Modes of mantle convection and the removal of heat from the Earth's interior

Abstract

Earth thermal histories based on two‐layer and whole‐mantle parameterized convection models have been explored to determine the influence of the mode of mantle convection on the content and distribution of mantle radioactive heat sources and on the temperatures and heat transfer properties of the mantle. Two‐layer mantle convection thermally insulates the lower mantle and is less efficient at heat removal than whole mantle convection. Consequently, two‐layer mantle convection requires a lower mantle depleted of radioactive heat sources contrary to the undepleted lower mantles of recent geochemical models. Whole‐mantle convection removes 3 times more primordial heat from the earth's interior and 6 times more heat from the core in 4.5 billion years than does two‐layer convection. The average mantle volumetric heat generation rates for both modes of convection are comparable to that of a potassium‐depleted chondrite. Thus no potassium is necessary in the core to balance the observed present‐day surface heat flux. However, most of the radioactives are concentrated in the upper mantle for two‐layer convection. Model geotherms for whole‐mantle convection compare well with independent estimates of temperature at various depths, including the core‐mantle boundary, while the lower mantle is too hot for two‐layer convection models. Thermal histories of the upper mantle in two‐layer models are similar to those of whole mantle models. Thermal histories for the lower mantle in two‐layer models are similar to those of the core in whole mantle convection models. This is because the lower mantle is thermally insulated by the upper mantle if it convects separately, just as the core is thermally insulated by the whole mantle. The differences between whole and two‐layer mantle convection are mostly due to the thermal insulation of the lower mantle and the smaller efficiency of heat removal of the upper mantle compared to that of the whole mantle, which is much larger in volume. A whole‐mantle convection model reconciling the need for distinct geochemical reservoirs with the physics of mantle convection is presented.