The origin of volatiles in the Emeishan large igneous province, China: Constraints from chemical and C–He–Ar isotopic compositions of volatiles in picrites and basalts

Abstract

The Emeishan large igneous province (LIP) consists of Permian flood basalts and coeval intrusive rocks with compositions varying from mafic–ultramafic rocks to syenites and granites. The volatiles in the Emeishan LIP are dominated by H2O with high H2, indicating that the plume‐derived magma was generated under a reduced condition. Picrite has minor contents of H2S without detectable SO2, and the basalt samples show no detectable H2S and SO2, indicating that the basaltic magma may have undergone sulphur‐saturation during magma evolution. The δ13C values of CO2 and CH4 in picrites and basalts are between the values of mantle and crustal organic carbon. This, together with the distribution patterns among CH4 homologue, indicates contribution of recycled sediments to the Emeishan mantle‐derived magma. The 3He/4He ratios of volatiles extracted from olivine and pyroxene phenocrysts of picrites vary from 0.01 to 0.66 Ra, and those from pyroxene separates of olivine websterite samples in the Zhubu intrusion vary from 0.92 to 1.05 Ra. The low 3He/4He ratios of the picrites from Emeishan LIP are lower than those from the Siberian LIP, but within the values of continental crust and oceanic crust. The positive correlation between 4He/40Arc and 4He contents in Emeishan picrite suggests that the isotopic ratios were dramatically affected by crustal contamination rather than degassing. The He and Ar isotopes indicate that Emeishan picrite experienced addition of subducted oceanic crustal materials, and the Zhubu magma was contaminated by continental crustal materials.