The feedback between surface mobility and mantle compositional heterogeneity: Implications for the Earth and other terrestrial planets

- Surface mobility decreases when compositionally dense material is present.
- Upwellings are stronger at the edges of compositional piles rather than on pile tops.
- Surface mobility always persists if the enriched material volume is 1% or less.
- If enriched material envelops the core, surface mobility is eventually lost.

Planetary surface mobility depends on lithospheric stresses arising from the mantle's convective vigor. Using a model of thermochemical convection featuring force-balanced plates we examine the effect on surface mobility of different fractions of compositionally dense mantle material. Specifically, we introduce a uniform thickness compositionally enriched basal layer in a system with mobile-lid tectonics and monitor whether an active lid is subsequently maintained. We find that long-term surface mobility decreases when enriched material is present. High mobility is always maintained if the total material volume is no more than 1% of the mantle volume. For the inferred volume of the Large Low Shear Velocity Provinces in the present-day Earth surface mobility is dependent on the buoyancy ratio of the enriched material. If the compositionally dense material self-organizes into provinces, both surface mobility and mantle upwelling vigor become more variable. Generally, upwellings that form at the edges of provinces are more buoyant relative to upwellings that form on the tops of provinces. If enriched material envelops the core, upwelling vigor is diminished so that plates are consumed more quickly than they can fragment, and surface mobility is eventually lost.