Sharpness of the D″ discontinuity beneath the Cocos Plate: Implications for the perovskite to post‐perovskite phase transition

Abstract

The depth extent, or sharpness, of the D″ seismic velocity discontinuity is determined by modeling distance‐dependence of reflected S‐wave signals sampling the lowermost mantle beneath the Cocos Plate. The data, spanning distances from 64° to 83° and grouped in two small turning‐point bins from 5° to 10°N and 10° to 15°N, are compatible with a sharp 1 to 2% shear velocity increase near 300 km above the core‐mantle boundary, but not with reflection from a transition zone extending more than ∼30 km in depth. Interpretation of the velocity structure as the result of magnesium‐silicate perovskite undergoing a phase transition to post‐perovskite requires limited presence of Al and Fe, which will broaden the phase transition. Current experiments indicate that the phase transition may occur over ∼5 GPa (90 km) in pyrolitic material, requiring either distinct chemistry or dynamical mechanisms to sharpen the phase transition for this high‐velocity sub‐circum‐Pacific region.