Fourier transform infrared spectroscopy determinations of intragranular water content in quartz-bearing rocks: implications for hydrolytic weakening in the laboratory and within the earth

Fourier transform infrared (FTIR) spectroscopy has been used to measure intragranular water contents of quartz (and feldspar) within fine-grained quartzites, granite, and naturally-deformed mylonites. Calibrations and tests of methods developed for this application using natural and synthetic quartz standards indicate that hydrogen concentrations down to 30 ppm are detectable (within apertured regions 100-200 μm in diam.) and concentrations of ≥ 400 ppm measurable to an accuracy of 30%. Quartzite and novaculite starting materials, used in previous studies of mechanical properties, contain substantial intragranular water contents (2400-3900 ppm), much larger than water concentrations commonly found in large clear natural crystals and more comparable to those of wet synthetic crystals. However, unlike molecular water within rapidly-grown synthetic quartz, most of this water is freezable, resembling fluid inclusion water of natural milky quartz crystals. Likewise, quartz and feldspar grains within granite and mylonites deformed at greenschist to upper amphibolite facies conditions contain large amounts of water (from 500 to 8000 ppm) as fine (often submicron size) fluid inclusions and IR spectra are dominated by a freezable O-H absorption band. Comparisons between dry natural and wet synthetic quartz crystals have formed the basis for our understanding of hydrolytic weakening. However, mechanisms by which water-related defects access dislocation cores within quartzites deformed in the laboratory and mylonites deformed within natural shear zones may differ from those within idealized natural and synthetic single crystals, and their mechanical properties may not be directly comparable.