Quantifying the influence of cooling and shear rate on the disequilibrium rheology of a trachybasaltic melt from Mt. Etna

Abstract

Magmas often experience severe disequilibrium conditions during their migration through the Earth's crust and the subsequent emplacement on its surface. During their transport, magmas are subjected to a wide range of cooling (q) and deformation rates (γ˙), generating physico-chemical perturbations in the magmatic system able to inhibit or promote crystallization processes. Quantifying the magnitude and timescale of kinetic effects is essential to correctly constrain the rheological evolution of magmas and their ability to flow. Here we present a suite of cooling deformation experiments (CDE) conducted on a basalt from Mt. Etna (Sicily, Italy) to disentangle and model the concurrent effects of q (from 1 to 10°C/min) and γ˙ (from 1 to 10 s−1) on the rheology of the system. The analysis of the temporal evolution of viscosity indicates that both q and γ˙ strongly affect the onset of crystallization and achievement of a rheological cut-off over time, which represents the steep viscosity increase responsible for inhibiting magma flow. Both these rheological thresholds occur at lower T and earlier in time with increasing q, as well as at higher T and earlier in time with increasing γ˙. To reproduce the observed effects of crystallization on the apparent viscosity, we adopt a stretched exponential function that identifies two main crystallization regimes: i) a first shear-induced crystallization regime, characterized by a gentle viscosity increase and ii) a second cooling-dominated regime, marked by a steeper viscosity increase. The relative extent of these crystallization regimes strictly depends on the interplay between q and γ˙ on the crystallization kinetics and suggest a first order control of q and a subordinate role of γ˙.