HR: 1340h
AN: T13B-1338    [Abstracts]
TI: Anisotropic plasticity of single crystals of wet synthetic quartz
AU: * Muto, J
EM: Jun_Muto@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook street, Box 1846, Providence, RI 02912, United States
AU: Tullis, J
EM: Jan_Tullis@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook street, Box 1846, Providence, RI 02912, United States
AB: Anisotropic plasticity (e.g., crystallographic preferred orientation patterns) for quartz has been reported more than for any other mineral in deformed rocks. In natural quartzites, with increasing deformation temperature, the dominant slip system has been observed to change from basal , to prism , to prism [c]. However, the flow stress at which each slip system is activated and the effect of water content on slip system activity has not been fully investigated. We have undertaken an experimental study to examine the effects of temperature and water content on the slip system activity and strength of single crystals of wet synthetic quartz deformed at 1.5 GPa confining pressure, 10^-5/s strain rate, and 700-900^oC using a Griggs apparatus. From FTIR measurements, the synthetic quartz shows a broadband absorption around 3400 cm^-1 that can be assigned to the vibration of molecular H₂O. Samples of synthetic quartz with two different water contents were utilized: ~ 400 and ~ 700 ppm H/Si. Two orientations of axial compression were chosen: O⁺ (45^o to a and c) and c\bot (normal to a and c), and plastic strains up to 40{%} shortening were achieved. Optical microscope observations indicate that samples compressed in the O⁺ orientation deform by basal single slip at 700^oC and by a combination of basal and prism [c] slip at 900^oC, and samples compressed in the c\bot orientation deform by prism double slip. The stress-strain curves for both orientations show gradual strain hardening out to ~ 25{%} strain. The difference in water content between 400 and 700 ppm H/Si has no marked effect on the strength of either orientation, indicating that this difference in water content is not important for the dislocation creep strength of synthetic quartz at our experimental conditions. At 700^oC, the c\bot orientation (prism slip) is somewhat stronger than the O⁺ orientation (basal slip). At 900^oC, the c\bot orientation has approximately the same strength as the O⁺ orientation (~ 100 MPa at 40{%} strain). Our experimental results confirm that basal slip is easier at lower temperature (700^oC), whereas basal , prism , and prism [c] slip have about same strength at higher temperature (900^oC). These results are in partially agreement with previous experimental results conducted at 1.5 GPa confining pressure (e.g., Blacic, 1975) but do not agree with results on synthetic quartz at 300 MPa confining pressure (e.g., Hobbs et al., 1972). Our experimental study provides more accurate mechanical data convincing anisotropic rheology of crustal rocks and the role of fluids.
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 8030 Microstructures
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2007 Fall Meeting