Melting and phase relations in the system Mg2SiO4-MgSiO3 at 20 GPa under hydrous conditions

An experimental study was performed on the melting and subsolidus phase relations in a synthetic magnesium silicate system at a pressure of about 20 GPa and over the temperature range of 1700∼2500°C under hydrous conditions. The modified spinel, Mg₂SiO₄, is stable up to 2250°C, above which it melts incongruently into phase B, Mg₂₃Si₈O₄₂H₆, and liquid. At higher temperatures, phase B melts again incongruently into periclase and liquid. The liquid coexisting with periclase has an atomic ratio Mg/Si∼1.5 at 2360°C and ∼1.7 at 2500°C. For MgSiO₃ composition the ilmenite is stable at lower temperatures, and it transforms to a garnet with increasing temperatures to 2000°C. At above 2200°C, clinopyroxene replaces the garnet and then melts congruently at 2250°C. The triple point of clinopyroxene, garnet, and liquid is closely located at 20 GPa and 2250°C. The binary system of Mg₂SiO₄-MgSiO₃ shows a eutectic melting between modified spinel and garnet with a eutectic point located at 2150°C and Mg/Si∼1.3.