Seismic evidence for a thermo-chemical boundary at the base of the Earth'smantle

We report seismic evidence for a unique 300 km thick layer at the base of t he mantle beneath the south Atlantic ocean. with a steeply dipping edge. an omalously low sheer wave velocities linearly decreasing from 2% (top) to 10 -12% (bottom), and a maximum P velocity decrease of 3% relative to the prel iminary reference Earth model (PREM). These characteristics can be best exp lained by partial melt driven by a compositional change produced early in t he Earth's history and a vertical thermal gradient within the lever. This b oundary layer may provide an explanation for the distinctive isotope geoche mical DUPAL anomaly observed at some surface ocean islands. (C) 2001 Elsevi er Science B.V. All rights reserved.