RB-SR AND SM-ND ISOTOPES IN GARNET PYROXENITE XENOLITHS FROM SIBERIAN KIMBERLITES: AN INSIGHT INTO LITHOSPHERIC MANTLE

Abstract

Rb-Sr and Sm-Nd isotope compositions of seven garnet pyroxenite xenoliths from Udachnaya and Mir kimberlites, Siberia, are presented. Five of them are websterites, and the other two are clinopyroxenites. The websterite suite is divided into two groups (Group A and Group B) according to their mineral chemistry. Group B websterites are characterized by presence of jadeite-rich clinopyroxene and pyropepoor garnet. The Garnet-orthopyroxene geobarometry indicates that Group A were produced in deep positions of the cratonic mantle (up to diamond stability field) and Group B websterites are formed at lower pressure (ca. 19 kb). Probably, the Rb-Sr isotope system was re-equilibrated at the time of kimberlite emplacement at 367 Ma. The initial Sr isotope compositions differs from one group to the other. The initial ratio 87Sr/86Sr (0.70272-0.70338) for Group A is lower than that of Group B (0.70485-0.70737). The Sm-Nd isotope system in all samples gives apparent isochron ages considerably older than that of the emplacement of host kimberlites. Group A websterites and clinopyroxenites are apparently younger (1223-582 Ma) than those of Group B (1550-1465 Ma). The origin of garnet pyroxenites is suggested as crystal accumulation from LREE-enriched melts which episodically added into peridotite-dominated lithospheric mantle. These melts played also a role of metasomatic agent to the host peridotites. A delaminated lower crustal protolith has probably been involved in Group B origin. The Sm-Nd isotope study reveals a systematic increase of initial 143Nd/144Nd with time. A vairable Sm-Nd isotope data suggest that peridotites and pyroxenites in lithospheric mantle beneath the Udachnaya area were in isotopic equilibrium, but not beneath the Mir area where peridotites depleted in initial Nd isotope composition.