Geochimica et Cosmochimica Acta最新文献

{"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":"10.1016/j.gca.2025.05.012","url":null,"abstract":"<div><div>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.</div><div>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 , Yicheng Sun , Guoxin Xia , Xiandong Liu , Guo-Guang Wang , Xiancai Lu","doi":"10.1016/j.gca.2025.05.015","DOIUrl":"10.1016/j.gca.2025.05.015","url":null,"abstract":"<div><div>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<sub>2</sub>SiO<sub>4</sub>-H<sub>2</sub>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<sub>2</sub>SiO<sub>4</sub>-H<sub>2</sub>O system, water and OH are primarily bonded to Mg, forming Mg-H<sub>2</sub>O<em><sub>m</sub></em> and Mg-OH species. The results demonstrate that as the water content increases, the viscosity of the Mg<sub>2</sub>SiO<sub>4</sub>-H<sub>2</sub>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<sub>2</sub>SiO<sub>4</sub>-H<sub>2</sub>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<sub>2</sub>SiO<sub>4</sub>-H<sub>2</sub>O fluid, we observed that Q<sup>0</sup> and Q<sup>1</sup> species are the predominant types. In the water content range of 20 wt% to 30 wt%, the proportional distribution of Q<sup>n</sup> species in the supercritical Mg<sub>2</sub>SiO<sub>4</sub>-H<sub>2</sub>O fluid is: Q<sup>1</sup> > Q<sup>0</sup>. However, when the water content exceeds 30 wt%, the order of Q<sup>n</sup> abundance shifts to: Q<sup>0</sup> > Q<sup>1</sup>. The content of Q<sup>2</sup> species is relatively low, while the concentrations of Q<sup>3</sup> and Q<sup>4</sup> 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 , Zaicong Wang , Liang Guo , Zongqi Zou , Fei Wu , Wei Dai , Jingliang Guo , Kang Chen , Lanping Feng , Haihong Chen , Ming Li , Yongsheng Liu","doi":"10.1016/j.gca.2025.04.033","DOIUrl":"10.1016/j.gca.2025.04.033","url":null,"abstract":"<div><div>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 (<em>δ</em><sup>44/40</sup>Ca). The upper continental crust (UCC) shows low <em>δ</em><sup>44/40</sup>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 <em>δ</em><sup>44/40</sup>Ca. However, due to limited exposure of middle-lower arc crustal sections, the <em>δ</em><sup>44/40</sup>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 <em>δ</em><sup>44/40</sup>Ca of these rocks and mineral separates to understand the Ca isotope compositions of deep continental arc crust.</div><div>The garnet metagabbros were formed from granulite-facies metamorphism of cumulative gabbros, which overall exhibit low <em>δ</em><sup>44/40</sup>Ca (from 0.59 ‰ to 0.85 ‰, with a mean value of 0.71 ± 0.06 ‰, 2SD, N = 10). Two samples with higher <em>δ</em><sup>44/40</sup>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 <em>δ</em><sup>44/40</sup>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 <em>δ</em><sup>44/40</sup>Ca (0.68–0.82 ‰). These results indicate limited Ca isotope fractionation (< 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 <em>δ</em><sup>44/40</sup>Ca value of 0.69 ± 0.07 ‰, indistinguishable from the upper crust. The estimated <em>δ</em><sup>44/40</sup>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)<sub>N</sub> 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 <em>δ</em><sup>44/40</sup>Ca, indicating the negligible effect of garnet during island arc magmatism. The distinct <em>δ</em><sup>44/40</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":"Oxygen and hydrogen isotope compositions in dentine collagen correlate strongly with oxygen isotope compositions of enamel carbonate","authors":"L.M. Reynard,&nbsp;M.J. Kohn","doi":"10.1016/j.gca.2025.05.009","DOIUrl":"10.1016/j.gca.2025.05.009","url":null,"abstract":"<div><div>The isotopic composition of tooth and bone has long been exploited in ecological, environmental, and archaeological studies. Increasingly, hydrogen and oxygen isotopes in collagen are used as tracers of environment and diet. However, the isotopic relationship between hydrogen and oxygen in collagen and oxygen in tooth enamel is poorly described. A new investigation of varied wild large herbivores shows that δ¹⁸O and δ²H in collagen are both strongly correlated to δ¹⁸O in tooth enamel carbonate (r² = 0.96 and r² = 0.86, respectively), with a δ¹⁸O collagen vs. δ¹⁸O carbonate slope ∼ 0.8 and a δ²H collagen vs. δ¹⁸O carbonate slope ∼ 6. Given that enamel carbonate δ¹⁸O generally reflects environmental water isotopic composition, δ²H and δ¹⁸O in collagen also reflect isotopic compositions in environmental water and may be used analogously to tooth enamel for environmental or dietary studies.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 204-213"},"PeriodicalIF":4.5,"publicationDate":"2025-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144133759","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":"Compound class carbon isotope characterization of organic matter sources, transport, and degradation in sediments of large river-influenced coastal oceans","authors":"Fuqiang Wang ,&nbsp;Shuwen Sun ,&nbsp;Weikun Xu ,&nbsp;Wenjing Fu ,&nbsp;Honghua Shi ,&nbsp;Meixun Zhao","doi":"10.1016/j.gca.2025.05.008","DOIUrl":"10.1016/j.gca.2025.05.008","url":null,"abstract":"&lt;div&gt;&lt;div&gt;The Yangtze River Estuary and the adjacent inner shelf are significant areas for both the burial and degradation of terrestrial organic matter (OM). However, the impact of hydrodynamics on the degradation of different types of organic compound classes has not been fully elucidated. This study measured dual carbon isotopes (δ&lt;sup&gt;13&lt;/sup&gt;C, Δ&lt;sup&gt;14&lt;/sup&gt;C) of organic compound classes [total hydrolysable amino acids (THAAs), total lipids, humic acids (HAs) and acid-base insoluble OM] in both surface and core sediment samples to evaluate the influences of hydrodynamic processes on the fates of organic compound classes in the Yangtze River Estuary and the adjacent inner shelf. In surface sediments, the δ&lt;sup&gt;13&lt;/sup&gt;C (−28.6 ± 0.6 ‰ in lipids; −21.8 ± 0.5 ‰ in THAAs) and Δ&lt;sup&gt;14&lt;/sup&gt;C (−598 ± 41 ‰ in lipids; −143 ± 47 ‰ in THAAs) values of lipids and THAAs from the mobile-muds are lower than those from the offshore areas (δ&lt;sup&gt;13&lt;/sup&gt;C: −27.9 ± 0.4 ‰ in lipids; −20.9 ± 0.8 ‰ in THAAs; Δ&lt;sup&gt;14&lt;/sup&gt;C: −559 ± 20 ‰ in lipids; −64 ± 7 ‰ in THAAs), indicating replenishment of marine source may be the primary factor contributing to the rejuvenation of lipids and THAAs during seaward transport. In contrast, both HAs and acid-base insoluble OM have higher Δ&lt;sup&gt;14&lt;/sup&gt;C values in mobile muds (−298 ± 28 ‰ and −322 ± 21 ‰, respectively) than those in offshore areas (−443 ± 14 ‰ and −533 ± 30 ‰, respectively), suggesting significant aging even for the refractory OM during transport processes. The three end-member model of organic compound classes shows that lipids and THAAs in mobile-muds have higher marine source than offshore areas, while HAs show opposite change. These results indicate organic compound classes undergo different transport and degradation mechanisms during seaward transport, then affecting the contributions of marine and terrestrial sources to marine sediments. The different correlations between Δ&lt;sup&gt;14&lt;/sup&gt;C values of labile (THAAs) and refractory (HAs and acid-base insoluble OM) OM with specific mineral surface areas (SSA) also indicate different transport and degradation mechanisms of organic compound classes in surface sediments, and causing rejuvenation of THAAs and gradually aging of HAs and acid-base insoluble OM during seaward transport. In core QT1 samples, the younger &lt;sup&gt;14&lt;/sup&gt;C ages and large variations in the accumulation rates of labile THAAs since 2003 indicate that the construction of the TGD probably altered the hydrodynamic and depositional environment in these areas, and causing the degradation of THAAs in the sediment cores. In contrast, acid-base insoluble OM have stable &lt;sup&gt;14&lt;/sup&gt;C ages and accumulation rates in cores TQ1 and QT3, suggesting minor impacts from human activities and physical processes. These carbon isotopic results suggest that labile and refractory OM respond differently to hydrodynamics, with refractory OM playing a key role in the burial of terrestrial carbon in marine sediments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 190-203"},"PeriodicalIF":4.5,"publicationDate":"2025-05-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144083377","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 formation of the trinitites unveiled by their oxygen and silicon isotopic compositions","authors":"Nathan Asset ,&nbsp;Marc Chaussidon ,&nbsp;Christian Koeberl ,&nbsp;Johan Villeneuve ,&nbsp;François Robert","doi":"10.1016/j.gca.2025.05.011","DOIUrl":"10.1016/j.gca.2025.05.011","url":null,"abstract":"<div><div>During the world’s first nuclear explosion, in 1945, glassy melts called “trinitites”, mostly derived from the sands at the surface of the test site, formed and were deposited at or near the hypocenter. The processes of formation of this fallout remain unclear. Here, we show how the oxygen and silicon isotopic compositions of three trinitites allow to refine their formation scenario. The three samples are typical of trinitites, being composed of various crystalline phases and of glassy phases divided into three chemical groups (CaMgFe, alkali, silica) that are mixed in various proportions in the three samples. The three samples show a large range of oxygen and silicon isotopic variations (−10.9 ± 0.6 &lt; δ³⁰Si &lt; 4.2 ± 0.6 ‰, and 2.3 ± 0.4 &lt; δ¹⁸O &lt; 24.2 ± 0.5 ‰). At variance with the Hiroshima fallout deposits, no oxygen mass-independent isotopic fractionation was found in the three trinitites. The chemical and isotopic compositions of the chemical groups reveal that they result from different processes: the silica phases are molten fragments of the site material, while the CaMgFe and alkali phases are produced by the mixing of condensates and molten site material. Models show that the observed silicon isotopic variations resulted from Rayleigh distillation during condensation of the gaseous species injected into the cloud, while the variability in composition of the site materials also played an important role for controlling the oxygen isotopic compositions. From these observations, a general scenario, beginning with the vaporization of the site surface, producing a depression, is proposed. The vaporized material condensed and grew by agglomeration with other condensates and liquid materials. These agglomerates rained on the surface and quenched, forming the trinitites. This scenario is different from the formation of the Hiroshima glasses but shows some similarities to tektite formation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 214-226"},"PeriodicalIF":4.5,"publicationDate":"2025-05-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144255390","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":"Tin isotope systematics in subduction zones","authors":"Xueying Wang ,&nbsp;Oliver Nebel ,&nbsp;Alexandra Churchus ,&nbsp;Peter A. Cawood","doi":"10.1016/j.gca.2025.05.004","DOIUrl":"10.1016/j.gca.2025.05.004","url":null,"abstract":"<div><div>The origin of slab components in arc lavas remains controversial with proposed sources including direct sediment melts, sediment-mantle mélanges, or crustal fluids. Stable isotope distributions of fluid-borne metals and those immobile in subduction components have the potential to trace the respective contribution of these components if placed into the context of additional magmatic differentiation and in conjunction with radiogenic isotopes. Here, we present the first high-precision double-spike tin abundances and isotope compositions in twenty-six arc rocks sampled along the Indonesian Sunda arc, a subduction zone with the highest subducted sediment volume globally. <em>δ</em>^122/118Sn_3161a in these samples ranges from 0.13‰ to 0.46‰ and we find that Sn isotope compositions in Sunda arc rocks were affected by magmatic differentiation, superimposed on additional influences from components derived from the subducting slab. A step change in <em>δ</em>^122/118Sn_3161a at 4–5wt.% MgO suggests the removal of heavier Sn isotopes, likely due to higher compatibility of isotopically heavier Sn⁴⁺ in Fe-Ti oxides along the liquid line of descent. This change of ∼0.1‰, however, is dwarfed by Sn isotope variations consistent with sediment melt contribution coupled with fluid addition from altered oceanic crust, both of which introduce lighter Sn isotopes. Sediment melt contamination increases with depth (temperature), but some exceptions at shallow sub-arc sources (depth ≤ 130 km) indicate additional melting of a mélange-style source. Our findings suggest the coexistence of mélange contributions, and deep fluid-induced melting plus partial sediment melts in the Sunda arc. Mélanges contribute only at shallow depths (≤130 km), consistent with their diapiric nature, whereas deeper levels favor fluid-fluxed melting and, at higher temperatures and pressures, partial sediment melting. This study provides insights into the complex interactions between fluids, sediments, and mantle wedge in subduction zones, highlighting the depth-dependent nature of slab contributions to arc magmatism. It also demonstrates the utility of Sn isotopes for tracing these processes and advancing our understanding of subduction zone geochemistry.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 18-34"},"PeriodicalIF":4.5,"publicationDate":"2025-05-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143947247","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":"Large Zn isotope variations in lower oceanic crust offer insights into oceanic crustal magma plumbing system","authors":"Wei-Qi Zhang ,&nbsp;Chuan-Zhou Liu ,&nbsp;Zong-Qi Zou ,&nbsp;Xiao-Ni Li","doi":"10.1016/j.gca.2025.05.005","DOIUrl":"10.1016/j.gca.2025.05.005","url":null,"abstract":"<div><div>Constraining mid-ocean ridge magmatic processes is crucial for understanding the geochemical evolution of magmas traversing the oceanic crust. A crucial archive for investigating these magmatic processes is the chemical composition of lower oceanic crust. This study presents Zn isotope data for primitive to highly evolved cumulates from the Mid-Atlantic Ridge (Kane) and the Southwest Indian Ridge (IODP Hole U1473A), which are compared with published mid-ocean ridge (MOR) lava data. Previous studies have indicated that high-degree magma differentiation results in small Zn isotope fractionation (&lt;0.1 ‰) in the mafic to felsic MOR lavas. Unlike the MOR lavas, significant δ⁶⁶Zn variations (0.1 − 0.5 ‰) are observed in the Kane and U1473A cumulates, which cannot be explained by crystal accumulation alone but reflect the variable incorporation of trapped melts. These trapped melts exhibit heavy δ⁶⁶Zn (&gt;0.5 ‰) and strongly fractionated trace element characteristics, indicative of extensive melt-rock interactions. Furthermore, the bulk normal mid-ocean ridge basalt (N-MORB)-type lower oceanic crust has δ⁶⁶Zn values (0.24 ± 0.11 ‰) that are close to the average N-MORB (0.26 ± 0.06 ‰), suggesting minor Zn isotope fractionation during the crustal processing of MOR lavas. It is proposed that the small δ⁶⁶Zn variations in MOR magmas result from the homogenization and dilution of reactive signals within the eruptive reservoir. In contrast, prolonged processing of trapped melts through melt-rock interactions leads to significant Zn isotope fractionation in the lower oceanic crust. Thus, the evolution of oceanic crustal magma plumbing systems is more complex than inferred from MORB alone. Despite these complexities, efficient homogenization within eruptive reservoirs results in limited Zn isotopic variations in the erupted magmas. Hence, the Zn isotopic variations in MORB, except for the highly evolved ones, are predominantly linked to the source heterogeneity. Our findings also suggest heterogeneous δ⁶⁶Zn in subducted oceanic crusts, which has implications for mantle Zn isotope heterogeneity.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 158-171"},"PeriodicalIF":4.5,"publicationDate":"2025-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144083378","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}