Application of nuclear microprobe to the study of metasomatism in mantle peridotites

Abstract

In recent years, some models have been proposed to explain the chemical heterogeneity of the earth mantle. Among them the metasomatism by the mantle fluid and the mantle melt is the most popular one. The trace elemental behavior of the mantle minerals from different areas in micron scales can provide important information on the understanding of the mantle metasomatism. This paper describes the study of the metasomatism in mantle peridotites from Raobazhai, Anhui Province of China by the nuclear microprobe. The concentrations and distributions of trace elements in the metasomatized mantle peridotite were determined quantitatively by the micro-PIXE software package TTSPM. The internal standard and the integrated iterative method were used for the calculation. The matrix elements in the minerals were analyzed by EPMA. The accuracy of the quantitative analysis was examined by three geochemical standards of rocks (GSR). The results were better than 15% for most elements. According to the elemental maps, the trace element distributions in the amphibole, which was crystallized from the mantle fluid, displayed obvious heterogeneity and zoning. The enrichment of Ni, Zn, Ga, Rb, Sr, Y, Zr and Pb in the mineral could be associated with the metasomatism of the mantle fluid. The local coexistence of Ni, Cu, Pb, Fe and S in the amphibole suggested that there were xenoliths trapped in the minerals during their crystallization. The characteristics of the trace element composition and distribution in the mantle peridotites demonstrated that the metasomatism by the mantle fluid and the mantle melt might change the chemical composition of the mantle peridoties and thus resulted in the chemical heterogeneity of the earth mantle.