Abstract

We present a mechanical model for flow and seepage in a deforming, shear-dilatant sensitive magma mush that enables estimates of the excess pore fluid pressures and flow rates in both the melt and solid phase to be captured simultaneously as a function of stress rate1. Calculations are relevant to crystallizing magma in the solidosity range 0.5 to 0.8 (50-20% melt). Composition is expressed through the viscosity of the fluid phase, making the model generally applicable to a wide range of magma types. A natural scaling emerges that allows results to be presented in nondimensional form. Using mush permeability as a proxy, we show that the greatest maximum excess pore pressures develop consistently in rhyolitic (high viscosity) magmas at high rates of shear, implying that during deformation, the mechanical behaviour of basaltic and rhyolitic magmas will differ. Transport parameters of the granular framework including tortuosity and the ratio of grain size to layer thickness (a/H), exert a strong effect on the mechanical behaviour of the layer at a given rate of strain. For dilatant materials under shear, flow of melt into the granular layer is implied. At rates of loading comparable with emplacement of some magmatic intrusions (c. 10-10s-1), melt velocities greatly exceed effects due to instabilities arising from local changes in density and composition. Such a flow carries particulates with it, and we speculate these may become trapped in the granular layer depending on their sizes. Shear induced flow may be locally important in transporting heat and chemical components from the mainly fluid region beneath the capture front, upwards into the solidification zone. We argue that some types of chemical zonation and resorbtion seen in plagioclase grains in plutonic and volcanic rocks may be due to the local flow of melt into the expanding pore space of the crystal mush, as opposed to larger, chamber-wide processes. For the extreme case of near instantaneous shear arising from earthquakes, flow rates of up to 1 ms-1 are predicted. These high flow rates will result in liquefaction of the mush, and extremely rapid segregation of evolved interstitial fluid, the bulk composition of which may resemble continental rhyolites.