The role of convective mixing in degassing the Earth's mantle
Résumé
Because the chemical evolution of the mantle depends on the way rocks are processed at the surface of the Earth and on the efficiency of convective mixing, the goal of the present paper is to quantify how mixing and processing depend on convection parameters (Rayleigh number, heating mode and viscosity stratification). An extensive set of simple 2D convection simulations with passive tracers is built to compute bulk (a) Lagrangian strain rates to evaluate mixing efficiency and (b) processing histories. At high Rayleigh number, the strain rate is only a function of the velocity of the flow, whatever the heating mode or viscosity stratification. Though this is not the case in low Rayleigh number experiments with basal heating where a transition regime is observed with inefficient mixing though chaotic. The simulations show that the processing efficiency depends a lot on the heating mode : the probability of sampling primordial rocks is larger with internal heating than with basal heating. Scaling laws are proposed to parameterize mantle processing histories and it is predicted that in a 300~K hotter Earth, free convection can account for the mantle early degassing (more than 90% in less than 100~Myrs).