Effect of high pressure on the melting relation in the system Mg₂SiO₄-MgSiO₃ part I. Eutectic relation up to 7 GPa

The melting temperature and eutectic relation of the system Mg₂SiO₄ (forsterite)-MgSiO₃ (enstatite) have been experimentally determined at pressures of 3 and 7 GPa. The melting temperatures are; 1,975°C and 2,125°C for forsterite; 1,825°C and 2,050°C for enstatite; and 1,775°C and 2,000°C for eutectic points of the binary system at 3 GPa and 7 GPa, respectively. On the basis of the liquid composition coexisting with the end member minerals, the eutectic composition is determined as Fo12En88 and Fo21En79 in mol percent, at each pressure. The difference of the melting temperatures between forsterite and enstatite decreases with a rise in pressure. Its decreasing rate is correlated with the shift of the eutectic composition towards the forsterite-rich side of the system. The observed trend in eutectic composition suggests that the eutectic composition in this binary system does not go much toward the magnesiumrich side at higher pressures.