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

Abstract

The relative changes between shear and compressional velocities ( R SP = ∂ ln V S /∂ ln V P ), bulk sound and shear velocities ( R CS = ∂ ln V C /∂ ln V S ), and density versus shear wave velocity ( R ρS = ∂ ln ρ/∂ ln V S ) in response to thermal and chemical variations were investigated for the pyrolitic lower mantle. For heterogeneities with thermal origins, R SP increases from 1.7 to 2.0 together with R ρS decreasing from 0.4 to 0.2 and R CS = ∼0.27 from the top to the bottom of the lower mantle. In comparison, chemical variations (bulk iron or silica contents) are characterized by R SP < 1.5 and R CS > 0.5 at lower mantle depths. Negative values of R ρS and R CS 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.