Mineral physics constraints on ultra-low velocity zones in the lowermost mantle

Ultra-low velocity zones (ULVZs) at the base of Earth's lower mantle inferred from seismic inversions exhibit up to 50% lower shear velocity (Vs) and up to 25% lower compressional wave velocity (Vp) with respect to the average seismic velocities. Since the inversion procedure is not unique, the outcomes of the inversions remain inconclusive without a realistic cause supported by mineral physics data. In this review, we highlight the elastic properties of hypothesized candidates that were derived primarily from experimental in-situ measurements and some theoretical calculations. These candidates include partial melting of ordinary mantle material, ironenriched solid phases (bridgmanite, post-perovskite, and ferropericlase), eutectic melting of iron-carbon systems, and hydrogen-bearing ferric oxyhydroxide. We summarize and clarify the relative velocity changes and density changes as a function of the amount of each candidate mixing with the surrounding silicate-rich PREM mantle at 10% increments. The database provides constraints for seismic inversions in mapping seismic heterogeneities in the deep lower mantle. Although a comprehensive database of the elastic properties for each proposed candidate is unavailable, this review highlights or derives the material properties at the pressure and temperature conditions (- 136 GPa and - 3500 K) near the core-mantle boundary (CMB), readily to be used in determining acoustic impedance for seismic inversions.