Chemical and carbon isotope compositions of fluid inclusions in peridotitexenoliths and eclogites from eastern China: Geodynamic implications

Chemical and carbon isotope compositions of fluid inclusions were measured for peridotite xenoliths enclosed in Cenozoic basalts and Triassic ultrahig h pressure eclogites from the Dabie terrane in eastern China to provide ins ight into the nature of their related fluids. The results show that the inc lusions contain different amounts of gaseous CO2, N-2, H2S, CO, CH4 and H-2 , some of which have significant amounts of H-2. This may reflect chemical heterogeneity in mantle fluids of eastern China and possible metasomatism t o the mantle-derived rocks during their eruption and exhumation to surface. There is a variation in delta C-13 from -25.1 to 0.7 parts per thousand fo r the fluid inclusions in the peridotite xenoliths. Although the higher del ta C-13 values may be either derived from decarbonation of sedimentary carb onates or responsible for the primary mantle carbon remained in the mantle- derived rocks, the lower delta C-13 values below -15 parts per thousand sug gest the incorporation of organic carbon by plate subduction into the conti nental lithospheric mantle in eastern China. The fluid inclusions in the eclogites from the Dabie terrane also exhibit a large delta C-13 variation from -18.5 to 4.6 parts per thousand, similar t o previously reported 5 13C features measured for bulk carbon in eclogites from the Dabie-Sulu terranes. While the low delta C-13 values result from h ydrothermal alteration to the eclogite protoliths by C-13-depleted surface fluid before plate subduction, the high delta C-13 values indicate overprin ting of C-13-rich CO2-bearing fluid derived from leaching marble lithologie s subsequent to the ultrahigh pressure metamorphism. A geodynamic model is proposed to account for the geochemical. recycling of carbon in the process es of plate subduction and prograde metamorphism as well as plate break-off and exhumation. It appears that the C-13-depleted slab may be broken off a t mantle depths and undergone partial melting to generate basaltic magma an d peridotite melt. This provides a genetic links between the C-13-depleted peridotites and eclogites. (C) 2001 Elsevier Science Ltd. All rights reserv ed.