Signatures of chemical heterogeneity in the lowermost mantle from full-spectrum seismic tomography

Abstract:

A large data set comprising surface-wave phase anomalies, body-wave travel times, normal-mode splitting functions and long-period waveforms is used to evaluate the relationship between large-scale variations in anisotropic shear velocity, density and compressional velocity in the Earth’s mantle. The mantle-wave waveforms provide additional constraints on density and velocity scaling ratios (ν = dlnv_S/dlnv_P) in the lower mantle and are compatible with the estimates obtained from remaining datasets. We develop a methodology that allows construction of joint models with various levels of scaling complexity. Our preferred model of anisotropic shear and compressional velocity fits P-wave travel times and v_P-sensitive modes up to 40 percent better than our recent anisotropic shear-velocity model S362ANI+M, which was constructed with a constant v_P-v_S relationship throughout the mantle. The RMS shear-velocity variations in the transition zone and lowermost mantle are slightly reduced from S362ANI+M (~0.1%) when v_P-sensitive data are included. Several interesting properties of the lowermost mantle reported in earlier tomographic studies persist in our analysis; anti-correlation between bulk-sound and shear velocities, the associated increase in ν, and a lack of correlation between shear velocity and density are robust results that are largely independent of the regularization scheme. When positive correlation between density and shear-velocity variations is imposed in the lowermost mantle, variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40 percent. Recent splitting measurements of ₀S₂, in particular, are largely incompatible with perfectly correlated shear-velocity and density heterogeneity throughout the mantle. A way to fit concurrently the various data sets in our inversions is by allowing independent density perturbations in the lowermost mantle. Our preferred joint model consists of denser-than-average anomalies (~1% peak-to-peak) at the base of the mantle roughly coincident with low-velocity superplumes. The relative behavior of anisotropic velocities and density disfavor a purely thermal contribution to heterogeneity in the lowermost mantle, which has important implications for the long-term stability and evolution of superplumes.