Experimental Determination of Eutectic Liquid Compositions in the MgO-SiO2 System to the Lowermost Mantle Pressures

Abstract

We have performed melting experiments on the MgO-SiO2 binary system to 139 GPa in a diamond anvil cell. Both MgO-MgSiO3 and MgSiO3-SiO2 eutectic compositions were examined by textural/chemical characterizations of recovered samples. Results demonstrate that the MgO-MgSiO3 eutectic becomes more enriched in MgO with increasing pressure and is close in composition to Mg2SiO4 at the core-mantle boundary (CMB) pressure. On the other hand, the MgSiO3-SiO2 eutectic is more SiO2-rich at higher pressure and exhibits SiO2/(MgO + SiO2) = 0.66 at the CMB. These have profound implications for a large-scale compositional stratification possibly formed upon solidification of a magma ocean. The MgO-MgSiO3 eutectic liquid being more MgO-rich than previously predicted suggests that only bridgmanite crystallizes up to 60 wt% solidification from a fully molten state of a pyrolitic lowermost mantle. In contrast, an SiO2 layer may develop when melting/crystallization occurs in rocky objects with silica-rich enstatite-chondritic compositions.