GEOPHYSICAL INFERENCES OF THERMAL-CHEMICAL STRUCTURES IN THE LOWER MANTLE

Lateral variations of the temperature field in the lower mantle have b een reconstructed using new results in mineral physics and seismic tom ographic data. We show that, with the application of high-pressure exp erimental values of thermal expansivity and of sound velocities, the s low seismic anomalies in the lower mantle under the Pacific and Africa can be converted into realistically looking plume structures with lar ge dimensions of 0(10(3) km). The outer fringes of the plumes have an excess temperature of around 400 K. In the core of the plumes are foun d tongue-like structures with extremely high thermal anomalies. These values can exceed 1200 K and are too high to be explained on the basis of thermal anomalies alone. We suggest that these major plumes in the deep mantle may be driven by both thermal and chemical buoyancies or that enhanced conductive heat-transfer may be important there.