[SIT22-P25] Melting relations in the MgO-MgSiO3 system under the lower mantle conditions
The experiments were performed up to 115 GPa using a CO2 laser heated diamond anvil cell. The quenched samples were polished and analyzed by a dualbeam focused ion beam (FIB) and a field emission scanning electron microscope (FE-SEM), respectively. The eutectic compositions and liquidus phase were determined on the basis of chemical and textual analyses of sample cross sections. Our experimental results show that the eutectic composition is Si/Mg molar ratio of ~0.76 at around 35 GPa and it decreases with increasing pressure below 45 GPa. Above 45 GPa, it becomes relatively constant at about 0.64-0.65 Si/Mg molar ratio. Additionally, the eutectic composition was described by thermodynamic calculation under the whole lower mantle conditions. We obtained the Si/Mg molar ratio of ~0.64 at the base of the mantle. The liquidus phase changes from MgO-periclase to MgSiO3-bridgmanite at around 35 GPa in the Fe-free simplified pyrolite composition (~0.7 Si/Mg molar ratio). In the other model rock composition such as chondrite (~0.84 Si/Mg molar ratio), MgSiO3-bridgmanite becomes the liquidus phase in the entire lower mantle. Thus MgSiO3-bridgmanite should be the dominant phase to crystallize from a deep global magma ocean in the lower mantle.