High Pressure‐Temperature Study on the Thermal Equations of State of Seifertite andCaCl2‐Type SiO2

Abstract

AbstractIn this study, we have determined the thermal equations of state (EoS) of CaCl2‐type SiO2 and seifertite (α‐PbO2‐type SiO2) from 55 to 147 GPa up to 3500 K using X‐ray diffraction in laser‐heated diamond anvil cells. The phase transition from stishovite to CaCl2‐type SiO2 is in good agreement with previous experimental results, whereas the transition from CaCl2‐type SiO2 to seifertite was observed to strongly rely on the starting material. For CaCl2‐type SiO2, we obtained a bulk modulus, KT0 = 245(7) GPa with a fixed KT′ = 4, V0 = 48.1(2) Å3, γ0 = 1.3(3), q = 1(fixed), and θD0 = 1100(400) K. Seifertite has a greater KT0 = 290(10) GPa (KT′ = 4 fixed) than CaCl2‐type SiO2. Other thermoelastic parameters of seifertite are V0 = 92.3(5) Å3, γ0 = 1.6(2), q = 1 (fixed), and θD0 = 1600(200) K. Using the obtained results and phase boundary in previous studies, we have found that the phase transition from stishovite to CaCl2‐type SiO2 cannot cause a noticeable change in density but can lower the bulk sound velocity (VФ) by ~10%. This phase transition in the subducted oceanic crust with 20 vol.% SiO2 was estimated to generate a ~2% discontinuity in VФ, which could be related to the observed seismic velocity anomalies at depths of 1,400–1,800 km. At the bottom of the lower mantle, CaCl2‐type SiO2 to seifertite phase transition can lead to a 0.3% jump in ρ and a 0.8% jump in VΦ considering 20 vol.% SiO2 in the subducted mid‐ocean ridge basalt, which may contribute to the formation of the D″ discontinuity above the core‐mantle boundary.