3 research outputs found
Synchrotron FTIR imaging of OH in quartz mylonites
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