In situ observations of phase transition between perovskite and CaIrO3-type phase in MgSiO3 and pyrolitic mantle composition

In situ observations of the perovskite-CaIrO3 phase transition in MgSiO3 and in pyrolitic compositions were carried out using a laser-heated diamond anvil cell interfaced with a synchrotron radiation source. For pure MgSiO3, the phase boundary between the orthorhombic Mg-perovskite and CaIrO3-type phases in the temperature range of 1300-3100 K was determined to be P (GPa) = 130 (± 3) + 0.0070 (± 0.0030) × (T − 2500) (K) using platinum as a pressure calibrant. We confirmed that the CaIrO3-type phase remained stable up to pressures of at least 156 GPa and temperatures of 2600 K. The consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the CaIrO3-type phase. The phase change from an orthorhombic Mg-perovskite to a CaIrO3-type bearing assemblage in a pyrolitic mantle composition was also observed at P = 125 GPa, which corresponds to the same mantle depth as the seismic discontinuity. The phase boundary between the orthorhombic Mg-perovskite and CaIrO3-type bearing assemblage was determined to be P (GPa) = 124 (± 4) + 0.008 (± 0.005) × (T − 2500) (K) using gold as a pressure calibrant. This transition boundary indicates that the temperature at a depth of 2700 km is about 2600 K, and the adiabatic temperature gradient in the lower mantle is estimated to be 0.31 K/km. The partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic MgSiO3 perovskite and CaIrO3-type MgSiO3 were estimated from ab initio calculations. Our experimental and theoretical results indicate that the D″ layer consists of a CaIrO3-type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the Earth's mantle.