Identifying shock microstructures in quartz from terrestrial impacts: new scanning electron microscopy methods

Terrestrial impact craters and ejecta layers are widely studied, both as an analogue to craters on other rocky planets, moons and asteroids and for their relevance to the geology of the Earth. Therefore, the correct identification of impact structures and ejecta layers on the Earth is essential, but recognition of the characteristic circular crater morphology is often hindered by erosion and deformational processes. One of the most reliable impact indicators is the presence of shock metamorphosed minerals in target rocks and in material ejected out of the crater. Of the minerals that can show these shock effects quartz is most widely studied. A wide variety of shock effects can form in quartz at different peak shock pressures. Especially planar deformation features (PDFs - originally amorphous microlamellae) are universally accepted as the most reliable form of impact evidence. In a light microscope distinction between PDFs and other, non-shock related (sub-)planar microstructures in quartz is not always possible. To prove the shock origin of planar microstructures in quartz, time-consuming, difficult and expensive transmission electron microscopy (TEM) analysis of the lamellae is often required. Scanning electron microscopy (SEM) techniques, on the other hand, are relatively easy and quick. The aim of the research presented in this thesis is to develop SEM methods for the reliable identification and characterisation of shock microstructures in quartz, in particular PDFs, as an alternative and addition to existing TEM techniques. The SEM techniques include cathodoluminescence (CL) and forescattered electron (FSE) imaging and electron backscatter diffraction (EBSD) mapping. Focussed ion beam preparation of TEM foils allows direct correlation of information obtained using the SEM to the microstructures analysed using TEM. The combination of CL, FSE and EBSD techniques and more standard applications in the SEM, such as backscattered and secondary electron imaging, provides a powerful and easy to use, non-destructive integrated approach for studying shock microstructures in quartz. Direct correlation with light microscopy is possible, because standard petrographic thin sections can be studied in both the light microscope and the SEM. The combined SEM techniques described in this thesis bridge the gap between highly detailed TEM analysis and general observations in light microscopy that are relevant for larger sample volumes. Combined SEM and TEM observations on shocked quartz grains show combinations of microstructures that can be related to different stages of healing in grains shocked to moderate and high pressure and suggest that mechanical twinning in the low shock pressure regime may play a more important role in PDF development than previously assumed. For the identification and general characterisation of shock microstructures in quartz, SEM analysis using CL and FSE imaging and EBSD mapping is sufficient. This conclusion is a major step forward for terrestrial impact research and will contribute to the reliable identification of proposed impact structures and ejecta layer