The role of chemical boundary layers in regulating the thickness of continental and oceanic thermal boundary layers

Abstract

An important feature of continents and oceans is that they are underlain by chemically distinct mantle, made intrinsically buoyant and highly viscous by melt depletion and accompanying dehydration, respectively. Of interest here are the influences of these preexisting chemical boundary layers on small-scale convective processes (as opposed to large-scale processes, which govern the drift of continents and the eventual fate of oceanic thermal boundary layers, e.g., subduction) at the base of the oceanic and continental thermal boundary layers. This manuscript explores the endmember in which dehydrated and melt-depleted boundary layers are assumed to be strong (in the viscous sense) and chemically buoyant enough that they do not partake in any secondary convection, that is, vertical heat transfer through these lids occurs purely by conduction. This assumption implies that the only part of the thermal boundary layer that participates in secondary convection resides beneath the strong chemical boundary layer. For oceans, this leads to the condition that the onset time of convective instability is suppressed until after the thermal boundary layer has cooled through the base of the strong chemical boundary layer, whose thickness is defined at the outset by the depth at which the solid mantle adiabat crosses the anhydrous peridotite solidus. A scaling law is presented that accounts for the presence of a preexisting strong chemical boundary layer and predicts that the onset time of convective instability beneath oceans correlates with the thickness of the chemical boundary layer, which itself correlates with the potential temperature of the mantle at the time of melting. Estimated paleo-potential temperatures required to generate old oceanic crust in the Pacific and Atlantic may in fact be correlated with onset time of seafloor flattening, but more data are needed to confirm these preliminary observations. Finally, for continents, recent numerical models suggest that the thickness of the convective sublayer, hence the total thermal boundary layer thickness, is also controlled by the thickness of a preexisting strong chemical boundary layer. Xenolith data from cratons are shown to be largely consistent with the model-predicted relationship between the thicknesses of the chemical and thermal boundary layers beneath continents. The conclusion of this study is that the nature of both oceanic and continental thermal boundary layers is likely to be linked to preexisting strong chemical boundary layers.