental properties of porous media in the subsurface such as porosity and permeability thus affecting ground water flow and solute transport. These processes occur at different time and length scales, and are often not easily captured by a single macroscopic continuum scale reactive transport model. Precipitation of even small amounts of solid phases along the major transport pathways may result in substantial change of rock permeability. A mechanistic description of these processes in the pore space is therefore a prerequisite. In order to bridge atomistic and macroscopic scales, it is necessary to understand the processes and subsequently to upscale the pore-scale results to macroscopic simulation codes. In this direction, we develop a methodology combining a sophisticated microfluidic experimental setup and multiscale modelling numerical diagnostics. This allows exploring the processes of mass transport coupled to nucleation, mineral precipitation and dissolution in confined space. For this purpose, a system with relatively few chemical species, but with well-defined kinetic parameters was chosen. As a result of the advective and diffusive mixing of solutions of Sr〈sup〉2+〈/sup〉 and SO〈sub〉4〈/sub〉〈sup〉2−〈/sup〉 ions, celestine precipitated inside the microfluidic chambers. The reactions inside the microfluidic experiment were marked by 4 stages: the induction period, the crystal growth, the clogging which prevented the mixing of the reactant solutions, and finally the dissolution of celestine crystals. The growth and dissolution rates of celestine inside the chambers were found to be in good agreement with literature data obtained in bulk experiments. The pore scale modelling of the processes inside the microfluidic reactor was based on the Lattice Boltzmann method and was used to spatially resolve the solution composition (not accessible experimentally), to predict the induction time, and to calculate the local saturation indices using full speciation. The information gained from the cross-scale pore-level model helped to gain insights into the underlying processes and to explain the preferential growth direction of the crystals during the experiment. The combination of microfluidic lab-on-chip experiments with complementary multiphysics numerical diagnostics is a promising methodology to provide insights into pore scale phenomena, to optimize the design of experiments and to parameterize appropriate constitutive equations for coupled processes in porous media.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 3 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Matthew S. Fantle, Elizabeth Griffith〈/p〉

Description: 〈p〉Publication date: Available online 13 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): A.E.S. Van Driessche, T.M. Stawski, M. Kellermeier〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The solution-mediated formation of calcium sulfate minerals, i.e. gypsum, anhydrite and bassanite, is a common process in both natural and engineering settings. It plays a key role in the global sulfur cycle and serves as an indicator of past environmental conditions on Earth and Mars. Products relying on the crystallization of these minerals have been employed since antiquity, and today they are an essential part of a wide array of industrial applications. Accordingly, the fundamental aspects of calcium sulfate mineralization have been the focus of intensive research during the past century. However, a recent flurry of studies addressing alternative, i.e. non-classical, nucleation and growth mechanisms has spurred a revisit of the precipitation pathway of the most common phase, gypsum. The newly obtained data sketch a far more complex picture of the mineralization process than previously assumed. This has important consequences for the interpretation of calcium sulfate deposits, both from a geochemical and industrial point of view. In order to shed light on this issue, we discuss in this review both recent and long-standing observations of abiotic formation routes of calcium sulfate minerals as a function of the physicochemical solution properties. By integrating both the classical and non-classical perspectives on crystallization we put forward a unified model for calcium sulfate crystallization. Using this model, we (re)-evaluate the phase stability and transformations taking place in the CaSO〈sub〉4〈/sub〉-H〈sub〉2〈/sub〉O system. Next, we look into the formation of calcium sulfate minerals occurring in close association with the biosphere. Employing the abiotic case scenario as a benchmarking tool, the possible influence and/or control exerted by biological activity (and its byproducts) on the precipitation pathway is critically reviewed. Finally, we point out the central issues that need to be resolved if we wish to fully understand and control the formation of calcium sulfate solids in natural and engineering environments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 528〈/p〉 〈p〉Author(s): Jinlong Wang, Qiangqiang Zhong, Mark Baskaran, Jinzhou Du〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The long-lived progeny of radon-222, lead-210 (〈sup〉210〈/sup〉Pb), polonium-210 (〈sup〉210〈/sup〉Po) and bismuth-210 (〈sup〉210〈/sup〉Bi), serve as a tool in the study of marine biogeochemical processes. Here, we report our investigations on the partitioning of 〈sup〉210〈/sup〉Pb, 〈sup〉210〈/sup〉Po and 〈sup〉210〈/sup〉Bi (〈sup〉207〈/sup〉Bi) between seawater and solids (twelve commonly-occurring mineral particles) under different values of pH, salinity and contact time. The K〈sub〉d〈/sub〉 values of 〈sup〉210〈/sup〉Pb and 〈sup〉207〈/sup〉Bi for MnO〈sub〉2〈/sub〉 varied widely between the synthetic and natural minerals, and thus the suitability of using synthetic material for their partitioning studies in marine system needs to be re-assessed. The K〈sub〉d〈/sub〉 values of 〈sup〉210〈/sup〉Pb in clay minerals were found to be in the order: smectite 〉 illite 〉 kaolinite. The sorption efficiencies (=the particulate activity divided by the sum of particulate and dissolved activity) of 〈sup〉210〈/sup〉Pb, 〈sup〉207〈/sup〉Bi and 〈sup〉210〈/sup〉Po on illite were above 81% when the contact time was 〉2 h. These radionuclides were more soluble when pH was ≤5, but show more particle-reactive nature for pH of 6 to 8. The fractionation factor between 〈sup〉210〈/sup〉Po and 〈sup〉210〈/sup〉Pb (F〈sub〉Po/Pb〈/sub〉) varied with salinity for different mineral components while the F〈sub〉Bi/Pb〈/sub〉 was less affected by mineral components, salinity and pH. The in-situ experimental results showed that the F〈sub〉Bi/Pb〈/sub〉 increased with increase in Chl-a concentration. Considering the much shorter half-life of 〈sup〉210〈/sup〉Bi (5.0 d) and the high K〈sub〉d〈/sub〉 values established in this study, we propose that 〈sup〉210〈/sup〉Bi/〈sup〉210〈/sup〉Pb activity ratio could potentially be used to estimate the particle export from phytoplankton bloom on time scale much shorter than 〈sup〉234〈/sup〉Th or 〈sup〉210〈/sup〉Po.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 8 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): G. Borghini, E. Rampone, A. Zanetti, C. Class, P. Fumagalli, M. Godard〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Deep melt intrusion and melt-peridotite interaction may introduce small-scale heterogeneity in the MORB mantle. These processes generate pyroxenite-bearing veined mantle that represent potential mantle sources of oceanic basalts. Natural proxies of such veined mantle are very rare and our understanding of mechanisms governing the chemical modification of mantle peridotite by MORB-type pyroxenite emplacement is very limited. We report the results of detailed spatially-controlled chemical profiles in pyroxenite-peridotite associations from the Northern Apennine ophiolitic mantle sequences (External Liguride Units, Italy), and investigate the extent and mechanism driving the local modification of peridotite by the interaction with pyroxenite-derived melt. Pyroxenites occur as cm-thick layers parallel to mantle tectonite foliation and show diffuse orthopyroxene-rich reaction rims along the pyroxenite-peridotite contact. Relative to distal unmodified peridotites, wall-rock peridotites show i) modal orthopyroxene enrichment at the expense of olivine, ii) higher Al, Ca, Si contents and slightly lower X〈sub〉Mg〈/sub〉, iii) Al-richer spinel and lower-X〈sub〉Mg〈/sub〉 pyroxenes. Clinopyroxenes from wall-rock peridotites exhibit variable LREE-MREE fractionation, always resulting in Sm〈sub〉N〈/sub〉/Nd〈sub〉N〈/sub〉 ratios lower than distal peridotites. From the contact with pyroxenite layers, peridotite clinopyroxenes record a REE compositional gradient up to about 15 cm marked by an overall REE increase away from the pyroxenite. Beyond 15 cm, and up to 23 cm, the MREE and HREE content decreases while the LREEs remain at nearly constant abundances. This REE gradient is well reproduced by a two-step numerical simulation of reactive melt percolation assuming variable amounts of olivine assimilation and pyroxene crystallization. Percolative reactive flow at decreasing melt mass and rather high instantaneous melt/peridotite ratio (initial porosity of 30%), combined with high extents of fractional crystallization (i.e. relatively low M〈sub〉a〈/sub〉/M〈sub〉c〈/sub〉 ratio), accounts for the overall REE enrichment in the first 15 cm. Change of melt-rock reaction regime, mostly determined by the drastic decrease of porosity (Φ〈sub〉i〈/sub〉 = 0.01) due to increasing crystallization rates, results in more efficient chemical buffering of the host peridotite on the HREE composition of the differentiated liquids through ion-exchange chromatographic-type processes, determining the observed increase of the LREE/HREE ratio. Emplacement of thin (cm-sized) pyroxenite veins by deep melt infiltration is able to metasomatize a much larger volume of the host peridotite. Hybrid mantle domains made by pyroxenite, metasomatized peridotite and unmodified peridotite potentially represent mantle sources of E-MORB. Results of this work stress the key role of melt-peridotite reactions in modifying the upwelling mantle prior to oceanic basalts production.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Guoqing Zhang, Zidan Yuan, Lei Lei, Jinru Lin, Xin Wang, Shaofeng Wang, Yongfeng Jia〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The redistribution and transformation of arsenic (As) during the Fe(II)-catalyzed transformation of As-adsorbed ferrihydrite remains unclear. This study investigated the re-crystallization process and products of As-adsorbed ferrihydrite accelerated by Fe(II) and the fate of arsenate and arsenite at pH 7 under anaerobic condition. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and synchrotron based X-ray absorption near edge structure (XANES) were utilized for the As and Fe speciation analysis. Results showed that both As(V) and As(III) can retard the transformation of ferrihydrite. The concentration and speciation of As strongly influenced the mineralogy and morphology of produced crystalline Fe oxyhydroxides. The higher As(V) loading (Fe/As = 40–100) resulted in the conversion of most ferrihydrite to lepidocrocite instead of goethite. The adsorbed As(III) showed weak effect on the re-crystallization of ferrihydrite at the early stage. A considerable fraction of As(III) (70–80%) was oxidized to As(V) during the Fe(II) catalyzed transformation of As(III)-adsorbed ferrihydrite. The oxidation of As(III) to As(V) in turn retarded the transformation of the remaining ferrihydrite to goethite and induced the production of a small amount of lepidocrocite. A temporary release of As(V) was observed during the first 5 days and the released As(V) was re-adsorbed back to the crystalline products, whereas no As was released to the aqueous phase in the As(III)-ferrihydrite systems. During the re-crystallization of ferrihydrite, 1 M phosphate-extractable As decreased quickly by up to 90% in the solid phase, indicating that partial As transformed to more stable phase. Our results suggested that Fe(II)-catalytic transformation of As-adsorbed ferrihydrite could help reducing the mobility and toxicity of As in anoxic aquifers.〈/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-S0009254119303705-ga1.jpg" width="328" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): L. Drugat, E. Pons-Branchu, E. Douville, L. Foliot, L. Bordier, M. Roy-Barman〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Speleothems represent a key source of information on climate variations in continental environments as they enable high temporal resolution reconstructions. The stalagmite salam3 presented in this study comes from the Salamandre Cave (SE France, Gard region). Its growth period, determined by 〈sup〉230〈/sup〉Th/〈sup〉234〈/sup〉U chronology and MOD-AGE extrapolation, falls between 10.91 ± 1.00 kyr BP and 13.43 ± 0.25 kyr BP, which corresponds to the transition between the last glacial period and the Holocene. This period is essential for the understanding of past and future climate variations.〈/p〉 〈p〉In this study, trace elements were measured along the growth axis of this stalagmite and results were compared with the elementary composition of bedrock and soil above the cave. Three periods were identified with high Li, Rb, Cs, Th and rare earth element (REE) concentrations, between 13.43 ± 0.25 and 13.11 ± 0.27, between 12.70 ± 0.34 and 12.30 ± 0.15 kyr, and between 11.31 ± 0.86 and 11.15 ± 0.87 kyr.〈/p〉 〈p〉Our results suggest that the alkali metals (alkalis) studied are either associated in the detrital phase with the clay and particle fraction (correlation with Th), or with the organic matter and colloids as complexed ligands (correlation with Mn). To attribute the origin of REE measured within the stalagmite, we compared their pattern with those of soil and bedrock, and two sources were deduced: i) periods with an enrichment in light REE and no cerium anomaly corresponding to mobilization from soil during weathering episodes such as the Bölling-Alleröd period; ii) from bedrock during a longer residence time of water in the epikarst such as during the Younger Dryas and the Holocene. Thus, as illustrated in this study, combined analysis of REE and alkalis can provide key information on soil weathering linked to climate/environmental change.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Michael W. Förster, Stephen F. Foley, Olivier Alard, Stephan Buhre〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The subduction of sediment connects the surface nitrogen cycle to that of the deep Earth. To understand the evolution of nitrogen in the atmosphere, the behavior of nitrogen during the subduction and melting of subducted sediments has to be estimated. This study presents high-pressure experimental measurements of the partitioning of nitrogen during the melting of sediments at sub-arc depths. For quantitative analysis of nitrogen in minerals and glasses, we calibrated the electron probe micro-analyzer on synthetic ammonium feldspar to measure nitrogen concentrations as low as 500 μg g〈sup〉−1〈/sup〉. Nitrogen abundances in melt and mica are used together with mass balance calculations to determine D〈sub〉N(Mica/Melt)〈/sub〉, D〈sub〉N(Fluid/Mica)〈/sub〉, and D〈sub〉N(Fluid/Melt)〈/sub〉. Calculated partition coefficients correspond to expected values for NH〈sub〉4〈/sub〉〈sup〉+〈/sup〉, which behaves similarly to Rb〈sup〉+〈/sup〉 due to its nearly identical size. Nitrogen partitioning between fluid and melt (D〈sub〉N(Fluid/Melt)〈/sub〉) and fluid and bulk residue (melt+mica) (D〈sub〉N(Fluid/Bulk)〈/sub〉) increase linearly with temperature normalized to pressure. This linear relationship can be used to calculate D〈sub〉N(Fluid/Melt)〈/sub〉 and D〈sub〉N(Fluid/Bulk)〈/sub〉 for slab melts from 800 to 1200 °C following: 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈msub〉〈mi mathvariant="normal"〉D〈/mi〉〈mrow〉〈mi mathvariant="normal"〉N〈/mi〉〈mfenced open="(" close=")"〉〈mrow〉〈mtext〉Fluid〈/mtext〉〈mo〉/〈/mo〉〈mtext〉Melt〈/mtext〉〈/mrow〉〈/mfenced〉〈/mrow〉〈/msub〉〈mo linebreak="goodbreak" linebreakstyle="after"〉=〈/mo〉〈mn〉0.012〈/mn〉〈mo linebreak="badbreak"〉×〈/mo〉〈mfrac〉〈mrow〉〈mi〉T〈/mi〉〈mspace width="0.5em"〉〈/mspace〉〈mfenced open="[" close="]"〉〈mrow〉〈mo〉°〈/mo〉〈mi mathvariant="normal"〉C〈/mi〉〈/mrow〉〈/mfenced〉〈/mrow〉〈mrow〉〈mi〉P〈/mi〉〈mspace width="0.5em"〉〈/mspace〉〈mfenced open="[" close="]"〉〈mi〉GPa〈/mi〉〈/mfenced〉〈/mrow〉〈/mfrac〉〈mo linebreak="badbreak"〉−〈/mo〉〈mn〉2.02〈/mn〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈msub〉〈mi mathvariant="normal"〉D〈/mi〉〈mrow〉〈mi mathvariant="normal"〉N〈/mi〉〈mfenced open="(" close=")"〉〈mrow〉〈mtext〉Fluid〈/mtext〉〈mo〉/〈/mo〉〈mtext〉Bulk〈/mtext〉〈/mrow〉〈/mfenced〉〈/mrow〉〈/msub〉〈mo linebreak="goodbreak" linebreakstyle="after"〉=〈/mo〉〈mn〉0.04〈/mn〉〈mo linebreak="badbreak"〉×〈/mo〉〈mfrac〉〈mrow〉〈mi〉T〈/mi〉〈mspace width="0.5em"〉〈/mspace〉〈mfenced open="[" close="]"〉〈mrow〉〈mo〉°〈/mo〉〈mi mathvariant="normal"〉C〈/mi〉〈/mrow〉〈/mfenced〉〈/mrow〉〈mrow〉〈mi〉P〈/mi〉〈mspace width="0.5em"〉〈/mspace〉〈mfenced open="[" close="]"〉〈mi〉GPa〈/mi〉〈/mfenced〉〈/mrow〉〈/mfrac〉〈mo linebreak="badbreak"〉−〈/mo〉〈mn〉8.48〈/mn〉〈/math〉.〈/p〉 〈p〉We used these partition coefficients to quantify the amount of N recycled into the mantle as 50 ± 6% of today's atmospheric N. Depending on the rate of mantle N degassing we calculated 4 different scenarios for atmospheric pN〈sub〉2〈/sub〉 evolution. All 4 scenarios estimate pN〈sub〉2〈/sub〉 to be 8–12% higher at the beginning of the Phanerozoic. These estimates diverge towards the past due to uncertainties in the mechanism and magnitude of N degassing from the mantle. Assuming degassing of N in the past was close to modern degassing rates from MORB, pN〈sub〉2〈/sub〉 was up to 40% higher at the onset of plate tectonics at 3–4 Ga. However, degassing rates were probably higher than this: assuming 10× and 20× times higher rates at the onset of plate tectonics leads to pN〈sub〉2〈/sub〉 within 20% of modern values. If N degassing from the mantle is increased to 40× the modern MORB rate, pN〈sub〉2〈/sub〉 in the Archean would have been 50% lower than today's, which is in accordance with observations from paleoatmospheric studies.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Limin Zhang, Qiang Zeng, Xi Liu, Ping Chen, Xiaoxuan Guo, Luyan Z. Ma, Hailiang Dong, Ying Huang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Microbial reduction of Fe(III) is an important biogeochemical process in anoxic or acidic environments. However, this process under oxic and pH-neutral conditions is understudied, and the iron reduction capability of the phylum 〈em〉Actinobacteria〈/em〉 is poorly known. In this study, we investigated the capacity of diverse actinobacteria to reduce Fe(III) in the presence of oxygen and at pH 7. From 277 actinobacterial strains isolated from deep sea and terrestrial soil, 109 strains (39.40%) belonging to 17 genera showed the capacity of reducing FeCl〈sub〉3〈/sub〉 under oxic and initially neutral pH conditions. Among the 109 strains, 49 spanning 13 genera also reduced ferric iron oxides under the same conditions. Four non-filamentous strains from four genera were selected for further analyses. All the four strains showed the capacity to reduce goethite under oxic and pH 7 and 8 conditions. Microbial fuel cell (MFC) experiments showed a current enhancement when the strains were cultured in the presence of goethite. Living actinobacterial cells and their metabolites played a key role in reducing Fe(III). The strains produced organic acids and siderophores, which likely promoted Fe(III) reduction due to altered reduction potentials of complexed Fe(III) and inhibition of Fe(II) oxidization by oxygen. SEM/EDS and XRD analyses detected vivianite in the culture of 〈em〉Kocuria oceani〈/em〉 FXJ8.057 with goethite, where, unexpectedly, 〈em〉K. oceani〈/em〉 FXJ8.057 produced nanowire-like structures. These results demonstrate that actinobacteria are able to reduce Fe(III) in both aqueous and solid forms under oxic and pH-neutral conditions. The mechanisms, however, still need further investigation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Kengo T.M. Ito, Yuki Hibiya, Yoshitaka Homma, Takashi Mikouchi, Tsuyoshi Iizuka〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Acid leaching has been widely used in Pb–Pb chronology especially on meteorite samples, as it enhances the precision and accuracy of the chronology by separating radiogenic Pb from non-radiogenic Pb in the samples. Yet, the detailed mechanism of the radiogenic Pb separation remains elusive. Here we report scanning electron microscopy observations of acid-leached minerals from three achondrites that were previously analyzed for pyroxene Pb〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb dating (D'Orbigny, Ibitira and Northwest Africa 6704), and one stony‑iron meteorite (Brenham). Moreover, we present chemical analysis of acid leachates and residues of pyroxene and whole-rock fractions from the three achondrites using inductively coupled plasma mass spectrometry. The combined results reveal the resistance of minerals to acid treatments with HNO〈sub〉3〈/sub〉, HCl, and HF and, in turn, allow us to explore the prospects and limitations of acid leaching for Pb〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb chronology. In particular, we show that (i) washing with dilute acids can efficiently liberate contaminant terrestrial Pb adsorbed on the mineral surface; (ii) hot and more concentrated (~6 M) HNO〈sub〉3〈/sub〉 and HCl can separate pyroxenes from sulfides and anorthitic plagioclase that are highly enriched in initial non-radiogenic Pb; (iii) albitic plagioclase and pyroxenes show limited dissolution during the HNO〈sub〉3〈/sub〉 and HCl treatments, but are progressively leached by hot 1 M HF, making it difficult to separate them from each other; and (iv) within single pyroxene grains having exsolved lamellar textures, high-Ca lamellae are more efficiently leached by the HF treatment than low-Ca ones. In the context of a two-stage Pb isotopic evolution model for pyroxene lamellae, we demonstrate that heterogeneous and incomplete dissolution of pyroxene lamellae with dilute HF can result in inaccurate Pb〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb age estimates. This may account for previously observed scattered Pb isotopic data for meteorite samples that were taken as evidence for isotope fractionation in a radiogenic Pb component during HF treatments.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Ronald J. Bakker〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The software 〈em〉AqSo_NaCl〈/em〉 (Bakker, 2018) can be used to characterize the properties of phases that may exist in the binary H〈sub〉2〈/sub〉O-NaCl system up to 1000 °C and 500 MPa. The model replaces all available correlation equations and other equation of state that are restricted to narrow temperature, pressure, and composition intervals. Liquid-vapour, liquid-solid, and vapour-solid immiscibility fields emerge with increasing salinity at relative high temperatures, and already occur at mass fractions of a few μg per gram. The liquid-vapour immiscibility is split in two fractions in solutions between 〈em〉x〈/em〉(NaCl) = 2·10〈sup〉−〈/sup〉〈sup〉5〈/sup〉 and 0.00025, at salinities that cannot be detected by microthermometry. Properties such as density, molar volume, composition, and volume fraction of liquid, vapour, and solid phases in fluid inclusions can be calculated in heating and freezing experiments. Isochores can be calculated within the homogeneous fluid field, within immiscibility fields, and along the 〈em〉SLV〈/em〉 (solid-liquid-vapour) curve. Iso-〈em〉T〈/em〉〈sub〉h〈/sub〉 lines are obtained by modifying the total volume of fluid inclusion with the volumetric properties of quartz. Bulk properties of fluid inclusions can be directly calculated from microthermometric data.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 525〈/p〉 〈p〉Author(s): Jianghanyang Li, Fan Wang, Greg Michalski, Benjamin Wilkins〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Hyper-arid areas such as the Atacama Desert accumulated significant amounts of insoluble dust and soluble salts from the atmosphere, providing minable salt deposits as well as mimicking the surface processes on Mars. The deposition rates, compositions and sources, however, were poorly constrained. Especially, the variabilities of atmospheric deposition in the Atacama Desert corresponding to a changing climate were unassessed. In this work, the atmospheric depositions collected using dust traps across a west-east elevation gradient in the Atacama (~23°S) from 1/2/2010 to 12/31/2011 were analyzed and compared to previous results in 2007–2009. The insoluble dust deposition rates in our sampling period were significantly higher than those of 2007–2009 in most dust traps, which was attributed to the changes in wind, highlighting the importance of long-term monitoring of insoluble dust fluxes. Soluble salts, instead, showed less distinct interannual variations in deposition rates, geochemical compositions or source contributions. At the coastal site (T1), soluble salts were originated from both primary sea-salt (SS) aerosols and non-sea-salt (NSS) sources such as anthropogenic emission, marine biogenic emission and biomass burning; the deposition rates of these salts largely depended on the wind speed and the amount of anthropogenic emissions. Sulfur isotopic evidence further showed that NSS sulfate at T1 was mainly originated from local SO〈sub〉2〈/sub〉 emission from local power plants. The inland sites (T2-T8) displayed much lower soluble salts deposition rates, and the salts were primarily sourced from entrainment of local surface minerals, including Na(Cl, NO〈sub〉3〈/sub〉), CaSO〈sub〉4〈/sub〉, and nitrate formed via atmospheric oxidation of anthropogenic NO〈sub〉x〈/sub〉. Sulfur isotopic compositions of sulfate deposited at T2-T8 were similar to those in local surface soil; however, three sites near the Chuquicamata mine showed slightly lower δ〈sup〉34〈/sup〉S〈sub〉sulfate〈/sub〉 values, indicating the presence of secondary sulfate originated from mining activities. The soluble salts deposited at the Andean site (T10) were dominantly from wet deposition that incorporated local lake salts, and therefore, the deposition rates were mainly controlled by the amount of precipitation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 528〈/p〉 〈p〉Author(s): Valery M. Savatenkov, Sergey V. Malyshev, Alexei V. Ivanov, Sebastien Meffre, Adam Abersteiner, Vadim S. Kamenetsky, Alexander M. Pasenko〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Lead isotope systematics is a powerful tool for the interpretation of magma sources. However, a significant problem in the study of Pb and U is their high mobility during weathering and secondary alteration of igneous rocks. Therefore, recalculations of the time-integrated lead isotopic ratios back to their initial values in ancient rocks should be approached with caution. A standard procedure for deriving initial lead isotopic ratios includes the analyses of minerals which have low U/Pb elemental ratios (e.g., potassium feldspar and/or sulfides). However, these minerals may not always be present in the sample of interest, too small and/or intergrown with other minerals, and thereby mechanically inseparable and/or altered. To circumvent the effects of secondary alteration, a conventional approach is the preliminary treatment of minerals by strong acids (e.g., concentrated HNO〈sub〉3〈/sub〉 or Aqua Regia) as well as whole-rock samples in order to remove altered components. The residual phases are then analyzed for initial lead isotopic ratios. However, this procedure does not always yield satisfactory results.〈/p〉 〈p〉In this study, we developed an advanced stepwise leaching technique in which the rock powder was first treated by 2.2 N HCl, then by more concentrated HCl, and finally by concentrated HNO〈sub〉3〈/sub〉. This procedure allows for the chemical separation of sulfides, which are dissolved by concentrated HNO〈sub〉3〈/sub〉, but unaffected by HCl. At least three leaching steps and two isotopic analyses per sample should be performed. We applied this stepwise leaching procedure to mafic intrusions of a known age (1386 ± 30 Ma) from the Mesoproterozoic Udzha paleo-rift in the northern part of the Siberian Craton. The Pb isotope results for the whole-rock and final leachate fractions are presented for three samples as well as their U, Th and Pb whole-rock data, which revealed completely closed U-Th-Pb isotopic behavior despite moderate alteration of the samples, thereby supporting the 1.38 Ga age of the magmatic event. The obtained initial Pb isotope composition of the mafic intrusion clearly indicates an origin from a long-lived enriched source.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 27 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Shawn B. Hood, Matthew J. Cracknell, Michael F. Gazley, Anya M. Reading〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Mass balance equations can be used to understand metasomatism. Here we use 2726 multi-element chemical analyses to quantify the effect of metasomatism associated with Au mineralisation at the Hamlet Gold Mine (Hamlet), Yilgarn Craton, Western Australia. Principal components analysis is first used to identify covarying elements and six groups of elements are identified: (1) relatively immobile elements, with Zr and Hf providing an appropriate reference frame for mass balance calculations; (2) elements associated with mafic rocks, such as Ca, Co, Fe, Ge, Mg, Mn, and Zn; (3) elements related to potassic alteration, Ba, Cs, K, Rb, and Tl; (4) elements related to sodic alteration, Be, Na, and Sr; (5) chalcophile elements, including Bi, weakly associated with Au; and (6) Au, Te, and S representing the main Au-mineralising event. A statistically-rigorous approach is then used to quantify element addition and depletion and provides 95% confidence limits for changes in mass. To achieve this, the basaltic host rocks are divided into four subunits based on Cr content. Sample subsets are then created to select altered rocks and least-altered protolith rocks, using discrimination diagrams. Pairings of least altered-altered samples from each basalt subunit are used to construct matrices via bootstrapping. Mass balance results from these matrices indicate similar element mobility associated with Au mineralisation across the four basalt subunits, with up to 24% mass loss. Key potassic and sodic group elements are enriched in the Au mineralised zones at the expense of elements with a mafic-association. Spatial patterns of mass change are linear and repetitive within the steeply-dipping Hamlet Shear Zone, interpreted to represent a mesh-like geometry of metasomatism within the plane of the shear zone. Localised enrichment of K, Na, and Bi are related to Au distribution while Ca depletion zones reflect the movement of reactive fluid along the shear plane.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 30 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): O.V. Vasyukova, A.E. Williams-Jones〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉In principle, potentially economic (fertile) granites can be distinguished from uneconomic (barren) granites based on the composition of their magmas. Ideally, this composition would be evaluated by analysing melt inclusions, but such inclusions are rarely preserved and difficult to analyse. Information on the composition of magmas can also be obtained from the composition of the minerals crystallising from these magmas, if the mineral-melt partition coefficients are known. In some cases, where these partition coefficients are not known, they have been calculated from lattice strain theory. Here we report arfvedsonite-melt partition coefficients for the rare earth elements (REE) calculated from the results of analyses of arfvedsonite, bulk rock and melt inclusions in granites and pegmatites in the Strange Lake peralkaline rare metal pluton. These coefficients were fitted to the lattice strain model from which values of the ideal partition coefficient (〈em〉D〈/em〉〈sub〉〈em〉0〈/em〉〈/sub〉), the ideal radius (〈em〉r〈/em〉〈sub〉〈em〉0〈/em〉〈/sub〉) and the elastic response (〈em〉E〈/em〉〈sub〉〈em〉M〈/em〉〈/sub〉) were determined. The light REE were shown to partition preferentially into the M4 site and the heavy REE into the M2 site, requiring that partition coefficients be calculated for each site. The partition coefficients for both sites were shown to decrease systematically with increasing evolution of the magma, i.e., from the early hypersolvus granite to the late pegmatites. In the case of the M4 site, 〈em〉D〈/em〉〈sub〉〈em〉0〈/em〉〈/sub〉, 〈em〉r〈/em〉〈sub〉〈em〉0〈/em〉〈/sub〉 and 〈em〉E〈/em〉〈sub〉〈em〉M〈/em〉〈/sub〉 vary linearly with the Ca content of the arfvedsonite. By contrast, these parameters for the M2 site vary linearly with temperature. The two relationships form the basis of a predictive model, in which the arfvedsonite-melt REE partition coefficients for any peralkaline granite can be estimated, provided that the Ca content of the amphibole and its crystallisation temperature are known. This model was applied to a peralkaline granitic pegmatite from the Amis complex (Namibia) for which data on the composition of the amphibole and corresponding magma (melt inclusions) have been reported. The model successfully predicts the concentrations of the various REE in this magma, thereby providing confidence that it can be used to discriminate between granites that are potentially fertile and those that are barren in respect to the REE.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 29 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Dong He, Zhenli Zhu, Lingyun Zhao, Nicholas S. Belshaw, Hongtao Zheng, Xiuli Li, Shenghong Hu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A procedure for the measurement of high precision Ca isotope ratios for calcium carbonate samples by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) is reported. Under instrumental low mass resolution mode, 〈sup〉42〈/sup〉Ca, 〈sup〉43〈/sup〉Ca and 〈sup〉44〈/sup〉Ca were measured directly without column chemistry and the measurement uncertainty was 0.08‰ (2sd, n = 32). The background interference on mass 42 u was investigated and minimized to an acceptable level by tuning the sweep gas and ICP torch position. In order to ensure the reliability of the method and quantify the tolerance of samples with incorporated Sr, the effect of Sr as an interference was evaluated for different concentrations, results showing no statistical difference with Sr/Ca 〈0.1, implying that most natural carbonate materials are accessible by this method. To validate the accuracy of this method, several stalagmite samples were analyzed in a comparative study in the Isotope Geochemistry Laboratory, Department of Earth Sciences, Oxford University. Results from both laboratories are in good statistical agreement which supports the use of this method as a simple, practical approach for Ca isotope measurement of calcium carbonate samples.〈/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-S0009254119301305-ga1.jpg" width="443" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Qibiao Sun, Jing Li, Roger D. Finlay, Bin Lian〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Ectomycorrhizal (ECM) fungi can promote soil mineral weathering through acidification and complexation by secreting oxalic acid. However, the metabolism of oxalate in the ectomycorrhizosphere and identity of the bacteria involved are still poorly understood. Here, we investigated the abundance and community structure of oxalotrophic bacteria in the ectomycorrhizospheres of 〈em〉Pinus massoniana〈/em〉 and 〈em〉Quercus serrata〈/em〉 trees using quantitative PCR and high-throughput sequencing. The secondary minerals formed in the degradation of oxalate were analyzed using SEM, XRD and HRTEM. Our results showed that oxalotrophic bacteria are abundant in the ectomycorrhizosphere (2.60–5.03 × 10〈sup〉8〈/sup〉/g soil) and that ECM fungi can influence oxalotrophic bacteria, resulting in communities that are compositionally distinct from those in non-mycorrhizosphere soils. The results also showed that approximately one third of these bacterial species were nitrogen-fixing, accounting for 43–60% of the total sequences of oxalotrophic bacteria. An oxalotrophic 〈em〉Streptomyces〈/em〉 NJ10 was isolated from the ectomycorrhizosphere of 〈em〉Pinus massoniana〈/em〉 and shown to be able to degrade calcium oxalate and induce the formation of carbonate minerals (calcite or dolomite). This study provides novel evidence that ECM fungi can enrich specific oxalotrophic bacteria in the ectomycorrhizosphere, that degrade oxalate using the oxalate-carbonate pathway, representing a potential long-term sink for photosynthetically fixed carbon derived from the atmosphere. These findings improve our understanding of the possible ecological functioning and environmental effects of plant-fungal-bacterial interactions in forests.〈/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-S0009254119301391-ga1.jpg" width="500" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Gerhard Brügmann, Michael Brauns, Roland Maas〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉We describe a technique for high-precision analysis of 〈sup〉109〈/sup〉Ag/〈sup〉107〈/sup〉Ag ratios in natural and archaeological gold (1.0–120 mg/g Ag) with particular emphasis on the control of Ag yield and effects of instrumental mass bias. After dissolution of mg-sized gold samples in aqua regia and conversion to chlorides, a miniature column filled with the anion exchange resin AG@1-X8 was used for removal of Au from the sample solutions. In a second miniature column made of the Triskem TBP resin Ag was converted from the chloride to the nitrate form. Isotopic analyses on the MC-ICP-MS employed the combination of Pd-doping and standard bracketing and considering the measurements of replicate dissolutions and chromatographic separations for SRM978a and CEZAg reference solutions, the combined analytical uncertainty (2 s) of the analytical method is better than 0.016‰. A solution prepared from several gold nuggets was characterised isotopically (CEZAg with δ〈sup〉109〈/sup〉Ag = 0.043 ± 0.015‰) for use as a silver-in‑gold reference material in Ag isotope studies of natural and processed gold.〈/p〉 〈p〉Results for gold nuggets from three continents indicate a wide variations in Ag concentrations (5 to 123 mg/g, Table 3) and this is matched by an equally wide range in δ〈sup〉109〈/sup〉Ag (−0.58 – +0.83‰, Fig. 5) range in isotopic compositions (δ〈sup〉109〈/sup〉Ag −0.58 to +0.83‰). This is larger than the variation found so far in in native gold from primary deposits (−0.42 to −0.5‰). Although small isotopic effects to lower δ〈sup〉109〈/sup〉Ag during strong Ag loss are possible within a single grain, most analysed nuggets are internally isotopically homogeneous. This suggests preservation of the primary δ〈sup〉109〈/sup〉Ag in the supergene environment and that gold was sourced from several distinct gold deposits in the catchment. Recent studies however, indicate that even single primary deposits may be isotopically heterogeneous implying that Ag isotope fractionation is caused by numerous deposition-dissolution cycles in hydrothermal systems. Fine-grained detrital gold from three placers along the Rhine river in Germany show only small differences in δ〈sup〉109〈/sup〉Ag (−0.005 ± 0.047 to +0.124 ± 0.007‰, despite being distributed along 480 km of river length. This may indicate thorough mixing of gold grain populations during transport.〈/p〉 〈p〉The small sample size required for Ag isotope work on gold opens the way for detailed micro-sampling approaches. These may be used to further examine the potential for within-grain isotopic variability related to fluviatile processing and can be used to correlate the composition of the placers with that of grains from primary sources.〈/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-S0009254119301135-ga1.jpg" width="232" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): P.-M. Zanetta, C. Le Guillou, H. Leroux, B. Zanda, R.H. Hewins, E. Lewin, S. Pont〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Numerous geosciences samples display a multi-scale mineralogical heterogeneity for which it is challenging to obtain spatially resolved quantitative chemical data. It is the case for chondritic meteorites, which can contain up to 10 different phases with grain size ranging from the nanometer to the millimeter. We developed a method providing multiple physical and chemical information by advanced scanning electron microscopy (SEM), hyperspectral energy dispersive X-ray spectroscopy (EDX) and electron probe micro-analyses (EPMA). The method includes: i) infra-micrometric low-voltage EDX mapping and innovative post-acquisition hyperspectral data analysis (based on both clustering and multiple linear least square fitting) which allow phase mapping and quantification of the modal abundances; ii) EPMA of chemical end-members to upgrade the phase map into a quantified chemical map; iii) physical modeling of the EDX background, used as a proxy of the density. Density maps can be obtained with a precision of ~10%; iv) determination of the bulk sample composition by combining modal abundances, chemical analysis and density measurements.〈/p〉 〈p〉The approach is applied to three well-known chondrites (Murchison, Paris and Orgueil), showing heterogeneous grain sizes and mineralogy. Areas of ~250 ∗ 250 μm〈sup〉2〈/sup〉 were mapped with a pixel size of 250 nm to determine the modal abundances, size distribution, circularity and densities of all phases, as well as the matrix bulk compositions. Taking bulk wet chemistry data as reference, ACADEMY leads to a better match than published defocused beam EPMA measurements. We demonstrate that choosing a Fe-rich, hydrated standard (a biotite) to quantify phyllosilicate by EPMA improves the quantification by up to 10%, and we ultimately retrieve the Mg/Si ratio with a 1% precision. We called this method ACADEMY for 〈strong〉A〈/strong〉nalyzing the 〈strong〉C〈/strong〉omposition, the modal 〈strong〉A〈/strong〉bundance and the 〈strong〉D〈/strong〉ensity using 〈strong〉E〈/strong〉lectron 〈strong〉M〈/strong〉icroscop〈strong〉Y〈/strong〉. A code was developed and was made available online so that ACADEMY can be applied to other materials.〈/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-S000925411930141X-ga1.jpg" width="500" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 28 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Ivan V. Podborodnikov, Anton Shatskiy, Anton V. Arefiev, Altyna Bekhtenova, Konstantin D. Litasov〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Subsolidus and melting phase relationships in the system Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉–CaCO〈sub〉3〈/sub〉–MgCO〈sub〉3〈/sub〉 have been studied at 6 GPa and 900–1250 °C using a Kawai-type multianvil press. At 900 and 1000 °C, the system has four intermediate compounds: Na〈sub〉2〈/sub〉Ca〈sub〉4〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉5〈/sub〉 burbankite, Na〈sub〉2〈/sub〉Ca〈sub〉3〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉4〈/sub〉, Na〈sub〉4〈/sub〉Ca(CO〈sub〉3〈/sub〉)〈sub〉3〈/sub〉, and Na〈sub〉2〈/sub〉Mg(CO〈sub〉3〈/sub〉)〈sub〉2〈/sub〉 eitelite. The Na-Ca compounds dissolve noticeable amounts of Mg component, whereas eitelite dissolves a few percent of Ca component: Na〈sub〉2〈/sub〉(Ca〈sub〉≥0.91〈/sub〉Mg〈sub〉≤0.09〈/sub〉)〈sub〉4〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉5〈/sub〉, Na〈sub〉2〈/sub〉(Ca〈sub〉≥0.94〈/sub〉Mg〈sub〉≤0.06〈/sub〉)〈sub〉3〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉4〈/sub〉, Na〈sub〉4〈/sub〉(Ca〈sub〉≥0.67〈/sub〉Mg〈sub〉≤0.33〈/sub〉)(CO〈sub〉3〈/sub〉)〈sub〉3〈/sub〉, and Na〈sub〉2〈/sub〉(Mg〈sub〉≥.93〈/sub〉Ca〈sub〉≤0.07〈/sub〉)(CO〈sub〉3〈/sub〉)〈sub〉2〈/sub〉. At 1050 °C, the system is complicated by an appearance of dolomite. Na-Ca burbankite decomposes at 1075 ± 25 °C to aragonite plus Na〈sub〉2〈/sub〉Ca〈sub〉3〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉4〈/sub〉. Na〈sub〉4〈/sub〉Ca(CO〈sub〉3〈/sub〉)〈sub〉3〈/sub〉 and eitelite disappear via congruent melting between 1200 and 1250 °C. Na〈sub〉2〈/sub〉Ca〈sub〉3〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉4〈/sub〉 remains stable through the whole studied temperature range. The liquidus projection of the studied ternary system has eight primary solidification phase regions for magnesite, dolomite, calcite-dolomite solid solutions, aragonite, Na〈sub〉2〈/sub〉Ca〈sub〉3〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉4〈/sub〉, Na〈sub〉4〈/sub〉Ca(CO〈sub〉3〈/sub〉)〈sub〉3〈/sub〉, and Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉 solid solutions. The system has five ternary peritectic reaction points and one minimum on the liquidus at 1050 °C and 48Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉·52(Ca〈sub〉0.75〈/sub〉Mg〈sub〉0.25〈/sub〉)CO〈sub〉3〈/sub〉. The minimum point resembles a eutectic controlled by a four-phase reaction, by which a liquid transforms into three solid phases upon cooling: Na〈sub〉2〈/sub〉(Ca〈sub〉0.94〈/sub〉Mg〈sub〉0.06〈/sub〉)〈sub〉3〈/sub〉(CO〈sub〉3〈/sub〉)〈sub〉4〈/sub〉, Na〈sub〉4〈/sub〉(Ca〈sub〉0.67〈/sub〉Mg〈sub〉0.33〈/sub〉)(CO〈sub〉3〈/sub〉)〈sub〉3〈/sub〉, and Na〈sub〉2〈/sub〉(Mg〈sub〉0.93〈/sub〉Ca〈sub〉0.07〈/sub〉)(CO〈sub〉3〈/sub〉)〈sub〉2〈/sub〉 eitelite. Since at 6 GPa, the system has a single eutectic, there is no thermal barrier preventing continuous liquid fractionation from alkali-poor toward Na-rich dolomitic compositions. Cooling of the Na-Ca-Mg carbonatite melt from 1400 to 1100 °C within the lherzolite substrate will be accompanied by magnesite crystallization and wehrlitization keeping calcium number of the melt at 40 and shifting the Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉 content to ≥40 mol%. In the case of the eclogitic wall rock the cooling will be accompanied by dolomite crystallization keeping calcium number of the melt at 60–65 and shifting the Na〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉 content to ≥30 mol%.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 504〈/p〉 〈p〉Author(s): Remco C. Hin, Antony D. Burnham, Diego Gianolio, Michael J. Walter, Tim Elliott〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Previous work has shown that Mo isotopes measurably fractionate between metal and silicate liquids, even at temperatures appropriate for core formation. However, the effect of variations in the structural environment of Mo in the silicate liquid, especially as a function of valence state, on Mo isotope fractionation remained poorly explored. We have investigated the role of valence state in metal-silicate experiments in a gas-controlled furnace at 1400 °C and at oxygen fugacities between 10〈sup〉−12.7〈/sup〉 and 10〈sup〉–9.9〈/sup〉, i.e. between three and 0.2 log units below the iron-wüstite buffer. Two sets of experiments were performed, both with a silicate liquid in the CaO-Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉-SiO〈sub〉2〈/sub〉 system. One set used molybdenum metal wire loops as the metal source, the other liquid gold alloyed with 2.5 wt% Mo contained in silica glass tubes. X-ray absorption near-edge spectroscopy analysis indicates that Mo〈sup〉6+〈/sup〉/ΣMo in the silicate glasses varies between 0.24 and 0.77 at oxygen fugacities of 10〈sup〉–12.0〈/sup〉 and 10〈sup〉–9.9〈/sup〉 in the wire loop experiments and between 0.15 and 0.48 at 10〈sup〉–11.4〈/sup〉 and 10〈sup〉–9.9〈/sup〉 in the experiments with Au-Mo alloys. Double-spiked analysis of Mo isotope compositions furthermore shows that Mo isotope fractionation between metal and silicate is a linear function of Mo〈sup〉6+〈/sup〉/ΣMo in the silicate glasses, with a difference of 0.51‰ in 〈sup〉98〈/sup〉Mo/〈sup〉95〈/sup〉Mo between purely Mo〈sup〉4+〈/sup〉-bearing and purely Mo〈sup〉6+〈/sup〉-bearing silicate liquid. The former is octahedrally and the latter tetrahedrally coordinated. Our study implies that previous experimental work contained a mixture of Mo〈sup〉4+〈/sup〉 and Mo〈sup〉6+〈/sup〉 species in the silicate liquid. Our refined parameterisation for Mo isotope fractionation between metal and silicate can be described as〈span〉〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈msup〉〈mi mathvariant="normal"〉Δ〈/mi〉〈mrow〉〈mn〉98〈/mn〉〈mo〉/〈/mo〉〈mn〉95〈/mn〉〈/mrow〉〈/msup〉〈msub〉〈mi mathvariant="italic"〉Mo〈/mi〉〈mrow〉〈mi mathvariant="italic"〉metal〈/mi〉〈mo〉–〈/mo〉〈mi mathvariant="italic"〉silicate〈/mi〉〈/mrow〉〈/msub〉〈mo〉=〈/mo〉〈mfrac〉〈mrow〉〈mo〉−〈/mo〉〈mn〉1.43〈/mn〉〈mfenced open="(" close=")"〉〈mrow〉〈mo〉±〈/mo〉〈mn〉0.14〈/mn〉〈/mrow〉〈/mfenced〉〈mo〉×〈/mo〉〈msup〉〈mn〉10〈/mn〉〈mn〉6〈/mn〉〈/msup〉〈mfenced open="[" close="]"〉〈mrow〉〈msup〉〈mi mathvariant="italic"〉Mo〈/mi〉〈mrow〉〈mn〉6〈/mn〉〈mo〉+〈/mo〉〈/mrow〉〈/msup〉〈mo〉/〈/mo〉〈mi mathvariant="normal"〉Σ〈/mi〉〈mi mathvariant="italic"〉Mo〈/mi〉〈/mrow〉〈/mfenced〉〈mo〉+〈/mo〉〈mn〉8〈/mn〉〈mfenced open="(" close=")"〉〈mrow〉〈mo〉±〈/mo〉〈mn〉6〈/mn〉〈/mrow〉〈/mfenced〉〈mo〉×〈/mo〉〈msup〉〈mn〉10〈/mn〉〈mn〉4〈/mn〉〈/msup〉〈/mrow〉〈msup〉〈mi〉T〈/mi〉〈mn〉2〈/mn〉〈/msup〉〈/mfrac〉〈/math〉〈/span〉〈/p〉 〈p〉Molybdenum isotope ratios therefore have potential as a proxy to constrain the oxygen fugacity during core formation on planetary bodies if the parameterisation of Mo〈sup〉6+〈/sup〉/ΣMo variation with oxygen fugacity is expanded, for instance to include iron-bearing systems. On Earth literature data indicate that the upper mantle is depleted in heavy Mo isotopes relative to the bulk Earth, as represented by chondrites. As previously highlighted, this difference is most likely not caused by core formation, which either enriches the mantle in heavy Mo isotopes or causes no significant fractionation, depending on temperature and, as we determined here, Mo〈sup〉6+〈/sup〉 content. We reaffirm that core formation does not account for the Mo isotope composition of the modern upper mantle, which may instead reflect the effect of fractionation during subduction as part of global plate recycling.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 1 December 2018〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Moritz Zieringer, Martin Frank, Roland Stumpf, Ed C. Hathorne〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Dissolved neodymium (Nd) and its radiogenic isotope composition (〈sup〉143〈/sup〉Nd/〈sup〉144〈/sup〉Nd, expressed as εNd) belong to the key parameters of the international GEOTRACES program, which aims to investigate the processes controlling the distribution of trace elements and their isotopes in the global ocean. We present Nd isotope and concentration ([Nd]) data from eleven full depth water column profiles from the eastern (sub)tropical Atlantic Ocean between 2°N and 29°N and from the Romanche Fracture Zone sampled during Meteor cruise M81/1 (GEOTRACES cruise GA11) in February/March 2010.〈/p〉 〈p〉The εNd signatures range from −12.9 in upper North Atlantic Deep Water (UNADW) at the equator to −8.1 in the mixed layer near the Canary Islands. Nd concentrations range from 11.9 pmol/kg observed within the Equatorial Undercurrent to 40.2 pmol/kg in Antarctic Bottom Water (AABW) in the Romanche Fracture Zone. Large variations in surface water Nd isotope compositions (−12.7 ≤ εNd ≤ −8.1) and concentrations (15.7 pmol/kg ≤ [Nd] ≤ 27.7 pmol/kg) are caused by partial dissolution of Saharan dust particles between 2°N and 13°N and volcanic island weathering (Canaries) between 25°N and 29°N. In contrast to dust inputs, which predominantly affect the Nd concentrations and isotope compositions of surface waters and underlying South Atlantic Central Water (SACW), contributions originating from the Canary Islands affect the Nd isotope composition of the entire surrounding water column without significantly altering Nd concentrations, thus confirming the concept of boundary exchange in its strictest sense.〈/p〉 〈p〉The results confirm that the composition of lower North Atlantic Deep Water (LNADW, εNd = −12.1) in the abyssal plains of the eastern North Atlantic is exclusively set by the mixing ratio of LNADW and Antarctic Bottom Water (AABW). Upper North Atlantic Deep Water (UNADW), in contrast, is characterized by εNd signatures between −12.7 and − 12.0 between 2°N and 10°N, whereas further north it is clearly affected by admixture of Mediterranean Water (MW) and radiogenic inputs from the Canary Islands and likely also the Cape Verde Islands.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 504〈/p〉 〈p〉Author(s): Yanzhang Li, Yan Li, Xiaoming Xu, Cong Ding, Ning Chen, Hongrui Ding, Anhuai Lu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Spinel ferrite minerals widely distribute in supergene environments. This study mainly focus on investigating the relationship between the crystal chemistry (structural disordering, oxygen vacancy) and semiconducting property (optical absorbance, band structure, photocatalytic activity) of spinel ferrites (Zn〈sub〉1−〈em〉x〈/em〉〈/sub〉Fe〈sub〉〈em〉x〈/em〉〈/sub〉)[Zn〈sub〉〈em〉x〈/em〉〈/sub〉Fe〈sub〉2−〈em〉x〈/em〉〈/sub〉]O〈sub〉4〈/sub〉 (〈em〉x〈/em〉 = 0, 0.03, 0.06, 0.15, 0.20). With the increase of inversion coefficient (〈em〉x〈/em〉) from 0 to 0.20, ZnFe〈sub〉2〈/sub〉O〈sub〉4〈/sub〉 becomes disordered and its lattice parameter descends from 8.4407 to 8.4320 Å. New polyhedrons ZnO〈sub〉6〈/sub〉 and FeO〈sub〉4〈/sub〉 appear since 〈em〉x〈/em〉 = 0.03, which induce three new Raman bands at 299–317 (F〈sub〉2g〈/sub〉〈sup〉(2)〈/sup〉), 482–484 (F〈sub〉2g〈/sub〉〈sup〉(3)〈/sup〉) and 690–696 cm〈sup〉−1〈/sup〉 (A〈sub〉1g〈/sub〉), respectively. The amount of oxygen vacancy and adsorbed hydroxyl species on ZnFe〈sub〉2〈/sub〉O〈sub〉4〈/sub〉 surface show linear increase with increasing 〈em〉x〈/em〉 (〈em〉R〈/em〉〈sup〉2〈/sup〉 = 0.943 and 0.997, respectively), as indicated by electron paramagnetic resonance and X-ray photoelectron spectroscopy. Both the spectroscopic analysis and density functional theory calculations suggest the valence band edge of ZnFe〈sub〉2〈/sub〉O〈sub〉4〈/sub〉 keeps unchanged as the structure goes disorderly, while its conduction band edge gradually goes lower. Therefore, the bandgap linearly decreases from 2.08 to 1.95 eV (〈em〉R〈/em〉〈sup〉2〈/sup〉 = 0.971) when 〈em〉x〈/em〉 increases from 0 to 0.20. Furthermore, two impurity levels are produced in oxygen-defective ZnFe〈sub〉2〈/sub〉O〈sub〉4〈/sub〉 and cause dual fluorescence signals at 670 and 740–850 nm. Compared with normal ZnFe〈sub〉2〈/sub〉O〈sub〉4〈/sub〉, the disordered ZnFe〈sub〉2〈/sub〉O〈sub〉4〈/sub〉 produces more hydroxyl radical (〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/rad"〉OH) under the same solar irradiation conditions. In summary, the structural disordering degree of spinel ferrites controls the concentration of oxygen vacancy, configuration of electronic structure and thus the photocatalytic activity, which endow natural bulk spinel ferrite minerals with surprising chemical reactivity on the surface of solid planets.〈/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-S0009254118305825-ga1.jpg" width="500" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 504〈/p〉 〈p〉Author(s): H.M. Xu, R.Y. Sun, J.J. Cao, Ru-Jin Huang, B. Guinot, Z.X. Shen, M. Jiskra, C.X. Li, B.Y. Du, C. He, S.X. Liu, T. Zhang, J.E. Sonke〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Atmospheric pollution by fine particulates and mercury (Hg) in emerging economies is a serious environmental concern. Here, we present Hg concentrations and isotope compositions of 24-hour integrated fine particulate matter (PM〈sub〉2.5〈/sub〉) samples collected during January 2014 at four large Chinese cities (three inland cities: Beijing, Changchun and Chengdu; one coastal city: Hong Kong), with an aim of identifying the potential Hg sources and transformation processes. Mean concentrations of PM〈sub〉2.5〈/sub〉 (171 ± 62 μg m〈sup〉−3〈/sup〉) and PM〈sub〉2.5〈/sub〉-bound Hg (1.3 ± 1.1 ng m〈sup〉−3〈/sup〉) in Chengdu were the highest. Overall, PM〈sub〉2.5〈/sub〉 samples in Chinese inland cities were characterized by moderately negative δ〈sup〉202〈/sup〉Hg (−1.08 ± 0.64‰, 1σ), slightly negative Δ〈sup〉199〈/sup〉Hg (−0.13 ± 0.28‰, 1σ) and insignificant Δ〈sup〉200〈/sup〉Hg (0.03 ± 0.05‰, 1σ). On average, δ〈sup〉202〈/sup〉Hg of PM〈sub〉2.5〈/sub〉 was the highest in Chengdu (−0.74 ± 0.67‰, 1σ), followed by Beijing (−1.11 ± 0.26‰, 1σ) and Changchun (−1.60 ± 0.45‰, 1σ). PM〈sub〉2.5〈/sub〉 from Beijing showed the most negative Δ〈sup〉199〈/sup〉Hg (−0.31 ± 0.40‰, 1σ) that was significantly lower than Changchun (−0.12 ± 0.21‰, 1σ) and Chengdu (−0.02 ± 0.15‰, 1σ). Coal combustion and cement production were identified to be the dominant sources of PM〈sub〉2.5〈/sub〉-bound Hg in these cities, with additional Hg sources from non-ferrous metal smelting in Chengdu. Besides, Hg emissions from biomass burning were evident in specific days for each city. We found that source materials and isotope fractionation during emission processes could not fully explain the observed Hg isotope compositions in PM〈sub〉2.5〈/sub〉, especially the large negative Δ〈sup〉199〈/sup〉Hg values (〈−0.3‰) in Beijing. The near-unity slope of Δ〈sup〉199〈/sup〉Hg 〈em〉vs.〈/em〉 Δ〈sup〉201〈/sup〉Hg in PM〈sub〉2.5〈/sub〉 samples from each studied city indicates that Hg〈sup〉II〈/sup〉 in the fine aerosols was likely photo-reduced to different degrees in the atmosphere following emission from sources.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 3 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): M.H. Köster, L.B. Williams, P. Kudejova, H.A. Gilg〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The mineralogical, chemical and isotopic analyses of smectites, with variable interlayer cation occupancies, from bentonite deposits in various depositional environments, reveal new insights into the boron sources and the fluids involved in bentonitization in marine and non-marine environments.〈/p〉 〈p〉Smectites from bentonites have non-exchangeable, structural boron concentrations of 0.2to 196 μg/g B. Smectites from sodium bentonites have higher boron concentrations (〉30μg/g) than those from magnesium or calcium bentonites. Most smectites have a small, interstratified illitic component that has a major influence on boron concentrations, requiring the use of modified fluid-mineral boron partitioning coefficients, which indicate that bentonites formed from fluids of highly variable boron concentrations of 〈0.1mg/L B to 〉100 mg/L B, and a chlorine content of 76.4 mg/L to 59,076 mg/L. The sodium bentonites formed from boron-rich saline fluids or brines whereas the fluids involved in calcium and magnesium bentonite formation have a more variable boron composition and salinity.〈/p〉 〈p〉The δ〈sup〉11〈/sup〉B values of the structural boron in tetrahedral sites of smectites range from −30.1‰ to +11.8‰. The smectites from terrestrial depositional settings have δ〈sup〉11〈/sup〉B values of −30.1‰ to about 0‰, whereas smectites from marine depositional settings have negative as well as positive δ〈sup〉11〈/sup〉B values. The boron isotope values indicate that all examined bentonites from terrestrial depositional settings as well as many bentonites from marine depositional settings formed from basinal fluids or brines.〈/p〉 〈p〉The boron isotope geochemistry of smectites is demonstrated to be a tool for elucidating the fluids involved in the formation of clay mineral deposits. It also has great potential for tracing fluids in other settings involving authigenic clay minerals such as sedimentary basins and surficial crystalline rocks, as well as man-made applications such as in disposal sites for highly active nuclear waste.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Chun Liu, Zhongwu Li, Asmeret Asefaw Berhe, Guangming Zeng, Haibing Xiao, Lin Liu, Danyang Wang, Hao Peng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Soil erosion is a key variable in the biogeochemical cycle of carbon (C) on the Earth's surface. However, questions remain about the roles of land use and erosion intensity on the composition, source, and fate of soil C eroded from terrestrial to fluvial systems. In this study, chemical characteristics of eroded water-extractable organic matter (WEOM) in soils and sediments, as well as subsequent source identification, were inferred from UV–Visible absorption and fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC) in study sites that include land uses and gully banks, experiencing three levels of erosion intensity in a semi-arid region of China. 〈sup〉13〈/sup〉C and 〈sup〉15〈/sup〉N isotopic signatures along with elemental ratios were also successfully employed to explore the source of bulk soil organic matter (SOM) in sediments. We found a much greater contribution of condensed aromatic structures and hydrophobic fraction of soluble organic compounds in forest soils compared to croplands at eroding sites, where these variables were greater than those of depositional sites. The results from fluorescence analysis in soil materials showed that erosion intensity has a negligible influence on WEOM quality. The EEM-PARAFAC with fluorescence indices indicated that biological production of soil substrates can also play a key role in the dynamics of WEOM induced by soil erosion. Our results from an isotopic mixing model analysis showed that gully bank soil was the primary sources of sedimentary SOM in all regions with different erosion intensities (mean probability estimate (MPE) 100% for the region with light erosion intensity, 36.18% for the region with high erosion intensity, and 99.25% for the region with extremely high erosion intensity). However, orchard and grassland were also the main contributors for the SOM in sediments in regions with high erosion intensity, accounting for MPE 29.93% and 33.89%, respectively. Our findings demonstrate that land use and erosion intensity have significant effect on nature of eroded OM.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): C. Glotzbach, K.A. Lang, N.N. Avdievitch, T.A. Ehlers〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Conventional (U-Th(-Sm))/He thermochronology requires accurate determination of the grain shape and dimension for calculation of 〈sup〉4〈/sup〉He loss by alpha particle ejection and diffusion. In routine practice, calculations of 〈sup〉4〈/sup〉He loss and diffusion are calculated assuming that measured grains approximate symmetric grain shapes with perfectly smooth crystal surfaces. This may introduce uncertainties that are virtually impossible to quantify if grains that do not pass the latter criteria are used. Alternatively, this leads to a selection bias for the analyses of detrital minerals if only grains with perfect crystal surfaces are measured. To address this limitation, we introduce a flexible, open-source numerical approach in Matlab™ to accurately determine the three-dimensional grain shape and numerically model He loss by alpha ejection and diffusion on individual grains. The three-dimensional surface of a mineral grain is interpolated from photomicrographs taken orthogonal to the principal grain axes. He ejection and diffusion is simulated using a coupled Monte Carlo and Brownian motion algorithm. Comparing our approach to high-resolution X-ray microtomography, we demonstrate that our “3D-He” approach improves the accuracy of He loss corrections when mineral grains do not approximate perfect crystal shapes by avoiding constraints inherent to standard manual-analytical approaches. Importantly, this method mitigates any bias introduced by user-dependent grain selection, measurements and assigning grains to a mineral shape for which analytical solutions of He production have been derived. In routine practice, we find that our 3D-He approach improves the accuracy of geometric measures (volume and surface area) and associated sphere-equivalent radii (SER) significantly, reducing the average deviation from true values from ~7–8% (manual-analytical approach) to ~2.5% (3D-He approach). The difference in SER significantly impacts the resulting apatite (U-Th-Sm)/He ages, which may deviate up to 40% when long held at temperatures where He production roughly matches diffusive loss of He. Using the 3D-He approach, the average deviation and overall spread of He loss corrections due to alpha ejection (F〈sub〉T〈/sub〉 factors) could be reduced from 1.6 ± 4.6% to 0.6 ± 0.8%. In summary, our 3D-He approach is a significant refinement to (U-Th(-Sm))/He age calculation considering relatively low analytical uncertainty in U, Th, Sm and He measurement (~2%). Importantly, our 3D-He approach should mitigate the selection bias introduced through manual grain selection. Toward this end, we discuss additional advantages of our numerical approach to handling broken grains or grains with known parent isotope zonation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 507〈/p〉 〈p〉Author(s): M.A. Fitzgerald, K.B. Knight, J.E. Matzel, K.R. Czerwinski〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Energetic events such as meteorite impacts or nuclear explosions in silicate-rich environments produce compositionally heterogeneous melts/vapors that quench to glasses such as tektites and fallout. These glassy byproducts constitute a compositional record of an unknown vapor combined with melted, compositionally heterogeneous environmental materials. The energetic formation process often results in incomplete mixing in the melt, being preserved as a glass. While this chemical heterogeneity can theoretically be interpreted to understand the starting conditions of the system, individual source materials may have overlapping element abundances and chemical behaviors that complicate interpretation using tools such as bivariate analysis. This paper develops and compares multivariate linear-least squares approaches to understand major element relationships in compositionally heterogeneous glasses. We compare classical least squares (CLS) and principal component analysis (PCA) with two multivariate curve resolution–alternating least squares (MCR-ALS) optimization techniques that are novel in application. These approaches are tested using synthetic data to understand sensitivities to the resulting determination of the abundances and compositions of the precursor components. We explore the impact of incomplete 〈em〉a priori〈/em〉 knowledge of the system prior to an energetic glass formation event on the resulting confidence and accuracy of the modeled solution (〈em〉i.e.〈/em〉, the number, composition, and abundances of the precursor materials). We conclude that a closure and equality constrained MCR-ALS approach is the most robust means of identifying precursor components, and is a suitable alternative to CLS-style approaches when the investigator has incomplete knowledge of the starting precursor chemical compositions. The MCR-ALS approach is then applied to measurements of fallout glass reported in the literature, demonstrating how multivariate approaches can be applied to objectively establish pre-event source term compositions and contributions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 507〈/p〉 〈p〉Author(s): Levente Patkó, Nóra Liptai, István János Kovács, László Előd Aradi, Qun-Ke Xia, Jannick Ingrin, Judith Mihály, Suzanne Y. O'Reilly, William L. Griffin, Viktor Wesztergom, Csaba Szabó〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The structural hydroxyl content of the nominally anhydrous minerals (olivine and pyroxenes) in the upper mantle is among the important attributes that influence the physical and chemical features of the upper mantle. In this study, we provide detailed Fourier-transform infrared (FTIR) data on 63 petrographically and geochemically well-defined upper mantle xenoliths from the Nógrád-Gömör Volcanic Field (Pannonian Basin, Central Europe). These xenoliths show extremely low average structural hydroxyl contents (~0, 31 and 185 ppm for olivine, orthopyroxene and clinopyroxene, respectively) compared to values reported regionally and worldwide. The studied xenoliths have anomalous types of FTIR spectra and high structural hydroxyl ratios between clinopyroxenes and orthopyroxenes (an average of ~8). Furthermore, there is usually no correlation between the structural hydroxyl content and other physical or chemical properties of the xenoliths. These specific FTIR characteristics suggest that the Nógrád-Gömör upper mantle xenoliths were exposed to significant modification of their structural hydroxyl contents, which may be linked to pre- and post-eruptive processes. Decompression during extension leads to lower ‘water’ activity, which is most likely to have played a key role. However, pre-eruptive mantle metasomatism with an agent having low water activity cannot be excluded either. The post-eruptive cooling can be significant as well, as suggested by the higher structural hydroxyl content in xenoliths hosted in more rapidly cooled volcanic facies (i.e. pyroclastics).〈/p〉 〈p〉Our study reveals how FTIR characteristics may evolve in continental rift settings in young extensional basins. Furthermore, novel applications of our study are the diagnostic features that indicate significant changes in structural hydroxyl properties. This contributes to distinguishing low structural hydroxyl contents linked to the pre-eruptive (i.e., low structural hydroxyl content in pyroxenes, anomalous partitioning and anomalous band characteristic in pyroxenes) or the post-eruptive (completely ‘dry’ olivines) periods.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Norma Pérez-Almeida, Aridane G. González, J. Magdalena Santana-Casiano, Melchor González-Dávila〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The redox interaction between iron and copper was experimentally studied in UV-seawater (UV-SW) and a kinetic model was developed to simultaneously reproduce the coupled Fe and Cu redox chemistry. The kinetic model included a combination of chemical reactions and their equilibrium and rate constants. The interactions between Cu(I) and Fe(II), Cu(I) and Fe(III) and Cu(II) and Fe(II) under different oxygen and H〈sub〉2〈/sub〉O〈sub〉2〈/sub〉 concentrations were considered. The Cu(I) oxidation rate under oxygen saturation conditions linearly decreased in the presence of Fe(II) and increased in the presence of Fe(III). The effect of Fe(II) was one order of magnitude higher than that for Fe(III).〈/p〉 〈p〉The kinetic model considered the competition between Fe and Cu in the UV-SW by reactive oxygen species, the direct reaction between Fe and Cu, and the formation of Fe-Cu species such as cuprous ferrite and cupric ferrite. This inorganic kinetic model is the previous step to be able to incorporate the role of organic binding ligands in the Fe and Cu redox interaction in natural waters and represents an advance in the Fe and Cu coupled marine biogeochemical cycles.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 507〈/p〉 〈p〉Author(s): Shili Liao, Chunhui Tao, Chuanwei Zhu, Huaiming Li, Xiaohu Li, Jin Liang, Weifang Yang, Yejian Wang〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The newly discovered Yuhuang-1 hydrothermal field (YHF) on the Southwest Indian Ridge primarily contains two different sulfide distribution areas: the southwest sulfide (SWS) and the northeast sulfide (NES) distribution areas. In this study, Zn isotope compositions and element ratios of Zn, Fe, Cu, and Cd in the surface sulfides were analyzed to characterize Zn isotopic fractionation, ore-forming conditions, and mineralization processes of the YHF. The results indicate that sulfide-rich samples in the YHF exhibit intensive Zn isotopic fractionation, with δ〈sup〉66〈/sup〉Zn values ranging from −0.43‰ to 1.24‰. These values are not only larger than those currently reported for seafloor sulfides but are also larger than the values reported for the deposits on land. Remarkably, the obtained δ〈sup〉66〈/sup〉Zn value (−0.43‰) is the lowest one observed in seafloor sulfides (−0.26‰–1.17‰), currently. A study of Zn isotope systematics revealed that Zn isotope variations of sulfide-rich samples in the YHF were controlled by the sphalerite and pyrite content; the study also revealed that early minerals are rich in light Zn isotopes, but later minerals are rich in heavy Zn isotopes. A comparative study of previous data on Zn isotopes shows that Zn isotope fractionations in the YHF are likely attributed to Rayleigh fractionation, caused by mineral precipitation and multiple generations of pyrite. Further, Zn/Cd ratios of sulfide-rich samples indicate that the ore-forming temperature of the silicified sulfide-rich samples in the SWS probably formed under lower temperatures as compared with the silicified sulfide-rich chimney in the NES. Furthermore, sulfur and Zn isotope compositions in the SWS are significantly different from those in sulfide-rich samples of the NES, which suggests pervasively different metallogenic conditions, further indicating that they are probably products of separate mineralization episodes.〈/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-S0009254118306296-ga1.jpg" width="347" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Zeyang Sun, Xiangli Wang, Noah Planavsky〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Stable chromium (Cr) isotopes (expressed in δ〈sup〉53〈/sup〉Cr as the deviation of 〈sup〉53〈/sup〉Cr/〈sup〉52〈/sup〉Cr in samples from the international standard NIST SRM 979) are an emerging paleoredox proxy that has been used to track Earth's redox evolution over a range of timescales. There have been several major advances in our understanding of the Cr isotope system, but we are still developing a basic knowledge of the global Cr isotope mass balance and there are key Cr fluxes with poorly constrained isotopic compositions. The Cr isotope behavior in estuaries is one such poorly constrained aspect of the modern Cr cycle. Therefore, we collected water and suspended particulate samples from the estuary of the Connecticut River, and present the first Cr isotope dataset from a salinity gradient. As pH and salinity increases from brackish to salt water, dissolved Cr increases from 118.1 ± 25.7 ng/kg (1σ, n = 6) to 225.1 ± 26.0 ng/kg (1σ, n = 8), while particulate Cr decreases from 341.3 ± 319.8 ng/kg (1σ, n = 6) to 208.4 ± 364.5 ng/kg (1σ, n = 8); dissolved δ〈sup〉53〈/sup〉Cr decreases from 1.30 ± 0.27‰ (1σ, n = 6) to 0.64 ± 0.16‰ (1σ, n = 8), whereas particulate δ〈sup〉53〈/sup〉Cr remained stable at 0.11 ± 0.07‰ (1σ, n = 9). These data suggest that Cr is lost from particles to solution when transitioning from brackish to salt water in the Connecticut River estuary. If these observations can be confirmed in other estuaries, estuary systems can potentially enhance riverine dissolved Cr fluxes and thus lower the Cr oceanic residence time, which has important implications for the Cr isotope system as a paleoceanographic redox proxy. There is a clear inverse correlation between dissolved Cr and δ〈sup〉53〈/sup〉Cr in the Connecticut River estuary, which yields a Cr isotope fractionation factor of ~0.89‰ that is indistinguishable from previous estimates based on open seawater samples. Large δ〈sup〉53〈/sup〉Cr variation in oxygenated open and estuary seawater suggests that marine δ〈sup〉53〈/sup〉Cr variations need not necessarily be exclusively tied to Cr(VI) reduction in low oxygen zones. Therefore, interpreting δ〈sup〉53〈/sup〉Cr in the sedimentary record requires taking multiple processes into consideration.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 5 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 507〈/p〉 〈p〉Author(s): G. Jacques, F. Hauff, K. Hoernle, R. Werner, G. Uenzelmann-Neben, D. Garbe-Schönberg, M. Fischer〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Mozambique Ridge (MOZR) is one of several bathymetric highs formed in the South African gateway shortly after the breakup of the supercontinent Gondwana. Two major models have been proposed for its formation - volcanic plateau and continental raft. In order to gain new insights into the genesis of the Mozambique Ridge, R/V SONNE cruise SO232 carried out bathymetric mapping, seismic reflection studies and comprehensive rock sampling of the igneous plateau basement. In this study, geochemical data are presented for 55 dredged samples, confirming the volcanic origin of at least the upper (exposed) part of the plateau. The samples have DUPAL-like geochemical compositions with high initial 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr (0.7024–0.7050), low initial 〈sup〉143〈/sup〉Nd/〈sup〉144〈/sup〉Nd (0.5123–0.5128) and low initial 〈sup〉176〈/sup〉Hf/〈sup〉177〈/sup〉Hf (0.2827–0.2831), and elevated initial 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb and 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb at a given 〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb (Δ7/4 = 2–16; Δ8/4 = 13–167). The geochemistry, however, is not consistent with exclusive derivation from an Indian MORB-type mantle source and requires a large contribution from at least two components. Ratios of fluid-immobile incompatible elements suggest the addition of an OIB-type mantle to the ambient upper mantle. The MOZR shares similar isotopic compositions similar to mixtures of sub-continental lithospheric mantle end members but also to long-lived, mantle-plume-related volcanic structures such as the Walvis Ridge, Discovery Seamounts and Shona hotspot track in the South Atlantic Ocean, which have been proposed to ascend from the African Large Low Shear Velocity Province (LLSVP), a possible source for DUPAL-type mantle located at the core-mantle boundary. Interestingly, the MOZR also overlaps compositionally with the nearby Karoo-Vestfjella Continental Flood Basalt province after filtering for the effect of interaction with the continental lithosphere. This geochemical similarity suggests that both volcanic provinces may be derived from a common deep source. Since a continuous hotspot track connecting the Karoo with the MOZR has not been found, there is some question about derivation of both provinces from the same plume. In conclusion, two possible models arise: (1) formation by a second mantle upwelling (blob or mantle plume), possibly reflecting a pulsating plume, or (2) melting of subcontinental lithospheric material transferred by channelized flow to the mid-ocean ridge shortly after continental break-up. Based on geological, geophysical and geochemical observations from this study and recent published literature, the mantle-plume model is favored.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Wei Wang, Duc Huy Dang, Breda Novotnik, Thai T. Phan, R. Douglas Evans〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Temporal and vertical variations in uranium (U) concentrations and U isotope (δ〈sup〉238〈/sup〉U, ‰) signatures were examined in sediment cores collected seven times over a one year period, from two lakes in Ontario, Canada, which are contaminated with U by historical mining activities. Bow Lake is holomictic, experiencing seasonal anoxia, while the sediments of meromictic Bentley Lake are permanently anoxic. Average annual peak concentrations of U in Bow Lake subsurface sediments were approximately 300 μg L〈sup〉−1〈/sup〉 and 600 μg g〈sup〉−1〈/sup〉 in porewater and bulk sediments, respectively. Similar ranges of concentrations (900 μg L〈sup〉−1〈/sup〉 and 600 μg g〈sup〉−1〈/sup〉, respectively) were observed in Bentley Lake sediments. The exceedingly high levels of U observed in the porewaters of both lakes, as well as the seasonal variability in U levels, challenge the traditional paradigm regarding U chemistry, i.e., that reduced U(IV) should be insoluble under anoxic conditions.〈/p〉 〈p〉The average annual δ〈sup〉238〈/sup〉U ‰ values at the sediment-water interface of both lakes were similar (i.e., 0.47 ± 0.09‰ and 0.50 ± 0.16‰, relative to IRMM-184). The deep sediments in both Bentley Lake and Bow Lake record U isotope composition with a typical fractionation of 0.6‰ relative to the surface water, confirming authigenic U accumulation, i.e., negligible contribution of particulate material from the tailings. Also, the δ〈sup〉238〈/sup〉U values in porewater have an average offset of ca. −0.1‰ relative to bulk sediments in anoxic zones and are reversed in the oxic sediment layer.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 January 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 504〈/p〉 〈p〉Author(s): 〈/p〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Ze-Rui Ray Liu, Mei-Fu Zhou, Anthony E. Williams-Jones, Wei Wang, Jian-Feng Gao〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Titanium (Ti) is typically hosted in detrital minerals in marine sediments and has long been considered to be immobile during diagenesis. In this study, the authigenic titania minerals, brookite and anatase, are observed in early Cambrian carbonaceous shales from the Meishucun and Zhajin sections of South China, respectively. Black shales in the Meishucun section have total organic carbon (TOC) contents from 1.6 to 3.9 wt% and HI (hydrogen index) values from 3.8 to 20 mg HC/g TOC, whereas black shales in the Zhajin section have much higher TOC (7.1–15.6 wt%) but lower HI (〈2.0 mg HC/g TOC) and contain abundant bitumen (~3 vol%). Brookite in black shales from the Meishucun section crystallized invariably along cleavages of detrital biotite. This intimate association suggests that the Ti required to form titania minerals was derived from detrital biotite and that Ti was mobilized only on a nano- to micro-meter scale. In contrast, anatase aggregates in black shales from the Zhajin section are intergrown with bitumen. It is proposed that Ti in these shales was mobilized in low pH organic-rich fluids and, subsequently, preferentially precipitated as anatase with increasing pH. The mobilization of inert Ti in black shales is indicative of a possible diagenetic or post-diagenetic elemental redistribution in carbonaceous rocks. Our study further demonstrates that proxies based on geochemical component of black shales can be employed to estimate redox state of ancient oceans, but must be applied with caution.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 30 March 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 509〈/p〉 〈p〉Author(s): Maximilian Hansen, Denis Scholz, Bernd R. Schöne, Christoph Spötl〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Here we present novel cave-analogue experiments directly investigating stable carbon and oxygen isotope fractionation between the major involved species of the carbonate system (HCO〈sub〉3〈/sub〉〈sup〉−〈/sup〉, CO〈sub〉2〈/sub〉, CaCO〈sub〉3〈/sub〉 and H〈sub〉2〈/sub〉O). In these experiments, which were performed under controlled conditions inside a climate box, a thin film of solution flew down an inclined marble or glass plate. After different distances of flow and, thus, residence times on the plate, pH, electrical conductivity, supersaturation with respect to calcite, precipitation rate as well as the δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C values of the dissolved inorganic carbon (DIC) and the precipitated CaCO〈sub〉3〈/sub〉 were obtained.〈/p〉 〈p〉Progressive precipitation of CaCO〈sub〉3〈/sub〉 along the plate is accompanied by degassing of CO〈sub〉2〈/sub〉 and stable isotope fractionation, and the system is driven out of isotope equilibrium. We observe a strong enrichment of the δ〈sup〉13〈/sup〉C values with increasing residence time and a smaller enrichment in δ〈sup〉18〈/sup〉O. The temporal evolution of the δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C values of both the DIC and the precipitated CaCO〈sub〉3〈/sub〉 can be explained by a Rayleigh fractionation model, but the observed enrichment in δ〈sup〉13〈/sup〉C values is much larger than expected based on isotope equilibrium fractionation factors.〈/p〉 〈p〉Our setup enables to determine the fractionation between CaCO〈sub〉3〈/sub〉 and HCO〈sub〉3〈/sub〉〈sup〉−〈/sup〉, i.e., ε〈sub〉CaCO3/HCO3−〈/sub〉. Carbon isotope fractionation, 〈sup〉13〈/sup〉ε〈sub〉CaCO3/HCO3−〈/sub〉, is strongly negative for all experiments and much lower than equilibrium isotope fractionation (0–1‰). In addition, 〈sup〉13〈/sup〉ε〈sub〉CaCO3/HCO3−〈/sub〉 decreases with increasing residence time on the plate, and thus decreasing supersaturation with respect to calcite. Thus, isotope fractionation depends on precipitation rate and consequently occurs under kinetic conditions. This is in contrast to previous studies, which found no rate-dependence and no or even a positive carbon isotope fractionation between CaCO〈sub〉3〈/sub〉 and HCO〈sub〉3〈/sub〉〈sup〉−〈/sup〉. Oxygen isotope fractionation, 〈sup〉18〈/sup〉ε〈sub〉CaCO3/HCO3−〈/sub〉, is also negative and dependent on precipitation rate. Since no literature values for 〈sup〉18〈/sup〉ε〈sub〉CaCO3/HCO3−〈/sub〉 are available, we calculated 〈sup〉18〈/sup〉ε〈sub〉CaCO3/HCO3−〈/sub〉 using equilibrium oxygen isotope fractionation factors between water and calcite and water and HCO〈sub〉3〈/sub〉〈sup〉−〈/sup〉, respectively. At the beginning of the plate, the fractionation is in agreement with the fractionation calculated using fractionation factors determined in cave systems.〈/p〉 〈p〉The observed fractionation between CaCO〈sub〉3〈/sub〉 and water, 1000ln〈sup〉18〈/sup〉α, is also in good agreement with the values determined in cave systems and shows a very similar temperature dependence 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"〉〈mn〉1000〈/mn〉〈msup〉〈mo mathvariant="italic"〉ln〈/mo〉〈mn〉18〈/mn〉〈/msup〉〈mi〉α〈/mi〉〈mo〉=〈/mo〉〈mn〉16.516〈/mn〉〈mfenced open="(" close=")"〉〈mrow〉〈mo〉±〈/mo〉〈mn〉1.267〈/mn〉〈/mrow〉〈/mfenced〉〈mo〉∗〈/mo〉〈mfrac〉〈msup〉〈mn〉10〈/mn〉〈mn〉3〈/mn〉〈/msup〉〈mi〉T〈/mi〉〈/mfrac〉〈mo〉−〈/mo〉〈mn〉26.141〈/mn〉〈mfenced open="(" close=")"〉〈mrow〉〈mo〉±〈/mo〉〈mn〉4.356〈/mn〉〈/mrow〉〈/mfenced〉〈/math〉. However, with progressive precipitation of CaCO〈sub〉3〈/sub〉 along the plate, the system is forced out of isotope equilibrium with the water, and 1000ln〈sup〉18〈/sup〉α increases.〈/p〉 〈p〉The large, negative, rate-dependent isotope fractionations observed in this study suggest that precipitation of speleothem calcite is strongly kinetically controlled and may, thus, have a large effect on speleothem δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C values. Since these values may erroneously be interpreted as reflecting changes in past temperature, precipitation and/or vegetation density, these results have important implications for paleoclimate reconstructions from speleothems.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Alexandra M. Horne, Matthijs C. van Soest, Kip V. Hodges〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Paired U/Pb and (U-Th-Sm)/He dating of individual detrital apatite crystals has potential as a valuable tool for constraining the high- and low-temperature thermal evolution of sediment source terrains. Here we present the results of exploratory applications of the laser ablation double-dating (LADD) method – originally developed for detrital zircon research – to the widely available Durango fluorapatite standard in order to evaluate the practicality of the method. Sixty-two laser-ablation analyses of a single large crystal of Durango fluorapatite yielded an inverse-variance weighted mean 〈sup〉206〈/sup〉Pb/〈sup〉238〈/sup〉U date of 31.46 ± 0.48 Ma and an inverse-variance weighted mean (U-Th-Sm)/He date of 31.75 ± 0.60 Ma, both of which are in good agreement with previously published conventional dates. While these results are encouraging, several factors suggest that LADD may be less useful for detrital apatites than for detrital zircons given widely available analytical instrumentation. These principally reflect the propensity for apatites to have comparatively lower U + Th concentrations, and thus lower radiogenic He and Pb concentrations, as well as high concentrations of common Pb. These factors contribute to substantially higher analytical imprecision for most LADD U/Pb dates for apatite, occasionally too high for the dates to be geologically useful. Reasonably precise laser ablation (U-Th-Sm)/He dating of detrital apatites requires relatively large crystal sizes (≥ 100 μm in the shortest dimension), with the minimum useful size increasing with decreasing (U-Th-Sm)/He apparent age. In contrast to the geological interpretation of LADD datasets for detrital zircons, the interpretation of datasets for detrital apatites is less straightforward. In particular, researchers should consider carefully the possibility that (U-Th-Sm)/He apparent age distributions are biased by the need to analyze only larger apatites.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Clarisse Balland-Bolou-Bi, Emile B. Bolou-Bi, Nathalie Vigier, Christian Mustin, Anne Poszwa〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Phlogopite weathering experiments were carried out under aerobic conditions in a closed system to study Mg isotopic fractionation during the dissolution and processes involved in the presence of bacterial strains. Four different bacterial strains with different metabolisms were chosen. Biotic experiments were performed in batch reactors at 24 °C for 12 days. In parallel, abiotic phlogopite weathering experiments were also performed using organic and nitric acids. Citric and gluconic acids were used to model the effects of chelating agents produced by bacteria, and nitric acid was used to model the effects of acidifying agents. The results indicate that both decreases in pH and the production of metabolites during heterotrophic bacterial activities significantly accelerate the release of Mg and Si in solution. Also, at a given pH, the fraction of elements released in solution is greater in the presence of bacteria or citric acid compared to that released in the presence of nitric and gluconic acids. Magnesium isotopic analyses indicate that the solutions obtained using nitric acid and acidifying bacterial strains display, on average, δ〈sup〉26〈/sup〉Mg values that are close to or slightly heavier (by 0.4‰) than those of fresh phlogopite (−1.2‰ ± 0.08). In contrast, in experiments performed with citric acid, the Mg leached into solution is slightly enriched in light isotopes by −0.3‰ relative to the initial phlogopite, while its binding to organics is expected to be related to its preferential enrichment in heavy isotopes. Despite its small range of variations, the δ〈sup〉26〈/sup〉Mg values of solutions vary inversely with pH, thus suggesting that secondary phases preferentially enriched in 〈sup〉26〈/sup〉Mg at the highest pH values may play a key role.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Lindsay Krall, Lena Z. Evins, Ellen Kooijman, Martin Whitehouse, Eva-Lena Tullborg〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉U-Pb isotope systems have been used to constrain the timing of formation, alteration, and oxidation of U minerals from the meta-granitic bedrock at Forsmark, eastern Sweden. Secondary ion mass spectrometry (SIMS) has been used to collect U-Pb data from uraninite. Discordant data suggest a ~1.8 Ga emplacement of uraninite-bearing pegmatites and an event of uraninite alteration at ~1.6 Ga. The latter age is contemporaneous with the Gothian orogeny in Scandinavia, which was associated with hydrothermal fluid circulation in the Fennoscandian Shield. Ca-uranyl silicates haiweeite and uranophane predominately formed 1.3–1.2 Ga, contemporaneous with the emplacement of the Satakunta complex of the Central Scandinavian Dolerite Group. A Palaeozoic group of Ca-U(VI)-silicates is also present, which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time. Low Pb concentrations in the U(VI) silicates of several samples are compatible with a recent (〈100 Ma) alteration or precipitation of these minerals in connection to reaction with carbonate-rich fluids. The results support a geologically early oxidation of U(IV) to U(VI) and provide insight into the palaeoredox conditions that may impart an on-going influence on the mobility of natural U in the Forsmark fracture network.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): T.N. Stokes, G.D. Bromiley, N.J. Potts, K.E. Saunders, A.J. Miles, EIMF〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Oxygen fugacity and melt composition are both known to have a strong influence on the partitioning of trace elements between coexisting minerals and melt. Previous work has suggested that Mn partitioning between apatite and silicate melt may be strongly affected by oxygen fugacity and could, therefore, act as an oxybarometer. Here, we present a new study on the partitioning of Mn between apatite and melt at high temperature (1400–1250 °C) and 1 GPa pressure, for various melt compositions and oxygen fugacities (NNO +4.7 to NNO -10). We find that there is no demonstrable variation in the partition coefficient for Mn between apatite and silicate melt (D〈sub〉Mn〈/sub〉〈sup〉Ap-m〈/sup〉) across the range of 〈em〉f〈/em〉O〈sub〉2〈/sub〉 conditions studied here. Instead, we find that D〈sub〉Mn〈/sub〉〈sup〉Ap-m〈/sup〉 varies significantly with melt composition and that in particular, the proportion of non-bridging oxygens strongly influences partitioning of Mn between apatite and melt. We propose that variations in the Mn content of natural apatite, previously thought to reflect variations in 〈em〉f〈/em〉O〈sub〉2〈/sub〉, are instead related to the degree of melt polymerisation. These findings are consistent with the results of Mn K-edge XANES spectroscopy, which demonstrate that Mn in coexisting apatite and silicate glass is present predominantly as Mn〈sup〉2+〈/sup〉 regardless of 〈em〉f〈/em〉O〈sub〉2.〈/sub〉 Furthermore, XANES spectra from a series of silicate glasses synthesised at various oxygen fugacities demonstrate that Mn〈sup〉2+〈/sup〉 is the predominant species, and that the average Mn oxidation state does not vary over a wide range of 〈em〉f〈/em〉O〈sub〉2〈/sub〉-T conditions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 February 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 506〈/p〉 〈p〉Author(s): Fanny Cattani, Pierre-Yves Gillot, Xavier Quidelleur, Anthony Hildenbrand, Jean-Claude Lefèvre, Claire Boukari, Fréderic Courtade〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Absolute dating is needed to check and calibrate the relative Martian chronology presently available from meteoritic crater counting. For such purpose, a new analytical system (KArMars) has been developed to experiment the feasibility of in-situ K-Ar for future landing planetary missions. It consists in a laser ablation-based system built to vaporize a reproducible volume of rock. Potassium content is measured by laser-induced breakdown spectroscopy (LIBS) and argon by Quadrupole Mass Spectrometry (QMS). Instrument calibration, and checking of the reliability of the measurements for Martian analyses, requires terrestrial analogues. For such purpose, total chemistry, electron microprobe analyses, flame absorption spectrometry and mass spectrometry have been performed in order to qualify the stoichiometry, the mineralogy, the K concentration and the Ar isotopic composition from a collection of old terrestrial rocks. These new analyses coupled with published data allowed us to select mineral phases (e.g. feldspars, phlogopite, muscovite, amphibole) showing a large range of K content (0.3–8.5 wt%). All these mineral phases display a K-Ar age older than 300 Ma. Hence, the content of radiogenic Ar atoms per gram is within the range of Martian samples (on the order of 1 Ga for 0.4 wt% of K). We have shown that the ablated mass can be precisely estimated by measurement of Ar extracted from a similar mineral of known amount of radiogenic Ar content per gram. Then, it has been possible to derive a well-defined relationship between the ablation time and the sample mass extracted from UV-laser ablations for different minerals. Using an isochron approach from multiple ablations of several samples analyzed as unknowns, we have shown that KArMars can be successfully used for dating samples with different compositions, and with ages varying by an order of magnitude from 0.3 to 3 Ga. In-situ K-Ar ages obtained here for plagioclase and amphibole crystals display uncertainties as low as about 10%, and with accuracy, i.e. difference with the reference age, of only 10%. Such results further support in-situ K-Ar dating based on QMS and LIBS instruments as a reliable approach to be implemented onboard Martian rovers. In case a return mission is made possible, KArMars can also be a valuable tool to select the most relevant samples for return and ensure onsite which samples are representative of most of the surface at a given landing site.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 12 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Lindsay Krall, Luis Auqué-Sanz, Jordi Garcia-Orellana, Giada Trezzi, Eva-Lena Tullborg, Juhani Suksi, Don Porcelli, Per Andersson〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉〈sup〉223〈/sup〉Ra, 〈sup〉224〈/sup〉Ra, 〈sup〉226〈/sup〉Ra, and 〈sup〉228〈/sup〉Ra isotopes have been measured in groundwaters from depths ranging 50–900 m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150 μg/L) aqueous uranium (U〈sub〉aq〈/sub〉) at 400–650 m depth. Ra isotope data is interpreted alongside previously reported 〈sup〉222〈/sup〉Rn, 〈sup〉234〈/sup〉U, and 〈sup〉238〈/sup〉U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [〈sup〉223〈/sup〉Ra/〈sup〉226〈/sup〉Ra]〈sub〉GW〈/sub〉-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [U〈sub〉aq〈/sub〉] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [〈sup〉223〈/sup〉Ra/〈sup〉228〈/sup〉Ra]〈sub〉corr〈/sub〉 (where “corr” subscript refers to a correction applied to compare [〈sup〉223〈/sup〉Ra/〈sup〉228〈/sup〉Ra]〈sub〉GW〈/sub〉 to the more commonly reported [〈sup〉226〈/sup〉Ra/〈sup〉228〈/sup〉Ra]〈sub〉GW〈/sub〉) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca〈sub〉2〈/sub〉UO〈sub〉2〈/sub〉CO〈sub〉3〈/sub〉〈sup〉0〈/sup〉 complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [〈sup〉223〈/sup〉Ra/〈sup〉228〈/sup〉Ra]〈sub〉corr〈/sub〉 and [〈sup〉224〈/sup〉Ra/〈sup〉228〈/sup〉Ra]〈sub〉GW〈/sub〉 also suggest that U(VI)〈sub〉aq〈/sub〉 precipitates as UO〈sub〉2+X〈/sub〉 at the interface between marine and non-marine groundwaters. From these data, local [U〈sub〉aq〈/sub〉] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: Available online 12 September 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Lukáš Krmíček, Lukáš Ackerman, Jakub Hrubý, Jindřich Kynický〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Orogenic lamproites represent a group of peralkaline, ultrapotassic and perpotassic mantle-derived igneous rocks that hold the potential to sample components with extreme compositions from highly heterogeneous orogenic mantle. In our pilot study, we present highly siderophile element (HSE) and Re〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Os isotope systematics of Variscan orogenic lamproites sampled in the territories of the Czech Republic, Austria and Poland, i.e., from the termination of the Moldanubian and Saxo-Thuringian zones of the Bohemian Massif. Orogenic lamproites of the Bohemian Massif are distinguished by variably high contents of SiO〈sub〉2〈/sub〉, high Mg# and predominant mineral associations of K-rich amphibole and Fe-rich microcline. The HSE show (i) consistently very low contents in all investigated orogenic lamproites compared to the estimated concentrations in majority of mid-ocean ridge basalts, hotspot-related volcanic rocks (e.g., ocean island basalts, continental flood basalts, komatiites, some intraplate alkaline volcanic rocks such as kimberlites and anorogenic lamproites) and arc lavas, and (ii) marked differences in relative and absolute HSE abundances between the samples from the Moldanubian and Saxo-Thuringian Zone. Such a regional dependence in HSE from mantle-derived melts is exceptional. Orogenic lamproites have highly variable and high initial suprachondritic 〈sup〉187〈/sup〉Os/〈sup〉188〈/sup〉Os values (up to 0.631) compared with rather chondritic to subchondritic Os isotope values of the young lithospheric mantle below the Bohemian Massif. The highly radiogenic Os isotope component in orogenic lamproites may be derived from preferential melting of metasomatised vein assemblages sitting in depleted peridotite mantle. This process appears to be valid generally in the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area. As a specific feature of the orogenic lamproites from the Bohemian Massif, originally ultra-depleted mantle component correlative with remnants of the Rheic Ocean lithosphere in the Moldanubian Zone was metasomatised by a mixture of evolved and juvenile material, whereas the lithospheric mantle in the Saxo-Thuringian Zone was enriched through the subduction of evolved crustal material with highly radiogenic Sr isotope signature. As a result, this led to observed unique regionally dependent coupled HSE, Rb〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Sr and Re〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Os isotope systematics.〈/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-S0009254119303973-ga1.jpg" width="301" alt="Unlabelled Image" title="Unlabelled Image"〉〈/figure〉〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 30 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 530〈/p〉 〈p〉Author(s): M. Dolores Basallote, Carlos R. Cánovas, Manuel Olías, Rafael Pérez-López, Francisco Macías, Sergio Carrero, Carlos Ayora, José Miguel Nieto〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Efflorescent sulfate salts constitute a transient storage of acidity and metals during the dry season in mining areas affected by acid mine drainage, especially under semiarid climates. The main goal of this work was to study the metal partitioning among the dissolved and solid phases through the evaporative precipitation sequence of extremely metal-rich mine waters. The evaporative sequence was induced in the laboratory under controlled conditions for 24 days. The loss of water caused a progressive decrease of pH values (from 2.71 to 1.33) and increase of metal concentrations (e.g., from 1826 to 17800 mg/L of Al, 836 mg/L to 9783 mg/L of Fe and 301 to 2879 mg/L of Zn), which caused the precipitation of efflorescent sulfate salts. The precipitated salts were mainly composed of a mixture of minerals of Ca (gypsum, CaSO〈sub〉4〈/sub〉·2H〈sub〉2〈/sub〉O), Al (alunogen, Al〈sub〉2〈/sub〉(SO〈sub〉4〈/sub〉)〈sub〉3〈/sub〉·17H〈sub〉2〈/sub〉O), Fe (copiapite, Fe〈sup〉2+〈/sup〉Fe〈sup〉3+〈/sup〉〈sub〉4〈/sub〉(SO〈sub〉4〈/sub〉)〈sub〉6〈/sub〉(OH)〈sub〉2〈/sub〉·20H〈sub〉2〈/sub〉O and melanterite Fe〈sup〉2+〈/sup〉SO〈sub〉4〈/sub〉·7H〈sub〉2〈/sub〉O) and Mg (hexahydrite, MgSO〈sub〉4〈/sub〉·6H〈sub〉2〈/sub〉O), although the proportion of each mineral phase varied throughout the experiment. A preferential precipitation of Al and Mg sulfate salts, together with melanterite, was observed until the second week of the experiment, with evaporation rates lower than 70%. The precipitation of gypsum predominated with higher evaporation rates, reaching values higher than 80% of the mineral assemblage. The evaporative precipitation sequence was modeled using PHREEQC, obtaining good agreement with experimental data for gypsum, but failing in turn to accurately reproduce the evaporative sequence of other minerals such as hexahydrite, alunogen and melanterite. A metal partitioning pattern was observed during the evaporative precipitation sequence. Fe-sulfate minerals (e.g., copiapite and melanterite) present a higher affinity for Cu, Y and Th (and other trace metals such as Mn, Cd or Sc) while Al and Mg sulfate salts would retain Zn, Co, Ni and to a lesser extent Cr. In the case of gypsum, found prominently in the mineral assemblage during the experiment, it seems to have affinity for REE. This metal partitioning pattern is not observed in field data reported in literature. Such discrepancies, as well as those observed between modeled and experimental results, must be investigated in further studies.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0009254119304528-ga1.jpg" width="457" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 16 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Adam C. Denny, Ian J. Orland, John W. Valley〈/p〉 〈h5〉ABSTRACT〈/h5〉 〈div〉〈p〉Diagenetic minerals preserve records of burial processes that overprint records of seawater chemistry and impact reservoir porosity. The Mississippian-Devonian aged Bakken Formation in the Williston Basin is a reservoir rock of economic importance whose productivity is affected by diagenetic carbonates, particularly dolomite-ankerite-series carbonates. To investigate how diagenetic carbonate alteration manifests in the Bakken and how that might change across the basin, a combined δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C isotope dataset for diagenetic carbonates was collected by in situ SIMS analysis of 10-μm spots from nine drill holes covering a ∼250 km transect of the middle Bakken member. Observed core-to-rim isotopic variability in these small Fe-zoned dolomites and calcites frequently exceeds 10‰ in both δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C, indicating significant changes in thermal and chemical conditions during cementation. Individual ankeritic growth bands can be correlated across the basin by systematic similarities in minor-element compositions and isotope ratios. In the central part of the basin, δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C trends at sub-mm-scale decline consistently towards the rims of ankerite-series carbonates, which is interpreted to reflect mineral growth coincident with rising temperatures and an increasing organic contribution to inorganic carbon during burial. The most abrupt shifts in δ〈sup〉18〈/sup〉O and δ〈sup〉13〈/sup〉C (changes as large as 12‰ for δ〈sup〉18〈/sup〉O and 6.5‰ for δ〈sup〉13〈/sup〉C within distances 〈 15 µm) are observed along the basin margin, and are believed to preserve signals of hydrocarbon expulsion and fluid infiltration along basin margins. Based on the data, we conclude that ankerite-series carbonates preserve records of prolonged thermal and chemical processes that operated basinwide. The isotopic trends presented here show diagenetic rock evolution from shallow dolomite formation to deep burial ankerites, as well as the isotopic responses of carbonates to changing fluids and availability of organic δ〈sup〉13〈/sup〉C. These results provide important constraints on how carbonate isotope records may be altered during burial in organic-rich sedimentary rocks and emphasize the need for caution in using bulk powder samples for geochemical analysis in carbonate systems.〈/p〉〈/div〉

Description: 〈p〉Publication date: 30 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 530〈/p〉 〈p〉Author(s): Hiroyuki Hoshi, Hideki Iwano, Tohru Danhara, Hikoma Oshida, Hiroki Hayashi, Yukito Kurihara, Yukio Yanagisawa〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The age assigned to the Miocene N7/N8 (M4/M5) planktonic foraminifera zone boundary is markedly different from the astronomically tuned Neogene time scales published in 2004 (ATNTS2004) and 2012 (ATNTS2012). This difference is primarily because of the lack of reliable assignment of the zone boundary to magnetostratigraphy, which needs to be resolved for the better refinement of the geologic time scale. Our previous biostratigraphic results from an onland sedimentary sequence of the upper Ichishi Group in central Japan allow location of the stratigraphic position of the lowest occurrence datum (LOD) of 〈em〉Praeorbulina sicana〈/em〉, a key planktonic foraminifera species for defining the N7/N8 (M4/M5) zone boundary. In this study, we performed laser ablation-multiple collector-inductively coupled plasma mass spectrometry U–Pb dating on zircons from two felsic tuff beds in the Ichishi sequence. The age of the zone boundary is approximated by a U–Pb age of 17.03 ± 0.11 Ma for a tuff bed ∼2 m above the boundary. We also conducted a magnetostratigraphic analysis in which site-mean characteristic remanent magnetization directions were determined for 19 sites and showed a reverse–normal–reverse (R–N–) stratigraphic change in polarity. The N7/N8 (M4/M5) zone boundary is within the upper reverse polarity zone. The R–N– sequence can be correlated with a stratigraphic portion corresponding to chrons C5Dr–C5Dn–C5Cr; hence, the age of the zone boundary is within Chron C5Cr. Our results are thus compatible with the age (16.97 Ma) assigned to the boundary in ATNTS2004 rather than ATNTS2012 and should be considered when the Neogene time scale is updated. A normal polarity subzone found in the upper reverse polarity zone can also be dated at ∼17.0 Ma and is probably correlated with a short normal polarity episode in Chron C5Cr. In terms of tectonic rotation, the site-mean magnetization directions determined in this study suggest an ∼35° clockwise rotation in Ichishi with respect to the North China Block in the Asian continent since ∼17.2 Ma (the upper boundary age of Chron C5Dn). Clockwise rotation of Southwest Japan, which occurred in association with backarc opening of the Japan Sea, was probably in progress during deposition of the upper Ichishi Group sediments.〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Frank S. Spear, Oliver M. Wolfe〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The composition of a porphyroblast such as garnet is not governed by equilibrium relations when it nucleates and grows after considerable overstepping. The approach followed here is to assume the composition is controlled by the maximum driving force (MDF) or parallel tangent model and to examine the potential pitfalls for assuming equilibrium. The MDF model ensures Fe-Mg partitioning equilibrium between the matrix phases and growing porphyroblast, but the net transfer equilibrium controlling grossular content has a different stoichiometry from the equilibrium relations. An important result of this study is that garnet zoning profiles generated using this model and assuming chemical fractionation are nearly identical to zoning profiles grown assuming continuous equilibrium and it is impossible to discern whether garnet grew under near-equilibrium conditions or only after considerable overstepping by examination of the zoning profile alone.〈/p〉 〈p〉Pressure–temperature paths calculated using the method of intersecting isopleths from simulations in which garnet was grown after overstepping and assuming the MDF model typically display nearly isothermal loading or loading with minor heating paths, even when the garnet was grown under isothermal and isobaric conditions. Garnet-plagioclase barometry on the garnet core and rim also suggests loading during garnet growth, even for crystals grown isothermally and isobarically. Garnet-biotite Fe-Mg exchange thermometry does, however, recover the correct temperature to within 10 °C for both the garnet core and rim.〈/p〉 〈p〉Comparison of natural samples to the theoretical calculations suggests that the local effective bulk composition in the vicinity of garnet is depleted in Na〈sub〉2〈/sub〉O, in addition to FeO, MnO and CaO thus rendering calculation schemes that rely on knowledge of the bulk composition difficult to administer. To the extent that garnet has nucleated and grown after considerable overstepping, application of methods of extracting P–T paths from the chemical zoning in garnet will yield incorrect results if those methods assume continuous equilibration with the matrix. The major discrepancy comes from calculation of the P–T conditions of the garnet core. Simulations and natural samples suggest that under typical scenarios the garnet rim is in near equilibrium with the matrix assemblage so equilibrium approaches (thermobarometry or intersecting isopleths) should recover the peak metamorphic conditions with reasonable assuredness.〈/p〉 〈/div〉

Description: 〈p〉Publication date: 20 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 529〈/p〉 〈p〉Author(s): D.J. Cherniak〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Diffusion of helium was characterized in natural zircon that had been implanted with Kr at a range of energies to simulate the effects of alpha recoil in natural radiation damage. Polished slabs of zircon, oriented either normal or parallel to c, were implanted with Kr〈sup〉+〈/sup〉 ions at a series of energies to produce a damaged region ∼ 1 μm in thickness, then implanted with 100 keV 〈sup〉3〈/sup〉He at a dose of 1 × 10〈sup〉15 3〈/sup〉He/cm〈sup〉2〈/sup〉, which placed the 〈sup〉3〈/sup〉He within the region damaged by the Kr ion implantation. The implanted zircons were annealed isothermally in Pt capsules in 1-atm furnaces at temperatures ranging from 51 to 252 °C and times ranging from 15 min to 3 weeks. 〈sup〉3〈/sup〉He distributions following experiments were measured with Nuclear Reaction Analysis using the reaction 〈sup〉3〈/sup〉He(d,p)〈sup〉4〈/sup〉He. For diffusion in zircon normal to c, we obtain the following Arrhenius relation:〈/p〉 〈p〉D = 1.7 × 10〈sup〉−8〈/sup〉 exp(−71 ± 2 kJ mol〈sup〉−1〈/sup〉/RT) m〈sup〉2〈/sup〉 sec〈sup〉−1〈/sup〉.〈/p〉 〈p〉This activation energy for diffusion is about half of the value for He diffusion in zircon from the same sample locality but without Kr implantation, measured with an otherwise similar experimental and analytical approach (Cherniak et al., 2009). He diffusivities in the Kr-implanted material are also significantly faster. Further, He diffusivities normal and parallel to c in the Kr-implanted zircon are similar, in contrast to the marked diffusional anisotropy (with differences of two orders of magnitude for diffusion normal and parallel to c) observed for He diffusion in this zircon in samples not implanted with Kr.〈/p〉 〈p〉These data indicate that heavily damaged zircon will have very limited retentivity of helium. Only those zircons larger than a few hundred μm will have mean closure temperatures exceeding typical earth surface temperatures; mean closure temperatures for He in highly damaged zircon will be about ∼200 °C lower than those for relatively undamaged zircon. At 200 °C, equivalent amounts of He will be lost from severely damaged zircon at a rate about a million times faster than from relatively undamaged zircon. For retentivity of He comparable to that for low-damage zircon at 200 °C, the severely damaged zircon would require residence temperatures of ∼ -10 °C.〈/p〉 〈/div〉

Description: 〈p〉Publication date: Available online 15 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Richard W. Carlson, Marion Garçon, Jonathan O’Neil, Jesse Reimink, Hanika Rizo〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Recycling of crust into the mantle has left only small remnants at Earth’s surface of crust produced within a billion years of Earth formation. Few, if any, of these ancient crustal rocks represent the first crust that existed on Earth. Understanding the nature of the source materials of these ancient rocks and the mechanism of their formation has been the target of decades of geological and geochemical study. This traditional approach has been expanded recently through the ability to simultaneously obtain U-Pb age and initial Hf isotope data for zircons from many of these ancient, generally polymetamorphic, rocks. The addition of information from the short-lived radiometric systems 〈sup〉146〈/sup〉Sm-〈sup〉142〈/sup〉Nd and 〈sup〉182〈/sup〉Hf-〈sup〉182〈/sup〉W allows resolution of some of the ambiguities that have clouded the conclusions derived from the long-lived systems. The most apparent of these is clear documentation that Earth experienced major chemical differentiation events within the first tens to hundreds of millions of years of its formation, and that Earth’s most ancient crustal rocks were derived from these differentiated sources, not from primitive undifferentiated mantle. Eoarchean rocks from the North Atlantic Craton and the Anshan Complex of the North China Craton have sources in an incompatible-element-depleted mantle that dates to 4.4-4.5 Ga. Hadean/Eoarchean rocks from two localities in Canada show the importance of remelting of Hadean mafic crust to produce Eoarchean felsic crust. The mafic supracrustal rocks of the Nuvvuagittuq Greenstone Belt are a possible example of the Hadean mafic basement that is often called upon to serve as the source for the high-silica rocks that define continental crust. Many, but not all, ancient terranes show a shift in the nature of the sources for crustal rocks, and possibly the physical mechanism of crust production, between 3.0-3.6 Ga. This transition may reflect the initiation of modern plate tectonics. Eoarchean/Hadean rocks from some terranes, however, also display compositional characteristics expected for convergent margin volcanism suggesting that at least some convergent margin related magmatism began in the Hadean. The persistence of isotopic variability in 〈sup〉142〈/sup〉Nd/〈sup〉144〈/sup〉Nd into the mid-Archean, and the eventual reduction in that variability by the end of the Archean, provides new information on the efficiency by which mantle convection recombined the products of Hadean silicate-Earth differentiation. The rate of crust production and recycling in the Hadean/Archean, however, is not resolved by these data beyond the observation that extreme isotopic compositions, such as expected for Hadean evolved, continent-like, crust are not observed in the preserved Eoarchean rock record. The lack of correlation between 〈sup〉142〈/sup〉Nd/〈sup〉144〈/sup〉Nd and 〈sup〉182〈/sup〉W/〈sup〉184〈/sup〉W variation in Archean rocks suggests that these two systems track different processes; the Sm-Nd system mantle-crust differentiation while Hf-W is dominated by core formation. The major silicate differentiation controlling Sm/Nd fractionation occurred at ∼4.4 Ga, possibly as a result of the Moon-forming impact, after the extinction of 〈sup〉182〈/sup〉Hf.〈/p〉〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S0009254119304280-ga1.jpg" width="301" alt="Graphical abstract for this article" title=""〉〈/figure〉〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 17 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Michael M. Goodman, Gregory T. Carling, Diego P. Fernandez, Kevin A. Rey, Colin A. Hale, Barry R. Bickmore, Stephen T. Nelson, Jeffrey S. Munroe〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Atmospheric deposition to urban areas and mountain snowpack often contains a mixture of playa dust and anthropogenic aerosols, yet the relative contribution of trace elements from each source is not well understood. To evaluate dust contributions from regional playas to an urban area, we sampled playa dust sources, urban dust deposition, and snow dust deposition across the Wasatch Front in northern Utah, USA. Dust samples were analyzed for trace and major element chemistry, grain size distribution, and mineralogy. Playa, urban, and snow dust samples contained similar mineralogy, dominated by silicate, carbonate, and evaporite minerals. Grain size distribution between playa, urban, and snow dust samples was also similar, suggesting that playas are a primary dust source for the region. Principal component analysis and enrichment diagrams revealed similar chemistry of playa, urban, and snow dust with some exceptions. Compared with playa dust, urban and snow dust had lower concentrations of Li, Na, Sr, U, Mg, and Ca and higher concentrations of Fe, Al, Be, Sb, Se, Mo, Cr, La, and Cu. The first set of elements are found in evaporite- and carbonate-minerals while the second set of elements are sourced from anthropogenic activities. Mass balance calculations suggest that 90% of the dust mass deposited on the Wasatch Front is from playas, with small additions from local sources that alter the dust chemistry. Sequential leaching of dust samples showed that some elements were readily leached with weak acids, including B, K, Na, Sr, Ca, U, Mo, Cd, and Se, suggesting that they are environmentally available and may serve as important macronutrients or contaminants to ecosystems. This is the first study to directly compare the geochemistry of playa dust sources with dust deposition to urban areas and mountain snowpack in the western US, with implications for understanding how water diversions, land use changes, and population growth may affect the regional dust cycle in the future.〈/p〉〈/div〉

Description: 〈p〉Publication date: 30 December 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 530〈/p〉 〈p〉Author(s): Seema Kumari, Debajyoti Paul, Andreas Stracke〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge–Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ∼30% and ∼75% of the present-day mass of the CC, respectively. Models proposing formation of most (∼90%) of the present-day CC during the initial 1 Gyr or nearly 50–60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (∼ 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).〈/p〉〈/div〉

Description: 〈p〉Publication date: Available online 5 October 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology〈/p〉 〈p〉Author(s): Nathan Cogné, David M. Chew, Raymond A. Donelick, Claire Ansberque〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The LA-ICP-MS method is becoming increasingly popular for uranium determinations in fission track dating of apatite, zircon or titanite. This is because the approach has several advantages over the classical external detector method (EDM), including faster sample throughput, simultaneous acquisition of additional data (such as U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb age information and trace element abundances), while removing the need for neutron irradiation. Two different approaches are used to determine U contents in LA-ICP-MS fission track dating: an absolute dating approach, or a zeta-based determination analogous to the classical EDM. Absolute age dating by LA-ICP-MS potentially suffers from small but 〈em〉systematic〈/em〉 deviations in apatite U contents, which in turn propagate through to minor systematic deviations in the accuracy of absolute fission track age determinations. A zeta-based approach typically requires time-consuming counting of large numbers of zeta-standard grains (usually Durango apatite) so as to yield a precise zeta factor for every LA-ICP-MS session containing unknowns. The modification of the zeta-based approach proposed here has two major advantages. Firstly, it employs just one large primary LA-ICP-MS session to determine a precise primary zeta factor on a large number of counted Durango primary zeta grains. During subsequent secondary LA-ICP-MS sessions with unknowns, no further fission track counting of the primary zeta standard is required. This is because we reanalyse a subset of the primary zeta grains to calculate a session-specific zeta fractionation factor, which is related to variations in the instrumental operating conditions (primarily plasma tuning) between primary and secondary LA-ICP-MS sessions. This enables us to ‘reuse’ the primary zeta factor, and thus avail of its precision derived from the large spontaneous track count. The second advantage is that reusing the primary zeta grains by applying a session-specific zeta fractionation factor allows us to verify that background and drift corrections applied during the secondary LA-ICP-MS session were fully appropriate. This method has been successfully tested by dating samples of known apatite fission track age, by comparing EDM and LA-ICP-MS data from the same sample and by participating in a round robin test between international fission track laboratories where ‘blind’ fission track dating of two unknown samples was undertaken. Our LA-ICP-MS apatite fission track dating approach is also easily modifiable for fission track dating of zircon or titanite if suitable age standards are employed.〈/p〉〈/div〉

Description: 〈p〉Publication date: 5 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 531〈/p〉 〈p〉Author(s): Céline Dessert, Clémentine Clergue, Alain Rousteau, Olivier Crispi, Marc F. Benedetti〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The important fertilizing role of atmospheric dust, and particularly African dust, in tropical rainforests is increasingly recognized but still poorly quantified. To better evaluate dust input into the Caribbean basin, we sampled critical zone compartments of a small forested volcanic catchment in Guadeloupe (soils, parent rock, atmospheric dust, plants, soil solutions, stream and rain waters). The aims of this study are to track sources of cation nutrients (Ca, Mg, K, Sr) developed on highly weathered soil in the rainforest of Guadeloupe, to quantify plant recycling of these nutrients, and to identify constraints on regolith development and its associated nutrient pool.〈/p〉 〈p〉In the Quiock Creek catchment, a large isotopic range of 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr and εNd values was observed despite the small scale of observation. Sr isotopic composition of the dissolved load varied from 0.7084 in rainfall to 0.7110 in soil solution, whereas it ranges between 0.7068 and 0.7153 for soil samples and between 0.7096 and 0.7102 for plants. The Nd isotopic composition varied between -8.39 in near-surface soil samples to 2.71 in deeper soil. All samples had an intermediate signature between that of the bedrock endmember (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.7038; εNd = 4.8) and the atmospheric endmember (sea salt: 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.7092 and Saharan dust: 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.7187, εNd=-11.5).〈/p〉 〈p〉The regolith was built on pyroclastic deposits, but, because of extreme leaching, the regolith has lost its original bedrock signature and inherited an exogenous atmospheric signature. Our results show that only the chemical weathering of the fresh near-surface minerals can provide nutrients to the ecosystem (first 30 cm). However, this dust weathering is too low to sustain the tropical forest ecosystem on its own. The cationic mass balance at the catchment scale, as well as the Sr isotopic signature, show that cation and Sr fluxes are of atmospheric origin only and that original bedrock no longer participates in nutrient cycles. The vegetation reflects the 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr of the dissolved pool of atmospheric Sr.〈/p〉 〈p〉At the soil-plant scale, the cation-nutrient fluxes provided by vegetation (litter fall + leaf excretion) are major compared to input and output fluxes. The annual Ca, K, Sr and Mg fluxes within the vegetation are, respectively, 31, 28, 20 and 3 times greater than the exported fluxes at the outlet of the basin. The residence time of nutrients in the vegetation is 16 years for K and close to 45 years for Sr, Ca and Mg. These results emphasize the highly efficient vegetative turnover that dominates the nutrient cycle in the Quiock Creek catchment.〈/p〉 〈p〉This first characterization of biogeochemical cycles in the Guadeloupean rainforest suggests that the forest community of Quiock Creek is sustained by a small near-surface nutrient pool disconnected from the deep volcanic bedrock. We also demonstrated that, even with efficient nutrient recycling, Saharan dust plays a significant role in maintaining ecosystem productivity in Guadeloupe over long-time scales.〈/p〉 〈/div〉

Description: 〈p〉Publication date: 20 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 532〈/p〉 〈p〉Author(s): Benjamin F. Walter, Manuel Scharrer, Mathias Burisch, Olga Apukthina, Gregor Markl〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Mixing of compositionally contrasting fluids is widely accepted as the major ore-forming process in unconformity-related hydrothermal Pb-Zn-fluorite-barite veins. Although the general process is relatively well understood, the temporal evolution of such fluid systems which may result in distinct mineralogical changes of the ore and gangue minerals precipitating are insufficiently constrained. One specific example is the occurrence of late-stage siderite-chalcopyrite-gersdorffite mineralization in 28 hydrothermal veins of the Schwarzwald, SW Germany post-dating the major fluorite-barite-galena-sphalerite phase which is very common in the whole area. To investigate the underlying process for this late-stage Cu-Ni-stage, published fluid inclusion data have been complemented by detailed confocal Raman micro-spectroscopy of fluid inclusions, petrographic observations and electron microprobe analyses.〈/p〉 〈p〉Petrography as well as mineral compositions of sphalerite, gersdorffite, pyrite, arsenopyrite, fahlore and galena record a gradual, but significant influx of Cu and Ni into a prevailing Pb-Zn hydrothermal system. This influx caused a distinct shift towards lower fO〈sub〉2〈/sub〉 and Pb + Zn + Sb + Ag (+ S) in the hydrothermal system at the transition from the Pb-Zn to the Cu-Ni mineralization stage. This transition in mineralogy and the related gradual change of the hydrothermal fluid can be explained by changes in the relative proportion of involved fluid components. However, the transition to the Cu-Ni mineralization stage by fluid influx is significantly different compared to the common rapid fluid mixing from below (which is thought to be responsible for the typical Pb-Zn veins) and probably records the gradual transition from long-term stable deep fluid reservoirs with constant chemical compositions towards more shallow and perturbed fluid reservoirs during rifting and exhumation.〈/p〉 〈/div〉

Description: 〈p〉Publication date: 20 January 2020〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 532〈/p〉 〈p〉Author(s): A.M. Balbas, K.A. Farley〈/p〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The geomagnetic field prevents a portion of incoming cosmic rays from reaching Earth’s atmosphere. During magnetic reversals and excursions, the field strength can decrease by up to 90% relative to the modern-day value. During such anomalies, cosmic ray bombardment to Earth’s atmosphere increases as evident from atmospheric 〈sup〉10〈/sup〉Be anomalies recorded in sediment and ice cores. However, how the flux of cosmic rays to Earth’s surface varies during such geomagnetic anomalies is not well constrained. We measured fossil cosmogenic 〈sup〉3〈/sup〉He in olivine from the tops of two pairs of 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar age-dated Tahitian lava flows that erupted during the Matuyama-Brunhes reversal precursor event. We corrected these raw values for the diffusive loss of helium caused by heating from the overlying flow with a diffusion model using cooling rates and maximum temperature conditions based on field measurements of active lava flows from Kilauea, Hawaii. We assume the maximum temperature suggested by field measurements and thus present a limiting case for the highest diffusive loss corrections and thus the highest paleo-production rates. Based on paleomagnetic field strength reconstructions and scaling factor models, the upper limits of the corrected 〈em〉in situ〈/em〉 〈sup〉3〈/sup〉He paleo-production rates (100 ± 23, 144 ± 35 atoms g〈sup〉−1〈/sup〉 a〈sup〉−1〈/sup〉) are in agreement with those expected during the period of a geomagnetic field strength low when these flow tops were exposed. However, the more plausible contact temperatures (〈700°C maximum temperature in diffusion model) are associated with diffusion corrected paleo-production rates lower than those predicted by scalar models. This potential underestimation is likely a function of changes in local non-dipole field components, atmospheric density and/or an overestimation of the dipole field strength reduction during the M-B precursor event.〈/p〉〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Pierre Josso, Ian Parkinson, Matthew Horstwood, Paul Lusty, Simon Chenery, Bramley Murton〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Accurate age models for marine ferromanganese (Fe-Mn) crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity. However, no absolute method exists for dating these deposits beyond the age of 10 Myr, which requires the combination of a number of approaches. Here, we present a composite age model for a 15 cm thick Fe-Mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m, on the summit of Tropic Seamount, in the north-east Atlantic. The age model is based on cross-validation of laser-ablation U-Pb dating, Co-chronometry and Os isotopes. These enable robust calibration of the age-depth model using the Bayesian statistical modelling of Markov Chain Monte Carlo (MCMC) simulations. The results show that this Fe-Mn crust commenced growth in the Late Cretaceous between 73 and 77 Ma, and grew at a rate between 1 and 24 mm/Myr, averaging 4 mm/Myr. The phosphatised carbonate substrate, capping Tropic Seamount and underlying most of the Fe-Mn crusts, yields a U-Pb age of 84 ± 4 Myr, and provides the upper age limit for the model. Less radiogenic excursions of 〈sup〉188〈/sup〉Os/〈sup〉187〈/sup〉Os in the vertical profile through the crust permit the identification of key inflection points in the Os isotope seawater curve at the Eocene-Oligocene and Cretaceous-Paleogene transitions. Growth rates estimated from the empirical Co-chronometer are combined with the age envelope defined by the Os data and used to validate the MCMC simulations. The model identifies five hiatuses that occurred during the Pliocene (2.5 ± 1.9–5.3 ± 1.7 Ma), Early Miocene (16 ± 1–27 ± 2 Ma), Oligocene (29 ± 2–32 ± 1 Ma), Eocene (41 ± 2–52 ± 0.6 Ma), and the Late Paleocene (55 ± 1–59 ± 1.4 Ma). A major phosphatisation event affecting the Fe-Mn core can be dated to the Late Eocene (38 ± 1.2 Ma), which coincides with a recorded change in the global oceanic system, from warm and sluggish circulation to cold and vigorous thermohaline-driven meridional overturn at the onset of Antarctic glaciation.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Xunli Shi, Shide Mao, Jiawen Hu, Jia Zhang, Jingxu Zheng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉A simple and general relation between the solubility of quartz and the density of solution is derived rigorously. Based on this relation and the pressure-volume-temperature-composition model of Mao et al. (2010), an accurate density-based model is developed for the solubility of quartz in aqueous NaCl and/or CO〈sub〉2〈/sub〉 solution up to 1273 K and 20,000 bar. The model parameters are regressed with carefully assessed experimental data. Compared to a large number of experimental data, the average absolute deviations of calculated quartz solubilities in water, aqueous NaCl solution and aqueous CO〈sub〉2〈/sub〉 solution are 5.50%, 5.24% and 7.55%, respectively, which are within experimental uncertainties, and are much better than the most competitive models in literature. Particularly, this model can predict the experimental solubility of quartz in aqueous NaCl and CO〈sub〉2〈/sub〉 solution without using any additional parameter. This model can be incorporated in software for accurate geochemical modeling. The strategy of this model should be promising for the solubilities of other minerals in water or multicomponent aqueous solutions.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Peter W. Crockford, Marcus Kunzmann, Andrey Bekker, Justin Hayles, Huiming Bao, Galen P. Halverson, Yongbo Peng, Thi H. Bui, Grant M. Cox, Timothy M. Gibson, Sarah Wörndle, Robert Rainbird, Aivo Lepland, Nicholas L. Swanson-Hysell, Sharad Master, Bulusu Sreenivas, Anton Kuznetsov, Valery Krupenik, Boswell A. Wing〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉The Proterozoic Eon spans Earth's middle age during which many important transitions occurred. These transitions include the oxygenation of the atmosphere, emergence of eukaryotic organisms and growth of continents. Since the sulfur and oxygen cycles are intricately linked to most surface biogeochemical processes, these transitions should be recorded in changes to the isotopic composition of marine and terrestrial sulfate minerals. Here we present oxygen (∆〈sup〉17〈/sup〉O, δ〈sup〉18〈/sup〉O) and sulfur (∆〈sup〉33〈/sup〉S, δ〈sup〉34〈/sup〉S) isotope records of Proterozoic sulfate from currently available data together with new measurements of 313 samples from 33 different formations bearing Earth's earliest unambiguous evaporites at 2.4 Ga through to Ediacaran aged deposits. This record depicts distinct intervals with respect to the expression of sulfate isotopes that are not completely captured by established intervals in the geologic timescale. The most salient pattern is the muted ∆〈sup〉17〈/sup〉O signatures across the 〈em〉GOE〈/em〉, late Proterozoic and Ediacaran with values that are only slightly more negative than modern marine sulfate, contrasting with highly negative values across the mid-Proterozoic and Cryogenian. We combine these results with estimates of atmospheric composition to produce a gross primary production (〈em〉GPP〈/em〉) curve for the Proterozoic. Through these results we argue that changes in 〈em〉GPP〈/em〉 across Earth history likely help account for many of the changes in the Proterozoic Earth surface environment such as rising atmospheric oxygen, large fluctuations in the size of the marine sulfate reservoir and variations in the isotopic composition of sedimentary sulfate.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): John F. Slack, Wayne C. Shanks, W. Ian Ridley, Cynthia Dusel-Bacon, Joel W. DesOrmeau, Jahandar Ramezani, Mostafa Fayek〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The Dry Creek Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit, in east-central Alaska, occurs in a Late Devonian sequence of peralkaline rhyolite tuff, minor graphitic argillite, and local peralkaline quartz-porphyry rhyolite intrusions. Principal mineralized facies are semi-massive and massive sulfide in variably silicified and graphitic rhyolite tuff, massive sulfide in graphitic argillite, disseminated sulfides in graphitic and non-graphitic rhyolite tuff, and vein-hosted sulfides in a subvolcanic, peralkaline quartz-porphyry intrusion. 〈em〉In situ〈/em〉 analysis of the sulfur isotope composition of sulfide minerals from all facies of the deposit shows a total range in δ〈sup〉34〈/sup〉S values from −48.0 to 23.1‰. This remarkable 71.1‰ variation is more than twice the largest range known for sulfides in nearly all individual VMS deposits, both modern and ancient, which typically is 〈25‰. Based on results of geochemical reaction modeling presented here, extreme sulfur isotope systematics in the Dry Creek deposit is attributed to two main processes: low-temperature (〈100 °C) kinetically controlled bacterial sulfate reduction (BSR) in anoxic to locally sulfidic bottom waters and pore fluids, and higher temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater sulfate during water-rock reactions. Deposition of organic-rich sediments prior to and during mineralization provided the key environment that promoted microbially mediated fractionations with Δ〈sup〉34〈/sup〉S〈sub〉sulfate-sulfide〈/sub〉 of up to 70‰. During BSR reaction progress modeling, residual sulfate evolved by Rayleigh distillation in pore water, producing δ〈sup〉34〈/sup〉S〈sub〉SO4〈/sub〉 values up to 58‰; normal Late Devonian seawater sulfate had δ〈sup〉34〈/sup〉S〈sub〉SO4〈/sub〉 = 22‰. BSR produced FeS precipitates with δ〈sup〉34〈/sup〉S values from −50 to −15‰. Integrated textural and δ〈sup〉34〈/sup〉S data also suggest that isotopically light H〈sub〉2〈/sub〉S produced in pore fluids by BSR was incorporated into hydrothermal fluids and early disseminated sulfide minerals during the emplacement of peralkaline quartz-porphyry rhyolite intrusions in the shallow subsurface, broadly coeval with VMS mineralization. Modeling of moderate- to high-temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater-rock interaction predicts sulfide minerals with δ〈sup〉34〈/sup〉S values from −8 to 19‰.〈/p〉 〈p〉High-precision CA-ID-TIMS U-Pb geochronology of zircons from a synvolcanic peralkaline quartz porphyry intrusion yields a weighted mean 〈sup〉206〈/sup〉Pb/〈sup〉238〈/sup〉U date of 363.02 ± 0.43 Ma (2σ total uncertainty). This date indicates that the formation of graphitic and sulfidic sediments at Dry Creek, and contemporaneous VMS mineralization, occurred at least 3.6 m.y. before, and hence are unrelated to, widespread black shale deposition during the Hangenberg Event at ca. 359 Ma.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Ying-Kui Xu, Yan Hu, Xin-Yang Chen, Tian-Yi Huang, Ronald S. Sletten, Dan Zhu, Fang-Zhen Teng〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉There is a renewed interest in K isotope geochemistry driven by the advances in analytical precision and it is emerging as a useful tracer in a variety of disciplines ranging from Earth sciences to biology. However, high-quality K isotopic data for reference materials are still limited but highly needed. Here, we report high-precision stable K isotopic compositions (δ〈sup〉41〈/sup〉K) for 23 commercially available international reference materials, including igneous, sedimentary, and metamorphic rocks, as well as an in-house seawater standard. Potassium in digested samples was separated by cation-exchange chromatography with Bio-Rad AG50W-X8 (200–400 mesh) resin in 0.5 N HNO〈sub〉3〈/sub〉 media. Potassium isotopes were measured on a Nu Plasma II high-resolution MC-ICPMS. The reproducibility of K isotopic analysis, based on over one year of measurements of pure K solutions and rock standards, was ≤0.06‰ (95% confidence interval). Synthetic solutions made by mixing single element standards to represent various rock matrices confirmed the accuracy of our methods. The 23 reference materials have δ〈sup〉41〈/sup〉K values ranging from −0.562‰ to −0.253‰ and the seawater standard has a much higher δ〈sup〉41〈/sup〉K value of 0.143‰. The comprehensive dataset presented here provides a reference for quality control and inter-laboratory comparison of high-precision K isotopic analyses for future studies.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Qinghai Guo, Britta Planer-Friedrich, Mingliang Liu, Ketao Yan, Geng Wu〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Tibet is characterized by extremely high terrestrial heat flow and widely distributed hydrothermal systems, among which many are discharging geothermal waters with extremely high arsenic concentrations from over 10 mg/L to up to 126 mg/L. The distribution of these high arsenic waters is basically in accordance with the location of the Indian continental subduction zone. A detailed analysis indicates that host rock leaching alone cannot explain the observed arsenic anomaly. Instead, substantial contribution of arsenic from the underlying magma chambers is postulated, which are likely mantle-derived intrusions that have been severely contaminated by the very thick crustal material below southern Tibet, possibly including deep-seated arsenic-rich sedimentary rocks. Alternatively, the partial melts at crustal depths reflected by the detected low-velocity and/or high-conductivity anomalies in southern Tibet, serving as the magmatic heat source of the high‑arsenic hydrothermal systems, may also form indirectly under the high-temperature conditions generated by mantle upwelling or due to crustal thickening. Low sulfide concentrations further promote high dissolved arsenic concentrations with little formation of thioarsenates and no precipitation of limiting arsenic-sulfide phases. In contrast, mantle magmas less affected by crustal contamination, serving as the heat source of many hydrothermal systems in rift zones and hot spots worldwide where the crust is thinner, are releasing magmatic fluids lower in arsenic and higher in mantle sulfur species. Overall, the existence of a magmatic heat source alone does not ensure the formation of geothermal waters with very high arsenic concentrations, and geothermal waters from sedimentary rock-hosted systems are neither necessarily arsenic-rich. The most critical factor controlling the arsenic concentration of geothermal water discharging from a magmatic hydrothermal system is the geological genesis of the magma fluid and its chemical composition.〈/p〉〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Liam Courtney-Davies, Simon R. Tapster, Cristiana L. Ciobanu, Nigel J. Cook, Max R. Verdugo-Ihl, Kathy J. Ehrig, Allen K. Kennedy, Sarah E. Gilbert, Daniel J. Condon, Benjamin P. Wade〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉Hematite (α-Fe〈sub〉2〈/sub〉O〈sub〉3〈/sub〉) is a common iron-oxide mineral known to incorporate U into its crystal lattice at up to wt% concentrations and has been previously used to date ore formation within iron-oxide copper gold and banded iron formation deposits. However, there has been no detailed evaluation of the potential challenges this novel mineral geochronometer may present for accurate temporal interpretation. We report a multi-technique U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb geochronological study comprising laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), sensitive high-resolution ion microprobe (SHRIMP) and the first isotope dilution-thermal ionization mass spectrometry (ID-TIMS) procedure and analyses using an in-situ grain micro-sampling technique. We assess intra- and inter-sample data reproducibility, and examine the compatibility of spatial and analytical resolutions to texturally and compositionally diverse samples.〈/p〉 〈p〉Five samples of hydrothermal, primary, oscillatory-zoned hematite were analysed from distinct lithologies throughout the ~1.59 Ga Olympic Dam Cu-U-Au-Ag deposit, with typical U concentrations ranging between ~20 and 1000 ppm. LA-ICP-MS mapping of isotope distributions across mm-sized grains allows pinpointing of U-rich and relatively U/Pb isotopically ‘homogeneous’ domains. Micro-domains were extracted via laser-cut square shaped trenches (100 × 100 μm). U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb datum range in complexity, with concordant age domains measured by ID-TIMS indicating that hematite can retain a near-closed U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb system over ~1.6 Ga, while also demonstrating that younger Pb-loss events may be recorded. Significant reverse discordance is common in the data, likely to be a real feature due to both internal decoupling and mobility of Pb within a single grain and U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb fractionation during microbeam analysis. These effects will obscure the interpretation of real ages for Proterozoic samples where non-zero age Pb-loss events occur, and restricts the evaluation of common Pb mixing components. As a result, microbeam analyses may be biased younger or older in terms of 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb dates in samples with complex histories. Microbeam datum compare favourably with high precision ID-TIMS ages. Although reproducible ID-TIMS U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb dates were generally not obtained, in the best-case example, a 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb weighted mean age of 1589.91 ± 0.91 Ma (MSWD = 1.3, 〈em〉n〈/em〉 = 5), within 0.2% of the Olympic Dam granite host rock age (1593.87 ± 0.21 Ma) was generated. All five hematite samples dated by LA-ICP-MS and SHRIMP yield weighted mean 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb dates within 2% of the host granite age after small degrees of data rejection. The results demonstrate that with careful sample petrography, screening and data interpretation, hematite can be considered a very useful U〈img src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"〉Pb mineral geochronometer, with potential application to all U-bearing Fe-oxide rich mineral systems.〈/p〉 〈/div〉 〈/div〉

Description: 〈p〉Publication date: 20 May 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Chemical Geology, Volume 513〈/p〉 〈p〉Author(s): Christopher J. Stefano, Samuel B. Mukasa, Joan A. Cabato〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉 〈p〉The Yellowstone hotspot track in the northwestern US has a 17-myr history that began with eruption of the voluminous Columbia River basalts (CRB), and produced an eastward track with the bimodal rhyolite and basaltic eruptive centers of the Snake River Plain (SRP) as the North American Plate drifted westwards. We have generated the first Hf isotopic data on basalts in this large igneous province along with bulk-rock trace-element abundance patterns and Sr-Nd isotopic ratios to assess the hypothesized kinship with co-located rhyolites throughout the hotspot track. The isotopic compositions for all three elements span much of the compositional range observed in mantle-derived basalts worldwide. Varying between a high of +11.3 in parts of the CRB and a low of −8.3 in some of the late SRP basalts, the new ε〈sub〉Hf〈/sub〉 values are geographically controlled, similar to the shift first documented with Sr and Nd isotopes across the “0.706 Sr line,” which marks the boundary between thin- accreted young ocean lithosphere and arc terranes in the west to thick, old cratonized crust in the east. This observation shows that there is significant lithospheric control on the isotopic compositions of both basalts and rhyolites associated with the Yellowstone hotspot track.〈/p〉 〈p〉Modeling of assimilation and fractional crystallization (AFC) processes in seven basaltic lava flows filling the 2-Ma Big Bend Ridge Caldera at Mesa Falls, Idaho, near the eastern end of the hotspot track suggests that the crustal contribution to the Nd and Hf concentration budgets increases systematically with time and was significant. A comparison of new and previously published bulk-rock major-oxide concentrations (e.g., the alkalis Na〈sub〉2〈/sub〉O and K〈sub〉2〈/sub〉O) with our previously published data for H〈sub〉2〈/sub〉O, Cl, F and S in olivine-hosted melt inclusions suggests that the volatiles are sub-crustal, possibly from the lithospheric mantle, and degas extensively during magma ascent and crystallization.〈/p〉 〈/div〉 〈/div〉