Ground state properties for Mg isotopes,including binding energies,one- and two-neutron separation energies,pairing energies,nuclear matter radii and quadrupole deformation parameters,are obtained from the selfconsist...Ground state properties for Mg isotopes,including binding energies,one- and two-neutron separation energies,pairing energies,nuclear matter radii and quadrupole deformation parameters,are obtained from the selfconsistent relativistic mean field(RMF) model with the pairing correlations treated by a shell-mode-like approach(SLAP),in which the particle-number is conserved and the blocking effects are treated exactly.The experimental data,including the binding energies and the one- and two-neutron separation energies,which are sensitive to the treatment of pairing correlations and block effects,are well reproduced by the RMF+SLAP calculations.展开更多
Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater.Mg isotopes are significantly(up to 5%)fractionated during carbonate precipitation by preferential incorporati...Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater.Mg isotopes are significantly(up to 5%)fractionated during carbonate precipitation by preferential incorporation of ^(24)Mg,while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of^( 26)Mg.Thus,the marine Mg cycle could be reflected by seawater Mg isotopic composition(δ^(26)Mg_(sw)),which might be recorded in marine carbonate.However,carbonates are both texturally and compositionally heterogeneous,and it is unclear which carbonate component is the most reliable for reconstructing δ^(26)Mg_(sw).In this study,we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China.Based on petrographic studies,four carbonate components were recognized:micrite,marine cement,brachiopod shell,and mixture.The four components had distinct δ^(26)Mg:(1)micrite samples ranged from -2.86% to -2.97%;(2)pure marine cements varied from -3.40% to -3.54%,while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range(-3.27% to-3.75%);(3)values for the mixture component were-3.17% and -3.49%;and (4)brachiopod shells ranged from -2.20% to -3.07%,with the thickened hinge area enriched in ^( 24)Mg.Due to having multiple carbonate sources,neither the micrite nor the mixture component could be used to reconstruct δ^(26)Mg_(sw).In addition,the marine cement was homogenous in Mg isotopes,but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ^(26)Mg_(sw).Furthermore,brachiopod shells had heterogeneous C and Mg isotopes,suggesting a significant vital effect during growth.Overall,the heterogeneous δ^(26)Mg of the Huangjin limestone makes it difficult to reconstruct δ^(26)Mg_(sw)using bulk carbonate/calcareous sediments.Finally,δ^(26)Mg_(sw)was only slightly affected by the faunal composition of carbonate-secreting organisms,even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range(0.2%–4.8%)of fractionation during biogenic carbonate formation.展开更多
Seafloor hydrothermal systems play a significant role in the oceanic Mg cycle due to ubiquitous deposits of secondary Mg-rich clays during the strong fluid-rock reactions.However,the magnitude of net Mg enrichment and...Seafloor hydrothermal systems play a significant role in the oceanic Mg cycle due to ubiquitous deposits of secondary Mg-rich clays during the strong fluid-rock reactions.However,the magnitude of net Mg enrichment and Mg isotopic fractionation,particularly within the medium-high temperature hydrothermal systems in felsic-hosted settings,are not well studied yet.Here we report elemental and isotopic compositions of Mg in hydrothermal chlorite-rich sediments,volcanic materials,and terrigenous sediments collected during the IODP Expedition 331 drilled to the thick sediment-covered and felsic-hosted middle Okinawa Trough(Iheya North Knoll) in the West Pacific.We investigate the sources of Mg in chlorite and Mg isotopic behavior at medium-high temperature hydrothermal alteration.After 1 mol/L HCl leaching,Mg isotopic compositions of chlorite-rich sediments present overall similar values in the residual fractions and bulk samples albeit with slightly higher values in the leachates.Mineralogical differentiation primarily determines the Mg isotopic compositions,showing that siliciclastic residues have slightly higher δ^(26) Mg values than the leachates dominated by carbonates and oxides/hydroxides.Significant Mg isotopic fractionation happened in the medium-high temperature(~150°C to 260°C) felsic-hosted hydrothermal system,with Δ^(26)MgChl-SW ranging from 0.15‰ to 0.71‰ and yielding a negative correlation with temperature.This observation suggests the preferential incorporation of heavy Mg isotopes by the secondary chlorite precipitation.We infer that the medium-high temperature hydrothermal systems can take up about 8–14% of riverine input of Mg in the arc and back-arc regions.Incomplete removal of aqueous Mg in porewater and vent fluids by the medium-high temperature hydrothermal alterations in the arc and back-arc basins provides constraints on the Mg budget and isotopic composition of seawater.展开更多
Dolomite genesis is a century-old mystery in sedimentology.To reveal the mechanism of dolomite genesis,two core problems need to be addressed.The first is the origin and migration mechanism of Mg^(2+)-rich fluids duri...Dolomite genesis is a century-old mystery in sedimentology.To reveal the mechanism of dolomite genesis,two core problems need to be addressed.The first is the origin and migration mechanism of Mg^(2+)-rich fluids during the dolomitization process.The second is the kinetic barrier caused by Mg^(2+)hydration during dolomite precipitation at low temperatures.To address these problems,our study,based on detailed petrological,sedimentological,geochemical(major and trace elements),and isotopic(C-O-Mg)analysis,clarified the source and migration of Mg^(2+)-rich fluids and the kinetic barrier mechanism of lowtemperature dolomite precipitation in the Upper Sinian Qigebulake Formation and the Lower Cambrian Xiaoerbulake Formation in the Tarim Basin.First,we found that the Mg^(2+)-rich fluids required for the dolomitization of dolomite in the Xiaoerbulake Formation were primarily derived from the Early Cambrian marine fluid.At the interface of the sedimentary cycle,δ26Mg values fluctuated considerably,indicating that the sequence interface was the starting point and channel for the migration of dolomitized fluids.Sea level variation plays a major role in controlling the dolomitization process of the Xiaoerbulake Formation.Second,the Qigebulake Formation contains low-temperature dolomite with Mg^(2+)-rich fluids supplied by seawater,microorganisms,and sedimentary organic matter.Comprehensive analysis shows that the dolomite of the Qigebulake Formation was formed by microbial induction by anaerobic methane bacteria.Finally,the properties and sources of dolomitization fluids and the formation process of dolomite were the reasons for the difference in the Mg isotope composition of dolomite during the Sinian-Cambrian transition.This study reveals the genetic mechanism of the Sinian-Cambrian dolomite in the Tarim Basin and establishes a new method to explain the genesis of microbial dolomite by C-O-Mg isotopes,providing a reference for the reconstruction of the formation and evolution of dolomites.展开更多
To better understand the mechanism of Mg isotopic variation in magma systems, here we report high precision Mg isotopic data of 17 bulk rock samples including dunite, clinopyroxenite, hornblendite and gabbro and 10 pa...To better understand the mechanism of Mg isotopic variation in magma systems, here we report high precision Mg isotopic data of 17 bulk rock samples including dunite, clinopyroxenite, hornblendite and gabbro and 10 pairs of dunite-hosted olivine and chromite separates from the well-characterized Alaskan-type Xiadong intrusion in NW China, which formed by continuous and high degree of lithological differentiation from mafic magmas. Chromite separates have highly variable δ^(26)Mg values from -0.10‰ to 0.40‰, and are consistently heavier than coexisting olivine separates(-0.39‰ to -0.15 T‰). Both mineral δ^(26)Mg values and the degrees of inter-mineral fractionation are well correlated with geochemical indicators of magma differentiation, indicating that these inter-sample and inter-mineral Mg isotope fractionations are caused by magma evolution. The δ^(26)Mg values range from -0.20‰ to -0.02‰ in the dunite,-043‰ in the clinopyroxenite,-043‰ to -0.28‰ in the hornblendite, 0.18 T‰ in the chromite-bearing hornblendite, and -0.56 T‰ to -0.16‰ in the gabbro. The Mg isotopic variations in different types of rocks are closely related to fractional crystallization and accumulation of different proportions of oxides vs. silicates. Chromite crystallization and accumulation is the most important factor in controlling Mg isotope fractionation during the formation of the Xiadong intrusion. Compared to basaltic and granitic magmas, differentiation of the Alaskan-type intrusions occurs at a relatively high oxygen fugacity, which favors chromite crystallization and consequently significant Mg isotope fractionations at both mineral and whole-rock scales. Therefore, Mg isotope systematics can be used to trace the degree of magma differentiation and related-mineralization.展开更多
基金Supported by NSFC(11465001,11275098,11275248,11505058,11165001)Natural Science Foundation of Inner Mongolia of China(2016BS0102)
文摘Ground state properties for Mg isotopes,including binding energies,one- and two-neutron separation energies,pairing energies,nuclear matter radii and quadrupole deformation parameters,are obtained from the selfconsistent relativistic mean field(RMF) model with the pairing correlations treated by a shell-mode-like approach(SLAP),in which the particle-number is conserved and the blocking effects are treated exactly.The experimental data,including the binding energies and the one- and two-neutron separation energies,which are sensitive to the treatment of pairing correlations and block effects,are well reproduced by the RMF+SLAP calculations.
基金supported by the Natural Science Foundation of China (41272017, 41322021, and 41172001)the China Postdoctoral Science Foundation (2014M55006)
文摘Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater.Mg isotopes are significantly(up to 5%)fractionated during carbonate precipitation by preferential incorporation of ^(24)Mg,while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of^( 26)Mg.Thus,the marine Mg cycle could be reflected by seawater Mg isotopic composition(δ^(26)Mg_(sw)),which might be recorded in marine carbonate.However,carbonates are both texturally and compositionally heterogeneous,and it is unclear which carbonate component is the most reliable for reconstructing δ^(26)Mg_(sw).In this study,we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China.Based on petrographic studies,four carbonate components were recognized:micrite,marine cement,brachiopod shell,and mixture.The four components had distinct δ^(26)Mg:(1)micrite samples ranged from -2.86% to -2.97%;(2)pure marine cements varied from -3.40% to -3.54%,while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range(-3.27% to-3.75%);(3)values for the mixture component were-3.17% and -3.49%;and (4)brachiopod shells ranged from -2.20% to -3.07%,with the thickened hinge area enriched in ^( 24)Mg.Due to having multiple carbonate sources,neither the micrite nor the mixture component could be used to reconstruct δ^(26)Mg_(sw).In addition,the marine cement was homogenous in Mg isotopes,but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ^(26)Mg_(sw).Furthermore,brachiopod shells had heterogeneous C and Mg isotopes,suggesting a significant vital effect during growth.Overall,the heterogeneous δ^(26)Mg of the Huangjin limestone makes it difficult to reconstruct δ^(26)Mg_(sw)using bulk carbonate/calcareous sediments.Finally,δ^(26)Mg_(sw)was only slightly affected by the faunal composition of carbonate-secreting organisms,even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range(0.2%–4.8%)of fractionation during biogenic carbonate formation.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41806229, 41730531, and 41991324)。
文摘Seafloor hydrothermal systems play a significant role in the oceanic Mg cycle due to ubiquitous deposits of secondary Mg-rich clays during the strong fluid-rock reactions.However,the magnitude of net Mg enrichment and Mg isotopic fractionation,particularly within the medium-high temperature hydrothermal systems in felsic-hosted settings,are not well studied yet.Here we report elemental and isotopic compositions of Mg in hydrothermal chlorite-rich sediments,volcanic materials,and terrigenous sediments collected during the IODP Expedition 331 drilled to the thick sediment-covered and felsic-hosted middle Okinawa Trough(Iheya North Knoll) in the West Pacific.We investigate the sources of Mg in chlorite and Mg isotopic behavior at medium-high temperature hydrothermal alteration.After 1 mol/L HCl leaching,Mg isotopic compositions of chlorite-rich sediments present overall similar values in the residual fractions and bulk samples albeit with slightly higher values in the leachates.Mineralogical differentiation primarily determines the Mg isotopic compositions,showing that siliciclastic residues have slightly higher δ^(26) Mg values than the leachates dominated by carbonates and oxides/hydroxides.Significant Mg isotopic fractionation happened in the medium-high temperature(~150°C to 260°C) felsic-hosted hydrothermal system,with Δ^(26)MgChl-SW ranging from 0.15‰ to 0.71‰ and yielding a negative correlation with temperature.This observation suggests the preferential incorporation of heavy Mg isotopes by the secondary chlorite precipitation.We infer that the medium-high temperature hydrothermal systems can take up about 8–14% of riverine input of Mg in the arc and back-arc regions.Incomplete removal of aqueous Mg in porewater and vent fluids by the medium-high temperature hydrothermal alterations in the arc and back-arc basins provides constraints on the Mg budget and isotopic composition of seawater.
基金supported by the Scientific Research and Technology Development Project“Research on Marine Carbonate Reservoir Forming Theory and Exploration Technology”(Grant No.2021DJ05)of CNPC。
文摘Dolomite genesis is a century-old mystery in sedimentology.To reveal the mechanism of dolomite genesis,two core problems need to be addressed.The first is the origin and migration mechanism of Mg^(2+)-rich fluids during the dolomitization process.The second is the kinetic barrier caused by Mg^(2+)hydration during dolomite precipitation at low temperatures.To address these problems,our study,based on detailed petrological,sedimentological,geochemical(major and trace elements),and isotopic(C-O-Mg)analysis,clarified the source and migration of Mg^(2+)-rich fluids and the kinetic barrier mechanism of lowtemperature dolomite precipitation in the Upper Sinian Qigebulake Formation and the Lower Cambrian Xiaoerbulake Formation in the Tarim Basin.First,we found that the Mg^(2+)-rich fluids required for the dolomitization of dolomite in the Xiaoerbulake Formation were primarily derived from the Early Cambrian marine fluid.At the interface of the sedimentary cycle,δ26Mg values fluctuated considerably,indicating that the sequence interface was the starting point and channel for the migration of dolomitized fluids.Sea level variation plays a major role in controlling the dolomitization process of the Xiaoerbulake Formation.Second,the Qigebulake Formation contains low-temperature dolomite with Mg^(2+)-rich fluids supplied by seawater,microorganisms,and sedimentary organic matter.Comprehensive analysis shows that the dolomite of the Qigebulake Formation was formed by microbial induction by anaerobic methane bacteria.Finally,the properties and sources of dolomitization fluids and the formation process of dolomite were the reasons for the difference in the Mg isotope composition of dolomite during the Sinian-Cambrian transition.This study reveals the genetic mechanism of the Sinian-Cambrian dolomite in the Tarim Basin and establishes a new method to explain the genesis of microbial dolomite by C-O-Mg isotopes,providing a reference for the reconstruction of the formation and evolution of dolomites.
基金financially supported by the National Key R&D Program of China (2017YF0601204)National Natural Science Foundation of China (41522203)National Science Foundation of United States(EAR-1747706)
文摘To better understand the mechanism of Mg isotopic variation in magma systems, here we report high precision Mg isotopic data of 17 bulk rock samples including dunite, clinopyroxenite, hornblendite and gabbro and 10 pairs of dunite-hosted olivine and chromite separates from the well-characterized Alaskan-type Xiadong intrusion in NW China, which formed by continuous and high degree of lithological differentiation from mafic magmas. Chromite separates have highly variable δ^(26)Mg values from -0.10‰ to 0.40‰, and are consistently heavier than coexisting olivine separates(-0.39‰ to -0.15 T‰). Both mineral δ^(26)Mg values and the degrees of inter-mineral fractionation are well correlated with geochemical indicators of magma differentiation, indicating that these inter-sample and inter-mineral Mg isotope fractionations are caused by magma evolution. The δ^(26)Mg values range from -0.20‰ to -0.02‰ in the dunite,-043‰ in the clinopyroxenite,-043‰ to -0.28‰ in the hornblendite, 0.18 T‰ in the chromite-bearing hornblendite, and -0.56 T‰ to -0.16‰ in the gabbro. The Mg isotopic variations in different types of rocks are closely related to fractional crystallization and accumulation of different proportions of oxides vs. silicates. Chromite crystallization and accumulation is the most important factor in controlling Mg isotope fractionation during the formation of the Xiadong intrusion. Compared to basaltic and granitic magmas, differentiation of the Alaskan-type intrusions occurs at a relatively high oxygen fugacity, which favors chromite crystallization and consequently significant Mg isotope fractionations at both mineral and whole-rock scales. Therefore, Mg isotope systematics can be used to trace the degree of magma differentiation and related-mineralization.
