Marine carbonates,the major carrier of carbon upon the upper crust,can be subducted into the Earth’s interior along with oceanic crust,and then returned to the surface through magmatism,which constitute the deep carb...Marine carbonates,the major carrier of carbon upon the upper crust,can be subducted into the Earth’s interior along with oceanic crust,and then returned to the surface through magmatism,which constitute the deep carbon cycle.This process plays an important role in modulating the CO_(2) concentrations in the atmosphere over geologic time,and thus the forming of the habitable earth.Therefore,identifying recycled marine carbonates in the mantle is critical to well understand the global deep carbon cycle.Calcium is one of the major constituent cations in marine carbonates and its isotopes may be a potential tracer for recycled marine carbonates in the mantle.To further evaluate the capability and challenges of Ca isotopes as such a geochemical tracer,we reviewed the Ca isotopic compositions in important reservoirs and the behavior of Ca isotopes during high-temperature geological processes that are related to the deep carbon cycle,including plate subduction,mantle metasomatism,mantle partial melting,magma differentiation,etc.Available studies show that carbonate-rich marine sediments have significantly lowerδ^(44/40) Ca than the Earth mantle,and metasomatism by such recycled materials can cause lighter Ca isotopic compositions in deep mantle-derived rocks than those of the depleted mantle and mid ocean ridge basalts.However,the Ca isotopic fractionation during partial melting of mantle peridotites is small(~0.10‰)and the Ca isotopic fractionation during plate subduction and intermediate-mafic magma evolution is indistinguishable.These investigations suggest that Ca isotopes have great advances in tracing such recycled materials in the mantle.However,other processes(such as the influence by partial melts of eclogites)may induce similar effects on mantle-derived rocks as subducted marine carbonates but still remains debated,and thus further investigations are strongly needed in the future.展开更多
To investigate the stable chromium(Cr)isotope variations during melt percolation in the mantle,we ana-lyzed the Cr isotopic compositions of fresh ultramafic rocks from the Balmuccia and Baldissero peridotite massifs l...To investigate the stable chromium(Cr)isotope variations during melt percolation in the mantle,we ana-lyzed the Cr isotopic compositions of fresh ultramafic rocks from the Balmuccia and Baldissero peridotite massifs located in the Italian Alps.These massifs represent fragments of the subcontinental lithospheric mantle.The samples collected included lherzolites,harzburgites,dunites,and pyroxenites.Lherzolites,formed through 5%-15%fractional melting of a primitive mantle source,exhibited δ^(53)Crvalues ranging from−0.13‰±0.03‰to−0.03‰±0.03‰.These values correlated negatively with Al_(2)O_(3)content,sug-gesting that partial melting induces Cr isotopic fractionation between the melts and residual peridotites.Harzburgites and dunites,influenced by the silicate melt percolation,displayed distinctδ^(53)Cr values.Notably,dunites not spatially associated with the pyroxenite veins exhibited slightly elevatedδ^(53)Cr val-ues(−0.05‰±0.03‰to 0.10‰±0.03‰)relative to lherzolites.This difference likely resulted from pyroxene dissolution and olivine precipitation during melt percolation processes.However,one dunite sample in direct contact with pyroxenite veins showed lowerδ^(53)Cr values(−0.26‰±0.03‰),possibly owing to the kinetic effects during silicate melt percolation.Pyroxenites are formed through the interac-tion of basaltic melts with the surrounding peridotite via a metasomatic reaction or crystallization in a vein.Most of theirδ^(53)Cr values(−0.26‰±0.03‰to−0.13‰±0.03‰)are positively correlated with MgO contents,suggesting that they were influenced by magmatic differentiation.However,two subsam-ples from a single clinopyroxenite vein exhibit anomalously lowδ^(53)Crvalues(−0.30‰±0.03‰and−0.43‰±0.03‰),which are attributed to kinetic isotopic fractionation during melt-percolation pro-cesses.Our findings suggest that melt percolation processes in the mantle contribute to the Cr isotopic heterogeneity observed within the Earth’s mantle.展开更多
The Longshan orogenic belt is located in the southwestern margin of Ordos Basin at the junction zone between the Western Qinling and Northern Qilian orogenic belt.Voluminous Early Paleozoic magmatism in this area is o...The Longshan orogenic belt is located in the southwestern margin of Ordos Basin at the junction zone between the Western Qinling and Northern Qilian orogenic belt.Voluminous Early Paleozoic magmatism in this area is of key significance for determining the Early Paleozoic tectonic evolution and deep crust-mantle structure.Previous studies mainly focused on the Paleozoic granites;the coeval mafic rocks in this area are still poorly understood.A set of Late Silurian intraplate tholeiitic basalts has been discovered in Longshan area,providing key evidence for the mantle source and deep geodynamic background in this area.The Late Silurian Angou basalt has similar geochemical features as intraplate tholeiitic basalt,with high Na_(2)O/K_(2)O ratios(5.22-8.25),enriched in large ion lithophile elements and LREE.In combination with their relatively evolved Sr-Nd isotopic composition[^(87)Sr/^(86)Sr(i)=0.7128-0.7140;ε_(Nd)(t)=-5.55 to-3.40],it is suggested that it originated from decompression melting of metasomatized enriched mantle in extensional setting.These results indicate that the mantle source in the junction zone of the West Qinling-North Qilian orogenic belt evolved from depleted to enriched with the continuation of Proto-Tethys subduction from the Cambrian to the Silurian.These results are of great significance to understanding the genesis of contemporaneous granite and the crust-mantle interaction in the junction zone between the Western Qinling and Northern Qilian orogenic belt.展开更多
基金Supported by the National Natural Science Foundation of China(Nos.42322302,42373048)the Youth Innovation Promotion Association,Chinese Academy of Sciences(No.2022207)+1 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB42020303)the Laoshan Laboratory(No.LSKJ202204100)。
文摘Marine carbonates,the major carrier of carbon upon the upper crust,can be subducted into the Earth’s interior along with oceanic crust,and then returned to the surface through magmatism,which constitute the deep carbon cycle.This process plays an important role in modulating the CO_(2) concentrations in the atmosphere over geologic time,and thus the forming of the habitable earth.Therefore,identifying recycled marine carbonates in the mantle is critical to well understand the global deep carbon cycle.Calcium is one of the major constituent cations in marine carbonates and its isotopes may be a potential tracer for recycled marine carbonates in the mantle.To further evaluate the capability and challenges of Ca isotopes as such a geochemical tracer,we reviewed the Ca isotopic compositions in important reservoirs and the behavior of Ca isotopes during high-temperature geological processes that are related to the deep carbon cycle,including plate subduction,mantle metasomatism,mantle partial melting,magma differentiation,etc.Available studies show that carbonate-rich marine sediments have significantly lowerδ^(44/40) Ca than the Earth mantle,and metasomatism by such recycled materials can cause lighter Ca isotopic compositions in deep mantle-derived rocks than those of the depleted mantle and mid ocean ridge basalts.However,the Ca isotopic fractionation during partial melting of mantle peridotites is small(~0.10‰)and the Ca isotopic fractionation during plate subduction and intermediate-mafic magma evolution is indistinguishable.These investigations suggest that Ca isotopes have great advances in tracing such recycled materials in the mantle.However,other processes(such as the influence by partial melts of eclogites)may induce similar effects on mantle-derived rocks as subducted marine carbonates but still remains debated,and thus further investigations are strongly needed in the future.
基金supported by National Natural Science Foundation of China(Grant No.42473017)Hong Kong RGC grants(JLFS/P-702/24 and 17308023)China Geological Survey project(Grant No.DD20242037).
文摘To investigate the stable chromium(Cr)isotope variations during melt percolation in the mantle,we ana-lyzed the Cr isotopic compositions of fresh ultramafic rocks from the Balmuccia and Baldissero peridotite massifs located in the Italian Alps.These massifs represent fragments of the subcontinental lithospheric mantle.The samples collected included lherzolites,harzburgites,dunites,and pyroxenites.Lherzolites,formed through 5%-15%fractional melting of a primitive mantle source,exhibited δ^(53)Crvalues ranging from−0.13‰±0.03‰to−0.03‰±0.03‰.These values correlated negatively with Al_(2)O_(3)content,sug-gesting that partial melting induces Cr isotopic fractionation between the melts and residual peridotites.Harzburgites and dunites,influenced by the silicate melt percolation,displayed distinctδ^(53)Cr values.Notably,dunites not spatially associated with the pyroxenite veins exhibited slightly elevatedδ^(53)Cr val-ues(−0.05‰±0.03‰to 0.10‰±0.03‰)relative to lherzolites.This difference likely resulted from pyroxene dissolution and olivine precipitation during melt percolation processes.However,one dunite sample in direct contact with pyroxenite veins showed lowerδ^(53)Cr values(−0.26‰±0.03‰),possibly owing to the kinetic effects during silicate melt percolation.Pyroxenites are formed through the interac-tion of basaltic melts with the surrounding peridotite via a metasomatic reaction or crystallization in a vein.Most of theirδ^(53)Cr values(−0.26‰±0.03‰to−0.13‰±0.03‰)are positively correlated with MgO contents,suggesting that they were influenced by magmatic differentiation.However,two subsam-ples from a single clinopyroxenite vein exhibit anomalously lowδ^(53)Crvalues(−0.30‰±0.03‰and−0.43‰±0.03‰),which are attributed to kinetic isotopic fractionation during melt-percolation pro-cesses.Our findings suggest that melt percolation processes in the mantle contribute to the Cr isotopic heterogeneity observed within the Earth’s mantle.
基金supported by the National Natural Science Foundation of China(42172010,42372071,41102037)。
文摘The Longshan orogenic belt is located in the southwestern margin of Ordos Basin at the junction zone between the Western Qinling and Northern Qilian orogenic belt.Voluminous Early Paleozoic magmatism in this area is of key significance for determining the Early Paleozoic tectonic evolution and deep crust-mantle structure.Previous studies mainly focused on the Paleozoic granites;the coeval mafic rocks in this area are still poorly understood.A set of Late Silurian intraplate tholeiitic basalts has been discovered in Longshan area,providing key evidence for the mantle source and deep geodynamic background in this area.The Late Silurian Angou basalt has similar geochemical features as intraplate tholeiitic basalt,with high Na_(2)O/K_(2)O ratios(5.22-8.25),enriched in large ion lithophile elements and LREE.In combination with their relatively evolved Sr-Nd isotopic composition[^(87)Sr/^(86)Sr(i)=0.7128-0.7140;ε_(Nd)(t)=-5.55 to-3.40],it is suggested that it originated from decompression melting of metasomatized enriched mantle in extensional setting.These results indicate that the mantle source in the junction zone of the West Qinling-North Qilian orogenic belt evolved from depleted to enriched with the continuation of Proto-Tethys subduction from the Cambrian to the Silurian.These results are of great significance to understanding the genesis of contemporaneous granite and the crust-mantle interaction in the junction zone between the Western Qinling and Northern Qilian orogenic belt.
