The large low shear velocity provinces (LLSVPs) observed in the lowermost mantle are widely accepted as chemically distinct thermochemical 'piles', but their origin and long-term evolution remain poorly understood. The survival time and shape of the LLSVPs are thought to be mainly controlled by their compositional density, while their viscosity has beeen considered less important. Based on recent constraints on chemical reactions between mantle and core, a more complex viscosity structure of the lowermost mantle, possibly including high viscosity thermochemical pile material, seems reasonable. In this study, we use numerical models to identify a trade-off between compositional viscosity and density contrasts required for long-term stability of thermochemical piles, which permits lower-density and higher-viscosity piles. Moreover, our results indicate more restrictive stability conditions during periods of strong deformation-induced entrainment, e.g. during initial pile formation, which suggests long-term pile survival.
Stabilizing effect of compositional viscosity contrasts on thermochemical piles