Two‐layer mantle convection and the depletion of radioactive elements in the lower mantle

Abstract

Only a small percentage of the mantle’s total content of radioactive elements can be in the lower mantle if this region convects steadily and separately from the upper mantle. Recent geochemical models of a depleted upper mantle and an undepleted lower mantle are thus inconsistent with a steady thermal model involving distinct upper and lower mantle convection cells. Steady two‐layer mantle convection requires a depleted lower mantle and an undepleted upper mantle to match the observations of surface heat flow and mantle viscosity. The constraint on the abundance of radioactive elements in the lower mantle depends on the existence of at least one thermal boundary layer at the upper mantle‐lower mantle interface with thickness comparable to that of the surface thermal boundary layer. Since the temperature drop across an interface boundary layer is limited by the relatively small difference between the solidus temperature and the upper mantle temperature, the amount of heat which can be transferred across this layer is small compared with the surface heat flow. The upper limit on the concentration of radioactive elements in the lower mantle can be relaxed only if both the lower boundary layer of the upper mantle cell and the upper boundary layer of the lower mantle cell are extremely thin or absent altogether, which we consider, however, highly unlikely.