Geochimica et Cosmochimica Acta最新文献

{"title":"Subduction-related transfer of sulfur and chalcophile elements recorded in continental mantle wedge peridotites","authors":"Aiqing Ren , Zaicong Wang , Sonja Aulbach , Keqing Zong , Xiang Wang , Zongqi Zou , Yanan Shen , Huai Cheng , Zhaochu Hu , Zhaoxian Zhu","doi":"10.1016/j.gca.2025.04.016","DOIUrl":"10.1016/j.gca.2025.04.016","url":null,"abstract":"<div><div>Fluids or melts liberated from subducting slabs promote metasomatism of the overlying mantle wedge, enriching it in volatiles, such as sulfur and carbon, and possibly ore-forming chalcophile metals. Despite the genetic links, the redox evolution of arc mantle, mineralization at convergent margins, the S species in slab fluids and related metal transfer remain poorly understood, also because physicochemical properties vary at different stages of subduction. In this study, we conduct a comprehensive investigation in sulfide petrology, <em>in-situ</em> and bulk-rock S isotope compositions, PGE-Au-Cu-Ag-S abundances, and Re-Os isotope compositions of orogenic peridotites from the Sulu ultrahigh-pressure (UHP) metamorphic unit, documenting the three-stage transfer of S and of chalcophile and siderophile elements within the mantle wedge.</div><div>Refractory peridotites have Paleoproterozoic Re depletion ages (1.7–1.9 Ga) and show strong depletion of S, PPGE, Cu, Ag and Au, indicating that they are melt residues of the subcontinental lithosphere mantle beneath the North China Craton. Sulfur isotope signatures combined with detailed petrographic observations identify three distinct pulses of crustal S release and associated metal transfer at variable depths. First, mantle refertilization at UHP conditions (Stage 1) led to the precipitation of sulfides (δ<sup>34</sup>S: +0.15 ± 0.48 ‰) in garnet lherzolite and replenished PPGE, Cu, Ag, Au, and S contents to a level similar to the fertile mantle. The peridotites were further metasomatized by H<sub>2</sub>S-bearing crustal melts/fluids during early exhumation within or near the slab (Stage 2), as indicated by the formation of matrix sulfides (δ<sup>34</sup>S: +1.63 ± 0.32 ‰), amphiboles, and carbonates as well as by elevated bulk-rock <sup>187</sup>Os/<sup>188</sup>Os isotope ratios (up to 0.13927). During serpentinization (Stage 3), samples were metasomatized by CO<sub>2</sub>-SO<sub>4</sub><sup>2-</sup>-bearing, H<sub>2</sub>O-dominated fluids, causing elevated bulk S contents (161–713 μg/g, largely incorporated into serpentine) and high bulk-rock Fe<sup>3+</sup>/∑Fe ratios (0.39–0.59). This was accompanied by partial replacement of earlier-precipitated pentlandite grains (δ<sup>34</sup>S: +2.98 to + 5.24 ‰) by magnetite. Combined with moderately elevated δ<sup>34</sup>S, non-zero Δ<sup>33</sup>S<sub>sulfide</sub> values (up to + 0.05 ‰) suggest the contribution of surficial S to a mantle-S dominated reservoir. In contrast, the transfer of metals in COHS-bearing fluids in the exhumation channel (Stages 2–3) appears limited.</div><div>The results suggest that the efficacy of volatile and metal mobilization during metasomatic processes in continental subduction zones is depth-dependent along the slab-mantle interface. At high pressure, refertilization introduces reduced S and a modest amount of metals into the mantle wedge, while circulation of shallow CO<sub>2</sub>-SO<sub>4</sub><sup>2-</sup>-b","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 11-28"},"PeriodicalIF":4.5,"publicationDate":"2025-04-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143869278","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":"Sulfate reduction rates in Peru Margin sediments: From 1 cm to 100 m below seafloor","authors":"Jens Kallmeyer, Timothy G. Ferdelman, Bo Liu, R.John Parkes, Hans Røy, Bo Barker Jørgensen","doi":"10.1016/j.gca.2025.04.019","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.019","url":null,"abstract":"The Peruvian margin is one of the biologically most productive regions in the world ocean. The oxygen minimum zone impinges on the seabed over the outer continental shelf and across the upper slope to about 700 m water depth. Due to the partial or complete anoxia, microbial sulfate reduction is a predominant, terminal pathway of organic matter oxidation in the sediment. Sulfate reduction rates (SRR) were measured by <ce:sup loc=\"post\">35</ce:sup>S-radiotracer technique in multiple sediment cores retrieved by multicorer to 0.3 m depth, by gravity corer to 5 m depth, and by advanced piston corer (ODP, Ocean Drilling Program) to > 100 m depth. Hydrostatic pressure, back-reactions and experimental handling effects were considered and found to have small effect on measured rates. Therefore, the <ce:sup loc=\"post\">35</ce:sup>S-SRR yielded discrete data on organic carbon degradability from 1 cm to 100 m sediment depth in the Peru Margin. For all three coring intervals, SRR dropped with depth according to a power law. The mean SRR in the sulfatic zones of the sediment dropped more than a million-fold, from > 100 nmol SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2-</ce:sup> cm<ce:sup loc=\"post\">−3</ce:sup> d<ce:sup loc=\"post\">-1</ce:sup> at 1 cm depth to < 10<ce:sup loc=\"post\">-4</ce:sup> nmol SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2-</ce:sup> cm<ce:sup loc=\"post\">−3</ce:sup> d<ce:sup loc=\"post\">-1</ce:sup> in million-year-old sediment at 100 m depth. Even in the deep methanic zones of ODP cores active sulfate reduction was detected with rates of 10<ce:sup loc=\"post\">-7</ce:sup>-10<ce:sup loc=\"post\">-4</ce:sup> nmol SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2-</ce:sup> cm<ce:sup loc=\"post\">−3</ce:sup> d<ce:sup loc=\"post\">-1</ce:sup> and a sulfate turnover time of up to a million years. Half of the entire sulfate reduction in the continental shelf and upper slope sediments off Peru down to 100 mbsf took place in the top 5 cm. Such an extreme skewing towards the sediment surface is not detectable by diffusion–reaction modeling of sulfate. Nevertheless, the <ce:sup loc=\"post\">35</ce:sup>S-SRR matched the estimated flux of sulfate and burial rate of organic carbon in ODP cores beneath 1.5 m sediment depth. The depth-integrated areal SRR in the upper 0.2 m peaked at 85 m water depth with a high rate of 12 mmol SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2-</ce:sup> m<ce:sup loc=\"post\">−2</ce:sup> d<ce:sup loc=\"post\">-1</ce:sup> and dropped 40-fold to 0.3 mmol SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2-</ce:sup> m<ce:sup loc=\"post\">−2</ce:sup> d<ce:sup loc=\"post\">-1</ce:sup> at 800 m, at which depth nitrate reduction expectedly dominates. The mean turnover time of the total organic carbon pool in the top 0.05 m of sediment increased nearly 100-fold, from < 50 years on the inner shelf to a few thousand years on the upper slope. Beneath the seafloor, it increased steeply to 30 million years at 100 m sediment dept","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"109 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901851","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":"Unravelling chondrule formation processes: Clues from the potassium isotopic composition of chondrules from unequilibrated ordinary chondrites","authors":"Piers Koefoed, Kun Wang","doi":"10.1016/j.gca.2025.04.012","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.012","url":null,"abstract":"Understanding chondrule formation processes has been a major focus of the cosmochemistry community for many decades. In order to help further this understanding, here we apply high-precision K isotope analyses to chondrule fractions from the four Antarctic unequilibrated ordinary chondrites of QUE 97008 (L3.05), MET 00452 (L(LL)3.05), GRO 95658 (LL3.3), and GRO 95539 (LL3.2). The K isotope ratios of the chondrules fractions from all four of these samples lie within the range of −2.20 ‰ to 0.14 ‰ δ<ce:sup loc=\"post\">41</ce:sup>K, with QUE 97008, MET 00452, GRO 95658, and GRO 95539 showing chondrule fraction δ<ce:sup loc=\"post\">41</ce:sup>K ranges of −1.54 to 0.14 ‰, −0.76 to −0.28 ‰, −2.20 to −1.23 ‰, and −1.30 to −0.84 ‰, respectively. Overall, no strong correlations between K isotope ratio and K concentration are observed among the chondrule fractions for any of the four chondrites. Additionally, unlike what was seen previously for the LL4 Hamlet, no correlation between chondrule mass and K isotope ratio was observed. In conjunction with previous studies, the data here suggest that a combination of secondary parent body processes and nebular processes involved in chondrule formation are the dominant controls on the K isotope systematics of the chondrules from unequilibrated ordinary chondrites. The effects of secondary parent body processing vary significantly from chondrule to chondrule, however, the dominant effect is the migration of K from the K rich matrix to the K poor chondrules. As such, parent body alteration partially overprinted and disturbed the initial chondrule K compositions to various degrees. Nevertheless, even with the effects of parent body processing, the key observation that the vast majority of the chondrule fractions show δ<ce:sup loc=\"post\">41</ce:sup>K values lighter than, or equal to, their respective matrix and bulk compositions is best explained by these chondrules experiencing incomplete condensation in the solar nebula. This aligns with K isotope observations made for the carbonaceous chondrites where the matrix-dominated CI chondrites are enriched in heavier K isotopes and the chondrule-rich carbonaceous chondrites are enriched in lighter K isotopes. The K isotopes of individual chondrules in this study suggest that chondrules from ordinary chondrites were also formed via incomplete condensation from a supersaturated medium, agreeing with the previous conclusion drawn for carbonaceous chondrules. This means both CC and OC chondrules likely experienced incomplete condensation, making this chondrule formation process ubiquitous and widespread throughout both the inner and outer regions of early solar nebula.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"55 6 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901831","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 phytol εp-based core-top calibration to reconstruct past changes in atmospheric CO2","authors":"Olivia A. Graham, Caitlyn R. Witkowski, Mark A. Stevenson, Francien Peterse, B. David A. Naafs","doi":"10.1016/j.gca.2025.04.014","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.014","url":null,"abstract":"Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (<ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf>) are essential for understanding climate dynamics. A common method for reconstructing past <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> is based on the carbon isotopic fractionation during photosynthesis by Rubisco (ε<ce:inf loc=\"post\">p</ce:inf>). This proxy method is based upon the difference (ε<ce:inf loc=\"post\">p</ce:inf>) between the stable carbon isotopic composition (δ<ce:sup loc=\"post\">13</ce:sup>C) of dissolved CO<ce:inf loc=\"post\">2</ce:inf> and the δ<ce:sup loc=\"post\">13</ce:sup>C of marine photoautotroph biomass, which depends on the concentration of dissolved CO<ce:inf loc=\"post\">2</ce:inf>, related to <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> through Henry’s Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ<ce:sup loc=\"post\">13</ce:sup>C. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> across the Phanerozoic (∼450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the δ<ce:sup loc=\"post\">13</ce:sup>C of chlorophyll (as phytol) and the concentration of dissolved CO<ce:inf loc=\"post\">2</ce:inf> in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol ε<ce:inf loc=\"post\">p</ce:inf>) and dissolved CO<ce:inf loc=\"post\">2</ce:inf> concentration. This marks the first empirical calibration between phytol ε<ce:inf loc=\"post\">p</ce:inf> and the concentration of dissolved CO<ce:inf loc=\"post\">2</ce:inf> in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> reconstruction with estimates that are statistically like direct <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> across geological time.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"23 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901829","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 and triple-oxygen isotope effects of water adsorption on KGa-2 kaolinite with implications for pedological separation of soil water","authors":"Juske Horita, Xinying Ling, Changjie Liu, Osamu Abe, Ryu Uemura","doi":"10.1016/j.gca.2025.04.013","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.013","url":null,"abstract":"A series of detailed, systematic experiments were conducted to determine hydrogen and triple-oxygen isotope effects of water adsorption on KGa-2 kaolinite from the Clay Mineral Society at 30 °C as a function of the relative vapor pressure (<ce:italic>p/p<ce:inf loc=\"post\">o</ce:inf></ce:italic>). Both hydrogen and oxygen isotope fractionation factors between the adsorbed water and water vapor are smaller than those of the liquid water – water vapor (L-V) system even near the saturation. They gradually decrease with decreasing <ce:italic>p/p<ce:inf loc=\"post\">o</ce:inf></ce:italic>, approaching the isotopic property of water vapor. The ratio of hydrogen to oxygen fractionation factors increases from that of the L-V system (8.0) with decreasing <ce:italic>p/p<ce:inf loc=\"post\">o</ce:inf></ce:italic>. The exponent <ce:sup loc=\"post\">17</ce:sup><ce:italic>θ</ce:italic> for the triple oxygen isotope system also deviates from that of L-V (0.529) up to 0.585 well above the high-temperature limit (0.5305). Our experimental results clearly demonstrate that the isotopic property of water confined in pores and adsorbed on the mineral surface differs significantly from that of bulk liquid water with wide-ranging implications for stable isotope tracers of soil waters in the terrestrial and extraterrestrial hydrological systems, including a new concept of the ‘pedological separation.’","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"42 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901852","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":"Molybdenum isotopes record dehydrated slab components input to arc magmatism in subduction zones","authors":"Jin-Lei Sun, Zhong-Jie Bai, Wei-Guang Zhu, Xu Liu","doi":"10.1016/j.gca.2025.04.011","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.011","url":null,"abstract":"The traditional model that attributes the genesis of basaltic arc magma to the partial melting of metasomatized mantle wedges is increasingly being challenged by evidence highlighting the critical role of dehydrated oceanic crust. Molybdenum (Mo) isotopes (expressed as <ce:italic>δ</ce:italic><ce:sup loc=\"post\">98/95</ce:sup>Mo, relative to the NIST SRM 3134 standard) from subarc mantle-derived mafic cumulate rocks offer a novel perspective on this issue. This study reports <ce:italic>δ</ce:italic><ce:sup loc=\"post\">98/95</ce:sup>Mo values with whole-rock Sr-Nd isotopes for the Late Cretaceous Milin juvenile lower crustal mafic–ultramafic cumulates in the Gangdese belt, Tibet. The Milin samples exhibit low initial (<ce:sup loc=\"post\">87</ce:sup>Sr/<ce:sup loc=\"post\">86</ce:sup>Sr)<ce:inf loc=\"post\">i</ce:inf> ratios ranging from 0.70392 to 0.70454 and depleted εNd(t) values between 3.26 and 4.45. These samples display a significant variation in <ce:italic>δ</ce:italic><ce:sup loc=\"post\">98/95</ce:sup>Mo values (–0.64 to –0.05 ‰), with a lower mean value of –0.38 ‰ compared to depleted mantle values (–0.21 ± 0.02 ‰). The light Mo isotopes show no correlations with MgO content, Sr-Nd isotopes, or whole-rock hornblende content, suggesting that observed light Mo isotopes are associated with dehydrated oceanic crust rather than crustal processes (crustal contamination or fractional crystallization). The positive correlation between Ba/Th and Ba/La ratios and <ce:italic>δ</ce:italic><ce:sup loc=\"post\">98/95</ce:sup>Mo values indicates the overprinting of subduction fluids. According to the Mo-Sr-Nd isotopic mixing model, the Milin mantle source incorporated minor subduction fluids (∼1 %), reduced sediment melts (∼1 %) and less than 30 % dehydrated oceanic crust melts, which leads to its heterogeneity and significant variation in the light Mo isotopes within the Milin lower crustal mafic–ultramafic cumulates. We suggest that such lower crust with oceanic crust melts in their source can serve as an important light Mo isotopic reservoir. Integrating the Mo isotopic features in global subduction zones, we propose that the thermal structure of the subduction zones controls the input of dehydrated oceanic crust melts into basaltic arc magmas, resulting in the predominant participation of oceanic crust in the basaltic arc magma genesis within the hot arc and back-arc regions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"10 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901854","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 reconstruction of the H2O and F contents of the Erg Cech 002 parent body","authors":"Liam D. Peterson, Megan E. Newcombe, Conel M.O’D. Alexander, Jianhua Wang, Sune G. Nielsen","doi":"10.1016/j.gca.2025.04.009","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.009","url":null,"abstract":"Erg Cech 002 (EC 002) is an andesitic achondrite, the earliest formed achondrite identified to date, and is a rare sample of primary melts that formed crusts on the first generation(s) of planetesimals. Given that EC 002 represents a primary or primitive melt and that H and F are incompatible during silicate partial melting, EC 002 may be a H- and F-rich material relative to previously studied achondrites. We measured the H<ce:inf loc=\"post\">2</ce:inf>O (total H quantified as H<ce:inf loc=\"post\">2</ce:inf>O) and F contents of low-Ca pyroxene xenocrysts (∼4–12 µg/g H<ce:inf loc=\"post\">2</ce:inf>O; &lt;0.5 µg/g F), groundmass augite (∼5–10 µg/g H<ce:inf loc=\"post\">2</ce:inf>O; &lt;2.2 µg/g F), albitic feldspar (∼2–5 µg/g H<ce:inf loc=\"post\">2</ce:inf>O; &lt;0.5 µg/g F), and a silica-rich phase (∼28–30 µg/g H<ce:inf loc=\"post\">2</ce:inf>O; ∼0.7–2.5 µg/g F) in EC 002 by Nanoscale Secondary Ion Mass Spectrometry. We use a single-stage equilibrium batch melting model to provide a first-order reconstruction of the EC 002 parent body H<ce:inf loc=\"post\">2</ce:inf>O (∼7–200 µg/g H<ce:inf loc=\"post\">2</ce:inf>O) and F (∼0.44–2.4 µg/g F) contents, which are depleted relative to chondrites and the bulk Earth. This requires the first generation(s) of planetesimals to have either accreted from volatile-poor materials or undergone extensive volatile loss, supporting the idea that Earth acquired its H<ce:inf loc=\"post\">2</ce:inf>O budget from thermally primitive materials.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"23 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901853","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 and Oxygen Stable Isotope Compositions of Kaolinite Hydroxyl Water and their Paleoenvironmental Significance","authors":"Erik J.H. Oerter","doi":"10.1016/j.gca.2025.04.006","DOIUrl":"10.1016/j.gca.2025.04.006","url":null,"abstract":"<div><div>The phyllosilicate clay mineral kaolinite can preserve paleoclimate information on the hydrogen and oxygen stable isotope composition of the water the mineral formed from, as well as its formation temperature. Oxygen in kaolinite exists in three distinct bonded groups: Si-O-Si, Si-O-Al, and Al-OH; and it has been an outstanding analytical problem to feasibly and accurately measure the oxygen isotope compositions of the different groups. The ability to make δ¹⁸O_Al-OH measurements on kaolinite using thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS) is established herein. Using complementary fluorination and IRMS measurements, we add to the limited knowledge of intracrystalline oxygen isotope fractionation between that bound into Al-OH^- groups in kaolinite, and that of the bulk mineral, of which the average value is 1000 ln<span><math><mrow><msubsup><mo>∝</mo><mrow><mi>Total</mi><mo>-</mo><mi>O</mi><mi>H</mi></mrow><mi>O</mi></msubsup><mspace></mspace></mrow></math></span> = 18.9 ‰. Together with δ²H_Al-OH measurements on kaolinite, we demonstrate the extent to which hydrogen in the Eocene-age paleo-Oxisol Ione Fm has isotopically exchanged with water after its initial formation. At Mesa Alta, New Mexico, kaolinite hydrogen appears to be pristine since its initial formation 147 Ma BP (early Cretaceous), and we use δ²H_Al-OH, δ¹⁸O_Al-OH, and δ¹⁸O_Total measurements to determine a paleo-environmental temperature of 26.9 °C, which is significantly warmer than the modern MAT of 8.9 °C. These examples illustrate the utility of TGA-IRIS to yield paleoclimatic information on the globally ubiquitous kaolinite mineral.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 1-10"},"PeriodicalIF":4.5,"publicationDate":"2025-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143850618","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":"Chromium isotope fractionation during magmatic processes of island arc basalts","authors":"Li-Juan Xu, Chunyang Liu, Haibo Ma, Qianru Man, Ji Shen, Sheng-Ao Liu, Guochun Zhao, Gerhard Wörner","doi":"10.1016/j.gca.2025.04.008","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.008","url":null,"abstract":"We present the first stable chromium isotope dataset for a suite of island arc basalts (IABs) from Kamchatka to investigate chromium isotope fractionation during magmatic processes in a subduction setting. The chromium isotopic data (δ<ce:sup loc=\"post\">53</ce:sup>Cr) for Kamchatka arc magmas range from –0.09 ± 0.02 ‰ to –0.16 ± 0.02 ‰ (2σ, n = 22). These magmatic rocks exhibit a positive correlation between MgO content and Ni, Cr contents, as well as CaO/Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf> ratios, which suggests that they undergo olivine and pyroxene fractional crystallization. However, δ<ce:sup loc=\"post\">53</ce:sup>Cr shows no correlation with Cr, Ni, or MgO contents, nor with CaO/Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf> ratios. Ionic modeling and Rayleigh fractionation modeling reveals minimal effects of fractional crystallization on the δ<ce:sup loc=\"post\">53</ce:sup>Cr factors in these arc magmas (Δ<ce:sup loc=\"post\">53</ce:sup>Cr<ce:inf loc=\"post\">crystal-melt</ce:inf>: +0.0001 ‰ to + 0.009 ‰), and probably IABs in general. Furthermore, the lack of correlation between δ<ce:sup loc=\"post\">53</ce:sup>Cr and the melting fraction suggests that partial melting also has a limited influence on Cr isotopic compositions. The δ<ce:sup loc=\"post\">53</ce:sup>Cr values also show no correlation with <ce:sup loc=\"post\">87</ce:sup>Sr/<ce:sup loc=\"post\">86</ce:sup>Sr ratios, chromium contents, or fluid-mobile elements (e.g., Ba/Th, B/Nb, As*/Ce), indicating that slab-derived fluids have a minimal effect on the chromium isotope composition of the mantle source of arc magmas. Notably, however, the majority of these samples show δ<ce:sup loc=\"post\">53</ce:sup>Cr similar to Bulk Silicate Earth, and are statistically higher compared to those observed in mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). Melting and Rayleigh fractionation models suggest that the high δ<ce:sup loc=\"post\">53</ce:sup>Cr values in these arc magmas may be attributed to partial melting and fractional crystallization under higher magmatic oxygen fugacity conditions compared to those in MORBs and OIBs. This is consistent with their higher Fe<ce:sup loc=\"post\">3+</ce:sup>/Σ Fe ratios and Δ log fO<ce:inf loc=\"post\">2</ce:inf> (QFM) values, emphasizing the role of varying oxygen fugacity on chromium isotope fractionation in terrestrial basalts. This study highlights the potential of chromium isotopes as a powerful tool for tracing planetary oxygen fugacity environments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"34 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143877859","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":"Rubidium and potassium isotope compositions of enstatite meteorites: implications for the evolution of their parent body(ies)","authors":"Baoliang Wang ,&nbsp;Frédéric Moynier ,&nbsp;Yan Hu","doi":"10.1016/j.gca.2025.04.007","DOIUrl":"10.1016/j.gca.2025.04.007","url":null,"abstract":"<div><div>Enstatite meteorites, including enstatite chondrites and enstatite achondrites (e.g., aubrites), formed under highly reducing conditions in the solar system. Enstatite chondrites underwent progressive thermal metamorphism from petrologic type 3 to type 6, potentially leading to vaporization and redistribution of volatile elements. Coupled Rb and K isotopic analyses of enstatite meteorites could provide complementary insights into the inherent isotopic variability and volatile depletion processes. In this study, we present Rb and K isotopic compositions for a suite of enstatite meteorites, including sixteen enstatite chondrites spanning metamorphic grades from 3 to 6, as well as four aubrites. Type 3 enstatite chondrites exhibit isotopic compositions similar to those of Earth for both Rb and K, which further underscores the isotopic resemblance between Earth and enstatite chondrites. From type 3–4 to type 5–6, the examined enstatite chondrites generally show a trend towards heavier Rb and K isotopic compositions, indicating volatilization and redistribution of Rb and K during open system thermal metamorphism of the parent body(ies). One EH5 (St. Marks) and two EL6 (Pillistfer and Atlanta) samples deviate from this trend with light K isotope compositions, which may result from an interplay of evaporation, vapor transport and recondensation. On the other hand, the Rb and K isotopic variations in aubrites—which originated from the melting and fractional crystallization of enstatite chondrite-like parent body(ies)—likely reflect more complex processes, possibly involving a combination of plagioclase-bearing melt extraction, magmatic differentiation, core segregation, and the back-condensation of volatiles after impact volatilization.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 63-74"},"PeriodicalIF":4.5,"publicationDate":"2025-04-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143838916","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}