High-pressure and high-temperature phase relations in the system Mg2SiO4-Fe2SiO4 were thoroughly reinvestigated at pressures up to 21 GPa and at temperatures of 1200 ¿C and 1600 ¿C using a uniaxial split-sphere apparatus. Both normal and reverse experiments were performed on the univariant reactions. Both exsolving and dissolving experiments were performed to determine the divariant loops at 1600 ¿C. The run products were examined using microfocused X ray diffractometry, scanning electron microscopy, and electron probe microanalysis. The phase diagrams thus constructed were compared with those previously proposed using an experimental or thermochemical method. Phase changes in mantle olivine with Mg number of 89 occur in the following order: (α)-(α+β)-(β)-(β+&ggr;)-(&ggr;). The α+β loop narrows with increasing temperature. The applicability of an isochemical peridotite mantle model is discussed in the light of the location and thickness of the 400-km discontinuity. ¿ American Geophysical Union 1989