Water in MgSiO3-majorite at high temperatures and pressures: Incorporation mechanisms and thermoelastic properties

As the major component of garnet, the second most abundant phase in Earth’s transition zone, MgSiO₃-majorite (Mgmj) may be an important reservoir of water. Previous works at ambient conditions show that water is incorporated in Mgmj by substituting Si with H in the Si-O tetrahedra. Due to the challenges of experiments and simulations, there are still very limited data on hydrous MgSiO₃-majorite at simultaneously high temperatures and pressures.

In this study, we have carried out extensive first principles calculations to determine the properties of hydrous MgSiO₃-majorite up to 2000 K and 40 GPa. We systematically considered all possible incorporation mechanisms with seven substitutional reactions. By solving the equations of equilibrium constants of the reactions, we obtained the ratios of various polyhedral substitutions and their temperature and pressure dependences. Our results show that the occupations of water (hydrogen) in Mgmj change mainly with temperature and are generally pressure-independent. Almost all of hydrogen occupy in the Si-O tetrahedra at 300 K, but the ratio of Mg-O dodecahedra increases remarkably with temperature and is the highest at 2000 K, which suggests that water in Mgmj under mantle conditions would be different from that observed at ambient conditions and implies the importance of in-situ measurements of high-TP experiments.

We have also calculated the elastic velocities and anisotropies of two types of hydrous Mgmj which caused by substitutions of Mg and Si. Our results show that ~1 wt% water in Si vacancy would decrease wave velocities of Mgmj by ~3% in both V_P and V_S, while the same amount of water in Mg vacancy would more effectively decrease the velocities by ~6%. In addition, water in Mg vacancy would noticeably improve the seismic anisotropy and the dlnV_s/dlnV_P of Mgmj while water in Si vacancy shows much smaller effects.