Precise determination of post-stishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle

The phase transition boundary between stishovite and CaCl2-type structure in pure SiO2 was investigated at 45–75 GPa and 300–2490 K based on the in situ X-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). Results show that the boundary has a positive dP/dT slope of 11.1 MPa/K and the transition occurs about 70 GPa at 2200 K along the typical mantle geotherm. This corresponds to a depth of 1700 km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850 km in the upper to middle layers of the lower mantle. The post-stishovite phase transition is known to be ferroelastic-type, which causes a considerable reduction in shear modulus, leading to a large slow shear velocity anomaly. The almost pure SiO2 phase included in subducted crustal materials may be therefore responsible for the relatively deep mid-lower mantle seismic heterogeneities. The shallow lower mantle anomalies are possibly caused by the similar phase transition in SiO2 phase containing Al2O3 and H2O or by the phase transition in Al-bearing CaSiO3 perovskite from cubic to tetragonal structure.