Field And Experimental Constraints on the Rheology of Silicic Magma

The microphysics governing the rheology of crystal-rich silicic magma remains an open question. We conduct detailed field measurements of the Pleasant Bay layered gabbro-diorite intrusion (Coastal Maine, USA), which records the temporal evolution of a silicic crystal-liquid mush periodically injected with mafic material (Wiebe 1993). Comparison of field measurements of kinematic indicators including crystal alignments and textural gradients around the margins of stoped basaltic blocks, as well as at interfaces between basaltic and silicic layers, with scaling analyses drawn from thin viscous sheet theory suggest that the silicic magma had a viscoplastic rheology with a yield stress. A key outstanding issue, however, is the relationship of this macroscopic rheology to the microphysics governing the deformation of the crystal-rich magma. In particular, how the microtextures we observe in plutonic rocks can be related quantitatively to the coupled crystal-liquid dynamics that ultimately govern the rheological response of the magma is unclear. To understand how a magma's microstructure might evolve to produce the textures we observe in the field, we perform a series of quasi-two-dimensional lab experiments on the responses of mono- and poly-disperse mixtures of plastic particles (analogue crystals) and viscous liquid to an imposed simple shear using a rotating cylinder-Couette setup under laminar flow conditions. We aim to understand phenomena such as particle alignment and particle migration in terms of the three-way (solid-liquid, liquid-solid, solid-solid) solid-fluid coupling in our experiments. We use particle image velocimetry and video analysis to characterize the microstructural response to imposed shear for a broad range of strain rates, particle concentrations and shape distributions. Stress-strain rate curves are estimated and the corresponding rheologies are applied in a simple numerical model to understand measured velocity profiles.