Experimental Deformation of Ti-saturated and -undersatuated Quartz Porphyroclasts

Quartz crystals were grown from a hydrostatic fluid, embedded in a quartz aggregate, and deformed at 900 °C to high shear strain to study re-equilibration of Ti solubilities during dynamic recrystallization. The initial Ti contents of quartz grown at temperatures above (925 °C) and below (875 °C) the deformation temperature are 50 ppm Ti higher or lower than the equilibrium solubility, respectively. These out-of-equilibrium quartz crystals were deformed as porphyroclasts by embedding them in a matrix composed of either dry (crushed Herkimer) or wet (silica gel-derived) quartz. Selecting dry or wet quartz as the aggregate matrix induces two end-member deformation regimes, high-stress/low-strain or high-strain/low-stress, respectively. In high-stress experiments, quartz porphyroclasts exhibit deformation lamellae and undulose extinction, with some evidence for grain boundary bulging (BLG) and subgrain rotation (SGR) recrystallization. In high-strain experiments, some quartz porphyroclasts deform with limited dynamic recrystallization, resulting in high aspect ratio ribbon grains, whereas others recrystallized during dislocation creep, as indicated by microstructures illustrating SGR and grain boundary migration (GBM) recrystallization. The deformation style and extent of recrystallization are dictated by the original orientation of crystals with respect to the imposed deformation. The deformation style also influences the extent of Ti equilibration, with more recrystallized grains exhibiting greater equilibration. By comparing quartz microstructures (identified both optically and with electron back-scattered diffraction) with maps of Ti contents from cathodoluminescence we illustrate that dynamic recrystallization of quartz via BLG, SGR, and GBM is effective at re-equilibrating quartz to the ambient conditions of deformation.