The origin of Permian Pobei ultramafic complex in the northeastern Tarim craton, western China: Evidences from chemical and C-He-Ne-Ar isotopic compositions of volatiles

The large Pobei Permian layered mafic-ultramafic complex in the northeastern margin of Tarim craton, western China is related temporally and spatially to adjacent Tarim Large Igneous Province (LIP), and hosts Ni-Cu sulfide mineralization. The chemical composition, carbon and noble gas isotopic compositions of volatiles in olivine, pyroxene and plagioclase minerals of the Pobei complex have been measured to reveal the composition and origin of magmatic volatiles, mantle information and dynamic settings. The magmatic volatiles in the Pobei complex are composed of dominant H2O (av. 3284.3 mm3·STP/g, STP = standard temperature and pressure), H2 (449.1) and CO2 (449.4), indicated a H2O-rich reduced magmatic condition. The δ13C values of CH4, C2H6, C3H8 and C4H10 show normal or (partial) reversal distribution pattern with carbon number. The δ13C values of CO2 and CH4 range among mantle, crust (methane oxidation) and thermogenic origins. The 3He/4He (1.13 to 6.15 Ra) and 40Ar/36Ar (326.4 to 1004.3) ratios are plotted on a trend of the mantle plume to atmosphere. The high 20Ne/22Ne and low 21Ne/22Ne ratios fall along the mass fractionation line. The volatile compositions indicated that the Pobei mafic-ultramafic complex could be originated from the staged partial melting of heterogeneous H2O-rich mantle sources triggered by a mantle plume, the air and crust components from subducted altered oceanic crust have been added into the Pobei magma.