Phase relations in the systems MgO-Al2O3-SiO2 and CaO-MgO-SiO2 have been investigated experimentally at 166--226 kbar pressures and 1350¿--2500¿C temperatures with a split sphere anvil apparatus (USSA 2000). MgSiO3 ilmenite is stable to lower pressures (175 kbar) than found previously. CaSiO3 perovskite forms at P(bar)=15T(¿C) +151,000. Three new high-pressure phases are reported: a phase with the diopsidic composition (the CM phase), a high-pressure polymorph of Al2O3, and superphase B with the tentative composition Mg10Si3O14 (OH, F)4. The invariant point for coexisting majorite (Mg4Si4O12), perovskite (MgSiO3), beta phase (Mg2SiO4), and liquid has been located at 224 kbar, 2430 ¿C, with the eutectic melt composition on the enstatite-forsterite join at 40--44 wt% of forsterite. The melting temperature of MgSiO3 perovskite at 225 kbar is predicted to be 2600 ¿C. The new data made possible the calculation of a temperature-pressure phase diagram for the system CaO-MgO-Al2O3-SiO2 (CMAS) in the pressure range 0--280 kbar. The mineralogy of the Earth's mantle and the possible mechanisms for producing large seismic velocity gradients in the transition zone are discussed on the basis of the derived phase relations. ¿ American Geophysical Union 1990