Thermoelastic Properties of Seifertite at High Pressures and Temperatures: Implications for Negative Velocity Discontinuities in the D" Layer.

Negative velocity discontinuities are often observed at the base of the D'' layer, yet their formation mechanisms remain elusive. Here, we present the first investigation of thermoelastic properties of SiO2‐seifertite under 30–200 GPa and 1,000–4,000 K using first‐principles molecular dynamics simulations. We find that the compressional and shear wave velocities of seifertite are 2.0%–4.3% and 7.4%–11.3% lower, respectively, than those of CaCl2‐type SiO2 in the D'' layer. The reductions in VS across the phase transition are significantly larger than previous estimates from density functional theory results. Incorporating the elastic properties of other minerals, we demonstrate that the presence of SiO2 in the accumulated subducted oceanic crust and the associated VS reductions can contribute to the negative velocity discontinuities observed in the D'' layer. The observed low seismic velocities at the base of the lower mantle can be matched if 19–27 vol.% SiO2 is present in the D'' region. Plain Language Summary: Geochemical and seismic observations suggest that subducted oceanic crust can accumulate at the core‐mantle boundary, but its effects on the seismic velocity of the D'' layer remain poorly constrained. In fact, the thermoelastic properties of seifertite, which is a major mineral in the deeply subducted oceanic crust at the lowermost mantle, have not been previously investigated at high pressure‐temperature conditions. Using first‐principles molecular dynamics simulations, we computed the thermoelastic properties of seifertite under high pressures and temperatures up to 200 GPa and 4000 K. Our findings shows that seifertite has lower compressional and shear wave velocities than the CaCl2‐type SiO2, but has slightly higher density and bulk sound velocity. Thus, the reductions in VS of SiO2‐bearing mantle assemblages, such as the subducted oceanic crust, upon transition from CaCl2‐type SiO2 to seifertite, could contribute to the negative velocity discontinuities observed in certain regions of the D'' layer. Key Points: Thermoelastic properties of seifertite were determined using first‐principles molecular dynamics simulationsSeifertite has lower compressional and shear wave velocities than the CaCl2‐type SiO2 in the D'' layerCombined effects of SiO2 phase transition and temperature increase may lead to large shear velocity reductions in the D'' layer [ABSTRACT FROM AUTHOR]