Geochimica et Cosmochimica Acta最新文献

{"title":"Crystal-scale heterogeneity of Sn isotopes in Cassiterite: implications for reconstructing ore-forming processes in magmatic-hydrothermal systems","authors":"Liang Han, Guo-Guang Wang, Pei Ni, Jun-Yi Pan, Jia-Xin She, Wei Tan, Weiqiang Li","doi":"10.1016/j.gca.2025.09.016","DOIUrl":"https://doi.org/10.1016/j.gca.2025.09.016","url":null,"abstract":"Cassiterite, as the primary ore mineral for critical metal tin, often forms in a variety of granite-related magmatic-hydrothermal systems. Sn isotopes in cassiterite provide unique insights into the mechanisms of tin precipitation and fluid evolution in magmatic-hydrothermal systems, but Sn isotope data based on traditional sample dissolution methods or <ce:italic>in situ</ce:italic> on cassiterite fragments have led to ambiguous interpretations. In this study, we first select a well-developed cassiterite crystal from the Weilasituo deposit in North China to perform <ce:italic>in situ</ce:italic> Sn isotope and trace element analyses, accompanied by scanning electron microscopy-cathodoluminescence (SEM-CL) imaging and electron backscatter diffraction (EBSD). The cassiterite sample reveals distinct primary sector zonations, including CL-bright and CL-dark zones. EBSD analyses indicate that the CL-bright zones are commonly developed within each individual crystallographic plane, while CL-dark zones are formed along the boundaries of different planes such as {001} and {201}. Moreover, the CL-dark zones are predominantly hosted in the {001}, with minor overlaps into {201} domains. Our results reveal significant enrichment of trace elements (W, Nb, and Ta) and heavier Sn isotopes in the CL-dark sectors relative to the CL-bright sectors. In contrast to traditional protosite model predictions, this enrichment is primarily attributed to structural complexities along the plane boundaries. Elevated lattice defects and dislocation in these structurally disturbed regions likely promote selective incorporation of trace elements and the retention of heavier Sn isotopes. In both dark and bright sectors, Sn isotopes decreased similarly from the center outwards. The decline of Sn isotope ratio and trace element concentrations across the oscillatory growth bands from the core to the rim, caused by element depletion in hydrothermal fluids through progressive mineral precipitation, is mainly attributed to Rayleigh fractionation. Challenging the conventional view, this study reveals that the heavier Sn isotopes in dark sectors do not result from the superposition of multi-stage fluids. This finding reshapes our understanding of cassiterite formation mechanisms and has important implications for reconstructing ore-forming processes.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"18 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-09-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145103482","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Observed seasonal trends of diatom-derived C20 highly branched isoprenoids (HBIs): implications for paleoclimate studies","authors":"Megan C. Corcoran ,&nbsp;Aaron F. Diefendorf ,&nbsp;Nicholas Wiesenberg ,&nbsp;Thomas V. Lowell ,&nbsp;Gregory C. Wiles ,&nbsp;Mark A. Wilson ,&nbsp;Broxton W. Bird ,&nbsp;Hans Naake ,&nbsp;Watts L. Dietrich","doi":"10.1016/j.gca.2025.09.017","DOIUrl":"10.1016/j.gca.2025.09.017","url":null,"abstract":"<div><div>The hydrogen isotopic composition of lake water (δ²H_lw) contains hydrologic information and can be used as a recorder of lake water hydrology, including the extent of evaporation of the lake system. Initial studies indicate that the hydrogen isotopes of highly branched isoprenoids (δ²H_HBI), synthesized by lake diatoms and preserved in lake sediments are a promising proxy for constraining past δ²H_lw values that are free from terrestrial influences. However, there are many aspects of this proxy, including the seasonality of HBI production, that are unknown and need to be addressed more fully before the proxy can by widely applied. To determine when HBIs are produced throughout the year, and whether there are seasonal biases in δ²H_lw reconstructions, we deployed two sediment traps at Brown’s Lake, in northeastern Ohio. We present HBI concentrations, δ²H_HBI values, HBI carbon isotopes and bulk sediment carbon isotopes from sediment traps collected monthly for 26 months to investigate seasonality of HBIs. We observed HBIs in each of the monthly sediment traps throughout the study interval with an increase in HBI concentration during September and October, suggesting that HBIs are made throughout the year with greater production during fall. We calculated the difference between δ²H_HBI and δ²H_lw values (ε²H_HBI/lw) and observe a range in ε²H_HBI/lw values of up to 64‰, which we speculate is related to changes in the diatom communities that synthesize HBIs throughout the year and between different years. Different diatom communities may have different biosynthetic pathways or metabolisms that result in isotope effects. This study is the first that examines the seasonality of HBIs in lake sediments and provides framework for interpreting the seasonality of hydroclimate records generated from δ²H_HBI values in temperate eutrophic lakes.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 211-223"},"PeriodicalIF":5.0,"publicationDate":"2025-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145103483","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Using carbon isotopes to trace the origin of volatiles on Earth and Mars","authors":"Damanveer S. Grewal ,&nbsp;Sujoy Mukhopadhyay","doi":"10.1016/j.gca.2025.09.014","DOIUrl":"10.1016/j.gca.2025.09.014","url":null,"abstract":"&lt;div&gt;&lt;div&gt;The distinct accretionary histories of Earth and Mars – with Earth experiencing protracted growth and small contributions from outer solar system (carbonaceous, CC) materials, and Mars undergoing rapid growth with building materials drawn almost exclusively from the inner solar system (non-carbonaceous, NC) – highlight key differences in planetary formation. These contrasts underscore the importance of a comparative planetology framework for understanding the origin of volatiles in terrestrial planets. In this study, we examined the relationship between the carbon (C) isotopic compositions of planetary and planetesimal reservoirs to trace the origin of volatiles on Earth and Mars. The mean δ&lt;sup&gt;13&lt;/sup&gt;C value of magmatic C in Martian meteorites (−20 ‰) is significantly lower than that of the bulk silicate Earth (BSE), with a canonical value of −5 ‰. While basaltic achondrites, magmatic iron meteorites, and ordinary chondrites from the NC reservoir display δ&lt;sup&gt;13&lt;/sup&gt;C values similar to Martian meteorites, the BSE δ&lt;sup&gt;13&lt;/sup&gt;C value is comparable to volatile-rich CC chondrites such as CI, CM, and CR, as well as with enstatite chondrites and ureilites from the NC reservoir. If Martian magmas underwent minimal C isotopic fractionation during degassing or degassed under kinetic conditions, then the δ&lt;sup&gt;13&lt;/sup&gt;C value of the Martian mantle likely reflects accretion from thermally processed undifferentiated (ordinary chondrite-like) and differentiated NC materials. In contrast, if extensive degassing occurred via Rayleigh fractionation under equilibrium conditions, the δ&lt;sup&gt;13&lt;/sup&gt;C value of the Martian mantle would have a higher δ&lt;sup&gt;13&lt;/sup&gt;C value (−12 to −10 ‰) than that recorded in Martian meteorites – though still lighter than that of the canonical BSE δ&lt;sup&gt;13&lt;/sup&gt;C. This implies a contribution from relatively &lt;sup&gt;13&lt;/sup&gt;C-rich NC materials, potentially similar to enstatite chondrites. For BSE, although the canonical δ&lt;sup&gt;13&lt;/sup&gt;C value of –5 ‰ overlaps with those of enstatite chondrites and ureilites, the late-stage delivery of volatile-rich CC materials during the main phase of Earth’s growth, which was critical for establishing its water and nitrogen inventories, likely biased its C isotopic composition towards a CC-like signature. However, a lower mean δ&lt;sup&gt;13&lt;/sup&gt;C value of −8.4 ‰ of the MORB mantle, as proposed by recent studies, could mean that Earth’s mantle still preserves the primordial signature of &lt;sup&gt;13&lt;/sup&gt;C-poor, thermally processed NC materials accreted during the early stages of the planet’s growth. The observed heterogeneity in mantle C isotopic compositions, similar to that seen in H and N isotopes, could therefore reflect a mixed contribution from both NC and CC materials. Thus, the δ&lt;sup&gt;13&lt;/sup&gt;C value of the BSE could be lower than the canonical estimate and may align more closely with the proposed value for the MORB mantle. Taken together, these findings suggest that the contrasting accret","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"408 ","pages":"Pages 12-27"},"PeriodicalIF":5.0,"publicationDate":"2025-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145156604","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Triple iron isotopes inform iron mineralization and sedimentation at the TAG hydrothermal mound","authors":"Andy W. Heard, Margaret K. Tivey, Christopher R. German, Jerzy S. Blusztajn, Sune G. Nielsen","doi":"10.1016/j.gca.2025.09.019","DOIUrl":"https://doi.org/10.1016/j.gca.2025.09.019","url":null,"abstract":"Hydrothermal seafloor massive sulfide deposition at ocean spreading centers modifies the flux of iron from vents while producing an archive of these processes in both active and fossil hydrothermal complexes. Despite hopes of stable iron isotopes tracing mineral formation and iron cycling at these sites, the competing fractionations accompanying various mineralization processes have presented an obstacle to confidently interpreting iron isotopic datasets. We have developed a triple iron isotope proxy that can resolve some of these previously indistinguishable mineral formation histories, and applied it to a suite of samples from the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound on the Mid-Atlantic Ridge. We show that massive pyrite-dominated sulfides formed near the TAG mound surface retain primary kinetic isotope signatures indicative of their rapid formation, likely from an iron monosulfide intermediate. Pyrite-anhydrite breccias retain mixed isotopic signatures of reworked primary massive sulfides together with secondary pyrite, grown in confined conditions, in some cases in equilibrium with subsurface hydrothermal fluid. We also suggest that iron oxyhydroxide-rich chert precipitation across parts of the mound surface trapped isotopically evolved fluid within the mound, and a later generation of sulfides precipitated from this evolved fluid. Metalliferous sediments from a core recovered near the base of the TAG mound mostly fall along a primary mass fractionation law defined by mound sulfides. This is consistent with a dominant contribution of collapsed mound debris and/or black smoker sulfide fallout to the seafloor near the base of the TAG mound. However, the triple iron isotopic composition of surface sediment from this core suggests it may also contain iron oxyhydroxide fallout from the dispersing non-buoyant plume that had already undergone extensive sulfide precipitation. Triple iron isotope studies of non-buoyant plume sediments may be used to quantify the mineral fate of iron vented to the oceans, estimate variations in iron to sulfide ratios of primary vent fluids, and resolve the relative importance of low and high temperature hydrothermal flow to basin-scale iron-rich plumes in the global oceans.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"1 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145182921","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sulfur cycling at two Mars-relevant saline lakes underscores the importance of dynamic metabolic substrate concentrations for isotopic biosignatures","authors":"Christopher J. Tino, Serhat Sevgen, Alec M. Hutchings, Jaxon Dii Horne, Liam E. Walters, Timothy W. Lyons, Benjamin M. Tutolo","doi":"10.1016/j.gca.2025.09.011","DOIUrl":"https://doi.org/10.1016/j.gca.2025.09.011","url":null,"abstract":"The presence of magnesium sulfate minerals (Mg-sulfates) within the sulfate-bearing unit of Gale Crater on Mars may coincide with a major planetary drying transition at ∼ 3.5 Ga that would have had negative consequences for surface habitability. Here, we investigate the formation conditions and biosignature potential of lacustrine Mg-sulfates by contrasting the sulfur biogeochemistry of two saline lakes of the Interior Plateau of British Columbia, Canada: Basque Lake 2 and Goodenough Lake. Basque Lake 2 is characterized by magnesium-sulfate-chloride-type hypersalinity with evaporative Mg-sulfate precipitation. It exhibits invariant dissolved sulfate isotope ratios (δ<ce:sup loc=\"post\">34</ce:sup>S) throughout its porewater profiles. This uniformity is best explained by the lake’s extremely high sulfate-to-sulfide ratio (&gt;1900:1 on average), which nullifies the isotopic influence of dissimilatory microbial sulfate reduction (MSR) on the sulfate reservoir. In contrast, Goodenough Lake is alkaline sodium-carbonate-chloride type with unambiguous evidence of temporally and spatially dynamic sulfur cycling, as indicated by high porewater sulfate δ<ce:sup loc=\"post\">34</ce:sup>S and low chromium-reducible sulfide δ<ce:sup loc=\"post\">34</ce:sup>S values, as well as porewater sulfide concentrations that vary across field seasons and meter-scale distances. At both lakes, the upper bound on MSR appears related to physiological limitations of sulfate reducing bacteria. Specifically, Basque Lake 2 waters reach inhibitory salinity levels, while Goodenough Lake porewaters can approach toxic sulfide concentrations. Importantly, these contrasting systems indicate that the sulfate isotopic signature of MSR is masked in Mg-sulfate-precipitating lakes, challenging the use of δ<ce:sup loc=\"post\">34</ce:sup>S values in Mg-sulfate minerals to interpret the ancient sulfur cycle and/or paleohabitability on Mars. However, abundant sulfate with high δ<ce:sup loc=\"post\">34</ce:sup>S values that co-occurs with mineral sulfides exhibiting low δ<ce:sup loc=\"post\">34</ce:sup>S values in a low-temperature environment would match the MSR profile of Basque Lake 2 and defy an abiotic explanation. In the context of evaluating potential biosignatures, isotopic systems defined by sufficient, dynamic metabolic substrate reservoirs are preferable to substrate-replete conditions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"4 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145182889","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Unravelling the extent of ocean euxinia during the late Paleoproterozoic: Constraints from Re–Os and Mo isotopes","authors":"Achyuth Venugopal ,&nbsp;Gyana Ranjan Tripathy ,&nbsp;Vineet Goswami ,&nbsp;Tavheed Khan ,&nbsp;Lukáš Ackerman","doi":"10.1016/j.gca.2025.09.013","DOIUrl":"10.1016/j.gca.2025.09.013","url":null,"abstract":"<div><div>Oceanic euxinia was one of the prime constraints for the Proterozoic nutrient availability and hence, eukaryotic expansion. Although available studies mostly point to the occurrence of sulfidic to ferruginous conditions in the Paleoproterozoic ocean, data on the areal extent of sulfidic settings are scarce. In this study, geochemical (major and trace elements, and iron-speciation), and Re–Os and Mo isotopic data for black shales from the Cumbum Formation, Cuddapah Basin (India) are presented. These datasets are used to constrain the depositional age and environment, and euxinic extent for the late Paleoproterozoic ocean. The Re–Os isochron for these shales provides a depositional (Model 3) age of 1658 ± 50 Ma (2σ, n = 10), with an initial ¹⁸⁷Os/¹⁸⁸Os (Os_i) ratio of 0.03 ± 0.53. Distribution of enrichment factors (EF) for redox-sensitive elements and their (Mo_EF/U_EF) ratio for the Cumbum shales indicates shale deposition in an anoxic basin. Further, high Fe_HR/Fe_T (0.61 ± 0.18 (1σ)) and Fe_Py/Fe_HR (0.72 ± 0.14 (1σ)) ratios confirm euxinic to ferruginous bottom-water conditions. The δ⁹⁸Mo values of these shales (+0.68 ± 0.13 ‰ (1σ)) are comparable to Paleoproterozoic shales (+0.51 ± 0.57 ‰ (1σ)) and systematically lower in comparison to the present-day average seawater (+2.34 ± 0.10 ‰ (1σ)) and euxinic sediments (+1.8 ± 0.4 ‰ (1σ)) values. A mass balance modelling using Mo isotopic values of euxinic shales, their major sources and sinks, and related fractionation factors suggests an order of magnitude higher extent of ocean euxinia (∼5% of seafloor area) at 1.66 Ga than that observed for modern oceans (0.1 to 0.3 %). Such extensive euxinia in shallow-water environments under a weakly oxygenated atmosphere (≥0.1 % PAL to ≤40 % PAL) could have played a major role in delaying the emergence of eukaryotes and multicellular life.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 158-173"},"PeriodicalIF":5.0,"publicationDate":"2025-09-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145045902","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The authigenic and detrital neodymium isotopic composition in the modern Mediterranean Sea: implications for paleocirculation proxy","authors":"Kazuyo Tachikawa, François Beny, Marine Cornuault, Vincent Guarinos, Abel Guihou, Pierre Deschamps, Hartmut Schulz, Francisco J. Sierro, Marie Boye","doi":"10.1016/j.gca.2025.09.012","DOIUrl":"https://doi.org/10.1016/j.gca.2025.09.012","url":null,"abstract":"Neodymium isotopic composition (<ce:sup loc=\"post\">143</ce:sup>Nd/<ce:sup loc=\"post\">144</ce:sup>Nd or ε<ce:inf loc=\"post\">Nd</ce:inf>) has been used as a tracer of water mass provenance in the modern and the past oceans. However, recent studies have questioned its reliability because some phases of detrital sediments are reactive and a partial dissolution of these phases could alter the initial authigenic signals through porewater Nd. We examined this possibility using core-top sediments from the Mediterranean Sea, an ideal region, because the water circulation and the ε<ce:inf loc=\"post\">Nd</ce:inf> distribution of seawater and detrital fractions are relatively well constrained. Furthermore, smectite, one of the most reactive minerals, is abundant in the easternmost basin due to Nile sediment input. We obtained paired authigenic (leachates and foraminiferal tests) and detrital ε<ce:inf loc=\"post\">Nd</ce:inf> signatures from core-top sediments along the zonal transect from 2°W to 33°E. The authigenic ε<ce:inf loc=\"post\">Nd</ce:inf> values generally agreed with the bottom water signals, except in the easternmost Levantine and the Aegean Seas, where they were higher than the water values. These areas are characterised by high smectite abundance and enclosed nature, making local Nd source influence more visible. The authigenic ε<ce:inf loc=\"post\">Nd</ce:inf> values in these areas were stable at about −5 to −4, while the corresponding detrital values ranged between −10 and −4. The compilation of modern seawater and detrital ε<ce:inf loc=\"post\">Nd</ce:inf> values shows that seawater ε<ce:inf loc=\"post\">Nd</ce:inf> covaries with detrital signals at 0–200 m water depth, and that the correlation is insignificant at depths greater than 1500 m. These results argue against a ubiquitous control of bottom water ε<ce:inf loc=\"post\">Nd</ce:inf> by the benthic flux in the present Mediterranean Sea. Sensitivity tests suggest that the ɛ<ce:inf loc=\"post\">Nd</ce:inf> values of Mediterranean Sea water are affected by the isotopic composition of Atlantic inflow, deep-water formation zones and local benthic sources, as well as by the size of the Atlantic inflow flux, deep-water formation and zonal water exchange. These results support the use of Nd isotopic composition as a tracer of water mass provenance in the Mediterranean Sea although a combination of isotopic data from various basins is required to distinguish the influence of these factors.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"28 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-09-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145103519","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lithium isotope fractionation in the mid-lower Changjiang basin: Insights from riverine geochemistry and the role of authigenic oxyhydroxides","authors":"Chengfan Yang ,&nbsp;Jiacheng Peng ,&nbsp;Junjie Guo ,&nbsp;Qiang Hao ,&nbsp;Jinniu Chen ,&nbsp;Shouye Yang","doi":"10.1016/j.gca.2025.09.007","DOIUrl":"10.1016/j.gca.2025.09.007","url":null,"abstract":"<div><div>Lithium (Li) isotopes have been extensively studied as a key proxy for silicate weathering. However, our understanding of Li isotope fractionation in lowlands and/or floodplains, particularly regarding authigenic phases, needs further refinement. In this study, we present comprehensive geochemical data, including elemental, ionic concentrations, as well as lithium and strontium (Sr) isotopic compositions, obtained from river water, suspended particulate matter (SPM), and associated oxyhydroxide fractions collected across the mid-lower Changjiang basin. The dissolved δ⁷Li values (+8.4 ‰ to +17.1 ‰) contrast with suspended particulate matter (SPM: −2.7 ‰ to −0.3 ‰) and its oxyhydroxide fraction (−15.9 ‰ to −7.0 ‰), revealing significant isotope fractionation during secondary phase formation. An inverse mixing model identifies silicate weathering (50.5 ± 4.9 %) and evaporite dissolution (35.4 ± 4.0 %) as the dominant sources of the dissolved Li, followed by urban sewage contributions of 10.4 ± 1.5 %. After correcting for non-silicate sources, dissolved δ⁷Li values conform to a steady-state batch fractionation model, reflecting the dynamic balance between primary mineral dissolution and secondary mineral (mainly clays) formation. The nearly consistent ⁸⁷Sr/⁸⁶Sr ratios of SPM oxyhydroxide fraction with those of river water strongly demonstrate their authigenic precipitation within the riverine environment. We further infer that the formation of authigenic oxyhydroxide likely played a significant role in fractionating Li isotope in the geological past, albeit secondary to clays. However, in the context of intense dam constructions, their impacts cannot be easily observed in modern river lowlands/floodplains due to reduced SPM concentrations. According to the updated global average, we hypothesize that floodplains exert little influence on elevating riverine δ⁷Li values, at least in present-day river basins surrounding the Tibetan Plateau. These findings refine our understanding of river chemistry in a highly-populated river basin, highlighting authigenic processes as a potential control on Li isotopes, warranting re-evaluation in high erosion and free-flowing river systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 121-132"},"PeriodicalIF":5.0,"publicationDate":"2025-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145045785","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multicomponent diffusion in basaltic melts: A temperature-independent eigenvector matrix, and a multicomponent diffusion calculator","authors":"Bobo Bai,&nbsp;Youxue Zhang","doi":"10.1016/j.gca.2025.09.002","DOIUrl":"10.1016/j.gca.2025.09.002","url":null,"abstract":"<div><div>Multicomponent diffusion in natural silicate melts is a fundamental process in magma mixing and evolution. In <span><math><mrow><mi>N</mi></mrow></math></span>-component silicate melts, multicomponent diffusion is characterized by an <span><math><mrow><mi>N</mi><mo>-</mo><mn>1</mn></mrow></math></span> square matrix, termed the diffusion matrix <span><math><mrow><mo>[</mo><mi>D</mi><mo>]</mo></mrow></math></span>. Eigenvectors of <span><math><mrow><mo>[</mo><mi>D</mi><mo>]</mo></mrow></math></span> define <span><math><mrow><mi>N</mi><mo>-</mo><mn>1</mn></mrow></math></span> eigen-components that diffuse independently of each other. Diffusion eigenvectors in an 8-component SiO₂-TiO₂-Al₂O₃-FeO-MgO-CaO-Na₂O-K₂O basalt appear to be roughly temperature independent (e.g., Guo and Zhang, 2020). For additional verification of the temperature independence and for improving the accuracy of the eigenvector matrix, we use a single eigenvector matrix, <span><math><mrow><mo>[</mo><mi>Q</mi><mo>]</mo></mrow></math></span>, to simultaneously fit concentration profiles from 26 diffusion couple experiments at three temperatures from Guo and Zhang, (2018, 2020), and one additional experiment conducted in this work. The goodness of fit using one single eigenvector matrix in this work is about the same as that using three different eigenvector matrices in Guo and Zhang (2018, 2020), further supporting the temperature independence of <span><math><mrow><mo>[</mo><mi>Q</mi><mo>]</mo></mrow></math></span> in basaltic melts. The eigenvalues (i.e., the diffusion coefficients for individual eigenvector components) follow the Arrhenius relation. Using the extracted eigenvector matrix and eigenvalues, we present an open-access calculator for the community to compute multicomponent diffusion profiles. The calculator is applied to examine multicomponent diffusion in eigen-component space, and minor compositional dependence of eigen-component diffusivities (i.e., eigenvalues) is revealed.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 133-143"},"PeriodicalIF":5.0,"publicationDate":"2025-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145045786","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Disorder and Mg concentration effects on equilibrium C-O isotope fractionations in MgxCa1-xCO3 solid solutions (0 ≤ x ≤ 0.5): A crystallographic and spectroscopic study of magnesian calcite and dolomite","authors":"Yunfan Miao, Xi Zhu, Yu Ye, Zhengrong Wang, Qingbo Wang, Dan Liu, Sha Wang, Yancheng Hu, Zhen Wu","doi":"10.1016/j.gca.2025.09.005","DOIUrl":"https://doi.org/10.1016/j.gca.2025.09.005","url":null,"abstract":"Magnesian calcites and dolomite are critical carbonate minerals that archive valuable environmental and geochemical records. In this study, Mg&lt;ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;x&lt;/ce:inf&gt;&lt;/ce:italic&gt;Ca&lt;ce:inf loc=\"post\"&gt;1-&lt;/ce:inf&gt;&lt;ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;x&lt;/ce:inf&gt;&lt;/ce:italic&gt;CO&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt; solid solutions (0 ≤ &lt;ce:italic&gt;x&lt;/ce:italic&gt; ≤ 0.5) were synthesized at 1100 °C and 1.5 GPa using mixtures of natural calcite and dolomite as starting materials. Their structural and vibrational properties were analyzed using single-crystal X-ray diffraction (XRD) technique, Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The homogeneity of the products was checked by electron microprobe analysis (EMPA) and XRD on multiple crystals, and the minor compositional variations detected on sample chips do not measurably affect lattice parameters or vibrational modes. The synthesized magnesian calcites adopt the calcite-type structure (&lt;mml:math altimg=\"si2.svg\"&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;R&lt;/mml:mi&gt;&lt;mml:mover accent=\"true\"&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;3&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mo stretchy=\"false\"&gt;¯&lt;/mml:mo&gt;&lt;/mml:mrow&gt;&lt;/mml:mover&gt;&lt;mml:mi&gt;c&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;/mml:math&gt;). Dolomite, initially refined in &lt;mml:math altimg=\"si1.svg\"&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;R&lt;/mml:mi&gt;&lt;mml:mover accent=\"true\"&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;3&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mo stretchy=\"false\"&gt;¯&lt;/mml:mo&gt;&lt;/mml:mrow&gt;&lt;/mml:mover&gt;&lt;/mml:mrow&gt;&lt;/mml:math&gt; symmetry, undergoes an order–disorder transition upon heating, with the order parameter (&lt;ce:italic&gt;s&lt;/ce:italic&gt;) decreasing from 0.79 to 0.02. Increasing Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; content linearly reduces the unit-cell volume and average Ca(Mg)-O bond lengths, while all major vibration modes shift to higher frequencies. More interestingly, we show that vibration frequencies of Mg&lt;ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;x&lt;/ce:inf&gt;&lt;/ce:italic&gt;Ca&lt;ce:inf loc=\"post\"&gt;1-&lt;/ce:inf&gt;&lt;ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;x&lt;/ce:inf&gt;&lt;/ce:italic&gt;CO&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt; (0 ≤ &lt;ce:italic&gt;x&lt;/ce:italic&gt; ≤ 0.5) solid solutions are volume-controlled, with lattice compression governing the shifts of the vibration frequencies rather than C-O bond shortening. Equilibrium 10&lt;ce:sup loc=\"post\"&gt;3&lt;/ce:sup&gt;·ln&lt;ce:italic&gt;β&lt;/ce:italic&gt; values for carbon and oxygen isotope fractionations, calculated from the observed vibration frequencies, rise linearly with Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; substitution at a given temperature. Using these &lt;ce:italic&gt;β&lt;/ce:italic&gt; factors, equilibrium carbon and oxygen isotope fractionation factors are further calculated for calcite/dolomite − CO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;/H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O systems and compared with previous experimental data. Our results show that thermally-induced structural disorder expands the unit-cell volume of dolomite by ∼0.4 % at ambient conditions, reducing 10&lt;ce:sup loc=\"post\"&gt;3&lt;/ce:sup&gt;·ln&lt;ce:italic&gt;β&lt;/ce:italic&gt; values. The &lt;ce:italic&gt;ab initio&lt;/ce:italic&gt; calculations confirm these reduct","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"73 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145103520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}