Water weakening in experimentally deformed milky quartz single crystals

Abstract:

Natural single crystals of quartz have been experimentally deformed in two orientations: (1) normal to one prism-plane, (2) In O+ orientation at temperatures of 900 and 1000°C, pressures of 1.0 and 1.5 GPa, and strain rates of ~1 x 10-6s-1. The starting material is milky quartz, consisting of dry quartz (H₂O contents of <150 H/10⁶Si) with fluid inclusions (FI). During pressurization many FI´s decrepitate. Cracks heal and small neonate FI´s form, increasing the number of FI´s drastically. During subsequent deformation, the size of FI´s is further reduced (down to ~10 nm). Sample deformation occurs by dominant dislocation glide on selected slip systems, accompanied by some dynamic recovery. Strongly deformed regions show FTIR spectra with a pointed broad absorption band in the ~3400 cm-1 region as a superposition of molecular H₂O bands and three discrete absorption bands (at 3367, 3400, and 3434 cm-1). In addition, there is a discrete absorption band at 3585 cm-1, which only occurs in deformed regions. The 3585 cm-1 band is reduced or even disappears after annealing. This band is polarized and represents structurally bound H, its H-content is estimated to be 1-3% of the total H₂O-content and appears to be associated with dislocations. The H₂O weakening effect in our FI-bearing natural quartz crystals is assigned to the processes of dislocation generation and multiplication at small FI´s. The deformation processes in these crystals represent a recycling of H₂O between FI´s, dislocation generation at very small fluid inclusions, incorporation of structurally bound H into dislocation cores, and release of H₂O from dislocations back into FI´s during recovery. Cracking and crack healing play an important role in the recycling process and imply a close interrelationship between brittle and crystal plastic deformation. The H₂O weakening by this process is of a disequilibrium nature and thus depends on the amount of H₂O available.