Study Of Ca Isotope Of Continental Basalts From Eastern China

Carbon dioxide?CO₂?is one of the most important greenhouse gas in the atmosphere.Marine carbonates as an important sink of atmospheric CO₂ can be transported into the mantle by subduction and returned to the surface by degassing of volcanoes,which is termed as deep carbon cycling.Deep carbon cycling may control CO₂ fluctuation of the atmosphere over geological time,and thus influence long-term climate.Therefore,it is important to trace deep carbon cycle over past.Calciuim isotopes have been proposed as a potential tool to trace deep carbon recycling,while magma processes and other recycled crustal components can also lead to Ca isotopic variations in igneous rocks.Therefore,it needs further studies how to use Ca isotopes tracing deep carbon cycle.Here,we report Ca isotopic data of 26 well characterized Mesozoic to Cenozoic basalts from eastern China,to discuss geological processes affecting Ca isotopic compositions of basalts.Mesozoic and Cenozoic basalts have variable?^44/42Ca ranging from 0.28 to 0.41 and from 0.24 to 0.40 respectively,both systematically lower than that of the mean upper mantle?0.46-0.51?and the mean oceanic basalts?0.42±0.01?.The study suggested that post-magmatic alteration,magma differentiation and mantle melting can not explain Ca isotopic variations in basalts from eastern China,but at least partially reflect mantle source heterogeneity.The following two processes are likely to contribute to the low?^44/42Ca of basalts in eastern China:?i?Mantle metasomatism by recycled carbonates.A binary mixing model show that metasomatic agency should have extremely low?^44/42Ca?-0.5^-1.5?,which can't be explained directly by sedimentary carbonates(almost all the sedimentary carbonate:?^44/42Ca>0).This mechanism requires that metasomatic agency have significant Ca isotopic fractionation relative to the sedimentary carbonate.The fractionation can occur in the process of dehydration of subducted slabs and/or partial melting of the carbonate-bearing oceanic crust at the depth of the mantle transition zone.?ii?Mantle metasomatism by silicate melts.High silicate melts by partial melting of eclogitic sources at high pressure can have light Ca isotopic compositions.?^44/42Ca of the melts can further decrease by interacting with mantle peridotites.Partial melting of mantle sources that have hybridized by the resulting lower?^44/42Ca melts can explan the low?^44/42Ca Mesozoic basalts,and also provides a new possibility for the origin of low?^44/42Ca of some Cenozoic basalts.This study based on Ca isotopic data of continental basalts from eastern China suggests that origin of low?^44/42Ca basalts is complicated.A combination with other elemental and isotopic indices is required to use Ca isotopes to trace deep carbon cycling.