Pub Date : 2024-10-26DOI: 10.1016/j.gca.2024.10.020
Dionysis I. Foustoukos
A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H2 between H2O-saturated sodium aluminosilicate melts and H2O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H2 between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H2 in silicate melts saturated with H2O decreases as the temperature increases, with control exerted by the mole fraction of H2O in the melt. The dissolution of H2 in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:
{"title":"Molecular H2 in silicate melts","authors":"Dionysis I. Foustoukos","doi":"10.1016/j.gca.2024.10.020","DOIUrl":"https://doi.org/10.1016/j.gca.2024.10.020","url":null,"abstract":"A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H<ce:inf loc=\"post\">2</ce:inf> between H<ce:inf loc=\"post\">2</ce:inf>O-saturated sodium aluminosilicate melts and H<ce:inf loc=\"post\">2</ce:inf>O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H<ce:inf loc=\"post\">2</ce:inf> between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H<ce:inf loc=\"post\">2</ce:inf> in silicate melts saturated with H<ce:inf loc=\"post\">2</ce:inf>O decreases as the temperature increases, with control exerted by the mole fraction of H<ce:inf loc=\"post\">2</ce:inf>O in the melt. The dissolution of H<ce:inf loc=\"post\">2</ce:inf> in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"257 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-10-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142718839","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}
Pub Date : 2024-10-26DOI: 10.1016/j.gca.2024.10.024
Andrey V. Plyasunov, Elena V. Cherkasova
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
了解了水溶液离子的部分摩尔体积，就可以准确计算平衡常数的压力依赖性、矿物的溶解度等，从而有助于热液过程的热力学建模。本研究分析了关联和预测 1-1 种电解质和单电荷离子在高温度和高压力下无限稀释时的部分摩尔体积（V2o）值的方法。由于水的介电常数的精确实验值只测到 873 K，我们只对关联 V2o 数据的非静电方法感兴趣。首先，我们汇编了以下 1-1 种电解质在 T > 373 K 时的 V2o 值：HCl、LiCl、LiI、LiNO3、LiOH、NaF、NaCl、NaBr、NaI、NaNO3、NaOH、NaHCO3、NaClO4、NaH2PO4、NaTr（Tr 代表三盐酸盐）、KF、KCl、KBr、KI、KNO3、KOH、CsBr 和 NH4Cl。数据分析采用了 "密度 "模型和波动解理论（FST）的关系。得出的结论是，根据目前对 V2o 的了解，建议使用电解质的 FST 关系，主要是为了剔除偏差较大的实验异常值。然而，"密度 "模型提供了一种简单而相当准确的方法来描述所汇编的数据集，每种离子只有两个参数，即 n 和 Vhc：H+、Li+、Na+、K+、Cs+、NH4+、F-、Cl-、Br-、I-、OH-、NO3-、H2PO4-、HCO3-、ClO4-、Tr-（Tr = 三酸盐）。根据 Mesmer 等人（1988 年）的研究，我们认为拟合参数 Vhc 和 n 分别与离子的固有体积和从体水转移到离子周围水合壳的水分子数有关。
{"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}
Pub Date : 2024-10-24DOI: 10.1016/j.gca.2024.10.019
Bin Hu, Barbara Etschmann, Denis Testemale, Weihua Liu, Qiushi Guan, Harald Müller, Joël Brugger
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 in-situ 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 (TaF94−) to heptafluoridotantalate dianion (TaF72−) complexes was observed as a function of temperature in solutions containing ≥1 m fluoride. Conversely, our findings indicate a negligible role for chloride in Ta complexation even in high Cl (∼6 m) 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 in-situ XAS results. This confirms that Ta(OH)5(aq) predominates in solutions containing <0.02 m fluoride; oxyfluoridotantalate anions such as [TaF3(OH)3−] dominate in solutions containing intermediate fluoride concentrations (0.02–1 m), and the fluoridotantalate anions [TaF94− to TaF72−] occur in more concentrated fluoride solutions (>1 m) 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.
了解钽（Ta）在热液系统中的行为对于了解其地球化学富集过程和通过湿法冶金进行经济提取至关重要。然而，钽在热液系统中的行为特征仍然不甚明了。本研究利用原位高能量分辨率荧光检测 X 射线吸收光谱（HERFD-XAS），研究了五价 Ta（V）在富含氟化物（F）和氯化物（Cl）的热液溶液（温度高达 413 °C、压力高达 800 巴）中的配位化学、标示和溶解度。研究结果表明，在所研究的最高温度下，富氟流体溶液中的高阶氟对钽酸盐络合物具有稳定性，这突出表明了氟在水溶液中通过形成稳定的氟对钽酸盐络合物来提高钽溶解度的重要作用。在含氟量≥1 m的溶液中，随着温度的变化，可观察到从非氟阳离子四阴离子（TaF94-）向七氟阳离子二阴离子（TaF72-）络合物的转变。相反，我们的研究结果表明，即使在高Cl（∼6 m）的水溶液中，氯化物在Ta络合中的作用也微乎其微，这表明氯化Ta络合物不会对热液系统中的Ta迁移产生重大影响。现有的溶解度数据是根据整合了原位 XAS 结果的最新标示模型重新解释的。这证实了在含0.02 m氟化物的溶液中，Ta(OH)5(aq)占主导地位；在含中等氟化物浓度（0.02-1 m）的溶液中，氧氟阳离子（如[TaF3(OH)3-]）占主导地位；在热液条件下（∼100-400 °C），氟阳离子[TaF94- 至 TaF72-]出现在浓度较高（1 m）的氟化物溶液中。推导出的这些物种的热力学数据有助于更好地理解热液中Ta的迁移并建立地球化学模型，突出了富氟流体迁移大量Ta的潜力。
{"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 <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 (>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}
Pub Date : 2024-10-24DOI: 10.1016/j.gca.2024.10.014
J. Javier Rey-Samper, Ryan Mathur, Fernando Tornos
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 (δ65Cu ≈ −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 ‰ δ65Cu) and upwards into the former Cu-depleted gossan that originally capped the cementation zone (−7.79 ± 0.11 ‰ to −1.32 ± 0.11 ‰ δ65Cu). 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. δ65Cu 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.
{"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}
Pub Date : 2024-10-24DOI: 10.1016/j.gca.2024.10.015
Kristy Guerin , David Murphy , Stefan C. Löhr , Luke Nothdurft
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.
野火对地球表面的影响很大，而且所占比例越来越大。有记录的土壤表面温度高达 ∼ 850 °C，野火可能会从根本上改变土壤和风化岩的矿物学和地球化学，而传统上认为这主要是低温风化过程。在此，我们采用实验方法测试温度对火成矿物形成的影响，以及对火灾后化学风化过程中主要、微量和稀土元素的分布和流动性的影响。我们重点研究了土壤中常见的氧化铁胶结成分--铁质结核，在野火条件下，这些结核从非磁性转变为含磁铁矿的磁性结核。这些转变提供了火灾影响的宝贵记录，有助于研究热过程和元素流动性。我们的研究结果表明，加热会产生由赤铁矿、磁铁矿、偏高岭土和过渡氧化铝组成的典型火成矿物组合。在 900 °C 时，形成了高温 Fe2O3 多晶体 luogufengite，这种多晶体从未在受自然火灾影响的基质中出现过，因此为土壤-火灾界面的古野火温度设定了上限。我们采用化学浸出法来模拟火灾后风化的影响，结果表明，这些火成矿物的形成和随后的分解导致多种元素的流动性增加，包括锂、硅、钪、铬、钴、铜、锌、铷、铯、腊、铅和铀。最后，我们讨论了这些结果对火灾后潜在有毒金属释放的意义，确定了最有可能用作古野火地球化学代用指标的痕量元素（与其他稀土元素相比，碱金属浓度降低、U/Th 比值降低和 La 降低），以及野火对全球地球化学循环的潜在影响。
{"title":"Experimental constraints on the role of temperature and pyrogenic mineral assemblage in wildfire-induced major and trace element mobilisation","authors":"Kristy Guerin , David Murphy , Stefan C. Löhr , 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 Fe<sub>2</sub>O<sub>3</sub> 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}
Pub Date : 2024-10-21DOI: 10.1016/j.gca.2024.10.018
Z.E. Wilbur, J.J. Barnes, S.A. Eckley, T. Erickson, R.A. Zeigler, K. Domanik
Volcanic products returned from the Apollo missions over 50 years ago provide a unique perspective into the magmatic evolution of the Moon. However, questionsremain 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.
{"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}
Pub Date : 2024-10-20DOI: 10.1016/j.gca.2024.10.017
D. Gilliard, D.J. Janssen, N. Schuback, S.L. Jaccard
Over the last decades, the chromium (Cr) stable isotope system (referred to as δ53Cr) 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 δ53Cr 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−1 (n = 68) and δ53Cr 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 δ53Cr show only a weak correlation (δ53Cr vs. Ln([Cr]) R2 = 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 δ53Cr 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.
{"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}
Pub Date : 2024-10-19DOI: 10.1016/j.gca.2024.10.011
C. Michael B. Henderson
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 O2– 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, H2O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both K and L-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, XIIK2VIMg6IV[Si6Al2]O20(OH)4. 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 WO2−) 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 %; TiO2 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 Ba2+ and Ti4+ replacing K+ and Mg2+.
富含钛和钡的三八面体云母出现在碎屑基性火成岩、变质地幔橄榄岩、变质辉长岩/碳酸盐岩以及热液蚀变矿床中。电子微探针分析（EMP）将所有的铁都报告为 FeO，在 1970/80 年代被广泛用于解释 Ti 和 Ba 的置换机制，其依据是 22 O2- 单胞计算，这意味着阳离子空位出现在八面体和/或片间位点。1996 年，通过对铁和总铁、H2O、（OH）的化学和物理分析，以及特定元素铁的 X 射线吸收光谱（K 边和 L 边），EMP 确定了铁和钛的价态以及阳离子位点占有率，确定了 ∼ 50 % 的 O 取代（OH）分子，并确定了 24 个阴离子晶胞公式显示不存在阳离子空位。这里改进了辉石云母的晶胞公式计算规程，并根据无空位辉石的化学计量公式 XIIK2VIMg6IV[Si6Al2]O20(OH)4 对结果进行了测试。 根据与特定混合价元素置换相关的固定空位含量计算出的假想硅酸盐片状成分证实，天然云母固体的可靠晶胞公式要求必须考虑到每个化学计量空位。如果将可靠的 "过量 O"（表示为 WO2-）估计值分配给 EMP 分析，就可以确定云母固体溶液中氧云母成分的比例。我们使用已发表的分析结果对这种方法进行了测试，这些分析结果来自分馏的碱性和超基性火山火成岩（氧云母范围为 2.5-45%；TiO2 高达 14%；BaO 高达 23%）、上地幔橄榄岩（等值为 7-18%；6%；0.7%）和变质上地幔（2-37%；9%；23%）中富含钛和钡的生物岩。云母中Ti和Ba的富集显然与中和取代K+和Mg2+的高电荷Ba2+和Ti4+所需的额外氧电荷有关。
{"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}
Pub Date : 2024-10-19DOI: 10.1016/j.gca.2024.10.016
Cassidy J. Stegner, Richard M. Gaschnig, Samuel Marshall, Shelby T. Rader, Gray E. Bebout, Sarah C. Penniston-Dorland
The molybdenum (Mo) isotope composition (defined as δ98Mo 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.
{"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}
Pub Date : 2024-10-19DOI: 10.1016/j.gca.2024.10.013
Paul Vosteen , Michael Kossack , Christoph Vogt , Chloe H. Andersen , Sonja Geilert , Matthias Zabel , Florian Scholz
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.
We investigated sedimentary Fe cycling along two transects across the Namibian mud belt with variable terrigenous sedimentation (23°S < 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.
{"title":"Iron redox shuttling and uptake by silicate minerals on the Namibian mud belt","authors":"Paul Vosteen , Michael Kossack , Christoph Vogt , Chloe H. Andersen , Sonja Geilert , Matthias Zabel , 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 < 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<sup>−2</sup> d<sup>−1</sup> 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}
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