Hotspots, mantle plumes and core heat loss

Abstract

The heat flux at the core–mantle boundary (CMB) is a key parameter for core dynamics since it controls its cooling. However, it is poorly known and estimates range from 2 TW to 10 TW. The lowest bound comes from estimates of buoyancy fluxes of hotspots under two assumptions: that they are surface expression of mantle plumes originating from the base of the mantle, and that they are responsible for the totality of the heat flux at the CMB. Using a new procedure to detect plumes in a numerical model of Rayleigh–Bénard convection (convection between isothermal horizontal planes) with internal heating, it is shown that many hot plumes that start from the bottom boundary do not reach the top surface and that the bottom heat flux is primarily controlled by the arrival of cold plumes. Hot plumes easily form at the bottom boundary but they are mostly due to the spreading of cold plume heads that allow the concentration of hot matter. These plumes are generally not buoyant enough to cross the whole system and the hot plumes that reach the top surface result from an interaction between several hot plumes. According to this simple dynamical behavior, the heat flux at the bottom boundary is shown to be strongly correlated with the advection due to cold plumes and not with advection by hot plumes that arrive at the surface. It is then inferred that the heat flux out of hotspots can only give a lower bound to the heat flow at the CMB and that knowing the advection by subducted plates would give a better estimate.