EVIDENCE FOR INTRACRYSTALLINE WATER PROMOTING C-SLIP DURING LOW-TEMPERATURE DEFORMATION OF QUARTZ

Crystallographic fabric transitions in quartz are generally interpreted to reflect changes in deformation temperature or strain. However, intracrystalline water has been shown to influence slip system activity in some minerals (e.g., olivine), and in quartz high water content has been proposed to promote slip on the prism <c> system. We investigate the influence of water on intracrystalline deformation in quartz through detailed study of a quartzite from the Antietam Formation of the Blue Ridge in Virginia that was deformed at low temperatures (~250-280°C) and a low strain rate (10^-15 s^-1) during long-term convergence between North America and Africa. For a large population of grains (968), water content was measured using Fourier transform infrared spectroscopy (FTIR) and crystallographic orientation was determined via electron backscatter diffraction (EBSD). “Dry” (< 150 ppm H₂O) and “wet” (> 500 ppm H₂O) subsets reveal distinct crystallographic preferred orientation patterns. The c-axes in dry grains define a cross-girdle oriented perpendicular to the X direction, while those for wet grains tend to cluster around the periphery of the pole figure. We relate the distribution of crystallographic orientations within individual crystals to activity of known quartz slip systems, finding basal <a>, prism <a>, and prism <c> to be most common. When compared to the distribution of grains deformed basal <a> or prism <a>, the population of prism <c> grains shows higher water content and lower aspect ratios. This result supports recent studies arguing for dehydration of quartz during deformation. While dehydration in these cases has been related to recrystallization, our grains generally lack evidence for this process. Instead, we observe deformation lamellae often decorated with fluid inclusions, most conspicuous in high aspect ratio grains, that may represent pathways for fluid migration and deyhydration.