Heat-blanketed convection: numerical simulations and implications for the evolution of the continental lithosphere

Abstract:

Earth's continents are characterized by a strong enrichment in long-lived radioactive isotopes. Recent estimates suggest that they contribute to 33 % of the heat produced, while occupying less than 1 % of the Earth's mantle. This distinctive feature has profound implications for the underlying mantle as it impacts its thermal structure and heat transfer. Despite its importance, a systematic investigation of this feedback is still missing. Here, we conduct a preliminary investigation in this direction by considering a simplified convective system, a mixed heated fluid where all the internal heating is concentrated in a top layer of thickness d_HL, which we refer to as ''heat-blanketed convection''. In that aim, we perform 24 numerical simulations in 3D Cartesian geometry for four specific conditions, representative of a large range of parameters, and various values of d_HL. The numerical results are then used to describe thoroughly the effects of d_HL on the convective system. In particular, we find that these effects strongly depend on the relative thickness of the heated layer (d_HL) compared to the thickness of the thermal boundary layer (δ_TBL) in the homogeneous heating case (d_HL = 1.0). More specifically, for d_HL > δ_TBL, these effects are modest, while, for d_HL < δ_TBL, the properties of the convective system are strongly altered as d_HL gets smaller. When applied to Earth's continents, these results suggest that the vertical distribution of heat producing elements plays a key role on mantle dynamics. For instance, this distribution may significantly impact the thermal structure of the system or its convective vigour. However, in contrast with previous studies, our results indicate that continents have only a modest effect on Earth's cooling rate.