A new way to confirm meteorite impact produced planar features in quartz: combining Universal Stage and Electron Backscatter Diffraction techniques

Abstract

As recognised from the geological record, meteorite impact events can have a severe influence on (local) geology, climate and life. Solid evidence for these events is therefore important to obtain. The most convincing evidence comes from microstructures in quartz. Upon impact, Planar Fractures (PFs) and Planar Deformation Features (PDFs) form parallel to specific crystallographic planes in quartz. Non-impact formed (tectonic) Deformation Lamellae (DL) may be hard to distinguish qualitatively from PFs or PDFs with the optical microscope, but do not form parallel to crystallographic planes. Quantitative methods using the Universal Stage (U-Stage) on the optical microscope have therefore been applied widely to (dis)confirm this parallelism. With the method, the quartz c-axis and poles to planar features are measured and plotted. An improved technique requires so-called indexing of the measured orientations using a stereographic projection template. Even when these techniques are applied, some proposed impact structures remain debated. An important reason for this is the U-stage can not provide the full crystal orientation of quartz.

The goal of this thesis was to check the classical U-stage techniques for quantitatively confirming impact planar features in quartz, and to see whether the addition of Electron Backscatter Diffraction (EBSD, on the Scanning Electron Microscope: SEM) and Cathodoluminescence (CL, on the SEM) can provide more solid evidence. Six previously confirmed impact and three non-impact samples were studied. U-stage analysis was performed like mentioned above. EBSD (providing the full crystal orientation) and CL data (providing 2D planar feature orientations) was gathered for the same grains, combined with the U-stage data, and analysed.

Unlike previous reports, the U-stage analysis (including the indexing method) proves to give equivocal results: plots for impact and non-impact samples are similar and can not be distinguished. Indexing of combined U-stage and EBSD data gives unequivocal results: PF/PDF crystallographic planes can be confirmed (>80% correspondence) for impact sample RI37, and not (<10% correspondence) for non-impact sample SAPP2. The combination of U-stage and SEM-EBSD may therefore provide an important new technique for definitively confirming shocked quartz, hence impact. Comparison of the 2D orientation of PDFs observed by CL, with PDF poles measured by U-stage, did not provide additional information.