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

Jun-Ichi Fukuda,Ichiko Shimizu

doi:10.1186/s40623-019-1117-4

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.

Highlights

Quartz is widespread in the Earth’s crust and is a control on rheology
We examined previously reported IR calibrations for determining the water contents of quartz, and we unified these previous calibrations as an absorption coefficient in L/mol H2O cm2
The value of this coefficient varies from 24,100 (Stipp et al 2006) to 89,000 (Thomas et al 2009), which roughly corresponds to the increase in the components of structural –OH in IR spectra and to the decrease in components of molecular H 2O. (This could be orientated in some cases.) We used the calibration of Paterson (1982) for molecular H2O

Summary

Introduction

Quartz is widespread in the Earth’s crust and is a control on rheology. It is well known that water affects the plastic deformation of quartz, as confirmed by many experimental works (e.g., Griggs and Blacic 1965; Griggs1967; Parrish et al 1976; Jaoul et al 1984; Kronenberg and Tullis 1984; Koch et al 1989; Post et al 1996; Chernak et al 2009; Holyoke and Kronenberg 2013; Fukuda et al 2018) and theoretical models (Fukuda and Shimizu 2017). It is well known that water affects the plastic deformation of quartz, as confirmed by many experimental works Fukuda and Shimizu E arth, Planets and Space (2019) 71:136 experiments, the water contents of naturally deformed quartz have been measured (Kronenberg and Wolf 1990; Kronenberg et al 1990; Kronenberg 1994; Nakashima et al 1995; Niimi et al 1999; Muto et al 2004, 2005; Gleason and DeSisto 2008; Menegon et al 2011; Fukuda 2012; Finch et al 2016; Kilian et al 2016; Kronenberg et al 2017), it is still difficult to relate the measured water content to the development of plastic deformation in nature. IR mapping measurements can be correlated with the textures of the sample, and they are useful for evaluating the distribution of water in the area of analysis (e.g., Ito and Nakashima 2002; Fukuda 2012; Fukuda et al 2012; Kronenberg et al 2017)

Methods

Results

Discussion

Conclusion