Geochimica et Cosmochimica Acta最新文献

{"title":"The molecular weight-dependent redox capacity of soil dissolved organic matter: Roles of aromaticity and organic sulfur","authors":"Jiang Xiao ,&nbsp;Zhenqing Shi ,&nbsp;Fu Liu ,&nbsp;Qianting Ye ,&nbsp;Lanlan Zhu ,&nbsp;Quan Gao ,&nbsp;Xin Yang ,&nbsp;Paul G. Tratnyek","doi":"10.1016/j.gca.2025.05.018","DOIUrl":"10.1016/j.gca.2025.05.018","url":null,"abstract":"<div><div>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.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 174-189"},"PeriodicalIF":4.5,"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":"The link between seawater magnesium concentrations and anhydrite formation in the ocean crust","authors":"Angus Fotherby ,&nbsp;Harold J. Bradbury ,&nbsp;Benjamin Tutolo ,&nbsp;Danielle Santiago Ramos ,&nbsp;Eoin P. Walsh ,&nbsp;Alexandra V. Turchyn","doi":"10.1016/j.gca.2025.05.016","DOIUrl":"10.1016/j.gca.2025.05.016","url":null,"abstract":"<div><div>Subseafloor hydrothermal systems exert a strong control on the chemical composition of the ocean. Likewise, the chemical composition of the ocean impacts the chemical and physical reactions that happen during hydrothermal circulation, although this has been less well considered. We present a 2D model of basalt alteration under hydrothermal conditions, exploring how changes in major seawater ion concentration over geologic time affect anhydrite (CaSO₄) formation in the oceanic crust. Anhydrite precipitation plays a key role in influencing the permeability structure of the ocean crust and acts as a sink for sulfur in the global biogeochemical sulfur cycle, one of the major biogeochemical cycles that regulates Earth’s redox state over geologic time. We develop a fully-coupled 2D reactive transport model to simulate the alteration of fresh mid-ocean ridge basalt by circulating seawater. We verify the model by comparing it to measured vent fluid chemistry and associated alteration mineralogy observed in modern drill cores. We then conduct a series of experiments, systematically changing the chemical composition of seawater to evaluate the impact these changes have on alteration and anhydrite formation. Model results support that the largest controls on the amount of anhydrite precipitation in the oceanic crust are sulfate and calcium concentrations in the ocean, and show that magnesium concentrations exert a strong control on the depth and distribution of anhydrite precipitation. The model results suggest that with the chemistry of certain oceans—in particular low magnesium-calcium ratios and higher magnesium and sulfate concentrations—there may have been significantly shallower anhydrite precipitation, with implications for the permeability structure of the crust and therefore extent of hydrothermal alteration. We suggest that the shallowing of the depth of anhydrite precipitation due to higher magnesium concentrations is via the impact of higher seawater magnesium concentrations on clay formation, which also modulates the pH of fluids during hydrothermal circulation. We speculate that, over Earth’s history, changes in the seawater magnesium, sulfate, and calcium concentrations may have influenced the amount and distribution of anhydrite in hydrothermally altered ocean crust, thus affecting crustal permeability structures, with consequences for key global biogeochemical cycles (e.g. sulfur, calcium).</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 160-173"},"PeriodicalIF":4.5,"publicationDate":"2025-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144237377","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 ,&nbsp;Changxun Yu ,&nbsp;Pasi Heikkilä ,&nbsp;Yann Lahaye ,&nbsp;Matti Kurhila ,&nbsp;Wei‐Li Hong ,&nbsp;Aivo Lepland ,&nbsp;Sten Suuroja ,&nbsp;Volker Liebetrau ,&nbsp;Joonas J. Virtasalo","doi":"10.1016/j.gca.2025.05.012","DOIUrl":"10.1016/j.gca.2025.05.012","url":null,"abstract":"&lt;div&gt;&lt;div&gt;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.&lt;/div&gt;&lt;div&gt;The lateral distribution and morphology of concretions are influenced by local hydrodynamic conditions, sedimentation dynamics, and redox fluctuations. An important factor is the periodic cover of a very organic-rich “fluffy” mud layer, which is driven by near-bottom currents, imports detrital minerals and modifies redox conditions, impacting microbial activity within the concretions. The higher occurrence of detrital minerals in Fe-rich concretions, particularly in the crust morphotype, suggests formation under stronger terrigenous influence in high-energy sedimentation conditions as opposed to more Mn containing concretions (mainly discoidal and spheroidal) forming in a relatively tranquil depositional setting and deeper water. The maturity of the detrital mineral fraction generally increases from crust to discoidal to spheroidal concretions. The Fe-rich concretions contain greater proportion of micas, clay minerals and K-feldspar to plagioclase, while the more Mn-containing concretions have proportionally high quartz contents. The detrital minerals likely act as nucleation sites promoting Fe precipitation and are redistributed diagenetically toward the interfaces dominated by Fe phases, which are slightly more tolerant to reductive dissolution than Mn phases. The preferential reductive dissolution of Mn phases results in thick Fe-rich growth layers and relative enrichment of the detrital mineral f","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 227-247"},"PeriodicalIF":4.5,"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 ,&nbsp;Yicheng Sun ,&nbsp;Guoxin Xia ,&nbsp;Xiandong Liu ,&nbsp;Guo-Guang Wang ,&nbsp;Xiancai Lu","doi":"10.1016/j.gca.2025.05.015","DOIUrl":"10.1016/j.gca.2025.05.015","url":null,"abstract":"&lt;div&gt;&lt;div&gt;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;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&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;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O system, water and OH are primarily bonded to Mg, forming Mg-H&lt;sub&gt;2&lt;/sub&gt;O&lt;em&gt;&lt;sub&gt;m&lt;/sub&gt;&lt;/em&gt; and Mg-OH species. The results demonstrate that as the water content increases, the viscosity of the Mg&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&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;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&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;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O fluid, we observed that Q&lt;sup&gt;0&lt;/sup&gt; and Q&lt;sup&gt;1&lt;/sup&gt; species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Q&lt;sup&gt;n&lt;/sup&gt; species in the supercritical Mg&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O fluid is: Q&lt;sup&gt;1&lt;/sup&gt; &gt; Q&lt;sup&gt;0&lt;/sup&gt;. However, when the water content exceeds 30 wt%, the order of Q&lt;sup&gt;n&lt;/sup&gt; abundance shifts to: Q&lt;sup&gt;0&lt;/sup&gt; &gt; Q&lt;sup&gt;1&lt;/sup&gt;. The content of Q&lt;sup&gt;2&lt;/sup&gt; species is relatively low, while the concentrations of Q&lt;sup&gt;3&lt;/sup&gt; and Q&lt;sup&gt;4&lt;/sup&gt; species are negligible. This study reveals that the s","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 279-291"},"PeriodicalIF":4.5,"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":"Calcium isotope composition of the Gangdese continental arc deep crust and implications for making continental crust","authors":"Meiling Wang ,&nbsp;Zaicong Wang ,&nbsp;Liang Guo ,&nbsp;Zongqi Zou ,&nbsp;Fei Wu ,&nbsp;Wei Dai ,&nbsp;Jingliang Guo ,&nbsp;Kang Chen ,&nbsp;Lanping Feng ,&nbsp;Haihong Chen ,&nbsp;Ming Li ,&nbsp;Yongsheng Liu","doi":"10.1016/j.gca.2025.04.033","DOIUrl":"10.1016/j.gca.2025.04.033","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Magmatism in deep continental arcs is crucial for understanding the formation of andesitic continental crust. Garnet, pyroxene and amphibole are typical major mafic phases in deep arcs, with garnet potentially causing noticeable variations in stable isotopes of major elements such as Ca (&lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca). The upper continental crust (UCC) shows low &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca value (0.70 ± 0.02 ‰, 2SE) relative to oceanic crust (0.85 ± 0.09 ‰), which is inferred to result from accumulation of garnet-rich mafic rocks with high &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca. However, due to limited exposure of middle-lower arc crustal sections, the &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca values of the deep continental crustal rocks are lacking, and the effect of magmatism in the deep crust is poorly understood. A juvenile (90–80 Ma) and continuous (∼ 42 to 17 km in depth) continental arc lower crust section is exposed in the Gangdese arc, southern Tibet. This section comprises garnet metagabbros, overlying mafic-intermediate sequence with a minor later ultramafic sequence. Here we present the &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca of these rocks and mineral separates to understand the Ca isotope compositions of deep continental arc crust.&lt;/div&gt;&lt;div&gt;The garnet metagabbros were formed from granulite-facies metamorphism of cumulative gabbros, which overall exhibit low &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca (from 0.59 ‰ to 0.85 ‰, with a mean value of 0.71 ± 0.06 ‰, 2SD, N = 10). Two samples with higher &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca values could be ascribed to higher proportions of coarse garnet grains in analyzed fractions. The mafic-intermediate rocks with different chemical compositions show similar &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca (0.58–0.72 ‰, with an average of 0.66 ± 0.06 ‰, 2SD, N = 12). The ultramafic sequence with variable degree of accumulation also displays consistent &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca (0.68–0.82 ‰). These results indicate limited Ca isotope fractionation (&lt; 0.1 ‰) during the continental arc magmatism related to pyroxene, plagioclase and amphibole. Based on these lithologies, the Gangdese continental deep crust of variable depths shows a mean &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca value of 0.69 ± 0.07 ‰, indistinguishable from the upper crust. The estimated &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca for the continental crust is thus lighter by ∼ 0.1 ‰ than the oceanic crust and by ∼ 0.2 ‰ than the mantle. Such difference could be explained by crystallization and segregation of garnet-bearing (∼ 20 %) cumulates in deeper Gangdese arc crust, consistent with high (Dy/Yb)&lt;sub&gt;N&lt;/sub&gt; of the most primitive mafic-intermediate rocks relative to MORBs (e.g., 1.4 versus 1.1). Therefore, our data supports the importance of accumulation of garnet-rich mafic rocks in the formation of thick continental arc crust. Conversely, island arc lavas show MORB-like &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/sup&gt;Ca, indicating the negligible effect of garnet during island arc magmatism. The distinct &lt;em&gt;δ&lt;/em&gt;&lt;sup&gt;44/40&lt;/s","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 248-264"},"PeriodicalIF":4.5,"publicationDate":"2025-05-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144188841","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 ,&nbsp;Guodong Chen ,&nbsp;Rong Mao ,&nbsp;Xin Luo ,&nbsp;Jiu Jimmy Jiao ,&nbsp;Meiqing Lu ,&nbsp;Tianwei Wang ,&nbsp;Wenli Hu ,&nbsp;Xingxing Kuang","doi":"10.1016/j.gca.2025.05.014","DOIUrl":"10.1016/j.gca.2025.05.014","url":null,"abstract":"<div><div>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 ²²⁴Ra/²²⁸Th and concentration profiles of additional constituents (SO₄^2-, NH₄⁺, NO₃^– and NO₂^–) through simulation and Bayesian theorem, enabling the estimation of bio-irrigation coefficient (<em>α</em>) and corresponding reaction rates. Based on the sediment columns collected from different segments of the lake, the model demonstrates vertical variability of <em>α</em>, with an average range of 0.60 × 10^-4 to 5.01 × 10^-4 s⁻¹. The calculated reaction rates and microbial taxa indicate the dominance of chemoheterotrophy, where NO₃^– serves as an electron acceptor during the degradation of organic matter in the anoxic sediment environment to generate NH₄⁺. 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 ²²⁴Ra/²²⁸Th disequilibrium to near-surface sediments in freshwater lacustrine systems, advancing the technique in tracing proxies for benthic fluxes in lakes.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 265-278"},"PeriodicalIF":4.5,"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}

{"title":"Experimental degassing of moderately volatile chalcophile elements from silicate melt with and without sulfur present","authors":"William F. Heck ,&nbsp;Achim D. Herrmann ,&nbsp;Colin R.M. Jackson","doi":"10.1016/j.gca.2025.05.007","DOIUrl":"10.1016/j.gca.2025.05.007","url":null,"abstract":"<div><div>We quantify the evaporation of moderately volatile chalcophile elements (MVCEs, here Cu, Zn, Ge, As, Ag, Cd, In, and Pb) from silicate melts. Our approach is to subject MVCE-bearing silicate melt to variable temperature (1480–1773 K), <em>f</em>O₂ (10⁻⁹ to atmospheric) and <em>f</em>S₂ (0 or 10⁻³ to 10⁻²), and duration (5 min to 24 h) using a gas-mixing furnace where evaporation can occur. Chemical analyses are completed using core to rim transects for major elements and MVCEs. We process our MVCE data through layered inverse models to empirically determine evaporation reaction stoichiometry, apparent thermodynamic data, and MVCE diffusivity for experiments that undergo diffusion-limited evaporative loss. The diffusivity values recovered for MVCEs between 1481 and 1673 K range from 6.5 × 10⁻¹² to 1.62 × 10⁻⁹ m²/s. Apparent evaporation enthalpies range from 213 to 1099 kJ/mol and entropies range from 85 to 621 J/mol K. We observe sulfur to have element-specific effects on MVCE volatility (Cu, Zn, Ge, and In). The volatility of Cu and Zn decreases with the presence of S, while Ge and In volatility increase with S present. We report previously unquantified evaporation thermodynamic data for Ag and As from silicate melt. Our thermodynamic data applied to magma oceans suggest that degassing under relatively cool (∼1573 K), highly reduced conditions (IW-3 to −5) can produce MVCE depletions broadly similar to those associated with incomplete nebular condensation, but key fractionations remain, particularly in In/Cd ratios. We also predict that magmatic evaporation under more oxidized conditions yields MVCE fractionations highly dissimilar to those associated with nebular condensation but broadly consistent with observations of volcanic outgassing.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"402 ","pages":"Pages 153-172"},"PeriodicalIF":4.5,"publicationDate":"2025-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144633942","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}