Phase transition in Al-bearing CaSiO3 perovskite: implications for seismic discontinuities in the lower mantle

Phase transitions in CaSiO3(+5.9 wt.% Al2O3) and pure CaSiO3 perovskites were investigated at 24–75 GPa and 300–2250 K on the basis of in situ X-ray diffraction measurements in a laser-heated diamond anvil cell (LHDAC). Results demonstrate that CaSiO3(+5.9 wt.% Al2O3) has a GdFeO3-type orthorhombic perovskite structure at low temperatures and undergoes phase transition to a cubic structure above 1840 K at 50 GPa. The transition boundary determined in a pressure range from 32 to 75 GPa shows strong temperature-dependence with a slightly positive P/T slope. The structure of end-member CaSiO3 is slightly distorted from cubic symmetry at low temperatures and similarly transforms to a cubic structure with increasing temperature. The transition occurs at about 580 K and 52 GPa, which is a much lower temperature than for the Al2O3-bearing composition at equivalent pressure. These distorted CaSiO3-rich perovskites may be ferroelastic and have large elastic anomalies near the structural transition to the cubic phase. Part of the seismic discontinuities in the upper- to middle-layers of the lower mantle, which are often observed beneath convergent margins, may be attributed to the phase transition in Al2O3-bearing CaSiO3 perovskite included within relatively cold subducting slabs.