Potential causes for the non‐Newtonian rheology of crystal‐bearing magmas

Experimental studies indicate that crystal‐bearing magma exhibits non‐Newtonian behavior at high strain rates and solid fractions. We use a zero‐dimensional (0‐D) inversion model to reevaluate rheological parameters and shear heating effects from laboratory data on crystal‐bearing magma. The results indicate non‐Newtonian behavior with power law coefficients of up to n = 13.5. It has been speculated that finite strain effects, shear heating, power law melt rheology, or plasticity are responsible for this non‐Newtonian behavior. We use 2‐D direct numerical crystal‐scale simulations to study the relative importance of these mechanisms. These simulations demonstrate that shear heating has little effect on aggregate (bulk) rheologies. Finite strain effects result in both strain weakening and hardening, but the resulting power law coefficient is modest (maximum n = 1.3). For simulations with spherical crystals the strain weakening and hardening behavior is related to rearrangement of crystals rather than strain rate related weakening. Finite strain effects were insignificant in a numerical simulation with naturally shaped crystals. Strain partitioning into the melt phase may induce microscopic stresses that are adequate to provoke a nonlinear viscous response in the melt. Large differential stresses and low effective stresses revealed by the simulations are sufficient to cause crystals to fail plastically. Numerical experiments that account for plastic failure show large power law coefficients ( n ≈ 50 in some simulations). We conclude that this effect is the dominant cause of the strong nonlinear viscous response of crystal‐bearing magmas observed in laboratory experiments.