Pub Date : 2024-09-12DOI: 10.1016/j.gca.2024.09.005
Maria P. Asta , Sarah Bonilla-Correa , Aurélie Pace , Martin Dietzel , Antonio García-Alix , Torsten Vennemann , Anders Meibom , Arthur Adams
<div><div>Crystalline calcium carbonate isotope compositions have been widely used to reconstruct past environments. However, if their isotopic compositions are modified because of crystallization from an amorphous precursor, their reliability as paleo-geochemical proxies can be compromised. This study explored the changes in the oxygen isotope compositions during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate under different conditions of relative humidity (RH of 33–95 %), temperature (T of 5 °C and 20 °C) and in the presence/absence of atmospheric CO<sub>2</sub>. The data showed that at low RH and T (e.g., RH ≤ 45 % and 5 °C) when a complete ACC-crystalline carbonate transformation did not take place then the original ACC δ<sup>18</sup>O values (δ<sup>18</sup>O<sub>CaCO3</sub> = −15.9 ± 1.0 ‰, VPDB) were preserved throughout the experimental runtime (up to 144 days). In contrast, in fully crystallized CaCO<sub>3</sub> (e.g., at RH ≥ 60 %) the δ<sup>18</sup>O<sub>CaCO3</sub> values increased rapidly over the first few days, followed by a slower and gradual increase. By the end of the experiments (i.e., after 103–144 days) the crystalline δ<sup>18</sup>O<sub>CaCO3</sub> values ranged from −10.4 ‰ to −8.1 ‰ in the presence of atmospheric CO<sub>2</sub> and from −12.6 ‰ to −9.5 ‰ in the CO<sub>2</sub>-free experiments. These changes in oxygen isotope compositions of the CaCO<sub>3</sub> reaction products (calcite and/or vaterite) were mainly driven by exchange with H<sub>2</sub>O from the hydrated ACC i.e. the synthesis fluid. In CO<sub>2</sub>-present experiments, oxygen isotope fractionation factors between the CaCO<sub>3</sub> reaction products and the synthesis fluid (<sup>18</sup>α<sub>c–w</sub>) approached or exceeded oxygen isotope equilibrium values. This could be explained by a decrease in the initially high pH of the aqueous fluid released from ACC dissolution during CO<sub>2</sub> hydration/hydroxylation, which would have increased the oxygen isotope exchange kinetics between H<sub>2</sub>O and dissolved inorganic carbon (DIC). In some experiments, the hydration/hydroxylation of <sup>18</sup>O-enriched CO<sub>2</sub>, due to isotopic salt-effects, might have also resulted in <sup>18</sup>O-enriched calcium carbonates and calculated fractionation factors that exceeded equilibrium values. In the CO<sub>2</sub>-free experiments, isotopic equilibrium between the crystalline phase and the synthesis fluid was not reached. This oxygen isotope disequilibrium suggests that without the pH lowering effect of the hydroxylation/hydration of CO<sub>2</sub>, the CO<sub>3</sub><sup>2−</sup> released during ACC/calcite dissolution-reprecipitation may have not isotopically equilibrated with the high pH synthesis fluid due to the long equilibration times required to reach isotope equilibrium at high pH values, leading to the self-buffering of δ<sup>18</sup>O<sub>CaCO3</sub> values. The results suggest that the oxygen iso
{"title":"Oxygen isotope fractionation during amorphous to crystalline calcium carbonate transformation at varying relative humidity and temperature","authors":"Maria P. Asta , Sarah Bonilla-Correa , Aurélie Pace , Martin Dietzel , Antonio García-Alix , Torsten Vennemann , Anders Meibom , Arthur Adams","doi":"10.1016/j.gca.2024.09.005","DOIUrl":"10.1016/j.gca.2024.09.005","url":null,"abstract":"<div><div>Crystalline calcium carbonate isotope compositions have been widely used to reconstruct past environments. However, if their isotopic compositions are modified because of crystallization from an amorphous precursor, their reliability as paleo-geochemical proxies can be compromised. This study explored the changes in the oxygen isotope compositions during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate under different conditions of relative humidity (RH of 33–95 %), temperature (T of 5 °C and 20 °C) and in the presence/absence of atmospheric CO<sub>2</sub>. The data showed that at low RH and T (e.g., RH ≤ 45 % and 5 °C) when a complete ACC-crystalline carbonate transformation did not take place then the original ACC δ<sup>18</sup>O values (δ<sup>18</sup>O<sub>CaCO3</sub> = −15.9 ± 1.0 ‰, VPDB) were preserved throughout the experimental runtime (up to 144 days). In contrast, in fully crystallized CaCO<sub>3</sub> (e.g., at RH ≥ 60 %) the δ<sup>18</sup>O<sub>CaCO3</sub> values increased rapidly over the first few days, followed by a slower and gradual increase. By the end of the experiments (i.e., after 103–144 days) the crystalline δ<sup>18</sup>O<sub>CaCO3</sub> values ranged from −10.4 ‰ to −8.1 ‰ in the presence of atmospheric CO<sub>2</sub> and from −12.6 ‰ to −9.5 ‰ in the CO<sub>2</sub>-free experiments. These changes in oxygen isotope compositions of the CaCO<sub>3</sub> reaction products (calcite and/or vaterite) were mainly driven by exchange with H<sub>2</sub>O from the hydrated ACC i.e. the synthesis fluid. In CO<sub>2</sub>-present experiments, oxygen isotope fractionation factors between the CaCO<sub>3</sub> reaction products and the synthesis fluid (<sup>18</sup>α<sub>c–w</sub>) approached or exceeded oxygen isotope equilibrium values. This could be explained by a decrease in the initially high pH of the aqueous fluid released from ACC dissolution during CO<sub>2</sub> hydration/hydroxylation, which would have increased the oxygen isotope exchange kinetics between H<sub>2</sub>O and dissolved inorganic carbon (DIC). In some experiments, the hydration/hydroxylation of <sup>18</sup>O-enriched CO<sub>2</sub>, due to isotopic salt-effects, might have also resulted in <sup>18</sup>O-enriched calcium carbonates and calculated fractionation factors that exceeded equilibrium values. In the CO<sub>2</sub>-free experiments, isotopic equilibrium between the crystalline phase and the synthesis fluid was not reached. This oxygen isotope disequilibrium suggests that without the pH lowering effect of the hydroxylation/hydration of CO<sub>2</sub>, the CO<sub>3</sub><sup>2−</sup> released during ACC/calcite dissolution-reprecipitation may have not isotopically equilibrated with the high pH synthesis fluid due to the long equilibration times required to reach isotope equilibrium at high pH values, leading to the self-buffering of δ<sup>18</sup>O<sub>CaCO3</sub> values. The results suggest that the oxygen iso","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 96-109"},"PeriodicalIF":4.5,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142321642","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 : 2024-09-11DOI: 10.1016/j.gca.2024.09.010
Zhongxing Sun , Mingdi Gao , Xiaolin Xiong
<div><div>Zn/Fe<sup>T</sup> (Fe<sup>T</sup>=Fe<sup>2+</sup> + Fe<sup>3+</sup>) ratios in primitive melts have been proposed as a redox proxy to assess the redox states of the upper mantle. However, to effectively use the melt Zn/Fe<sup>T</sup> ratio as a redox proxy, it is necessary to compare variations of melt Zn/Fe<sup>T</sup> ratios induced by changes in oxygen fugacity (<em>f</em>O<sub>2</sub>) with variations due to changes in Zn-Fe contents and mineralogy of the sources. Here we show that the melt Zn/Fe<sup>T</sup> ratio variation caused by <em>f</em>O<sub>2</sub> change can be expressed as Δ(Zn<sub>m</sub>/<span><math><msubsup><mtext>Fe</mtext><mrow><mtext>m</mtext></mrow><mtext>T</mtext></msubsup></math></span>) = (Zn<sub>per</sub>/<span><math><msubsup><mtext>Fe</mtext><mrow><mtext>per</mtext></mrow><mtext>T</mtext></msubsup></math></span>)* <span><math><mrow><mi>Δ</mi><mo>(</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msubsup><mo>/</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mi>T</mi></msubsup><mo>)</mo></mrow></math></span>/(<span><math><msubsup><mtext>D</mtext><mrow><mtext>Zn</mtext></mrow><mtext>per/m</mtext></msubsup></math></span>/<span><math><msubsup><mtext>D</mtext><mrow><msup><mrow><mtext>Fe</mtext></mrow><mtext>2+</mtext></msup></mrow><mtext>per/m</mtext></msubsup></math></span>). Zn/Fe<sup>T</sup> ratios in most arc and MORB peridotites (Zn<sub>per</sub>/<span><math><msubsup><mtext>Fe</mtext><mrow><mtext>per</mtext></mrow><mtext>T</mtext></msubsup></math></span>) are 9.0 ± 1.0*10<sup>−4</sup>, and melt Fe<sup>2+</sup>/Fe<sup>T</sup> ratio variation resulted from <em>f</em>O<sub>2</sub> change [<span><math><mrow><mi>Δ</mi><mo>(</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msubsup><mo>/</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mi>T</mi></msubsup><mo>)</mo></mrow></math></span>] can be easily obtained by the existing model. Hence, if the Zn and Fe<sup>2+</sup> partition coefficients (<span><math><msubsup><mtext>D</mtext><mrow><mtext>Zn</mtext></mrow><mtext>per/m</mtext></msubsup></math></span> and <span><math><msubsup><mtext>D</mtext><mrow><msup><mrow><mtext>Fe</mtext></mrow><mtext>2+</mtext></msup></mrow><mtext>per/m</mtext></msubsup></math></span>) between melt and peridotite are known, the melt Zn/Fe<sup>T</sup> ratio variation resulting from <em>f</em>O<sub>2</sub> change can be estimated. In this study, we determined D<sub>Zn</sub> between olivine, orthopyroxene, clinopyroxene and basaltic melts at 0.75–2.5 GPa and 1250–1450 ℃. Our data show that melt composition (expressed as MgO content) dominantly controls the mineral-melt Zn partitioning under peridotite melting conditions. These data, along with published mineral-melt <span><math><msub><mtext>D</mtext><msup><mrow><mtext>Fe</mtext></mrow><mtext>2+</mtext></msup></msub></math></span> data, enable us to
{"title":"Zinc partitioning between mantle minerals and basaltic melts: Application to revisit the Zn/FeT redox proxy","authors":"Zhongxing Sun , Mingdi Gao , Xiaolin Xiong","doi":"10.1016/j.gca.2024.09.010","DOIUrl":"10.1016/j.gca.2024.09.010","url":null,"abstract":"<div><div>Zn/Fe<sup>T</sup> (Fe<sup>T</sup>=Fe<sup>2+</sup> + Fe<sup>3+</sup>) ratios in primitive melts have been proposed as a redox proxy to assess the redox states of the upper mantle. However, to effectively use the melt Zn/Fe<sup>T</sup> ratio as a redox proxy, it is necessary to compare variations of melt Zn/Fe<sup>T</sup> ratios induced by changes in oxygen fugacity (<em>f</em>O<sub>2</sub>) with variations due to changes in Zn-Fe contents and mineralogy of the sources. Here we show that the melt Zn/Fe<sup>T</sup> ratio variation caused by <em>f</em>O<sub>2</sub> change can be expressed as Δ(Zn<sub>m</sub>/<span><math><msubsup><mtext>Fe</mtext><mrow><mtext>m</mtext></mrow><mtext>T</mtext></msubsup></math></span>) = (Zn<sub>per</sub>/<span><math><msubsup><mtext>Fe</mtext><mrow><mtext>per</mtext></mrow><mtext>T</mtext></msubsup></math></span>)* <span><math><mrow><mi>Δ</mi><mo>(</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msubsup><mo>/</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mi>T</mi></msubsup><mo>)</mo></mrow></math></span>/(<span><math><msubsup><mtext>D</mtext><mrow><mtext>Zn</mtext></mrow><mtext>per/m</mtext></msubsup></math></span>/<span><math><msubsup><mtext>D</mtext><mrow><msup><mrow><mtext>Fe</mtext></mrow><mtext>2+</mtext></msup></mrow><mtext>per/m</mtext></msubsup></math></span>). Zn/Fe<sup>T</sup> ratios in most arc and MORB peridotites (Zn<sub>per</sub>/<span><math><msubsup><mtext>Fe</mtext><mrow><mtext>per</mtext></mrow><mtext>T</mtext></msubsup></math></span>) are 9.0 ± 1.0*10<sup>−4</sup>, and melt Fe<sup>2+</sup>/Fe<sup>T</sup> ratio variation resulted from <em>f</em>O<sub>2</sub> change [<span><math><mrow><mi>Δ</mi><mo>(</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msubsup><mo>/</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mi>m</mi></mrow><mi>T</mi></msubsup><mo>)</mo></mrow></math></span>] can be easily obtained by the existing model. Hence, if the Zn and Fe<sup>2+</sup> partition coefficients (<span><math><msubsup><mtext>D</mtext><mrow><mtext>Zn</mtext></mrow><mtext>per/m</mtext></msubsup></math></span> and <span><math><msubsup><mtext>D</mtext><mrow><msup><mrow><mtext>Fe</mtext></mrow><mtext>2+</mtext></msup></mrow><mtext>per/m</mtext></msubsup></math></span>) between melt and peridotite are known, the melt Zn/Fe<sup>T</sup> ratio variation resulting from <em>f</em>O<sub>2</sub> change can be estimated. In this study, we determined D<sub>Zn</sub> between olivine, orthopyroxene, clinopyroxene and basaltic melts at 0.75–2.5 GPa and 1250–1450 ℃. Our data show that melt composition (expressed as MgO content) dominantly controls the mineral-melt Zn partitioning under peridotite melting conditions. These data, along with published mineral-melt <span><math><msub><mtext>D</mtext><msup><mrow><mtext>Fe</mtext></mrow><mtext>2+</mtext></msup></msub></math></span> data, enable us to","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"385 ","pages":"Pages 141-155"},"PeriodicalIF":4.5,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142321785","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 : 2024-09-10DOI: 10.1016/j.gca.2024.09.006
Silvio Mollo , Fabrizio Di Fiore , Alice MacDonald , Teresa Ubide , Alessio Pontesilli , Gabriele Giuliani , Alessandro Vona , Claudia Romano , Piergiorgio Scarlato
Most of the solidification history of magmas beneath active volcanoes takes place in chemically and physically perturbed plumbing systems where the growth of crystals is collectively governed by a range of kinetic processes related to the dynamics of crustal reservoirs and eruptive conduits. In this context, we have experimentally investigated the partitioning of major, minor, and trace cations between plagioclase and trachybasaltic melt under conventional static (no physical perturbation) and dynamic (melt stirring) crystallization regimes. Slow interface reaction kinetics are established between the advancing crystal surface and the adjacent melt, as the result of the combined control of a small degree of effective undercooling, prolonged diffusive relaxation, and convective homogenization. The kinetic aspects of plagioclase growth influence the partitioning of trace cations during transport of structural units across the crystal-melt interface, with consequent departure from macroscopic equilibrium in the system. The type and number of charge–balanced and −imbalanced configurations produced by the accommodation of trace cations into the coordination polyhedron can be thermodynamically rationalized in terms of lattice strain and electrostatic partitioning energetics. However, the overall solution energy accompanying trace cation kinetic substitutions cannot be entirely deconvoluted from major component activities in both melt and plagioclase phases. The emerging view that slow interface kinetic processes may lead to strong compositional dependence for the partition coefficient in dynamic subvolcanic environments contrasts markedly with the conventional idea that the energetics of cation partitioning are dominantly controlled by the effect of isothermal changes in the bulk system.
{"title":"Thermodynamics and kinetics of cation partitioning between plagioclase and trachybasaltic melt in static and dynamic systems: A reassessment of the lattice strain and electrostatic energies of substitution","authors":"Silvio Mollo , Fabrizio Di Fiore , Alice MacDonald , Teresa Ubide , Alessio Pontesilli , Gabriele Giuliani , Alessandro Vona , Claudia Romano , Piergiorgio Scarlato","doi":"10.1016/j.gca.2024.09.006","DOIUrl":"10.1016/j.gca.2024.09.006","url":null,"abstract":"<div><p>Most of the solidification history of magmas beneath active volcanoes takes place in chemically and physically perturbed plumbing systems where the growth of crystals is collectively governed by a range of kinetic processes related to the dynamics of crustal reservoirs and eruptive conduits. In this context, we have experimentally investigated the partitioning of major, minor, and trace cations between plagioclase and trachybasaltic melt under conventional static (no physical perturbation) and dynamic (melt stirring) crystallization regimes. Slow interface reaction kinetics are established between the advancing crystal surface and the adjacent melt, as the result of the combined control of a small degree of effective undercooling, prolonged diffusive relaxation, and convective homogenization. The kinetic aspects of plagioclase growth influence the partitioning of trace cations during transport of structural units across the crystal-melt interface, with consequent departure from macroscopic equilibrium in the system. The type and number of charge–balanced and −imbalanced configurations produced by the accommodation of trace cations into the coordination polyhedron can be thermodynamically rationalized in terms of lattice strain and electrostatic partitioning energetics. However, the overall solution energy accompanying trace cation kinetic substitutions cannot be entirely deconvoluted from major component activities in both melt and plagioclase phases. The emerging view that slow interface kinetic processes may lead to strong compositional dependence for the partition coefficient in dynamic subvolcanic environments contrasts markedly with the conventional idea that the energetics of cation partitioning are dominantly controlled by the effect of isothermal changes in the bulk system.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"384 ","pages":"Pages 27-43"},"PeriodicalIF":4.5,"publicationDate":"2024-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.sciencedirect.com/science/article/pii/S0016703724004642/pdfft?md5=8c47cdf9d3ca6a5258b0773e9473f477&pid=1-s2.0-S0016703724004642-main.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142171551","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 : 2024-09-10DOI: 10.1016/j.gca.2024.09.007
Harry-Luke Oliver McClelland, Renee B.Y. Lee, Ann Pearson, Rosalind E.M. Rickaby
The geochemical characterization of phytoplankton-derived organic compounds found in marine sediments has been widely used to reconstruct atmospheric pCO2 throughout the Cenozoic. This is possible owing to a well-established relationship between the carbon isotope ratios of phytoplankton biomass and CO2 concentration in the ambient seawater. An ideal molecular target for such proxy reconstructions would be degradation resistant on geologic timescales and unambiguously associated with known, experimentally tractable, organisms, so that species-specific models can be developed, calibrated, and applied to appropriate material. However, existing organic matter targets do not meet these criteria, primarily owing to ambiguity in the source species of recalcitrant compounds in deep time. Here we explore the potential of a novel organic carbon target for isotopic analysis: acidic polysaccharides extracted from the calcite plates (coccoliths) that are produced by all calcifying haptophytes. Carbohydrates are usually rapidly remineralized in sediments, but coccolith-associated polysaccharides (CAPs) are mechanically protected from diagenesis within the coccolith calcite lattice. Coccoliths can be taxonomically separated by size and identified, often to species level, prior to CAP extraction, providing a species-specific record. Coccolith morphology and composition are important additional sources of information, which are then unambiguously associated with the extracted CAPs. We found that carbon isotope ratios of CAPs changed in response to the environmental changes associated with a glacial cycle, which we attribute to temperature-driven changes in average growth rate. Once the underlying biosynthetic processes and the associated isotope effects are better understood, this archive of pristine organic matter has the potential to provide insight into phytoplankton growth rates and atmospheric pCO2 far beyond the Cenozoic, to when the first coccolithophores inhabited the surface ocean over 200 million years ago.
海洋沉积物中浮游植物产生的有机化合物的地球化学特征已被广泛用于重建整个新生代的大气 pCO2。之所以能够做到这一点，是因为浮游植物生物量的碳同位素比值与环境海水中的二氧化碳浓度之间存在着公认的关系。这种代用重构的理想分子目标应在地质时间尺度上具有抗降解性，并与已知的、可进行实验的生物体明确相关，从而可以开发、校准特定物种模型，并将其应用于适当的材料。然而，现有的有机物目标并不符合这些标准，主要原因是深部时间中难降解化合物的来源物种不明确。在此，我们探索了一种新型有机碳目标的同位素分析潜力：从所有钙化七叶树产生的方解石板（茧石）中提取的酸性多糖。碳水化合物通常会在沉积物中迅速再矿化，但茧石相关多糖（CAPs）在茧石方解石晶格内受到机械保护，不会被成岩作用所破坏。在提取 CAP 之前，可按大小对球果石进行分类和鉴定，通常可鉴定到物种级别，从而提供物种特异性记录。钙钛矿的形态和组成是重要的附加信息来源，可与提取的钙钛矿明确关联。我们发现，CAP 的碳同位素比值随冰川周期相关的环境变化而变化，我们将其归因于温度驱动的平均生长率变化。一旦对基本的生物合成过程和相关的同位素效应有了更好的了解，这一原始有机物档案就有可能让人们深入了解浮游植物的生长速率和大气中的 pCO2，甚至远至新生代，即 2 亿多年前第一批茧石藻类栖息在表层海洋时的情况。
{"title":"Stable carbon isotope ratios of pristine carbohydrates preserved within nannofossil calcite","authors":"Harry-Luke Oliver McClelland, Renee B.Y. Lee, Ann Pearson, Rosalind E.M. Rickaby","doi":"10.1016/j.gca.2024.09.007","DOIUrl":"https://doi.org/10.1016/j.gca.2024.09.007","url":null,"abstract":"The geochemical characterization of phytoplankton-derived organic compounds found in marine sediments has been widely used to reconstruct atmospheric pCO<mml:math altimg=\"si20.svg\" display=\"inline\"><mml:msub><mml:mrow></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math> throughout the Cenozoic. This is possible owing to a well-established relationship between the carbon isotope ratios of phytoplankton biomass and CO<mml:math altimg=\"si20.svg\" display=\"inline\"><mml:msub><mml:mrow></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math> concentration in the ambient seawater. An ideal molecular target for such proxy reconstructions would be degradation resistant on geologic timescales and unambiguously associated with known, experimentally tractable, organisms, so that species-specific models can be developed, calibrated, and applied to appropriate material. However, existing organic matter targets do not meet these criteria, primarily owing to ambiguity in the source species of recalcitrant compounds in deep time. Here we explore the potential of a novel organic carbon target for isotopic analysis: acidic polysaccharides extracted from the calcite plates (coccoliths) that are produced by all calcifying haptophytes. Carbohydrates are usually rapidly remineralized in sediments, but coccolith-associated polysaccharides (CAPs) are mechanically protected from diagenesis within the coccolith calcite lattice. Coccoliths can be taxonomically separated by size and identified, often to species level, prior to CAP extraction, providing a species-specific record. Coccolith morphology and composition are important additional sources of information, which are then unambiguously associated with the extracted CAPs. We found that carbon isotope ratios of CAPs changed in response to the environmental changes associated with a glacial cycle, which we attribute to temperature-driven changes in average growth rate. Once the underlying biosynthetic processes and the associated isotope effects are better understood, this archive of pristine organic matter has the potential to provide insight into phytoplankton growth rates and atmospheric pCO<mml:math altimg=\"si20.svg\" display=\"inline\"><mml:msub><mml:mrow></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math> far beyond the Cenozoic, to when the first coccolithophores inhabited the surface ocean over 200 million years ago.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"64 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142245681","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 : 2024-09-07DOI: 10.1016/j.gca.2024.09.009
S.A. Gibson , J.C. Crosby , J.A.F. Day , F.M. Stuart , L. DiNicola , T.R. Riley
<div><p>Helium isotopes are unrivalled tracers of the origins of melts in the Earth’s convecting mantle but their role in determining melt contributions from the shallower and rigid lithospheric mantle is more ambiguous. We have acquired new <sup>3</sup>He/<sup>4</sup>He data for olivine and pyroxene separates from 47 well-characterised mantle xenoliths from global on- and off-craton settings. When combined with existing data they demonstrate a new systematic relationship between fluid-hosted <sup>3</sup>He/<sup>4</sup>He and major and trace element composition of host minerals and whole rock. We show that a significant proportion (>70 %) of mantle peridotites from continental off-craton settings with depleted major element compositions (e.g., olivine Mg# ≥ 89.5) have <sup>3</sup>He/<sup>4</sup>He in the range of modern-day mid-ocean ridge basalt (MORB) source mantle (7–9 R<sub>a</sub>) and we propose that they represent underplated melt residues, which initially formed in the convecting upper mantle. Furthermore, we observe that off-craton mantle xenoliths with signatures often attributed to enrichment by melts or fluids from ‘ancient’ subducted oceanic lithosphere have lower <sup>3</sup>He/<sup>4</sup>He (<7 R<sub>a</sub>). Modest correlations between <sup>3</sup>He/<sup>4</sup>He and whole rock incompatible trace element signatures commonly used as proxies for metasomatism by small-fraction carbonatite and silicate melts or C-O-H fluids characterise lithospheric mantle with <sup>3</sup>He/<sup>4</sup>He ranging from 5 to 8 R<sub>a</sub>.</p><p>Using a numerical model that integrates temperature-dependent melt extraction from the upper mantle with <em>in-situ</em> radiogenic ingrowth of <sup>4</sup>He in the continental mantle we show that the initial <sup>3</sup>He/<sup>4</sup>He of continental lithosphere mantle has decreased over time. This is consistent with previous observations demonstrating that ancient (2.5–3.5 Ga) cratonic mantle has a depleted mineral chemistry (e.g., olivine Mg# = 91–94) and low <sup>3</sup>He/<sup>4</sup>He (0.5–6.7 R<sub>a</sub>), while continental off-craton mantle (<2.5 Ga) is more fertile (olivine Mg# = 88–92) and has less radiogenic <sup>3</sup>He/<sup>4</sup>He (4–8.8 R<sub>a</sub>). This relationship defines a ‘global lithospheric mantle array’ for intraplate peridotites on plots of <sup>3</sup>He/<sup>4</sup>He vs olivine Mg#. Peridotites influenced by past and present subduction fluids, including those that contain amphibole, plot off this array. Our findings have broad implications for the <sup>3</sup>He/<sup>4</sup>He signatures observed in continental magmas. Many of Earth’s deepest melts, i.e. proto-kimberlites, are characterised by relatively low <sup>3</sup>He/<sup>4</sup>He. We attribute this to assimilation and incorporation of low <sup>3</sup>He/<sup>4</sup>He cratonic mantle material during ascent of carbonate-rich melts through thick lithosphere, which overprints the original signatures.
{"title":"Systematic behaviour of 3He/4He in Earth’s continental mantle","authors":"S.A. Gibson , J.C. Crosby , J.A.F. Day , F.M. Stuart , L. DiNicola , T.R. Riley","doi":"10.1016/j.gca.2024.09.009","DOIUrl":"10.1016/j.gca.2024.09.009","url":null,"abstract":"<div><p>Helium isotopes are unrivalled tracers of the origins of melts in the Earth’s convecting mantle but their role in determining melt contributions from the shallower and rigid lithospheric mantle is more ambiguous. We have acquired new <sup>3</sup>He/<sup>4</sup>He data for olivine and pyroxene separates from 47 well-characterised mantle xenoliths from global on- and off-craton settings. When combined with existing data they demonstrate a new systematic relationship between fluid-hosted <sup>3</sup>He/<sup>4</sup>He and major and trace element composition of host minerals and whole rock. We show that a significant proportion (>70 %) of mantle peridotites from continental off-craton settings with depleted major element compositions (e.g., olivine Mg# ≥ 89.5) have <sup>3</sup>He/<sup>4</sup>He in the range of modern-day mid-ocean ridge basalt (MORB) source mantle (7–9 R<sub>a</sub>) and we propose that they represent underplated melt residues, which initially formed in the convecting upper mantle. Furthermore, we observe that off-craton mantle xenoliths with signatures often attributed to enrichment by melts or fluids from ‘ancient’ subducted oceanic lithosphere have lower <sup>3</sup>He/<sup>4</sup>He (<7 R<sub>a</sub>). Modest correlations between <sup>3</sup>He/<sup>4</sup>He and whole rock incompatible trace element signatures commonly used as proxies for metasomatism by small-fraction carbonatite and silicate melts or C-O-H fluids characterise lithospheric mantle with <sup>3</sup>He/<sup>4</sup>He ranging from 5 to 8 R<sub>a</sub>.</p><p>Using a numerical model that integrates temperature-dependent melt extraction from the upper mantle with <em>in-situ</em> radiogenic ingrowth of <sup>4</sup>He in the continental mantle we show that the initial <sup>3</sup>He/<sup>4</sup>He of continental lithosphere mantle has decreased over time. This is consistent with previous observations demonstrating that ancient (2.5–3.5 Ga) cratonic mantle has a depleted mineral chemistry (e.g., olivine Mg# = 91–94) and low <sup>3</sup>He/<sup>4</sup>He (0.5–6.7 R<sub>a</sub>), while continental off-craton mantle (<2.5 Ga) is more fertile (olivine Mg# = 88–92) and has less radiogenic <sup>3</sup>He/<sup>4</sup>He (4–8.8 R<sub>a</sub>). This relationship defines a ‘global lithospheric mantle array’ for intraplate peridotites on plots of <sup>3</sup>He/<sup>4</sup>He vs olivine Mg#. Peridotites influenced by past and present subduction fluids, including those that contain amphibole, plot off this array. Our findings have broad implications for the <sup>3</sup>He/<sup>4</sup>He signatures observed in continental magmas. Many of Earth’s deepest melts, i.e. proto-kimberlites, are characterised by relatively low <sup>3</sup>He/<sup>4</sup>He. We attribute this to assimilation and incorporation of low <sup>3</sup>He/<sup>4</sup>He cratonic mantle material during ascent of carbonate-rich melts through thick lithosphere, which overprints the original signatures.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"384 ","pages":"Pages 44-64"},"PeriodicalIF":4.5,"publicationDate":"2024-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.sciencedirect.com/science/article/pii/S0016703724004666/pdfft?md5=63a27091b2add4d3117712e8de19cd91&pid=1-s2.0-S0016703724004666-main.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142228649","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 : 2024-09-07DOI: 10.1016/j.gca.2024.09.008
Gábor Újvári , Ramona Schneider , Thomas Stevens , László Rinyu , Gabriella Ilona Kiss , Jan-Pieter Buylaert , Andrew Sean Murray , Amélie J.M. Challier , Redzhep Kurbanov , Farhad Khormali , Judit Benedek , Marjan Temovski , Danny Vargas , László Palcsu
<div><div>Pleistocene loess records of the Khovaling Loess Plateau (KLP) in Tajikistan provide rich collections of lithic artifacts demonstrating past human presence in the region. To understand the timing of human activity and environmental conditions prevailing at that time U–Th dating and clumped/stable C/O isotope measurements have been applied to modern and Pleistocene soil carbonates (SCs) collected at several sites on the KLP and surroundings. U–Th ages were corrected by two methods: 1) assuming an initial [<sup>230</sup>Th/<sup>232</sup>Th] activity ratio of 0.85 ± 0.25 based on gamma spectrometry of loess/paleosol samples, and 2) the isochron technique using leachates and fully dissolved subsamples. Diagenetic alteration and potential U/Th mobilization and related isotope fractionation due to alpha-recoil was also modelled and found to be minor in the studied soil carbonates. Compared to model ages as references, uncorrected <sup>230</sup>Th ages are only acceptable if measured [<sup>230</sup>Th/<sup>232</sup>Th] activity ratios of leachates are high (>30), while <sup>230</sup>Th ages derived using method 1 are mostly overcorrected. It appears that SCs can be reliably dated by the U-series disequilibrium method in this sedimentary setting, but isochron dating cannot be spared. Application of the isochron method is required to derive <sup>230</sup>Th<sub>model</sub> ages, which ensures that the non-zero initial <sup>230</sup>Th and possible U–Th gain/loss due to alpha-recoil can be simultaneously corrected and reliable U–Th ages obtained. U–Th ages of Pleistocene SCs clearly demonstrate post-pedogenic ingrowth of multiple, non-contemporaneous populations of SCs within loess/paleosol units, and that SC formation happened in many cases under cold, presumably dry glacial climate conditions. Considering that U–Th ages of SCs provide minimum ages of the sediment in which they form, these ages can be useful in developing loess stratigraphic models and for correlation of paleosols with marine isotope stages. This implies that the age of a given paleosol and any lithic artifacts it may contain, indicating human activity, cannot be younger than the age of SCs formed in that paleosol. This is due to the nature of soil carbonates, which can be the product of both syn- and post-depositional processes.</div><div>Clumped isotope thermometry of SCs collected from modern soils at three sites in Tajikistan provide evidence for SCs dominantly recording summer season soil temperatures, while the calculated soil water oxygen isotope signatures reflect annual signals and carbonate precipitation from source waters incorporating rainfall from prior to and during SC formation. In contrast, some Pleistocene SCs record soil temperatures and stable isotope compositions more appropriate to glacial conditions, confirming the findings of U–Th ages, and highlighting the primary role of aridity-driven soil moisture changes in SC precipitation in this setting. Conside
{"title":"Absolute 230Th/U chronologies and Δ47 thermometry paleoclimate reconstruction from soil carbonates in Central Asian loess over the past 1 million years","authors":"Gábor Újvári , Ramona Schneider , Thomas Stevens , László Rinyu , Gabriella Ilona Kiss , Jan-Pieter Buylaert , Andrew Sean Murray , Amélie J.M. Challier , Redzhep Kurbanov , Farhad Khormali , Judit Benedek , Marjan Temovski , Danny Vargas , László Palcsu","doi":"10.1016/j.gca.2024.09.008","DOIUrl":"10.1016/j.gca.2024.09.008","url":null,"abstract":"<div><div>Pleistocene loess records of the Khovaling Loess Plateau (KLP) in Tajikistan provide rich collections of lithic artifacts demonstrating past human presence in the region. To understand the timing of human activity and environmental conditions prevailing at that time U–Th dating and clumped/stable C/O isotope measurements have been applied to modern and Pleistocene soil carbonates (SCs) collected at several sites on the KLP and surroundings. U–Th ages were corrected by two methods: 1) assuming an initial [<sup>230</sup>Th/<sup>232</sup>Th] activity ratio of 0.85 ± 0.25 based on gamma spectrometry of loess/paleosol samples, and 2) the isochron technique using leachates and fully dissolved subsamples. Diagenetic alteration and potential U/Th mobilization and related isotope fractionation due to alpha-recoil was also modelled and found to be minor in the studied soil carbonates. Compared to model ages as references, uncorrected <sup>230</sup>Th ages are only acceptable if measured [<sup>230</sup>Th/<sup>232</sup>Th] activity ratios of leachates are high (>30), while <sup>230</sup>Th ages derived using method 1 are mostly overcorrected. It appears that SCs can be reliably dated by the U-series disequilibrium method in this sedimentary setting, but isochron dating cannot be spared. Application of the isochron method is required to derive <sup>230</sup>Th<sub>model</sub> ages, which ensures that the non-zero initial <sup>230</sup>Th and possible U–Th gain/loss due to alpha-recoil can be simultaneously corrected and reliable U–Th ages obtained. U–Th ages of Pleistocene SCs clearly demonstrate post-pedogenic ingrowth of multiple, non-contemporaneous populations of SCs within loess/paleosol units, and that SC formation happened in many cases under cold, presumably dry glacial climate conditions. Considering that U–Th ages of SCs provide minimum ages of the sediment in which they form, these ages can be useful in developing loess stratigraphic models and for correlation of paleosols with marine isotope stages. This implies that the age of a given paleosol and any lithic artifacts it may contain, indicating human activity, cannot be younger than the age of SCs formed in that paleosol. This is due to the nature of soil carbonates, which can be the product of both syn- and post-depositional processes.</div><div>Clumped isotope thermometry of SCs collected from modern soils at three sites in Tajikistan provide evidence for SCs dominantly recording summer season soil temperatures, while the calculated soil water oxygen isotope signatures reflect annual signals and carbonate precipitation from source waters incorporating rainfall from prior to and during SC formation. In contrast, some Pleistocene SCs record soil temperatures and stable isotope compositions more appropriate to glacial conditions, confirming the findings of U–Th ages, and highlighting the primary role of aridity-driven soil moisture changes in SC precipitation in this setting. Conside","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 110-126"},"PeriodicalIF":4.5,"publicationDate":"2024-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142245682","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 : 2024-09-07DOI: 10.1016/j.gca.2024.09.004
Hai Hoang, Khac Hieu Ho, Anne Battani, James Alexander Scott, Julien Collell, Magali Pujol, Guillaume Galliero
This study explores the estimation of solubility-induced elemental fractionation of noble gases in hydrocarbon-based oils through existing empirical and theoretical models, complemented by a novel molecular simulation-based approach. Quantifying such fractionation is essential for a deeper understanding of fluid processes and migration in subsurface geological resources, an area currently lacking in experimental data. To address this, the research introduces a predictive model that employs the Peng-Robinson equation of state and a fugacity-coefficient-based method to assess noble gas elemental fractionation in hydrocarbons, including normal alkanes, cycloalkanes, and aromatics. However, this model struggles with precise quantitative predictions, prompting the introduction of adjusted cross-interaction parameters to enhance its performance. Furthermore, molecular simulations, in conjunction with the refined equation of state, are shown to offer a novel method for calculating noble gas fractionation coefficients across different hydrocarbon solvents. A key finding is the identification of a universal master curve, demonstrating that noble gas solubility fractionation at low pressure in simple hydrocarbon solvents can be quantitatively determined by temperature and density alone, without the need for detailed compositional information. Consequently, a new correlation is proposed for deriving elemental fractionation coefficients based solely on oil temperature and density, offering significant improvements over existing empirical methods for a wide range of temperatures. Although limitations are noted when applying this approach to oils rich in complex heavy components like resins and asphaltenes, it allows to address inconsistencies observed in traditionally used experimental correlations at high temperatures.
{"title":"Modeling Solubility Induced Elemental Fractionation of Noble Gases in Oils","authors":"Hai Hoang, Khac Hieu Ho, Anne Battani, James Alexander Scott, Julien Collell, Magali Pujol, Guillaume Galliero","doi":"10.1016/j.gca.2024.09.004","DOIUrl":"https://doi.org/10.1016/j.gca.2024.09.004","url":null,"abstract":"This study explores the estimation of solubility-induced elemental fractionation of noble gases in hydrocarbon-based oils through existing empirical and theoretical models, complemented by a novel molecular simulation-based approach. Quantifying such fractionation is essential for a deeper understanding of fluid processes and migration in subsurface geological resources, an area currently lacking in experimental data. To address this, the research introduces a predictive model that employs the Peng-Robinson equation of state and a fugacity-coefficient-based method to assess noble gas elemental fractionation in hydrocarbons, including normal alkanes, cycloalkanes, and aromatics. However, this model struggles with precise quantitative predictions, prompting the introduction of adjusted cross-interaction parameters to enhance its performance. Furthermore, molecular simulations, in conjunction with the refined equation of state, are shown to offer a novel method for calculating noble gas fractionation coefficients across different hydrocarbon solvents. A key finding is the identification of a universal master curve, demonstrating that noble gas solubility fractionation at low pressure in simple hydrocarbon solvents can be quantitatively determined by temperature and density alone, without the need for detailed compositional information. Consequently, a new correlation is proposed for deriving elemental fractionation coefficients based solely on oil temperature and density, offering significant improvements over existing empirical methods for a wide range of temperatures. Although limitations are noted when applying this approach to oils rich in complex heavy components like resins and asphaltenes, it allows to address inconsistencies observed in traditionally used experimental correlations at high temperatures.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"26 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142321786","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 : 2024-09-04DOI: 10.1016/j.gca.2024.08.029
Anxu Sheng , Yurong Deng , Yuefei Ding , Linxin Cheng , Yuyan Liu , Xiaoxu Li , Yuji Arai , Juan Liu
Biotransformation of ferrihydrite (Fh) by dissimilatory iron-reducing bacteria (DIRB) into various secondary minerals assemblages widely occurs in anaerobic environments. While respiration-driven supply rates of Fe(II) have been proposed as a primary factor controlling kinetics and mineral products of this process, the specific mechanism by which DIRB respiration rates regulate Fh biotransformation remains elusive. Here, to minimize the complex effects of microbial cells, we conducted Fh transformation using 1 mM biogenic Fe(II) (BioFe(II)), added at different rates to mimic diverse respiration-driven supply rates of Fe(II) by DIRB. For comparison, transformation experiments with FeSO4 alone and FeSO4 plus citrate (CitFe(II)) added at the corresponding supply rates were performed to decouple the specific effects of Fe(II) addition rates and extracellular polymeric substances (EPS) associated with BioFe(II). Decreasing FeSO4 supply rates favored the transformation of Fh to lepidocrocite (Lp) over to Gt and the subsequent transformation of Lp to magnetite (Mt), altering the transformation pathway from Fh → Lp/Gt → Gt to Fh → Lp/Gt → Mt/Gt. These results underscore the significant effect of aqueous Fe(II) supply rates on the competition of olation and oxolation of labile Fe(III) intermediates into different secondary minerals. In the experiments with BioFe(II) and CitFe(II), although EPS or citrate slightly increased Fe(II) adsorption and Fe(III)labile generation, the increase in sorbed Fe(II) was minimal compared to the variations in aqueous Fe(II) concentrations caused by the different Fe(II) supply rates. At the same Fe(II) supply rates, EPS or citrate notably inhibited the transformation of Fh to Gt and the further conversion of Lp, altering the pathway from Fh → Mt/Gt/Lp to primarily Fh → Lp. These effects became more pronounced with the decrease of BioFe(II) and CitFe(II) supply rates. Our findings provide new insights into how DIRB respiration rates control kinetics, pathways, and mineral products of Fh transformation, which is crucial for elucidating the relevant biogeochemical cycling of nutrients and (im)mobilization of contaminants.
{"title":"Regulation of ferrihydrite biotransformation by Fe(II) supply rates and extracellular polymeric substances","authors":"Anxu Sheng , Yurong Deng , Yuefei Ding , Linxin Cheng , Yuyan Liu , Xiaoxu Li , Yuji Arai , Juan Liu","doi":"10.1016/j.gca.2024.08.029","DOIUrl":"10.1016/j.gca.2024.08.029","url":null,"abstract":"<div><div>Biotransformation of ferrihydrite (Fh) by dissimilatory iron-reducing bacteria (DIRB) into various secondary minerals assemblages widely occurs in anaerobic environments. While respiration-driven supply rates of Fe(II) have been proposed as a primary factor controlling kinetics and mineral products of this process, the specific mechanism by which DIRB respiration rates regulate Fh biotransformation remains elusive. Here, to minimize the complex effects of microbial cells, we conducted Fh transformation using 1 mM biogenic Fe(II) (BioFe(II)), added at different rates to mimic diverse respiration-driven supply rates of Fe(II) by DIRB. For comparison, transformation experiments with FeSO<sub>4</sub> alone and FeSO<sub>4</sub> plus citrate (CitFe(II)) added at the corresponding supply rates were performed to decouple the specific effects of Fe(II) addition rates and extracellular polymeric substances (EPS) associated with BioFe(II). Decreasing FeSO<sub>4</sub> supply rates favored the transformation of Fh to lepidocrocite (Lp) over to Gt and the subsequent transformation of Lp to magnetite (Mt), altering the transformation pathway from Fh → Lp/Gt → Gt to Fh → Lp/Gt → Mt/Gt. These results underscore the significant effect of aqueous Fe(II) supply rates on the competition of olation and oxolation of labile Fe(III) intermediates into different secondary minerals. In the experiments with BioFe(II) and CitFe(II), although EPS or citrate slightly increased Fe(II) adsorption and Fe(III)<sub>labile</sub> generation, the increase in sorbed Fe(II) was minimal compared to the variations in aqueous Fe(II) concentrations caused by the different Fe(II) supply rates. At the same Fe(II) supply rates, EPS or citrate notably inhibited the transformation of Fh to Gt and the further conversion of Lp, altering the pathway from Fh → Mt/Gt/Lp to primarily Fh → Lp. These effects became more pronounced with the decrease of BioFe(II) and CitFe(II) supply rates. Our findings provide new insights into how DIRB respiration rates control kinetics, pathways, and mineral products of Fh transformation, which is crucial for elucidating the relevant biogeochemical cycling of nutrients and (im)mobilization of contaminants.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"385 ","pages":"Pages 87-99"},"PeriodicalIF":4.5,"publicationDate":"2024-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142321643","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 : 2024-09-04DOI: 10.1016/j.gca.2024.08.027
Timothé Mahlé , Yves Marrocchi , Julia Neukampf , Johan Villeneuve , Emmanuel Jacquet
Among carbonaceous chondrites, the chondrules of CB and CH stand out by being dominated by skeletal barred olivine and cryptocrystalline textures. These non-porphyritic chondrules are thought to have formed within an impact-generated plume resulting from large-scale asteroidal collisions late in disk history. Porphyritic chondrules are also present, if rare, in CB and CH chondrites and might correspond to nebular objects formed earlier in the disk. We report on the mineralogy, petrology, and oxygen isotopic compositions of porphyritic chondrules in the Isheyevo CH/CBb chondrite. These chondrules show minor element variations at both the chondrule and individual olivine grain scales, which are similar to those observed in other chondrites. In terms of oxygen isotopes, individual chondrules show contrasting behavior with either negligible, mass-dependent or mass-independent O-isotopic variations. They also display different average Δ17O, ranging from −6 ‰ to +4 ‰, anticorrelated with size, with most chondrules (8/13) showing Δ17O > 0 ‰. Our results show that porphyritic chondrules in CB (and CH) chondrites are of nebular origin and do not result from the collisional impact at the origin of other CB components. We propose that CB porphyritic chondrules originate from the chondritic impactor involved in the collision, similarly to hydrated matrix-rich clasts reported in Isheyevo. Altogether, this shows that two chondrule populations, formed by both nebular and planetary processes, co-exist in CB and CH chondrites. Isheyevo thus represents an archetypal chondrite lying at the transition between two dominant chondrule-forming regimes, nebular and impact-related.
{"title":"The last generation of nebular chondrules possibly sampled in the CH/CBb chondrite Isheyevo","authors":"Timothé Mahlé , Yves Marrocchi , Julia Neukampf , Johan Villeneuve , Emmanuel Jacquet","doi":"10.1016/j.gca.2024.08.027","DOIUrl":"10.1016/j.gca.2024.08.027","url":null,"abstract":"<div><div>Among carbonaceous chondrites, the chondrules of CB and CH stand out by being dominated by skeletal barred olivine and cryptocrystalline textures. These non-porphyritic chondrules are thought to have formed within an impact-generated plume resulting from large-scale asteroidal collisions late in disk history. Porphyritic chondrules are also present, if rare, in CB and CH chondrites and might correspond to nebular objects formed earlier in the disk. We report on the mineralogy, petrology, and oxygen isotopic compositions of porphyritic chondrules in the Isheyevo CH/CB<sub>b</sub> chondrite. These chondrules show minor element variations at both the chondrule and individual olivine grain scales, which are similar to those observed in other chondrites. In terms of oxygen isotopes, individual chondrules show contrasting behavior with either negligible, mass-dependent or mass-independent O-isotopic variations. They also display different average Δ<sup>17</sup>O, ranging from −6 ‰ to +4 ‰, anticorrelated with size, with most chondrules (8/13) showing Δ<sup>17</sup>O > 0 ‰. Our results show that porphyritic chondrules in CB (and CH) chondrites are of nebular origin and do not result from the collisional impact at the origin of other CB components. We propose that CB porphyritic chondrules originate from the chondritic impactor involved in the collision, similarly to hydrated matrix-rich clasts reported in Isheyevo. Altogether, this shows that two chondrule populations, formed by both nebular and planetary processes, co-exist in CB and CH chondrites. Isheyevo thus represents an archetypal chondrite lying at the transition between two dominant chondrule-forming regimes, nebular and impact-related.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"385 ","pages":"Pages 74-86"},"PeriodicalIF":4.5,"publicationDate":"2024-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142245683","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 : 2024-09-04DOI: 10.1016/j.gca.2024.09.002
Huijuan Li , Zilong Wang , Zhenyu Chen , Wei Tian , Wei-(RZ) Wang , Guibin Zhang , Lifei Zhang
<div><div>Apatite is ubiquitous in lunar samples and has been used widely for estimating volatile abundances in the lunar interior. However, apatite compositional and isotopic variations within and between samples have resulted in varying and ambiguous results. Understanding apatite petrogenesis will help with both identifying the appropriate composition for volatile estimation and interpreting isotopic variations. Here we report a comprehensive petrogenetic investigation of apatite in Chang’E-5 (CE5) basaltic sample CE5C0800YJYX013GP. Apatite displays both intra-grain and inter-grain compositional variations with F and Cl contents falling in the ranges of 0.97–2.47 wt% and 0.24–1.09 wt%, respectively. These apatite compositions show relatively low F and high Cl characteristics in comparison to apatites of Apollo high-Ti and low-Ti mare basalts, but are similar to those reported for lunar meteorites LAP 04841 and MIL 05035. We discern three zoning profiles: fractional crystallization (FC)-dominated, degassing-induced and a third indicated by REE-enriched cores, which are interpreted as representing different generations of apatite. FC-dominated zoning is characterized with decreasing F and increasing Cl and S contents from core to rim; while the opposite is true for the degassing-induced zoning. Regardless of the zoning patterns, apatite Cl and S contents display positive correlations, with S contents up to ∼ 3000 ppm, much higher than previous reports for Apollo samples (up to ∼ 600 ppm). We demonstrate that the fractional crystallization model proposed by <span><span>Boyce et al. (2014)</span></span> in combination with H<sub>2</sub>O degassing and high S contents in melt (likely at sulfide saturation) can explain these high Cl and S contents observed in CE5 apatite.</div><div>Based on the core composition of the FC-dominated zoning profile, which has the lowest incompatible element concentrations, bulk F, Cl and H<sub>2</sub>O contents in the parental melt are estimated to be ∼ 72 ± 21, ∼43 ± 14 and ∼ 1576 ± 518 ppm, respectively. These estimates have lower F/Cl ratios than those measured in olivine-hosted melt inclusions from Apollo mare basalts. By adopting the petrogenetic model for CE5 basalt proposed by <span><span>Su et al. (2022)</span></span>, i.e., 10 % partial melting of a hybrid mantle source, followed by ∼ 30–70 % fractional crystallization (∼50 % for our sample), we estimate the F, Cl, H<sub>2</sub>O and S contents in the mantle source are in the ranges of ∼ 2.5–4.6, ∼0.7–1.4, ∼53–105 and ∼ 38–125 ppm, respectively, similar to estimates for both depleted Earth mantle and primitive lunar mantle. However, by adopting the model of <span><span>Tian et al. (2021)</span></span>, 2–3 % partial melting of a mantle source composed of 86 PCS+2% TIRL (PCS, percent crystallized solid; TIRL, trapped instantaneous residual liquid), followed by 43–88 % fractional crystallization, these estimates will be 5–10 times lower. To be certain whether th
磷灰石在月球样本中无处不在，被广泛用于估算月球内部的挥发物丰度。然而，由于样本内部和样本之间的磷灰石成分和同位素差异，导致了不同和模糊的结果。了解磷灰石成岩过程将有助于确定估算挥发物的适当成分和解释同位素变化。在此，我们报告了对嫦娥五号（CE5）玄武岩样本 CE5C0800YJYX013GP 中磷灰石岩石成因的全面研究。磷灰石显示出晶粒内和晶粒间的成分变化，F和Cl的含量范围分别为0.97-2.47 wt%和0.24-1.09 wt%。与阿波罗高钛和低钛泥质玄武岩的磷灰石相比，这些磷灰石成分显示出相对较低的 F 和较高的 Cl 特征，但与所报道的月球陨石 LAP 04841 和 MIL 05035 的磷灰石成分相似。我们发现了三种分带剖面：以分数结晶（FC）为主的分带、脱气诱导的分带以及由富含 REE 的核心显示的第三种分带，这些分带被解释为代表不同世代的磷灰石。FC主导型分带的特点是从岩心到岩缘F含量降低，Cl和S含量升高；而脱气诱导型分带的特点则恰恰相反。不管是哪种分带模式，磷灰石的Cl和S含量都呈正相关，S含量高达ppm∼3000，远高于之前关于阿波罗样本的报道（ppm∼600）。我们证明，Boyce等人（2014年）提出的部分结晶模型与H2O脱气和熔体中高S含量（可能达到硫化物饱和）相结合，可以解释在CE5磷灰石中观察到的这些高Cl和S含量。
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