Experimental Modelling of Interstitial Melt Convection in Cumulus Piles

Abstract

Fluid dynamic processes occur during the formation of cumulate rocks and thus model experiments and theory are a valuable counterpart to geological work in studying magma chamber processes. We describe experiments on crystallisation in porous media. Crystal growth from saturated aqueous solutions produced light, chemically-depleted fluid which could potentially convect out of the porous medium and be replaced by additional saturated solution from above. As long as the convective velocity in the pore space remained greater than the rate of solidification, the pore space became enriched in chemical components entering the growing crystals relative to the initial solution. This convection is a possible mechanism of adcumulus crystal growth in magma chambers, and might produce cumulate rocks with low trapped melt fraction in large mafic and ultramafic intrusions. When the porous media had low initial permeabilities, interstitial melt motion was greatly reduced or completely suppressed. An impermeable crystal layer grew on top of the porous medium in these circumstances. Trapping of dynamically-unstable melt beneath this layer occurred in the experiments and may be important geologically. If during magma chamber replenishment, dense melt is emplaced above a porous cumulus pile, it can sink into the pore space and replace the less dense intercumulus liquid. Experiments indicate that convective exchange takes the form of downward penetrating “fingers”. Quantitative results suggest that exchange may proceed at rates of metres to tens of metres per year in mafic to ultramafic magma chambers — this conclusion strongly depends on cumulus pile porosity and is less sensitive to magma viscosity. Re-equilibration and resorption of cumulus crystals could result from this process. When intercumulus melt exchange is incomplete, lateral geochemical gradients may be preserved in cumulate rocks. We interpret geological data from cyclic unit 10 of the Rhum Layered Intrusion in the light of these two experimental studies.