The thicknesses and Clapeyron slopes of mineral phase boundaries strongly influence the seismic detectability of the boundaries, as well as convection in the Earth’s mantle. In recent years, Mg-silicate perovskite, the dominant mineral in the lower mantle, was found to undergo a structural transition to the post-perovskite phase near 125 gigapascals, a pressure consistent with the D'' seismic discontinuity. Composition is expected to affect the pressure at which the transition occurs and whether it is a sharp or broad transition. We have investigated the effects of iron and aluminum as well as coexisting minerals on the perovskite to post-perovskite transition using in situ synchrotron X-ray diffraction in the laser-heated diamond-anvil cell. Composition is found to strongly affect both the depth and thickness of the transition, suggesting that the D'' discontinuity may only occur in regions of the Earth’s lowermost mantle, where subducted oceanic lithosphere is present.
