The components and evolution of subcontinental lithospheric mantle beneath the North China Craton and the Yangtze Craton is a current topic in the geological study of China and the carbon isotopic composition of diamo...The components and evolution of subcontinental lithospheric mantle beneath the North China Craton and the Yangtze Craton is a current topic in the geological study of China and the carbon isotopic composition of diamond is one of the most direct probes into cratonic lithospheric mantle processes.In this paper,in-situ SIMS(Secondary Ion Mass Spectrometry) techniques were used to analyze the carbon isotope compositions at different internal growth zones of diamonds from Shandong and Liaoning in the North China Craton and Hunan in the Yangtze Craton.It was found that the carbon isotopic range of diamonds from the North China Craton are rather distinct from those of the Yangtze Craton;the former has a range of 6.0‰ to 2.0‰(relative to VPDB) with an average value of 3.0‰ in their core areas,which is consistent with global peridotitic diamonds;the diamonds from the Yangtze Craton,however,have a carbon isotopic range from 8.6‰ to 3.0‰ with an average value of 7.4‰ in their core areas,being more consistent with global eclogitic diamonds.The variations of carbon isotope ratios between different internal growth zones in individual diamonds were different in the three diamond localities studied.There was a clear correlation between changes in carbon isotopic composition and phases of diamond dissolution and new growth,while no correlation was observed between δ13C and internal inclusions.The variations suggest that the carbon isotopic compositions of mantle fluids were changing during the process of diamond crystallization,and that the heterogeneity of the carbon isotopic composition in mantle carbon reservoirs was a more important factor than carbon isotope fractionation in controlling the carbon isotopic compositions and their variation in diamonds.In addition,the preliminary results of in-situ nitrogen analyses demonstrated that the variation of carbon isotopic compositions between the core and outer growth zones does not correlate with nitrogen abundances,implying either that diamonds crystallized in an open environment or that the carbon isotopic composition and nitrogen contents in mantle fluids were controlled by other,not yet understood factors.The experimental results provide hints that the isotopic composition of carbon and its original sources were different in metasomatic fluids controlling diamond formation in the mantle beneath the North China Craton and the Yangtze Craton.展开更多
Aluminum-rich chondrules(ARCs),which share mineralogic and chemical properties with both Ca,Al-rich inclusions(CAIs)and ferromagnesian chondrules,play an important role in revealing their temporal and petrogenetic rel...Aluminum-rich chondrules(ARCs),which share mineralogic and chemical properties with both Ca,Al-rich inclusions(CAIs)and ferromagnesian chondrules,play an important role in revealing their temporal and petrogenetic relationships.In this work,seven ARCs were found in three ordinary chondrites GRV 022410(H4),GRV 052722(H3.7)and Julesburg(L3.6).They contain bulk Al2O3~17%-33%and exhibit igneous textures composed of olivine,high-and low-Ca pyroxene,plagioclase,spinel and glass.In situ SIMS analyses show that ARCs have oxygen isotopic compositions(δ18O=–6.1‰–7.1‰;δ17O=–4.5‰–5.1‰)close to ferromagnesian chondrules but far more depleted in 16O than CAIs(δ18O=-40‰;δ17O=-40‰).Most ARCs plot close to the terrestrial mass fractionation(TF)line,and a few between the TF and carbonaceous chondrite anhydrous mixing(CCAM)lines.Plagioclase,nepheline and glass suffered O-isotopic exchanges during the metamorphism processes in the parent body.Spinel,olivine and pyroxene represent the primary O-isotopic compositions of ARCs,and define a fitted line with a slope of^0.7±0.1.Compared with the results of previous studies,shallower slope as well as more depleted16O compositions further demonstrates that ARCs in ordinary chondrites are not a simple mixing product of ferromagnesian chondrules and CAIs.Instead,they probably experienced higher-degree oxygen isotope exchange with a 16O-poor nebular gas reservoir during multiple melting episodes.展开更多
基金supported by the Public Research Project of the Ministry of Land and Resources of the People’s Republic of China(200811012)the National Natural Science Foundation of China(41272086)
文摘The components and evolution of subcontinental lithospheric mantle beneath the North China Craton and the Yangtze Craton is a current topic in the geological study of China and the carbon isotopic composition of diamond is one of the most direct probes into cratonic lithospheric mantle processes.In this paper,in-situ SIMS(Secondary Ion Mass Spectrometry) techniques were used to analyze the carbon isotope compositions at different internal growth zones of diamonds from Shandong and Liaoning in the North China Craton and Hunan in the Yangtze Craton.It was found that the carbon isotopic range of diamonds from the North China Craton are rather distinct from those of the Yangtze Craton;the former has a range of 6.0‰ to 2.0‰(relative to VPDB) with an average value of 3.0‰ in their core areas,which is consistent with global peridotitic diamonds;the diamonds from the Yangtze Craton,however,have a carbon isotopic range from 8.6‰ to 3.0‰ with an average value of 7.4‰ in their core areas,being more consistent with global eclogitic diamonds.The variations of carbon isotope ratios between different internal growth zones in individual diamonds were different in the three diamond localities studied.There was a clear correlation between changes in carbon isotopic composition and phases of diamond dissolution and new growth,while no correlation was observed between δ13C and internal inclusions.The variations suggest that the carbon isotopic compositions of mantle fluids were changing during the process of diamond crystallization,and that the heterogeneity of the carbon isotopic composition in mantle carbon reservoirs was a more important factor than carbon isotope fractionation in controlling the carbon isotopic compositions and their variation in diamonds.In addition,the preliminary results of in-situ nitrogen analyses demonstrated that the variation of carbon isotopic compositions between the core and outer growth zones does not correlate with nitrogen abundances,implying either that diamonds crystallized in an open environment or that the carbon isotopic composition and nitrogen contents in mantle fluids were controlled by other,not yet understood factors.The experimental results provide hints that the isotopic composition of carbon and its original sources were different in metasomatic fluids controlling diamond formation in the mantle beneath the North China Craton and the Yangtze Craton.
基金supported by the Natural Science Foundation of Jiangsu Province(Grant No.BK20131040)the National Natural Science Foundation of China(Grants Nos.41403056,41173076,41273079,41003026)the Minor Planet Foundation of China
文摘Aluminum-rich chondrules(ARCs),which share mineralogic and chemical properties with both Ca,Al-rich inclusions(CAIs)and ferromagnesian chondrules,play an important role in revealing their temporal and petrogenetic relationships.In this work,seven ARCs were found in three ordinary chondrites GRV 022410(H4),GRV 052722(H3.7)and Julesburg(L3.6).They contain bulk Al2O3~17%-33%and exhibit igneous textures composed of olivine,high-and low-Ca pyroxene,plagioclase,spinel and glass.In situ SIMS analyses show that ARCs have oxygen isotopic compositions(δ18O=–6.1‰–7.1‰;δ17O=–4.5‰–5.1‰)close to ferromagnesian chondrules but far more depleted in 16O than CAIs(δ18O=-40‰;δ17O=-40‰).Most ARCs plot close to the terrestrial mass fractionation(TF)line,and a few between the TF and carbonaceous chondrite anhydrous mixing(CCAM)lines.Plagioclase,nepheline and glass suffered O-isotopic exchanges during the metamorphism processes in the parent body.Spinel,olivine and pyroxene represent the primary O-isotopic compositions of ARCs,and define a fitted line with a slope of^0.7±0.1.Compared with the results of previous studies,shallower slope as well as more depleted16O compositions further demonstrates that ARCs in ordinary chondrites are not a simple mixing product of ferromagnesian chondrules and CAIs.Instead,they probably experienced higher-degree oxygen isotope exchange with a 16O-poor nebular gas reservoir during multiple melting episodes.
