Synchrotron FTIR imaging of OH in quartz mylonites

Abstract. Methods of measuring OH absorption bands of fluid inclusions and hydrogen defects in deformed quartz rocks at high spatial resolution are described, using synchrotron infrared IR radiation coupled with a Fourier transform infrared FTIR microscope, and applied to imaging OH in mylonites of the Moine Thrust (from the Stack of Glencoul, NW Scotland Caledonides) and the Main Central Thrust (from the Himalayan front, Sutlej Valley, NW India). Previous measurements of water in deformed quartzites using conventional FTIR instruments, through apertures of 50–100 μm for specimens ~ 100 μm in thickness 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 IR radiation is brighter and more collimated than offered by conventional FTIR globar light sources, and we have been able 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 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