Distinction Between Planar Deformation Features and Tectonic Deformation Lamellae
The most important characteristics of PDFs and deformation lamellae are summarized in Tables 1 and 2. Our results show that CL imaging is a very promising method to distinguish between shock and tectonic deformation lamellae, even on a visual basis alone. In cases where only one or two sets of lamellar microstructures are present, which could lead to misidentification in a light microscope, the CL characteristics of the structures clearly show whether they are PDFs or tectonic deformation lamellae (Figs. 2–5): PDFs are thin, straight lines that are dark in grayscale CL images, and red or nonluminescent in composite color CL images, whereas tectonic deformation lamellae are less well defined, thicker, and slightly wavy, with varying thickness, and can show varying CL colors in composite color images.
The physical basis of the difference in CL emission between shocked and tectonically deformed grains remains unclear, and requires further research into the relationship between the CL emission and the nature of the different microstructures. However, even without knowing the exact cause of the CL signal, SEM-CL imaging appears to be a useful technique to distinguish between tectonic and shock lamellae, and to identify PDFs in quartz.
Spacing and (apparent) thickness of planar microstructures in quartz have been mentioned as characteristic features to distinguish between tectonic and shock lamellae (Stöffler and Langenhorst 1994; Grieve et al. 1996; French and Koeberl 2010), but are in practice not often used quantitatively. In general, tectonic deformation lamellae are thicker, more widely spaced, and usually slightly curved, whereas PDFs are extremely thin, closely spaced, and straight (Alexopoulos et al. 1988; Lyons et al. 1993). However, these are not definitive criteria; a range of different types of tectonic deformation lamellae has been recognized (Christie and Raleigh 1959; White 1973; Christie and Ardell 1974; Drury 1993; Vernooij and Langenhorst 2005). For both PDFs and tectonic deformation lamellae, there are many cases in which spacing, thickness, and straightness deviate from the “standard” values. Even in a relatively small sample set such as ours, these characteristics show considerable variation, which is obvious from Fig. 2 to 5. Whereas the PDF spacing depends on the impact pressure, the spacing of tectonic deformation lamellae depends on stress level (Koch and Christie 1981). Tectonic lamellae may occur that are as closely spaced as PDFs (see for example the tectonic lamellae in Fig. 5d, e, and f). Furthermore, spacing and thickness measurements will depend on the imaging method used; in SEM-CL images, for example, more and thinner individual lamellae can be recognized than in light microscopic images, and thus the results of spacing and thickness measurements could differ if the size and spacing of the PDFs are below the spatial resolution for the imaging method used. The spatial resolution of a standard light microscope is theoretically limited to approximately 200–400 nm (Nesse 2004), but is worse in practice. It is known from TEM measurements that many PDFs and some types of tectonic deformation lamellae are thinner than this limit (McLaren et al. 1970; Langenhorst 1994; Stöffler and Langenhorst 1994), and some of these lamellae, therefore, will not be detected in a light microscope. As already mentioned in the section Composite Color Cathodoluminescence, it is questionable whether it is at all possible to perform reliable measurements of spacing and thickness on tectonic deformation lamellae. Tectonic deformation lamellae are not as clearly defined as PDFs. In CL images, it is not always evident which part of the grain is lamella and which part is host quartz (see for example Fig. 3c, 5b, 5c, and 5e). McLaren et al. (1970) also pointed out that in a light microscope, tectonic deformation lamellae are most easily observed when the microscope is focused on the upper surface of the thin section, and that when the lamellae are exactly in focus (which would be required for accurate thickness measurements), they are almost invisible. Thus, measurements on tectonic deformation lamellae are difficult and unreliable.
The presence of multiple, differently oriented sets of (indexed) planar features in a quartz grain of course remains a good indicator for shock, but in cases where light microscopy shows only one or two sets of lamellae, and when it is not immediately obvious whether these are shock or tectonic lamellae, CL imaging can distinguish between the two. In addition to the shape criteria described above, CL images often show more sets of PDFs than can be observed in light microscopy. This is readily seen when comparing, for example Fig. 1c and Fig. 4c, or Fig. 1b and Fig. 4a, which show the same grain in a light microscopic and color CL image, respectively.
Filtered, color, and cryo-CL imaging and CL spectroscopy can all provide extra information on the nature of planar microstructures in quartz. However, unfiltered grayscale CL images will, in many cases, be sufficient to distinguish PDFs from tectonic deformation lamellae and show features that are unclear in light microscopy.
All our shocked samples are from impact structures in predominantly crystalline, nonporous target rocks. As a result of the porosity of sedimentary target rocks, the shock wave energy is distributed much more heterogeneously than in crystalline targets. As a result, shock effects representative of different shock stages in the classification for nonporous rock types can occur together in porous rocks (Grieve et al. 1996). However, the same types of shock effects occur, and there is essentially no structural difference between PDFs in quartz grains from crystalline (nonporous) or from sedimentary (porous) target rocks (Kieffer 1971; Kieffer et al. 1976). We therefore do not expect that the CL characteristics from PDFs in quartz from sedimentary rocks differ so much from those in crystalline rocks, and that the distinction between PDFs and tectonic deformation lamellae becomes impossible. Future research is needed to show how the extra heat production involved in impacts into porous rocks might affect the CL emission of (parts of) shocked quartz grains.