SELF-LUBRICATED MANTLE CONVECTION - 2-DIMENSIONAL MODELS

We Present results from convection models allowing for self-lubricatio n of downflows. Models impose a line source of chemically light, low v iscosity material at the top of a convecting layer of temperature-depe ndent viscosity material. Low viscosity surface material serves as an analog to hydrated sediment/crust and the high viscosity upper portion of the convecting layer as an analog to mantle lithosphere. Slow near surface motion in the convecting layer entrains low viscosity materia l into zones of downflow, which has a lubricating effect. Once entrain ed lubricant is deeper than the cold high viscosity portion of the con vecting layer, rapid upper boundary layer overturn occurs and system p roperties change (e.g., heat flux doubles). This marks transition to a lubricated state. Before and after transition, transport properties a re dominantly determined by, respectively, the viscosity of mantle lit hosphere and that of interior mantle. Lubricated and nonlubricated sta tes appear as distinct regions in system output space suggesting that exchange between them is akin to a phase transition. That such exchang e depends on a near surface lubricant implies that the geodynamics of planets lacking such lubricants may fundamentally differ from that of planets possessing them.