Stability of Hydrous Phase H MgSiO2(OH)2 Under the Lower Mantle Conditions

Abstract:

Recent ab-initio calculation [1] and high pressure experiment [2] reported existence of a new high pressure hydrous phase, phase H with a composition of MgSiO₂(OH)₂. This phase has a structure similar to that of phase δ, AlOOH [3]. Here we report the stability field of this high pressure hydrous phase, phase H, and its implications for water transport into the deep lower mantle. We conducted in situ X-ray diffraction studies at high pressure and temperature on the stability field of hydous phase H, MgSiO₂(OH)₂. The high pressure and temperature were generated by using X-ray synchrotron radiation combined with the double-sided laser heating diamond anvil cell. We observed existence of hydrous phase H in the pressure range around 50 GPa, and it is stable up to 60 GPa and below 1610 K. The present results together with the previous works [1, 2, 3] indicate that the pure phase H MgSiO₂(OH)₂ has a very narrow stability field in a pressure range from 35 to 60 GPa, in the upper part of the lower mantle. The stability field expands significantly towards higher pressure and temperature by dissolution of hydous AlOOH component [4]. The hydrous phase H-phase δ solid solution (aluminous phase H), (MgSi,Al₂)O₂(OH)₂, is potentially the most important hydrous phase under the deep lower mantle conditions. References: [1] Tsuchiya, J., 2013. Geophys. Res. Lett., 20, 4579-4573. [2] Nishi, M. et al., 2014. Nature Geoscience 7, 224-227. [3] Suzuki, A. et al., 2000. Phys. Chem. Miner.27, 689–693. [4] Ohira, I. et al., 2014. Earth Planet. Sci. Lett., 2014, in press.