Redox heterogeneity of picritic lavas with respect to their mantle sources in the Emeishan large igneous province

Abstract

The oxygen fugacity (fO2) of the Earth’s mantle is a key parameter that regulates fundamental geological processes such as planetary differentiation, mantle melting, and volatile cycling. Large igneous provinces (LIPs) are among the most voluminous mafic volcanic eruptions in the Earth’s history. Constraining the fO2 of LIP rocks is critical for understanding the redox state of the mantle and migration of volatiles from the deep Earth to the atmosphere. Here, we investigated primitive picrites from the Dali and Lijiang areas belonging to the Emeishan LIP, which contain early-crystallized olivine phenocrysts with Fo contents up to 93.5 and 91.9, respectively. The fO2 conditions of primary and derivative melts are determined by the olivine-spinel and olivine-melt V oxybarometers, aided by temperature estimates from olivine-spinel Al and olivine-melt Sc/Y thermometers. The calculated fO2 values are negatively correlated with olivine Fo contents due to fractionation of mafic minerals in a system closed to oxygen. The fO2s of Dali and Lijiang primary melts are −0.8 ± 0.4 and + 0.6 ± 0.3 (1σ) ΔFMQ log units, respectively, more variable than those of primitive MORBs, indicating significant fO2 fluctuations in primary magmas from the Emeishan LIP caused by changes in both redox state of the source rock and depth of melt extraction. Melt barometer constrains the Dali and Lijiang primary melts generated at 4.7 and 3.0 GPa, respectively. After correcting for the depth effect, the Dali and Lijiang sources are more oxidized (with higher Fe3+/∑Fe ratios), in varying degrees, than the ambient upper mantle, likely due to incorporation of recycled surface materials. Under such oxidized conditions, mantle carbon is efficiently extracted during melting in the form of CO2 and released to the atmosphere during massive magma emplacement, which may have played an important role in causing the climatic and environmental changes and triggering the end-Guadalupian mass extinction.