Thermal equation of state of CaSiO3 perovskite

A comprehensive pressure-volume-temperature data set has been obtained for CaSiO3 perovskite up to 13 GPa and 1600 K, using synchrotron X ray diffraction with a cubic-anvil, DIA-6 type apparatus (SAM-85). For each volume measurement, nonhydrostatic stress is determined from the relative shift in the diffraction lines of NaCl, within which the sample was embedded. Heating to above 973 K greatly reduced the strength of NaCl (to below 0.05 GPa), making the measurements hydrostatic. At room temperature the cubic perovskite structure remains metastable at pressures as low as 1 GPa, below which the sample transforms into an amorphous phase as indicated by a large background, a marked decrease in diffractian signals, and an anomalous volume decrease of the remaining crystalline phase. Because our experimental uncertainties are significantly smaller than those in previous measurements, the new data provide a tighter constraint on the zero pressure bulk modulus for CaSiO3 perovskite. A new set of room temperature equation of state parameters are identified so that both our data and the diamond cell data of Mao et al. [1989] are compatible [K-T0 = 232(8) GPa, K'(T0) = 4.8(3), and V-0 = 45.58(4) Angstrom(3)]. Volume measurements along several isotherms under both stable and metastable pressure conditions allow isochoric and isobaric interpolations within the range of experimental pressure and temperature conditions. Analyses using various approaches yielded consistent results for (partial derivative K-T/partial derivative T)(p) of -0.033(8) GPa K-1, and (partial derivative alpha/partial derivative P), of -6.3 x 10(-7) GPa(-1) K-1, with a zero-pressure thermal expansion alpha(0) of 3.0 x 10(-5) K-1. The thermal pressure is found to be virtually independent of volume, and thus the Anderson-Gruneisen parameter delta(T) = K'(T0) = 4.8. These results are used to predict the bulk modulus and density of CaSiO3 perovskite under lower mantle conditions. Along an adiabat with the foot temperature of 2000 K, the density of the perovskite agrees with that of the preliminary reference Earth model (PREM) within 1% throughout the lower mantle. The bulk modulus shows a smaller pressure dependence along the adiabat; it matches that of PREM at the top of the lower mantle but is about 10% too low near the core-mantle boundary.