Mantle Dynamics in Super-Earths: Post-Perovskite Rheology and Self-Regulation of Viscosity

摘要

Simple scalings suggest that super-Earths are more likely than an equivalentEarth-sized planet to be undergoing plate tectonics. Generally, viscosity andthermal conductivity increase with pressure while thermal expansivitydecreases, resulting in lower convective vigor in the deep mantle. According toconventional thinking, this might result in no convection in a super-Earth'sdeep mantle. Here we evaluate this. First, we here extend the densityfunctional theory (DFT) calculations of post-perovskite activation enthalpy ofto a pressure of 1 TPa. The activation volume for diffusion creep becomes verylow at very high pressure, but nevertheless for the largest super-Earths theviscosity along an adiabat may approach 1030 Pa s in the deep mantle. Second,we use these calculated values in numerical simulations of mantle convectionand lithosphere dynamics of planets with up to ten Earth masses. The modelsassume a compressible mantle including depth-dependence of material propertiesand plastic yielding induced plate tectonics. Results confirm the likelihood ofplate tectonics and show a novel self-regulation of deep mantle temperature.The deep mantle is not adiabatic; instead internal heating raises thetemperature until the viscosity is low enough to facilitate convective loss ofthe radiogenic heat, which results in a super-adiabatic temperature profile anda viscosity increase with depth of no more than ~3 orders of magnitude,regardless of the viscosity increase that is calculated for an adiabat.Convection in large super-Earths is characterised by large upwellings andsmall, time-dependent downwellings. If a super-Earth was extremely hot/moltenafter its formation, it is thus likely that even after billions of years itsdeep interior is still extremely hot and possibly substantially molten with a"super basal magma ocean" - a larger version of (Labrosse et al., 2007).