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The Earth’s mantle in a microwave oven : thermal convection driven by a heterogeneous distribution of heat sources
Past laboratory experiments of thermo chemical convection have dealt with systems involving fluids with different intrinsic densities and viscosities in a Rayleigh-Bénard setup. Although these experiments have greatly improved our understanding of the Earth’s mantle dynamics, they neglect a fundamental component of planetary convection : internal heat sources.
We have developed a microwave-based method in order to study convection and mixing in systems involving two layers of fluid with different densities, viscosities, and internal heat production rates, allowing to reach the high Rayleigh-Roberts and Prandtl numbers that are relevant for planetary convection.
I will illustrate the potential of our technique with a few examples of convection experiments of an initially stably stratified system where the lower layer has higher internal heat production and density than the upper layer. Due to mixing, the amount of enriched material gradually decreases to zero over a finite time. Thermal steady-state is reached before mixing has proceeded to completion. The temperature contrast between the two fluids reaches a maximum during the mixing phase, as the volume of lower fluid is decreasing.
Based on more than 30 experiments, I will present the derived scaling laws that relate the lifetime of an enriched reservoir and its temperature contrast to the layer thickness ratio, to the density, viscosity and internal heat production rate contrasts between the two layers, and to the temperature scale imposed by the global heating rate. I will discuss the differences with respect to Rayleigh-Bénard convection.
In the end I will speculate on the geophysical implications of upscalling our results to the Earth mantle.