Penetrative Superplumes in the Mantle of Large Super‐Earth Planets: A Possible Mechanism for Active Tectonics in the Massive Super‐Earths

Abstract

AbstractRecent theoretical studies suggest that the physical and rheological mantle properties in massive rocky planets fall outside conventional behaviors inferred for mantle properties at the Earth's mantle pressures. The vacancy diffusion occurring at low pressures is assumed to be followed by interstitial diffusion above ∼0.1 TPa resulting in viscosity reduction at higher pressures. In addition, the dissociation transition of MgSiO3 post‐perovskite (pPv) into new phases of minerals at 0.9 and 2.1 TPa, both with large negative Clapeyron slopes, has further impact(s) on the style of circulation in the mantle of super‐Earth planets. Further, the electronic contribution of conductivity increases exponentially with temperature at temperatures ∼5000 K and higher. We employ 3D‐controlled volume spherical convection models to explore the style of mantle circulation in large rocky super‐Earth planets with different core temperatures. Our numerical models resembling a GJ 876 d size super‐Earth reveal that due to the buffering influence of the pPv‐dissociation transition at ∼0.9 TPa, for deep mantle viscosities lower than ∼1022 Pa.s a small‐scale convective layer may develop at the top of the core‐mantle boundary (CMB). Penetrative superplumes originating from deep mantle‐layered regions can maintain the heat flux from the CMB required for the planet's geodynamo, and can survive for billions of years reaching shallow depths of the mantle without significant lateral migration. The strength of the focused penetrative superplumes that can potentially sustain surface volcanism and plate tectonics is enhanced with increasing CMB temperature, but diminished by higher rates of internal heating.