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 Rad iation Sources (GSECARS) sector of the Advanced Photon Source. An analysis of the temperature error sources in laser heating reveals that the axial an d radial thermal gradients are the greatest error source, We have used meas urements where the combined temperature error (1 sigma) 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 CaS iO3 perovskite was determined to be 14-16 GPa, in contrast to the results o f 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 in terior. No evidence of phase transformation or break down to oxides was obs erved. The proposed tetragonal distortion, and hence the phase transformati on from distorted phase to cubic, was not observed. The combined data set o f this study and earlier LVP measurements was fit to a Birch-Murnaghan-Deby e equation. By fixing V-0 = 27.45 cm(3)/mol, K-T0 = 236 GPa, and K-T0' = 3. 9 from recent static compression data and <theta>(0) = 1000 K, we obtain ga mma (0) = 1.92 +/- 0.5 and q = 0.6 +/- 0.3. Although data to 69 GPa and 238 0 K were used in the fitting, this result is also consistent with measureme nts to 96 GPa. This result yields not only density and bulk modulus but als o higher-order thermoelastic parameters, such as thermal expansivity and te mperature dependence of bulk modulus, at lower mantle P-T condition in an i nternally consistent way. This direct measurement of the equation of state at lower mantle condition verifies that the density and bulk modulus of CaS iO3 perovskite at lower mantle P-T conditions are very close to seismic val ues (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.