Mantle Avalanches in Venus-Like Stagnant Lid Planet
Madeleine Kerr, University of California, San Diego
Stagnant lid planets are characterized by a globe-encircling, conducting lid that is thick and strong, which leads to reduced global surface heat flows. Consequently, the mantles of such planets can have warmer interiors than Earth, and interestingly, a pyrolitic mantle composition under warmer conditions is predicted to have a distinctly different mantle transition zone compared to the present-day Earth. Instead of olivine primarily transforming into its higher pressure polymorphs such as Wadsleyite and then Ringwoodite, at pressures corresponding to 410km and 520km depth in Earth, respectively, it instead transforms into a mineral assemblege of Wadsleyite, Garnet, and Ferropericlase (WGF), and then to Garnet + Ferropericlase (GF), before finally transforming into Bridgmanite at pressures corresponding to 660km depth in Earth. Convective motions in stagnant lid planets are dominated by small-scale instabilities (cold drips) forming within the mobile rheological sublayer under the rigid lid. Using the geodynamic code ASPECT and a thermodynamic model of a pyrolitc mantle composition generated by the Gibbs free energy minimization software HeFESTo, we show that under certain conditions, the small drips can pond atop the WGF-GF mineral phase transition. The barrier to convective flow arises from an exotic property of WGF assemblage having a negative thermal expansivity. In contrast to mobile lid planets that recycle their entire lithosphere via large-scale downwellings which pass through the WGF zone without difficulty, the WGF zone in stagnant lid planets is capable of causing an ephemeral layering of the mantle. Our numerical models show that in stagnant lid planets with mantle potential temperatures that exceed 1900K, the smaller, cold drips from the lid continue to pile up until enough of them have coalesced that they collectively avalanche as a larger instability into the deeper interior.