The ultra-low velocity zone (ULVZ) is a thin ( ~10 km) layer in some regions of the lower-most mantle immediately above the boundary with the outer core that is characterized by a dramatic reduction in seismic wave speeds. The cause of this reduction in wave speed is uncertain, but likely possibilities include a small degree of mantle melting and chemical heterogeneity created by reactions between silicate minerals in the mantle with iron in the core. Previous, lower resolution studies that characterized the geographic extent of the ULVZ have hinted at large regional patches, however, recent, higher resolution observations made by members of this group have revealed a potentially smaller-scale structure than originally thought. A small, isolated pocket of more-dense ULVZ material was discovered in a region that was previously thought to contain a much larger, continuous ULVZ layer. Work proposed here involves a collaboration of seismologists and both numerical and laboratory geodynamicists at Arizona State University and the University of California Berkeley. The ultimate goal of this work is to determine how upwelling mantle plumes originating from the core-mantle boundary affect the local geometry of the ULVZ, and one exciting possibility to examine is whether seismically detectable pockets of ULVZ can be used as markers to determine the source region for nearly seismically-invisible mantle plumes. On the seismology front, the ULVZ will be studied at much higher resolution than before with the goal of determining whether previously-thought-continuous ULVZ regions are instead composed of isolated pockets of more-dense regions. Geodynamically, numerical and laboratory experiments will be used to determine whether observed isolated pockets of ULVZ material are related to mantle plumes, and if so, what is the relationship between the morphology of ULVZ material and the flow patterns associated with mantle plume source regions? Finally, proposed work will focus on determining whether the dynamically-predicted and observational constraints can be used to differentiate between competing hypotheses of partial melting and chemical heterogeneity as a cause of the ULVZ.
