LOWER MANTLE THERMAL STRUCTURE DEDUCED FROM SEISMIC TOMOGRAPHY, MINERAL PHYSICS AND NUMERICAL MODELING

The long-wavelength thermal anomalies in the lower mantle have been ma pped out using several seismic tomographic models in conjunction with thermodynamic parameters derived from high-pressure mineral physics ex periments. These parameters are the depth variations of thermal expans ivity and of the proportionality factor between changes in density and seismic velocity. The giant plume-like structures in the lower mantle under the Pacific Ocean and Africa have outer fringes with thermal an omalies around 300-400 K, but very high temperatures are found in the center of the plumes near the base of the core-mantle boundary. These extreme values can exceed +1500 K and may reflect large hot thermal an omalies in the lower mantle, which are supported by recent measurement s of high melting temperatures of perovskite and iron. Extremely cold anomalies, around -1500 K, are found for anomalies in the deep mantle around the Pacific rim and under South America. Numerical simulations show that large negative thermal anomalies of this magnitude can be pr oduced in the lower mantle, following a catastrophic flushing event. C old anomalies in the mid-lower mantle have modest magnitudes of around -500 K. A correlation pattern exists between the present-day location s of cold masses in the lower mantle and the sites of past subduction since the Cretaceous. Results from correlation analysis show that the slab mass-flux in the lower mantle did not conform to a steady-state n ature but exhibited time-dependent behavior.