Synchrotron FTIR imaging of OH in quartz mylonites

Abstract

Abstract. Previous measurements of water in deformed quartzites using conventional Fourier transform infrared spectroscopy (FTIR) instruments have shown that water contents of larger grains vary from one grain to another. However, the non-equilibrium variations in water content between neighboring grains and within quartz grains cannot be interrogated further without greater measurement resolution, nor can water contents be measured in finely recrystallized grains without including absorption bands due to fluid inclusions, films, and secondary minerals at grain boundaries.Synchrotron infrared (IR) radiation coupled to a FTIR spectrometer has allowed us to distinguish and measure OH bands due to fluid inclusions, hydrogen point defects, and secondary hydrous mineral inclusions through an aperture of 10 µm for specimens > 40 µm thick. Doubly polished infrared (IR) plates can be prepared with thicknesses down to 4-8 µm, but measurement of small OH bands is currently limited by strong interference fringes for samples < 25 µm thick, precluding measurements of water within individual, finely recrystallized grains. By translating specimens under the 10 µm IR beam by steps of 10 to 50 µm, using a software-controlled x − y stage, spectra have been collected over specimen areas of nearly 4.5 mm 2 . This technique allowed us to separate and quantify broad OH bands due to fluid inclusions in quartz and OH bands due to micas and map their distributions in quartzites from the Moine Thrust (Scotland) and Main Central Thrust (Himalayas).Mylonitic quartzites deformed under greenschist facies conditions in the footwall to the Moine Thrust (MT) exhibit a large and variable 3400 cm −1 OH absorption band due to molecular water, and maps of water content corresponding to fluid inclusions show that inclusion densities correlate with deformation and recrystallization microstructures. Quartz grains of mylonitic orthogneisses and paragneisses deformed under amphibolite conditions in the hanging wall to the Main Central Thrust (MCT) exhibit smaller broad OH bands, and spectra are dominated by sharp bands at 3595 to 3379 cm −1 due to hydrogen point defects that appear to have uniform, equilibrium concentrations in the driest samples. The broad OH band at 3400 cm −1 in these rocks is much less common. The variable water concentrations of MT quartzites and lack of detectable water in highly sheared MCT mylonites challenge our understanding of quartz rheology. However, where water absorption bands can be detected and compared with deformation microstructures, OH concentration maps provide information on the histories of deformation and recovery, evidence for the introduction and loss of fluid inclusions, and water weakening processes.

The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

“…Using this approach, we calculate values of a(TiO 2 ) for Moine psammites in the range 0.20-0.35 (Table 2), considerably lower than the broad estimates suggested by Ghent and Stout [143]. They are however in agreement with thermodynamic modeling by Ashley and Law [78] who calculate a(TiO 2 ) for greywacke (i.e., psammitic) compositions of <0.3 and by Kidder et al [92] who suggest a value of aðTiO 2 Þ = 0:1 for mylonites from the Alpine Fault. These lower activities result in higher temperatures for a given Ti concentration.…”

“…A general trend emerges in the shape of the CPO through the structural section (Figure 12 [90,91] or enhanced hydrolytic weakening (e.g. [84,92]).…”

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

“…However, the quantitative relationship between water content and the plastic deformation of quartz has not been established, even though a rough link between the plastic weakness of quartz and water content is well known. In addition to studies of quartz in deformation experiments, the water contents of naturally deformed quartz have also been measured Kronenberg 1994;Nakashima et al 1995;Niimi et al 1999;Muto et al 2004Muto et al , 2005Gleason and DeSisto 2008;Menegon et al 2011;Fukuda 2012;Finch et al 2016;Kilian et al 2016;Kronenberg et al 2017), although it is still difficult to relate the measured water content to the development of plastic deformation in nature. To measure the water contents in quartz and to assess the nature of the water species (i.e., as molecular H 2 O or -OH coupled with cations), infrared (IR) spectroscopy is used.…”

“…To measure the water contents in quartz and to assess the nature of the water species (i.e., as molecular H 2 O or -OH coupled with cations), infrared (IR) spectroscopy is used. 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).…”

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

“…3). After separating these absorptions, integrating the 3400 cm −1 OH absorption, and applying the IR calibration of Aines et al (1984), broad-band OH contents of quartz grains vary widely within individual samples (see the Data Repository), much as documented by recent FTIR imaging studies (Kronenberg et al, 2017). However, variations in OH content show no apparent correlation with deformed detrital grain shape.…”