The post-spinel transformation in Mg₂SiO₄ and its relation to the 660-km seismic discontinuity

The 660-km seismic discontinuity in the Earth's mantle has long been identified with the transformation of (Mg,Fe)₂SiO₄ from γ-spinel (ringwoodite) to (Mg, Fe)SiO₃-perovskite and (Mg, Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures. But the first in situ examination of this phase transformation in Mg₂SiO₄ using a multi-anvil press indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to ∼600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg₂SiO₄ at pressures of 20-36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from γ-Mg₂SiO₄ to MgSiO₃-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study, we find that the pressure and temperature of the post-spinel transformation in Mg₂SiO₄ is consistent with seismic observations for the 660-km discontinuity.