Impact of grain size on the convection of terrestrial planets

Abstract

This article presents a set of simulations of mantle convection, using a new model of grain size‐dependent rheology. In the present paper, it is shown that this rheology behaves in many ways as a visco‐plastic rheology. I use a model of grain size evolution which has been calibrated on experimental data in a previous paper. In this physical model, the grain size is directly related to the stress state, following a temperature‐dependent piezometric law. The rheology used here allows both diffusion and dislocation creep, depending on the grain size. At low stress, the grain size is high and forces the rheology to be dislocation dominated. For sufficiently high stresses, the equilibrium regime reached by the grains is located in the diffusion creep. In this case, the viscosity is linked to a stress‐dependent grain size, which actually makes the rheology more non‐Newtonian than it is in dislocation creep. This experimentally calibrated model allowed me to perform a set of numerical experiments of convection in which the rheology may be diffusion or dislocation creep dominated, depending on the state of the stress tensor. Then, The stress exponent varies from 3 to 5 because of grain size, which has a large impact on the temperature dependence of the viscosity. The present paper shows the impact of this new model on the convection regimes of terrestrial planets. In particular, for a wide range of parameters, I observe the episodic regime which is thought to govern the dynamics of Venus. This process of episodic resurfacing was obtained in previous simulations using visco‐plastic rheologies is a tight range of parameters. I obtain it here without using an ad hoc plasticity law, only using a viscous rheology based on laboratory measurements. In these simulations, I show that the cooling rate of the terrestrial planets may be largely modified by the consideration of a grain size‐dependent rheology.