Plate Tectonics in Massive Rocky Super-Earth Planets: A Consequence of the Dissociation Transition of Post-Perovskite at 0.9 TPa

The possibility of plate tectonics in massive rocky planets is a contentious issue. Variable estimations suggested by different studies rely on uncertain mantle properties at very high mantle pressures for which there are no experimental constraints. Unlike the previous interpretations, new theoretical studies suggest a reduction in the mantle viscosity of rocky planets at very high pressures due to the inclusion of quantum mechanical and other pressure dependent effects for deep mantle minerals. Also, magnesium silicate post-perovskite (pPv) dissociates into new phases of minerals at ~ 0.9 TPa with a highly negative Clapeyron slope which is expected to influence the style of convection in massive exoplanets. In this study using compressible 3D-spherical convection models, we show that the pPv-dissociation transition along with a reduction in viscosity at the transition depth can cause deep mantle layering at the bottom of massive super-Earth planets (m > 7M⊕). Superplumes originating from this small-scale convective layer by penetration from weak zones at the top of the layer can reach the surface without significant loss in momentum. These penetrative focused superplumes (PFS) with lateral extent exceeding 10,000 km and radial velocities exceeding 20 cm/yr are highly stable with no significant lateral migration, and can sustain active plate tectonics and an active core dynamo for billions of years.