Seismic evidence for olivine phase changes at the 410-and 660-kilometer discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)(2)SiO4 spinel. and postspinel transformations.