The excess temperature of plumes rising from the core-mantle boundary

We study the rise of conduit-type thermal plumes from the core-mantle boundary (CMB) in a simplified numerical model. Viscosity is a strong function of temperature and pressure. Assuming that all plumes start with the same excess temperature at the CMB, we find that their temperature in the upper mantle is controlled by the plume flux and is insensitive to variations in viscosity or other material parameters or to the kind of flow law (Newtonian or non-linear). The reduction of excess temperature with height is stronger for weaker plumes. Plumes with a buoyancy flux of less than 1000 kg/s should have cooled so much that they would not melt beneath old lithosphere. The existence of such hot-spots possibly indicates that not all plumes rise from the CMB.