Geochimica et Cosmochimica Acta最新文献

{"title":"Partial molar volumes of 1–1 electrolytes at high T and P: correlations and predictions","authors":"Andrey V. Plyasunov, Elena V. Cherkasova","doi":"10.1016/j.gca.2024.10.024","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.024","url":null,"abstract":"Knowledge of the partial molar volumes of aqueous ions allows accurate calculation of the pressure dependence of equilibrium constants, solubility of minerals, etc., thus being useful for thermodynamic modeling of hydrothermal processes. This study analyzed methods to correlate and predict the values of the partial molar volumes at infinite dilution, <mml:math altimg=\"si6.svg\"><mml:mrow><mml:msubsup><mml:mi>V</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mi mathvariant=\"normal\">o</mml:mi></mml:msubsup></mml:mrow></mml:math>, for 1–1 electrolytes and singly charged ions at elevated T and P. Since the precise experimental values of the dielectric constant of water are measured only up to 873 K, we were interested only in non-electrostatic ways to correlate <mml:math altimg=\"si6.svg\"><mml:mrow><mml:msubsup><mml:mi>V</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mi mathvariant=\"normal\">o</mml:mi></mml:msubsup></mml:mrow></mml:math> data. First of all, we compiled the <mml:math altimg=\"si6.svg\"><mml:mrow><mml:msubsup><mml:mi>V</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mi mathvariant=\"normal\">o</mml:mi></mml:msubsup></mml:mrow></mml:math> values at T > 373 K for the following 1–1 electrolytes: HCl, LiCl, LiI, LiNO<ce:inf loc=\"post\">3</ce:inf>, LiOH, NaF, NaCl, NaBr, NaI, NaNO<ce:inf loc=\"post\">3</ce:inf>, NaOH, NaHCO<ce:inf loc=\"post\">3</ce:inf>, NaClO<ce:inf loc=\"post\">4</ce:inf>, NaH<ce:inf loc=\"post\">2</ce:inf>PO<ce:inf loc=\"post\">4</ce:inf>, NaTr (Tr stands for triflate), KF, KCl, KBr, KI, KNO<ce:inf loc=\"post\">3</ce:inf>, KOH, CsBr, and NH<ce:inf loc=\"post\">4</ce:inf>Cl. Relations, following from the “density” model and from the Fluctuation Solution Theory (FST) were employed to analyze data. It was concluded that at the current state of knowledge of <mml:math altimg=\"si6.svg\"><mml:mrow><mml:msubsup><mml:mi>V</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mi mathvariant=\"normal\">o</mml:mi></mml:msubsup></mml:mrow></mml:math> the FST-relations for electrolytes are recommended mainly to reject strongly deviating experimental outliers. However, the “density” model provides a simple and fairly accurate way to describe the compiled set of data with only two parameters for each ion, <mml:math altimg=\"si4.svg\"><mml:mrow><mml:mi>n</mml:mi></mml:mrow></mml:math> and <mml:math altimg=\"si5.svg\"><mml:mrow><mml:msub><mml:mi>V</mml:mi><mml:mtext>hc</mml:mtext></mml:msub></mml:mrow></mml:math>, values of which were evaluated for the following singly-charged ions: H<ce:sup loc=\"post\">+</ce:sup>, Li<ce:sup loc=\"post\">+</ce:sup>, Na<ce:sup loc=\"post\">+</ce:sup>, K<ce:sup loc=\"post\">+</ce:sup>, Cs<ce:sup loc=\"post\">+</ce:sup>, NH<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">+</ce:sup>, F<ce:sup loc=\"post\">-</ce:sup>, Cl<ce:sup loc=\"post\">-</ce:sup>, Br<ce:sup loc=\"post\">-</ce:sup>, I<ce:sup loc=\"post\">-</ce:sup>, OH<ce:sup loc=\"post\">–</ce:sup>, NO<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">–</ce:sup>, H<ce:inf loc=\"post\">2</ce:in","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"74 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673650","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":"Electron transfer at birnessite/organic compound interfaces: mechanism, regulation, and two-stage kinetic discrepancy in structural rearrangement and decomposition","authors":"Huan Ye, Zhaoyang Hu, Rongzhang Yin, Teak D. Boyko, Yunpeng Liu, Yanzhang Li, Chunjiang Li, Anhuai Lu, Yan Li","doi":"10.1016/j.gca.2024.10.009","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.009","url":null,"abstract":"Electron transfer between birnessite and organic compounds (OC) plays a dominant role in the coupling cycle of manganese (Mn) and carbon across diverse environmental settings. While previous studies have extensively investigated individual processes of interface Mn reduction, surface Mn<ce:sup loc=\"post\">2+</ce:sup> adsorption, and surface-to-interior electron transfer, the dynamic interplay among these reactions and the mechanisms regulating subtle changes in surface and interior Mn states remained poorly understood. Additionally, existing models have not adequately captured electron transfer kinetics in multivariable systems involving pH, Mn<ce:sup loc=\"post\">2+</ce:sup> concentration, electron donor type, etc. In this study, we investigated the reduction kinetics of birnessite under the influence of multiple environmental variables by employing three typical OC: formic acid (HCOOH), formaldehyde (HCHO), and methanol (CH<ce:inf loc=\"post\">3</ce:inf>OH). Time-series analysis revealed kinetic discrepancy and time lag between the alteration of the average Mn oxidation state (AMOS) within the solid and the release of Mn<ce:sup loc=\"post\">2+</ce:sup> from the reductive dissolution of birnessite, indicating a two-stage electron transfer mechanism occurring at the interface between birnessite and OC. X-ray absorption fine structure spectra revealed a rapid increase in corner-sharing MnO<ce:inf loc=\"post\">6</ce:inf> octahedra and a decline in AMOS during the initial stage, followed by a slight decrease in AMOS and substantial mineral dissolution to release Mn<ce:sup loc=\"post\">2+</ce:sup> in the subsequent stage. The transition point between the two stages is primarily influenced by the concentration of surface Mn<ce:sup loc=\"post\">II</ce:sup> under pH regulation, as confirmed by soft X-ray absorption spectroscopy and density functional theory calculations. Based on these findings, the adsorption equilibrium and electron transfer rate were modeled by a machine learning framework (JAX), which is influenced by three main factors: pH, Mn<ce:sup loc=\"post\">2+</ce:sup> concentration, and OC types. The adsorption equilibrium constant for HCHO was one order of magnitude lower than for HCOOH, yet displayed a faster reaction rate due to higher electron transfer rates. Competitive adsorption of OC and Mn<ce:sup loc=\"post\">2+</ce:sup> on reactive sites was influenced by both pH and Mn<ce:sup loc=\"post\">2+</ce:sup> concentrations. Combining these parameters, we created a 3D surface plot that comprehensively considered the interplay between different elementary reactions, including competitive adsorption and redox reaction rates, thereby visualizing the kinetic regulation mechanisms in multivariable systems. Furthermore, a comprehensive rate equation for the reduction of birnessite by OC was developed to predict its behavior in natural settings. With an electron storage capacity of 2.7×10<ce:sup loc=\"post\">23</ce:sup> electrons/mol Mn before structural decompositio","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"16 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142763014","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":"Tantalum in hydrothermal fluids","authors":"Bin Hu, Barbara Etschmann, Denis Testemale, Weihua Liu, Qiushi Guan, Harald Müller, Joël Brugger","doi":"10.1016/j.gca.2024.10.019","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.019","url":null,"abstract":"Understanding the behaviour of tantalum (Ta) in hydrothermal systems is pivotal for understanding its geochemical enrichment processes and economic extraction via hydrometallurgy. Yet, its behaviour in hydrothermal systems remains poorly characterised. This study investigates the coordination chemistry, speciation, and solubility of pentavalent Ta(V) in fluoride (F) − and chloride (Cl) −rich hydrothermal solutions up to 413 °C and 800 bar, utilising <ce:italic>in-situ</ce:italic> High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS). The results reveal the stability of high order fluoridotantalate complexes in fluoride-rich fluids solutions up to the highest investigated temperature, highlighting fluoride’s paramount role in enhancing Ta solubility through the formation of stable fluoridotantalate complexes in aqueous solutions. A transition from nonafluoridotantalate tetraanion (TaF<ce:inf loc=\"post\">9</ce:inf><ce:sup loc=\"post\">4−</ce:sup>) to heptafluoridotantalate dianion (TaF<ce:inf loc=\"post\">7</ce:inf><ce:sup loc=\"post\">2−</ce:sup>) complexes was observed as a function of temperature in solutions containing ≥1 <ce:italic>m</ce:italic> fluoride. Conversely, our findings indicate a negligible role for chloride in Ta complexation even in high Cl (∼6 <ce:italic>m</ce:italic>) aqueous solutions, suggesting that Ta chloride complexes do not contribute significantly to Ta transport in hydrothermal systems. Existing solubility data were reinterpreted based on an updated speciation model that integrates the <ce:italic>in-situ</ce:italic> XAS results. This confirms that Ta(OH)<ce:inf loc=\"post\">5</ce:inf>(aq) predominates in solutions containing &lt;0.02 <ce:italic>m</ce:italic> fluoride; oxyfluoridotantalate anions such as [TaF<ce:inf loc=\"post\">3</ce:inf>(OH)<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">−</ce:sup>] dominate in solutions containing intermediate fluoride concentrations (0.02–1 <ce:italic>m</ce:italic>), and the fluoridotantalate anions [TaF<ce:inf loc=\"post\">9</ce:inf><ce:sup loc=\"post\">4−</ce:sup> to TaF<ce:inf loc=\"post\">7</ce:inf><ce:sup loc=\"post\">2−</ce:sup>] occur in more concentrated fluoride solutions (&gt;1 <ce:italic>m</ce:italic>) at hydrothermal conditions (∼100–400 °C). Derived thermodynamic data for these species enable better understanding and geochemical modelling of Ta transport in hydrothermal fluids, highlighting the potential of F-rich fluids to transport significant amounts of Ta.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"19 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673598","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":"Bio-mediated enhancement of supergene copper mineralization: Evidence from Cu isotope geochemistry","authors":"J. Javier Rey-Samper, Ryan Mathur, Fernando Tornos","doi":"10.1016/j.gca.2024.10.014","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.014","url":null,"abstract":"The relationship between microbial activity and the supergene modification of ore systems has been a major subject of debate. Here, we present isotopic evidence of microbial-driven secondary copper mineralization in the active cementation zone of the Las Cruces deposit, a volcanogenic massive sulfide deposit located in Iberian Pyrite Belt, Spain. Copper isotopic data show that the lower isotopic ratios (δ<ce:sup loc=\"post\">65</ce:sup>Cu ≈ −9.2 ± 0.11 ‰, the lowest value measured worldwide in a supergene environment) are found in the upper part of the cementation zone, the same zone where the maximum copper grades are found and where there is direct evidence of extremophilic microbial activity. There is a tendency towards higher values downwards through the cementation zone (−9.2 ± 0.11 ‰ to + 1.67 ± 0.11 ‰ δ<ce:sup loc=\"post\">65</ce:sup>Cu) and upwards into the former Cu-depleted gossan that originally capped the cementation zone (−7.79 ± 0.11 ‰ to −1.32 ± 0.11 ‰ δ<ce:sup loc=\"post\">65</ce:sup>Cu). As microbes preferentially sequester the lighter isotope when incorporating intracellular Cu, this distribution indicates that microbes played a major role in the formation of the high-grade zones. Water arrives to the deposit enriched in isotopically heavy copper, likely because it has leached other ore bodies upstream. δ<ce:sup loc=\"post\">65</ce:sup>Cu values of water currently flowing into the system are remarkably more positive than those in the ore, indicating that microbial activity is a major cause of copper isotope fractionation. At least half of the copper transported by the incoming waters remains within the ore body. Our best interpretation is that the large and high-grade cementation zone at Las Cruces is of biogenic origin, and that the primary mineralization acted as a trap for copper transported by groundwater, leading to the formation of an exotic mineralization distal to sub-eroded massive sulfides located upstream.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"6 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673849","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Experimental constraints on the role of temperature and pyrogenic mineral assemblage in wildfire-induced major and trace element mobilisation","authors":"Kristy Guerin ,&nbsp;David Murphy ,&nbsp;Stefan C. Löhr ,&nbsp;Luke Nothdurft","doi":"10.1016/j.gca.2024.10.015","DOIUrl":"10.1016/j.gca.2024.10.015","url":null,"abstract":"<div><div>Wildfires impact a large and increasing proportion of the Earth’s surface. With documented soil surface temperatures of up to ∼850 °C, wildfires may fundamentally alter the mineralogy and geochemistry of soils and regolith, more conventionally thought to be dominated by low temperature weathering processes. Here we use an experimental approach to test the effect of temperature on the formation of pyrogenic minerals, and on the distribution and mobility of major, trace and rare earth elements following post-fire chemical weathering. We focus on ferruginous nodules, common Fe-oxide cemented components of soils, which transform from non-magnetic to maghemite-bearing, magnetic nodules under wildfire conditions. These transformations provide a valuable record of fire impacts and facilitate the study of thermal processes and element mobility. Our results show heating produces a typical pyrogenic mineral assemblage of hematite, maghemite, metakaolin and transition alumina. At 900 °C the high temperature Fe₂O₃ polymorph luogufengite forms, which has never been reported in natural fire-affected substrates and therefore places an upper boundary on palaeowildfire temperatures at the soil-fire interface. Chemical leaching, employed to simulate the impacts of post-fire weathering, demonstrates that formation and subsequent breakdown of these pyrogenic minerals results in increased mobility of several elements including Li, Si, Sc, Cr, Co, Cu, Zn, Rb, Cs, La, Pb and U. Further, we propose that incongruent dissolution of pyrogenic metakaolin may be responsible for the formation of fusic material, an aluminous cement commonly found in soils. We conclude by discussing the significance of these results for the release of potentially toxic metals following a fire, identify trace elements that have the greatest potential to be used as palaeowildfire geochemical proxies (decreased alkali metal concentrations, decreased U/Th ratios, and decreased La compared to other rare earth elements), and the potential impact of wildfire on global geochemical cycles.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 18-32"},"PeriodicalIF":4.5,"publicationDate":"2024-10-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142532708","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":"Evaluating the crystallization and eruptive histories of low-titanium basalts with 2D and 3D studies","authors":"Z.E. Wilbur, J.J. Barnes, S.A. Eckley, T. Erickson, R.A. Zeigler, K. Domanik","doi":"10.1016/j.gca.2024.10.018","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.018","url":null,"abstract":"Volcanic products returned from the Apollo missions over 50 years ago provide a unique perspective into the magmatic evolution of the Moon. However, questions<ce:hsp sp=\"0.25\"></ce:hsp>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.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"8 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-21","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":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Controls on the distribution of dissolved Cr in the upper water column of the Atlantic Basin","authors":"D. Gilliard, D.J. Janssen, N. Schuback, S.L. Jaccard","doi":"10.1016/j.gca.2024.10.017","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.017","url":null,"abstract":"Over the last decades, the chromium (Cr) stable isotope system (referred to as δ<ce:sup loc=\"post\">53</ce:sup>Cr) has emerged as a proxy to reconstruct past oxygenation changes in Earth’s atmosphere and oceans. Although Cr is a promising paleoproxy, uncertainties remain as to the modern marine Cr cycle, and limited data are yet available in large swaths of the ocean, including the Atlantic Ocean. Here we present dissolved seawater Cr concentrations ([Cr]) and δ<ce:sup loc=\"post\">53</ce:sup>Cr along a meridional transect from the North to the South Atlantic (AMT 29). Chromium concentrations range from of 2.51 to 3.96 nmol kg<ce:sup loc=\"post\">−1</ce:sup> (n = 68) and δ<ce:sup loc=\"post\">53</ce:sup>Cr values range from +0.86 ± 0.04 ‰ (2SEM) to +1.20 ± 0.02 ‰ (2SEM) (n = 68). In contrast to data from other ocean basins [Cr] and δ<ce:sup loc=\"post\">53</ce:sup>Cr show only a weak correlation (δ<ce:sup loc=\"post\">53</ce:sup>Cr vs. Ln([Cr]) R<ce:sup loc=\"post\">2</ce:sup> = 0.17), inconsistent with a closed-system Rayleigh distillation model. These results can mainly be explained by horizontal advection and water mass mixing, which our data demonstrate are the dominant processes controlling [Cr] and δ<ce:sup loc=\"post\">53</ce:sup>Cr distributions throughout much of the Atlantic, while the impact of in situ biogeochemical cycling is comparatively minor. There is, indeed no clear impact of biological productivity nor of dysoxic environments in the (sub)tropical Atlantic on the cycling of Cr along the transect. This is likely explained by insufficiently depleted oxygen concentrations and relatively low biological productivity, resulting in these processes being of secondary importance relative to water mass mixing in shaping the distribution of Cr in the low- to mid-latitude Atlantic Ocean.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"99 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673652","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":"Ti- and Ba-rich phlogopitic micas in alkaline basic and upper mantle igneous rocks; stoichiometry, stability, and Fe valence estimation reassessed and rationalised","authors":"C. Michael B. Henderson","doi":"10.1016/j.gca.2024.10.011","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.011","url":null,"abstract":"Ti- and Ba-rich tri-octahedral micas occur in fractionated basic igneous rocks, metasomatized mantle peridotites, metamorphosed pelites/carbonates, and hydrothermally altered mineral deposits. Electron microprobe analyses (EMP), with all iron reported as FeO, were widely used in the 1970/80s to interpret Ti and Ba substitution mechanisms, based on 22 O<ce:sup loc=\"post\">2–</ce:sup> unit cell calculations, implying that cation vacancies occur in octahedral and/or intersheet sites. In 1996 EMP with chemical and physical analyses for ferric and total Fe, H<ce:inf loc=\"post\">2</ce:inf>O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both <ce:italic>K</ce:italic> and <ce:italic>L</ce:italic>-edges) established valence states for Fe and Ti and cation site occupancies, that ∼50 % O replaces (OH) molecules, and that 24 anion cell formulae show the absence of cation vacancies. Cell formula calculation protocol for phlogopitic micas is refined here and results tested against the stoichiometric formula for vacancy-free phlogopite, <ce:sup loc=\"post\">XII</ce:sup>K<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">VI</ce:sup>Mg<ce:inf loc=\"post\">6</ce:inf><ce:sup loc=\"post\">IV</ce:sup>[Si<ce:inf loc=\"post\">6</ce:inf>Al<ce:inf loc=\"post\">2</ce:inf>]O<ce:inf loc=\"post\">20</ce:inf>(OH)<ce:inf loc=\"post\">4</ce:inf>. Hypothetical sheet silicate compositions, calculated with fixed contents of vacancies linked to particular mixed-valence element substitutions, confirm that reliable unit cell formulae for natural mica solids require that each stoichiometric vacancy must be accounted for. If reliable estimates for ‘excess O’ (denoted <ce:sup loc=\"post\">W</ce:sup>O<ce:sup loc=\"post\">2−</ce:sup>) are assigned to EMP analyses, the proportion of the oxy-mica component in a mica solid solution can be defined. This approach is tested using published analyses for Ti- and Ba-rich biotites from fractionated basic and ultramafic volcanic igneous rocks (oxymica range 2.5–45 %; TiO<ce:inf loc=\"post\">2</ce:inf> up to 14 %; BaO up to 23 %), upper mantle peridotites (equivalent values 7–18 %; 6 %; 0.7 %), and metasomatised upper mantle (2–37 %; 9 %; 23 %). Enrichments of Ti and Ba in micas are clearly linked to the extra oxygen charge required to neutralise the more highly charged Ba<ce:sup loc=\"post\">2+</ce:sup> and Ti<ce:sup loc=\"post\">4+</ce:sup> replacing K<ce:sup loc=\"post\">+</ce:sup> and Mg<ce:sup loc=\"post\">2+</ce:sup>.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"99 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673571","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Molybdenum isotope behavior during subduction zone metamorphism","authors":"Cassidy J. Stegner, Richard M. Gaschnig, Samuel Marshall, Shelby T. Rader, Gray E. Bebout, Sarah C. Penniston-Dorland","doi":"10.1016/j.gca.2024.10.016","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.016","url":null,"abstract":"The molybdenum (Mo) isotope composition (defined as δ<ce:sup loc=\"post\">98</ce:sup>Mo measured per mil relative to NIST-3134) of many modern arc systems and the upper continental crust is heavier than the mantle and most subducting slab lithologies. This observation has led to a model whereby fluids leaving the slab transfer isotopically heavy Mo preferentially to the mantle wedge, leaving the residual slab isotopically lighter. We explore this model via an Mo isotope study of the metasedimentary and mélange lithologies of the Catalina Schist in California. These rocks record subduction zone metamorphism over a wide range of high-pressure/low-to-medium temperature conditions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"36 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673653","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 redox shuttling and uptake by silicate minerals on the Namibian mud belt","authors":"Paul Vosteen ,&nbsp;Michael Kossack ,&nbsp;Christoph Vogt ,&nbsp;Chloe H. Andersen ,&nbsp;Sonja Geilert ,&nbsp;Matthias Zabel ,&nbsp;Florian Scholz","doi":"10.1016/j.gca.2024.10.013","DOIUrl":"10.1016/j.gca.2024.10.013","url":null,"abstract":"<div><div>Anoxic marine sediments represent an important source of bioavailable iron (Fe) to the ocean. The highest sedimentary Fe fluxes are observed in open-marine oxygen minimum zones where anoxic bottom waters are in contact with ferruginous surface sediments. Here, sedimentary Fe release, transport and re-deposition (i.e., Fe shuttling) may generate a lateral pattern of sedimentary Fe enrichment and depletion, which can be used to trace redox-related Fe mobility in the paleo-record. However, depending on the balance between terrigenous and authigenic (i.e., shuttle-related) Fe flux, the stability of bottom water redox conditions as well as post-depositional processes of mineral alteration, the sedimentary fingerprint of an Fe redox shuttle may be obscured and difficult to identify.</div><div>We investigated sedimentary Fe cycling along two transects across the Namibian mud belt with variable terrigenous sedimentation (23°S &lt; 25°S) and during two seasons with opposing bottom water redox conditions (oxic in austral winter versus anoxic to sulfidic in austral summer). On both transects, substantial benthic Fe fluxes up to −50 µmol m⁻² d⁻¹ were inferred based on pore water profiles. The magnitude of these fluxes is comparable to those reported for other open-marine oxygen minimum zones. On the transect at 23°S, Fe source areas with ferruginous surface sediments were clearly separated from Fe sink areas with highly sulfidic surface sediments. Therefore, Fe redox shuttling was reflected by a lateral pattern of reactive Fe depletion and enrichment relative to the terrigenous background sedimentation. By contrast, on the transect at 25°S, benthic Fe fluxes were temporally and spatially more variable and surface sediments were ferruginous or only weakly sulfidic. Therefore, sedimentary Fe depletion and enrichment was less pronounced at 25°S. In the Fe sink area at 23°S, hydrogen sulfide was present at the sediment surface during both sampling campaigns and solid phase data suggest that Fe sulfide minerals represented the main burial phase of reactive Fe. By contrast, at 25°S excess Fe was associated with potassium (K) rather than reduced sulfur. While a differing sediment provenance cannot be ruled out entirely, combined evidence from pore water silica profiles, K to Fe stoichiometric relationships and electron microprobe images suggest that laterally derived excess Fe was incorporated into pre-existing and/or authigenic clay minerals during early diagenesis. Iron uptake by clay minerals may be supported by frequent redox oscillations and sediment mixing preventing preservation of Fe sulfide minerals and promoting Fe and K fixation in clay minerals. The burial fluxes of excess Fe associated with sulfide minerals at 23°S and silicate minerals at 25°S were similar. Our findings thus underscore that neoformation or alteration of silicate minerals can be important processes within the low-temperature marine Fe cycle.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 1-17"},"PeriodicalIF":4.5,"publicationDate":"2024-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142532707","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}