Elasticity of Phase H Under the Mantle Temperatures and Pressures: Implications for Discontinuities and Water Transport in the Mid‐Mantle

Abstract

AbstractPhase H (MgSiO4H2), one of the lower mantle's dense hydrous magnesium silicates (DHMSs), may form and exist in cold slabs and is crucial in carrying water into the deep mantle. Its sound velocities and density are crucial for inferring the mid‐mantle water cycling via seismic approaches. Here we obtain the elastic and thermodynamic properties of phase H under lower‐mantle conditions using first‐principles calculations and discuss the effect of the Mg‐Si disorder on elasticity. The density of phase H is ∼15% and ∼6% lower than that of bridgmanite and periclase, respectively. The dehydration reaction from phase H to bridgmanite, which may occur at the depth of ∼1,300–1,700 km in cold slabs, will cause an increase of 1.0%, 2.7%, and 15% at 1,500 km on VP, VS, and density, respectively. The dehydration of phase H in subduction zones could produce a seismic VS impedance contrast of ∼17% in the mid‐mantle, which can provide an explanation for some seismic discontinuities detected by previous studies. Meanwhile, phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones. Collectively, our results suggest that some seismic observations in mid‐mantle slabs may be related to the presence of phase H formed via the deep water cycle, further constraining the potential water content in local regions of the subducted slabs.