A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz |
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| Authors: | Gayle C Gleason Stephanie DeSisto |
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| Institution: | aGeology Department, SUNY Cortland, P.O. Box 2000, Cortland, NY 13045, USA |
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| Abstract: | Water content of quartz in and around a greenschist facies mylonitic shear zone located in the western Adirondacks was analyzed by micro-FTIR spectroscopy. The shear zone is within a pegmatitic dike, which cuts across a granitic gneiss. The thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south, along strike. Microstructures, including quartz ribbons and recrystallized grains, indicate quartz and feldspar within the mylonite underwent dislocation creep. Infrared spectral analysis was carried out using a Nicolet micro-FTIR on mylonitic quartz ribbons, pegmatitic quartz and gneissic quartz. A small aperture size (56 μm by 50 μm) for the IR beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries. The smallest dimension of the quartz ribbons is 0.3 mm, whereas the pegmatitic quartz has a grain size of 3 to 5 cm. Results show mylonitic quartz ribbons contain the most water (320 H:106 Si average, range of 50 to 1120 H:106 Si); pegmatite quartz contains much less water (30 H:106 Si average, range of 20–40 H:106 Si) and the gneissic quartz contained an intermediate amount (200 H:106 Si average, range of 20 to 870 H:106 Si). These data indicate that water was preferentially incorporated into the deformed quartz ribbons. |
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| Keywords: | Quartz Mylonite Water Micro-FTIR Hydrolytic weakening Dislocation creep |
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