Geochimica et Cosmochimica Acta最新文献

{"title":"The multicomponent exchange of metals between magmatic fluids and silicate melts","authors":"Austin M. Gion, Fabrice Gaillard","doi":"10.1016/j.gca.2025.01.033","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.033","url":null,"abstract":"Magmatic fluids are an integral part of volcanic eruptions and the transport of metals through the crust. In order to understand this transport and the evolution of magmatic fluids, we performed experiments on rhyolitic melts saturated with an aqueous fluid at 800 °C and 200 MPa and measured the major and trace element composition, as well as the chlorine and fluorine content of coexisting fluids and melts. We find that most trace elements are largely fluid immobile, i.e. partition coefficients of < 1, with the exception of some transition metals, such as Cr, Ni, Cu, and Zn. Fluid mobility is primarily affected by the chlorine concentration of the fluid where increasing chlorine concentration in the fluid increases metal mobility. Such experimental observations have been previously parameterized using empirical relationships between partition coefficients and fluid salinity; however, such relationships do not consider metal speciation or fully capture the fluid-melt exchanges in which cations (major and trace elements) compete for available ligands (Cl, F, OH, etc.). In order to better characterize the behavior of metal in magmatic fluids we utilize existing thermodynamic databases and experimental fluid compositions to calculate the equilibrium concentration of aqueous hydroxide, chloride, and fluoride species in the fluid phase. The equilibrium concentrations of each species were then used to calculate apparent equilibrium constants and characterize the exchange of 42 cations between the fluid and silicate melts for 129 aqueous species. We find that these apparent equilibrium constants vary as a function of the HCl and HF content of the experimental fluid. We further present a model based on these experimentally determined apparent equilibrium constants that is capable of calculating fluid-melt equilibria. This model can subsequently be used to predict fluid-melt partition coefficients for metals, as well as chlorine and fluorine, over a wide range of P-T-X conditions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"80 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-01-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143462868","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":"Carbonation processes in Oman peridotite: temperature-dependent reactions and Mg isotope composition of reacting fluids","authors":"Giuseppe D. Saldi, Thierry Decrausaz, Vasileios Mavromatis, Pascale Bénézeth","doi":"10.1016/j.gca.2025.01.029","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.029","url":null,"abstract":"The study of natural analogues of CO<ce:inf loc=\"post\">2</ce:inf> mineral sequestration combined with the experimental quantification of carbonation reactions constitutes a fundamental approach to understand the spatial and structural distribution of carbonated bodies and the time scales by which large amounts of CO<ce:inf loc=\"post\">2</ce:inf> can be stored in solid form into geologic formations. To better quantify the carbonation rates of ultramafic rocks and study the evolution of dissolved Mg isotope composition during their interaction with CO<ce:inf loc=\"post\">2</ce:inf>-rich fluids, a series of batch carbonation experiments using a partially serpentinized harzburgite from the Semail ophiolite (Oman) was conducted at 90–180 °C and at constant CO<ce:inf loc=\"post\">2</ce:inf> partial pressures (∼ 15–20 bar). The yield of the carbonation reaction increased from ∼ 0 at 90 °C to a maximum of 31 mol % at 150 °C, decreasing to 12 mol % at 180 °C over a period of one month. Magnesites containing 3–9 wt% of Fe and silica polymorphs (SiO<ce:inf loc=\"post\">2(am)</ce:inf> and chalcedony) were the main reaction products, with a fraction of secondary Mg-silicates that increased with increasing temperature, significantly reducing the carbonation extent at 180 °C. The aqueous fluid became progressively enriched in heavy isotopes with the progress of the carbonation reaction. The apparent Mg isotope fractionations between the rock and bulk fluid (Δ<ce:sup loc=\"post\">26</ce:sup>Mg = δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">solid</ce:inf> − δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">fluid</ce:inf>) varied from −1.6 ‰ at 120 °C to −0.9 ‰ at 180 °C, consistent with the preferential uptake of <ce:sup loc=\"post\">24</ce:sup>Mg by carbonate minerals and the decrease of isotope fractionation with increasing temperature. The average magnesite isotope compositions (−1.6 ‰ ≤ δ<ce:sup loc=\"post\">26</ce:sup>Mg ≤ -0.3 ‰) derived from mass-balance calculations were found to be within the range of δ<ce:sup loc=\"post\">26</ce:sup>Mg values reported for Oman listvenites, suggesting that the carbonation processes in this geological unit took place within the temperature range considered in this study (∼120–180 °C).","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"41 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143125392","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":"Gold(I)-bisulfite complexation in hydrothermal nanodroplets: A molecular dynamics study","authors":"Wallace C.H. Hui, Kono H. Lemke","doi":"10.1016/j.gca.2025.01.015","DOIUrl":"https://doi.org/10.1016/j.gca.2025.01.015","url":null,"abstract":"Water nanodroplets present a unique environment for gold hydrothermal transport, with fluid properties in aqueous nanodroplets distinct from bulk liquid and vapor phases. By performing classical and <ce:italic>ab initio</ce:italic> molecular dynamics simulations, we have probed the stability of water nanodroplets (H<ce:inf loc=\"post\">2</ce:inf>O)<ce:italic><ce:inf loc=\"post\">n</ce:inf></ce:italic> (<ce:italic>n</ce:italic> = 100, 1000) at 25 °C and 100 °C. The solvation and complexation of gold(I)-bisulfite AuHSO<ce:inf loc=\"post\">3</ce:inf> in nanodroplet environments were also examined, with a particular focus on surface and interior solvation. Classical TIP4P/2005 molecular dynamics simulations reveal extreme densities in the interior of (H<ce:inf loc=\"post\">2</ce:inf>O)<ce:inf loc=\"post\">100</ce:inf> and (H<ce:inf loc=\"post\">2</ce:inf>O)<ce:inf loc=\"post\">1000</ce:inf> nanodroplets compared to droplet surface regions. At 25 °C, the interior region of (H<ce:inf loc=\"post\">2</ce:inf>O)<ce:inf loc=\"post\">100</ce:inf> exhibits fluctuating densities at 1.016–1.079 g/cm<ce:sup loc=\"post\">3</ce:sup>, with two maxima at 1.079 g/cm<ce:sup loc=\"post\">3</ce:sup> and 1.074 g/cm<ce:sup loc=\"post\">3</ce:sup>, corresponding to pressures of ∼ 2.23 kbar and 2.07 kbar, respectively; Reduced densities are predicted for the larger (H<ce:inf loc=\"post\">2</ce:inf>O)<ce:inf loc=\"post\">1000</ce:inf> systems, these being 1.013 g/cm<ce:sup loc=\"post\">3</ce:sup> (25 °C, 370 bar) and 0.968 g/cm<ce:sup loc=\"post\">3</ce:sup> (100 °C, 220 bar). The outer regions, on the other hand, featured densities intermediate between saturated liquid and vapor conditions, as part of a transition from liquid to vapor-like densities at the edge of the droplet. Born-Oppenheimer molecular dynamics simulations at 100 °C show that the gold(I)-bisulfite complex H<ce:inf loc=\"post\">2</ce:inf>O-AuHSO<ce:inf loc=\"post\">3</ce:inf> maintains a near linear solvation structure (θ<ce:inf loc=\"post\">O-Au-S</ce:inf> = 172°-174°) in bulk aqueous fluids and at surface and interior sites of (H<ce:inf loc=\"post\">2</ce:inf>O)<ce:inf loc=\"post\">100</ce:inf> nanodroplets. Distance constrained simulations reveal that, upon extension of the gold(I)-bisulfite Au-S contact (equilibrium <ce:italic>r</ce:italic><ce:inf loc=\"post\">Au-S</ce:inf> = 2.3 Å), HSO<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">−</ce:sup> is displaced by a water molecule, forming a two-water solvation shell around Au<ce:sup loc=\"post\">+</ce:sup>. Thermodynamic integration gives gold(I)-bisulfite dissociation energies (ΔG) of 17.65 ± 0.37 kcal/mol (bulk), 20.22 ± 0.38 kcal/mol (nanodroplet surface), and 18.31 ± 0.31 kcal/mol (nanodroplet interior). Our <ce:italic>ab initio</ce:italic> molecular dynamics results demonstrate that water nanodroplets are stable at hydrothermal conditions and would play an important role in the speciation and transport of gold in volcanic and hydrothermal vapors.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"31 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-01-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143462870","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Processes controlling nickel and its isotopes in anoxic sediments of a seasonally hypoxic bay","authors":"Sarah Fleischmann ,&nbsp;Florian Scholz ,&nbsp;Jianghui Du ,&nbsp;Jan Scholten ,&nbsp;Derek Vance","doi":"10.1016/j.gca.2025.01.016","DOIUrl":"10.1016/j.gca.2025.01.016","url":null,"abstract":"<div><div>Nickel (Ni) is a biologically active metal whose reactivity and isotope fractionation in the marine realm are strongly influenced by biological and redox-related processes, giving the stable isotope system potential for studying past ocean environments. Reducing, organic-rich, sediments constitute an important sink of Ni from the modern ocean. Importantly, at open ocean upwelling margins, these kinds of sediment record the isotope composition of the modern deep ocean. Thus, records of their Ni isotope composition in the past have the potential to record the past deep ocean isotope composition and the oceanic isotope mass balance. However, the detailed processes controlling the upwelling sink are not fully understood. Here, we address this issue through data for sediments, porewaters and the water column of Kiel Bight in the Western Baltic Sea. This setting preserves sediments that have similar characteristics to those of open ocean upwelling margins, allowing us to study specific controlling processes in a well constrained setting.</div><div>In common with sediments from open-ocean upwelling settings, Ni is well-correlated with carbon in solid sediment, suggesting delivery of Ni via rain of organic carbon from the water column. Overall, porewaters at all sites studied show increasing Ni concentrations from around 10 nM near the sediment–water interface to as high as 50 nM at 25 cm depth. This increase is correlated with increases in ammonia concentrations, suggesting release of Ni from anaerobic respiration of organic matter. However, porewater Ni/NH₄ ratios are always lower than Ni:N of water column suspended particulate matter, suggesting an additional process that removes Ni from the porewater. Porewater sulphide also increases with depth, from as low as zero at the sediment–water interface to levels as high as 3 mM at 25 cm. Overall, porewater Ni isotopes become heavier with depth, from bottom water δ⁶⁰Ni around +0.5 to +1‰, to values as high as +2.3‰ at depth. All these observations strongly suggest that Ni is removed from porewater into a solid sulphide. Mass balance indicates that over 90% of the Ni delivered in organic material to the sediment–water interface is transferred from organic matter into solid sulphide. Upward diffusive fluxes lead to the loss of a small amount back to the water column via a benthic flux. Given the large proportion of Ni retained within the sediment, the loss of such Ni does not strongly impact the isotope composition of the buried pool. These data are crucial in clarifying the processes controlling the size and isotope composition of organic-rich sediments on upwelling margins.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"391 ","pages":"Pages 1-15"},"PeriodicalIF":4.5,"publicationDate":"2025-01-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143055247","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":"Microstructural analysis of phosphorus (P)-bearing assemblages in type 3 chondrites: Implications for P condensation and processing in the early solar nebula","authors":"M.C. Benner ,&nbsp;V.R. Manga ,&nbsp;B.S. Prince ,&nbsp;L.M. Ziurys ,&nbsp;T.J. Zega","doi":"10.1016/j.gca.2025.01.012","DOIUrl":"10.1016/j.gca.2025.01.012","url":null,"abstract":"<div><div>As the limiting element in the development of living systems, it is crucial to understand the history of phosphorus (P), from its stellar origins to its arrival on planetary surfaces. A key component in this cycle is understanding the forms of P delivered to the presolar nebula and their subsequent evolution on planetary bodies, including meteorites. Here, we report on the P distribution in the Bishunpur (LL3.15), Queen Alexandra Range (QUE) 97008 (L3.05), and Allan Hills (ALHA) 77307 (CO3.0) chondrites to determine its origins and secondary processing in the solar protoplanetary disk and on meteorite parent bodies using a coordinated analytical approach. In support of the microstructural characterization, we used density functional theory (DFT) to calculate the Gibbs free energy of the Fe₃P – Ni₃P binary under non-ideal mixing conditions in its entire range of composition and temperature space and performed equilibrium condensation modeling. We identified 106 P-bearing regions in these petrologic type-3 chondrites and find that the major P-bearing minerals are schreibersite ((Fe, Ni)₃P) and merrillite (Ca₉NaMg(PO₄)₇). Bishunpur predominately contains merrillite, which occurs in rims on chondrules and as hopper crystals. QUE 97008 primarily contains merrillite in association with metal and sulfides. Microstructural evaluation of merrillite in Bishunpur suggests igneous origins within the chondrule-forming region, whereas merrillite in QUE 97008 formed via condensation. In comparison, the dominant P-bearing phase in ALHA 77307 is P-bearing metal, including two Ni-rich schreibersite grains that are composed of 45 and 52.5 at.% Ni, far higher than predicted by equilibrium condensation. The equilibrium thermodynamic model, including our newly described non-ideal schreibersite solid solution, predicts the formation of a miscibility gap where (Fe_0.63, Ni_0.37)₃P and Ni₃P form via nebular condensation. We therefore suggest that Ni-rich schreibersite formed through non-equilibrium condensation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 207-222"},"PeriodicalIF":4.5,"publicationDate":"2025-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020254","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":"Evaluating the crystallization and eruptive histories of low-titanium basalts with 2D and 3D studies","authors":"Z.E. Wilbur ,&nbsp;J.J. Barnes ,&nbsp;S.A. Eckley ,&nbsp;T. Erickson ,&nbsp;R.A. Zeigler ,&nbsp;K. Domanik","doi":"10.1016/j.gca.2024.10.018","DOIUrl":"10.1016/j.gca.2024.10.018","url":null,"abstract":"<div><div>Volcanic products returned from the Apollo missions over 50 years ago provide a unique perspective into the magmatic evolution of the Moon. However, questions<!--> <!-->remain regarding the volatile loss, crystallization, and emplacement histories of lunar lavas. To address gaps in our understanding of the eruptive histories of lunar lavas, we investigate phase chemistry and 3D morphologies of low-titanium Apollo 15 basalts belonging to the olivine-normative and quartz-normative suites. We report the 2D and 3D petrography, mineral chemistry, and 3D void space morphologies of 15499, 15555, 15556, and the lesser studied 15495 and 15608 basalts. Quantitative apatite chemistry shows a wide range of apatite volatile compositions and that low-Ti basalt 15495 may contain the most OH-rich compositions measured from the Moon. Analyses of metal grains within the low-Ti basalts have expanded the field of expected Ni and Co metal concentrations for Apollo 15 mare basalts and are used to determine the petrogenesis of two of the studied samples. Coupling 2D chemistry with nondestructive 3D morphologic analyses provides critical insights on the relative timing of volatile exsolution in low-titanium lavas. Through the analysis of vesicles and vugs from X-ray computed tomographic data, we report the first 3D void space volume percentages for a suite of low-Ti basalts and show that these basalts degassed before the onset of mesostasis (e.g., apatite) crystallization. We use calculated cooling rates and 3D morphologic analyses to show that the studied basalts crystallized at various depths in separate lava flows, and 15608 represents the quenched margin of a mare flow. Our work highlights the value of combining 2D and 3D analytical techniques to study the emplacement history of basalts that lack geological context.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 110-124"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673574","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":"Adsorption pathways of boron on clay and their implications for boron cycling on land and in the ocean","authors":"Simon J. Ring ,&nbsp;Michael J. Henehan ,&nbsp;Roberts Blukis ,&nbsp;Friedhelm von Blanckenburg","doi":"10.1016/j.gca.2024.08.014","DOIUrl":"10.1016/j.gca.2024.08.014","url":null,"abstract":"<div><div>Reversible adsorption and isotope fractionation of boron on the surface of clay minerals is a key process that impacts boron isotope cycling in porewater, rivers and the ocean. However, the differences in boron isotope fractionation factors between various clay minerals and their dependence on fluid chemistry are not well known. We performed two sets of experiments, using solutions of pure water with added boron and seawater, to explore the isotope behavior during adsorption of boron onto kaolinite, smectite and illite. We found that the amount of sorbed boron increases with ionic strength of solutions and is proportional to the cation exchange capacity of a given clay mineral. Maximum adsorption is observed in alkaline seawater, which we attribute to the efficient fixation of magnesium-borate ion pairs onto negatively charged surface sites. Isotopic fractionation is modestly different between clays and demonstrates that clay surfaces preferentially sorb borate, even when the concentration of borate in solution is low. In both pure water and seawater, adsorbed complexes retain the isotopic composition of their dissolved precursors (borate or boric acid) with minimal isotopic fractionation. In other words, isotopic composition of adsorbed boron is set by the ability of clays to adsorb boron from an already fractionated boron pool rather than specific fractionation associated with the complexation reaction. Our experimental results allow us to provide revised constraints on the adsorbed boron being transported in terrestrial fluids and the ocean.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 74-83"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142090198","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":"Association of tungsten with aluminosilicate mineral colloids and silicotungstates in soil porewaters: Insights into the unexpectedly high tungsten mobility in soil","authors":"Lei Lu ,&nbsp;Jing Sun ,&nbsp;Yu Dai ,&nbsp;Yaoyu Zhou ,&nbsp;Haojie Cui ,&nbsp;Ming Lei ,&nbsp;Huihui Du","doi":"10.1016/j.gca.2024.11.032","DOIUrl":"10.1016/j.gca.2024.11.032","url":null,"abstract":"<div><div>As an emerging contaminant, tungsten (W) displays unexpectedly high mobility in soil despite its extremely low solubility, challenging current scientific understanding. This paradox underscores the limited knowledge regarding the specific W species responsible for its high mobility. In this study, a series of field and incubation experiments were conducted across multiple soil types to investigate the distribution and speciation of W in soil porewater, widely known as ″the most mobile fraction″. Ultrafiltration analysis revealed that W in soil porewater predominantly existed in colloidal-size (5 kDa–0.45 μm) phases rather than the ″truly-dissolved″ phase (&lt;5 kDa). Especially in deeper soil layers approaching shallow groundwater, colloidal W content exceeded 93 %. XANES spectra showed that colloidal W was mainly in the hexavalent state (W^VI), and insoluble W metal (W⁰) entering the soil could rapidly oxidize into W^VI through biotic or abiotic processes. Advanced characterizations, including STEM-EDS-SAED, SEM-EDS, and SR-XRF, identified aluminosilicate mineral colloids as the primary carrier for W in soil porewater. Within these W-bearing aluminosilicate mineral colloids, W was primarily present as Al₂(WO₄)₃ precipitates with a W–Al distance of ∼ 3.64 Å, as confirmed by EXAFS. Additionally, a minor fraction of silicotungstates was also detected in the colloidal fraction using XAS and STEM-EDS-SAED. These two species were further substantiated through geochemical modeling and density functional theory (DFT) analysis. Importantly, this study hypothesizes that the associations of W with aluminosilicate mineral colloids and silicotungstates are widespread across different soil types. The finding suggests that colloid-associated W mobility is a dominant yet previously overlooked process, helping to explain why W, despite its low solubility, exhibits exceptionally high mobility in soils.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 1-13"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142804467","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":"Cadmium isotope fractionation and neutron capture effects in lunar samples","authors":"W. Abouchami ,&nbsp;F. Wombacher ,&nbsp;S.J.G. Galer","doi":"10.1016/j.gca.2024.12.001","DOIUrl":"10.1016/j.gca.2024.12.001","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Early pioneering studies of Apollo lunar soils revealed a geochemical dichotomy reflecting a dominance of mare and highland lithologies, with variable additions of Procellarum KREEP Terrane material. Here, we use the moderately volatile element cadmium to identify the sources and processes responsible for mass-dependent Cd stable isotope variations in the lunar regolith. In addition, capture of thermal neutrons by &lt;sup&gt;113&lt;/sup&gt;Cd, resulting from galactic cosmic rays (GCR) impacting the lunar surface, provides a means of reconstructing the exposure history of the regolith.&lt;/div&gt;&lt;div&gt;We report TIMS double spike Cd stable isotope data on samples from the Apollo 12, 16 and 17 missions, consisting of twelve soils of varying maturity, two ferroan anorthosites, and orange glass 74220. Cadmium abundances are generally lower in mare (12 to 79 ng/g) than highland soils (∼70 to 95 ng/g). Cadmium stable isotope compositions, expressed as ε&lt;sup&gt;112/110&lt;/sup&gt;Cd, display a larger range in mare (∼0 to + 106) than in highland (+60 to + 97) soils. The two anorthosites exhibit contrasting ε&lt;sup&gt;112/110&lt;/sup&gt;Cd values (−107 &lt;em&gt;vs.&lt;/em&gt; + 47) and Cd concentrations similar to those of highland soils. Orange glass 74220 is Cd-rich (290 ng/g) and has a light Cd isotopic composition (ε&lt;sup&gt;112/110&lt;/sup&gt;Cd = -27) due to condensation of Cd vaporized during lava fountaining.&lt;/div&gt;&lt;div&gt;A broad trend of decreasing Cd abundance and increasing heavy isotope enrichment with increasing maturity is observed for mare soils but is not apparent for the highland soils. These characteristics might arise from space weathering, including micrometeorite bombardment, but simple mass balance indicates that meteoritic addition has a negligible effect on the lunar regolith Cd. Likewise, neutron capture on &lt;sup&gt;113&lt;/sup&gt;Cd tends to increase with maturity in mare soils while being greater and relatively uniform in highland soils, reflecting a longer exposure history and more extensive reworking of the highland regolith. Neutron capture effects were not resolved for immature mare soils, orange glass and one anorthosite, indicating these samples experienced only short near-surface exposure to GCR.&lt;/div&gt;&lt;div&gt;The relationships between Cd abundances and isotope effects reveal three distinct correlations for the highland soils and the mature and immature mare soils, respectively. These are best explained by simple binary mixing between isotopically distinct components. The “heavy” Cd components of mare and highland soils have variable but overall low Cd contents while the cadmium-rich component is always isotopically “light”, and common, at least, to all mare soils. The strong correlation between Cd stable isotopic composition and neutron capture effects in mare soils constrains the ε&lt;sup&gt;112/110&lt;/sup&gt;Cd of the neutron capture-free component to be −4.9 ± 2.3, that is marginally lighter than that of the Bulk Silicate Earth (0.01 ± 0.94). This component is predominantly found in immature, K","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 43-58"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841588","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":"Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet","authors":"Getachew Agmuas Adnew ,&nbsp;Thomas Röckmann ,&nbsp;Thomas Blunier ,&nbsp;Christian Juncher Jørgensen ,&nbsp;Sarah Elise Sapper ,&nbsp;Carina van der Veen ,&nbsp;Malavika Sivan ,&nbsp;Maria Elena Popa ,&nbsp;Jesper Riis Christiansen","doi":"10.1016/j.gca.2024.11.009","DOIUrl":"10.1016/j.gca.2024.11.009","url":null,"abstract":"<div><div>Clumped isotopes of methane (CH₄), specifically <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>13</mn></mrow></msup></math></span>CH₃D and <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂, provide additional information to constrain its sources and sink processes. These isotopes complement interpretations of CH₄ provenance based on bulk isotopes. However, interpreting the origin of CH₄ using isotopes becomes challenging when the isotopic signature is altered by post-generation processes. In this study, we measured, for the first time, the bulk and clumped isotopic composition of sub-glacial CH₄ samples. These samples were collected from the air-filled headspace of the glacier portal (ice cave) at the edge of the Isunnguata Sermia glacier (ISG), located at the western margin of the Greenland ice sheet (GrIS). Our goal was to identify the processes underlying the sub-glacial production and potential processing of CH₄. The <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>13</mn></mrow></msup></math></span>CH₃D and <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ values are close to those of atmospheric CH₄. The elevated <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ values can be attributed to the alteration of the source’s isotope signal by aerobic oxidation. This conclusion is supported by previous studies at this site, which reported the presence of methanotrophic bacteria and dissolved oxygen close to saturation in the meltwater. Our results confirm that the correlation between <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>13</mn></mrow></msup></math></span>CH₃D and <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ is a useful tool for deciphering oxidation pathways. Our results support the inference that aerobic CH₄ oxidation can strongly modify the <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ isotope signal, which must be considered when determining the source signatures of environmental samples.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 249-264"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841590","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}