Sound Velocities of the Hydrous Iron‐Rich HH1‐Phase: Implications for Lower Mantle Seismic Heterogeneities

We investigated the compressional and shear velocities, VP and VS, of the HH1‐phase by combining the inelastic x‐ray scattering and x‐ray diffraction experiments. In the pressure range of 64.7–79.6 GPa and at room temperature, the VP and VS were measured to be 9.66–11.05 and 4.9–6.4 km·s⁻1, respectively. We employed a simplified model to quantitatively discuss the sound velocity differences between hydrous and anhydrous phase assemblages in the mid‐lower mantle, revealing that the hydrous assemblage exhibits increased VS and a reduced VP/VS ratio compared to its anhydrous counterpart. This suggests formation of seismic scatterers in the mid‐lower mantle exhibiting high VS anomalies may be related to the heterogeneous distribution of water. Moreover, the interpretation of a large VP/VS ratio as indicating the presence of water in the upper mantle and crust may not be applicable for detecting water in the lower mantle. Plain Language Summary Water, which is heterogeneously distributed in the Earth's interior, is one of the most important volatiles of our planet. Incorporation of water into a multicomponent system of the Earth's lower mantle can dramatically alter its phase relations. In particular, the formation of hydrous phases can lead to changes in the seismic velocities of the system. The HH1‐phase is an iron‐rich hydrous hexagonal phase of Fe16.72O18Hx (x∼4.5), which can be formed through reactions in basaltic or peridotitic compositions with water under the pressure and temperature conditions of the deep lower mantle. We have performed inelastic X‐ray scattering and X‐ray diffraction experiments in diamond anvil cells, and measured the sound velocities of HH1‐phase. We thus investigate the sound velocity differences between hydrous and anhydrous phase assemblages in the mid‐lower mantle. Our study can provide a better understanding of the seismic anomalies in the lower mantle. Key Points VP and VS of the HH1‐phase were obtained by inelastic x‐ray scattering and x‐ray diffraction measurements at 64.7–79.6 GPa and 300 K Some hydrous regions of pyrolite lower mantle may exhibit higher VS and lower VP/VS ratio than the dry regions Heterogeneous distribution of water may provide an explanation for the seismic scatterers in the lower mantle