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.