Water distribution in quartz schists of the Sanbagawa Metamorphic Belt, Japan: infrared spectroscopic mapping and comparison of the calibrations proposed for determining water contents

Abstract

We evaluated water distributions in deformed quartz in schists along the Asemi River, Central Shikoku, in the Sanbagawa Metamorphic Belt, Japan, using infrared spectroscopic (IR) mapping. The water trapped in quartz as molecular H2O showed a broad IR absorption band at 2800–3750 cm−1. A necessary step before assessing the quartz water content was to evaluate and compare six previously proposed IR calibrations in terms of the molar absorption coefficients of H2O (L/mol H2O cm2). The coefficients vary from 24,100 to 89,000 L/mol H2O cm2, and the values of the coefficients show a rough increase with increasing component of structural –OH in the IR spectra. We used Paterson’s calibration, which does not require input regarding the mineral species, but which was modified in his paper for measurements of molecular H2O in quartz. The absorption coefficient is 38,000 L/mol H2O cm2. IR mapping was performed on Sanbagawa metamorphic rocks with increasing grades of metamorphism, where the mean grain size of quartz increases from ~ 40 to ~ 120 µm. The absorption bands that are only from the quartz can be distinguished on the basis of microstructural observations and the corresponding mapping results. The IR spectra of quartz commonly show dominant molecular H2O bands at 2800–3750 cm−1 with no additional bands associated with crystalline –OH when only quartz is measured. The water contents of quartz in all our samples were 40–310 ppm, and these values are about one-third of previously reported values measured using point analyses with the unified Paterson’s calibration. This difference seems to reflect the incorporation of phyllosilicates in previous measurements that showed a broad band around 3600 cm−1. The lowest and highest water contents in our quartz samples are associated with intragranular water and grain boundary water, respectively. We estimated the grain boundary widths to be at most ~ 10 nm on the basis of the water contents at grain boundaries.[Figure not available: see fulltext.]