The olivine(α)-modified spinel(β)-spinel (&ggr;) transitions in the system Mg2SiO4-Fe2SiO4 were studied by high-temperature solution calorimetry. Enthalpies of the β-&ggr; and α-&ggr; transitions in Mg2SiO4 at 975 K and the α-&ggr; transition in Fe2SiO4 at 298 K were measured. The &ggr; solid solution showed a positive enthalpy of mixing. Phase relations at high pressures and high temperatures were calculated from these thermochemical data including correction for the effect of nonideality of α, β, and &ggr; solid solutions. The calculated phase diagrams agree well with those determined experimentally by Katsura and Ito very recently. The α- (Mg0.89, Fe0.11)2SiO4 transforms to β through a region of α+β without passing through the α+&ggr; phase field at around 400 km depth in the mantle with an interval of about 18(¿5) km. Temperatures at 390 and 650 km depths are estimated to be about 1673 and 1873 K, respectively, assuming an adiabatic geotherm. ¿ American Geophysical Union 1989