Seismic evidence for a rapidly varying compositional anomaly at the base of the Earth's mantle beneath the Indian Ocean

Seismic observations recorded by an African seismic array reveal a low velocity anomaly at the base of the mantle beneath the Indian Ocean, with steeply dipping edges, rapidly varying thicknesses and geometries, and anomalously low shear wave velocities decreasing from -2% at 200 km above the core-mantle boundary to -9% to -12% at the core-mantle boundary (relative to the preliminary reference Earth model). These characteristics unambiguously suggest that it is a compositional anomaly and its velocity structures can be well explained by partial melt driven by a compositional change produced early in the Earth's history. This chemical anomaly geographically coincides with the DUPAL geochemical anomaly observed in island volcanoes around the Indian Ocean and may provide an explanation for its distinctive isotope characteristics observed at the Earth's surface. (C) 2001 Elsevier Science B.V. All rights reserved.