Stereographic Reconstruction of Impact Microstructures using Optical and Electron Microscopy
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Stereographic Reconstruction of Impact Microstructures using Optical and Electron Microscopy Amar Agarwal1,*, Boris Reznik2 and Deepak C. Srivastava3 1
Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur - 208 016, India. Structural Geology and Tectonophysics, Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany. 3 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667, India. *E-mail: [email protected]
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ABSTRACT This study reconstructs the pre-impact orientation of impact-generated µm-sized microstructures. The approach is illustrated using a sample from the basement dolerites of the Lockne impact structure, Sweden. Under the polarised light microscope, the microstructure appears as alternating augiteplagioclase wedges. The top-view of the microstructure is presented in the polished thin-section using light and scanning electron microscopy, while, the side-view is observed in focused-ionbeam trenches in a field emission scanning electron microscope. The top- and side view of the wedges are projected on a stereo net, which allows the determination of their 3D orientation. The wedges are thus found to have a dip of 08° towards 280°. The stereo net projections are then used to reconstruct the orientation of pre-impact boundary between augite-plagioclase, which had a dip of 15° towards 237°. This approach of combining light and electron microscopy with FIB trenching and projecting the data on a stereo net can be especially useful for reconstructing the pre-deformed state of impact-induced or tectonic µm-sized microstructures, which cannot be adequately analysed using a light microscope. INTRODUCTION With the application of stresses, rocks, and mineral grains deform and change their shape and orientation. Determining the original undeformed shape and orientation is critical to understand the stress regime. For example, in case of tectonic deformation, such comparison provides stress direction and intensity (e.g., Agarwal et al., 2017a; Gay and Weiss, 1974; Joshi et al., 2017; Naylor et al., 1986; Oncken, 1988). In case of impact cratering, dynamic stresses are implied by the shock waves, and comparison of deformed and pre-deformation grain shape and orientation may reveal the direction of shock wave propagation (e.g., Agarwal et al., 2019; Hörz, 1970; Hörz and Ahrens, 1969; Kenkmann et al., 2000). In the basement dolerites of the Lockne Impact crater (Fig. 1), shock waves have generated wedge-shaped microstructures (Figs. 2, 3), due to protrusion of augite and plagioclase grains into each other (Agarwal et al., 2017b, 2016). In this study, post-impact orientations of these wedges, and pre-impact orientation of grain boundary between augite and plagioclase grains are reconstructed. To determine the postimpact orientation of the wedges, this study first uses a new combination of well-known techniques of stereographic projection, light microscopy, scanning electron microscopy (SEM) and focussedion-beam milling (FIB). During t
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