Influence of water on crystallographic preferred orientation patterns in a naturally-deformed quartzite

Abstract

Abstract. Laboratory experiments demonstrate that intragranular water exerts an important control on deformation within quartz, causing weakening and promoting plasticity. The role of water on natural quartz deformation, however, remains unclear, as recent studies find an inverse relationship between water content and the magnitude of plastic strain. Furthermore, little work has investigated the effects, if any, of water on the relative activity of various slip systems in quartz. We focus on a naturally-strained quartzite from the Antietam Formation of the Blue Ridge in Virginia, USA. Quartz water content ranges from <50 to >2000 ppm H₂O. Water content and crystallographic data were correlated for 968 grains, enabling us to explore the relationship between water content and quartz crystallographic preferred orientation (CPO) patterns. “Dry” (<150 ppm H₂O) and “wet” (>500 ppm H₂O) subsets show distinct CPOs; c-axes of dry grains define a cross-girdle oriented perpendicular to the extension direction (X), whereas c-axes of wet grains are concentrated along the perimeter of the pole figure. All water content subsets show grains clustered near the direction of maximum shortening (Z), consistent with activity of the basal ˂a˃ slip system. The cross girdle in the driest grains suggests activity of prism ˂a˃ and possibly rhomb ˂a˃, whereas the orientation of the wettest grains implies a contribution from prism ˂c˃ slip. These slip system interpretations are supported by analyses of intragranular misorientations. These results indicate that water content impacts the relative activity of various slip systems in natural quartz, potentially affecting application of the quartz opening angle thermometry.