Water loss during dynamic recrystallization of Moine thrust quartzites, northwest Scotland

Abstract

Infrared absorption measurements of molecular water in sheared Cambrian quartzites in the footwall to the Moine thrust reveal a decrease in water content from 4080 to 1570 ppm with increasing recrystallization traced toward the overlying thrust at the Stack of Glencoul in northwest Scotland. These results are contrary to the expected correlation between shear strain and water content for quartz deformed by dislocation creep and water-weakening processes. The observed inverse correlation indicates that fluid inclusions and hydrous defects within grains were lost by mobile grain boundary sweeping and grain boundary diffusion. Although reduced water contents might lead to hardening as chemical weakening is diminished, quartz mylonites in the immediate footwall (5 mm) to the thrust are characterized by intense strain localization and contain the least water, and there is little evidence of shear zone widening. Water weakening appears to have been important throughout the quartz mylonites, controlled by the presence of water, not by water concentration. Fluids present within relict inclusions and at grain boundaries may have governed the high water fugacities critical for water weakening.

Partial melting in polycrystalline ice: Pathways identified in 3D neutron tomographic images

“…In addition to rheological weakening, meltwater concentrations may influence the relative 230 activity of particular slip systems activated during deformation. This has been clearly identified in quartz (Kronenberg et al, 2020) which is an ice analogue (Wilson et al, 2014).…”

Water Recycling in the Deep Earth: Insights From Integrated μ‐XRF, THz‐TDS Spectroscopy, TG, and DCS of High‐Pressure Granulite

“…As mentioned above, the hydrous species may be preserved in the garnet, hornblende, and plagioclase (Figure 8). Indeed, quartz may also contain a considerable amount of water as the OH − species via the reaction of H 2 O + Si–O–Si →2SiOH (Aines et al., 1984; Kronenberg et al., 2020). In addition, ilmenite can also absorb significant amounts of water (Lu et al., 2016; Reiss et al., 2019).…”

“…The rutile‐bearing garnet amphibolite shows that dehydration of molecular water was conducted at a higher temperature compared to the garnet amphibolite and amphibolite, which is consistent with the petrological modeling that shows a decreasing trend in water content with the temperature increases (Figure 8). However, the relatively lower content of molecular water and weight loss of whole‐rock of the garnet amphibolite and amphibolite compared to the rutile‐bearing garnet amphibolite at a temperature of 35–1,000°C indicate that some water might migrate into a greater depth of the mantle or stored in the mantle wedge (Kronenberg et al., 2020).…”

Evidence of Dislocation Mixed Climb in Quartz from the Moine and Main Central Thrusts: an Electron Tomography Study

“…Moine thrust specimen SG-10 (MT) consists of sheared ribbon quartz grains with smooth undulatory extinction observed in conventional thin sections, and well-developed subgrain walls that are apparent in ultra-thin sections (~5 µm) (Kronenberg et al, 2020), consistent with optical microscopy of etched sections and transmission electron microscopy (TEM) of nearby mylonitic Cambrian quartzites (Ord & Christie, 1984). Ribbon quartz grains are recrystallized at their margins, with microstructures due to grain boundary bulging (BLG) and subordinate subgrain rotation (SGR) (Stipp et al, 2002b;Law, 2014); over 70 % of this specimen consists of recrystallized grains (with a 21 µm mean size; Weathers et al, 1979;Kronenberg et al, 2020) but the new TEM observations of this study are from original ribbon quartz grains.…”

“…Moine thrust specimen SG-10 (MT) consists of sheared ribbon quartz grains with smooth undulatory extinction observed in conventional thin sections, and well-developed subgrain walls that are apparent in ultra-thin sections (~5 µm) (Kronenberg et al, 2020), consistent with optical microscopy of etched sections and transmission electron microscopy (TEM) of nearby mylonitic Cambrian quartzites (Ord & Christie, 1984). Ribbon quartz grains are recrystallized at their margins, with microstructures due to grain boundary bulging (BLG) and subordinate subgrain rotation (SGR) (Stipp et al, 2002b;Law, 2014); over 70 % of this specimen consists of recrystallized grains (with a 21 µm mean size; Weathers et al, 1979;Kronenberg et al, 2020) but the new TEM observations of this study are from original ribbon quartz grains. Crystallographic c-axes of ribbon and recrystallized quartz grains exhibit nearly symmetric crossed-girdle patterns aligned with respect to grain shape foliation and lineation (Law et al, 2010), consistent with significant plane-strain foliation-normal shortening by glide on multiple slip systems with Burgers vectors parallel to a and to c (and other crystallographic slip vectors with components in a and c).…”