PRESSURE-VOLUME-TEMPERATURE BEHAVIOR OF GAMMA-FE2SIO4 (SPINEL) BASED ON STATIC COMPRESSION MEASUREMENTS AT 400-DEGREES-C

Thirteen energy-dispersive x-ray diffraction spectra for gamma-Fe2SiO4 (spinel) collected in situ at 400 degrees C and pressures to 24 GPa c onstitute the basis for an elevated-temperature static compression iso therm for this important high-pressure phase. A Murnaghan regression o f these molar volume measurements yields 177.3 (+/- 17.4) GPa and 5.4 (+/- 2.5) for the 400 degrees C, room pressure values of the isotherma l bulk modulus (K-P0) and its first pressure derivative (K-P0'), respe ctively. When compared to the room-T determinations of K-P0 available in the literature, our 400 degrees C KP0 yields -4.1 (+/- 6.2) x 10-2 GPa/degree for the average value of (partial derivative K/partial deri vative T)(P0) over the temperature interval 25 degrees C < T < 400 deg rees C. A five-parameter V(P, T) equation for gamma-Fe2SiO4 based on s imultaneous regression of our data combined with the elevated P-T data of Yagi et al. (1987) and the extrapolated thermal expansion values f rom Suzuki et al. (1979) yields isochores which have very little curva ture [(partial derivative(2)T/partial derivative P-2)(V) congruent to 0], in marked contrast to the isochores for fayalite (Plymate and Stou t 1990) which exhibit pronounced negative curvature [(partial derivati ve(2)T/partial derivative P-2)(V)<0]. Along the fayalite/gamma-Fe2SiO4 reaction boundary Delta V-R varies from a minimum of approximately 8. 3% at approximately 450 degrees C to approximately 8.9% at 1200 degree s C. Extrapolation of the fayalite and gamma-Fe2SiO4 V(P, T) relations hips to the temperature and pressure of the 400 km discontinuity sugge sts a Delta V-R of approximately 8.4% at that depth, approximately 10% less than the 9.3% Delta V-R at ambient conditions.