Shear thickening dilatancy in crystal-rich flows

Rheological models of molten rock are mostly based on behaviours in which viscosity is constant with respect to strain rate (e.g. Newtonian, Bingham plastic) or viscosity decreases with increasing strain rate (pseudo-plastic or shear thinning). Rheological models in which viscosity increases with increasing strain rate (shear thickening or dilatant) models have rarely been used, yet such behaviour is common in particulate systems. Whereas magmas with low crystal content may be appropriately modelled by the former behaviours, greater crystal content may lead to particle interactions resulting in shear thickening behaviour. The effect is illustrated using a powdered rhyolitic rock mixed with water which exhibits extreme shear thickening behaviour caused by particles moving apart as they move past each other. The expansion of the grain fabric during flow is opposed by the applied loads such that dilatancy is invariably associated with shear thickening rheology. Crystal-rich lava may exhibit shear thickening dilatancy, a proposition supported by observations of glass-rich microscopic shear zones disrupting crystal fabrics. Plutonic crystal mush may also exhibit shear thickening dilatancy, which can explain synplutonic intrusion involving brittle fracture followed by ductile disruption. In general, strain rate is an important contributor to the transition from ductile to brittle behaviour in crystal-rich igneous rocks.