Chemical and carbon isotope compositions of fluid inclusions in peridotite xenoliths and eclogites from eastern China: geodynamic implications

Abstract

Chemical and carbon isotope compositions of fluid inclusions were measured for peridotite xenoliths enclosed in Cenozoic basalts and Triassic ultrahigh pressure eclogites from the Dabie terrane in eastern China to provide insight into the nature of their related fluids. The results show that the inclusions contain different amounts of gaseous CO₂, N₂, H₂S, CO, CH₄ and H₂, some of which have significant amounts of H₂. This may reflect chemical heterogeneity in mantle fluids of eastern China and possible metasomatism to the mantle-derived rocks during their eruption and exhumation to surface. There is a variation in δ ¹³C from −12.1 to 0.7‰ for the fluid inclusions in the peridotite xenoliths. Although the higher δ ¹³C values may be either derived from decarbonation of sedimentary carbonates or responsible for the primary mantle carbon remained in the mantle-derived rocks, the lower δ ¹³C values below −15‰ suggest the incorporation of organic carbon by plate subduction into the continental lithospheric mantle in eastern China.

The fluid inclusions in the eclogites from the Dabie terrane also exhibit a large δ ¹³C variation from −18.5 to 4.6‰, similar to previously reported δ ¹³C features measured for bulk carbon in eclogites from the Dabie-Sulu terranes. While the low δ¹³C values result from hydrothermal alteration to the eclogite protoliths by ¹³C-depleted surface fluid before plate subduction, the high δ¹³C values indicate overprinting of ¹³C-rich CO₂-bearing fluid derived from leaching marble lithologies subsequent to the ultrahigh pressure metamorphism. A geodynamic model is proposed to account for the geochemical recycling of carbon in the processes of plate subduction and prograde metamorphism as well as plate break-off and exhumation. It appears that the ¹³C-depleted slab may be broken off at mantle depths and undergone partial melting to generate basaltic magma and peridotite melt. This provides a genetic links between the ¹³C-depleted peridotites and eclogites.