STABILITY OF ORTHORHOMBIC MGSIO3 PEROVSKITE IN THE EARTHS LOWER MANTLE

MAGNESIUM-rich silicate perovskite is thought to be the primary consti tuent of the Earth's lower mantle: experiments have shown1 MgSiO3 pero vskite to be stable at lower-mantle pressures, and the elastic propert ies of perovskite-dominated assemblages agree well with seismological observations2-4. It has also been suggested5-8 that the observed ortho rhombic structure will undergo displacive phase transitions to higher- symmetry structures at lower-mantle conditions. The presence of such t ransitions would have important consequences for mantle convection9, a nd could provide an explanation for some of the weak seismic discontin uities observed10-12 in the lower mantle. However, the determination o f the phase behaviour Of MgSiO3 perovskite at lower-mantle conditions has so far eluded both experimental and theoretical efforts. Here we r eport the results of electronic-structure calculations of the energeti cs of displacive phase transitions in MgSiO3 perovskite, and demonstra te that the lower-symmetry orthorhombic phase should be highly favoure d throughout the lower mantle. Our results are consistent with recent experiments13 on MgSiO3 perovskite encompassing the temperatures and p ressures of the uppermost regions of the lower mantle.