Fine-scale Ultra-low velocity zone layering at the core-mantle boundary and superplumes

Ultra-low velocity layering at the Earth's core-mantle boundary (CMB) has now been detected using a variety of seismic probes. P- and S-wave velocity reductions of up to 10's of percent have been mapped in a thin (5-50 km) layer, which commonly underlies reduced seismic shear wave speeds in the overlying few 100 km of the mantle. Ultra-low velocity zones (ULVZ) contain properties consistent with partial melt of rock at the very base of the mantle. Strong evidence now exists for a significant density increase in the layer (∼5-10% greater than reference models), which must be included in dynamical scenarios relating ULVZ partial melt to deep mantle plume genesis. 3-D geodynamical calculations involving an initially uniform dense layer in the lowermost few 100 km of the mantle result in thermo-chemical piles that are geographically well-correlated with seismic tomography low velocities, when past plate motions are imposed as a surface boundary condition. The hottest lower mantle regions underlay edges of the dense thermo-chemical piles.Ascenario is put forth where these piles geographically correlate with ultra-low velocity zones, and subsequent mantle plume genesis.