Phase H: A new high pressure phase of dense hydrous magnesium silicates in the lower mantle

Abstract:

It has been believed that water is carried into the deep Earth’s interior by hydrous minerals such as the dense hydrous magnesium silicates (DHMSs) in the descending cold plate. Recently, the new high pressure phase of DHMSs is theoretically predicted (Tsuchiya 2013) and experimentally confirmed in lower mantle pressure conditions above ~45 GPa (Nishi et al. 2014). This phase has MgSiO₄H₂ chemical composition and named as phase H.

At the lower mantle pressure conditions, Al and H-bearing SiO₂, δ-AlOOH, ε-FeOOH and phase H may be the potential hydrous phases in the subducting slabs. Interestingly, the crystal structure of these hydrous phases are almost same and have CaCl₂ type structure. This suggests that these hydrous phases may be able to make the wide range of solid solution. Some experimental studies already reported that Al preferentially partitioned into phase H and the stability of phase H drastically increased by incorporation of Al (Nishi et al. 2014, Ohira et al. 2014). The density of subducted MORB is reported to be denser than that of pyrolite in the lower mantle. Therefore, there is a possibility that phase H containing Al and Fe in subducted MORB survive down to the bottom of lower mantle and the melting of phase H at the core mantle boundary may contribute to the cause of ultra-low velocity zones.

In this study, we report the effects of Al and Fe on the stability of phase H, elasticity and seismic anisotropy of this new hydrous mineral using first principles calculation techniques and discuss the possible effects of this hydrous phase at the bottom of lower mantle.