Stability of orthorhombic MgSiO₃ perovskite in the Earth's lower mantle

Abstract

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 observations^2–4. It has also been suggested^5–8 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 observed^10–12 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 experiments¹³on MgSiO₃ perovskite encompassing the temperatures and pressures of the uppermost regions of the lower mantle.