Rüdiger Kilian

Water redistribution in experimentally deformed natural milky quartz single crystals – implications for H2O-weakening processes

Natural quartz single crystals were experimentally deformed in two orientations: (1) ⊥ to one prism plane and (2) in O + orientation at 900 and 1000°C, 1.0 and 1.5 GPa, and strain rates of ~1 × 10 A6 s A1. In addition, hydrostatic and annealing experiments were performed. The starting material was milky quartz, which consisted of dry quartz with a large number of fluid inclusions of variable size up to several 100 μm. During pressurization fluid inclusions decrepitated producing much smaller fluid inclusions. Deformation on the sample scale is anisotropic due to dislocation glide on selected slip systems and inhomogeneous due to an inhomogeneous distribution of fluid inclusions. Dislocation glide is accompanied by minor dynamic recovery. Strongly deformed regions show a pointed broad absorption band in the ~3400 cm A1 region consisting of a superposition of bands of molecular H 2 O and three discrete absorption bands (at 3367, 3400, and 3434 cm A1). In addition, there is a discrete absorption band at 3585 cm A1 , which only occurs in deformed regions and reduces or disappears after annealing, so that this band appears to be associated with dislocations. H 2 O weakening in inclusion-bearing natural quartz crystals is assigned to the H 2 O-assisted dislocation generation and multiplication. Processes in these crystals represent recycling of H 2 O between fluid inclusions, cracking and crack healing, incorporation of structurally bound H in dislocations, release of H 2 O from dislocations during recovery, and dislocation generation at very small fluid inclusions. The H 2 O weakening by this process is of disequilibrium nature because it depends on the amount of H 2 O available.

Dislocation creep of dry quartz

Small-scale shear zones within the Permian Truzzo meta-granite developed during the Alpine orogeny at amphibolite facies conditions. In these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250–750 μm, consistent with flow at low differential stresses. Fourier transform infrared (FTIR) spectroscopy reveals very low water contents in the interior of recrystallized grains (in the form of discrete OH peaks, ~20 H/10 6 Si and very little broad band absorption, <100 H/10 6 Si). The spectral characteristics are comparable to those of dry Brazil quartz. In FTIR spectra, magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry. Drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation. Loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening. These FTIR measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses. Low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength, and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz. It is suggested that dislocation creep of quartz in the Truzzo meta-granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions.