Geochimica et Cosmochimica Acta最新文献

{"title":"Lithium isotopic composition of fluid inclusions provides insights into fluid–rock interaction in subducted sediments and sources of mud volcano fluids","authors":"Kristijan Rajič ,&nbsp;Antonin Richard ,&nbsp;Hugues Raimbourg ,&nbsp;Tomáš Magna ,&nbsp;Clément Herviou ,&nbsp;Catherine Lerouge ,&nbsp;Romain Millot","doi":"10.1016/j.gca.2026.01.026","DOIUrl":"10.1016/j.gca.2026.01.026","url":null,"abstract":"<div><div>Lithium isotopes have been widely used to investigate fluid-mediated processes in subduction zones. However, direct analysis of pore fluids in metamorphic rocks remains challenging due to prohibitive depths at which these processes occur. To evaluate the potential of Li concentration and isotopic composition recorded in fluid inclusions, we employed a crush-leach technique to extract fluid inclusion leachates, complemented by analyses of host quartz and wall rocks of <em>syn</em>-subduction veins from three paleo-accretionary complexes–The Kodiak complex (Alaska), Shimanto Belt (Japan), and Western Alps (France–Italy)–formed in the temperature range of 250–400℃.</div><div>Our results reveal that Li concentrations in fluid inclusion leachates range between 2 and 24 ppm. Compared to seawater and pore fluids of modern deep-sea sediments, the leachates show an enrichment of at least an order of magnitude. The mixing model suggests that such an increase in Li concentration in metasedimentary pore fluids requires the release of less than 1% Li from the surrounding rock in a closed system, unexpectedly low for metasediments experiencing temperatures up to 400°C. The δ⁷Li values of leachates span from −1.5‰ to +17.1‰, varying between localities, and are generally lower compared to paired quartz residues (+10.9‰ to +22.6‰). The Li-δ⁷Li mixing model between quartz and fluid inclusions suggests minimal to negligible impact of post-entrapment re-equilibration on Li concentrations and δ⁷Li composition of fluid inclusions, confirming the preservation of the initial pore fluid δ⁷Li values. The variation in δ⁷Li values of pore fluids between Kodiak (+8.1‰ to +17.1‰), the Western Alps (−1.5‰ to +9.5‰), and Shimanto (+2.5‰ to +10.4‰) is primarily governed by the presence of Li-bearing mineral phases during vein formation–particularly chlorite, white mica, and paragonite–and their capacity to retain lithium. In closed systems, δ⁷Li values of pore fluids can be explained by Rayleigh fractionation during the crystallization of chlorite, illite/phengite, and paragonite, which dominantly incorporate ⁶Li, leaving the fluids enriched in ⁷Li. Conversely, in open systems, continuous Li removal from the rocks results in decreasing δ⁷Li values of pore fluids, though their Li concentrations remain comparable to those in closed systems. Similarities in Li concentrations and δ⁷Li values between leachates and mud volcano fluids in modern examples of subduction zone forearcs further confirm that such fluids dominantly originate from subducted sediments. Collectively, this study demonstrates the capability of Li isotope systematics of fluid inclusions to effectively contribute to the understanding of fluid–rock interactions in subducted lithologies.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 279-293"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146033634","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":"Processes controlling the authigenic 10Be/9Be ratio in the Arctic Ocean","authors":"A. Ollive ,&nbsp;W. Geibert ,&nbsp;J. Matthiessen ,&nbsp;M. Alscher ,&nbsp;M. Frank ,&nbsp;J. Lachner ,&nbsp;K. Stübner ,&nbsp;F. Adolphi","doi":"10.1016/j.gca.2025.12.047","DOIUrl":"10.1016/j.gca.2025.12.047","url":null,"abstract":"<div><div>The authigenic ¹⁰Be/⁹Be ratio in marine sediments is a promising proxy for reconstructing Arctic paleoceanography, particularly the mixing of riverine and marine inputs which differ in their ¹⁰Be/⁹Be by an order of magnitude. This study examines the modern spatial variability of authigenic ¹⁰Be/⁹Be ratios in surface sediments from the Kara Sea, the Laptev Sea, and the adjacent deep Eurasian Basin, tracing the changes from river mouths and estuaries to the open ocean. Our results show that sedimentary authigenic ¹⁰Be/⁹Be reflects different inputs: low ratios of riverine origin dominate on the shelves, while values increase toward the deep sea under the influence of waters of North Atlantic origin. However, unlike other pelagic oceanic settings, where authigenic sedimentary ¹⁰Be/⁹Be generally matches the dissolved ¹⁰Be/⁹Be of the water column, Arctic deep-sea sediments display a clear offset, characterized by lower authigenic ¹⁰Be/⁹Be ratios ([1.5 – 3] × 10^-8) than expected from the overlying water column ([5–8] × 10^-8). We hypothesize that this offset originates from the preservation of preformed Fe-Mn oxides carrying a low riverine ¹⁰Be/⁹Be signal which are efficiently transported via sea-ice to the central Arctic basins. Additional processes, such as depth dependent scavenging efficiencies in the water column and benthic beryllium fluxes from sediments may also contribute to this offset. Our study thus highlights the contribution of preformed phases to the reactive ¹⁰Be/⁹Be in Arctic Ocean sediments. We demonstrate that the relevance of preformed phases is not limited to near-coastal settings (<span><span>Wittmann et al. 2017</span></span>), likely due to large fractions of the sediment being transported by sea-ice. These processes likely also apply to other isotope systems measured in authigenic coatings of Arctic Ocean sediments such as radiogenic Nd isotopes.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 305-318"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145796046","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":"Hydrogen in natural inclusions: geological legacy or laboratory artifact?","authors":"Haokun Lian ,&nbsp;Xiaolin Wang ,&nbsp;Matthew Steele-MacInnis ,&nbsp;Dongquan Sun ,&nbsp;Wenxuan Hu ,&nbsp;I-Ming Chou","doi":"10.1016/j.gca.2026.01.002","DOIUrl":"10.1016/j.gca.2026.01.002","url":null,"abstract":"<div><div>Molecular hydrogen (H₂) is a highly reactive volatile that plays a fundamental role in Earth’s evolution. <em>In situ</em> Raman spectroscopic analysis of H₂ in natural inclusions offers crucial insights into its behavior during geological processes. However, H₂ preservation within inclusions remains poorly constrained. Here, we demonstrate the formation of H₂ within natural gas and aqueous inclusions in fluorite, dolomite and quartz, and synthetic fused silica capillary capsules containing CH₄ and carbonaceous materials during Raman spectroscopic analyses at ambient temperature. Under laser irradiation (with wavelengths of 473 nm, 532 nm and 633 nm), common carbonaceous materials (e.g., pyrobitumen, graphite, and diamond) catalyze CH₄ decomposition, producing H₂ within seconds to tens of seconds. This photocatalytic reaction may lead to false-positive identification of H₂ as a primary fluid inclusion component. Our findings emphasize the necessity of preventing artificial H₂ generation during fluid inclusion studies to avoid misinterpretations of the inferred water/H₂ cycle and the physicochemical conditions governing H₂ percolation in subduction zones and magmas.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 54-68"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145902353","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":"Depositional settings and speciation control vanadium isotopic fractionation in black shales","authors":"Bojidar Mandjoukov ,&nbsp;Anthony Chappaz ,&nbsp;Ashley N. Martin ,&nbsp;Kaarel Mänd ,&nbsp;Johannes Vind ,&nbsp;Kaarel Lumiste ,&nbsp;Kalle Kirsimäe","doi":"10.1016/j.gca.2026.01.006","DOIUrl":"10.1016/j.gca.2026.01.006","url":null,"abstract":"<div><div>Vanadium (V) stands out as a relatively abundant and remarkably redox-sensitive metal that can be found in multiple oxidation states. In this study, we combine X-ray Absorption Near Edge Structure (XANES) speciation analyses and stable V isotope composition measurements with vanadium bulk concentration data. By applying this approach to samples from the extensively studied Cambrian-Ordovician Alum Shale Formation in Baltic Paleobasin, we refined our understanding of the V use as a paleo-redox proxy. Vanadium concentrations in the studied samples ranged from 798 to 2286 mg/kg, whilst bulk δ⁵¹V values were highly variable, ranging from −0.01‰ to −0.95‰. XANES analyses revealed two main V species, V(+IV)-S and V(+III)-O, whereas ca. 26% of the latter is present in an early diagenetic V-rich illite-type clay mineral structure. In addition, we report the first direct V(+IV)-O-porphyrin detection in black shale using XANES analysis. Our results show a negative correlation between δ⁵¹V values and V(+III)-O species, and a positive correlation between δ⁵¹V values and V(+IV)-S species that also covaries positively with the increasing basinal restriction. This highlights the effects of depositional conditions and burial pathways on V isotope fractionation in shales. Additionally, our data imply the presence of two distinct environmental zones in the eastern facies of the Alum Shale formation in Estonia: in the west zone the sediments were initially formed under mild to weakly euxinic conditions, which later became more oxic; in the east zone, sedimentation was primarily controlled by the combination of increasing basinal restriction and temporal redox changes.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 82-93"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145956588","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":"A bacteriohopanetetrol stereoisomer in Western Pacific oxycline indicates anammox-driven nitrogen loss and warm eddy dynamics","authors":"Liqin Duan ,&nbsp;Jinming Song ,&nbsp;Meiling Yin ,&nbsp;Xiaowei Wang ,&nbsp;Chuanyu Liu ,&nbsp;Jiawei Kan ,&nbsp;Zhuoyue Wang ,&nbsp;Feng Zhao ,&nbsp;Xuegang Li ,&nbsp;Huamao Yuan","doi":"10.1016/j.gca.2026.01.025","DOIUrl":"10.1016/j.gca.2026.01.025","url":null,"abstract":"<div><div>A unique bacteriohopanetetrol isomer (BHT-<em>x</em>) serves as a distinctive biomarker for marine anammox, a critical process in oceanic nitrogen loss. While anammox has been well-studied in extreme oxygen-deficient zones, its occurrence in moderately oxygen-deficient waters remains less explored. Here, we investigated anammox in the Western Pacific (WP) based on vertical and latitudinal distributions of the anammox-specific 16S rRNA gene (<em>amx</em>-16S rRNA gene) and BHT-<em>x</em>. Both the <em>amx</em>-16S rRNA<!--> <!-->gene and BHT-<em>x</em> were detected across the study region, indicating the presence of anammox in these moderately oxygen-deficient waters. The <em>amx</em>-16S rRNA<!--> <!-->gene copy numbers peaked in the oxycline or core oxygen minimum depths (OMDs; 60–100 μmol/L O₂), whereas BHT-<em>x</em> abundance reached maxima in the core or lower OMDs, generally at greater depths. The vertical offset suggests that BHT-<em>x</em> produced by marine anammox bacteria in the oxycline or core OMDs is transported downward via particle association. This downward transport appears to be enhanced by warm eddies within the North Equatorial Counter Current (NECC) region. BHT-<em>x</em> ratios (BHT-<em>x</em>/total BHT) at stations outside the NECC, where hydrodynamic conditions are more stable, exhibited a stronger negative correlation with dissolved oxygen levels than at all stations combined. This confirms the utility of the BHT-<em>x</em> ratio as a more reliable redox proxy in marine regions with minimal eddy influence.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 265-278"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146033629","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":"Speciation of Niobium in hydrothermal fluids","authors":"Bin Hu ,&nbsp;Barbara Etschmann ,&nbsp;Denis Testemale ,&nbsp;Huan Chen ,&nbsp;Richen Zhong ,&nbsp;Andrew G. Tomkins ,&nbsp;Harald Müller ,&nbsp;Wei Li ,&nbsp;Angela Isaura Santos Costa ,&nbsp;Joël Brugger","doi":"10.1016/j.gca.2026.01.021","DOIUrl":"10.1016/j.gca.2026.01.021","url":null,"abstract":"<div><div>Niobium (Nb) is classified as a ‘<em>critical mineral</em>’ due to its irreplaceable applications in superalloys and superconductors but limited supply chain. This study employs high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) at the Nb <em>K</em>-edge to investigate Nb(V) mobility in fluids containing various ligands, namely fluoride [F^-], chloride [Cl^-], carbonate [CO^3–], and hydroxide [NaOH], under hydrothermal conditions (25-413 ˚C; 800 bar). <em>Ab initio</em> simulations of X-ray absorption near-edge structure (XANES) spectra were combined with shell-by-shell modelling of extended X-ray absorption fine structure (EXAFS) data to elucidate the nature and geometry of the aqueous Nb(V) complexes. Key findings are that Nb(V) predominantly forms eight-coordinated hydroxyfluoride complexes (e.g., [NbF₆O₂] moiety) in acidic F-rich solutions, whereas high-chloride acidic conditions favour six-coordinated chlorohydroxy complexes (e.g., [NbCl₄O₂] moiety). Competitive ligand interactions between F^- and Cl^- are modulated by temperature and pH, with higher temperature or low pH promoting Cl-dominated coordination. Carbonate solutions exhibit retrograde Nb solubility and promote lower solubility than acidic F/Cl solutions above 282 ˚C, with spectral features suggesting the presence of [NbO₆]-like moieties in polynuclear clusters and/or nanoparticles. Comparative analysis with tantalum (Ta) highlights distinct geochemical behaviours: Ta dissolution depends strictly on F concentration, whereas Nb demonstrates greater coordination flexibility in Cl-bearing solution systems. These findings challenge conventional views that dismiss the role of chloride in Nb transport. Differences in Nb and Ta complexation provide a geochemical discriminator, leading to Nb/Ta ratios that are low near intrusions, increase outwards in intrusion-related hydrothermal systems, and are highest in metamorphic fluids.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 198-220"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146033632","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":"New empirical kinetics of iron oxidation by CO2 applied to micrometeorites imply a CO2-rich Archean atmosphere","authors":"Danqiu Chen,&nbsp;David C. Catling","doi":"10.1016/j.gca.2026.01.020","DOIUrl":"10.1016/j.gca.2026.01.020","url":null,"abstract":"<div><div>The oxidation state of iron-type micrometeorites preserved in 2.7 Ga sedimentary rocks offers a potential proxy for reconstructing Archean atmospheric CO₂ levels. However, the reaction kinetics of CO₂-driven oxidation during atmospheric entry lack empirical constraints. Here, we present high-temperature experiments using a custom-built, inductively heated vacuum furnace to quantify the reaction kinetics between heated iron and CO₂ under low-pressure anoxic conditions. The newly measured kinetic parameters were incorporated into a one-dimensional micrometeorite entry model, updated with Archean atmospheric profiles from photochemical simulations. Despite being a weaker oxidant than O₂, our results demonstrate that CO₂ alone can oxidize micrometeorites efficiently in the early upper atmosphere. Moreover, photolysis of CO₂ generates atomic oxygen, and subsequent recombination produces molecular O₂, both of which could also contribute to micrometeorite oxidation. By integrating experimentally derived reaction rates with modeled entry trajectories, we estimate that surface CO₂ concentrations in the Archean atmosphere exceeded 9 vol%, with an upper bound between 21 vol% and 60 vol%, to reproduce the oxidation observed in one best-preserved Archean micrometeorite. Our results also indicate that when CO₂ levels are below 20 vol%, nearly all incident CO₂ molecules during atmospheric entry are kinetically able to react with micrometeorites, using extrapolated laboratory-derived kinetic parameters. These findings suggest that high CO₂ levels in the Archean atmosphere drove observed micrometeorite oxidation, which is consistent with an inferred need for strong greenhouse warming to counter a faint, young Sun.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 183-197"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146001265","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":"Combined Ba-Cd isotope tracer in microbialites records biogeochemical metal cycling changes as a response to the Middle Miocene Badenian Salinity Crisis","authors":"Simon V. Hohl ,&nbsp;Yi-Bo Lin ,&nbsp;Philipp Gleissner ,&nbsp;Eva E. Stüeken ,&nbsp;Yuxiang Jiang ,&nbsp;Sebastian Viehmann","doi":"10.1016/j.gca.2026.01.027","DOIUrl":"10.1016/j.gca.2026.01.027","url":null,"abstract":"<div><div>Stromatolitic carbonates are vital archives for understanding the complex interactions among the microbial biosphere, hydrosphere, and atmosphere. At the Rabenkopf section in the Oberpullendorf Basin of the Parathetys, planar, undulated, and domal stromatolites of the Ritzing Formation record the Middle Miocene Badenian Salinity Crisis. We used the combined Ba-Cd isotope tracer along with trace element systematics to infer nutrient availability and primary productivity in a Neogene microbial environment under extreme environmental stress caused by basin restriction and related evaporation. We find that while productivity in microbial habitats during restricted conditions does not cease, the microbial communities adapt to changing nutrient conditions, developing isotopic signatures that are increasingly controlled by heterotrophic over phototrophic metabolism. Under restricted and freshwater input conditions, stromatolitic carbonates record lower Cd, Ba, and S isotopic conditions at variable, but negative, C isotopic conditions. Together with elevated Ba and Cd concentrations these hint at progressive organic matter remineralization via bacterial sulphate reduction or halophilic bacteria-mediated barite dissolution. In contrast, more open basin conditions correlate with heavier Ba, Cd and S isotopic conditions, arguing for the formation of micro-barites on extra polymeric substances from a marine sulphate pool and Rayleigh-type Cd isotope fractionation in ambient fluids to values similar to modern productive open surface ocean conditions. This work emphasises the importance of macro- and micronutrient availability and the adaptability of microbial environments during severe climatic and environmental changes in Earth’s history. Examining novel stable metal isotope systems, especially the combined Ba-Cd tracer, can serve as a blueprint for better understanding both ancient and modern microbial habitats affected by the ongoing climate crisis.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 294-304"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146033627","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":"REE mineralization controlled by decarbonation-driven melt alkalinity and ligand availability in carbonatite and associated fenite systems","authors":"Fan Yang ,&nbsp;Wei Chen ,&nbsp;Jue Lu ,&nbsp;Yuancan Ying ,&nbsp;Sam Broom-Fendley ,&nbsp;Antonio Simonetti ,&nbsp;Kuidong Zhao ,&nbsp;Shaoyong Jiang","doi":"10.1016/j.gca.2026.01.022","DOIUrl":"10.1016/j.gca.2026.01.022","url":null,"abstract":"<div><div>Alkali metasomatism is common in carbonatite hosted rare-earth element (REE) deposits, but the relationship between REE mineralization and fenite is complex and influenced by the chemistry of the metasomatic agents involved. This study examines the role of fenitization in controlling the enrichment and spatial distribution of REEs, focusing on the distinct sodic fenite zones of the Wu carbonatite dyke at Bayan Obo. In situ isotopic analyses reveal elevated ⁸⁷Sr/⁸⁶Sr ratios (0.70346 to 0.70620) and lower δ¹³C values (−4.16 to −9.87‰) for calcite from sodic fenite, relative to that from carbonatite. The elevated ⁸⁷Sr/⁸⁶Sr ratios indicate mixing between evolved alkali-carbonatite melts and the surrounding quartz conglomerate, while the progressive depletion in δ¹³C reflects decarbonation reactions during fenitization. These reactions result in aegirine crystallization in high-grade sodic fenite, while riebeckite and minor phlogopite are more common in medium- and low-grade fenite, fitting an interpreted transition from alkaline to near-neutral or weakly acidic conditions. The high- to low-grade sodic fenites exhibit significant REE enrichment (∼7,510–48,500 ppm), with light REEs primarily concentrated in high-grade zones (LREE: ∼13,400–46,700 ppm) and heavy REEs enriched in medium-grade zones (HREE: ∼1,400–1,530 ppm). Alkalis, sulfate, and carbonate together govern the light and heavy REE transport. The precipitation of LREEs and HREEs is controlled by carbonate and phosphate due to the distinct alkalinity-dependent solubilities of their complexes. LREEs largely occur in monazite in high- and medium-grade sodic fenites and in bastnäsite and parisite in low-grade type, while HREEs are predominantly hosted in apatite in medium-grade fenite. These results demonstrate that decarbonation-driven changes in melt alkalinity and ligand availability affect REE enrichment and LREE-HREE fractionation during fenitization, a mechanism potentially common to carbonatite-associated REE systems worldwide.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 221-237"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146033631","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":"Iron and calcium isotopic fractionation during calcic carbonatite–silicate liquid immiscibility","authors":"Yan Liu ,&nbsp;Haoyu Luo ,&nbsp;Hongjie Wu ,&nbsp;Zengqian Hou","doi":"10.1016/j.gca.2026.01.017","DOIUrl":"10.1016/j.gca.2026.01.017","url":null,"abstract":"<div><div>The role of liquid immiscibility between carbonatitic and silicate melts in forming carbonatite-associated rare earth element (REE) deposits is controversial. Iron and Calcium isotopes have significant potential for identifying liquid immiscibility, particularly in natural calcic carbonatite–silicate systems that host the most economically significant REE mineralization. This paper presents high-precision Fe and Ca isotopic data, along with major and trace element and C–O–Sr–Nd isotopic data, for unaltered calcic carbonatites and spatially associated syenites from the Oligocene Maoniuping REE deposit, China. Uniform Sr–Nd isotopic compositions, low loss-on-ignition values, an absence of crustal contamination features, and mineralogical evidence confirm that the observed isotopic variations are primary magmatic features that did not result from fractional crystallization, source heterogeneity, or post-magmatic processes. The carbonatites have systematically lighter Fe isotopic compositions (δ⁵⁶Fe =  −0.26‰ to +0.09‰; average =  −0.13‰) than the syenites (δ⁵⁶Fe = +0.10‰ to +0.36‰; average = +0.24‰), with an average inter-phase fractionation of Δ⁵⁶Fe_carb–sil =  −0.37‰ ±0.31‰. Similarly, the carbonatites have slightly lighter Ca isotopic compositions (<em>δ</em>^44/42Ca = +0.26‰ to +0.33‰; average = +0.28‰) than the syenites (<em>δ</em>^44/42Ca = +0.37‰ to +0.42‰; average = +0.39‰), with Δ^44/42Ca_carb–sil =  −0.11‰ ± 0.07‰. Based on Zr saturation thermometry (1082 ± 35 K), the measured Δ⁵⁶Fe_carb–sil and Δ^44/42Ca_carb–sil values are consistent with experimental predictions for equilibrium fractionation at magmatic temperatures, supporting their origin as coeval products of liquid immiscibility. This study confirms that equilibrium Fe–Ca isotopic fractionation occurred during calcic carbonatite–silicate liquid immiscibility in the Maoniuping REE deposit. In addition, the low <em>δ</em>^44/42Ca values of both rock types suggest recycled marine carbonates contributed to their mantle source, which is consistent with regional geodynamic models for the Mianning–Dechang REE belt.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"416 ","pages":"Pages 17-28"},"PeriodicalIF":5.0,"publicationDate":"2026-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145995729","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}