Interaction Between LLSVPs and ULVZs and Its Implication for the Origin of ULVZs

Seismic tomography reveals two large low shear velocity provinces (LLSVPs) in the lowermost mantle, one beneath the central Pacific Ocean and the other beneath Africa and southern Atlantic Ocean. LLSVPs have been hypothesized to be caused by long-lived compositional reservoirs. Seismic studies have also detected much smaller scale ultra-low velocity zones (ULVZs) on the core mantle boundary. However, it is unclear whether ULVZs are caused by partial melting within hottest regions of LLSVPs or compositional heterogeneities that are different from LLSVPs. Furthermore, it is unknown whether ULVZs caused by compositional heterogeneity should form a continuous ribbon along the margins of LLSVPs or whether they should form discrete regional accumulations. Here, we performed high-resolution 3D thermochemical calculations to study the morphology and distribution of ULVZs and the interaction between ULVZs and LLSVPs. In some cases, a thin layer of ULVZ material is initially introduced at the lowermost 5-10 km of the mantle, and the ULVZ material is later advected by mantle flows. We found that, if ULVZs have a different composition from LLSVPs (no matter they are partially melted or not), they will preferentially accumulate into discontinuous patches with variable size and shape, along the edges of LLSVPs; however, if ULVZs are simply caused by partial melting in hottest regions of LLSVPs, they would be located within LLSVPs, inboard from their margins, where temperature is highest. Although the detailed dynamics of ULVZs are governed by parameters such as density and viscosity of ULVZ materials, Rayleigh number and temperature dependent viscosity, the general characteristics regarding the morphology and distribution of ULVZs remain unchanged. Thus, our results suggest that ULVZs along the edges of LLSVPs are caused by compositional heterogeneity with different composition as LLSVPs, while ULVZs in other regions of LLSVPs may be better explained by partial melting.