Re–Os isotopic compositions of picrites from the Emeishan flood basalt province, China

Abstract

Picrites from the Lijiang area, in the western part of the Emeishan flood basalt province, are considered to be one of the most important indicators of the Emeishan mantle plume, and may represent initial mantle plume activity. This paper presents the first Re–Os isotope data for picrites from the province. Eighteen geochemically well characterised samples, including 12 picrites and 6 basalts from two sections near Lijiang, have been analyzed for Os isotopes. The picrites from the two sections have overlapping Re and Os concentrations and initial Os isotopic compositions. They have higher Os concentrations (1.5–3 parts per billion), and slightly lower Re concentrations (< 0.07 ppb) than those from many other flood basalt provinces (FBPs) and ocean island picrites. The picrites have distinctly unradiogenic Os isotopes with γ Os( t) values ( t = 259 Ma) of − 0.9 to − 2.4, but the associated basalts have suprachondritic Os isotope compositions with γ Os( t) of 1.8 to 73.3, which overlap with published data for basalts from other localities in the province. The high γ Os( t) values of the basalts could be the result of crustal contamination, which is enabled by the low Os concentrations in the parental basalts. The low γ Os( t) values of the picrites are unusual in terms of plume-related magmatism. There is no clear evidence for recycled crustal material in the mantle source of the Emeishan picrites, or for a significant degree of interaction of the picrites with ancient subcontinental lithospheric mantle of the Yangtze craton. The low γ Os( t) values relative to many other plume-related picrites (from ocean islands or flood basalt provinces) indicate that there was no clear Os isotope evidence of involvement of the Earth's core or recycled crustal components in the formation of Emeishan flood basalt province.