Upper mantle compositional variations and discontinuity topography imaged beneath Australia from Bayesian inversion of surface‐wave phase velocities and thermochemical modeling

Here we discuss the nature of velocity heterogeneities seen in seismic tomography images of Earth's mantle whose origins and relation to thermochemical variations are yet to be understood. We illustrate this by inverting fundamental‐mode and higher‐order surface‐wave phase velocities for radial models of the thermochemical and anisotropic structure of the mantle to 450 km depth. Dispersion data are linked to thermochemical parameters through a thermodynamic formalism for computing mantle mineral phase equilibria and physical properties. The inverse problem is solved using a probabilistic inference approach whereby robust uncertainty estimates are obtained. We find that both compositional and thermal anomalies are required if observations are to be satisfied. Mantle thermochemical variations extend to 250 km depth beneath western and central Australia and are characterized by increased Mg/Fe and Mg/Si values relative to surrounding mantle. Correlated herewith are thermal variations that closely follow surface tectonics. We also observe a strong contribution to lateral variations in structure and topography across the “410 km” seismic discontinuity from thermochemically induced phase transformations that appear much stronger than lateral variations immediately above and below. Inside the transition zone, a general decoupling of structure relative to that of the upper mantle occurs driven by a reversal in Mg/Si, while thermal anomalies are smoothed out. Comparison of presently derived shear‐wave tomography models with other regional models is encouraging. Radial anisotropy is strongest at 150/200 km depth beneath oceanic/continental areas, respectively, and appears weak and homogeneous below. Finally, geoid anomalies are computed for a subset of sampled model and compared to observations.