Convection in multicomponent silicate melts driven by coupled diffusion

Abstract

THE simplest convecting systems are driven by buoyancy forces arising from the density variations produced by the diffusion of a single quantity, usually heat. In such systems, the necessary condition for the onset of convection is that density decreases downwards. But the convection process can be very different if there are two or more diffusing quantities that affect the density^1–5, a situation often encountered in geological settings. For example, multicomponent convection plays an important role in the convec-tive evolution and differentiation of terrestrial magmas^6–8. A typical terrestrial silicate magma is made up of a number of diffusing components, which not only have different diffusion rates but may also exhibit diffusive coupling — that is, the diffusive flux of one component can be strongly dependent on the spatial gradients of other components^9–12. We have investigated convection in a geologically relevant fluid — a multicomponent silicate melt in the system CaO–Al₂O₃–SiO₂ — and we find that diffusive coupling plays a key role in promoting convection. Perhaps the most striking feature of these experiments is that convection can occur regardless of whether the density decreases or increases with depth.