Conference paper

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 high-pressure and low-temperature conditions. This allowed to estimate characteristic frequencies (nu(i)) and other mode parameters as a function of both pressure and temperature. The cross derivatives partial derivative(2)nu(i)/partial derivative T partial derivative P were measured for the first time, thus providing new insights into the lattice dynamics of Mg-pv. These parameters 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 other modes (+3 X 10(-4) to +5 X 10(-4) cm(-1) GPa(-1) K-1). These measurements were combined with previously published vibrational density of states (VDoS) for deriving entropy, specific heat, thermal pressure, equation of state (EoS), and various thermoelastic parameters of Mg-pv at mantle P-T conditions. The calculations were performed using a general anharmonic formulation including the spectroscopically measured parameters, It is shown that anharmonic effects are relatively small in this compound under geophysically relevant conditions especially for the EoS. The model is used to discuss the discrepancies in the pressure and temperature calibrations in diamond-anvil cells and multianvil presses. Finally, a complete thermodynamic data set for (Mg0.9Fe0.1)SiO3-perovskite is proposed along lower mantle geotherms, It is shown that a pure perovskite lower mantle is unlikely to exist. (C) 2000 Elsevier Science B.V, All rights reserved.


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