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.Plain Language Summary MgO-SiO 2 is the most important binary system to understand melting/crystallization processes in the Earth's crust and mantle as well as those in other rocky objects. Its melting phase relations, in particular, eutectic compositions, also have profound implications for a large-scale compositional stratification possibly formed upon solidification of a magma ocean. However, the eutectic compositions in the MgO-MgSiO 3 and MgSiO 3 -SiO 2 systems have not been examined experimentally at the deep lower mantle conditions yet. Here we performed melting experiments and determined both of these eutectic compositions to 135 GPa, corresponding to the pressure at the bottom of the mantle, by a combination of high-pressure-temperature generation in a laser-heated diamond anvil cell and textural/compositional characterization of recovered samples. The results demonstrate that the MgO-MgSiO 3 eutectic liquid is more MgO-rich than previously predicted. It suggests that a large volume of MgSiO 3 bridgmanite-dominant-layer may have formed during crystallization of a deep magma ocean and is possibly still preserved up to the present in the Earth's lower mantle. On the other hand, the MgSiO 3 -SiO 2 eutectic liquid implies that an SiO 2 -phase-rich layer could develop when extensive melting occurs in rocky objects with silica-rich enstatite-chondritic compositions.
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