Convection in an Internally-Heated Two-Layer System
Abstract
The Earth's mantle is chemically heterogeneous and includes primordial material inherited from the first planetary processes which probably determined an initial depth-dependent composition of incompatible, radioactive elements. One consequence is that its internal heat sources are not distributed homogeneously. Mantle convection induces mixing, such that the flow pattern, the distribution of heterogeneities and the thermal structure are continuously evolving. We studied these phenomena in the laboratory using a unique microwave-based experimental set-up for convection in internally-heated systems. We characterize the development of convection and the progression of mixing in an initially stratified fluid made of two layers with different physical properties and heat production rates. In analogy to the Earth's mantle, the upper layer is thicker and depleted in heat sources compared to the lower one. Two different convection regimes are identified. In the dome regime, large domes of lower fluid invade the upper layer and remain stable for long time-intervals due to their enhanced heat production. In the stratified regime, cusp-like upwellings develop at the topography edges. Because of mixing, the volume of lower fluid decreases to zero over a finite time. Empirical scaling laws for the mixing rate allow extrapolation to planetary mantles.