Pub Date : 2024-12-17DOI: 10.1016/j.gca.2024.12.019
C. Laskar, E.F. Bazarkina, G.S. Pokrovski
The solubility of palladium sulfide (PdS(s)) has been measured in H2S/HS– aqueous solutions across wide ranges of sulfur concentrations (0.5–1.2 molal) and pH (5–8) at temperatures from 50 to 300 °C and pressures from 90 to 600 bar, using a hydrothermal flexible-cell reactor allowing controlled fluid injection and sampling. Combined with thermodynamic modeling and analysis of available literature data, our results demonstrate that palladium tetrahydrosulfide, PdII(HS)42–, is the dominant complex in sulfide-rich moderate-temperature hydrothermal fluids. The equilibrium constants of PdS(s) dissolution reaction, PdS(s) + 3 H2S0(aq) = Pd(HS)42– + 2H+ (Ks4,Pd), generated in this study can be described by the function log10Ks4,Pd = (196.4 ± 60.0) – (11384 ± 3567)/T(K) – (71.24 ± 19.52) × log10T(K), valid over the temperature range 25–300 °C and from saturated vapor pressure to 600 bar. Our results, combined with recent analogous PtS(s) solubility data, enabled the derivation of a self-consistent set of thermodynamic properties of Pd(HS)42– and Pt(HS)42– in the framework of the HKF model. Solubility predictions of PdS(s) using these properties yield maximum solubility values of 1 ppb Pd, as the tetrahydrosulfide complex, in H2S-bearing hydrothermal fluids (>0.01 m S) at moderate temperatures (50–350 °C) and near-neutral pH (6–7), whereas chloride complexes are predominant at acidic pH (<4). The Pd/Pt atomic ratio in typical H2S-bearing hydrothermal fluids at 300 °C in equilibrium with PdS(s) and PtS(s) varies from > 104 at pH < 2 to 10–2 at pH > 3, corresponding to the change from chloride- to sulfide-dominated speciation for both metals. However, the absolute solubilities of both chloride and sulfide complexes are too small over the whole pH range to significantly contribute to Pd vs. Pt fractionations observed in hydrothermal environments. Among different factors that may lead to such fractionations, the role of polysulfide sulfur species, including trisulfur radical ions, should be considered in future studies.
{"title":"The role of sulfur in palladium transport and fractionation from platinum by hydrothermal fluids","authors":"C. Laskar, E.F. Bazarkina, G.S. Pokrovski","doi":"10.1016/j.gca.2024.12.019","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.019","url":null,"abstract":"The solubility of palladium sulfide (PdS<ce:inf loc=\"post\">(s)</ce:inf>) has been measured in H<ce:inf loc=\"post\">2</ce:inf>S/HS<ce:sup loc=\"post\">–</ce:sup> aqueous solutions across wide ranges of sulfur concentrations (0.5–1.2 molal) and pH (5–8) at temperatures from 50 to 300 °C and pressures from 90 to 600 bar, using a hydrothermal flexible-cell reactor allowing controlled fluid injection and sampling. Combined with thermodynamic modeling and analysis of available literature data, our results demonstrate that palladium tetrahydrosulfide, Pd<ce:sup loc=\"post\">II</ce:sup>(HS)<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2–</ce:sup>, is the dominant complex in sulfide-rich moderate-temperature hydrothermal fluids. The equilibrium constants of PdS<ce:inf loc=\"post\">(s)</ce:inf> dissolution reaction, PdS<ce:inf loc=\"post\">(s)</ce:inf> + 3 H<ce:inf loc=\"post\">2</ce:inf>S<ce:sup loc=\"post\">0</ce:sup><ce:inf loc=\"post\">(aq)</ce:inf> = Pd(HS)<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2–</ce:sup> + 2H<ce:sup loc=\"post\">+</ce:sup> (K<ce:inf loc=\"post\">s4,Pd</ce:inf>), generated in this study can be described by the function log<ce:inf loc=\"post\">10</ce:inf>K<ce:inf loc=\"post\">s4,Pd</ce:inf> = (196.4 ± 60.0) – (11384 ± 3567)/<ce:italic>T</ce:italic>(K) – (71.24 ± 19.52) × log<ce:inf loc=\"post\">10</ce:inf><ce:italic>T</ce:italic>(K), valid over the temperature range 25–300 °C and from saturated vapor pressure to 600 bar. Our results, combined with recent analogous PtS<ce:inf loc=\"post\">(s)</ce:inf> solubility data, enabled the derivation of a self-consistent set of thermodynamic properties of Pd(HS)<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2–</ce:sup> and Pt(HS)<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2–</ce:sup> in the framework of the HKF model. Solubility predictions of PdS<ce:inf loc=\"post\">(s)</ce:inf> using these properties yield maximum solubility values of 1 ppb Pd, as the tetrahydrosulfide complex, in H<ce:inf loc=\"post\">2</ce:inf>S-bearing hydrothermal fluids (>0.01 m S) at moderate temperatures (50–350 °C) and near-neutral pH (6–7), whereas chloride complexes are predominant at acidic pH (<4). The Pd/Pt atomic ratio in typical H<ce:inf loc=\"post\">2</ce:inf>S-bearing hydrothermal fluids at 300 °C in equilibrium with PdS<ce:inf loc=\"post\">(s)</ce:inf> and PtS<ce:inf loc=\"post\">(s)</ce:inf> varies from > 10<ce:sup loc=\"post\">4</ce:sup> at pH < 2 to 10<ce:sup loc=\"post\">–2</ce:sup> at pH > 3, corresponding to the change from chloride- to sulfide-dominated speciation for both metals. However, the absolute solubilities of both chloride and sulfide complexes are too small over the whole pH range to significantly contribute to Pd vs. Pt fractionations observed in hydrothermal environments. Among different factors that may lead to such fractionations, the role of polysulfide sulfur species, including trisulfur radical ions, should be considered in future studies.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"28 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142936031","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}
Potassium ions (K+) are abundant in ore fluids of lode gold deposits, largely illustrated by pervasive potassic alteration and commonly expressed as K-feldspar and muscovite on both sides of individual gold lodes. However, their potential roles in gold mineralization remain elusive. Here, we present results from ab initio molecular dynamics simulation and geochemical modeling to address this question. Molecular dynamics simulation results show that the ability of K+ pairing with Au(HS)2– complex over a wide temperature–pressure range has a negative linear correlation to fluid density. In high-density liquid-like fluids, little K+ is coordinated with the Au(HS)2– complex. In contrast, this complex can be nearly neutralized by ion association with K+ in low-density, vapor-like fluids, but such a neutral complexation is not very stable, even if under conditions typical of lode gold mineralization. Thus, K+ has a limited role in the complexing and transporting of Au in hydrothermal fluids forming lode gold deposits. We conducted geochemical modeling that integrates geological context and mineral paragenesis. The results reveal that potassic alteration in lode gold deposits, characterized by the transition from K-feldspar to muscovite, occurs alongside decreasing temperature, pH, and oxygen fugacity of the ore-forming fluids. Among these factors, the drop in temperature is the most significant mechanism driving potassic alteration, while also causing the destabilization of Au-bisulfide complex and spatially associated deposition of gold. These results suggest that gold mineralization during potassic alteration is primarily driven by the cooling of ore fluids, which also explains the transition from K-feldspar to muscovite alteration. The combination of molecular simulation and geochemical modeling indicates that the role of potassic alteration in lode gold mineralization reflects the influence of fluid evolution particularly fluid cooling on gold precipitation, rather than a direct control of K+ on Au transport in lode gold deposits. Therefore, potassic alteration can serve as an effective indicator for lode gold exploration and has been widely applied in practical fieldwork.
{"title":"The role of potassium in lode gold mineralization: insights from ab initio molecular dynamics and geochemical modeling","authors":"Gao-Hua Fan, Jian-Wei Li, Yuan Mei, Si-Yu Hu, Ri-Chen Zhong, Chang Yu, Xiao-Dong Deng, Hao Cui, Wen-Sheng Gao","doi":"10.1016/j.gca.2024.12.022","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.022","url":null,"abstract":"Potassium ions (K<ce:sup loc=\"post\">+</ce:sup>) are abundant in ore fluids of lode gold deposits, largely illustrated by pervasive potassic alteration and commonly expressed as K-feldspar and muscovite on both sides of individual gold lodes. However, their potential roles in gold mineralization remain elusive. Here, we present results from <ce:italic>ab initio</ce:italic> molecular dynamics simulation and geochemical modeling to address this question. Molecular dynamics simulation results show that the ability of K<ce:sup loc=\"post\">+</ce:sup> pairing with Au(HS)<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">–</ce:sup> complex over a wide temperature–pressure range has a negative linear correlation to fluid density. In high-density liquid-like fluids, little K<ce:sup loc=\"post\">+</ce:sup> is coordinated with the Au(HS)<ce:inf loc=\"post\">2</ce:inf><ce:sup loc=\"post\">–</ce:sup> complex. In contrast, this complex can be nearly neutralized by ion association with K<ce:sup loc=\"post\">+</ce:sup> in low-density, vapor-like fluids, but such a neutral complexation is not very stable, even if under conditions typical of lode gold mineralization. Thus, K<ce:sup loc=\"post\">+</ce:sup> has a limited role in the complexing and transporting of Au in hydrothermal fluids forming lode gold deposits. We conducted geochemical modeling that integrates geological context and mineral paragenesis. The results reveal that potassic alteration in lode gold deposits, characterized by the transition from K-feldspar to muscovite, occurs alongside decreasing temperature, pH, and oxygen fugacity of the ore-forming fluids. Among these factors, the drop in temperature is the most significant mechanism driving potassic alteration, while also causing the destabilization of Au-bisulfide complex and spatially associated deposition of gold. These results suggest that gold mineralization during potassic alteration is primarily driven by the cooling of ore fluids, which also explains the transition from K-feldspar to muscovite alteration. The combination of molecular simulation and geochemical modeling indicates that the role of potassic alteration in lode gold mineralization reflects the influence of fluid evolution particularly fluid cooling on gold precipitation, rather than a direct control of K<ce:sup loc=\"post\">+</ce:sup> on Au transport in lode gold deposits. Therefore, potassic alteration can serve as an effective indicator for lode gold exploration and has been widely applied in practical fieldwork.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"258 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841589","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-16DOI: 10.1016/j.gca.2024.12.015
Maxime Enrico, Isabelle Le Hécho, Hervé Carrier, Guillaume Galliero, Anélia Petit, Romain Vermorel
Compound-specific isotope analysis is widely used for evaluating the degradation of organic compounds in aquifers. We investigated the potential of aqueous diffusion of benzene under aquifer conditions (310 K, 70 bars) to cause significant isotope fractionation that might bias the estimates of biodegradation. Diffusion coefficients were computed using external field – non-equilibrium molecular dynamics simulations for benzene isotopologues with fictive masses to investigate the influence of molecular mass on aqueous diffusion. Our findings reveal variable power-law relations between molecular masses and diffusion coefficients depending on mass distribution, none of them being consistent with the kinetic theory often used to address isotopic effects on solute diffusion, even in dense solvents. Fictive isotopologues with mass substitutions not affecting the symmetry of the molecule consistently align with a power-law exponent βa = 0.038. In contrast, mass substitutions on a single atom within the benzene molecule (either hydrogen or carbon) result in a smaller mass dependency with βs = 0.021. This result implies that isotopically-labelled molecules such as perdeuterated benzene might not behave similarly to naturally-occurring mono-substituted molecules, an observation that is of importance for the use of these compounds in laboratory experiments. The resulting fractionation factor for mono-substituted benzene isotopologues is −0.3 ‰, and hardly contributes to the variations in benzene isotope composition under aquifer conditions, which validates the use of isotope ratios for evaluating biodegradation of organic compounds in aquifers.
{"title":"Carbon and hydrogen isotope fractionation during aqueous diffusion of benzene","authors":"Maxime Enrico, Isabelle Le Hécho, Hervé Carrier, Guillaume Galliero, Anélia Petit, Romain Vermorel","doi":"10.1016/j.gca.2024.12.015","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.015","url":null,"abstract":"Compound-specific isotope analysis is widely used for evaluating the degradation of organic compounds in aquifers. We investigated the potential of aqueous diffusion of benzene under aquifer conditions (310 K, 70 bars) to cause significant isotope fractionation that might bias the estimates of biodegradation. Diffusion coefficients were computed using external field – non-equilibrium molecular dynamics simulations for benzene isotopologues with fictive masses to investigate the influence of molecular mass on aqueous diffusion. Our findings reveal variable power-law relations between molecular masses and diffusion coefficients depending on mass distribution, none of them being consistent with the kinetic theory often used to address isotopic effects on solute diffusion, even in dense solvents. Fictive isotopologues with mass substitutions not affecting the symmetry of the molecule consistently align with a power-law exponent β<ce:inf loc=\"post\">a</ce:inf> = 0.038. In contrast, mass substitutions on a single atom within the benzene molecule (either hydrogen or carbon) result in a smaller mass dependency with βs = 0.021. This result implies that isotopically-labelled molecules such as perdeuterated benzene might not behave similarly to naturally-occurring mono-substituted molecules, an observation that is of importance for the use of these compounds in laboratory experiments. The resulting fractionation factor for mono-substituted benzene isotopologues is −0.3 ‰, and hardly contributes to the variations in benzene isotope composition under aquifer conditions, which validates the use of isotope ratios for evaluating biodegradation of organic compounds in aquifers.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"13 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884630","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 : 2024-12-16DOI: 10.1016/j.gca.2024.12.016
L.G. Vacher, J. Eschrig, L. Bonal, W. Fujiya, L. Flandinet, P. Beck
Non-carbonaceous (NC) meteorites, such as enstatite and ordinary chondrites, are regarded as potential building blocks of terrestrial planets, possibly delivering volatile elements to the inner solar system. However, their parent bodies underwent intense thermal metamorphism during planet formation, raising questions about whether planets accreted volatile-rich or volatile-poor materials. Ordinary chondrites-like materials may have contributed significantly to the formation of Mars, but the impact of thermal metamorphism on their initial volatile content and isotopic composition is unclear. This study reports the bulk-rock hydrogen, carbon, and nitrogen abundances and isotopic compositions (δD, δ13C, δ15N) of unequilibrated ordinary chondrites (UOCs) across petrologic subtypes (PT) 3.00 to 3.9. Upon removing terrestrially contaminated samples, we found that the matrix-normalized hydrogen, carbon, and nitrogen concentrations are inversely correlated with the Raman spectral parameters (FWHMD), a tracer of thermal metamorphism in type 3 chondrites. Only δD shows a correlation with FWHMD, suggesting that δ13C and δ15N were not fractionated despite carbon and nitrogen being outgassed from the interior of the planetesimal. With increasing metamorphism, we proposed that less-metamorphosed UOCs (PT < 3.2) progressively lost deuterium (D) due to the breakdown of D-rich phyllosilicates above 300 °C, as supported by our FTIR analyses. We conducted thermal modeling to better understand how thermal metamorphism influences the delivery of water to terrestrial planets. Our results suggest that UOC-like precursors did not significantly contribute to Mars’ accretion due to the rapid progression of thermal metamorphism within ordinary chondrite planetesimals. However, volatile-rich UOCs may have supplied most of the hydrogen to Mars, implying that Mars’ primitive mantle may have recorded and retained a strong D-rich reservoir in its interior.
{"title":"Thermal metamorphism and volatile evolution in unequilibrated ordinary chondrites: Implications for the delivery of hydrogen to terrestrial planets","authors":"L.G. Vacher, J. Eschrig, L. Bonal, W. Fujiya, L. Flandinet, P. Beck","doi":"10.1016/j.gca.2024.12.016","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.016","url":null,"abstract":"Non-carbonaceous (NC) meteorites, such as enstatite and ordinary chondrites, are regarded as potential building blocks of terrestrial planets, possibly delivering volatile elements to the inner solar system. However, their parent bodies underwent intense thermal metamorphism during planet formation, raising questions about whether planets accreted volatile-rich or volatile-poor materials. Ordinary chondrites-like materials may have contributed significantly to the formation of Mars, but the impact of thermal metamorphism on their initial volatile content and isotopic composition is unclear. This study reports the bulk-rock hydrogen, carbon, and nitrogen abundances and isotopic compositions (δD, δ<ce:sup loc=\"post\">13</ce:sup>C, δ<ce:sup loc=\"post\">15</ce:sup>N) of unequilibrated ordinary chondrites (UOCs) across petrologic subtypes (PT) 3.00 to 3.9. Upon removing terrestrially contaminated samples, we found that the matrix-normalized hydrogen, carbon, and nitrogen concentrations are inversely correlated with the Raman spectral parameters (FWHM<ce:inf loc=\"post\">D</ce:inf>), a tracer of thermal metamorphism in type 3 chondrites. Only δD shows a correlation with FWHM<ce:inf loc=\"post\">D</ce:inf>, suggesting that δ<ce:sup loc=\"post\">13</ce:sup>C and δ<ce:sup loc=\"post\">15</ce:sup>N were not fractionated despite carbon and nitrogen being outgassed from the interior of the planetesimal. With increasing metamorphism, we proposed that less-metamorphosed UOCs (PT < 3.2) progressively lost deuterium (D) due to the breakdown of D-rich phyllosilicates above 300 °C, as supported by our FTIR analyses. We conducted thermal modeling to better understand how thermal metamorphism influences the delivery of water to terrestrial planets. Our results suggest that UOC-like precursors did not significantly contribute to Mars’ accretion due to the rapid progression of thermal metamorphism within ordinary chondrite planetesimals. However, volatile-rich UOCs may have supplied most of the hydrogen to Mars, implying that Mars’ primitive mantle may have recorded and retained a strong D-rich reservoir in its interior.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"68 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142911881","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 : 2024-12-15DOI: 10.1016/j.gca.2024.12.014
Yin-Chuan Li, Ya-Ru Zhou, Hai-Zhen Wei, Martin R. Palmer, Fa-Yang Guo, Yong-Hui Li, Yong-An Qi, Da-Ping Xia
The chemical weathering of continental silicates affects the global climate and the geochemical cycle of elements, and because of their significant fractionation during chemical weathering, boron isotopes have great potential to trace chemical weathering processes. However, our understanding of boron isotope fractionation in the weathering environment is limited. In this study, the equilibrium boron isotope fractionation between the kaolinite (010) surface and aqueous fluids is investigated using quantum mechanics calculations (density functional theory, DFT). On the basis of the computation of binding energy and interface-structure optimization, a three-step reaction mechanism of boron adsorption on kaolinite is proposed: i) the free hydronium ions bond to the surface hydroxyl groups of kaolinite to form water molecules at mineral surface; ii) both B(OH)3 and B(OH)4− replace the water molecule on the mineral surface via the coordination hydroxyl group; iii) the free hydroxyl groups capture the hydrogen ions from bridging oxygen to form free water molecules. We first verified the influence of the B-O bond length and the O-B-O angle distortions on boron isotope fractionation and observed negative correlations between the magnitude of modelled boron isotope fractionation and both the average B-O bond length and the degree of structure distortion. Comparison between theoretical and experimental results indicates that the BO4 structure dominates the boron isotope fractionation at the kaolinite (010) surface. This, in turn, allows for a more detailed understanding of boron isotope fractionation in the surface weathering environment at different T, pH and salinity conditions.
{"title":"Equilibrium boron isotope fractionation during kaolinite adsorption and applications to chemical weathering processes","authors":"Yin-Chuan Li, Ya-Ru Zhou, Hai-Zhen Wei, Martin R. Palmer, Fa-Yang Guo, Yong-Hui Li, Yong-An Qi, Da-Ping Xia","doi":"10.1016/j.gca.2024.12.014","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.014","url":null,"abstract":"The chemical weathering of continental silicates affects the global climate and the geochemical cycle of elements, and because of their significant fractionation during chemical weathering, boron isotopes have great potential to trace chemical weathering processes. However, our understanding of boron isotope fractionation in the weathering environment is limited. In this study, the equilibrium boron isotope fractionation between the kaolinite (010) surface and aqueous fluids is investigated using quantum mechanics calculations (density functional theory, DFT). On the basis of the computation of binding energy and interface-structure optimization, a three-step reaction mechanism of boron adsorption on kaolinite is proposed: i) the free hydronium ions bond to the surface hydroxyl groups of kaolinite to form water molecules at mineral surface; ii) both B(OH)<ce:inf loc=\"post\">3</ce:inf> and B(OH)<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">−</ce:sup> replace the water molecule on the mineral surface via the coordination hydroxyl group; iii) the free hydroxyl groups capture the hydrogen ions from bridging oxygen to form free water molecules. We first verified the influence of the B-O bond length and the O-B-O angle distortions on boron isotope fractionation and observed negative correlations between the magnitude of modelled boron isotope fractionation and both the average B-O bond length and the degree of structure distortion. Comparison between theoretical and experimental results indicates that the BO<ce:inf loc=\"post\">4</ce:inf> structure dominates the boron isotope fractionation at the kaolinite (010) surface. This, in turn, allows for a more detailed understanding of boron isotope fractionation in the surface weathering environment at different T, pH and salinity conditions.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"22 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142935973","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 : 2024-12-15DOI: 10.1016/j.gca.2024.12.007
Christopher Emproto, Ryan Mathur, Mingguang Sun, Adam C. Simon, Linda Godfrey
Titanium (Ti) typically exhibits low mobility in geologic fluids due to the low aqueous solubility of common (Fe-)Ti oxide minerals. Consequently, Ti isotope variations (δ49/47Ti, given as δ49Ti) in geologic systems are primarily attributed to magmatic differentiation. Thus, the potential for fluid-mineral fractionation has received less attention. However, ligand-rich fluids are capable of mobilizing Ti as observed in natural systems and laboratory studies. As hydrothermal ore mineralization is commonly associated with ligand-rich brines capable of transporting significant quantities of metals, Ti isotopes may aid in understanding mineralization and alteration in complex hydrothermal systems. Here we present data from computational modeling of various Ti coordination complexes theorized to exist in geologic systems and/or under relevant experimental conditions as well as computed fractionation factors for various Ti-bearing crystalline phases to investigate the basic mechanics of equilibrium fluid-mineral Ti isotope fractionation. These results indicate that equilibrium fluid-mineral Ti isotope exchange between our modeled Ti complexes and phases with 6-coordinated Ti is predicted to generally lead to enrichment of heavy Ti isotopes in the fluid. Because minerals with 6-coordinated Ti (such as magnetite and ilmenite) are the most important reservoirs of Ti in the solid Earth, Ti isotope equilibration between terrestrial rocks and fluids can be generalized to enrich the fluid in heavy Ti isotopes. We also performed magnetite-ülvospinel leaching experiments to investigate fluid-mineral Ti isotope fractionation in this phase. Mineral leaching experiments varying acid strength, leaching temperature, and reaction time with HCl and HF qualitatively support the prediction that the fluid phase will become enriched in heavy Ti isotopes during fluid-mineral interactions that approach equilibrium with Ti-rich magnetite. Additionally, the leaching data also suggest that the fluid becomes slightly enriched in lighter Ti isotopes when Ti exchange is limited—potentially due to kinetic effects. Therefore, magnetite from natural systems may be depleted in heavy Ti isotopes during regenerative mineral replacement involving equilibration with fluids or may possibly become depleted in light Ti isotopes under a kinetic fractionation regime—leading to mineral δ49Ti values that are insufficiently explained by magmatic differentiation or inter-mineral fractionation. These results are a first look at fluid-mineral interactions that may affect Ti isotope fractionation in hydrothermal mineral systems, and Ti isotopes should be further studied as a potential method of understanding aqueous metal transport and tracing alteration in mineral deposits.
{"title":"Fluid-mineral titanium isotope fractionation: Computational and empirical results with implications for mineral deposits","authors":"Christopher Emproto, Ryan Mathur, Mingguang Sun, Adam C. Simon, Linda Godfrey","doi":"10.1016/j.gca.2024.12.007","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.007","url":null,"abstract":"Titanium (Ti) typically exhibits low mobility in geologic fluids due to the low aqueous solubility of common (Fe-)Ti oxide minerals. Consequently, Ti isotope variations (δ<ce:sup loc=\"post\">49/47</ce:sup>Ti, given as δ<ce:sup loc=\"post\">49</ce:sup>Ti) in geologic systems are primarily attributed to magmatic differentiation. Thus, the potential for fluid-mineral fractionation has received less attention. However, ligand-rich fluids are capable of mobilizing Ti as observed in natural systems and laboratory studies. As hydrothermal ore mineralization is commonly associated with ligand-rich brines capable of transporting significant quantities of metals, Ti isotopes may aid in understanding mineralization and alteration in complex hydrothermal systems. Here we present data from computational modeling of various Ti coordination complexes theorized to exist in geologic systems and/or under relevant experimental conditions as well as computed fractionation factors for various Ti-bearing crystalline phases to investigate the basic mechanics of equilibrium fluid-mineral Ti isotope fractionation. These results indicate that equilibrium fluid-mineral Ti isotope exchange between our modeled Ti complexes and phases with 6-coordinated Ti is predicted to generally lead to enrichment of heavy Ti isotopes in the fluid. Because minerals with 6-coordinated Ti (such as magnetite and ilmenite) are the most important reservoirs of Ti in the solid Earth, Ti isotope equilibration between terrestrial rocks and fluids can be generalized to enrich the fluid in heavy Ti isotopes. We also performed magnetite-ülvospinel leaching experiments to investigate fluid-mineral Ti isotope fractionation in this phase. Mineral leaching experiments varying acid strength, leaching temperature, and reaction time with HCl and HF qualitatively support the prediction that the fluid phase will become enriched in heavy Ti isotopes during fluid-mineral interactions that approach equilibrium with Ti-rich magnetite. Additionally, the leaching data also suggest that the fluid becomes slightly enriched in lighter Ti isotopes when Ti exchange is limited—potentially due to kinetic effects. Therefore, magnetite from natural systems may be depleted in heavy Ti isotopes during regenerative mineral replacement involving equilibration with fluids or may possibly become depleted in light Ti isotopes under a kinetic fractionation regime—leading to mineral δ<ce:sup loc=\"post\">49</ce:sup>Ti values that are insufficiently explained by magmatic differentiation or inter-mineral fractionation. These results are a first look at fluid-mineral interactions that may affect Ti isotope fractionation in hydrothermal mineral systems, and Ti isotopes should be further studied as a potential method of understanding aqueous metal transport and tracing alteration in mineral deposits.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"14 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884585","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 : 2024-12-14DOI: 10.1016/j.gca.2024.12.011
Vasileios Mavromatis, Jean-Michel Brazier, Katja E. Goetschl, Sylvia Riechelmann, Martin Dietzel, Jacques Schott
Experimentally defined metal isotope fractionation factors between carbonate minerals and fluids are routinely used to interpret the isotopic compositions of natural samples. The majority of experimental works, however, have been conducted in simplified and rather diluted background electrolyte solutions although most carbonate proxies were applied in seawater. Thus the impacts of important inorganic ligands such as SO42− on metal isotope fractionation between carbonate minerals and fluids are poorly known. It is however nowadays well accepted that metal–ligand interactions strongly affect the incorporation and isotope composition of cations in the mineral lattice by either the formation of aqueous complexes or their direct incorporation in the solid. In order to shed light on the role SO42−(aq) holds on the Mg isotope fractionation between calcite and fluid, in this study we have measured the δ26Mg composition of Mg-calcites and forming fluids reported by Goetschl et al. (2019). The obtained results suggest that all calcites formed in the presence of sulfate exhibit systematically smaller Mg isotope fractionation values (i.e. Δ26Mgcalcite-fluid = δ26Mgcalcite-δ26Mgfluid) compared to those formed in the absence of this ligand. The increase of Δ26Mgcalcite-fluid values can be assigned to two processes. The first is the formation of aqueous MgSO4o complexes that enhance the exchange rate of water molecules between Mg2+ hydration sphere and bulk water. The formation of MgSO4o results in a reduction of the absolute value of Δ26Mgcalcite-fluid by allowing the incorporation of less hydrated Mg in calcite. The second is the incorporation in the solid of a significant fraction of Mg as MgSO4 which reduces the average Mg-O bond length in the crystal lattice, making the presence of 24Mg less abundant compared to solids that do not contain sulfate. The significance of these results is discussed with respect to the interpretation of Mg isotope compositions of natural carbonates and our understanding of the mechanisms controlling isotope fractionation of trace metals in CaCO3 phases.
{"title":"Effect of sulfate on the kinetic and equilibrium Magnesium isotope fractionation between low Mg-calcite and fluid","authors":"Vasileios Mavromatis, Jean-Michel Brazier, Katja E. Goetschl, Sylvia Riechelmann, Martin Dietzel, Jacques Schott","doi":"10.1016/j.gca.2024.12.011","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.011","url":null,"abstract":"Experimentally defined metal isotope fractionation factors between carbonate minerals and fluids are routinely used to interpret the isotopic compositions of natural samples. The majority of experimental works, however, have been conducted in simplified and rather diluted background electrolyte solutions although most carbonate proxies were applied in seawater. Thus the impacts of important inorganic ligands such as SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2−</ce:sup> on metal isotope fractionation between carbonate minerals and fluids are poorly known. It is however nowadays well accepted that metal–ligand interactions strongly affect the incorporation and isotope composition of cations in the mineral lattice by either the formation of aqueous complexes or their direct incorporation in the solid. In order to shed light on the role SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2−</ce:sup>(aq) holds on the Mg isotope fractionation between calcite and fluid, in this study we have measured the δ<ce:sup loc=\"post\">26</ce:sup>Mg composition of Mg-calcites and forming fluids reported by <ce:cross-ref ref>Goetschl et al. (2019)</ce:cross-ref>. The obtained results suggest that all calcites formed in the presence of sulfate exhibit systematically smaller Mg isotope fractionation values (i.e. Δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">calcite-fluid</ce:inf> = δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">calcite</ce:inf>-δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">fluid</ce:inf>) compared to those formed in the absence of this ligand. The increase of Δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">calcite-fluid</ce:inf> values can be assigned to two processes. The first is the formation of aqueous MgSO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">o</ce:sup> complexes that enhance the exchange rate of water molecules between Mg<ce:sup loc=\"post\">2+</ce:sup> hydration sphere and bulk water. The formation of MgSO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">o</ce:sup> results in a reduction of the absolute value of Δ<ce:sup loc=\"post\">26</ce:sup>Mg<ce:inf loc=\"post\">calcite-fluid</ce:inf> by allowing the incorporation of less hydrated Mg in calcite. The second is the incorporation in the solid of a significant fraction of Mg as MgSO<ce:inf loc=\"post\">4</ce:inf> which reduces the average Mg-O bond length in the crystal lattice, making the presence of <ce:sup loc=\"post\">24</ce:sup>Mg less abundant compared to solids that do not contain sulfate. The significance of these results is discussed with respect to the interpretation of Mg isotope compositions of natural carbonates and our understanding of the mechanisms controlling isotope fractionation of trace metals in CaCO<ce:inf loc=\"post\">3</ce:inf> phases.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"1 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142911920","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 : 2024-12-13DOI: 10.1016/j.gca.2024.12.012
Mao Tan, Yiping Yang, Xiao-Wen Huang, Jiaxin Xi, Nuo Li, Yu-Miao Meng, Liang Qi
Molybdenite has been established as a robust mineral for Re-Os isotope dating. However, higher-precision Re-Os molybdenite dating is necessary to accurately determine the absolute timing of metal mineralization and duration of ore-forming hydrothermal systems. To improve the precision and accuracy of molybdenite Re-Os dating, molybdenite from the Longmendian deposit—with its old age, heterogeneous distribution of Re, compression deformation, and complex polytypes—serves as a reference for enhancing the precision of in-situ Re-Os dating or grain-scale sampling during solution Re-Os isotope dating of molybdenite. High-resolution scanning transmission electron microscopy (TEM) analysis demonstrates that Re, Os, Pb, Bi, Cu, and Fe are incorporated into the molybdenite crystal through isomorphic substitution for Mo. Electron probe analysis shows that a single molybdenite grain exhibits heterogeneous textures consisting of Re-rich (∼0.29–0.82 wt%) and Re-poor (<0.29 wt%) domains. Some molybdenite grains have undergone compression deformation. Rhenium can be enriched in either the deformed or the undeformed domains of molybdenite grains. Compression deformation in some grains induces delamination cracks in Re-rich domains, facilitating ore-forming fluid infiltration and leading to an inhomogeneous distribution of Re and other elements in molybdenite grains. Re-rich domain in molybdenite is enriched in other metals, including Fe, Co, Zn, Pb, and Bi, due to the overprint of the hydrothermal fluids with a lower temperature and a relatively high oxygen fugacity, leading to the formation of heterogeneous molybdenite. Micro-X-ray diffraction (μXRD) and TEM analyses have revealed that both the Re-rich and Re-poor domains belong to the 2H1 polytype, indicating that Re concentration and distribution are not directly related to the polytype of molybdenite. The Re-poor and deformed domain of molybdenite shows the coexistence of 2H1 and 3R polytypes (in a ratio of 9:1), suggesting that compression deformation led to polytype transformation. Therefore, the diverse characteristics of molybdenite in the Longmendian deposit present challenges for obtaining primary Re-Os age information. Nondestructive pre-characterization of molybdenite is essential before dating to ensure age homogeneity. Molybdenite samples with an undeformed and uniform distribution of elements (Re) within molybdenite grains are suitable candidates for analysis. Our findings collectively offer strategies to enhance precision while advancing the understanding of elemental and isotopic geochemical behavior in the contexts of metamorphism, deformation, and fluid flow environments.
辉钼矿已被确定为 Re-Os 同位素测年的可靠矿物。然而，要准确确定金属成矿的绝对时间和成矿热液系统的持续时间，就需要更高精度的辉钼矿Re-Os年代测定。为了提高辉钼矿Re-Os定年的精度和准确性，龙门甸矿床的辉钼矿--其年代久远、Re分布不均、压缩变形和复杂的多型性--可作为提高辉钼矿原位Re-Os定年或溶液Re-Os同位素定年过程中晶粒尺度取样精度的参考。高分辨率扫描透射电子显微镜（TEM）分析表明，Re、Os、Pb、Bi、Cu 和 Fe 是通过同构取代 Mo 的方式融入辉钼矿晶体的。电子探针分析表明，单个辉钼矿晶粒呈现出由富Re（0.29-0.82 wt%）和贫Re（0.29 wt%）域组成的异质纹理。一些辉钼矿晶粒经历了压缩变形。铼可在辉钼矿晶粒的变形或未变形域中富集。某些晶粒的压缩变形会在富铼域诱发分层裂纹，促进成矿流体的渗入，导致钼矿晶粒中的铼和其他元素分布不均。由于热液温度较低，氧富集度相对较高，导致形成异质辉钼矿，因此辉钼矿中的富集域富含其他金属，包括铁、钴、锌、铅和铋。显微 X 射线衍射（μXRD）和 TEM 分析表明，富 Re 域和贫 Re 域都属于 2H1 多晶型，表明 Re 的浓度和分布与辉钼矿的多晶型没有直接关系。辉钼矿的再贫域和变形域显示出 2H1 和 3R 多型共存（比例为 9:1），表明压缩变形导致了多型转化。因此，龙门甸矿床中辉钼矿的多样化特征为获取原始Re-Os年龄信息带来了挑战。在测年之前，必须对辉钼矿进行非破坏性预表征，以确保年龄的均一性。辉钼矿样品中的元素（Re）在辉钼矿晶粒内未变形且分布均匀，适合进行分析。我们的研究结果共同为提高精确度提供了策略，同时也推进了对变质、变形和流体流动环境下元素和同位素地球化学行为的理解。
{"title":"Rhenium residency in molybdenite, compressional textures and relationship to polytypism","authors":"Mao Tan, Yiping Yang, Xiao-Wen Huang, Jiaxin Xi, Nuo Li, Yu-Miao Meng, Liang Qi","doi":"10.1016/j.gca.2024.12.012","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.012","url":null,"abstract":"Molybdenite has been established as a robust mineral for Re-Os isotope dating. However, higher-precision Re-Os molybdenite dating is necessary to accurately determine the absolute timing of metal mineralization and duration of ore-forming hydrothermal systems. To improve the precision and accuracy of molybdenite Re-Os dating, molybdenite from the Longmendian deposit—with its old age, heterogeneous distribution of Re, compression deformation, and complex polytypes—serves as a reference for enhancing the precision of in-situ Re-Os dating or grain-scale sampling during solution Re-Os isotope dating of molybdenite. High-resolution scanning transmission electron microscopy (TEM) analysis demonstrates that Re, Os, Pb, Bi, Cu, and Fe are incorporated into the molybdenite crystal through isomorphic substitution for Mo. Electron probe analysis shows that a single molybdenite grain exhibits heterogeneous textures consisting of Re-rich (∼0.29–0.82 wt%) and Re-poor (<0.29 wt%) domains. Some molybdenite grains have undergone compression deformation. Rhenium can be enriched in either the deformed or the undeformed domains of molybdenite grains. Compression deformation in some grains induces delamination cracks in Re-rich domains, facilitating ore-forming fluid infiltration and leading to an inhomogeneous distribution of Re and other elements in molybdenite grains. Re-rich domain in molybdenite is enriched in other metals, including Fe, Co, Zn, Pb, and Bi, due to the overprint of the hydrothermal fluids with a lower temperature and a relatively high oxygen fugacity, leading to the formation of heterogeneous molybdenite. Micro-X-ray diffraction (μXRD) and TEM analyses have revealed that both the Re-rich and Re-poor domains belong to the 2<ce:italic>H</ce:italic><ce:inf loc=\"post\">1</ce:inf> polytype, indicating that Re concentration and distribution are not directly related to the polytype of molybdenite. The Re-poor and deformed domain of molybdenite shows the coexistence of 2<ce:italic>H</ce:italic><ce:inf loc=\"post\">1</ce:inf> and 3<ce:italic>R</ce:italic> polytypes (in a ratio of 9:1), suggesting that compression deformation led to polytype transformation. Therefore, the diverse characteristics of molybdenite in the Longmendian deposit present challenges for obtaining primary Re-Os age information. Nondestructive pre-characterization of molybdenite is essential before dating to ensure age homogeneity. Molybdenite samples with an undeformed and uniform distribution of elements (Re) within molybdenite grains are suitable candidates for analysis. Our findings collectively offer strategies to enhance precision while advancing the understanding of elemental and isotopic geochemical behavior in the contexts of metamorphism, deformation, and fluid flow environments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"5 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841532","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-13DOI: 10.1016/j.gca.2024.12.008
Chadlin M. Ostrander, Andy W. Heard, Elizabeth D. Swanner, Yunchao Shu, Wang Zheng, Yaqiu Zhao, Sune G. Nielsen
Precambrian oceans were overwhelmingly anoxic and rich in dissolved ferrous iron (Fe2+), conditions referred to as ‘ferruginous’. Yet, few paleoceanographic tools widely applied to the Precambrian sedimentary record are investigated in detail in the few available ferruginous settings today. We conducted a detailed thallium (Tl) isotope investigation of Deming Lake, a meromictic ferruginous lake in Itasca State Park (Minnesota, USA). Only slight changes were observed in dissolved Tl concentrations (Tldiss) and isotopic compositions (ε205Tldiss) between spring, summer, and winter months. This was despite the development of a winter ice cap, a dramatic shoaling of the oxycline, and large swings in dissolved and particulate Fe concentrations. Isotopic compositions of authigenic Tl (ε205TlA) leached from surface sediments across a shallow-to-deep gradient averaged –2.2 ± 0.9‱ (2SD) and were at all depths slightly lower than corresponding waters, which averaged 0.0 ± 1.3‱ (2SD). We estimate that the Tl isotope fractionation process responsible for driving this effect has an associated fractionation factor (α) of 0.9998 to 0.9999. Processes behind this fractionation could be preferential 203Tl removal by biomass and organic sulfur. Thallium removal from the uppermost water column during the spring and summer months had an associated α up to 1.0005, most likely due to the preferential sorption of 205Tl onto Mn oxide minerals. This process plays little to no role in setting sedimentary ε205TlA values, probably because Mn oxide minerals are unstable in sediments throughout the lake. We find no evidence of strongly coupled Tl and Fe cycling in Deming Lake, and by extension suspect that ferruginous conditions did not play an important direct role in ancient seawater Tl isotope mass-balance. What the lake does tell us, however, is that organic-rich and sulfur-poor sediments can slightly fractionate Tl isotopes. Ancient sediments formed under comparable conditions may be unreliable water column ε205Tl archives. And if these sediment types were widely distributed on the ancient seafloor, they could have exerted a minor effect on global seawater Tl isotope mass-balance. Our results provide novel insights into how Tl and its isotopes are cycled under ferruginous conditions, permitting more accurate application of the Tl isotope paleoredox proxy to the Precambrian sedimentary record.
{"title":"Thallium isotope cycling in a ferruginous Precambrian ocean analogue","authors":"Chadlin M. Ostrander, Andy W. Heard, Elizabeth D. Swanner, Yunchao Shu, Wang Zheng, Yaqiu Zhao, Sune G. Nielsen","doi":"10.1016/j.gca.2024.12.008","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.008","url":null,"abstract":"Precambrian oceans were overwhelmingly anoxic and rich in dissolved ferrous iron (Fe<ce:sup loc=\"post\">2+</ce:sup>), conditions referred to as ‘ferruginous’. Yet, few paleoceanographic tools widely applied to the Precambrian sedimentary record are investigated in detail in the few available ferruginous settings today. We conducted a detailed thallium (Tl) isotope investigation of Deming Lake, a meromictic ferruginous lake in Itasca State Park (Minnesota, USA). Only slight changes were observed in dissolved Tl concentrations (Tl<ce:inf loc=\"post\">diss</ce:inf>) and isotopic compositions (ε<ce:sup loc=\"post\">205</ce:sup>Tl<ce:inf loc=\"post\">diss</ce:inf>) between spring, summer, and winter months. This was despite the development of a winter ice cap, a dramatic shoaling of the oxycline, and large swings in dissolved and particulate Fe concentrations. Isotopic compositions of authigenic Tl (ε<ce:sup loc=\"post\">205</ce:sup>Tl<ce:inf loc=\"post\">A</ce:inf>) leached from surface sediments across a shallow-to-deep gradient averaged –2.2 ± 0.9‱ (2SD) and were at all depths slightly lower than corresponding waters, which averaged 0.0 ± 1.3‱ (2SD). We estimate that the Tl isotope fractionation process responsible for driving this effect has an associated fractionation factor (α) of 0.9998 to 0.9999. Processes behind this fractionation could be preferential <ce:sup loc=\"post\">203</ce:sup>Tl removal by biomass and organic sulfur. Thallium removal from the uppermost water column during the spring and summer months had an associated α up to 1.0005, most likely due to the preferential sorption of <ce:sup loc=\"post\">205</ce:sup>Tl onto Mn oxide minerals. This process plays little to no role in setting sedimentary ε<ce:sup loc=\"post\">205</ce:sup>Tl<ce:inf loc=\"post\">A</ce:inf> values, probably because Mn oxide minerals are unstable in sediments throughout the lake. We find no evidence of strongly coupled Tl and Fe cycling in Deming Lake, and by extension suspect that ferruginous conditions did not play an important direct role in ancient seawater Tl isotope mass-balance. What the lake does tell us, however, is that organic-rich and sulfur-poor sediments can slightly fractionate Tl isotopes. Ancient sediments formed under comparable conditions may be unreliable water column ε<ce:sup loc=\"post\">205</ce:sup>Tl archives. And if these sediment types were widely distributed on the ancient seafloor, they could have exerted a minor effect on global seawater Tl isotope mass-balance. Our results provide novel insights into how Tl and its isotopes are cycled under ferruginous conditions, permitting more accurate application of the Tl isotope paleoredox proxy to the Precambrian sedimentary record.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"31 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884586","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 : 2024-12-13DOI: 10.1016/j.gca.2024.12.010
Xiangrong Yang, Detian Yan, David J. Wilson, Philip A.E. Pogge von Strandmann, Xianyi Liu, Chunyao Liu, Hui Tian, Mu Liu, Liwei Zhang, Bao Zhang, Daizhao Chen
The Ordovician-Silurian transition (OST) was characterised by climatic fluctuations (warming in the Katian and glaciation in the Hirnantian) and mass extinctions. However, the mechanisms driving the climatic and biological variability remain under debate. In order to reveal the relationships between volcanism, climate, and continental weathering, we measured lithium (Li) isotopes and mercury (Hg) concentrations in a carbonate-dominated marine section from South China. The reconstructed δ7Liseawater values were generally ∼ 21 ‰ during the Ordovician-Silurian transition, with negative excursions towards ∼ 16 ‰ in the latest Katian and the latest Hirnantian intervals. We infer that changes in continental weathering affected dissolved riverine Li fluxes and δ7Li values, and thereby exerted a major control on the seawater δ7Li variations, while changes in temperature that influenced isotope fractionation during weathering and reverse weathering exerted a secondary control. In the Late Katian, intense volcanic activity (high Hg/TOC ratios and low δ13C values) likely initiated the climatic warming (late Boda warming), which was sustained by enhanced clay formation (δ7Liseawater values of ∼ 21 ‰). The intense volcanism also contributed to the high primary productivity and expansion of ocean anoxia, accounting for the Katian extinction. In the latest Katian and latest Hirnantian, enhanced and more congruent weathering (δ7Liseawater values of 16 ‰) likely contributed to the initiation of global cooling and further glaciation. Meanwhile, the weathering-induced expansion of euxinic seawater could have driven the Late Ordovician Mass Extinction (LOME) events. During the Hirnantian glacial intervals, decreased and incongruent weathering could have contributed to reduced CO2 drawdown, ultimately allowing warming and climatic recovery. Overall, the climatic fluctuations during the OST were related to changes in continental weathering, while the multi-phase biotic extinctions could be attributed to volcanism and/or weathering-induced oceanic anoxia.
{"title":"Lithium isotope and mercury evidence for enhanced continental weathering and intense volcanism during the Ordovician-Silurian transition","authors":"Xiangrong Yang, Detian Yan, David J. Wilson, Philip A.E. Pogge von Strandmann, Xianyi Liu, Chunyao Liu, Hui Tian, Mu Liu, Liwei Zhang, Bao Zhang, Daizhao Chen","doi":"10.1016/j.gca.2024.12.010","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.010","url":null,"abstract":"The Ordovician-Silurian transition (OST) was characterised by climatic fluctuations (warming in the Katian and glaciation in the Hirnantian) and mass extinctions. However, the mechanisms driving the climatic and biological variability remain under debate. In order to reveal the relationships between volcanism, climate, and continental weathering, we measured lithium (Li) isotopes and mercury (Hg) concentrations in a carbonate-dominated marine section from South China. The reconstructed δ<ce:sup loc=\"post\">7</ce:sup>Li<ce:inf loc=\"post\">seawater</ce:inf> values were generally ∼ 21 ‰ during the Ordovician-Silurian transition, with negative excursions towards ∼ 16 ‰ in the latest Katian and the latest Hirnantian intervals. We infer that changes in continental weathering affected dissolved riverine Li fluxes and δ<ce:sup loc=\"post\">7</ce:sup>Li values, and thereby exerted a major control on the seawater δ<ce:sup loc=\"post\">7</ce:sup>Li variations, while changes in temperature that influenced isotope fractionation during weathering and reverse weathering exerted a secondary control. In the Late Katian, intense volcanic activity (high Hg/TOC ratios and low δ<ce:sup loc=\"post\">13</ce:sup>C values) likely initiated the climatic warming (late Boda warming), which was sustained by enhanced clay formation (δ<ce:sup loc=\"post\">7</ce:sup>Li<ce:inf loc=\"post\">seawater</ce:inf> values of ∼ 21 ‰). The intense volcanism also contributed to the high primary productivity and expansion of ocean anoxia, accounting for the Katian extinction. In the latest Katian and latest Hirnantian, enhanced and more congruent weathering (δ<ce:sup loc=\"post\">7</ce:sup>Li<ce:inf loc=\"post\">seawater</ce:inf> values of 16 ‰) likely contributed to the initiation of global cooling and further glaciation. Meanwhile, the weathering-induced expansion of euxinic seawater could have driven the Late Ordovician Mass Extinction (LOME) events. During the Hirnantian glacial intervals, decreased and incongruent weathering could have contributed to reduced CO<ce:inf loc=\"post\">2</ce:inf> drawdown, ultimately allowing warming and climatic recovery. Overall, the climatic fluctuations during the OST were related to changes in continental weathering, while the multi-phase biotic extinctions could be attributed to volcanism and/or weathering-induced oceanic anoxia.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"8 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884587","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}
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