Geochimica et Cosmochimica Acta最新文献

{"title":"The role of potassium in lode gold mineralization: insights from ab initio molecular dynamics and geochemical modeling","authors":"Gao-Hua Fan, Jian-Wei Li, Yuan Mei, Si-Yu Hu, Ri-Chen Zhong, Chang Yu, Xiao-Dong Deng, Hao Cui, Wen-Sheng Gao","doi":"10.1016/j.gca.2024.12.022","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.022","url":null,"abstract":"Potassium ions (K<ce:sup loc=\"post\">+</ce:sup>) are abundant in ore fluids of lode gold deposits, largely illustrated by pervasive potassic alteration and commonly expressed as K-feldspar and muscovite on both sides of individual gold lodes. However, their potential roles in gold mineralization remain elusive. Here, we present results from <ce:italic>ab initio</ce:italic> molecular dynamics simulation and geochemical modeling to address this question. Molecular dynamics simulation results show that the ability of K<ce:sup loc=\"post\">+</ce:sup> pairing with Au(HS)<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">–</ce:sup> complex over a wide temperature–pressure range has a negative linear correlation to fluid density. In high-density liquid-like fluids, little K<ce:sup loc=\"post\">+</ce:sup> is coordinated with the Au(HS)<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">–</ce:sup> complex. In contrast, this complex can be nearly neutralized by ion association with K<ce:sup loc=\"post\">+</ce:sup> in low-density, vapor-like fluids, but such a neutral complexation is not very stable, even if under conditions typical of lode gold mineralization. Thus, K<ce:sup loc=\"post\">+</ce:sup> has a limited role in the complexing and transporting of Au in hydrothermal fluids forming lode gold deposits. We conducted geochemical modeling that integrates geological context and mineral paragenesis. The results reveal that potassic alteration in lode gold deposits, characterized by the transition from K-feldspar to muscovite, occurs alongside decreasing temperature, pH, and oxygen fugacity of the ore-forming fluids. Among these factors, the drop in temperature is the most significant mechanism driving potassic alteration, while also causing the destabilization of Au-bisulfide complex and spatially associated deposition of gold. These results suggest that gold mineralization during potassic alteration is primarily driven by the cooling of ore fluids, which also explains the transition from K-feldspar to muscovite alteration. The combination of molecular simulation and geochemical modeling indicates that the role of potassic alteration in lode gold mineralization reflects the influence of fluid evolution particularly fluid cooling on gold precipitation, rather than a direct control of K<ce:sup loc=\"post\">+</ce:sup> on Au transport in lode gold deposits. Therefore, potassic alteration can serve as an effective indicator for lode gold exploration and has been widely applied in practical fieldwork.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"258 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841589","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":"Rhenium residency in molybdenite, compressional textures and relationship to polytypism","authors":"Mao Tan, Yiping Yang, Xiao-Wen Huang, Jiaxin Xi, Nuo Li, Yu-Miao Meng, Liang Qi","doi":"10.1016/j.gca.2024.12.012","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.012","url":null,"abstract":"Molybdenite has been established as a robust mineral for Re-Os isotope dating. However, higher-precision Re-Os molybdenite dating is necessary to accurately determine the absolute timing of metal mineralization and duration of ore-forming hydrothermal systems. To improve the precision and accuracy of molybdenite Re-Os dating, molybdenite from the Longmendian deposit—with its old age, heterogeneous distribution of Re, compression deformation, and complex polytypes—serves as a reference for enhancing the precision of in-situ Re-Os dating or grain-scale sampling during solution Re-Os isotope dating of molybdenite. High-resolution scanning transmission electron microscopy (TEM) analysis demonstrates that Re, Os, Pb, Bi, Cu, and Fe are incorporated into the molybdenite crystal through isomorphic substitution for Mo. Electron probe analysis shows that a single molybdenite grain exhibits heterogeneous textures consisting of Re-rich (∼0.29–0.82 wt%) and Re-poor (&lt;0.29 wt%) domains. Some molybdenite grains have undergone compression deformation. Rhenium can be enriched in either the deformed or the undeformed domains of molybdenite grains. Compression deformation in some grains induces delamination cracks in Re-rich domains, facilitating ore-forming fluid infiltration and leading to an inhomogeneous distribution of Re and other elements in molybdenite grains. Re-rich domain in molybdenite is enriched in other metals, including Fe, Co, Zn, Pb, and Bi, due to the overprint of the hydrothermal fluids with a lower temperature and a relatively high oxygen fugacity, leading to the formation of heterogeneous molybdenite. Micro-X-ray diffraction (μXRD) and TEM analyses have revealed that both the Re-rich and Re-poor domains belong to the 2<ce:italic>H</ce:italic><ce:inf loc=\"post\">1</ce:inf> polytype, indicating that Re concentration and distribution are not directly related to the polytype of molybdenite. The Re-poor and deformed domain of molybdenite shows the coexistence of 2<ce:italic>H</ce:italic><ce:inf loc=\"post\">1</ce:inf> and 3<ce:italic>R</ce:italic> polytypes (in a ratio of 9:1), suggesting that compression deformation led to polytype transformation. Therefore, the diverse characteristics of molybdenite in the Longmendian deposit present challenges for obtaining primary Re-Os age information. Nondestructive pre-characterization of molybdenite is essential before dating to ensure age homogeneity. Molybdenite samples with an undeformed and uniform distribution of elements (Re) within molybdenite grains are suitable candidates for analysis. Our findings collectively offer strategies to enhance precision while advancing the understanding of elemental and isotopic geochemical behavior in the contexts of metamorphism, deformation, and fluid flow environments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"5 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841532","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 solubility of La hydroxide and stability of La3+ and La hydroxyl complexes at acidic to mildly acidic pH from 25 to 250 °C","authors":"Kevin Padilla, Alexander P. Gysi","doi":"10.1016/j.gca.2024.12.006","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.006","url":null,"abstract":"The mobility of rare earth elements (REE) in natural hydrothermal systems can be assessed using geochemical modeling, which requires reliable thermodynamic data of relevant aqueous species. In this study, we evaluate the controls of pH and temperature on La speciation and the role of hydroxyl complexes in REE transport at hydrothermal conditions. Batch-type hydrothermal solubility experiments were conducted using synthetic La hydroxide powders equilibrated in perchloric acid-based aqueous solutions at temperatures between 150 and 250 °C and starting pH of 2 to 5. The La hydroxide solubility is retrograde with temperature and displays a strong pH dependence with a decrease in La concentrations from acidic to mildly acidic pH spanning between 3 and 5 orders of magnitude (e.g. log La molality of −2.5 to −7.2 at 250 °C). Thermodynamic optimizations using GEMSFITS allow to retrieve the standard partial molal Gibbs energies for the La<ce:sup loc=\"post\">3+</ce:sup> aqua ion and the formation constants for the La hydroxyl species (i.e., LaOH<ce:sup loc=\"post\">2+</ce:sup>, La(OH)<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">+</ce:sup>, La(OH)<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">0</ce:sup>) between 25 and 250 °C. A comparison between the experimentally derived thermodynamic properties with the calculated values from the Helgeson-Kirkham-Flowers equation of state parameters indicates an increased divergence with temperature. Discrepancies in standard partial molal Gibbs energies range between ∼ 1 − 12 kJ/mol and result in a predicted La hydroxide solubility differing by up to 3 orders of magnitude at 250 °C. Speciation calculations indicate a higher stability of La<ce:sup loc=\"post\">3+</ce:sup> and LaOH<ce:sup loc=\"post\">2+</ce:sup> over the other La hydroxyl species in the studied pH range of 3.4 to 6. The optimized thermodynamic properties for La aqueous species have important implications for modeling the solubility of REE minerals such as monazite and the mobility of REE in hydrothermal systems.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"75 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841533","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, F. Wombacher, S.J.G. Galer","doi":"10.1016/j.gca.2024.12.001","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.001","url":null,"abstract":"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 <ce:sup loc=\"post\">113</ce:sup>Cd, resulting from galactic cosmic rays (GCR) impacting the lunar surface, provides a means of reconstructing the exposure history of the regolith.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"63 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","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":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Deciphering multiple episodes of partial melting, metasomatic and remelting processes in the Eastern Pyrenean orogenic mantle massif","authors":"Yujian Wang, Jingao Liu","doi":"10.1016/j.gca.2024.11.033","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.033","url":null,"abstract":"Secular variations in the composition of the subcontinental lithospheric mantle (SCLM) are intimately controlled by multiple geological processes, including diverse melting mechanisms and complex metasomatic processes. We present comprehensive analyses of whole rock and mineral chemistry, along with Re-Os isotopic system of the Eastern Pyrenean orogenic mantle massifs, supported by quantitative modeling through alphaMELTS thermodynamic software. The Eastern Pyrenean lherzolites display consistent negative correlations of TiO<ce:inf loc=\"post\">2</ce:inf>/Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf>, TiO<ce:inf loc=\"post\">2</ce:inf> versus MgO and relatively high and unfractionated heavy rare earth elements. These chemical features shed light on the diverse melting mechanisms responsible for these rocks, encompassing passive continuous melting in regions of lithospheric extension and decompression melting linked to the upwelling asthenospheric mantle. Some refractory harzburgites exhibit elevated TiO<ce:inf loc=\"post\">2</ce:inf>/Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf>, TiO<ce:inf loc=\"post\">2</ce:inf> contents and moderate light rare earth element enrichment. This suggests chromatographic metasomatism due to the reactive porous flow of evolved melts/fluids in the upper part of the lithosphere during their ascent to the surface. An isochron analogy between <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os and Al<ce:inf loc=\"post\">2</ce:inf>O<ce:inf loc=\"post\">3</ce:inf> yields an age of ∼ 1.5 Ga, constraining the timing of partial melting responsible for the formation of SCLM beneath the Eastern Pyrenean region. We then expand the scope of our study to encompass on– and off-cratonic SCLM on a global scale. This expanded analysis explores the variations in melting mechanisms across different tectonic settings and geological epochs and scrutinizes the role of diverse metasomatic processes in shaping the characteristics of the lithospheric mantle and its longevity on a broad scale. Silicate metasomatism typically produces fertile peridotites at the asthenosphere-lithosphere boundary, making them vulnerable to thermomechanical erosion, whereas cryptic metasomatism, commonly observed in refractory mantle rocks induced by evolved melts at decreasing melt-rock ratios (e.g., hydrocarbon-bearing silicate melt, carbonatite melt etc.), generally forms enrichment of highly incompatible elements but less significant influence on the mineral assemblages and major element geochemistry of the on-cratonic refractory SCLM, making them remain stable for extended periods.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"201 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841587","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":"Coupling stable isotope analyses and X-ray absorption spectroscopy to investigate the molecular mechanism of zinc sorption by calcite","authors":"Yiren Duan, Hongtao He, Wenchao Liu, Wenxian Gou, Zhao Wang, Peng Liu, Jing Zhang, Caroline L. Peacock, Wei Li","doi":"10.1016/j.gca.2024.12.005","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.005","url":null,"abstract":"Calcite plays a pivotal role in regulating the mobility and fate of zinc (Zn) in natural environments. Despite its significance, the mechanism of Zn sorption on calcite surfaces, particularly the transitional dynamics from surface adsorption to precipitation, remains unclear. This research studied the sorption behavior of Zn on calcite across a wide range of reaction times, pH values, and Zn concentrations. The underlying sorption mechanisms were examined through a combination of Zn stable isotope measurements and X-ray absorption fine structure (XAFS) spectroscopy. At pH 6.5 and a low Zn concentration of 5 μM, the surface coverage reached 0.9 μmol/m<ce:sup loc=\"post\">2</ce:sup>, accompanied by a pronounced Δ<ce:sup loc=\"post\">66</ce:sup>Zn<ce:inf loc=\"post\">sorbed-aqueous</ce:inf> of + 0.40 ‰, which is indicative of a tetrahedral inner-sphere surface complexation mechanism. Conversely, at pH ≥ 7.5 and a higher Zn concentration (100 μM), the surface coverage surpassed 57.6 μmol/m<ce:sup loc=\"post\">2</ce:sup>, resulting in diminished Zn isotope fractionation (+0.20 ‰), suggesting the formation of hydrozincite precipitates. These results, integrated with the XAFS analysis, revealed a continuous transition from inner-sphere tetrahedral surface complexes to hydrozincite precipitates as the pH and/or Zn concentration increased. Notably, the sensitivity of Zn isotope fractionation to distinct Zn sorption mechanisms was supported by an inverse linear relationship between Zn isotope fractionation and the Zn-O bond distance. This study significantly advances our understanding of Zn sorption mechanisms on calcite by demonstrating that the surface of calcite may have catalyzed hydrozincite precipitation when the bulk solution was undersaturated with respect to hydrozincite. The synergistic application of Zn stable isotopes and XAFS spectroscopy provides a robust framework for probing metal-mineral interactions under environmentally relevant conditions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"23 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841534","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":"Source composition or melting effect: New evidence from Archean komatiites concerning the origin of low highly siderophile element abundances in Earth’s mantle","authors":"Xiaoyu Zhou, Ratul Banerjee, Laurie Reisberg, Sisir K. Mondal","doi":"10.1016/j.gca.2024.12.004","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.004","url":null,"abstract":"Highly siderophile element (HSE) contents of komatiites have been widely used to estimate the HSE composition of Earth’s mantle. However, the interpretation of existing komatiite data is controversial, with some authors arguing that the Archean deep mantle komatiite source was impoverished in HSE due to slow admixture of a late accretion component, while others invoke a melting process that would allow observed komatiite abundances to be obtained from a mantle source with present-day HSE abundances. To obtain insight into this issue, we present new HSE abundance data for komatiites from the Gorumahishani greenstone belt of the Singhbhum Craton, eastern India. Our Sm-Nd and Re-Os isotope data indicate a ∼3.5 Ga age for these little-studied rocks, which provide extreme examples of Al-depleted and Ti-depleted komatiite varieties, juxtaposed over a short-length scale. The calculated parental melt compositions for the Al-depleted komatiites have 2.7 ± 0.2 ppb Ru, 3.4 ± 0.2 ppb Pt, and 3.2 ± 0.6 ppb Pd, whereas, for the Ti-depleted type these values are 4.4 ± 0.3 ppb Ru, 3.2 ± 0.6 ppb Pt, 3.0 ± 0.5 ppb Pd. These concentrations are similar to those found in most Archean komatiites at &gt;3.4 Ga. For the Al-depleted samples, these values would correspond to mantle abundances equivalent to ∼38 % of modern Bulk Silicate Earth (BSE) Ru contents and ∼24 and ∼21 % of BSE Pt and Pd contents, respectively, if it is assumed that simple extrapolation of the measured values to the MgO content of fertile peridotite provides an adequate approximation of the HSE composition of the BSE. To examine the alternative model that the low contents of Ru, Pd and Pt in Gorumahishani komatiites could be obtained from a mantle source with BSE-like HSE contents, we apply a simple two-stage critical melting model using current experimental HSE partitioning coefficients. The Ru abundances of the Gorumahishani Al-depleted komatiitic magmas can be produced from the pooled melts of a fertile source with BSE-like Ru and S contents during the first melting stage. The Ru abundances of the Ti-depleted komatiitic magmas can then be produced from remelting the residue left by this first melting stage. On the other hand, Pt and Pd abundances cannot be successfully modelled for either the Al-depleted or the Ti-depleted komatiites using available partition coefficients, though our current understanding of Pt and Pd partitioning after sulfide exhaustion is limited. The use of komatiites to characterize the abundance and distribution of HSE in the early mantle critically depends on developing a better understanding of the partitioning behaviors of these elements between mantle sources and komatiitic magmas.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"7 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841535","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":"Association of tungsten with aluminosilicate mineral colloids and silicotungstates in soil porewaters: Insights into the unexpectedly high tungsten mobility in soil","authors":"Lei Lu, Jing Sun, Yu Dai, Yaoyu Zhou, Haojie Cui, Ming Lei, Huihui Du","doi":"10.1016/j.gca.2024.11.032","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.032","url":null,"abstract":"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<ce:sup loc=\"post\">VI</ce:sup>), and insoluble W metal (W<ce:sup loc=\"post\">0</ce:sup>) entering the soil could rapidly oxidize into W<ce:sup loc=\"post\">VI</ce:sup> 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 Al<ce:inf loc=\"post\">2</ce:inf>(WO<ce:inf loc=\"post\">4</ce:inf>)<ce:inf loc=\"post\">3</ce:inf> 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.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"39 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-05","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":"Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet","authors":"Getachew Agmuas Adnew, Thomas Röckmann, Thomas Blunier, Christian Juncher Jørgensen, Sarah Elise Sapper, Carina van der Veen, Malavika Sivan, Maria Elena Popa, Jesper Riis Christiansen","doi":"10.1016/j.gca.2024.11.009","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.009","url":null,"abstract":"Clumped isotopes of methane (CH&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;), specifically &lt;mml:math altimg=\"si74.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;13&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;D and &lt;mml:math altimg=\"si75.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;12&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;D&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;, provide additional information to constrain its sources and sink processes. These isotopes complement interpretations of CH&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; provenance based on bulk isotopes. However, interpreting the origin of CH&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; 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 CH&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt; 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 CH&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;. The &lt;mml:math altimg=\"si74.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;13&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;D and &lt;mml:math altimg=\"si75.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;12&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;D&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The &lt;mml:math altimg=\"si75.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;12&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;D&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; values are close to those of atmospheric CH&lt;ce:inf loc=\"post\"&gt;4&lt;/ce:inf&gt;. The elevated &lt;mml:math altimg=\"si75.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;12&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;D&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; 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 &lt;mml:math altimg=\"si74.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;13&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;/mml:math&gt;CH&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;D and &lt;mml:math altimg=\"si75.svg\" display=\"inline\"&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi&gt;Δ&lt;/mml:mi&gt;&lt;/","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"30 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-04","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":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Iron isotope fractionation in highly evolved magmas results from ilmenite crystallization","authors":"Fengli Shao, Yaoling Niu, Haiquan Wei, Yu Zhang, Guodong Wang","doi":"10.1016/j.gca.2024.11.029","DOIUrl":"https://doi.org/10.1016/j.gca.2024.11.029","url":null,"abstract":"Globally, granites and rhyolites with SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; &gt; 70 wt% show large Fe isotope variation (δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe = −0.05 to +0.65 ‰) relative to less silicic igneous rocks in δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe vs. SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; space. The upper bound of the data tends to show δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe increase with increasing SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;. Granitic magma differentiation can be invoked to explain magma compositional variation, including Fe isotope variation, but clearly cannot explain the highly varied δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe values. The latter may result from magma differentiation of varying liquidus phases, magma mixing, assimilation and magma source compositional variation. To decipher how each of these and altogether explain the large δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe variation requires rigorous studies of varying well characterized sample suites. This paper is not to solve all these issues but demonstrates clearly using three sample suites with well-defined liquid lines of descent from alkaline basalts to peralkaline rhyolites to show that the δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe increases with continued magma differentiation (increasing SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;, SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt;/MgO and decreasing MgO). The rapid δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe increase for samples with SiO&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; &gt; 70 wt% results from ilmenite (vs. magnetite) fractionation. Among all the major liquidus phases, ilmenite has a distinctive affinity with light-Fe isotope, whose crystallization elevates δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe in the residual melts. This result demonstrates the affinity of isotopically heavy Fe with Fe&lt;ce:sup loc=\"post\"&gt;3+&lt;/ce:sup&gt; and the correlation of isotopically light Fe with Fe&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt; because δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe values of ilmenite (TiFe&lt;ce:sup loc=\"post\"&gt;2+&lt;/ce:sup&gt;O&lt;ce:inf loc=\"post\"&gt;3&lt;/ce:inf&gt;) ≪ δ&lt;ce:sup loc=\"post\"&gt;56&lt;/ce:sup&gt;Fe values of magnetite (&lt;mml:math altimg=\"si1.svg\"&gt;&lt;mml:mrow&gt;&lt;mml:msup&gt;&lt;mml:mrow&gt;&lt;mml:mi mathvariant=\"normal\"&gt;F&lt;/mml:mi&gt;&lt;mml:mi mathvariant=\"normal\"&gt;e&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;2&lt;/mml:mn&gt;&lt;mml:mo&gt;+&lt;/mml:mo&gt;&lt;/mml:mrow&gt;&lt;/mml:msup&gt;&lt;mml:mi mathvariant=\"normal\"&gt;O&lt;/mml:mi&gt;&lt;mml:mo&gt;∙&lt;/mml:mo&gt;&lt;mml:msubsup&gt;&lt;mml:mrow&gt;&lt;mml:mi mathvariant=\"normal\"&gt;F&lt;/mml:mi&gt;&lt;mml:mi mathvariant=\"normal\"&gt;e&lt;/mml:mi&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;2&lt;/mml:mn&gt;&lt;/mml:mrow&gt;&lt;mml:mrow&gt;&lt;mml:mn&gt;3&lt;/mml:mn&gt;&lt;mml:mo&gt;+&lt;/mml:mo&gt;&lt;/mml:mrow&gt;&lt;/mml:msubsup&gt;&lt;mml:msub&gt;&lt;mml:mi mathvariant=\"normal\"&gt;O&lt;/mml:mi&gt;&lt;mml:mn&gt;3&lt;/mml:mn&gt;&lt;/mml:msub&gt;&lt;/mml:mrow&gt;&lt;/mml:math&gt;). We can conclude that ilmenite solid solution is likely the major oxide liquidus phase at the late-stage felsic melt evolution for relatively dry magmas with low &lt;ce:italic&gt;f&lt;/ce:italic&gt;O&lt;ce:inf loc=\"post\"&gt;2&lt;/ce:inf&gt; such as peralkaline rhyolites we study here and mid-ocean ridge basalts. We further predict that magnetite (vs. ilmenite) sol","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"52 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841591","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}