STABILITY OF ORTHORHOMBIC MGSIO3 PEROVSKITE IN THE EARTHS LOWER MANTLE

MAGNESIUM-rich silicate perovskite is thought to be the primary constituent of the Earth's lower mantle: experiments have shown1 MgSiO3 perovskite to be stable at lower-mantle pressures, and the elastic properties of perovskite-dominated assemblages agree well with seismological observations2-4. It has also been suggested5-8 that the observed orthorhombic structure will undergo displacive phase transitions to higher-symmetry structures at lower-mantle conditions. The presence of such transitions would have important consequences for mantle convection9, and could provide an explanation for some of the weak seismic discontinuities observed10-12 in the lower mantle. However, the determination of the phase behaviour Of MgSiO3 perovskite at lower-mantle conditions has so far eluded both experimental and theoretical efforts. Here we report the results of electronic-structure calculations of the energetics of displacive phase transitions in MgSiO3 perovskite, and demonstrate that the lower-symmetry orthorhombic phase should be highly favoured throughout the lower mantle. Our results are consistent with recent experiments13 on MgSiO3 perovskite encompassing the temperatures and pressures of the uppermost regions of the lower mantle.