Effect of partial melting on small scale convection atop a mantle plume

Abstract:

A lithospheric plate passing atop a mantle plume is likely to be thermally thinned or “rejuvenated”. Geophysical data on the lithosphere-asthenosphere boundary (LAB) depth beneath active hotspots partly validate this prediction, but there is a large variation of the LAB upwelling estimated from different methods. Numerical simulations of plume-lithosphere interactions show that the development of small-scale convection (SSC) in the plume pancake spreading out along the base of the lithosphere is a mechanism able to rejuvenate the lithosphere, even for a fast-moving plate. The triggering of SSC has been shown to depend on the rheological behaviour of the unstable layer underlying the stagnant upper part of the thermal boundary layer (TBL), but the stability of the this layer may also be affected by partial melting.
We analyze, using a 2D petrological-thermo-mechanical numerical model, the influence of partial melting on the dynamics of time-dependent SSC instabilities and the resulting rejuvenation of a lithosphere passing atop a mantle plume. These models show a complex behavior, with either an acceleration, no change or a slight decceleration of the SSC onset, due to the competing effects of the latent heat of melting, which cools the plume material, and of the buoyancy increase associated with melting, among which the dominant effect is the depletion in heavy elements of the solid fraction. The viscosity reduction, though significant (up to 2 orders of magnitude) is too localized to affect the SSC dynamics. Despite the presence of partial melting, the mechanical lithosphere erosion in not enhanced significantly relatively to melt-free models.