5:15 PM - 7:15 PM
[SCG51-P06] Quadruple points of plagioclase in gabbroic ultramylonites: an example from the northern Fizh shear zone in the Oman ophiolite
Keywords:quadruple points, grain boundary sliding, gabbroic ultramylonite, Oman ophiolite
Grain-boundary sliding is one of the deformation mechanisms in polycrystalline materials, in which grains slide against each other at grain boundaries. Ashby and Verrall[1] proposed that quadruple points, where grains intersect at a single point on a plane, are formed during grain boundary sliding, which has also been directly observed experimentally[2]. In addition, quadruple points have also been observed in natural ultramylonites and are treated as a microstructure indicating grain boundary sliding[3-5], but their number and distribution are not clear. In this study, we conducted SEM-EBSD analysis on gabbroic ultamylonite from the northern Fizh shear zone[6,7] in the Oman ophiolite to reveal its microstructural development.
Highly polished thin section was prepared from the ultramylonite sample perpendicular to foliation and parallel to lineation, and SEM-EBSD analysis was conducted at the Rock and Mineral Laboratory, Nagoya University. Grains were reconstructed from the acquired map data using the MTEX Toolbox in MATLAB and analyzed for crystallographic preferred orientations and grain sizes. Furthermore, quadruple points were detected from the reconstructed grain data by considering the quadruple points as the midpoint between two triple points that were sufficiently close to each other. The detected quadruple points were then filtered by considering the loss of particles on the thin section and particle size. In addition, the area containing a quadruple point was cut using FIB from a highly polished thin section prepared on the same plane and observed using TEM at the Graduate School of Science, Kyoto University. The gabbroic ultramylonite is composed of plagioclase and green amphibole, but no clinopyroxene, which composes the protolith, is observed. Green amphibole was abundant not only in the matrix but also in the porphyroclasts, whereas plagioclase was abundant in the matrix and few in the porphyroclasts. The grain sizes of both minerals were only a few micrometers, and the degree of phase mixing was weak. SEM-EBSD analysis showed that the plagioclase fabric was random, the average grain size was approximately 5 μm, and the quadruple points were universally distributed in the matrix. TEM observations showed that dislocations were present around the quadruple point, but in small amounts. In this presentation, the microstructure of gabbroic ultramylonite is discussed based on a more detailed distribution of the quadruple points obtained in this study.
[References] [1] Ashby & Verrall 1973 Acta Metall, [2] Maruyama & Hiraga 2017 J. Geophys. Res, [3] Casini et al. 2021 J. Geophys. Res., [4] Precigout et al. 2017 Nat. Commun., [5] Menegon et al. 2013 J. Struct. Geol., [6] Boudier et al. 1988 Tectonophysics, [7] Michibayashi & Oohara 2013 Earth Planet. Sci. Lett.
Highly polished thin section was prepared from the ultramylonite sample perpendicular to foliation and parallel to lineation, and SEM-EBSD analysis was conducted at the Rock and Mineral Laboratory, Nagoya University. Grains were reconstructed from the acquired map data using the MTEX Toolbox in MATLAB and analyzed for crystallographic preferred orientations and grain sizes. Furthermore, quadruple points were detected from the reconstructed grain data by considering the quadruple points as the midpoint between two triple points that were sufficiently close to each other. The detected quadruple points were then filtered by considering the loss of particles on the thin section and particle size. In addition, the area containing a quadruple point was cut using FIB from a highly polished thin section prepared on the same plane and observed using TEM at the Graduate School of Science, Kyoto University. The gabbroic ultramylonite is composed of plagioclase and green amphibole, but no clinopyroxene, which composes the protolith, is observed. Green amphibole was abundant not only in the matrix but also in the porphyroclasts, whereas plagioclase was abundant in the matrix and few in the porphyroclasts. The grain sizes of both minerals were only a few micrometers, and the degree of phase mixing was weak. SEM-EBSD analysis showed that the plagioclase fabric was random, the average grain size was approximately 5 μm, and the quadruple points were universally distributed in the matrix. TEM observations showed that dislocations were present around the quadruple point, but in small amounts. In this presentation, the microstructure of gabbroic ultramylonite is discussed based on a more detailed distribution of the quadruple points obtained in this study.
[References] [1] Ashby & Verrall 1973 Acta Metall, [2] Maruyama & Hiraga 2017 J. Geophys. Res, [3] Casini et al. 2021 J. Geophys. Res., [4] Precigout et al. 2017 Nat. Commun., [5] Menegon et al. 2013 J. Struct. Geol., [6] Boudier et al. 1988 Tectonophysics, [7] Michibayashi & Oohara 2013 Earth Planet. Sci. Lett.