Experimental Determination of Eutectic Liquid Compositions in the MgO‐SiO 2 System to the Lowermost Mantle Pressures

We have performed melting experiments on the MgO‐SiO 2 binary system to 139 GPa in a diamond anvil cell. Both MgO‐MgSiO 3 and MgSiO 3 ‐SiO 2 eutectic compositions were examined by textural/chemical characterizations of recovered samples. Results demonstrate that the MgO‐MgSiO 3 eutectic becomes more enriched in MgO with increasing pressure and is close in composition to Mg 2 SiO 4 at the core‐mantle boundary (CMB) pressure. On the other hand, the MgSiO 3 ‐SiO 2 eutectic is more SiO 2 ‐rich at higher pressure and exhibits SiO 2 /(MgO + SiO 2 ) = 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‐MgSiO 3 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 SiO 2 layer may develop when melting/crystallization occurs in rocky objects with silica‐rich enstatite‐chondritic compositions.