Compressional and shear wave velocities of Fe-bearing silicate post-perovskite in Earth’s lowermost mantle

Abstract

The bridgmanite (Bgm) to silicate post-perovskite (PPv) phase transition is believed to be the main cause for the distinct seismic features observed in the D'' layer, the lowermost region of the Earth’s mantle. However, the transition depth and elasticity of the PPv phase have been highly debated, as the chemical complexity within the D'' layer can largely affect the Bgm-PPv transition pressure and the associated velocity contrast. Experimental measurements of sound velocities for PPv with different chemical compositions under relevant lowermost-mantle conditions are essential but remain limited. In this study, we have reliably measured both compressional wave velocity (VP), shear wave velocity (VS), and density, for two Fe-bearing PPv compositions [(Mg0.85Fe0.15)SiO3 and (Mg0.75Fe0.25)SiO3] at lowermost mantle pressures using Impulsive Stimulated Light Scattering (ISS), Brillouin Light Scattering (BLS), and X-ray Diffraction (XRD) in diamond anvil cells. Our results indicate that the velocities of Fe-bearing PPv at 120 GPa can be described by the following relationships: VS (km/s) = 7.65–2.8XFe and VP (km/s) = 14.11–3.8XFe, where XFe represents mole fraction of the Fe content. The variations in the Fe content of PPv may provide one of the explanations for the seismic lateral variations observed at the Earth’s core mantle boundary. By comparing our results with the high-pressure velocities of Bgm, our velocity model suggests significant discontinuities across the Bgm-PPv transition, characterized by a reduction in both VP and VΦ, and an increase in VS. These findings highlight the importance of considering the influence of chemical composition, particularly Fe content which could vary significantly at the D'' region, on the seismic properties of the PPv phase. The observed velocity contrasts across the Bgm-PPv transition may contribute to the complex seismic signatures observed in the D'' layer, underscoring the potential role of this phase transition in interpreting the seismic features of the lowermost mantle region.