THE ROLE OF CHEMICAL BOUNDARY LAYERS IN REGULATING THE THERMAL THICKNESS OF CONTINENTS AND OCEANS

The depth of the seafloor and the thickness of the underlying thermal boundary layer initially increase with the square root of seafloor age. These features have been explained by conductive cooling of an infinite half-space. However, the depth and thickness on average saturate at constant values after 70 Ma and the model breaks down. On the basis of thermobarometric studies of mantle xenoliths, continents also saturate at thicknesses less than that predicted for their age by conductive cooling. Continents and oceans are shown here to be underlain by chemically distinct mantle, which is intrinsically buoyant and viscous due to melt depletion and accompanying dehydration. These chemical boundary layers behave as highly viscous lids through which vertical heat transfer occurs purely by conduction and beneath which lie the convectively active portions of the thermal boundary layer. For continents, the thickness of this convective sublayer is strongly controlled by the thickness of pre-existing chemical boundary layers. For oceans, a simple analytical model is presented that predicts the specific time at which the base of the thermal and chemical boundary layers cross and how this cross-over time depends on initial chemical boundary layer thickness.