CALCULATIONS OF HIGH-PRESSURE PHASE-TRANSITIONS IN THE SYSTEM MGO-SIO2 AND IMPLICATIONS FOR MANTLE DISCONTINUITIES

By incorporating the knowledge of the observed high-pressure phase transformations, the measured equilibrium phase boundaries for certain transitions, and the measured thermochemical data for certain phases in the system MgOSiO2, the equilibrium phase boundaries for all the phase transitions in this system at 1000°C have been calculated on the basis of an assumption that volume change across a phase boundary is independent of temperature. Three major seismic velocity discontinuities in the mantle (420, 570, and 650 km) have been chosen for comparison with the measured and calculated equilibrium phase boundaries for the high-pressure phases observed in the system MgOSiO2. Complications of phase changes due to the addition of FeO and Al2O3 to the system have also been accounted for. It is suggested by the results of this study that the 420- and 570-km discontinuities are probably related to phase transitions observed in the system MgOSiO2, but that the 650-km discontinuity is not likely to be associated with any of the equilibrium phase boundaries observed in olivine, pyroxene, and garnet, but may instead be a chemical change. © 1979.