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Abstract

Crystal-bearing magmas are assumed to be intrinsically non-Newtonian with only qualitative or empirical descriptions of apparent shear-thinning behaviour. Similarly, the threshold for brittle fracture for these materials remains poorly constrained. Here, we compile existing data for the rheology of high-temperature synthetic silicate crystal-bearing magmas across a wide range of conditions, to test microphysical models for the real origin of shear-thinning effects. Our hypothesis is that shear thinning in these materials arises from unrelaxed shear thinning in the melt phase or viscous heating, or both. In order to test this, we define a ‘lever’ function L which scales for the amplification of strain rate in the melt phase between crystals. We show that when the strain rates are amplified by the L factor, a validated shear-thinning law for the melt phase accounts for the observed non-Newtonian behaviour. Taken together, our results provide a theoretical framework that predicts a microphysical mechanism for shear thinning. This implies that crystal-bearing magmas may in fact be Newtonian except for when the scaled conditions for viscous heating are met, or when the strain rates in the melt phase approach the inverse of the structural relaxation timescale. Additionally, these results imply that shear thinning due to non-Newtonian behaviour in the melt between crystals occurs only very close to the brittle threshold, and that it is therefore an effect that may be closely associated with failure, rather than being intrinsic or pervasive during magma ascent. Our analysis provides constitutive laws that can be upscaled to magma ascent conditions.