Geochimica et Cosmochimica Acta最新文献

{"title":"Of zircons and zircons: The tumultuous story of Zr-Hf and REE during cooling of peralkaline granites","authors":"Loïs Monnier, Oscar Laurent, Stefano Salvi, Cyrielle Bernard, Mathieu Leisen, Guillaume Estrade, Philippe de Parseval, Claudie Josse, Armel Descamps-Mandine, Zhenpeng Duan, Sophie Gouy, Caliméria Passos do Carmo, Camille Dusséaux","doi":"10.1016/j.gca.2025.05.039","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.039","url":null,"abstract":"Understanding the behavior of trace elements such as Zr-Hf, REE, and Th-U during magmatic and hydrothermal processes is crucial for unraveling the evolution of rare-metal peralkaline granites. These elements play key roles in geochemical cycles, mineral formation, and economic mineral deposits. The Evisa intrusion, a rare-metal peralkaline granite, is an ideal setting to study such processes due to its diverse zircon population, which records a continuous transition from early igneous to late hydrothermal precipitation and alteration. This evolution spans temperatures from more than 600 °C to approximately 150 °C, with zircon demonstrating exceptional diversity in textures and compositions. In-situ U-Pb dating of both igneous and hydrothermal zircon suggests they precipitated coevally at ∼ 270 Ma, indicating the synchronous formation and alteration of zircon during the crystallization and cooling of the Evisa granite. Hf isotopic compositions vary from 0 to + 10 εHf(t) for igneous zircon and from −5 to + 5 εHf(t) for hydrothermal zircon, reflecting a shift from a mantle-derived to a more crustal signature. Both U-Pb and Hf isotopic data support a rift-related context for the Corsica terrane during the late Permian. Zircon in this granite is highly enriched in REE (median value > 2 wt% for Y + REE) and variably enriched in U and Th (up to several wt.%), with igneous zircon exhibiting higher Ce anomalies and HREE/LREE ratios than hydrothermal zircon. Alteration of zircon results in the formation of pores (ranging from nm to µm scale) and the redistribution of trace metals between newly-formed mineral inclusions, newly-crystallized zircon, and the aqueous fluid. This redistribution, combined with the breakdown of other REE-minerals and availability of suitable ligands in the fluid, controlled the cyclic enrichment of REE in hydrothermal zircon and influenced their fate in the Evisa granite. Hydrothermal remobilization of Hf at low temperature led to significant Hf isotopic variations (up to > 20 εHf(t) unit) in newly crystallized zircon, likely due to mass-dependent kinetic fractionation. Discrepancies in the Zr/Hf ratio (from ∼ 0.5 to 2.5) in successive growth zones of hydrothermal zircon were interpreted as resulting from fractionation due to mass-independent effects on bond strength during transport, probably in the form of fluoride complexes. This study not only provides new insights into the behavior of Zr-Hf, Th-U and REE in rare-metal peralkaline granites but also underscores the need for caution when interpreting zircon compositions, particularly when using commonly employed geochemical tools, such as Ti-based thermometers.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"18 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144237350","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":"Systematics of helium diffusion sinks in apatite demonstrated by 4He/3He degassing experiments and modeling","authors":"Hongcheng Guo ,&nbsp;Marissa M. Tremblay ,&nbsp;Peter K. Zeitler ,&nbsp;Bruce D. Idleman ,&nbsp;Annia K. Fayon","doi":"10.1016/j.gca.2025.05.036","DOIUrl":"10.1016/j.gca.2025.05.036","url":null,"abstract":"<div><div>Widely reported overdispersion of apatite (U-Th)/He ages has encouraged efforts to explore He diffusion kinetics and systematics in apatite. Studies such as continuous ramped heating (CRH) of apatite have revealed complex laboratory He behavior that is often correlated with abnormally old (U-Th)/He ages. Stemming from these studies is a hypothesis that diffusion sinks within apatite grains can reversibly trap radiogenic ⁴He over both geologic time and during laboratory heating. This sink-related trapping is further hypothesized to be temperature dependent and can hence potentially provide thermal-history information. In this work, we conducted ⁴He/³He degassing experiments from proton-irradiated apatite samples using a CRH-like heating strategy. We demonstrate that the synthesized ³He also exhibits complex release behavior which (1) confirms that the proposed sink-related trapping occurs during laboratory heating, and (2) provides foundational evidence for possibly qualifying the nature and quantifying the abundance and/or distribution of diffusion sinks. Our combined ⁴He/³He observations show that an apatite sample’s thermal history controls the amount of trapping over geologic time. Building upon the dataset from one of the sink-bearing samples, we attempt to simulate the sink characteristics using a simple random-walk code that can simulate trapping. The forward modeling can reproduce the ³He release observed in the laboratory experiment if multiple sinks characterized by different kinetics are present. Using such sink kinetics and the sample’s known thermal history, we find that the prediction of laboratory ⁴He release, which reflects both geologic and laboratory volume diffusion and trapping processes, is comparable to our observations. This work confirms that a temperature-dependent trapping mechanism is required to explain the complex He diffusion behavior seen in many apatite grains, and also serves as a proof-of-concept that helium trapped in sinks can provide added thermal history information.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 115-128"},"PeriodicalIF":4.5,"publicationDate":"2025-05-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144212536","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":"From crustal protoliths to mantle garnet pyroxenites: insights from Os isotopes and highly siderophile elements","authors":"Alessandra Montanini, Ambre Luguet, David van Acken, Riccardo Tribuzio, Stephan Schuth, Geoff M. Nowell","doi":"10.1016/j.gca.2025.05.034","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.034","url":null,"abstract":"Recycled pyroxenites are a key component of mantle heterogeneity, influencing magma generation and contributing to the radiogenic Os signature of oceanic basalts. However, natural examples of pyroxenites as proxies for crust-derived heterogeneities in the convective mantle are rare and recycling mechanisms remain debated. Here we report abundances of highly siderophile (HSE: Os, Ir, Pt, Pd, Re) and chalcogen (S, Se, Te) elements, along with Os isotope compositions, for garnet clinopyroxenites and websterites enclosed in fertile peridotites from the Jurassic Northern Apennine ophiolites. The pyroxenites provide fractionated HSE patterns—depleted in Os and Ir and enriched in Pt, Pd, and Re compared to host lherzolites. Present-day <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os ratios range from moderately to highly radiogenic (0.154–2.475). Garnet clinopyroxenites have lower Os, higher Pd/Ir, and more radiogenic Os than websterites. Model calculations based on bulk HSE data indicate the garnet clinopyroxenites reflect sulfur saturation and base metal sulfide (BMS) crystallization from partial melts of eclogites derived from MORB-type gabbroic protoliths. Their Os isotope signature indicates long-term Re/Os enrichment and derivation from ancient mafic protoliths, likely older than 1.0 Ga. The HSE distribution and Os isotopic signature of the websterites, along with their sulfide mineralogy, support a hybrid origin involving contributions from both peridotite- and eclogite-derived components. The websterites may represent natural analogues of the second-stage pyroxenites involved in OIB magma genesis. Host lherzolites exhibit flat CI chondrite-normalized HSE patterns and near-chondritic Se/Te and Pd/Ir ratios, implying initial depletion of incompatible HSEs like Pd and Re, followed by BMS addition during melt percolation that also affected some pyroxenites. Se (78–539 ppb) and Te (13–44 ppb) are significantly enriched in the pyroxenites compared to the lherzolites. The low Se/Te ratios (6–15) in garnet clinopyroxenites could explain the Se–Te signature of plume-influenced E-MORB. We show that mantle pyroxenites with a crustal fingerprint display wide HSE and Os isotope variability, reflecting heterogeneity of crustal protoliths, melt–peridotite interactions, and late melt percolation during their decompression history.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"99 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304933","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lithium- and oxygen-isotope compositions of a Si-rich nebular reservoir determined from chondrule constituents in the Sahara 97103 EH3 chondrite","authors":"Torii Douglas-Song,&nbsp;Tsutomu Ota,&nbsp;Masahiro Yamanaka,&nbsp;Hiroshi Kitagawa,&nbsp;Ryoji Tanaka,&nbsp;Christian Potiszil,&nbsp;Tak Kunihiro","doi":"10.1016/j.gca.2025.05.038","DOIUrl":"10.1016/j.gca.2025.05.038","url":null,"abstract":"&lt;div&gt;&lt;div&gt;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&lt;sub&gt;2&lt;/sub&gt; (Mg&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt; + SiO&lt;sub&gt;2&lt;/sub&gt; → Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;), instead of sulfidation (Mg&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt; + S → 1/2 Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt; + MgS + 1/2 O&lt;sub&gt;2&lt;/sub&gt;). Using the equation Mg&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt; + SiO&lt;sub&gt;2&lt;/sub&gt; → Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt; and the O-isotope compositions of enstatite and olivine, the O-isotope composition of the additional SiO&lt;sub&gt;2&lt;/sub&gt; was estimated. Based on the O-isotope composition, we infer that there could be a Si-rich gas with an elevated Δ&lt;sup&gt;17&lt;/sup&gt;O value similar to, or greater than the second trend line (Δ&lt;sup&gt;17&lt;/sup&gt;O = 0.9 ‰) suggested by &lt;span&gt;&lt;span&gt;Weisberg et al. (2021)&lt;/span&gt;&lt;/span&gt;, 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&lt;sup&gt;7&lt;/sup&gt;Li) of enstatite is lower than that of whole rock EH3 chondrites, suggesting the existence of a phase with higher δ&lt;sup&gt;7&lt;/sup&gt;Li. Meanwhile, the Li-isotope composition and concentration (δ&lt;sup&gt;7&lt;/sup&gt;Li, [Li]) of enstatite is higher than that of olivine. The Li-isotope composition of the Si-rich gas was estimated to be δ&lt;sup&gt;7&lt;/sup&gt;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&lt;sup&gt;7&lt;/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":"Rutile petrochronology and titanium isotope compositions record multiple melt-fluid-rock interactions in a continental subduction zone","authors":"Xiaojia Jiang ,&nbsp;Xin Chen ,&nbsp;Hans-Peter Schertl ,&nbsp;Liam Hoare ,&nbsp;Aitor Cambeses ,&nbsp;David Hernández Uribe ,&nbsp;Rongke Xu ,&nbsp;Youye Zheng ,&nbsp;Wen Zhang","doi":"10.1016/j.gca.2025.05.024","DOIUrl":"10.1016/j.gca.2025.05.024","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Subduction zones act as pivotal engines for global element cycling. Titanium (Ti) isotopes, exhibiting mass-dependent fractionation, emerge as a robust geochemical tracer for deciphering complex processes in these dynamic settings. However, the behavior of Ti and its isotopes during metamorphic dehydration and partial melting of deeply subducted continental lithosphere remains poorly constrained. This study addresses this issue through an integrated investigation of age, elemental signatures, and Ti isotopic compositions of rutile from quartz-bearing and granitic felsic veins, eclogites, paragneisses, and orthogneisses within the North Qaidam orogen—a paleo-continental subduction zone where eclogite boudins are embedded in gneissic matrices. Petrochronological U-Pb dating reveals distinct temporal records: rutile in eclogites and gneisses yields metamorphic ages of 439 ± 3 Ma to 430 ± 11 Ma, aligning with regional eclogite-facies metamorphism and implicating rutile growth during dehydration of subducted continental crust. In contrast, rutile from quartz- and felsic veins documents protracted melt/fluid activities spanning 433 ± 3 Ma to 408 ± 1 Ma, reflecting melt/fluid generation under eclogite-facies conditions and subsequent exhumation. High-resolution Ti isotope and trace element analyses demonstrate minimal intra-sample isotopic variability among rutile grains within individual eclogites or gneisses, suggesting negligible Ti isotope fractionation during metamorphic dehydration. Similarly, rutile in eclogite-hosted felsic veins exhibits δ&lt;sup&gt;49&lt;/sup&gt;Ti values indistinguishable from their host eclogites or adjacent gneisses, further negating significant isotopic fractionation during partial melting. However, inter-sample δ&lt;sup&gt;49&lt;/sup&gt;Ti variations correlate systematically with whole-rock geochemical proxies: negative correlations with εNd(t) and positive correlations with (&lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr)i highlight protolith heterogeneity as the dominant control on Ti isotopic signatures. These findings collectively demonstrate that Ti isotopic compositions in deep subduction-related systems primarily inherit protolith characteristics rather than reflecting process-driven fractionation. Consequently, Ti isotopes serve as powerful tracers for identifying melt/fluid sources in subduction zones. Notably, cold subduction regimes promote localized Ti mobility via eclogite-derived melts/fluids, while warmer settings facilitate widespread Ti activation through partial melting of gneiss-eclogite mixtures, as evidenced by abundant rutile-bearing veins spanning 427–408 Ma. The study underscores that continental subduction zones—spanning thermal gradients from cold to warm—exhibit melt-mediated Ti mobilization influenced by melt abundance, source heterogeneity, and prolonged melt-crystal interaction. These insights from the North Qaidam orogen advance our understanding of Ti cycling in continental subduction systems globally, emphasizing the ","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 94-114"},"PeriodicalIF":4.5,"publicationDate":"2025-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144184921","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-isotope variations in refractory inclusions from Rumuruti-type chondrites: Evidence for nebular gas heterogeneity and parent-body exchange","authors":"Samuel Ebert, Kazuhide Nagashima, Alexander N. Krot, Addi Bischoff","doi":"10.1016/j.gca.2025.05.029","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.029","url":null,"abstract":"Refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] in unmetamorphosed chondrites (petrologic type ≤ 3.0) have typically uniform <ce:sup loc=\"post\">16</ce:sup>O-rich solar-like compositions. The origin of oxygen-isotope heterogeneity within individual refractory inclusions from weakly metamorphosed (petrologic type &gt; 3.0) chondrites remains controversial. It may reflect (<ce:italic>i</ce:italic>) condensation from a nebular gas having variable O-isotope composition, (<ce:italic>ii</ce:italic>) gas–solid or gas–melt O-isotope exchange with this gas, and/or (<ce:italic>iii</ce:italic>) O-isotope exchange with an <ce:sup loc=\"post\">16</ce:sup>O-depleted aqueous fluid in the host chondrite parent bodies. Here, we present the mineralogy, petrology and O-isotope compositions of refractory inclusions (12 CAIs and 2 AOAs) from the Rumuruti-type (R) chondrites of petrologic type 3 – Northwest Africa (NWA) 753, NWA 1471, and Dhofar 1123. The CAIs and AOAs are extensively altered: melilite is completely replaced by secondary minerals; perovskite is largely replaced by ilmenite; spinel and olivine are enriched in FeO. The polymineralic refractory inclusions have heterogeneous O-isotope compositions: Δ<ce:sup loc=\"post\">17</ce:sup>O ranges from ∼−25 ‰ to ∼5 ‰ (2σ = ±∼2‰). The only exception is a spinel-hibonite inclusion having uniform <ce:sup loc=\"post\">16</ce:sup>O-depleted composition (Δ<ce:sup loc=\"post\">17</ce:sup>O ∼ −14 ‰). Hibonite, most ferroan spinel, and some olivine and Al,(Ti)-diopside grains in Rumuruti-type chondrite (RC) fragments of low petrologic type (3.15 − 3.2) retained their initial Δ <ce:sup loc=\"post\">17</ce:sup>O values, which, however, range from −25 ‰ to ∼ −14 ‰, suggesting variations in O-isotope composition of nebular gas in the CAI-forming region. Most Al,Ti-diopside and some olivine and spinel grains in RC refractory inclusions are <ce:sup loc=\"post\">16</ce:sup>O-depleted compared to minerals most resistant to O-isotope exchange (hibonite and spinel) that retained their original compositions. The most <ce:sup loc=\"post\">16</ce:sup>O-depleted compositions of Al,Ti-diopside and ferroan olivine have Δ<ce:sup loc=\"post\">17</ce:sup>O of ∼ +5‰ that is similar to Δ<ce:sup loc=\"post\">17</ce:sup>O of the aqueously formed grossular and ferroan olivine. We infer that the <ce:sup loc=\"post\">16</ce:sup>O-depleted Al,Ti-diopside, olivine, and spinel in isotopically heterogeneous refractory inclusions experienced post-formation O-isotope exchange with aqueous fluids in the RC parent asteroid(s).","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"19 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144237352","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":"Isotopic dynamics between methane-derived authigenic carbonates and gas hydrates: Insights from two deep-sea cold seeps","authors":"Josué J. Jautzy, Martine M. Savard, Stéphanie Larmagnat, Benjamin R. Fosu, D. Calvin Campbell","doi":"10.1016/j.gca.2025.05.023","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.023","url":null,"abstract":"The use of clumped isotopes (Δ<ce:inf loc=\"post\">47</ce:inf>) as a geothermometer in carbonates has become widespread in geosciences because carbonates are ubiquitous in the environment and this technique also constrains the precipitating fluid δ<ce:sup loc=\"post\">18</ce:sup>O composition. However, in some contexts, Δ<ce:inf loc=\"post\">47</ce:inf> values cannot be used as a geothermometer. Instead, they provide insights into processes affecting the isotopic composition of CO<ce:inf loc=\"post\">2</ce:inf> and dissolved inorganic carbon (DIC) prior to and during precipitation, as observed in speleothems, corals, and cold seep-associated authigenic carbonates. Among all these precipitates, authigenic carbonates associated with cold seeps stand out, as the measurement of their clumped isotopic abundances has revealed an unprecedented range of isotopic ordering, and several explanations were put forward to explain the various signals observed. To get a firmer understanding of the cause of natural variability in modern cold seeps, here we report δ<ce:sup loc=\"post\">13</ce:sup>C, δ<ce:sup loc=\"post\">18</ce:sup>O and Δ<ce:inf loc=\"post\">47</ce:inf> results performed on carbonates from two deep-sea cold seeps offshore Nova Scotia, Canada, <ce:italic>i.e.,</ce:italic> &gt;2300 m water depth and within the gas hydrate stability zone. We report a broad Δ<ce:inf loc=\"post\">47</ce:inf> range (<ce:italic>i.e.,</ce:italic> 0.502–0.663 ‰ I-CDES) that differs from values expected at isotopic equilibrium at seafloor temperatures (<ce:italic>i.e.</ce:italic>, 0.671–0.672 ‰ I-CDES). Using micro computed X-ray tomography, micro-X-ray fluorescence, and micro sampling, we demonstrate that methane-derived authigenic carbonates at these deep-water sites are closely associated with colocalized methane-hydrate dissolution. The isotopic composition of these authigenic carbonates are influenced by the complex mixing of different dissolved inorganic carbon (DIC) pools at various stages of isotopic re-equilibration, resulting in fine-scale isotopic variability. We propose several mixing models and stages of DIC re-equilibration to explain this isotopic variability and apply them to evaluate the contributions of the proposed sources of DIC. The isotopic domains resulting from this exercise encompass the entire range of dual isotopic values (δ<ce:sup loc=\"post\">18</ce:sup>O and Δ<ce:inf loc=\"post\">47</ce:inf>) ever observed in cold seeps and suggests that the isotopic diversity observed in these environments may be broader than that observed to date.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"14 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144237374","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":"Barium isotope variation during granitic magma differentiation: implications for the discrimination of crystal fractionation and fluid-magma interaction","authors":"Nian-Qin Zhang, Kun Zhou, Yi-Xiang Chen, Fang Huang, Xiao-Feng Gu, Lin-Hui Dong, Gengxin Deng, Peng Gao","doi":"10.1016/j.gca.2025.05.030","DOIUrl":"https://doi.org/10.1016/j.gca.2025.05.030","url":null,"abstract":"The genesis and compositional diversity of granites are primarily controlled by multiple processes, including partial melting and magma differentiation. However, discriminating the individual effects of processes such as partial melting, fractional crystallization, and fluid-magma interaction on the evolution of granitic magma remains challenging. Barium isotopes serve as a potentially powerful tracer for discrimination between fractional crystallization and fluid-magma interaction processes, yet the effect of fractionation crystallization on Ba isotope composition variation remains poorly constrained. To tackle this challenge, we measured the Ba isotope compositions of &lt;ce:italic&gt;syn&lt;/ce:italic&gt;-exhumation granites from the Sulu orogen in eastern China. These high-Si granites exhibit significant variations in &lt;ce:italic&gt;δ&lt;/ce:italic&gt;&lt;ce:sup loc=\"post\"&gt;138/134&lt;/ce:sup&gt;Ba values (–0.69 ‰ to 0.36 ‰). Using phase equilibrium modeling, we estimate that the Ba isotope variation during partial melting process can result in an increase of less than ca. 0.20 ‰ between melts and protoliths, which cannot account for the observed light Ba isotope compositions. Additionally, the content of SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; is significantly correlated with contents of other major and trace elements, such as Al&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;, TiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;, Fe&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;O&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;t, Ba, Sr and Eu/Eu*, suggesting extensive fractional crystallization in these granites. Moreover, &lt;ce:italic&gt;δ&lt;/ce:italic&gt;&lt;ce:sup loc=\"post\"&gt;138/134&lt;/ce:sup&gt;Ba values exhibit good correlations with the compositions of such major and trace elements, indicating large Ba isotopic variation during the fractional crystallization of granitic magmas. In contrast, the Zr/Hf, K/Rb, Nb/Ta ratios and REE patterns of the &lt;ce:italic&gt;syn&lt;/ce:italic&gt;-exhumation granites indicate the absence of fluid-magma interaction. These observations indicate that fractional crystallization, rather than fluid-magma interaction, is the primary factor driving significant Ba isotope fractionation in the granites, resulting in &lt;ce:italic&gt;δ&lt;/ce:italic&gt;&lt;ce:sup loc=\"post\"&gt;138/134&lt;/ce:sup&gt;Ba values as low as −0.69 ‰. Using fractional crystallization modelling with rhyolite-MELTS, we demonstrate that the fractional crystallization of K-feldspar, biotite and muscovite controls the Ba isotope variation of granites, consistent with the petrological observations. Combined with previous results from leucogranites and strongly fractionated granites, we document that it should be taken caution when using Ba isotope compositions to trace the fluid-magma interaction. While crystal fractionation can lead to &lt;ce:italic&gt;δ&lt;/ce:italic&gt;&lt;ce:sup loc=\"post\"&gt;138/134&lt;/ce:sup&gt;Ba values as low as ca. −0.7 ‰ in granites, values lower than this can be more unambiguously attributed to fluid-magma interaction during the late-stage differentiation of granitic magma","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"34 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144188964","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":"Path-integral molecular dynamics predictions of H isotope fractionation between brucite and water at elevated temperatures and pressures","authors":"Caihong Gao ,&nbsp;Yun Liu","doi":"10.1016/j.gca.2025.05.032","DOIUrl":"10.1016/j.gca.2025.05.032","url":null,"abstract":"<div><div>Hydrogen (H) isotope fractionation during the dehydration of H-bearing minerals is crucial for tracing the subduction processes. Previous studies have shown that H isotope fractionation between brucite and water is highly sensitive to the change of pressure. However, the detailed behaviors of H isotopes at high temperatures and high pressures remain poorly understood. In this study, we investigate the D/H isotope fractionation between brucite and water at elevated temperatures (300–1273 K) and pressures (0–8 GPa) using path-integral molecular dynamics (PIMD) simulations to account for the nuclear quantum effects (NQEs). The deep potential models trained on the first-principles molecular dynamics (FPMD) data were used to accelerate the simulations. The calculated reduced partition function ratios (RPFRs) of water decrease with increasing pressure at all the temperatures studied in this work, while the RPFRs of brucite increase with pressure. Due to the opposing pressure effects on RPFRs of these two phases, the D/H isotope fractionation between brucite and water is highly sensitive to the pressure change, and the fractionations are inversed at high pressures (e.g., at 2 GPa and 200 °C). The reversal of D/H isotope fractionation is attributed to the distinct responses of H bonding environments in brucite and water to the pressure changes.</div><div>Using brucite as a proxy for H-bearing minerals in subduction zones, we modeled the δD values of both the water retained in the slab and the water released into the mantle in three different subduction zones. The results demonstrate that the δD values of both the slab water and the released water are closely related to the pressures and temperatures of the slabs. The modeled δD values of the released water mainly range from − 40 ‰ to − 70 ‰, overlapping with the average δD value of the depleted mantle (−60 ± 5 ‰). Due to the reversal of isotope fractionation, the water transported into the deep Earth (&gt;200 km) exhibit relatively high δD values (&gt;−50 ± 10 ‰), suggesting that the extremely low δD signatures (e.g., &lt; −200 ‰) reported in previous works may not be the results of the past deep subductions. The modeled results also indicate that recycled water transported into the deep Earth may have δD values distinct from those of primitive mantle reservoirs.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"401 ","pages":"Pages 1-16"},"PeriodicalIF":4.5,"publicationDate":"2025-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144212276","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}