DEFORMATION OF WATER-DEPLETED QUARTZITE

Wet quartz is weaker than most other silicates and likely controls much of the strength of the middle to lower continental crust. Recent field-based investigations of water content in quartz from granitic rocks deformed at amphibolite conditions indicate that quartz water contents are very low (<400 H/10⁶ Si), yet quartz is still weaker than the other phases in these rocks. In addition, recent experimental studies indicate that water concentration also affects the strength of single crystals of quartz. We have performed an experimental study to determine how water concentration affects the strength of quartzite. We made fine-grained quartzites from Brazilian quartz powder and deformed these quartzites with different water contents at P = 1500 MPa, T = 1300°C and strain rate of 10^-6/s. Strengths of samples decreased from 350-250 MPa as water contents increased from 160 – 400 H/10⁶ Si, as measured by FTIR analyses. Optical-scale microstructures observed in the samples include undulatory extinction and serrated grain boundaries, indicating deformation by dislocation creep. Flow laws based on experiments performed on wet (2000-4000 H/10⁶ Si) quartzites predict that the strength of quartzite at our experimental conditions should be an order of magnitude weaker than we observed using quartzite with low water contents. Our results indicate that the strength of quartzite is dependent on water content and that quartz can be deformed at the low water contents observed in quartz grains in some lower crustal rocks.