COOLING THE CORE AND MANTLE BY PLUME AND PLATE FLOWS

Coupled thermal histories of the mantle and core are computed under various assumptions about the way the core is cooled, using empirical constraints on the present amounts of heat being transported by various modes of mantle flow. The mantle is cooled mainly by the large-scale flow associated with plates, assumed here to have operated throughout Earth history. The core is cooled by removal of heat from the thermal boundary at the base of the mantle. Plumes (both tails and heads) will accomplish much of this removal, if they originate at the core-mantle boundary, while there may also be a significant effect from the plate-scale flow. Removal by non-plume upwelling from the lower boundary layer cannot be ruled out. The thermal history of the core and the relative importance of plumes depend strongly on whether the heat removal is controlled by the viscosity at the core-mantle boundary (as would be the case with plumes) or by the viscosity within the bulk of the mantle (as would be the case for plate-scale flow or non-plume flow). If plumes are the dominant means of core cooling, the core will not have cooled by more than about 300-degrees-C during Earth history. Plumes would have been established early and their heat flux would have been fairly constant. On the other hand, if non-plume upwelling or the plate-scale flow are significant at present, they and plumes could have been much more important in the past and the core could have cooled by up to 1000-degrees-C.