Mineral Physics Test of Lateral Heterogeneities with Thermal and Chemical Origins in Pyrolitic Lower Mantle

Seismic tomography reveals various degrees of lateral heterogeneities of seismic velocities in the lower mantle, especially at the bottom of the lower mantle under Central Pacific and beneath Africa. Defining the origins of these heterogeneities (e.g., temperature versus chemical variations), which are critical for understanding the dynamics of the convective mantle, requires knowledge of physical properties of the constituent phases of the Earth’s interior from mineral physics at relevant pressure and temperature conditions. The ratios of the relative change (scaling factor) of shear to compressional wave velocities, density to shear wave, and bulk sound speed to shear wave velocity are often used as diagnostic for discrimination the origins of lateral heterogeneities, however, obtaining conclusive results of these scalings from current mineral physics data is still a challenge due to the lack of direct measurements on the elastic and anelastic properties at the conditions of the lower mantle. As a result, different mechanisms, such as anelasticity effect, variation of calcium content, as well as enrichments of iron and silica, have all been suggested as candidates to explain the extreme values of RSP (e.g., RSP >2.7) or the anti-correlation between bulk sound and shear speeds. In this study, a comprehensive investigation has been conducted on the variation of these scaling factors due to thermal and chemical anomalies for a pyrolitic lower mantle based on recent thermoelasticity data. Corresponding to different assumptions used in the derivation of thermoelasticity data from different studies, low and high bounds for the scaling factors are obtained, and the characteristics values for heterogeneities originated from temperature anomalies and chemical variations (bulk iron and silica contents) are compared.