Methane-bearing melt inclusion in olivine phenocryst in Cenozoic alkaline basalt from Eastern China and its geological significance

Abstract

Alkaline basalts originate from the partial melting of the upper mantle or deeper mantle region, and their study can contribute important knowledge of mantle composition. Melt inclusions in these basalts are small natural droplets trapped during the formation of olivine, and can be isolated from later alterations. Therefore, they can effectively record information from the early stages of magma formation and evolution. In this study, the volatile composition of 73 melt inclusions in olivine phenocrysts of alkaline basalts from Yangzhuang in Shandong Province were analyzed by Laser Raman Spectroscopy. The major and trace element compositions of some host olivines were measured by LA-ICP-MS. The results show that these melt inclusions contain amounts of methane (CH4) and other hydrocarbons, and some carbonaceous minerals such as graphite and carbonate. These findings prove that CH4 can stably exist in the upper-mantle environment of high pressure and temperature, and indicate that such a mantle region with low oxygen fugacity does exist below Eastern China. The host olivine phenocrysts are characterized by high Ni content and a high Fe/Mn ratio, implying the presence of a pyroxenite component in the mantle source. The subduction of the Western Pacific Plate beneath the lithospheric mantle of Eastern China is a possible source for the pyroxenite, combined with other geochemical and geophysical data. CH4 is an important form of deep carbon and the subduction of oceanic crust is responsible for the recycling of deep carbon.