Geochimica et Cosmochimica Acta最新文献

{"title":"Corrigendum to “Mineral-melt calcium isotope fractionation factors constrained using ab initio molecular dynamics simulations and their implications to calcium isotope effects during partial melting in the upper mantle”. [Geochim. Cosmochim. Ac. 396 (2025) 51–70]","authors":"Yonghui Li , Justin Hardin , Wenzhong Wang , Zhongqing Wu , Shichun Huang","doi":"10.1016/j.gca.2025.07.005","DOIUrl":"10.1016/j.gca.2025.07.005","url":null,"abstract":"","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Page 253"},"PeriodicalIF":4.5,"publicationDate":"2025-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621779","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":"Stable Mo isotopic fractionation during complexation onto hematite surface: Molecular insights from Ab-initio MD and X-ray absorption spectroscopy","authors":"Xinyu Wang ,&nbsp;Ruilin Wang ,&nbsp;Luo Li ,&nbsp;David M. Sherman ,&nbsp;Tao Yang ,&nbsp;Mingcai Hou ,&nbsp;Yoshio Takahashi ,&nbsp;Wei Li","doi":"10.1016/j.gca.2025.06.031","DOIUrl":"10.1016/j.gca.2025.06.031","url":null,"abstract":"<div><div>Iron (hydr)oxides are key sorbents of molybdenum (Mo) in a wide range of environmental settings. Among them, hematite (α-Fe₂O₃) is the most stable iron oxide under surficial conditions; however, the extent and molecular-scale mechanisms of Mo isotopic fractionation on hematite across variable pH conditions have remained poorly constrained. In this study, we conducted a series of Mo adsorption experiments at pH 5.0–9.0 to investigate isotopic fractionation and identified two distinct fractionation regimes under acidic and alkaline conditions. Mo isotopic fractionation remained relatively constant between pH 4.0 and 7.0, with Δ⁹⁸Mo_{[Mo(aq)-Mo(hematite)]} ≈ 2.10‰, indicating minimal pH sensitivity. Above pH 7, however, fractionation decreased markedly, with values declining to 1.80‰ at pH 8 and 1.37‰ at pH 9.</div><div>Ab initio molecular dynamics (AIMD) simulations indicate that Mo is predominantly present as a five-fold coordinated complex at the water–hematite interface. This structural motif is supported by Mo K-edge X-ray absorption near-edge structure (XANES) spectra, which show that the local coordination environment of sorbed Mo differs from that of MoO₃ (octahedral geometry) and cannot be explained by a simple mixture of tetrahedral and octahedral species. Extended X-ray absorption fine structure (EXAFS) analysis yields an average Mo–O coordination number of 4.3 ± 0.9 at pH = 5.0 and 4.7 ± 0.9 at pH = 7.0, consistent with prior observations (5.3 ± 0.6), thereby corroborating the predominance of a five-fold Mo species.</div><div>Theoretical isotopic fractionation for this five-fold complex, calculated as Δ⁹⁸Mo_{[Mo(aq)-Mo(five-fold complex)]} = 2.01 ‰ at 20 °C, agrees well with our experimental results (Δ⁹⁸Mo_{[Mo(aq)-Mo(hematite)]} = 1.899 ± 0.322 ‰) and previous data (2.19 ± 0.54 ‰). A two-component model incorporating both five-fold and tetrahedral Mo complexes successfully reproduces the observed pH-dependent fractionation trend. Collectively, these findings demonstrate that Mo isotope fractionation on hematite is strongly pH-dependent and governed by changes in surface complexation geometry, with implications for the application of Mo isotopes as tracers in both natural and anthropogenically influenced systems, from continental weathering profiles to aqueous transport pathways.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 52-66"},"PeriodicalIF":4.5,"publicationDate":"2025-07-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144605582","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":"Closing in on carbonate uranium isotopes as a paleo-redox proxy based on continuous records from Late Oligocene to Holocene island carbonates","authors":"Guolin Xiong, Feifei Zhang, Yibo Lin, Guang-Yi Wei, Na Li, Mengchun Cao, Xuexue Jia, Jian Wang, Wen Yan, Hai Cheng, Shu-Zhong Shen","doi":"10.1016/j.gca.2025.07.011","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.011","url":null,"abstract":"Understanding the fractionation of uranium isotopes (&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U/&lt;ce:sup loc=\"post\"&gt;235&lt;/ce:sup&gt;U, commonly denoted as δ&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U) during carbonate diagenesis is crucial for quantitatively reconstructing the variations in global seafloor anoxic extent throughout Earth’s history using marine carbonates. While the existing diagenetic framework is primarily based on the Bahamian carbonate platform, its robustness and applicability need validation in other regions. The reliability of proxies for recognizing U isotope alteration during diagenesis remains inadequate due to the lack of comprehensive investigations into the influence of diagenetic stages, environments, and redox conditions on U isotope fractionation. In this study, island carbonate samples were continuously collected from two deep drill cores (∼600 m XK-1 core and ∼ 1000 m NK-1 core) in the South China Sea, spanning from the Late Oligocene to Holocene (ca. 25 Ma to present). These samples, which underwent intensive meteoric and marine diagenesis, were notably devoid of organic matter, providing an excellent opportunity to validate the Bahamian diagenetic framework and to explore new geochemical tools for recognizing U isotope alterations during diagenesis. The observed significant positive offset from contemporaneous seawater in the South China Sea (0.24 ‰ ± 0.24 ‰, 1SD, n = 152) is similar to that found in the Bahamas (0.24 ‰ ± 0.15 ‰, 1SD), indicating that U isotope fractionation during early diagenesis is likely consistent on a global scale. The δ&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U values associated with syndepositional diagenesis in the South China Sea exhibit a uniform offset of 0.25 ‰ ± 0.07 ‰ (1SD), consistent with those observed in the Bahamas (0.26 ‰ ± 0.10 ‰, 1SD), suggesting uranium isotope fractionation during syndepositional diagenesis occurs extensively and consistently across different redox environments in shallow-water carbonates. However, the δ&lt;ce:sup loc=\"post\"&gt;238&lt;/ce:sup&gt;U values undergo further alteration during post-depositional diagenesis, with these values during post-depositional marine diagenesis being mainly determined by the redox conditions of the diagenetic fluids. Based on the sequential enrichment of vanadium (V)–uranium (U) concentrations, we classified the redox conditions of the diagenetic fluids within marine diagenetic environments into upper suboxic and lower suboxic conditions in the South China Sea. The uranium isotope offset from contemporaneous seawater is nearly negligible under upper suboxic conditions but progressively increases by about 1 ‰ due to the authigenic U(IV) enrichment under lower suboxic conditions. However, the uranium isotope offset (0.18 ‰ ± 0.12 ‰, 1SD) during eogenetic meteoric diagenesis is primarily inherited from the earlier syndepositional diagenesis. Our new data from the South China Sea confirm a general diagenetic offset of 0.24 ‰ ± 0.24 ‰ (1SD) between carbonates and seawater, ","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"45 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622046","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":"Molecular insights into Yb(III) speciation in sulfate-bearing hydrothermal fluids from X-ray absorption spectra informed by ab initio molecular dynamics","authors":"Xiaodong Zhao ,&nbsp;Duo Song ,&nbsp;Sebastian T. Mergelsberg ,&nbsp;Micah P. Prange ,&nbsp;Daria Boglaienko ,&nbsp;Zihua Zhu ,&nbsp;Zheming Wang ,&nbsp;Carolyn I. Pearce ,&nbsp;Chengjun Sun ,&nbsp;Kevin M. Rosso ,&nbsp;Xiaofeng Guo ,&nbsp;Xin Zhang","doi":"10.1016/j.gca.2025.07.010","DOIUrl":"10.1016/j.gca.2025.07.010","url":null,"abstract":"<div><div>Rare earth elements (REEs) are critical for advanced technologies, yet in hydrothermal aqueous solutions the molecular level details of their interaction with ligands that control their geochemical transport and deposition remain poorly understood. This study elucidates the coordination behavior of Yb³⁺ in sulfate-rich hydrothermal fluids using <em>in situ</em> extended X-ray absorption fine structure (EXAFS) spectroscopy and <em>ab initio</em> molecular dynamics (AIMD) simulations. By integrating multi-angle EXAFS with AIMD-derived constraints, we precisely resolve Yb³⁺ coordination structures and ligand interactions under hydrothermal conditions. At room temperature, Yb³⁺ is coordinated by five water molecules and two sulfate ligands (coordination number, CN = 8), forming a distorted square antiprism geometry. Increasing temperature induces progressive dehydration, reducing the hydration shell and favoring stronger sulfate complexation. At 200°C, sulfate ligands reorganize around Yb³⁺, shifting its geometry to a capped octahedron (CN = 7). At 300 °C, sulfate binding dominates, leading to structural reorganization that parallels the onset of sulfate mineral precipitation, consistent with the retrograde solubility of REE sulfates. These findings provide direct molecular-scale evidence that sulfate acts as both a transport and deposition ligand, critically influencing REE mobility in geochemical environments. Our results can also help to refine thermodynamic models of REE speciation in high-temperature hydrothermal fluids and improve our understanding of REE ore formation processes in nature.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"407 ","pages":"Pages 240-252"},"PeriodicalIF":5.0,"publicationDate":"2025-07-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622049","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":"Exploring the affinity and selectivity of sedimentary mackinawite (FeS) towards natural organic matter","authors":"Kathryn Balind ,&nbsp;Milad Ezzati ,&nbsp;Aude Picard ,&nbsp;Yves Gélinas","doi":"10.1016/j.gca.2025.07.009","DOIUrl":"10.1016/j.gca.2025.07.009","url":null,"abstract":"<div><div>While iron oxides have been thoroughly explored in terms of their ability to sorb and sequester organic carbon (OC) in sediments, the role of iron sulfide (Fe-S) minerals in the long-term sequestration of OC remains poorly defined. In this study, we assessed the affinity of different types of natural organic matter (NOM) towards synthetic Fe-S minerals using sorption isotherms. We found affinities and sorption capacities varying in the following order: plankton NOM &gt; corn leaves NOM &gt; aged terrestrial NOM. Scanning electron microscopy showed that NOM increases the size and surface area of Fe-S aggregates, likely also influencing their surface reactivity. High NOM contents in Fe-S minerals protected Fe(II) from oxidation after exposure to atmospheric oxygen. Analysis of the synthetically prepared Fe-S-NOM complexes by synchrotron scanning transmission X-ray microscopy (STXM) coupled to near-edge X-ray absorption fine-edge structure (NEXAFS) spectroscopy revealed strong interactions between Fe-S minerals and NOM extracted from plankton, specifically with amide and carboxylic functional groups. We also attempted to identify and characterize interactions between OC and Fe-S minerals in natural sulfidic sediments from the St. Lawrence Estuary and the Saguenay Fjord, hence linking our work on synthetic iron sulfides to what is occurring in natural environments, although this effort proved more challenging owing to the presence of Fe(III) minerals even at depth and the difficulty in distinguishing FeS from other Fe(II) minerals. We present depth concentration profiles of dissolved OC, iron, and sulfur in the liquid-phase (pore water) along with speciation data from sequential extractions of sulfur in the solid-phase collected from sediment cores. We found a clear association between mixed Fe(II)/Fe(III) minerals and OC in sediments, which, combined with the results of the synthetic FeS experiment, suggests that Fe-S minerals can promote OC sequestration in sediments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"405 ","pages":"Pages 98-113"},"PeriodicalIF":5.0,"publicationDate":"2025-07-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621780","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":"Uranium reduction in modern and ancient marine carbonate settings – insights from anaerobic U extractions and high-energy resolution X ray spectroscopy","authors":"Kasper Primdahl Olesen ,&nbsp;Elvira Bura-Nakic ,&nbsp;Ivan N. Pidchenko ,&nbsp;Kristina O. Kvashnina ,&nbsp;Tais W. Dahl","doi":"10.1016/j.gca.2025.07.006","DOIUrl":"10.1016/j.gca.2025.07.006","url":null,"abstract":"<div><div>In the marine environment, hexavalent uranium, U⁶⁺, is incorporated into primary carbonate minerals with the same isotopic composition (δ²³⁸U) as the seawater in which they are formed. Yet, modern marine carbonate sediments carry heavier U isotope compositions. This enrichment of heavy U isotopes has been linked to biogenic U reduction in and below the Fe-reducing zone inside the sediment. Still, the oxidation state(s) of uranium in marine carbonate sediments undergoing syndepositional diagenesis has never been measured before. Here, we 1) present an anaerobic ion chromatographic technique based on the TEVA® resin to chemically separate and quantify abundances of tetravalent U⁴⁺ and hexavalent U⁶⁺ fractions in the carbonate, and 2) compare the results from ion chromatography to U L3 edge HERFD-XANES spectroscopic measurements of the total U in sediments to 3) estimate U oxidation states of fresh carbonate sediments from a modern seawater-fed lake and ancient limestones. We find that our anaerobic extraction technique can provide credible evaluations of reduced U⁴⁺ and oxidized U⁶⁺ contents, applicable to carbonate sediments and rocks. Our results show that U resides both in reduced and oxidized states in modern carbonate sediments and ancient carbonate rocks. By comparing air-exposed, oven-dried samples to samples always kept under strictly anaerobic condition, we find that the majority of authigenic U in modern carbonate sediments resides in oxidation-sensitive phases that accumulate with sediment depth, instead of being structurally bound in carbonate minerals (aragonite and calcite). We propose a model to account for the observed trends in U oxidation state, U phase associations, and U isotope fractionation, where a substantial fraction of U in the sediments is likely delivered via microbial reduction and precipitated as a non-crystalline, reduced form near the sediment–water interface. We suggest these oxidation-sensitive reduced U species participate in redox cycling where some U is re-oxidized and perhaps bio-reduced again later, for example in the presence of Fe(III) mineral surfaces that undergo reductive dissolution with depth. Simultaneously, a continued incorporation of recalcitrant and isotopically light (i.e. ²³⁸U-depleted) U from the pore fluids into diagenetic carbonate may occur. The determination of U oxidation states in modern carbonates in this study helps to bridge a gap in our knowledge of how U isotope signals are affected by <em>syn</em>-sedimentary diagenetic U transformations, opening new avenues for understanding sedimentary U cycling and improving the δ²³⁸U paleo redox proxy.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 134-149"},"PeriodicalIF":5.0,"publicationDate":"2025-07-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621781","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":"Coupled zinc and iron isotope fractionation in mantle xenoliths: records of carbonated melt–lithosphere interaction","authors":"Tian-Hao Wu, Sheng-Ao Liu","doi":"10.1016/j.gca.2025.07.007","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.007","url":null,"abstract":"Coupled zinc and iron isotopic variations in basaltic magmas are potential indices of recycled crustal materials that have modified their mantle sources. Nevertheless, how these isotopes fractionate in response to various types of mantle metasomatism is not well constrained as of yet. Here we present high-precision Zn (δ<ce:sup loc=\"post\">66</ce:sup>Zn<ce:inf loc=\"post\">JMC-Lyon</ce:inf>) and Fe (δ<ce:sup loc=\"post\">57</ce:sup>Fe<ce:inf loc=\"post\">IRMM-014</ce:inf>) isotope data and <ce:italic>in situ</ce:italic> chemistry for direct mantle samples (peridotites, pyroxenites, and their mineral separates), occurring as xenoliths within Cenozoic alkaline basalts from northeast China. Based on mineralogical and chemical compositions, these xenoliths are classified into four groups. Group I peridotites are almost non-metasomatized, whereas Group II peridotites exhibit typical characteristics of carbonated silicate melt metasomatism (e.g., high La/Yb, low Ti/Eu). Distinct from the normal mantle-like δ<ce:sup loc=\"post\">66</ce:sup>Zn (∼0.18 ‰) and δ<ce:sup loc=\"post\">57</ce:sup>Fe (∼0.03 ‰) values of Group I peridotites, Group II peridotites have highly variable and positively correlated δ<ce:sup loc=\"post\">66</ce:sup>Zn (−0.04 ‰ to 0.29 ‰) and δ<ce:sup loc=\"post\">57</ce:sup>Fe (−0.38 ‰ to 0.07 ‰; N = 6). The generally light Zn and Fe isotopic compositions of Group II peridotites are attributed to kinetic isotope fractionation during carbonated melt infiltration. Low-Mg# pyroxenites, characterized by Mg# (100 × atomic ratio of Mg/(Mg + Fe)) of &lt;89 and positive Eu anomalies in clinopyroxenes, have extremely high and positively correlated δ<ce:sup loc=\"post\">66</ce:sup>Zn (0.27 ‰–0.62 ‰) and δ<ce:sup loc=\"post\">57</ce:sup>Fe (0.19 ‰–0.84 ‰; N = 5). These pyroxenites are interpreted as cumulates from carbonated melts, akin to the host basalts originating from partial melting of the asthenospheric mantle containing recycled carbonate. By comparison, high-Mg# pyroxenites (Mg# &gt; 90) display gradual modal variations in the composite xenoliths, consistent with being the product of silicate melt-peridotite reaction. These pyroxenites have higher δ<ce:sup loc=\"post\">57</ce:sup>Fe (0.02 ‰–0.24 ‰) but lower δ<ce:sup loc=\"post\">66</ce:sup>Zn (−0.05 ‰ to 0.03 ‰; N = 5) than those of the normal mantle. The negatively correlated Zn and Fe isotopic ratios were caused by diffusion-induced fractionation during silicate melt metasomatism. Collectively, these observations demonstrate that asthenosphere-derived, carbonated and silicate melts could induce substantial and coupled Zn and Fe isotopic variations in the lithospheric mantle through reaction and diffusion during mantle metasomatism. Therefore, coupled variations of Zn and Fe isotopes in mantle-derived rocks or magmas provide valuable approaches for tracing carbonated melt–lithospheric mantle interaction.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"57 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621782","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":"Gallium isotope fractionation during granite weathering: Insights from two profiles in contrasting climatic conditions","authors":"Wei Yuan ,&nbsp;Ting Gao ,&nbsp;Zhengrong Wang ,&nbsp;Jiubin Chen ,&nbsp;Hongming Cai ,&nbsp;Tong Zhang ,&nbsp;Guanghui Yu ,&nbsp;Nana Feng ,&nbsp;Chengshuai Liu ,&nbsp;Congqiang Liu ,&nbsp;Jacques Schott","doi":"10.1016/j.gca.2025.07.004","DOIUrl":"10.1016/j.gca.2025.07.004","url":null,"abstract":"<div><div>The recently developed gallium (Ga) isotope systematics provides valuable insights into continental weathering, particularly under conditions of intense chemical weathering. However, the relationship between Ga isotope fractionation and the factors influencing the intensity of silicate weathering has yet to be systematically explored. In this study, we report Ga isotope compositions from two granite weathering profiles developed under contrasting climates: a temperate semiarid climate in the Beijing profile and a tropical moist monsoon climate in the Guangdong profile. Our study shows that the majority of Ga in these weathering profiles is retained within the crystal-lattice of Al-bearing minerals, followed by Fe-bearing minerals, with minimal surface adsorption. Mössbauer spectra and sequential extraction results show that the influence of Fe on Ga concentrations and isotope compositions becomes more significant as weathering intensity increases, driven by the transformation of dissolved ferrous Fe into ferric Fe at advanced weathering stages. Both granite weathering profiles exhibit a similar trend of slightly decreasing δ⁷¹Ga values from bedrock to surface (Beijing profile: from 0.71 ‰ to 0.61 ‰; Guangdong profile: from 0.70 ‰ to 0.54 ‰). This suggests that the lighter Ga isotope, ⁶⁹Ga, is preferentially enriched in regolith relative to the coexisting aqueous solution in the pore spaces of weathered rocks, consistent with observations from a basalt weathering profile. In the Beijing profile, a strong correlation between τ<em>_Ga,Th</em> (a weathering index) and δ⁷¹Ga values suggests that Ga isotope compositions are mainly controlled by the dissolution of primary minerals. This correlation indicates that variations in δ⁷¹Ga values can serve as effective tracers of weathering intensity during early stages of chemical weathering. However, this pattern is absent in the Guangdong profile, where Ga isotope compositions appear to be influenced not only by mineral dissolution but also by processes such as mineral adsorption and coprecipitation. A broader comparison of our results with previously published Ga isotope composition suggests the lithology of bedrock provides the major control on the extent of Ga isotope fractionation. Other factors, including the composition of secondary minerals and the climatic conditions, also play significant roles in influencing Ga mobility and Ga isotope fractionation during chemical weathering.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 115-133"},"PeriodicalIF":5.0,"publicationDate":"2025-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622047","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":"Multiple fluid-rock interactions in the subduction channel evidenced by magnesium isotopes","authors":"Yongxiang Yao ,&nbsp;Yuanyuan Xiao ,&nbsp;Yaoling Niu ,&nbsp;Weidong Sun","doi":"10.1016/j.gca.2025.07.003","DOIUrl":"10.1016/j.gca.2025.07.003","url":null,"abstract":"<div><div>Fluids from the subducted oceanic slab are closely associated with mass transfer in subduction zones, which significantly affect the composition of arc magmas and deep mantle. Previous studies have shown that magnesium isotopes can be used to identify the source of fluids and to trace carbonate cycling. However, the Mg isotope variation as a function of fluids in the subduction channel remains obscure. As large-scale fluid flows and strong fluid-rock interactions can significantly enhance elemental mobility, Mg isotope variations may be recorded in metabasaltic rocks with large-scale veins (which represent original fluid pathways). Here we present Mg isotope data for different parts of a metabasaltic block from Tianshan (ultra)high pressure metamorphic belt, southwest of China to reveal the Mg isotope composition of fluids and their effects in the subduction channel. All the samples have been divided into three groups depending on their position in each “pillow” of the metamorphic block, i.e., those from the omphacite-rich interior, glaucophane-rich vein and transitional part. The omphacite-rich interior is mainly composed of omphacite and phengite, with minor garnet, while the glaucophane-rich vein mainly consists of garnet, glaucophane and epidote. The transitional part between the omphacite-rich interior and glaucophane-rich vein is dominated by epidote and carbonate, and aragonite has been widely replaced by dolomite. Together with our previous study, we think that the metabasaltic block has experienced two stages of rehydration overprinting after eclogitization: the first stage rehydration led to the common enrichment of LILEs in the whole block compared to its protolith E-MORB; the second stage rehydration resulted in the carbonate reprecipitation at the transitional part and the redistribution of garnet component.</div><div>The δ²⁶Mg values of the omphacite-rich interior are lower (−0.42 ‰ to −0.34 ‰) than those of MORBs (−0.25 ± 0.06 ‰), which probably indicates that subduction dehydration can lead to lighter Mg isotope composition of the residual eclogite and heavier Mg isotope composition of the released fluids. Furthermore, the lighter Mg isotope composition of the transitional part (δ²⁶Mg = −0.67 ‰ ∼ −0.63 ‰) and glaucophane-rich vein (δ²⁶Mg = −0.71 ‰ ∼ −0.54 ‰) may be caused by the reprecipitation of dolomite and garnet through Ca-Mg exchange as the result of interaction with externally derived carbonate rich fluids. The residual fluid after the reprecipitation of carbonate minerals is also expected to show a heavier Mg isotope composition. In addition, the recrystallization of dolomite in the subduction channel could not only hamper the influence of subducted carbonate-derived fluids on the heavier Mg isotope composition of arc magmas compared to MORBs, but also facilitate the transfer of carbonate with lighter Mg isotope compositions into the deep mantle.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 67-78"},"PeriodicalIF":4.5,"publicationDate":"2025-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621783","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":"Limited mercury (Hg) partitioning into bitumen and efficient gaseous Hg reabsorption during early thermal maturation of organic-rich mudrocks","authors":"Asri O. Indraswari ,&nbsp;Joost Frieling ,&nbsp;Erdem Idiz ,&nbsp;Tamsin A. Mather ,&nbsp;Hugh C. Jenkyns ,&nbsp;Stuart A. Robinson ,&nbsp;Alexander J. Dickson","doi":"10.1016/j.gca.2025.07.001","DOIUrl":"10.1016/j.gca.2025.07.001","url":null,"abstract":"<div><div>Recent studies have looked into the impact of exposure to extremely high temperature, such as typifies contact metamorphism, on mercury (Hg) distributions in sediments, and observed significant Hg loss with increasing temperature. By contrast, sediment cores of Lower Jurassic organic-rich Posidonienschiefer (also known as Posidonia Shale) from the Lower Saxony Basin, Germany seemingly showed Hg enrichment after maturation related to basin subsidence and burial under more typical geothermal gradients. To investigate the apparent differences in Hg behaviour, we conducted a series of artificial maturation experiments on immature Posidonienschiefer samples, analysing Hg concentrations within rock residues and bitumen generated during early maturation stages. Thermal desorption profiles were used to track Hg speciation changes in the matured sediment. Our results show a progressive decrease in Hg concentrations in sediments with increasing thermal maturity throughout the experiments, which dominantly relates to released gaseous Hg with only a fraction of the Hg being partitioned into the bitumen (≤1% of the total initial Hg). Further experiments showed that gaseous Hg in closed vessels was rapidly (≤1 h) and efficiently (≥95 %) reabsorbed into the sediment during cooling. We speculate that our experiments may simulate some of the processes that drive Hg mobilisation and recapture occurring in contact aureoles, such as the rapid release and recapture of gaseous Hg. However, the Hg speciation changes that occur in our experiments and during natural burial maturation clearly differ. Specifically, the changes in Hg speciation in the natural system with burial-related maturation suggest that under those conditions organic matter associated Hg may instead transition into more thermally stable phases.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 240-251"},"PeriodicalIF":4.5,"publicationDate":"2025-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622048","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}