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Solid Earth An interactive open-access journal of the European Geosciences Union

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doi:10.5194/se-2017-23
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
30 Mar 2017
Review status
This discussion paper is under review for the journal Solid Earth (SE).
Synchrotron FTIR imaging of OH in quartz mylonites
Andreas K. Kronenberg1, Hasnor F. B. Hasnan1,a, Caleb W. Holyoke III1,b, Richard D. Law2, Zhenxian Liu3, and Jay B. Thomas4 1Center for Tectonophysics, Department of Geology and Geophysics, MS 3115, Texas A&M University, College Station, TX, 77843 - 3115 , USA
2Department of Geosciences, MC 0420, Derring Hall RM 4044, Virginia Polytechnic Institute and State University , Blacksburg, VA, 2406 1 , USA
3Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW Washington, D.C., 20015, USA
4Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, NY , USA 13244 , USA
anow at: Department of Advanced Geophysics, PETRONAS , Carigali Sdn. Bhd., PETRONAS Twin Towers, Kuala Lumpur City Centre , 50088 Kuala Lumpur, Malaysia
bnow at: Department of Geosciences, University of Akron, Akron, OH, USA 44325 - 4101 , USA
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 < 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 mm2. Integrated absorbance of the broad OH band at 3400 cm−1 due to fluid inclusions was mapped and water content contoured, after subtracting the 3600 cm−1 band due to micas. Maps of the 3600 cm-1 band absorbance show concentrations of micas at grain boundaries, as well as finely dispersed mica inclusions within quartz grains.

Moine Thrust mylonites deformed at greenschist conditions exhibit a large and variable 3400 cm−1 band absorbance, and maps of water content corresponding to fluid inclusions show that inclusion densities correlate with deformation and recrystallization microstructures. Water contents of Moine quartz grains are comparable to those of wet quartz varieties deformed in laboratory experiments. OH absorption bands of large clear quartz grains of Main Central Thrust mylonites deformed at upper amphibolite conditions are much smaller, dominated by sharp bands at 3595 cm−1 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 lack of detectable water in highly sheared Main Central Thrust mylonites offers a challenge to our understanding of quartz rheology. However, where water absorption bands can be detected and compared with deformation microstructures, OH concentration maps provide evidence of the histories of deformation and recovery, the introduction and loss of fluid inclusions, and water weakening processes.


Citation: Kronenberg, A. K., Hasnan, H. F. B., Holyoke III, C. W., Law, R. D., Liu, Z., and Thomas, J. B.: Synchrotron FTIR imaging of OH in quartz mylonites, Solid Earth Discuss., doi:10.5194/se-2017-23, in review, 2017.
Andreas K. Kronenberg et al.
Andreas K. Kronenberg et al.
Andreas K. Kronenberg et al.

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Short summary
Methods of measuring trace water contents of quartz at high resolution using synchrotron infrared IR radiation and spectroscopy are described and applied to deformed rocks of the Moine Thrust of NW Scotland and the Main Central Thrust of the Himalaya in NW India. By imaging OH absorption bands, variations in water content can be linked to deformation microstructures, providing information on the histories of deformation, recovery, introduction and loss of water, and water weakening.
Methods of measuring trace water contents of quartz at high resolution using synchrotron...
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