The origin of ULVZs more here

We run 3D mantle convection models to look at how the distribution of ULVZs is related to their origin. We find that the hottest regions are located well within the hot thermochemical piles, whereas the intrinsically dense material is preferentially located at the edges of thermochemical piles. ULVZs are detected in the subducting regions, near the edges of LLSVPs and within LLSVPs. Our results suggest that most of the ULVZs are consistent with a compositionally-distinct origin, although the ULVZs well within LLSVPs can be caused by partial melting alone.

Here is a video of numerical calculation showing how seismic detected ultra-low velocity zones (ULVZs, red) look like and interact with the large low shear velocity privinces (LLSVPs, blue). Initially, a thin layer of ULVZ materials is put on the core mantle boundary. Shortly after that, the ULVZ materials are moved to the edges of LLSVPs, and accumulate into discontinuous patches with variable size and shape. Here, we assume ULVZs have different composition as LLSVPs and ULVZs are more dense than LLSVPs.