THERMAL VARIATIONS IN THE MANTLE INFERRED FROM 660 KM DISCONTINUITY TOPOGRAPHY AND TOMOGRAPHIC WAVE SPEED VARIATIONS

Abstract

Variations in mantle temperatures cause changes in seismic wave speeds and changes in the pressure at which mineralogical reactions occur, changing the depth at which a seismic discontinuity occurs. As an experiment in using global seismological data sets in a statistical way to draw inferences concerning the Earth using angular correlation functions, we analyse wave speed variations in P - and S -wave tomographic studies and perturbations to the depth of the 660 km discontinuity, with the goal of inferring lateral temperature variations in the lower mantle. On account of discrepancies between the temperatures estimated using various data sets, the results are not particularly encouraging. We find variations in temperatures of 60 or 90-120 K at the top of the lower mantle, depending on the data set used, 60 K in the middle of the lower mantle, and potentially 60 or 300 K in the core-mantle boundary region. Based on 660 km depth perturbations, plumes might be 160 K warmer than the ambient lower mantle. Future improvements in angular correlation analyses could be obtained through finer gridding of the global data sets.