CAN SHARP SEISMIC DISCONTINUITIES BE CAUSED BY NONEQUILIBRIUM PHASE-TRANSFORMATIONS

A long-posed problem of the seismic discontinuities at the depths of 4 10 and 660 km in Earth's mantle is whether these discontinuities are c aused by phase transitions alone or together with chemical layering. A mong several tests, the sharpness of the boundary is one of the most c rucial. Recent data suggest that the transition region thickness is le ss than 4 km for both the 410 and the 660 km discontinuities. This may be smaller than predicted by phase equilibria. We suggest that if the re is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convectiv e flow, the transformation in rising or descending material does not t ake place until the metastable overshoot becomes equal to the nucleati on barrier. An avalanche-like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity. In this case, the topography of the phase boundary is also substantially modified.