ABSTRACT
An assessment is made of the proposal that an isolated, compositionally dense, heat source rich layer exists in the bottom half of the lower mantle. The Earth’s heat budget is reviewed and it is concluded that though the fraction of the Earth’s heat loss attributable to mantle radioactivity is uncertain, an isolated mantle reservoir, rich in radioactive elements, is not required to balance the thermal budget. The balance does require either a large flow of heat from the core, or a large contribution from secular cooling of the mantle, or a mantle-wide concentration of radioactive elements larger than predicted by geochemical and cosmochemical models. It is argued that the disappearance of descending slabs in seismic tomographic images of the lower half of the lower mantle is not necessarily due to ponding of the slabs on the top of a compositionally dense lower mantle layer. Other explanations are equally plausible including the gradual reabsorption of descending slabs into the background mantle circulation. Most importantly, it is emphasized that a compositionally dense, heat source rich layer in the bottom half of the lower mantle cannot be hidden from seismic view by offsetting effects of temperature and composition on seismic velocity. The thermal boundary layer at the top of the lower layer would produce a distinct triplication in short-period P waves; seismic data show no evidence of P-wave triplications due to any mid-mantle thermal boundary layers. The thermal boundary layer at the top of the lower layer would also inhibit cooling of the lower layer and make it hot enough to be partially molten, in disagreement with seismic evidence for the solidity of the lower mantle. It is concluded that there is no isolated, compositionally dense, heat source rich layer in the bottom half of the Earth’s lower mantle.
