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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.

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The MgSiO3 orthoenstatite‐clinoenstatite (OEn‐CEn) phase boundary has been reversed between pressures of 70 and 110 kbar and temperatures of 900°C and 1700°C with a split‐sphere anvil apparatus (USSA‐2000). Starting materials contained PbO‐PbF2 (1∶1) flux to promote equilibration of the charges and eliminate a potential effect of deviatoric stresses on the phase boundary. The phase boundary separating orthoenstatite from clinoenstatite at high pressures and temperatures can be described by the equation P (kbar) = 0.031 T (°C)+50. Our results are consistent with previous unreversed determinations of the boundary at these high‐pressure, high‐temperature conditions. However, the dP/dT slope determined in the present study is much smaller than that implied by earlier experimental studies of the orthoenstatite/low clinoenstatite boundary at low pressures and temperatures. We propose that the clinoenstatite observed at high pressures and temperatures is a new high‐pressure clinoenstatite phase which is possibly an analogue of MgGeO3 clinopyroxene.