Geochimica et Cosmochimica Acta最新文献

{"title":"Revisiting magnesium isotope constraints on the petrogenesis of the Fe-Ti oxide-bearing mafic–ultramafic intrusions","authors":"Ping-Ping Liu ,&nbsp;Ben Ma ,&nbsp;Fang-Zhen Teng ,&nbsp;Zhen-Chao Wang","doi":"10.1016/j.gca.2024.09.025","DOIUrl":"10.1016/j.gca.2024.09.025","url":null,"abstract":"<div><div>Stable isotopes of metals, such as magnesium (Mg) and iron (Fe), typically undergo minimal fractionation at magmatic temperatures. However, in Fe-Ti oxide-bearing intrusions, significant Mg isotope fractionation (Δ²⁶Mg values of up to 23 ‰) has been observed between silicates and Fe-Ti oxides. The mechanism responsible for this substantial fractionation remains unclear. This study presents Mg isotopic compositions for bulk rocks and mineral separates—including olivine, clinopyroxene, titanomagnetite, ilmenite—from sixteen Fe-Ti oxide ores in both the lower and the upper zones of the Baima layered mafic–ultramafic intrusion, as well as bulk rocks from three high-Ti basalts of the Emeishan large igneous province, SW China. Oxide ores from the lower zone and the lower section of the upper zone of the Baima intrusion are characterized by a low abundance of volatile-bearing minerals, such as apatite and hornblende. These ores exhibit δ²⁶Mg values ranging from −0.30 to −0.14 ‰ for olivine, −0.26 to −0.06 ‰ for clinopyroxene, 0.16 to 0.60 ‰ for titanomagnetite and −0.33 to −0.21 ‰ for ilmenite, indicative of near-equilibrium fractionation. In contrast, oxide ores from the upper section of the upper zone exhibit a high abundance of volatile-bearing minerals and significantly different δ²⁶Mg values, ranging from −0.19 to −0.05 ‰ for olivine, −0.25 to −0.10 ‰ for clinopyroxene, 0.57 to 0.60 ‰ for titanomagnetite and 0.86 to 2.34 ‰ for ilmenite, indicative of disequilibrium fractionation. A negative correlation between Fo content and δ²⁶Mg value in olivine suggests that Mg isotopes fractionate as olivine undergoes fractional crystallization. Trapped liquid fractions, calculated based on Ce contents in apatite, suggest that rocks with high volatile-bearing mineral content have undergone re-equilibration with trapped liquids during cooling. The large disequilibrium Mg isotopic fractionation between silicates and Fe-Ti oxides is likely due to sub-solidus re-equilibration between minerals and trapped liquids. We propose that the extent of sub-solidus, diffusion-driven kinetic Mg isotopic fractionation in Fe-Ti oxide-bearing intrusions likely correlates with the volume of trapped liquids, which may serve as a significant medium accelerating ion exchange between silicates and Fe-Ti oxides. Moreover, mass balance calculations indicate that data of Mg isotopes on their own are insufficient to determine the petrogenesis of Fe-Ti oxides in layered intrusions. This insufficiency is primarily due to the low MgO content in titanomagnetite and ilmenite, where their separation causes minimal Mg isotopic fractionation of the melts.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 100-109"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143095859","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The role of scapolite-bearing granulites in sequestering and releasing sulfur: Implications for S isotope signatures of crustal fluids during lower-crustal exhumation","authors":"Johannes Hammerli ,&nbsp;Anthony I.S. Kemp ,&nbsp;Anne-Sophie Bouvier ,&nbsp;Roberta L. Rudnick ,&nbsp;Pierre Boivin ,&nbsp;Robert M. Holder ,&nbsp;Thomas Chacko ,&nbsp;Kevin Blake","doi":"10.1016/j.gca.2025.01.010","DOIUrl":"10.1016/j.gca.2025.01.010","url":null,"abstract":"<div><div>To understand sulfur and carbon sequestration and release within the continental crust, scapolite minerals from a variety of granulite facies rocks were analyzed for their elemental composition and S isotope signatures. These high-grade scapolites host significant amounts of SO₃ and CO₂, up to approximately 5 wt% and 3 wt%, respectively, with δ³⁴S_VCDT from −3 to +10 ‰, and formed in relatively oxidizing environments characterized by low <em>a</em>H₂O in which scapolite may form as a primary igneous mineral or via metamorphic reactions involving sulfides and silicates. The range of scapolite sulfur isotope compositions mirrors those observed in mantle xenoliths, suggesting transport of S from the mantle into the lower crust via fluids and melts. Although scapolite’s contribution to the global S and C cycles may be modest, it is significant in the context of sulfur fluxing from the mantle to the lower crust, particularly in its oxidized form. We estimate that at least 10 % of lower crustal sulfur is sequestered within scapolite. The exhumation of scapolite-bearing lower crustal rocks can therefore liberate substantial quantities of sulfur species and CO₂, which may serve to both supply and compositionally buffer retrograde metamorphic fluids. These fluids may exhibit a range of S isotope compositions from mantle-like (δ³⁴S_VCDT ≈ 0 ‰) to relatively ³⁴S-enriched signatures. Consequently, retrograde fluids may have S isotope signatures indistinguishable from those of mantle fluids, even in the absence of direct mantle S input during fluid formation. Exhumation of scapolite-bearing lower crust may facilitate element mobilization through S and Cl complexing, particularly with respect to base metals, within exhumed lower crustal sections, thus providing sources of metals and fluids in mid- to high-grade metamorphic rocks. Globally, scapolite-bearing lower crust may help balance the global sulfur cycle through catch-and-release from scapolite.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 175-194"},"PeriodicalIF":4.5,"publicationDate":"2025-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143386854","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Uranium isotope systematics of a low-productivity ferruginous ocean analog: Implications for the uranium isotope record of early Earth","authors":"Geoffrey J. Gilleaudeau ,&nbsp;Xinming Chen ,&nbsp;Stephen J. Romaniello ,&nbsp;Sajjad A. Akam ,&nbsp;Chad Wittkop ,&nbsp;Sergei Katsev ,&nbsp;Ariel D. Anbar ,&nbsp;Elizabeth D. Swanner","doi":"10.1016/j.gca.2025.01.011","DOIUrl":"10.1016/j.gca.2025.01.011","url":null,"abstract":"<div><div>The uranium isotope (δ²³⁸U) paleo-redox proxy has emerged as a premier tool for understanding ocean oxygenation through Earth’s history. The fidelity of this important proxy depends, however, on our mechanistic understanding of the pathways that induce uranium isotope fractionation during U(VI) reduction to U(IV). Uranium reduction has been extensively studied in euxinic (anoxic + sulfidic) environments, yet relatively few constraints currently exist on δ²³⁸U fractionation in ferruginous (anoxic + iron-rich) environments, even though ferruginous conditions may have been a dominant feature of Earth’s oceans for much of the geologic past. Here, we present a comprehensive uranium isotope study of modern, meromictic, oligotrophic to mesotrophic, ferruginous Canyon Lake, Upper Peninsula, Michigan (USA), including investigation of a high-resolution profile of lake waters, lake inlet and outlet waters, and groundwater, as well as shallow cores through both oxic and ferruginous sediments. The key observation of this study is that the entire water column, oxic sediments, and ferruginous sediments have indistinguishable δ²³⁸U values near the composition of the upper continental crust. This implies a lack of δ²³⁸U fractionation in the low-productivity, ferruginous environments of Canyon Lake. We suggest that uranium cycling in Canyon Lake is dominated by adsorption and co-precipitation with iron oxides, with only a limited role for U(VI) reduction. These processes result partly from aqueous uranium speciation in the lake, with the dominance of UO₂-CO₃ complexes in the upper water column leading to a high partition coefficient of uranium during sorption to iron oxides. In addition, the dominance of CaUO₂(CO₃)₃^2– and Ca₂UO₂(CO₃)_3(aq) in bottom waters kinetically inhibits U(VI) reduction by Fe(II)_(aq). The lack of U(VI) reduction and hence δ²³⁸U fractionation in Canyon Lake, despite Fe(II)_(aq) concentrations &gt;1.5 mM, is potentially analogous to the lack of δ²³⁸U fractionation that occurred in the Archean and Proterozoic oceans, as indicated by the carbonate δ²³⁸U record. In contrast with predictions that U(VI) should be rapidly reduced and scavenged from the water column in the presence of Fe(II)_(aq), our data suggest a limited role for U(VI) reduction and δ²³⁸U fractionation under the low-nutrient, low-productivity, ferruginous conditions that characterized the oceans on the early Earth.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 195-206"},"PeriodicalIF":4.5,"publicationDate":"2025-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143055388","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":"NdPO4 solubility and aqueous Neodymium speciation in supercritical fluids: An experimental study at 500–700 °C and 1.7 kbar","authors":"Debarati Banerjee, Laura E. Waters, Nicole C. Hurtig, Alexander P. Gysi, Daniel Harlov, Chen Zhu, Artaches Migdisov","doi":"10.1016/j.gca.2025.01.004","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.004","url":null,"abstract":"A key aspect in the formation of rare earth elements (REE) deposits is the role of REE transport as aqueous REE complexes in supercritical hydrothermal solutions, where the nature of the aqueous complex is controlled by solution composition, temperature and pressure. Despite chloride being considered as one of the most abundant transporting ligands in magmatic-hydrothermal fluids, experimental investigations on the stability of aqueous REE chloride complexes are scarce above 300 °C. In this study, synthetic NdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; crystals were reacted with non-saline and saline (0, 0.05 and 0.5 mNaCl), acidic (0.01 mHCl) aqueous solutions in a series of solubility experiments conducted at 500 − 700 °C and 1.7 kbar, where the solubilities were determined using a stable Nd isotope (&lt;ce:sup loc=\"post\"&gt;145&lt;/ce:sup&gt;Nd isotope spike) dilution technique. NdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; solubility ranges between 28 ppm and 10,858 ppm, where solubility increases with both temperature and salinity. At 500 °C, log mNdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; increases from −3.93 to −1.60 and there is a strong correlation between NdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; solubility and NaCl concentrations (slope of 1.2 ± 0.3), indicating stabilization of the Nd chloride aqueous complexes with a stoichiometry corresponding to NdCl&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;. At 600 °C, this correlation is weaker (slope of 0.4, log mNdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; increases from −2.63 to −1.88) indicating the stabilization of both Nd chloride and hydroxyl species controlling solubility. At 700 °C, NdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; solubility is largely independent of NaCl concentration indicating that solubility is controlled by Nd hydroxyl complexes, where stoichiometry suggests the neutral Nd(OH)&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;&lt;ce:sup loc=\"post\"&gt;0&lt;/ce:sup&gt; species is dominant. The solubility product (&lt;ce:italic&gt;K&lt;/ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;sp&lt;/ce:inf&gt;) of NdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; is derived from experimental data with the relation: log &lt;ce:italic&gt;K&lt;/ce:italic&gt;&lt;ce:inf loc=\"post\"&gt;sp&lt;/ce:inf&gt; = -41.81 – 0.057&lt;ce:italic&gt;T&lt;/ce:italic&gt; – 20987/&lt;ce:italic&gt;T&lt;/ce:italic&gt;, with &lt;ce:italic&gt;T&lt;/ce:italic&gt; temperature in Kelvin. Comparison of the measured Nd phosphate solubility to thermodynamic predictions using the available Helgeson-Kirkham-Flowers equation of state parameters for aqueous Nd complexes indicate that predictions are up to three orders of magnitude lower compared to experimental observations. This discrepancy is most pronounced in saline solutions, suggesting that thermodynamic properties of the REE chloride species in supercritical fluids require revision. Numerical simulations of fluid-rock interaction between acidic, saline fluids and a Strange Lake felsic mineral assemblage demonstrates that NdPO&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; solubility predictions from models are four to six orders of magnitude lower than those calculated based on empirical fits from experime","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"40 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143055249","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The antimonite-thioantimonates-antimonate pathway: Insights from sulfidic hot springs and microbial culture experiments for a novel mechanism of abiotic antimonite oxidation","authors":"Qinghai Guo, Junbiao Qian, Yu Li, Luxia Wang, Yue Meng, Britta Planer-Friedrich","doi":"10.1016/j.gca.2025.01.007","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.007","url":null,"abstract":"Antimony speciation in natural waters is of great environmental significance, and mechanisms involved in antimonite oxidation have been a research hotspot during the past years. In the present study, a new antimonite oxidation mechanism, which could be responsible for occurrence of appreciable antimonate in sulfidic hot springs, was proposed. Within the investigated hydrothermal areas in the Yunnan-Sichuan-Tibet Geothermal Province, much more antimonate formed in the neutral to alkaline sulfidic hot springs with low E<ce:inf loc=\"post\">H</ce:inf> values than in the acidic hot springs low in sulfide and with high E<ce:inf loc=\"post\">H</ce:inf> values, and antimonite-oxidizing microorganisms were rare in both neutral to alkaline and acidic hot springs. Antimonite oxidation experiments designed according to the key hydrochemical and microbial parameters of these hot springs indicated that the antimonite oxidation rates were the highest under abiotic, anoxic and dark, and sulfidic conditions and at pH 10, and there was a significant positive relationship between the proportions of antimonate and thioantimonates in the experimental solutions. Moreover, antimonite was oxidized only to a quite low degree even under microbial culture conditions with microorganisms-bearing hot spring sediments being used. These results implied that the antimonite oxidation in the sulfidic hot springs could occur via thiolation of antimonite followed by further transformation of thioantimonates into antimonate. This antimonite-thioantimonates-antimonate pathway may be prevalent in some other sulfidic environments, like paddy soils, mining areas, or euxinic basins.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"25 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020256","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":"Revealing atomistic mechanism of lithium diffusion in montmorillonite structure: A molecular simulation study","authors":"Qin Li ,&nbsp;Xiandong Liu ,&nbsp;Kai Wang ,&nbsp;Yongxian Cheng ,&nbsp;Zhe Yin ,&nbsp;Rucheng Wang ,&nbsp;Xiancai Lu","doi":"10.1016/j.gca.2025.01.008","DOIUrl":"10.1016/j.gca.2025.01.008","url":null,"abstract":"<div><div>The Li-bearing clays have attracted increasing attention as an important resource for lithium (Li) recovery. But the mobility of Li in clay lattice structure remains unclear, which hinders understanding of Li enrichment in Li deposits and technical development of Li extraction from Li-bearing claystones. In this study, we employed molecular simulations to investigate Li⁺ diffusion between different sites in the montmorillonite lattice. The calculation indicates that Li⁺ diffusion from the montmorillonite interlayer space to the vacant octahedral site occurs on a time scale of hours at 200 °C, which well agrees with the available Hofmann-Klemen effect. Our results also reveal that isomorphic substitution plays an important role in the Hofmann-Klemen effect. At temperatures up to 200 °C, the structure of montmorillonite with octahedral substitutions collapses due to the diffusion of almost all of the interlayer Li⁺ into the octahedral sheets, whereas montmorillonite bearing tetrahedral substitutions could maintain 28 % Li⁺ in the interlayer and thus the interlayer space can be well kept. Heating the montmorillonite to 300 °C in a water vapor environment causes Li⁺ to reversibly diffuse out of the octahedral sheets. Besides, the dehydroxylation of montmorillonite significantly lowers the activation energy barrier of Li⁺ diffusion between the octahedral layer and the interlayer. The disclosed diffusion of Li in montmorillonite may partially explain the Li enrichment in Li-bearing claystone, such as swinfordite. The revealed Li⁺ diffusion mechanism in montmorillonite may also help design and improve the technology of extracting Li from Li-bearing clays.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 165-174"},"PeriodicalIF":4.5,"publicationDate":"2025-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020260","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 chondritic Martian mantle revealed by the heavy noble gas composition of the chassignite NWA 8694","authors":"Sandrine Péron,&nbsp;Sujoy Mukhopadhyay","doi":"10.1016/j.gca.2025.01.002","DOIUrl":"10.1016/j.gca.2025.01.002","url":null,"abstract":"<div><div>Accretion of volatile elements is a critical step to make a planet habitable. It is often assumed that terrestrial planets initially captured solar gases from the nebula, which are partially ingassed into their interior during the magma ocean phase, and then chondritic and/or cometary volatiles are delivered during the main accretion phase or after. Recent krypton isotopic measurements of the Martian meteorite Chassigny have however shown that chondritic volatiles were acquired on Mars in the first Myr of Solar System formation before nebular capture. Yet, Martian mantle is heterogeneous, with multiple reservoirs as evidenced with the hydrogen isotopic composition of shergottites, and it is unclear if this is also the case for noble gases. In this study, we investigate the noble gas (Ne, Ar, Kr, Xe) isotopic and elemental composition of the chassignite NWA 8694, which constitutes a link between chassignites and nakhlites, via laser step-heating in order to assess potential heterogeneities of the Martian mantle. Similar to Chassigny, we found evidence for high Ar, Kr and Xe abundances, potentially at least one order of magnitude higher than in the Earth’s mantle, in the NWA 8694 mantle source based on a low ⁴⁰Ar/³⁶Ar ratio. We also found a chondritic component and a Martian atmospheric component in NWA 8694, the latter with fractionated Ar/Kr/Xe elemental ratios compared to Mars’ atmosphere. This Martian atmosphere component was possibly introduced through aqueous alteration by surface fluids, as observed in MIL nakhlites. The chondritic component corresponds to the composition of the NWA 8694 mantle source and hence confirms previous observation in Chassigny. A chondritic Martian mantle is in stark contrast with the presence of solar Kr and Xe in the Martian atmosphere. This suggests that chondritic volatiles were delivered to terrestrial planets in the first Myr of Solar System formation in presence of the nebula. Solar gases in the atmosphere could have been captured from the nebula afterwards or delivered by material similar to comets. If captured from the nebula, it would require the solar gases to be trapped either in polar ice caps or the regolith so as not to be lost via hydrodynamic escape after the nebula dissipates. Alternatively, delivery of solar gases associated with comets could occur after cessation of hydrodynamic escape on Mars, but the one comet (67P/C-G) that has been measured so far does not show a pure solar-like Xe and Kr isotopic composition.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 137-147"},"PeriodicalIF":4.5,"publicationDate":"2025-01-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020258","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Mn(II)-induced phase transformation of Mn(IV) oxide in seawater","authors":"Peng Yang, Ke Wen, Kevin A. Beyer, Wenqian Xu, Mengqiang Zhu","doi":"10.1016/j.gca.2025.01.014","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.014","url":null,"abstract":"Manganese (Mn) oxides are key components of oceanic and lacustrine Mn nodules and influence metal cycling through oxidation and adsorption processes. Layered Mn oxides (LMOs) are the most common minerals in these nodules and the immediate products of microbially mediated Mn(II) oxidation by O&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;. LMOs can transform into tunneled Mn oxides (TMOs), Mn oxyhydroxides (MnOOH), or Mn(II,III) phase (Mn&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;O&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;). LMOs often concur with Mn(II) in the environment and the adsorption and oxidation of Mn(II) by LMOs can greatly promote the transformation of LMOs to those phases. However, the Mn(II)-promoted transformation of LMOs in seawater—rich in various cations (300 mM Na&lt;ce:sup loc=\"post\"&gt;+&lt;/ce:sup&gt;, 10 mM K&lt;ce:sup loc=\"post\"&gt;+&lt;/ce:sup&gt;, 50 mM Ca&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;, and 10 mM Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;) remains poorly understood. We examined the transformation of δ-MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; in artificial seawater (pH 8.2) under anoxic conditions with the Mn(II)/MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; ratio (r) ranging from 0.08 to 3.83. To assess the effect of ionic strength (IS), parallel experiments were conducted in a mixed 530 mM NaCl and 10 mM KCl solution (having seawater ionic strength but without Ca&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; and Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;) and in 100 mM NaCl solution as a control. At low r (0.08), δ-MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; transformed into triclinic birnessite and a 4 × 4 TMO in 100 mM NaCl solution, which, however, was suppressed in seawater due to strong interactions of Ca&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;/Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; with δ-MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;. In the mixed 530 mM NaCl and 10 mM KCl solution (the same ionic strength as of seawater), the transformation occurred extensively but the products had lower crystallinity compared to in 100 mM NaCl solution. At the high Mn(II)/MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; ratios (0.5 ≤ r ≤ 3.83), δ-MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; transformed extensively into MnOOH phases and hausmannite (Mn&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;O&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;) in 100 mM NaCl solution. The seawater suppressed the transformation, but the suppression became weaker with increasing Mn(II)/MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; ratio. For example, the transformation was completely suppressed at r = 0.5 but essentially negligible at r = 3.83. The suppression at these high Mn(II)/MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; ratios was mainly ascribed to the influence of Ca&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; and Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; rather than of the high IS, and the weaker suppression at the higher Mn(II)/MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; ratio suggests stronger competition of Mn(II) with Ca&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;/Mg&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; for interacting with δ-MnO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;. Moreover, the composition and crystallinity of the transformation products (i.e","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"10 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-01-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143055251","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":"High-precision SIMS analyses of initial 26Al/27Al in un-melted refractory inclusions: The search for multiple condensation episodes","authors":"Glenn J. MacPherson ,&nbsp;Alexander N. Krot ,&nbsp;Kazuhide Nagashima ,&nbsp;Marina Ivanova","doi":"10.1016/j.gca.2025.01.001","DOIUrl":"10.1016/j.gca.2025.01.001","url":null,"abstract":"<div><div>Refractory inclusions formed via high temperature events during the earliest stages of the solar system evolution. Studies of short-lived radionuclide systems in the inclusions provide constraints on the timing and nature of these thermal events. High-precision SIMS data for initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] in a suite of seven un-melted refractory inclusions (fine-grained spinel-rich and Fluffy Type A CAIs) from CV (Vigarano type) carbonaceous chondrites – which we interpret as primary nebular condensates or their very close derivatives – yield six values close to the canonical ratio of 5.2 × 10⁻⁵ and one marginally lower but still almost within error of 5.0 × 10^–5. We specifically looked for but did not find much lower values like those reported recently by <span><span>Kawasaki et al. (2020)</span></span>, as low as 3.4 × 10^–5. Interpreted in terms of chronology, the accumulated high precision data acquired by us and others within the past 15 years for normal, ²⁶Al-rich CAIs show no evidence for a significant condensation event that would correspond to (²⁶Al/²⁷Al)₀ of (3–4) × 10^–5. Rather, there appears to have been one major thermal event resulting in extensive evaporation and condensation in the CAI-forming region corresponding to (²⁶Al/²⁷Al)₀ of 5.2 × 10^–5 resulting in formation of most normal refractory inclusion precursors. Subsequent smaller events over the succeeding ∼200,000 years caused thermal modification and melting of many of them. Inclusions such as that studied by <span><span>Kawasaki et al. (2020)</span></span> could have formed either in an early event prior to significant isotopic mixing in the CAI-forming region, or later than most refractory inclusions during a thermal event that is not well represented in the meteorite record. Refractory inclusions characterized by low (²⁶Al/²⁷Al)₀, &lt;1 × 10^–5, such as FUN (Fractionation and Unidentified Nuclear effects) inclusions, PLACs (Platy Hibonite Crystals), and some corundum-, hibonite-, and grossite-rich CAIs formed during a much earlier heating event, likely prior to homogenization of ²⁶Al in the early solar system. The initial ²⁶Al/²⁷Al values of such objects provide no quantitative chronological constraints.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"391 ","pages":"Pages 291-311"},"PeriodicalIF":4.5,"publicationDate":"2025-01-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020259","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The chemical and isotopic characterization of the pyrite to pyrrhotite desulfidation reaction across the metamorphic gradient of a metasedimentary basin","authors":"Bruna Coldebella ,&nbsp;Crystal LaFlamme ,&nbsp;Isaac S. Malta ,&nbsp;Carl Guilmette ,&nbsp;Guillaume Barré ,&nbsp;Georges Beaudoin ,&nbsp;Laure Martin ,&nbsp;Dany Savard","doi":"10.1016/j.gca.2024.12.027","DOIUrl":"10.1016/j.gca.2024.12.027","url":null,"abstract":"<div><div>During prograde metamorphism, sulfur is released through the pyrite to pyrrhotite desulfidation reaction. This process is believed to supply sulfur and metals to metamorphic fluids, potentially contributing to the formation of orogenic gold deposits. A deeper understanding of this reaction can therefore help unravel the mobility of sulfur, gold and other metals in metamorphic fluids during prograde metamorphism. We present a novel in situ study of multiple sulfur isotopes combined with trace elements, focused on the texturally-constrained pyrite to pyrrhotite desulfidation reaction. This reaction is documented in detail along a prograde, Barrovian metamorphic field gradient − increasing from the biotite through the garnet, staurolite, and sillimanite zones − recorded in the Pontiac Group metasedimentary sequence, which is fault-bound to the southern Abitibi Subprovince and its orogenic gold deposits. The prograde pyrite to pyrrhotite reaction texture, where pyrrhotite (Po) replaces corroded pyrite (Py₁), is identified in rocks from the biotite and garnet metamorphic zones. Py₁ and Po coexist in the staurolite zone without a clear textural relationship, and past the sillimanite isograd, Py₁ is completely absent with Po in abundance. Py₁ and/or Po are replaced by euhedral to subhedral, inclusion-free, late/retrograde pyrite (Py₂) in all metamorphic zones. In situ trace element analysis of the sulfide assemblage reveals a decrease, with increasing metamorphic grade, in metal contents such as Au, Ag, Te, Bi, and Se, associated with the prograde replacement of Py₁ by Po. Arsenic and Co are preserved and enriched in Py₂, while Ni is concentrated in Po, and Cu is redistributed to chalcopyrite. In situ, multiple sulfur isotope analyses show no analytically distinguishable fractionation (δ³⁴S shift of −0.1‰ ± 0.9 and a Δ³³S shift of +0.06‰ ± 0.17) between Py₁ and Po. These results indicate that the H₂S produced by the desulfidation reaction retains the δ³⁴S-Δ³³S signature of Py₁ across the biotite, garnet, and staurolite zones. This detailed examination of the textural and chemical evolution of sulfides highlights the role of desulfidation in metal mobility. The pyrite to pyrrhotite reaction can be an efficient mechanism for supplying sulfur, Au, Ag, Te, Bi and Se, and if synchronous with the release of fluids and deformation, mobilizing Au through the Earth’s crust.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 119-136"},"PeriodicalIF":4.5,"publicationDate":"2025-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020016","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}