Abstract

Recent thermal and fluid dynamical models have shown that density-driven ascent of isoviscous granitic melt through the crust in narrow dykes is geologically instantaneous, provided the role of suspended crystals is ignored. However, it is well known that solids in suspension can affect significantly the rheology and hence flow properties of magmas, leading to the justified criticism that granite ascent and emplacement rates based on simple dyke transport models may be unrealistic. Currently, there is still much debate as to the specific timing of various rheological transitions (eg. Newtonian to Bingham) during magma flow, and whether or not magmatic suspensions have an inherent yield strength. Despite these uncertainties, it is clearly desirable to take into account the potential role of suspended solids in magma ascent models. In the last few years papers have appeared in the fluid mechanics and micromechanics literature on sheared suspensions that are directly applicable to the transport of magma in conduits. Using granular flow theory, a measure of the fluctuations of the particle theory is introduced which is regarded as a temperature field or granular temperature (T) where with the fluctuation in the velocity (v) of a particle (j) given by v~(j) = v(j) ­ <v>. Specimen calculations giving the granular 'temperature', solidosity and velocity profiles for ascending granitic magmas show that the energy associated with these irregular movements is much greater than that expected on purely Brownian grounds. Some important petrological and rheological consequences of granular flow in ascending granitic magmas will be discussed. Initial results suggest: 1) at crystal contents up to 30% flow remains Newtonian, although ascent velocities are reduced by a factor of ~ 60; 2) viscosity contrasts (up to 70) between regions of maximum and minimum shear act to lubricate the crystal-rich interior of the conduit from its margins, preventing lock-up and allowing flow to continue at relatively high solid concentrations; 3) flow differentiation by migration of particles towards the region of minimal shear (shear-enhanced diffusion) is an inevitable consequence of granular flow, thus providing a simple mechanistic explanation for the widely observed Bagnold effect.