On the fate of thermally buoyant mantle plumes at density interfaces

Abstract

The 660-km depth endothermic phase transition is expected to delay upwelling plumes and modify their morphology. The interaction of thermal starting plumes with a density boundary is investigated experimentally by using simultaneous visualizations of temperature, composition and velocity fields. In a tank initially stratified with two viscous fluids with different densities and viscosities, a thermal starting plume was generated by using a circular plate heater at the bottom of the tank. Its interaction mode with the inner interface depends on the local buoyancy number (BL: the ratio of the stabilizing chemical buoyancy to the plume thermal buoyancy at the interface), the Rayleigh number (Ra), the viscosity contrast between the chemical layers (γ), and the local viscosity ratio of the ambient material to the plume head (γp). For BL<0.6, the “pass-through mode” develops, whereby a large volume of the lower material rises through the upper layer and reaches the top surface, since the plume head has a large thermal buoyancy compared to the stabilizing density contrast between the two layers. When BL>0.6, the “rebirth mode” occurs, where the thermal plume ponds and spreads under the chemical boundary and secondary thermal plumes are generated from the interface. Depending on the magnitude of BL, these plumes can entrain upwards by viscous coupling a significant amount of lower layer material. Scaling laws are determined for the onset of secondary plumes, their number and wavelength, the amount of entrainment and the topography of the interface. Our results could explain the hotspots' distribution above superswells and the peculiar time evolution of La Réunion hotspot.