The influence of crystal size distributions on dynamic processes in magmatic systems

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Volcanic activity is driven by the flow of magma. Whether a volcano erupts effusively or explosively often only depends on differences in magma flow properties. For the assessment of hazards associated with a volcanic eruption, it is therefore of pivotal importance to predict and model the transport properties of magma as accurately as possible, i.e. to know the magma rheology. Despite significant progress in understanding the rheological properties of silicate melts, as well as those of the two-phase mixture melt-crystals, it remains largely unclear in what way and to what extent the crystal size distribution (CSD) affects the rheology of a magma. It is, however, precisely the size distribution which, at a given concentration, strongly affects the crystal packing arrangements and density, and hence also the bulk magma rheology. Virtually all available numerical eruption and conduit dynamics models, which do account for the rheological impact of the crystal phase, are based on suspensions with no size variation of the crystal phase. In nature, however, crystals are commonly distributed over a range of sizes (i.e., the magma is polydisperse). It is therefore imperative that the dependence of suspension rheology on the size distribution of the particles be well constrained, such that informed predictions of a magmas flow behaviour can be made. The aims of the project are: (a) to experimentally and systematically investigate (and scale), how particle size distributions affect the bulk rheological parameters of particle suspensions, and (b) to predict (using the newfound experimental scalings) the rheological paths a magma should follow when it is ascending and erupting from a volcanic conduit. In a novel and synergistic approach, we will combine HP/HT-petrology experiments on natural samples with analogue experiments, in order to develop a set of constitutive equations that will accurately describe the rheology of natural crystal-bearing magma. Accordingly, the project will be composed of two parts: (I) analogue experiments on polydisperse particle suspensions, in order to investigate the influence of CSDs on flow properties and to deduce fundamental rheological laws, and (II) petrology experiments to re-enact pressure/temperature conditions along a volcanic conduit and investigate the associated CSDs of natural samples. A central part of this combinational approach will be to use the petrology experiments to constrain the evolution of crystal sizes as a function of pressure and temperature in the volcanic conduit, and to use these resultant textures as templates allowing us to synthesize matching analogue suspensions to be used in further rheology measurements.