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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    Light-induced coupling process of Fe redox cycling and natural dissolved organic matters oxidative decomposition at goethite surface: Key role of reactive oxidative species

    Yang, ShanshanGe, XiaoLi, YanzhangDing, Hongrui...
    10页
    查看更多>>摘要：The interactions between widespread iron (oxyhydr)oxides (Fe oxides) and natural soil dissolved organic matters (DOM) are the dominant drivers in the geochemical behavior of Fe and the cycle of carbon in soil. Under visible light irradiation, their interactions become more complex due to the unique semiconducting structure of Fe oxides and complex components in DOM, which have not been deeply studied. In this study, the complex reactions occurred at the DOM-goethite interface under irradiation were investigated in detail. Fe-(aq)(2+) from goethite reductive dissolution was accumulated at the DOM-goethite interface, along with the generation of reactive oxidative species (ROS). The reasons could be ascribed to the enhanced generation of photoelectron from the conduct band of goethite, ligand-to-metal charge transfer and photolysis of Fe(III)-carboxylate complexes. Moreover, the generated ROS were the main driving forces in the coupling process of Fe redox cycle and oxidative decomposition of DOM at goethite surface, inducing the formation of secondary Fe minerals and chemical composition change of DOM. This work provided new insights into the light-induced coupling process of Fe semiconducting minerals reductive dissolution and dissolved organic matters oxidation in top soil.
      原文链接:
    • NSTL
    • Elsevier

    Spatial differentiation of sediment organic matter isotopic composition and inferred sources in a temperate forest lake catchment

    Keenan, BenjaminDouglas, Peter M. J.Stratigopoulos, EmeraldPark, Sanga...
    16页
    查看更多>>摘要：Freshwater sediments are important carbon reservoirs, but the extent to which different components of soil or aquatic organic matter (OM) are deposited in these sediments is not well defined. Bulk sediment carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios, as well as radiocarbon, are valuable tracers for sediment OM sources, but there are few studies comparing the isotopic composition of soil and sediment OM at the catchment scale. We analyzed spatial variation in delta C-13, delta N-15, C-14, and C:N ratios in OM from soils, stream and lake sediments, and aquatic plants and algae, in a temperate forest lake catchment in southern Quebec, and used a Bayesian model to estimate source mixtures for sediment OM. Sediments at the stream mouths entering the lake were characterized by high C:N ratios, high fraction modern carbon (Fm), and low delta C-13, indicating preferential deposition of plant-derived OM. In contrast, sediments sampled further upstream during a period of low streamflow indicated a larger proportion of microbial OM based on low C:N ratios and high delta N-15. In lake sediments we observed zonation of OM isotopic composition by water depth. Shallow sediments (0-1 m water depth) were characterized by high amounts of plant-derived OM, while intermediate-depth sediments (1-3 m) were characterized by high delta C-13, indicating an increased input of OM from aquatic plants. Deep lake sediments (> 4 m) were characterized by low delta C-13 and Fm values, which likely reflect greater input of phytoplankton OM. Stream sediments downstream of the lake exhibited high delta N-15 and low Fm values, implying a greater input of aged microbial biomass from soils. Our results indicate catchment-scale spatial differentiation in the source of OM in sediments, with zones of preferential deposition of terrestrial plant, aquatic plant, phytoplankton, and soil microbial biomass.
      原文链接:
    • NSTL
    • Elsevier

    Yan Bottinga (1932-2021)

    2页
      原文链接:
    • NSTL
    • Elsevier

    Covariation between molybdenum and uranium isotopes in reducing marine sediments

    Zhao, MingyuTarhan, LidyaShull, DavidWang, Xiangli...
    13页
    查看更多>>摘要：The isotopic compositions of redox-sensitive elements such as uranium (U) and molybdenum (Mo) have been extensively used to track the evolution of redox conditions on Earth???s surface. However, additional investigation of the behaviors of U and Mo in modern marine sediments???particularly in reducing sediments underlying oxic seawater???can bolster understanding and improve application of these systems as biogeochemical tracers. Here we present elemental and isotopic Mo and U data from reducing sediments from various sites in the Bering Sea where the extent of bioturbation is well characterized. Total Mo concentrations and total organic carbon contents (TOC) are positively correlated, as are authigenic Mo isotopic compositions and TOC, indicating the presence of authigenic Mo enrichments in these sediments. Similar to what has previously been observed in euxinic settings, we observe a positive correlation between authigenic U and Mo concentrations, and a negative correlation between the isotopic compositions of authigenic U and Mo (??238Uauth and ??98Moauth). These patterns likely reflect close associations of U and Mo reduction to organic carbon degradation. Observed variation in ??98Moauth values could reflect variations in Mo removal efficiency or Mo isotopic fractionation between particles and seawater related to porewater H2S levels. Conversely, we interpret variation in ??238Uauth values in these Bering Sea samples to be related to varying extents of U reduction and removal, coupled with anaerobic carbon degradation. There is no significant correlation between the extent of bioturbation and Mo or U isotope values, suggesting that bioturbation would likely not have a significant influence on sedimentary Mo and U records. Further, the array of U and Mo isotopic data from these sediments is distinct from those observed in anoxic sediments below euxinic seawater???suggesting that a combination of U and Mo isotopic compositions could help to more accurately resolve the redox state of depositional environments from past intervals of Earth???s history.
      原文链接:
    • NSTL
    • Elsevier

    Extreme geochemical heterogeneity beneath the North Tonga Arc: Interaction of a subduction zone with intraplate seamount chains

    Storch, BettinaRegelous, MarcelNoebel, Kristina M.Haase, Karsten M....
    11页
    查看更多>>摘要：We provide new geochemical and isotope data for lavas from six young submarine volcanoes from the northernmost oceanic Tonga island arc. These lavas have low concentrations of moderately incompatible trace elements, but are enriched in Nb and Ta compared to all other Tonga island arc lavas, and their Nd, Hf and Pb isotope compositions resemble those of rejuvenated lavas from the nearby Samoa hotspot. These unusual compositions indicate that the mantle source of North Tonga lavas was depleted by prior melting events, before re-enrichment by a partial melt enriched in highly incompatible elements with Nd, Hf and Pb isotope compositions similar to Samoan intraplate lavas. Samoa plume mantle is flowing southwards beneath the northern Tonga island arc and Lau Basin, in part guided by rifting of the Tonga plate lithosphere. A fluid component was later added from a mixture of subducted pelagic sediment and subducting Louisville Seamount Chain volcanic material. Our new data show that the Louisville component is only found in lavas from volcanoes occurring along similar to 100 km of the arc front, indicating that the oceanic crust loses Pb by dehydration over a relatively small depth interval during subduction. The compositions of lavas from the northern Tonga island arc are therefore influenced by material from two hotspots; southwards inflow of mantle from the active Samoa plume beneath the arc within the past 6 Ma, and a subduction component from >80 Ma old subducted Louisville Seamount Chain basalts.
      原文链接:
    • NSTL
    • Elsevier

    Nickel isotope fractionation in komatiites and associated sulfides in the hart deposit, Late Archean Abitibi Greenstone Belt, Canada

    Hiebert, R. S.Bekker, A.Houle, M. G.Rouxel, O. J....
    13页
    查看更多>>摘要：Extremely light and highly variable 860Ni values have been observed in komatiite-associated magmatic sulfides in recent studies. In this study, we examine the mechanisms of Ni isotope fractionation between silicate and sulfide liquids in the Hart komatiite-associated Fe-Ni-Cu-sulfide system. We assess the petrogenetic significance of these mechanisms using Ni isotope and concentration data. The concentration of Ni in bulk rock varies from 774 to 2690 ppm in komatiite samples with no sulfide minerals to 8380???39,300 ppm in samples almost entirely consisting of sulfide minerals. The 860Ni values vary from +0.14%o in komatiite samples with no sulfide minerals to -1.06%o in samples dominantly consisting of sulfide minerals. A theoretical model of fractionation between the komatiitic lava and sulfide xenomelt with nickel isotope exchange followed by fractional crystallization during crystallization of the sulfide melt can produce a range of 860Ni values from +0.17%o to -1.02%o in sulfiderich rocks depending on the extent of fractional crystallization and the amount of trapped melt between the sulfide mineral grains, which corresponds well with the range of values observed in these rocks. This proposed model requires fractionation of Ni isotopes between sulfide liquid and the earliest formed sulfide crystals during crystallization. Effects of later crystallization during peritectic reactions and subsolidus exsolution could be tested by in situ measurements of Ni isotopes in different textural varieties of pentlandite that formed over a large range of temperatures during cooling.
      原文链接:
    • NSTL
    • Elsevier

    The analysis of volcanic brines by freezing stage tandem LIBS-ICP-MS

    Berlo, Kimvan Hinsberg, VincentLauzeral, RomainZwillich, Florentine...
    11页
    查看更多>>摘要：Volcanic crater lakes pose unique hazards to the local population both during eruption and quiescence. The composition of crater lake fluids is therefore commonly monitored to aid hazard prediction and mitigation. Some of these fluids, such as at Kawah Ijen in Indonesia and Poas in Costa Rica, are hyper-acidic, concentrated brines (SO4-5 wt%, Cl - 2.5 wt%, Al - 5000 ppm). Analysis of such fluids requires extreme dilution due to the high total dissolved element load, which risks contamination and precipitate formation. To circumvent this undesired sample preparation and speed up the turn-around time on the analysis, we tested approaches which involved freezing the fluids to -30 degrees C and analysing them by simultaneous Laser Ablation-Inductively Coupled Plasma Quadrupole Mass Spectrometry (LA-ICP-QMS) and Laser-Induced Breakdown Spectroscopy (LIBS). LIBS is highly matrix sensitive, requiring matrix-matched reference materials. At the same time, LA-ICP-MS owes its success to being relatively insensitive to the matrix after accounting for differences in ablation yield. Analysing the fluids as a solid and sampling them by laser circumvents the usual dilution step. Fluid reference materials were prepared by adding the elements of interest to one of the crater lake fluid samples at known, but varying concentrations and by stepwise dilution of one of these fluids. Differences in ablation yield, resulting mainly from variations in the coupling of the laser with the sample, were accounted for by adding a known quantity of lithium as an internal standard. Working curves for both ICP-MS and LIBS were obtained for Al, Mg, Fe, Ca, K, Na, S and Cl, yielding relative trueness values for 4 crater lake samples of known composition of Al-7%, Ca-5%, Fe-1%, K < 1%, Mg-4%, Na-5%, S-3%, and Cl-6% for ICP-MS and Al-2%, Ca-9%, Fe-17%, K-10%, Mg-4%, Na < 1%, S-8%, and Cl-9% for LIBS. For these elements, ranging in concentration from around 400 mg/kg to 25 g/kg, LIBS yielded similar trueness results to ICP-MS. LIBS calibration without an internal standard produces acceptable results for Li only, allowing the cryo-LIBS to analyse the major elements while optimising the ICP-MS for trace-element analysis and obtaining a full suite of elements simultaneously without the need for sample dilution.
      原文链接:
    • NSTL
    • Elsevier

    Precipitation sequence of fracture-filling calcite in fractured granite and changes in the fractionation process of rare earth elements and yttrium

    Mizuno, TakashiMilodowski, Antoni EdwardIwatsuki, Teruki
    16页
    查看更多>>摘要：This study has focused on the paragenetic sequence, and variation in rare earth elements with yttrium (REY) composition, of fracture-filling calcite in the Toki Granite in the Mizunami area, central Japan. The morphological, chemical, and isotopic characteristics of the calcite and chemistry of fluid inclusions reveal that the calcite in the Toki Granite can be differentiated into four discrete generations: Calcite I (oldest) to Calcite IV (most recent). The precipitation history of calcite reflects the changes in the hydrogeochemical regime of paleogroundwaters, controlled by the evolution of groundwater by seawater infiltration associated with marine transgression and surface water infiltration associated with marine regression and uplift. The post-Archean average shale-normalized REY patterns in each generation of calcite show no significant Ce anomaly, negative Eu anomaly, and a light REY (LREY)-depleted pattern dominates. These features are also common to the Toki Granite. The consistency of the features in each generation of calcite indicates that REY was supplied from the Toki Granite by water-rock interaction. The lack of a Ce anomaly in the calcite demonstrates that groundwaters have maintained reducing conditions during the calcite precipitation. However, the fractionation of LREY and heavy REY in each generation of the calcite is more pronounced than in the granite. The fractionation process in the paleo-groundwaters from which each generation of calcite precipitated closely relates to the systematic variation of carbonate complex in the REY series and/or pH in palaeo-groundwater. The findings of this study will be important for assessing the long-term safety of geological disposal of high-level radioactive waste.
      原文链接:
    • NSTL
    • Elsevier

    Monazite as a control on Th/U in magmatic zircon

    Pineda, CamilaSchmitt, Axel K.Morata, Diego
    10页
    查看更多>>摘要：Zircon is one of the most widely studied minerals for geochronology, and it has been successfully used to determine the age of young eruptions (< 300 ka) through U-Th dating. However, the processes leading to Th/U variations between cogenetic zircon crystals that are often much larger than expected from partitioning considerations are still not fully understood, despite the critical importance of zircon Th/U variability for isochron dating, for the disequilibrium correction of radiometric ages, and for identifying different zircon generations. Here, we present (UTh)-U-238-Th-230 disequilibrium ages and trace element abundances of zircon from the Quaternary Pudahuel Ignimbrite, a caldera-forming eruption associated to the Maipo volcanic complex in central Chile. Zircon chemical signatures show a decrease of Th/U along with decreasing Ti-in-zircon temperatures that can be related to the presence of accessory minerals which incorporate Th over U into its structure. These variations can also be identified along individual depth profiles, suggesting that monazite co-crystallization is the key cause for a sharp rimward decrease in zircon Th/U. Apatite fractionation is expected to have little impact on Th/U in the remaining melt, and in consequence U-Th activity ratios in zircon because it mostly saturates at temperatures above zircon saturation, and partitioning differences between Th and U are minor. By contrast, monazite fractionation strongly controls Th/U in Pudahuel rhyolite melts, especially at low temperatures. The isochron age calculated from zircon with low Th/U (U-238/Th-232 activity >5.2) crystallized at low temperature below monazite saturation (167 +/- 8 ka; 1 sigma error) is younger than for zircon with high Th/U (U-238/Th-232 < 5.2). This presents the most precise age constraint available for Pudahuel Ignimbrite, albeit as a maximum eruption age. These results are particularly important in the study of young eruptions, where accurate ages are important to reliably interpret eruptive recurrence and volcanic chronostratigraphy, which in turn can affect the hazard awareness of volcanic systems.
      原文链接:
    • NSTL
    • Elsevier

    The importance of high precision in the evaluation of U-Pb zircon age spectra

    Gaynor, Sean P.Ruiz, MelissaSchaltegger, Urs
    9页
    查看更多>>摘要：We present high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon data from the Carboniferous Punteglias granodiorite from the Central Alpine basement in the Aar massif in Switzerland. These analyses yield complex age spectra consisting of both concordant and normally discordant dates, which form a linear array indicative of mixing of two age components within individual zircon crystals. While the upper intercept age reflects zircon growth during Variscan magmatism, the lower intercept of 24 +/- 11 Ma is coincident with the Alpine orogeny. There is poor evidence from cathodo-luminescence imagery that magmatic zircon may have younger overgrowths, therefore, we pretreated an aliquot of zircon with physical abrasion followed by chemical abrasion to remove any potential later zircon overgrowths, and to directly date the initial magmatic domains of the zircon. This pretreatment significantly reduced age dispersion and yielded an age of 335.479 +/- 0.041 Ma for the crystallization of zircon in the Punteglias granodiorite. Due to the effectiveness of this pretreatment, we interpret that the magmatic zircon was overgrown with younger domains during the Alpine orogeny. Detangling the observed complications in the Punteglias granodiorite zircons was largely possible due to the particularly high analytical precision of the ID-TIMS U-Pb dates that made it possible to detect discordance between the Pb-206/U-238 and Pb-207/U-235 systems. Lower-precision U-Pb ID-TIMS dates would not allow identification of such a two-component mixture and lead to analytically concordant points spreading over 10(5 )to 10(6) years, and potentially leading to erroneous scientific interpretation. As an example, we model two hypothetical situations of simple mixing of two age components in the U-Pb concordia space to demonstrate the effect of precision on the ability to accurately interpret geological models from complicated age spectra. Combining these new high-precision data and modeling results highlights the importance of choosing the optimal, physical and/or chemical sample pre-treatment and the challenge of accurately interpreting complicated U-Pb data in terms of crystallization age.
      原文链接:
    • NSTL
    • Elsevier