Characteristics of lateral heterogeneities with thermal and chemical origins in the pyrolitic lower mantle

The relative changes between shear and compressional velocities (RSP = ∂ ln VS/∂ ln VP), bulk sound and shear velocities (RCS = ∂ ln VC/∂ ln VS), and density versus shear wave velocity (RρS = ∂ ln ρ/∂ ln VS) in response to thermal and chemical variations were investigated for the pyrolitic lower mantle. For heterogeneities with thermal origins, RSP increases from 1.7 to 2.0 together with RρS decreasing from 0.4 to 0.2 and RCS = ∼0.27 from the top to the bottom of the lower mantle. In comparison, chemical variations (bulk iron or silica contents) are characterized by RSP < 1.5 and RCS > 0.5 at lower mantle depths. Negative values of RρS and RCS are indicative of chemical anomalies in the lower mantle, but a combination of thermal and chemical heterogeneities may be required to produce velocity and density anomalies at the magnitudes observed in seismic data. Further refinement of these characteristics requires data on the higher order pressure and temperature derivatives of the elastic moduli of the constituent phases.