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

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 degrees to 83 degrees and grouped in two small turning-point bins from 5 degrees to 10 degrees N and 10 degrees to 15 degrees 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 similar to 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 similar to 5 GPa (90 km) in pyrolitic material, requiring either distinct chemistry or dynamical mechanisms to sharpen the phase transition for this high-velocity subcircum-Pacific region.