The geophysical structure of the lower mantle may inform about Earth’s long-term evolution and dynamics. Among the most extreme geophysical anomalies in the lowermost mantle are the Ultra-Low Velocity Zones (ULVZ), small patches of rock at the core-mantle boundary (CMB), through which seismic waves travel much more slowly than through the ambient mantle. The properties of ULVZs indicate the presence of partial melt and/or iron-enriched rock, even though their specific composition remains elusive. The ULVZs have been interpreted as the remnants of the magma ocean, or alternatively, as resulting from present-day melting of chemical heterogeneity at the hot CMB. An ancient origin of ULVZ is corroborated by geochemical data, which points to the preservation of short-lived radionuclides and their daughter products in the largest ULVZs on Earth. ULVZs have been moreover identified as sites of potential core-mantle chemical interaction.
To further explore the compositional nature and origin of ULVZs, we will run 2D regional models of two-phase flow in the lowermost mantle that are coupled with a thermodynamic model for melting and crystallization. In contrast to previous work, we will account for compositional heterogeneity and melt-rock reaction. To test these predictions of coupled geodynamic-thermodynamic models, we will explore wave propagation through synthetic velocity structures that are computed from a mineral-physics database, and compare the results to seismic data. Better constraints on the composition and geometry of ULVZs will improve our understanding of chemical interaction near/across the CMB, lowermost-mantle thermochemical structure and long-term thermal evolution of our planet.