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

We report seismic evidence for a unique 300 km thick layer at the base of the mantle beneath the south Atlantic ocean. with a steeply dipping edge. anomalously low sheer wave velocities linearly decreasing from 2% (top) to 10-12% (bottom), and a maximum P velocity decrease of 3% relative to the preliminary reference Earth model (PREM). These characteristics can be best explained by partial melt driven by a compositional change produced early in the Earth's history and a vertical thermal gradient within the lever. This boundary layer may provide an explanation for the distinctive isotope geochemical DUPAL anomaly observed at some surface ocean islands. (C) 2001 Elsevier Science B.V. All rights reserved.