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

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.