Geochimica et Cosmochimica Acta最新文献

{"title":"Benthic deposition and burial of total mercury and methylmercury estimated using thorium isotopes in the high-latitude North Atlantic","authors":"Xiangming Shi ,&nbsp;Amber L. Annett ,&nbsp;Rhiannon L. Jones ,&nbsp;Rob Middag ,&nbsp;Robert P. Mason","doi":"10.1016/j.gca.2025.04.029","DOIUrl":"10.1016/j.gca.2025.04.029","url":null,"abstract":"<div><div>The high-latitude oceans are regions of high mercury (Hg) bioaccumulation, especially in the form of methylmercury (MeHg), which is of great concern in terms of human and ecosystem health. In the high-latitude North Atlantic (60-80°N), the deep water entrains the Hg-enriched Arctic water southwards, consequently influencing global Hg cycling. Whilst Hg removal has been proposed <em>en route</em> from the Arctic Ocean to the Atlantic Ocean, the factors and the mechanism underpinning this loss are poorly studied. Here, we constrained Hg behavior at the sediment–water interface during the GEOTRACES process study GApr16 in 2021 using radionuclide approaches. Excess thorium isotopes (²³⁴Th_ex and ²²⁸Th_ex) in the sediment provided evidence of significant deposition of suspended particles on top of the shallow shelves and ridges due to the flowing waters with intense nepheloid layers. The benthic deposition and burial fluxes of Hg species were then evaluated based on the ²³⁴Th_ex and ²²⁸Th_ex, respectively. This analysis showed that most of the total Hg deposited from the bottom water was buried into the sediment. The net scavenging flux was 14.8 ± 7.0 Mg y^-1, explaining half of the Hg removal proposed in previous studies. In comparison, the benthic scavenging flux of MeHg was 27 ± 10 kg y^-1. Due to the lower particle activity, MeHg deposited from the bottom water was not retained well in the sediment, and over half was estimated to be released again, mainly by diffusion and advection, to the water column. This efflux might account for the elevated MeHg in the overflow waters during their advection. Overall, benthic scavenging dominated the Hg loss from the water column of the high-latitude North Atlantic with the shallow shelves and ridge regions being the primary deposition zones.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 191-204"},"PeriodicalIF":4.5,"publicationDate":"2025-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144083384","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":"Potassium isotope composition in global loess: Origins and implications","authors":"Wenshuai Li, Xiao-Ming Liu, Kun Wang, Mohsen Shakouri, Catherine Chauvel","doi":"10.1016/j.gca.2025.04.023","DOIUrl":"https://doi.org/10.1016/j.gca.2025.04.023","url":null,"abstract":"Potassium (K) isotope compositions (δ<ce:sup loc=\"post\">41</ce:sup>K) in global loess deposits can provide valuable insights into the average upper continental crust (UCC). However, current knowledge of δ<ce:sup loc=\"post\">41</ce:sup>K in the UCC is limited due to the lithological complexity of the continental crust, which results in highly variable K isotope compositions. Accurately estimating the K isotope composition of the UCC remains a challenge. Here, we investigated the concentration, phase, and isotopic composition of K in loess samples from Asia, Europe, Oceania, North America, and South America to identify the dominant controls on loess δ<ce:sup loc=\"post\">41</ce:sup>K and better constrain the average composition of the UCC. Our results show that the δ<ce:sup loc=\"post\">41</ce:sup>K value in globally compiled loess ranges from −0.60 ± 0.08 ‰ to −0.33 ± 0.04 ‰, with an average of −0.46 ± 0.12 ‰ (2 S.D.). This loess-based δ<ce:sup loc=\"post\">41</ce:sup>K value of the average UCC is comparable to the average δ<ce:sup loc=\"post\">41</ce:sup>K value established from various crustal materials by Huang et al. (2020). Nonetheless, our new range is slightly narrower compared with the previous estimation. We infer the measured K isotope variation in loess reflects a result of eolian sorting rather than chemical weathering. The abundance of <ce:sup loc=\"post\">39</ce:sup>K-rich illite is the primary driver of the δ<ce:sup loc=\"post\">41</ce:sup>K variability in these loess samples. We suggest that the K isotope composition in bulk loess record provide more reliable information of the grain size sorting effect and thus inferring regional wind patterns (e.g., past monsoon variation).","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"33 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143932403","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":"First-principles calculations of equilibrium isotope fractionation of Cl, Ca, and O in apatites and amphiboles","authors":"Shanqi Liu ,&nbsp;Yongbing Li ,&nbsp;Zhiming Yang ,&nbsp;Huiquan Tian ,&nbsp;Jianming Liu","doi":"10.1016/j.gca.2025.04.025","DOIUrl":"10.1016/j.gca.2025.04.025","url":null,"abstract":"<div><div>Apatites and amphiboles are common hydrous minerals in igneous and metamorphic rocks, which can be found from the crust to the lithospheric mantle. These minerals can incorporate chlorine (Cl) and fluorine (F) by substituting for hydroxyl groups (OH). The isotopic compositions of Cl, Ca, and O of apatites and amphiboles are valuable geochemical tools for understanding the terrestrial and extraterrestrial processes over a wide range of temperatures. In the present study, first-principles methods were used to investigate the equilibrium isotope fractionation of Cl, Ca, and O in apatites and amphiboles. Our results reveal that amphiboles are generally more enriched in ³⁷Cl compared to apatites. In apatites, increasing the Cl/(Cl + F) and Cl/(Cl + OH) ratios reduces the <em>β</em>_Cl-factors of Cl-F-apatites and Cl-OH-apatites, respectively, while substitution between F and OH has a small impact. In amphiboles, Cl contents have little effect on the <em>β</em>_Cl-factors. For Ca isotopes, the <em>β</em>_Ca-factors are largely unaffected by substitutions among Cl, F, and OH in either mineral. For O isotopes, the <em>β</em>_O-factors vary depending on the type of oxygen atoms. In Cl-F-apatites and Cl-OH-apatites, the <em>β</em>_O-factors for total oxygen atoms decrease with increasing Cl/(Cl + F) ratios but increase with Cl/(Cl + OH) ratios, respectively. Hydroxyl oxygen sites in amphiboles exhibit higher <em>β</em>_O-factors than those in apatites, whereas silicate oxygen sites are more enriched in ¹⁸O than phosphate oxygen sites. Correlations between bond lengths and isotope fractionation properties are also observed: in both apatites and amphiboles, the <em>β</em>_Cl-factor is negatively correlated with the Cl–Cation bond length, except in the cases of actinolite and richterite, while the <em>β</em>_O-factors of phosphate oxygen and silicate oxygen correlate with the O–P and O–Si bond lengths, respectively. These results can enhance our understanding of Cl, Ca, and O isotope behavior in halogen-bearing phosphates and silicates, providing valuable insights into the role of apatites and amphiboles as indicators of evolutionary processes on Earth and in extraterrestrial bodies.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 144-155"},"PeriodicalIF":4.5,"publicationDate":"2025-04-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143932416","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":"Diffusion-driven zircon Zr isotopic fractionation during ultramafic–mafic magmatic differentiation","authors":"Li-Tao Ma ,&nbsp;Li-Qun Dai ,&nbsp;Shao-Bing Zhang ,&nbsp;Ren-Xu Chen ,&nbsp;Qiong-Xia Xia ,&nbsp;Zi-Fu Zhao","doi":"10.1016/j.gca.2025.04.020","DOIUrl":"10.1016/j.gca.2025.04.020","url":null,"abstract":"<div><div>Previous studies of natural samples have documented marked variations in zircon Zr stable isotopes. However, the petrological mechanisms responsible for Zr stable isotopic fractionation during magmatic differentiation remain unclear. To address this, <em>in situ</em> Zr stable isotopic analyses of magmatic zircons were carried out on a suite of mafic plutonic rocks from the southwestern Tianshan, west China. Our results suggest that diffusion-driven Zr stable isotopic fractionation during zircon growth produces per mil-level, mass-dependent isotopic fractionation. Zircon Zr stable isotopic compositions range from –4.50 ‰ to +1.48 ‰, spanning a variation of 5.98 ‰. Some zircon grains exhibit internal Zr stable isotopic zoning, with lighter Zr isotopes in the core and heavier Zr isotope toward the rim, with intra-grain variations of up to 2.82 ‰. In addition, these zircons exhibit significant intra-grain variations in Zr/Hf ratios and Ti contents, which generally decrease with increasing <em>δ</em>^94/90Zr values. While the Zr/Hf and Ti variations likely record fractional crystallization processes, the Zr stable isotopic variations cannot be attributed to mass-dependent, stable isotopic equilibrium fractionation, which would produce only ∼0.08 ‰ fractionation at 800 °C. As the temperature decreases and Zr supersaturation in the magma increases, the Zr stable isotopic fractionation becomes more pronounced. The <em>δ</em>^94/90Zr values exhibit increasing variability toward both higher and lower values, rather than trending in a single direction. These variable Zr stable isotopic compositions can be explained by the diffusion-limited crystallization (DLC), which is primarily controlled by the zircon crystallization temperature and Zr supersaturation in the melt. As temperature decreases during magmatic differentiation, the diffusion-driven zircon Zr isotopic fractionation is enhanced. Therefore, diffusion during crystal accumulation provides a plausible explanation for the large Zr stable isotopic variability observed in zircons.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"400 ","pages":"Pages 129-141"},"PeriodicalIF":4.5,"publicationDate":"2025-04-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901850","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":"Cobalt-rich porphyry deposits derived from multiple mafic magma injections","authors":"Peng-Fei Shan ,&nbsp;Ming-Jian Cao ,&nbsp;Dong-Mei Tang ,&nbsp;Zheng-Jie Qiu ,&nbsp;Noreen J. Evans ,&nbsp;Marina Lazarov ,&nbsp;Da-Chuan Wang ,&nbsp;Wei Hu ,&nbsp;Ke-Zhang Qin ,&nbsp;Ingo Horn ,&nbsp;Stefan Weyer","doi":"10.1016/j.gca.2025.04.022","DOIUrl":"10.1016/j.gca.2025.04.022","url":null,"abstract":"&lt;div&gt;&lt;div&gt;In addition to copper, gold, and molybdenum, porphyry deposits are important reservoirs of critical metals such as rhenium, selenium, tellurium, and platinum group elements (PGEs). However, enrichment of cobalt (Co) has received little attention. Several studies have shown that Co enrichment does occur in porphyry deposits, however, the source(s) of Co and the mechanism(s) responsible for its enrichment in the high-temperature hydrothermal systems that ultimately form Co-rich porphyry deposits, are poorly understood. In order to address this knowledge gap, we investigated the Jinchang porphyry deposit in Northeast China which is one of the most Co-enriched porphyry deposits worldwide. &lt;em&gt;In-situ&lt;/em&gt; elemental and Fe-S isotopic analysis, as well as electron backscatter diffraction, have been conducted on two types of pyrite (Py1 and Py2). Py1 exhibits a core-mantle-rim structure, with Co enrichment in the core (Avg. 4.5 wt%) and rim (Avg. 7.5 wt%). Py2 displays a distinct core-rim structure, with Co enrichment only in the rim (Avg. 8.4 wt%). The early Co-rich fluid led to the formation of the Co-rich Py1 core. As pyrite continued to grow, Co in the fluid was depleted, leading to the formation of the Co-poor Py1 mantle and Py2 core. The most significant changes in δ&lt;sup&gt;56&lt;/sup&gt;Fe values and Co contents were observed between the Py2 core and Py2 rim (δ&lt;sup&gt;56&lt;/sup&gt;Fe: Δ0.94 ‰, Co: Δ10.67 wt%). This significant variation was generated by the re-injection of Co-rich fluids, which led to the coupled dissolution-reprecipitation of pyrite, leading to the formation of the Co-rich Py1 rim and Py2 rim. Each injection of Co-rich fluid not only formed a Co-rich zone in pyrite, but also precipitated Co-bearing minerals, such as siegenite and cobaltite. The magmatic δ&lt;sup&gt;34&lt;/sup&gt;S isotope signature of pyrite and chalcopyrite (1.5–5.3 ‰) rules out the possibility that Co originated from a sedimentary source. Due to the low Co content in felsic magmas, the repeated injections of Co-rich mafic magma are the only plausible source for the formation of such Co-rich fluids. Besides other possible causes, the heavy δ&lt;sup&gt;56&lt;/sup&gt;Fe&lt;sub&gt;fluid&lt;/sub&gt; values derived from mafic magmas suggest the addition of serpentinized oceanic crust slab during subduction, which directly contributed to the formation of mafic magmas.&lt;/div&gt;&lt;div&gt;Multiple injections of mafic magma can significantly enhance the Co content in ore-forming fluids, which may be a critical prerequisite for Co enrichment in porphyry deposits worldwide. Early high-temperature and high-salinity fluids create an environment highly favourable to Co enrichment. As temperatures decrease, Co begins to precipitate, and breccia pipes, which experience rapid temperature drops due to fracturing, become favourable areas for Co deposition. The main precipitation stage of Co pre-dates the main stage of porphyry Cu-Au ore formation, which might be the reason that Co enrichment in porphyry deposits normally goes unde","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 125-143"},"PeriodicalIF":4.5,"publicationDate":"2025-04-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901849","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":"Survivability and pathways of amino acids in oceanic hydrothermal systems: A mechanistic study of phenylalanine","authors":"Alexandria Aspin,&nbsp;Rhys Ellis,&nbsp;Megan Stull,&nbsp;Ziming Yang","doi":"10.1016/j.gca.2025.04.024","DOIUrl":"10.1016/j.gca.2025.04.024","url":null,"abstract":"<div><div>The survivability and pathways of amino acids in oceanic hydrothermal systems can be affected by fluid chemistry and composition including the presence of inorganic materials, however, their specific effects and underlying mechanisms remain largely unexplored. Using phenylalanine as a model compound, we find that decarboxylation is the primary degradation pathway for amino acids followed by deamination, via either substitution or elimination. Amino acid reactivity is influenced by solution pH, with degradation being strongly inhibited under highly acidic or alkaline conditions. The presence of dissolved metal salts not only changes the distribution of decarboxylation and deamination products but also plays a protective role, inhibiting the decomposition of amino acids. Overall, this study suggests that both relatively extreme pH and surrounding metal salts could enhance the survivability of amino acids, which may provide new insights into understanding the preservation and distribution of amino acids in oceanic hydrothermal systems on and outside Earth.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 44-53"},"PeriodicalIF":4.5,"publicationDate":"2025-04-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143892260","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Experimental investigations of mineral-organic chondritic analogs under hydrothermal conditions: implications for carbonaceous asteroids","authors":"Coline Serra ,&nbsp;Olivier Grauby ,&nbsp;Daniel Ferry ,&nbsp;Fabrice Duvernay ,&nbsp;Grégoire Danger ,&nbsp;Johanna Marin-Carbonne ,&nbsp;Vassilissa Vinogradoff","doi":"10.1016/j.gca.2025.04.018","DOIUrl":"10.1016/j.gca.2025.04.018","url":null,"abstract":"<div><div>The occurrence of hydroxylated minerals in carbonaceous chondrites provides valuable insights of water –rocks interactions in hydrous asteroids. Yet, the evolution of water-organic-rock type experiments, applied to carbonaceous chondrites, remains understudied. Here, we present experimental studies of chondritic analogs containing a mixture of minerals and 4 wt% of OM under hydrothermal anoxic conditions at low temperature. The mineral composition is a mixture of anhydrous minerals (peridot, feldspar and troilite), combined with hexamethylenetetramine (C₆H₁₂N₄), a model organic molecule derived from interstellar grains. After being exposed to water at 80 °C for different duration time (up to 100 days) simulating early-stage alteration, a combination of analytical techniques (X-ray diffraction, transmission electron microscopy, infrared spectroscopy and elemental analyses) revealed significant changes in the mineral part of the chondritic analogs. The secondary minerals formed are diverse, consisting of hydroxylated phases such as phyllosilicates, but also iron oxides, which become more abundant with time. Amorphous silicate phases are also observed in abundance. The formation of these secondary minerals is strongly influenced by the presence of soluble organic matter, which can impact the nature of the formed hydroxylated phases and reduce the formation of iron oxides. The secondary phase assemblage obtained in our experiments exhibits similarities with carbonaceous chondrites that have undergone a low degree of aqueous alteration (CM-type). The results indicate that the presence of organic matter, even accounting for 4 wt%, is an important driver in the formation of secondary mineral phases during the initial stage of aqueous alteration. They highlight the importance of understanding the complex interactions between organic and inorganic materials in the context of hydrous asteroids.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 29-43"},"PeriodicalIF":4.5,"publicationDate":"2025-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143873278","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":"Subduction-related transfer of sulfur and chalcophile elements recorded in continental mantle wedge peridotites","authors":"Aiqing Ren ,&nbsp;Zaicong Wang ,&nbsp;Sonja Aulbach ,&nbsp;Keqing Zong ,&nbsp;Xiang Wang ,&nbsp;Zongqi Zou ,&nbsp;Yanan Shen ,&nbsp;Huai Cheng ,&nbsp;Zhaochu Hu ,&nbsp;Zhaoxian Zhu","doi":"10.1016/j.gca.2025.04.016","DOIUrl":"10.1016/j.gca.2025.04.016","url":null,"abstract":"&lt;div&gt;&lt;div&gt;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, &lt;em&gt;in-situ&lt;/em&gt; 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.&lt;/div&gt;&lt;div&gt;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&lt;sup&gt;34&lt;/sup&gt;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&lt;sub&gt;2&lt;/sub&gt;S-bearing crustal melts/fluids during early exhumation within or near the slab (Stage 2), as indicated by the formation of matrix sulfides (δ&lt;sup&gt;34&lt;/sup&gt;S: +1.63 ± 0.32 ‰), amphiboles, and carbonates as well as by elevated bulk-rock &lt;sup&gt;187&lt;/sup&gt;Os/&lt;sup&gt;188&lt;/sup&gt;Os isotope ratios (up to 0.13927). During serpentinization (Stage 3), samples were metasomatized by CO&lt;sub&gt;2&lt;/sub&gt;-SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;-bearing, H&lt;sub&gt;2&lt;/sub&gt;O-dominated fluids, causing elevated bulk S contents (161–713 μg/g, largely incorporated into serpentine) and high bulk-rock Fe&lt;sup&gt;3+&lt;/sup&gt;/∑Fe ratios (0.39–0.59). This was accompanied by partial replacement of earlier-precipitated pentlandite grains (δ&lt;sup&gt;34&lt;/sup&gt;S: +2.98 to + 5.24 ‰) by magnetite. Combined with moderately elevated δ&lt;sup&gt;34&lt;/sup&gt;S, non-zero Δ&lt;sup&gt;33&lt;/sup&gt;S&lt;sub&gt;sulfide&lt;/sub&gt; 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.&lt;/div&gt;&lt;div&gt;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&lt;sub&gt;2&lt;/sub&gt;-SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;-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 ,&nbsp;Timothy G. Ferdelman ,&nbsp;Bo Liu ,&nbsp;R.John Parkes ,&nbsp;Hans Røy ,&nbsp;Bo Barker Jørgensen","doi":"10.1016/j.gca.2025.04.019","DOIUrl":"10.1016/j.gca.2025.04.019","url":null,"abstract":"<div><div>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 ³⁵S-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 &gt; 100 m depth. Hydrostatic pressure, back-reactions and experimental handling effects were considered and found to have small effect on measured rates. Therefore, the ³⁵S-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 &gt; 100 nmol SO₄^2- cm⁻³ d^-1 at 1 cm depth to &lt; 10^-4 nmol SO₄^2- cm⁻³ d^-1 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^-7-10^-4 nmol SO₄^2- cm⁻³ d^-1 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 ³⁵S-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 SO₄^2- m⁻² d^-1 and dropped 40-fold to 0.3 mmol SO₄^2- m⁻² d^-1 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 &lt; 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 depth.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 111-124"},"PeriodicalIF":4.5,"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,&nbsp;Kun Wang (王昆)","doi":"10.1016/j.gca.2025.04.012","DOIUrl":"10.1016/j.gca.2025.04.012","url":null,"abstract":"<div><div>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 ‰ δ⁴¹K, with QUE 97008, MET 00452, GRO 95658, and GRO 95539 showing chondrule fraction δ⁴¹K 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 δ⁴¹K 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.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 163-177"},"PeriodicalIF":4.5,"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}