Oxygen isotope and trace element compositions of peridotite xenoliths from Nushan volcano, SE China and implications for mantle metasomatism.

Oxygen isotopic compositions and trace element contents of minerals from 9 peridotite xenoliths hosted by Cenozoic basalts in Nushan, SE China have been obtained by laser fluorination method and LA-ICP-MS respectively. Some samples don't show oxygen isotope equilibrium between pyroxene and olivine with the lowest △Cpx-Ol18O (= δ18OCpx-δ18OOl) of -0. 5‰. Clinopyroxenes from the amphibole-bearing xenoliths are enriched in LILE (Sr, Th, LREE) and depleted in HFSE (Nb, Zr, Ti). Oxygen isotope data suggest that the Nushan upper mantle experienced metasomatism by oceanic fluids from the subducted plate. Based on the negative correlation between △Cpx-Ol18O and formation depth of Nushan xenoliths, we propose an ascending + buffering model for mantle metasomatism in Nushan. The degree of oxygen isotopic disequilibrium is not related with LILE enrichment and HFSE depletion , suggesting at least two metasomatic events happened in the Nushan mantle, one was responsible to LILE enrichment/ HFSE depletion and another to oxygen isotopic disequilibrium.