Geodynamica

Most rocky planets operate in a stagnant lid convective regime. They are characterized by the strongly temperature-dependent viscosity that locks the cold mantle silicates near the surface into a stiff layer, or lid, which does not participate in mantle convection. Stagnant lid planets have lower surface heat flows than mobile lid planets like Earth; however, Earth-like heat flows have been inferred from elastic lithosphere thickness estimates for Venus leading researchers to wonder how these higher heat fluxes occur. Mineral phase transitions in the upper mantle can inhibit or enhance mass exchange through the mantle transition region at pressures and temperatures relevant for larger terrestrial mantles such as Earth’s or Venus’s. For a multiphase composition of anhydrous pyrolite, small cold plume instabilities originating at the base of the stagnant lid sink to a depth of about 600 km.  When a critical amount of material accumulates, the drips avalanche into the lower mantle as a large coherent downwelling, generating warm return flow into the upper mantle.  Previous work using 2D numerical models of mantle convection with an assumed anhydrous pyrolite composition showed wadsleyite transforms into majorite plus ferropericlase for temperatures warmer than 1950 K. This study explores how the endothermic WMF transition demonstrates weaker upper-lower mantle layering for increasingly higher Rayleigh numbers and higher mantle temperatures. This counterintuitive weakening induces more numerous and continuous flushing events (mantle avalanches) between the upper and lower mantle, inducing a state of sustained, higher-efficiency heat flow throughout the mantle, manifesting as a “jump” or upwards shift in the Rayleigh-Nusselt scaling compared to colder models that also exhibit the effects of mantle layering. This higher efficiency state of continuously- layered-and-flushed stagnant lid convection may be relevant for Venus which lacks apparent plate tectonics yet has regions which show evidence of Earth-like (high) surface heat fluxes.