IN-SITU HIGH P-T X-RAY-DIFFRACTION STUDIES ON 3 POLYMORPHS (ALPHA,BETA,GAMMA) OF MG2SIO4

Unit cell volumes of the beta (beta) and spinel (gamma) phases (Mg2SiO4) have been measured under simultaneous high pressure and high temperature using synchrotron X ray radiation, in a cubic anvil apparatus. With volume-temperature data at constant pressure, we determine the average volume thermal expansion coefficients of the beta phase from 724 to 872 K at 7.6 GPa to be 2.28(+/- 0.45) x 10(-5)/K and of the gamma phase from 759 to 962 K at 9.8 GPa to be 1.71(+/- 0.14) x 10(-5)/K. Thermodynamic relations are used to constrain the temperature derivative of the isothermal bulk modulus (K(T)) from the high-pressure thermal expansion data: (partial derivative K(T)/partial derivative T)P is found to be -2.7(+/- 0.5) x 10(-2) GPa/K for the beta phase and -2.8(+/- 0.3) x 10(-2) GPa/K for the gamma phase. Unit cell volumes of the olivine (a) phase, back-transformed from the beta and gamma phases at high temperatures, have been measured under pressure at temperatures above the Debye temperature; using thermal pressure equations, we find (partial derivative K(T)/partial derivative T)P for the alpha phase to be -2.1(+/- 0.2) x 10(-2) GPa/K. These new data on the temperature derivatives of the bulk modulus for the three phases are consistent with an upper mantle containing 60 to 65% olivine and the absence of a velocity signature for the beta to gamma phase transition near 520 km depth.