Melting phase equilibrium relations in the MgSiO₃-SiO₂ system under high pressures

Melting relations in the MgSiO₃-SiO₂ system have been investigated at 13.5 GPa using a Kawai-type multi-anvil apparatus. The system displays eutectic melting with the eutectic point located at SiO₂/(SiO₂+MgO) = 0.61 (in mol; which is denoted by X_Si hereafter) and at 2350 ± 50 °C. Taking into account the eutectic compositions at lower pressures reported in previous studies, i.e., 0.556 at 1 GPa (Hudon et al. 2005) and 0.60 at 5 GPa (Dalton and Presnall 1997), the eutectic composition is slightly enriched in SiO₂ with increasing pressure. The silica-rich eutectic composition is not consistent with the present peridotitic mantle composition (X_Si = 0.43). Considering Si incorporation into iron alloys in a magma ocean, however, mass-balance calculations based on an E-chondrite model demonstrate that the silicate magma ocean could have X_Si consistent with the present peridotitic mantle.