Carbon isotope composition of carbon dioxide in fluid inclusions from peridotite xenoliths and eclogites in East China.

The carbon isotope composition of carbon dioxide in fluid inclusions was an alysized by step-heating technique for peridotite xenoliths enclosed by Cen ozoic basalts in East China and for eclogites in the Dabie Mountains. The r esults show a large delta C-13 variation from -22.8 to +0.7 parts per thous and for the peridotite, being different from the uniformly low delta C-13 v alues of -28 to - 20% reported previously for Cenozoic basalts and enclosed peridotite xenoliths in East China. This indicates carbon isotope heteroge neity in CO2 from the mantle fluid in East China. The higher delta C-13 val ues may be probably responsible for the primary mantle carbon remained in t he mantle-derived rocks, whereas the lower delta C-13 values suggest that t he lithospheric mantle in East China contains abundant crustal organic carb on. The eclogites from the Dabie Mountains also exhibit a large delta C-13 variation from -18.5 to +4.6 parts per thousand, being different from the u niformly low delta C-13 values of -30 to -20 parts per thousand reported pr eviously for ultrahigh pressure metamorphic rocks in East Dabie and West Su lu. While the lower delta C-13 values for the eclogites are responsible for hydrothermal alteration to the eclogite protoliths by C-13-depleted surfac e fluids prior to plate subduction, the higher delta C-13 values greater th an - 3 parts per thousand observed in this study indicate the overprinting of C-13-enriched and CO2-rich fluid derived from leaching marbles subsequen t to the ultrahigh pressure metamorphism. A geodynamic model is proposed to accound for the geochemical recycling of carbon in the processes of plate subduction and prograde metamorphism as well as plate break-off and uplift. It appears that the C-13-depleted slab may be broken off in mantle depths and underwent either partial melting to generate mafic magmas or metasomati ze the mantle to produce peridotitic melts with crustal isotopic signature. This provides a genetic link between C-13-depleted basalts , enclosed peri dotites and eclogites.