A thermodynamic model for MgSiO3-perovskite derived from pressure, temperature and volume dependence of the Raman mode frequencies

Raman spectra of MgSiO3-perovskite (Mg-pv) were recorded at simultaneous hi gh-pressure and low-temperature conditions. This allowed to estimate charac teristic frequencies (nu(i)) and other mode parameters as a function of bot h pressure and temperature. The cross derivatives partial derivative(2)nu(i )/partial derivative T partial derivative P were measured for the first tim e, thus providing new insights into the lattice dynamics of Mg-pv. These pa rameters are negative for the two lowest frequency modes at 250 and 255 cm( -1) (approximate to-6 X 10(-4) cm(-1) GPa(-1) K-1) and positive for the oth er modes (+3 X 10(-4) to +5 X 10(-4) cm(-1) GPa(-1) K-1). These measurement s were combined with previously published vibrational density of states (VD oS) for deriving entropy, specific heat, thermal pressure, equation of stat e (EoS), and various thermoelastic parameters of Mg-pv at mantle P-T condit ions. The calculations were performed using a general anharmonic formulatio n including the spectroscopically measured parameters, It is shown that anh armonic effects are relatively small in this compound under geophysically r elevant conditions especially for the EoS. The model is used to discuss the discrepancies in the pressure and temperature calibrations in diamond-anvi l cells and multianvil presses. Finally, a complete thermodynamic data set for (Mg0.9Fe0.1)SiO3-perovskite is proposed along lower mantle geotherms, I t is shown that a pure perovskite lower mantle is unlikely to exist. (C) 20 00 Elsevier Science B.V, All rights reserved.