The stability and P-V-T equation of state of CaSiO₃ perovskite in the Earth's lower mantle

Energy dispersive X-ray diffraction measurements for polycrystalline CaSiO₃ perovskite were carried out at in situ transition zone and lower mantle P-T conditions (P=18-96 GPa, T=1238-2419 K) using the diamond anvil cell and double-sided laser heating at the GeoSoilEnviro Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source. An analysis of the temperature error sources in laser heating reveals that the axial and radial thermal gradients are the greatest error source. We have used measurements where the combined temperature error (1σ) from all sources is <150 K. By obtaining X-ray diffraction patterns at 8-22 GPa and 300-2200 K range, the high-temperature phase boundary between CaSi₂O₅+Ca₂SiO₄ and CaSiO₃ perovskite was determined to be 14-16 GPa, in contrast to the results of previous large-volume press (LVP) measurements (9-11 GPa). The stability of cubic CaSiO₃ perovskite was confirmed to 2300 km depth in the Earth's interior. No evidence of phase transformation or break down to oxides was observed. The proposed tetragonal distortion, and hence the phase transformation from distorted phase to cubic, was not observed. The combined data set of this study and earlier LVP measurements was fit to a Birch-Murnaghan-Debye equation. By fixing V₀ = 27.45 cm³/mol, K_T0 = 236 GPa, and K′_T0 = 3.9 from recent static compression data and θ₀ = 1000 K, we obtain γ₀ = 1.92 ± 0.5 and q = 0.6 ± 0.3. Although data to 69 GPa and 2380 K were used in the fitting, this result is also consistent with measurements to 96 GPa. This result yields not only density and bulk modulus but also higher-order thermoelastic parameters, such as thermal expansivity and temperature dependence of bulk modulus, at lower mantle P-T condition in an internally consistent way. This direct measurement of the equation of state at lower mantle condition verifies that the density and bulk modulus of CaSiO₃ perovskite at lower mantle P-T conditions are very close to seismic values (within 1.5 and 3.0%, respectively). These differences are sufficiently small that the abundance of CaSiO₃ perovskite will have negligible effects on density and bulk modulus profiles for the mantle.