Crystallization and compositional convection in a porous medium with application to layered igneous intrusions

Abstract

Growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components, and vigorous convective motions in the fluid can result. We report a quantitative experimental study, in which saturated aqueous solutions of CuSO4 and Na2SO4 in porous media of either glass balls or CuSO4•5H2O crystals were crystallized by cooling from below. The crystals growing in the pore space released light, chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above. We explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium, the cooling rate, and the initial composition of the solution. This type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile. Adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front. We scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions. When our porous media had a small permeability, compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate. Trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically. Small, shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur.