Trace Element Composition of Mantle-derived Carbonates and Coexisting Phasesin Peridotite Xenoliths from Alkali Basalts

Abstract

The trace element compositions of carbonate, clinopyroxene, amphibole and silicate glass were determined in four mantle lherzolite xenoliths in alkali basalts from Spitsbergen and Mongolia by laser ablation ICP-MS. Carbonates in the xenoliths occur in fine-grained pockets that appear to have been produced by reaction of carbonate-rich melts with the host peridotites. The carbonates are rich in Sr and Ba, but have low contents of rare earth elements. (Na,Al)-rich silicate glass commonly associated with the carbonates is a major host for many incompatible lithophile elements in the carbonate-bearing pockets. The carbonates in the xenoliths do not appear to represent quenched carbonatite liquids but probably are crystal cumulates from carbonate-rich melts. Trace element patterns estimated for liquids that may have produced the carbonate-bearing pockets are consistent with general characteristics of carbonate-related metasomatism (enrichments in light rare earth elements, Th, U, Ba and negative anomalies for high field strength elements). However, the absolute incompatible element abundances estimated for those liquids cannot provide the extremely strong enrichments invoked by some models of carbonate mantle metasomatism. Clinopyroxene and amphibole outside the carbonate-bearing pockets in the xenoliths from Spitsbergen have high contents of incompatible trace elements, indicating that the lherzolites also experienced metasomatic enrichment before the formation of the carbonates.