Are mantle plumes adiabatic?

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 shear 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 Earthʹs history and a vertical thermal gradient within the layer. This boundary layer may provide an explanation for the distinctive isotope geochemical DUPAL anomaly observed at some surface ocean islands.