Water redistribution in experimentally deformed natural milky quartz single crystals—Implications for H₂O‐weakening processes

Abstract

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 × 10−6 s−1. 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−1 region consisting of a superposition of bands of molecular H2O 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 and reduces or disappears after annealing, so that this band appears to be associated with dislocations. H2O weakening in inclusion‐bearing natural quartz crystals is assigned to the H2O‐assisted dislocation generation and multiplication. Processes in these crystals represent recycling of H2O between fluid inclusions, cracking and crack healing, incorporation of structurally bound H in dislocations, release of H2O from dislocations during recovery, and dislocation generation at very small fluid inclusions. The H2O weakening by this process is of disequilibrium nature because it depends on the amount of H2O available.

Synchrotron FTIR imaging of OH in quartz mylonites

“…The water contents of MT samples and other quartz mylonites deformed at greenschist conditions are comparable to (and even larger than) those of wet varieties of synthetic and natural milky quartz (350-4000 ppm) that exhibit water weakening in laboratory studies (e.g., Griggs and Blacic, 1965;Kekulawala et al, 1978;Stünitz et al, 2017). Highly deformed ribbon quartz grains, less deformed quartz augen, and recrystallized quartz grains of the MT exhibit OH bands of similar character to those of quartzites deformed in laboratory experiments, a result that validates applications of wet quartzite rheologies to evaluate rates of dislocation creep in middle to upper crustal shear zones (e.g., Hirth et al, 2001;Behr and Platt, 2011;Law, 2014).…”

“…MT quartz spectra show a broad OH stretching band at 3400 cm −1 of large but variable magnitude (Fig. 2a), characteristic of molecular water in fluid inclusions of milky quartz (Kekulawala et al, 1978;Stünitz et al, 2017). In addition to this absorption band, some quartz grains from MT samples show an additional absorption band (or subtle shoulder) at ∼ 3600 cm −1 (Fig.…”

“…We know little of this early brittle deformation, but these fluid inclusion arrays have microstructures similar to those generated during hydrothermal diffusional healing of cracks (Smith and Evans, 1984;Beeler and Hickman, 2015). As a result, early brittle deformation, infiltration of water along open cracks, and crack healing appear to be important to the early introduction of water to quartz grain interiors (Kronenberg et al, 1986FitzGerald et al, 1991;Diamond et al, 2010;Tarantola et al, , 2012Stünitz et al, 2017).…”

“…Processes of fluid inclusion decrepitation under deviatoric stresses have been studied experimentally , 2012, Stünitz et al, 2017, and they include simultaneous shrinkage of coarse (> 10 µm, optical-scale) inclusions, generation of dislocations at fluid inclusion walls, and formation of a new population of very fine (< 100 nm) fluid inclusions (visible only by transmission electron microscopy), which reside at dislocations and resemble water clusters and inclusions of deformed and heat-treated synthetic quartz (McLaren and Hobbs, 1972;White, 1973;Kirby and McCormick, 1979;Christie and Ord, 1980). The loss of coarse fluid inclusions and growth of fine inclusions require diffusive transport, which may occur along interconnected and mobile dislocations (McLaren et al, 1983Cordier et al, 1988Cordier et al, , 1994Jansen, 1990, 1994;Hollister, 1990;Tarontola et al, 2010Tarontola et al, , 2012Stünitz et al, 2017). Once formed, very fine fluid inclusions may also coarsen by pipe diffusion and processes documented by McLaren et al (1983) and Cordier et al (1988), leading to continuous changes in inclusion densities and size distributions.…”

“…The effects of water on dislocation creep of quartz, including the nucleation, glide, climb and recovery of dislocations, and recrystallization are well known from (1) experimental studies of natural crystals, in which water was introduced into grain interiors (e.g., Griggs, 1967;Blacic, 1975Blacic, , 1981FitzGerald et al, 1991), (2) studies of synthetic and natural quartz varieties with large initial water contents (e.g., Griggs and Blacic, 1965;Hobbs, 1968;Baeta and Ashby, 1970;Kekulawala et al, 1978;Kirby and McCormick, 1979;McLaren et al, 1983;Linker et al, 1984;Gerretsen et al, 1989;Muto et al, 2011;Holyoke and Kronenberg, 2013;Stünitz et al, 2017), and (3) quartzites and polycrystalline quartz aggregates with water added or removed before or during experiments (e.g., Jaoul et al, 1984;Kronenberg and Tullis, 1984;Tullis and Yund, 1989;Hirth and Tullis, 1992;Gleason and Tullis, 1995;Post et al, 1996;Chernak et al, 2009). IR spectroscopy has played a key role in experimental studies of water weakening, through the characterization and measurement of OH absorption bands due to different hydrogen defects and forms of molecular water within quartz interiors (e.g., Kats, 1962;Griggs and Blacic, 1965;Stipp et al, 2006).…”