Large Low-Velocity Provinces (LLVPs): a new model for their structure, origin, and evolution

The Large Low-Velocity Provinces (LLVPs) beneath West Africa and the Southern Pacific are characterized by low seismic wave velocities and are associated with plate-unrelated magmatism such as hotspots, large igneous provinces, and kimberlite. Despite their significance, the structure, origin, and feeding processes of LLVPs remain elusive.

Previous studies have suggested that the LLVPs have remained stationary for over 300 million years, but their morphology appears to have changed. While geodynamic simulations favor denser LLVPs, a recent free-oscillation analysis has suggested lighter ones. The High-Velocity Region (HVR), which surrounds the LLVPs, is located beneath present and past subduction zones. Plumes of varying morphology are imaged between hotspots and LLVP margins, with intensive plumes revealing ultra-low velocity zones (ULVZs) at their roots. Ocean island basalts (OIBs) from hotspots are geochemically enriched and originate from multiple reservoirs. Interestingly, OIB chemistry does not correlate with seismic plume imaging and differs between the two LLVPs. OIB near the African LLVP is influenced by fluid-related subducted materials.

In the light of these results, I propose the following new model for the LLVPs. The LLVPs are at higher temperatures than the HVR and the surrounding mantle. They are composed of Fe-enriched mono-mineral bridgmanite rock, called bridgmanitite. The large grain size of bridgmanitite results in high viscosity despite the high temperatures, thereby stabilizing the LLVPs. The LLVPs are block-structured and the relative movement of the blocks changes the LLVP morphology. Bridgmanitite was formed by the solidification of a primordial magma ocean. Its deposition at the core-mantle boundary forms LLVP precursors in the early mantle. These LLVP precursor blocks can be moved by the thrust and sweep of subducted slabs to form the present-day LLVPs. Erosion and plume formation have reduced the volume of the LLVPs and resulted in different LLVP heights. The HVR consists of harzburgite brought from the surface by subduction. It contains only a limited amount of basaltic rock because the basaltic rock was detached from slabs in the mid-mantle due to suppressed grain growth. As a result, the HVR material is high-density due to the low temperature but is intrinsically low-density due to its chemistry. The HVR material has been heated using the LLVP heat to form plumes. Plumes are geochemically depleted when they have formed in the deep mantle. However, they are enriched in incompatible elements and volatiles in the shallow mantle. This enrichment results from melt migration due to the temperature gradient around the plume. Thus, although LLVP heat drives plume formation, the plate-unrelated magmas such as OIB are not directly derived from the LLVPs.