[S-IT06] Interaction and Coevolution of the Core and Mantle

[SIT06-P15] Ab initio prediction of the incongruent melting relation in the MgO-SiO₂ system at multi-megabar

*Takashi Taniuchi¹, Taku Tsuchiya¹ (1.Geodynamics Research Center, Ehime University)

Keywords:ab initio calculation, MgO-SiO2 system, incongruent melting, multi-megabar

Magnesium silicates are thought to be the major components of the mantle of terrestrial planets and the core of giant planets (Guillot, 1999; Seager et al., 2007). However, the thermodynamic phase equilibrium in the MgO-SiO₂ system is still not well studied at multi-megabar, including melting relations. A recent laser shock experience reported two discontinuous phase changes in MgSiO₃ at 300-400 GPa (Spaulding et al., 2012), but an ab initio molecular dynamics study identified no clear transition in MgSiO₃ liquid (Militzer, 2013). Boates and Bonev (2013), on the other hand, examined a decomposition reaction of liquid MgSiO₃ into solid MgO and liquid SiO₂ and reported that liquid MgSiO₃ is dissociated at ~300 GPa. This result implies a possible incongruent melting. However, the reaction they considered is too simple and unrealistic. The detailed phase diagram in the MgO-SiO₂ system is therefore required to be clarified at multi-megabar. In this study, we perform ab initio free energy calculations based on the thermodynamic integration method (Kirkwood, 1935) and determine the melting phase relation in this binary system.