The effect of subduction zones on large-scale mantle convection and some perspectives on convection experiments based on colloidal systems
École Normale Supérieure
https://www.gotomeet.me/SeminairesGeosciencesENS Abstract: Several authors have suggested that mantle convection is primarily resisted by strong subduction zones, which if true implies small or even negative values of the exponent ? in the Nusselt number/Rayleigh number relation Nu ? Ra?. To evaluate this hypothesis, we use the boundary element method (BEM) to study the energetics of subduction in a two-dimensional system comprising two purely viscous plates, a subducting plate (SP) and an overriding plate (OP), immersed in an infinitely deep ambient fluid beneath a free-slip surface. For reasonable viscosity contrasts between the plates and the mantle, i.e. ?P/?M ? [250, 2500], our BEM solutions show that the dissipation is always dominated by the ambient mantle contribution. We demonstrate that a small value of the exponent ? follows from an overestimation of the viscous dissipation related to the deformation of the SP and, in particular, from the adoption of the minimum radius of curvature for the characterization of the SP bending. Using the correct length scale (the “bending length”; Ribe, 2010), we find ? ? [0.25, 0.34], which is not much different than the classical result of 1/3. We conclude that subduction zone dissipation is not large enough to change substantially the classical Nusselt number/Rayleigh number scaling law. As some recent studies suggest (e.g. Grigné & Combes, 2020; Seals & Lenardic, 2020), it is probably necessary to look elsewhere to reconcile geodynamical and geochemical arguments regarding the thermal history of the Earth. We, finally, run a convection experiment based on the drying of colloidal dispersions, which can generate an “Earth-like” thermal convection. This type of experiment seems to effectively captures the essence of Earth’s mantle convection and the particular features which characterized it as, for instance, the breakage of the upper boundary layer and the subsequent phenomenon of one-sided subduction. Exploring further these systems, especially for what concern the link between microscopic-scale phenomena, colloid rheology transitions and macroscopic properties of the material, seems a promising route to follow in order to get a clearer picture of how mantle convection works.