Long-term survival of large low velocity provinces (LLVPs) due to internal layering

Large low-velocity provinces (LLVPs) in Earth's lowermost mantle, characterized by a significant reduction of seismic wave velocities, are among the largest structures in Earth's deep mantle, and their long-term stability plays a crucial role in Earth's thermal and chemical evolution. The stability of LLVPs is greatly controlled by their density anomaly with respect to the background mantle. Although the density anomaly of the LLVPs remains a matter of debate, previous studies suggested that they may have a denser basal layer. In this study, we perform geodynamic simulations to investigate how this basal dense layer affects the long-term stability of LLVPs. We find that in models where LLVP-like thermochemical piles are relatively light and rapidly mix into the background mantle, the presence of a thin, intrinsically denser basal layer can help these piles to survive at the core-mantle boundary (CMB) over timescales comparable to Earth's history. Our results suggest that LLVPs do not need to be denser than the surrounding mantle at all depths to maintain long-term stability. Instead, their density can be stratified, increasing toward the CMB, with a basal dense layer.