High-pressure X-ray diffraction and analytical transmission electron microscopy of laser-heated diamond cell samples show temperature-dependent phase assemblages in mid-ocean ridge basalt (MORB) at lower mantle pressures. The assemblage observed below ~2250 K consists of only stishovite plus an orthorhombic perovskite phase (space group Pbnm or Cmcm) having a composition close to that of MORB but slightly depleted in SiO2. With increasing temperature, the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at ~3250 K to orthorhombic and cubic perovskites plus stishovite and the CaFe2O4 phase. All of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle. ¿ 2001 American Geophysical Union