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

Journal metrics

  • IF value: 3.495 IF 3.495
  • IF 5-year<br/> value: 3.386 IF 5-year
    3.386
  • CiteScore<br/> value: 3.70 CiteScore
    3.70
  • SNIP value: 0.783 SNIP 0.783
  • SJR value: 1.039 SJR 1.039
  • IPP value: 1.987 IPP 1.987
  • h5-index value: 20 h5-index 20
Solid Earth, 8, 1025-1045, 2017
https://doi.org/10.5194/se-8-1025-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
04 Oct 2017
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 24061, 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 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 44325-4101, USA
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 mm2. 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.


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, 8, 1025-1045, https://doi.org/10.5194/se-8-1025-2017, 2017.
Publications Copernicus
Download
Short summary
Methods of measuring trace water contents of quartz at high resolution using synchrotron infrared 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...
Share