Geochimica et Cosmochimica Acta最新文献

{"title":"Lithium- and oxygen-isotope compositions of a Si-rich nebular reservoir determined from chondrule constituents in the Sahara 97103 EH3 chondrite","authors":"Torii Douglas-Song, Tsutomu Ota, Masahiro Yamanaka, Hiroshi Kitagawa, Ryoji Tanaka, Christian Potiszil, Tak Kunihiro","doi":"10.1016/j.gca.2025.05.038","DOIUrl":"10.1016/j.gca.2025.05.038","url":null,"abstract":"<div><div>Here we report the in situ ion-microprobe analyses of the Li- and O-isotope compositions of enstatite, FeO-rich pyroxene, olivine, glass, and cristobalite grains from six chondrule-related objects from the Sahara 97103 EH3 chondrite. The O-isotope composition of the enstatite grains scattered around the intersection between the terrestrial fractionation and primitive chondrule minerals lines. Whereas, that of olivine varied along the primitive chondrule minerals line. Based on the mineralogy, we found cristobalite formed as a result of Si saturation, instead of the reduction of FeO-rich silicates, consistent with Si-enrichment of whole rock enstatite chondrites. Based on the mineralogy and O-isotope compositions, we infer that olivines in some chondrules are relict grains. In chondrules that contained olivine, no abundant niningerite [(Mg,Fe,Mn)S] was observed. Thus, enstatite formation can be explained by the interaction of an olivine precursor with additional SiO<sub>2</sub> (Mg<sub>2</sub>SiO<sub>4</sub> + SiO<sub>2</sub> → Mg<sub>2</sub>Si<sub>2</sub>O<sub>6</sub>), instead of sulfidation (Mg<sub>2</sub>SiO<sub>4</sub> + S → 1/2 Mg<sub>2</sub>Si<sub>2</sub>O<sub>6</sub> + MgS + 1/2 O<sub>2</sub>). Using the equation Mg<sub>2</sub>SiO<sub>4</sub> + SiO<sub>2</sub> → Mg<sub>2</sub>Si<sub>2</sub>O<sub>6</sub> and the O-isotope compositions of enstatite and olivine, the O-isotope composition of the additional SiO<sub>2</sub> was estimated. Based on the O-isotope composition, we infer that there could be a Si-rich gas with an elevated Δ<sup>17</sup>O value similar to, or greater than the second trend line (Δ<sup>17</sup>O = 0.9 ‰) suggested by <span><span>Weisberg et al. (2021)</span></span>, during chondrule formation. The variation in the Li-isotope compositions of enstatite and olivine grains from EH3 chondrules is smaller than that for the same phases from CV3 chondrules. The variation in the Li-isotope compositions of the enstatite and olivine grains from EH3 chondrules is also smaller than that of their O-isotope compositions. During the recycling of enstatite-chondrite chondrules, both Li- and O-isotope compositions were homogenized. Although enstatite is the major carrier of Li in EH3 chondrules, the Li-isotope composition (δ<sup>7</sup>Li) of enstatite is lower than that of whole rock EH3 chondrites, suggesting the existence of a phase with higher δ<sup>7</sup>Li. Meanwhile, the Li-isotope composition and concentration (δ<sup>7</sup>Li, [Li]) of enstatite is higher than that of olivine. The Li-isotope composition of the Si-rich gas was estimated to be δ<sup>7</sup>Li = 1 ‰, using a similar mass-balance calculation as applied for the O-isotope composition. The Li-isotope composition of the Si-rich gas from the enstatite-chondrite-chondrule forming-region, is consistent with that of whole rock EH3 chondrites, and differs significantly from that of the Si-rich gas from the carbonaceous-chondrite-chondrule forming-region (δ<sup>7</s","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 51-71"},"PeriodicalIF":4.5,"publicationDate":"2025-05-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144168325","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":"Modeling the controls on microbial iron and manganese reduction in methanic sediments","authors":"Racheli Neumann Wallheimer ,&nbsp;Itay Halevy ,&nbsp;Orit Sivan","doi":"10.1016/j.gca.2025.05.026","DOIUrl":"10.1016/j.gca.2025.05.026","url":null,"abstract":"<div><div>Microbial iron and manganese respiration processes have been observed in deep methanic sediments of lacustrine and marine environments, challenging the “classical” model of microbial respiration in aquatic systems. Nonetheless, assessments of the type and relative role of these respiration processes in the methanic zone are lacking. Here, we quantify both the thermodynamic and the kinetic controls of potential iron and manganese respiration processes in the diffusive controlled steady state methanic sediments of lacustrine and marine sites – Lake Kinneret (LK) and the Southeastern Mediterranean Sea (MedS). We consider the substrates (electron donors) and iron and manganese oxides (electron acceptors) at concentrations that have been measured at these sites. Using theoretical bioenergetic methods, we develop a nominal model to calculate catabolic rates, considering both kinetic and thermodynamic parameters. Then, we estimate the biomass growth rates from the catabolic rates, the energy generated in each reduction–oxidation (redox) reaction, the biomass yield from a given amount of energy, the number of cells participating in each reaction, and the energetic needs of the cells. Lastly, we estimate the microbial community sizes of expected iron and manganese reducers. Additionally, we perform a Monte Carlo simulation to account for variations in uncertain parameter values, along with a sensitivity analysis. Together, these calculations enable estimation of the expected total reaction rates of the various metabolic processes.</div><div>Our results indicate that the type of iron or manganese oxide, which determines its thermodynamic and kinetic properties, is more significant in influencing bioreaction rates than its concentration. Thus, bioreactions with amorphous manganese oxides are more favorable than those with highly reactive iron oxides. Among the iron oxides, the reduction of amorphous iron oxyhydroxide and ferrihydrite are the only reactions capable of generating biomass in the methanic sediments at both sites. In both environments, manganese oxide reduction by ammonium and methane oxidation are expected to be significant, while manganese oxide reduction by hydrogen and acetate oxidation are expected to be considerable only in LK. The most probable iron oxide reduction process in LK is hydrogen oxidation, followed by methane oxidation. In the MedS iron oxide reduction is most probably coupled to the oxidation of ammonium (Feammox) to molecular nitrogen (N₂), and in a few cases may be coupled to methane oxidation. The Monte Carlo simulation agrees with the nominal model results for manganese reduction, and additionally predicts that iron reduction may be possible with some combinations of parameter values. These findings improve our understanding of the thermodynamic and kinetic controls on the composition of microbial communities and their effect on the geochemistry of methanic sediments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 32-50"},"PeriodicalIF":4.5,"publicationDate":"2025-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144168350","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":"Reactive iron controls sulfur partitioning between pyrite and organic matter in sedimentary rocks","authors":"Hadar Cohen-Sadon ,&nbsp;Yoav Oved Rosenberg ,&nbsp;Simon Emmanuel ,&nbsp;Shimon Feinstein ,&nbsp;Alon Amrani","doi":"10.1016/j.gca.2025.05.027","DOIUrl":"10.1016/j.gca.2025.05.027","url":null,"abstract":"<div><div>Sulfur isotopic values (δ³⁴S) of pyrite and organic matter (OM) in sediments are widely used for the reconstruction of the sulfur and oxygen cycles as well as pathways of OM preservation. Currently, significant uncertainties persist regarding the mechanism by which sulfur is partitioned between pyrite and OM and its effect on the δ³⁴S record of rocks. Here, we present experimental analysis of iron, carbon, sulfur and δ³⁴S values in marine and lacustrine rock samples. The experimental data was compared with published data of rock samples from the Mesoproterozoic era (1.6 Ga) to the Paleogene period (23 Ma). We also developed a kinetic model to simulate the evolution of δ³⁴S of pyrite and OM under different environmental conditions. Our analysis reveals linear relationships between δ³⁴S value of pyrite and its isotopic difference from δ³⁴S value of organic sulfur (R² ranging from 0.43 to 0.96) for large δ³⁴S ranges of pyrite (110 ‰) and organic sulfur (93 ‰). These ranges and linear trends cannot be explained by variations in the δ³⁴S value of seawater sulfate or by the isotopic fractionation associated with microbial sulfate reduction. Rather, local conditions that change the ratio between reactive iron and sulfate are shown to control the δ³⁴S values of organic sulfur and pyrite and their isotopic gap. This is because reactive iron pyritization rapidly captures isotopically light H₂S generated by microbial sulfate reducers in the early stages of diagenesis, leaving behind heavy H₂S that reacts with OM. In some environments, the isotopic gap between organic sulfur and pyrite correlates with the OM content in the rock, reflecting the critical role of reactive iron in OM preservation via sulfurization. Hence, the role of iron on the partitioning of sulfur between OM and pyrite in sedimentary environments is essential for reconstructing the sulfur cycle and its interaction with the carbon cycle in geological sequences.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 18-31"},"PeriodicalIF":4.5,"publicationDate":"2025-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144168324","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 molecular weight-dependent redox capacity of soil dissolved organic matter: Roles of aromaticity and organic sulfur","authors":"Jiang Xiao, Zhenqing Shi, Fu Liu, Qianting Ye, Lanlan Zhu, Quan Gao, Xin Yang, Paul G. Tratnyek","doi":"10.1016/j.gca.2025.05.018","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.018","url":null,"abstract":"The redox capacities of soil DOM regulate the oxidation–reduction reactions by accepting or donating electrons, which influences a variety of critical redox processes such as greenhouse gas emissions and the (bio)geochemical cycle of contaminants. The composition and reactivity of DOM components vary with their molecular weight (MW), and, therefore, the redox capacities of soil DOM are expected to depend on the MW. However, the limited data that are available on how specific molecular compositions affect the MW-dependent soil DOM redox capacities are inconclusive. In this study, we investigated how soil DOM fractions, separated according to their MW, differed in redox capacity as quantified with electron accepting and donating capacity, and evaluated the relationships between redox capacity of DOM fractions and their optical indices and molecular compositions. As expected, with increasing MW, aromaticity of DOM fractions increased and aliphaticity and protein-like composition decreased, which were measured with Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS) and the optical methods. For the high MW fraction, electron-accepting capacity (EAC) as measured by mediated chronoamperometry was positively correlated with condensed aromatics and polyphenol content and negatively correlated with aliphatic content. More interestingly, our data show that electron-donating capacity (EDC) correlated with sulfur-containing components of the low MW fraction but did not show any correlations with other molecular compounds/properties of soil DOM. Our results highlight the importance of considering both aromaticity and organic sulfur and integrating the molecular information of soil DOM (e.g., MW) for predicting soil DOM redox capacities.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"23 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144133756","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":"Transition from kinetic to equilibrium Zr isotope fractionations during magma crystallization","authors":"Zhen-Xin Li ,&nbsp;Shao-Bing Zhang ,&nbsp;Yong-Fei Zheng ,&nbsp;Michael A. Antonelli ,&nbsp;Wen Zhang ,&nbsp;Liang Zhang ,&nbsp;Fang-Yuan Sun ,&nbsp;Ting Liang","doi":"10.1016/j.gca.2025.05.017","DOIUrl":"10.1016/j.gca.2025.05.017","url":null,"abstract":"<div><div>Zirconium (Zr) isotopes are an emerging tool to study igneous processes. However, Zr isotope fractionation mechanisms are debated, especially regarding the relative roles of kinetic and equilibrium effects. Here, we report high-precision <em>in-situ</em> Zr isotope (δ⁹⁴Zr) measurements in zircons from granitoids with ages of 3.4 to 2.5 Ga. The δ⁹⁴Zr values in all magmatic zircons range from −0.63 to + 0.41 ‰. These zircons show negative correlations between δ⁹⁴Zr values and Zr/Hf ratios, indicating that the crystallization of zircon plays an important role in Zr isotope variations. However, our calculations suggest that neither fractional crystallization of zircon under equilibrium conditions nor that with a fixed kinetic effect cannot explain the δ⁹⁴Zr values observed in our zircons. Combined with theoretical models of crystal growth, we propose that the fractionation mechanisms evolve from diffusion-dominated situations in the early stage to equilibrium-controlled conditions in the later stage of a zircon population’s crystallization history. Our modelling results suggest that a gradual shift from diffusive-kinetic (growth-related) fractionation to near-equilibrium fractionation best explains our results. Zr isotopes, therefore, have the potential to probe the crystallization kinetics of high-temperature processes that are otherwise inaccessible to direct observation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 1-17"},"PeriodicalIF":4.5,"publicationDate":"2025-05-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144116927","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 dual origin of granites from fluid-absent and fluid-fluxed melting of the juvenile lower crust: New constraints from Mo isotopes and phase equilibrium modeling","authors":"You-Shan Xia, Guo-Chao Sun, Peng Gao, Zi-Fu Zhao, Yao Zhou, Zhen-Xin Li, Gang Wen, Lei Qin","doi":"10.1016/j.gca.2025.05.013","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.013","url":null,"abstract":"Water plays a pivotal role in the differentiation of continental crust and the formation of granitic magmas. However, it is still hotly debated how to identify fluid regimes during crustal anatexis. Molybdenum (Mo) isotopes have shown the potential to trace fluid activity during magmatic and metamorphic processes and thus are a valuable tool to identify the presence of free fluid during crustal anatexis. This is exemplified by our previous study on the Early Paleozoic sodic adakitic rocks from the Dabie orogen, eastern China, which were explained to originate from fluid-fluxed melting of juvenile crust based on their Mo isotope characteristics. However, the Early Paleozoic potassic granites (K&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O/Na&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O = 0.81–1.13) in the Dabie orogen—spatially and temporally associated with sodic granites—exhibit distinct geochemical signatures (e.g., higher K&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O/Na&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O ratios and lower Sr/Y, La/Yb ratios). Sr–Nd isotope results indicate that the potassic granites originate from the juvenile lower crust, consistent with the sodic granites. To evaluate the controlling factors behind the geochemical differences between the two groups of granites, we conducted whole-rock Mo isotope analyses of the potassic granites and compared them with the sodic granites and coeval mafic rocks. The results reveal that the potassic granites exhibit δ&lt;ce:sup loc=\"post\"&gt;98&lt;/ce:sup&gt;Mo values (–0.41 to 0.23 ‰, median = 0.02 ‰) similar to adjacent contemporaneous mafic rocks (–0.17 to 0.21 ‰, median = 0.02 ‰), whereas the sodic granites display systematically higher δ&lt;ce:sup loc=\"post\"&gt;98&lt;/ce:sup&gt;Mo values (–0.13 to 0.74 ‰, median = 0.22 ‰). Simulation calculations indicate that neither pure fractional crystallization (FC) nor AFC (assimilation-FC) process can account for the observed differences in K&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O/Na&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O ratios and Mo isotope compositions between the potassic and sodic granites. Instead, such differences can be well explained by partial melting of the same mafic source rocks at different fluid regimes. Further phase equilibrium modeling indicates that the addition of fluids can promote preferential plagioclase breakdown and amphibole retention in the source rocks, thereby generating or amplifying high Sr/Y and La/Yb ratios in melts. In contrast, pressure variations alone cannot account for the observed K&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O/Na&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O ratio differences. These results indicate that distinct fluid regimes in the source region exert the primary control on the geochemical disparities between the two groups of granites. The fluid-fluxed melting could be caused by fluids exsolved from crystallization of the underplated hydrous mafic arc magmas or released by the breakdown of hydrous minerals in the juvenile lower crust. As a comparison, the fluid-absent melting of the mafic crust is primarily dri","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"296 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144083375","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":"Mineral phases and growth conditions of morphologically diverse shelfal ferromanganese concretions","authors":"Joonas Wasiljeff, Changxun Yu, Pasi Heikkilä, Yann Lahaye, Matti Kurhila, Wei‐Li Hong, Aivo Lepland, Sten Suuroja, Volker Liebetrau, Joonas J. Virtasalo","doi":"10.1016/j.gca.2025.05.012","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.012","url":null,"abstract":"Ferromanganese concretions in the shelf sea regions, such as the Baltic Sea, are of significant interest due to their geochemical properties, economic resource potential, and roles in benthic ecosystems. This study analyses the authigenic and detrital mineral phases and their provenance in the Baltic Sea concretions, as well as their formation mechanisms and diagenetic evolution. These concretions exist in three distinct morphotypes: crust, discoidal, and spheroidal. Using synchrotron-based techniques (µ-XRF and µ-XAS) paired with XRD, stable Pb isotope, and bulk geochemical analyses, we found that discoidal and spheroidal concretions consist of alternating Fe- and Mn-rich layers, whereas crust concretions are predominantly Fe-rich. The Mn phases primarily consist of birnessite-like phyllomanganates with columnar and branched dendritic growth patterns, indicative of microbially-mediated precipitation. In contrast, the Fe phases are represented by poorly crystalline ferrihydrite, the formation of which is influenced by admixing of detrital minerals. The three main components (Fe-rich, Mn-rich and detrital), each exhibit distinct trace element associations. The geochemical composition and morphology of the Baltic Sea concretions resembles other shelfal precipitates, indicating consistency in formation mechanisms across different shelf environments. Slightly negative to intermediate Ce anomaly values and the range in Nd contents in the samples suggest that early diagenetic processes contribute to the formation of all the morphotypes.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"34 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144133757","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":"Structures and transport properties of Mg2SiO4-H2O system under high temperature and pressure: insights for supercritical fluids in deep subduction zones","authors":"Yifan Lu, Yicheng Sun, Guoxin Xia, Xiandong Liu, Guo-Guang Wang, Xiancai Lu","doi":"10.1016/j.gca.2025.05.015","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.015","url":null,"abstract":"Understanding the structure and transport properties of supercritical fluids is crucial for gaining insights into their behavior in subduction zones. While previous research has provided an understanding of the properties of supercritical fluids derived from felsic melts, our knowledge regarding supercritical fluids formed from ultramafic melts in deep subduction zones remains limited. In this study, we employed first-principles molecular dynamics to systematically investigate the structure, speciation, self-diffusion, and viscosity of the Mg&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;SiO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;-H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O system at temperatures of 2000 K and 3000 K under a pressure of approximately 10 GPa, covering water contents ranging from 0 to 70 wt%. Our study confirmed the previous experimental observation that ultramafic melts can undergo polymerization due to dissolved water. The cause of the polymerization is the increase in 5-fold Si-O coordination, and it only occurs within specific ranges of water content at a given temperature. In the Mg&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;SiO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;-H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O system, water and OH are primarily bonded to Mg, forming Mg-H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O&lt;ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;m&lt;/ce:inf&gt;&lt;/ce:italic&gt; and Mg-OH species. The results demonstrate that as the water content increases, the viscosity of the Mg&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;SiO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;-H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O system exhibits a rapid initial decrease followed by a gradual reduction. The rapid decrease in viscosity observed at 2000 K is not due to the depolymerization of the structure; on the contrary, the structure of the system becomes more polymerized during this process. The crucial factor driving the rapid viscosity decrease is the increasing proportion of protonated silicate units. The low viscosity of supercritical Mg&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;SiO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;-H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O fluid allows its mobility to reach 2 to 3 orders of magnitude greater than that of basalt melt and 1.7 to 20 times greater than that of carbonate melt. By comparing these findings with supercritical fluids derived from felsic melts, we propose that supercritical fluids formed from different silicate components in subduction zones exhibit similarly low viscosities with minor differences. The ability of supercritical fluids to facilitate element migration primarily depends on the solubility of these elements within the supercritical fluids. In the supercritical Mg&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;SiO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;-H&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O fluid, we observed that Q&lt;ce:sup loc=\"post\"&gt;0&lt;/ce:sup&gt; and Q&lt;ce:sup loc=\"post\"&gt;1&lt;/ce:sup&gt; species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Q&lt;ce:sup loc=\"post\"&gt;n&lt;/ce:sup&gt; species in the supercritical Mg&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;SiO&lt;ce:inf lo","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"47 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144133758","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":"Bioturbation and the effects on benthic flux of nitrogen in a large eutrophic lake: Insights from 224Ra/228Th disequilibrium and inverse geochemical modelling","authors":"Xiaoyan Shi, Guodong Chen, Rong Mao, Xin Luo, Jiu Jimmy Jiao, Meiqing Lu, Tianwei Wang, Wenli Hu, Xingxing Kuang","doi":"10.1016/j.gca.2025.05.014","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.014","url":null,"abstract":"Benthic fluxes are critical pathways for constituent exchanges and biogeochemical interactions across sediment–water interface. To comprehensively evaluate the modulation of bioturbation on the benthic fluxes of nutrients, we present a novel inverse geochemical modelling framework in near-surface sediments of Lake Taihu, a highly eutrophic freshwater lake in eastern China. This approach incorporates the disequilibrium of <ce:sup loc=\"post\">224</ce:sup>Ra/<ce:sup loc=\"post\">228</ce:sup>Th and concentration profiles of additional constituents (SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2-</ce:sup>, NH<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">+</ce:sup>, NO<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">–</ce:sup> and NO<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">–</ce:sup>) through simulation and Bayesian theorem, enabling the estimation of bio-irrigation coefficient (<ce:italic>α</ce:italic>) and corresponding reaction rates. Based on the sediment columns collected from different segments of the lake, the model demonstrates vertical variability of <ce:italic>α</ce:italic>, with an average range of 0.60 × 10<ce:sup loc=\"post\">-4</ce:sup> to 5.01 × 10<ce:sup loc=\"post\">-4</ce:sup> s<ce:sup loc=\"post\">−1</ce:sup>. The calculated reaction rates and microbial taxa indicate the dominance of chemoheterotrophy, where NO<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">–</ce:sup> serves as an electron acceptor during the degradation of organic matter in the anoxic sediment environment to generate NH<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">+</ce:sup>. The estimation of nitrogen benthic flux reveals that bioturbation predominates the export from sediments to the lake, and the directions of the internal nutrient fluxes across sediment–water interface are primarily controlled by nutrient loadings in the overlying lake water. This study introduces a systematic quantification to advocate that bioturbation is crucial in regulating nutrient variability by influencing both the reaction and flux rates, and the workflow expands the application of <ce:sup loc=\"post\">224</ce:sup>Ra/<ce:sup loc=\"post\">228</ce:sup>Th disequilibrium to near-surface sediments in freshwater lacustrine systems, advancing the technique in tracing proxies for benthic fluxes in lakes.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"18 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144083376","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":"Genesis of strongly peraluminous granites facilitates nitrogen retention in the crust","authors":"Yunzhe Chen ,&nbsp;Jian Xu ,&nbsp;Xiao-Ping Xia ,&nbsp;Long Li","doi":"10.1016/j.gca.2025.05.010","DOIUrl":"10.1016/j.gca.2025.05.010","url":null,"abstract":"<div><div>Sediments/sedimentary rocks are a major sink of atmospheric N₂, which is mainly fixed into diazotrophic biomass and recycled into other organic matters or diagenetically transferred (in the form of NH₄⁺) into the crystal structures of phyllosilicate minerals in sediments. Driven by tectonic activities, sedimentary rocks may experience various degrees of metamorphism with some eventually being melted to form S-type granitoids. The destiny of sedimentary N during these processes, i.e., retained in the crust or returned to the atmosphere, determines the long-term N evolution in the crust and the atmosphere, which profoundly impacts not only volatile properties in the lithosphere but also the surface climatic and environmental conditions. Previous studies have reported various extents of N loss (from very subtle to up to 70 %) from regional <em>meta</em>-sedimentary rocks. However, detailed studies on the N-losing mechanism during crustal anatexis and S-type granitoid formation are rare. Here, we examined the Permian–Triassic strongly peraluminous granites (SPGs) in the Diancangshan-Ailaoshan area in SW China, which were formed by extensional decompression melting of Precambrian pelitic sediments in the upper–middle crust within a back-arc basin triggered by slab rollback of the subducted Paleo-Tethyan oceanic crust. The results show that the SPGs have a N-content range of 6.8 ppm to 58.2 ppm (mean = 37.9 ± 11.3 ppm; 1σ; n = 19). The δ¹⁵N values of the samples mostly cluster between 0.0 ‰ and + 3.1 ‰ with 3 samples showing higher values of + 4.2 ‰, +5.2 ‰ and + 7.5 ‰ (mean = +2.4 ± 1.7 ‰; 1σ; n = 19). For comparison, one altered sample has much higher N content of 128.7 ppm with a δ¹⁵N value of + 0.1 ‰. Compared with Precambrian sedimentary rocks, these SPGs contain statistically at least an order of magnitude less N. Data modeling suggests that decompression-induced magmatic N₂ degassing preponderates metamorphic N devolatilization for the major N loss from the Precambrian sedimentary rocks, which were also observed to control the N signatures of sediment-derived PGs in subduction-zone settings. Further compilation of the N contents of global granitoids show that SPGs contain 2–4 times higher N than non-SPGs, suggesting better retention of crustal N in SPGs. This can be attributed to the crystallization of hydrous peraluminous minerals (micas in particular, which have higher N-hosting capacities) in the H₂O-rich strongly peraluminous melts. We further estimated the N inventory in global SPGs as 0.9^{± 0.5} × 10¹⁷ kg N, which accounts for 4–9 % of the N budget in the upper continental crust.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 35-47"},"PeriodicalIF":4.5,"publicationDate":"2025-05-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144068694","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}