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.
Cathodoluminescence Characteristics of Planar Deformation Features in Quartz
In contrast to many other minerals, such as feldspars, calcite, or zircons, quartz does not exhibit high intensity CL emission at room temperature. At temperatures below −80 to −100 °C, the CL emission of quartz shows a dramatic increase in intensity by factors of 100–1000 (Marshall 1988) so to get the highest intensity signal, CL work is often carried out at low temperature. In our low temperature comparison, the intensity of the CL emission strongly increased, as expected, especially in the blue and green range. However, the overall results at cryogenic temperatures were the same as those found at room temperature: both red- and nonluminescent PDFs are present, and in most cases also the CL color of the host quartz does not change significantly. These results show that, although cryo-CL does give a higher intensity CL emission, cryo is not required to image shocked quartz grains and identify PDFs. On the contrary, as Boggs et al. (2001) already observed, the quality of the CL images is often better at room temperature than at cryogenic temperatures. Although cryo-CL can often provide extra information, even the simplest method of SEM-CL imaging, at room temperature, produces high quality images that are sufficient to identify PDFs in quartz and distinguish them from tectonic deformation lamellae.
Cathodoluminescence of Shocked Quartz
Boggs et al. (2001) reported that PDFs can be imaged with SEM-CL methods, and concluded that PDFs are visible in CL images because they are nonluminescent in contrast to the host quartz grain. Our results show that the latter is not the case; in fact, most PDFs do emit light, mostly in the red to infrared wavelength range (595–850 nm, red filter). PDFs often appear nonluminescent in (panchromatic) CL images, because the intensity of the (usually) blue light emitted by the surrounding quartz is often so much higher than the intensity of the signal coming from the material in the PDFs. In the case of Boggs et al. (2001), the apparent nonluminescence of the PDFs might also be a result of the CL detector they use, which has a detection range of about 185–700 nm, and detects (part of) the ultraviolet and the complete blue wavelength range, but does not detect all of the red range (620–750 nm), and none of the infrared wavelengths. The PDFs could appear nonluminescent because of the low intensity of the CL signal emitted by the PDFs and the limited range of the detector, although this would depend on the specific wavelength of the CL emission of the PDFs. In the images recorded with our blue-sensitive limited wavelength CL detector, PDFs are dark because they emit red light, which is not detected, and therefore the structures appear to be nonluminescent. Several studies have shown that shocked quartz is usually blue luminescent, with an additional emission band at 630–650 nm (Ramseyer et al. 1992; Ramseyer and Mullis 2000; Trepmann et al. 2005; Götte 2009; Okumura et al. 2009; Kayama et al. 2010). The red-luminescent PDFs observed in our samples could (partly) be the source of this band.
The two types of CL behavior observed in PDFs (red to infrared or nonluminescent) are possibly related to shock intensity. Nonluminescence of PDFs only occurs in grains with multiple sets of closely and regularly spaced, thicker PDFs (Fig. 4g and h), which indicate high shock pressure (Grieve et al. 1996; Ferrière et al. 2008). Okumura et al. (2009) found that shocked quartz grains from the Ries crater show the usual emission bands with maxima around 385 nm (violet) and around 650 nm (red). The 650 nm band is observed independent of shock pressure, whereas the 385 nm band dissappears in more highly shocked grains. This seems to be in line with the observation that nonluminescent PDFs usually occur in highly shocked, red-luminescent quartz grains. Red CL emission of PDFs is seen in grains with a lower number of sets of PDFs, which are more widely and less regularly spaced and are therefore interpreted as the result of lower shock pressure.
Most of the PDFs from the Vredefort impact structure occur in single sets and are of the basal, Brazil twin type (Carter 1965; Grieve et al. 1990; Leroux et al. 1994). In our Vredefort samples, we observed only grains with one set of PDFs so it is likely that most of these are basal PDFs. It is striking that the characteristic red CL emission is observed in features that have a fundamentally different structure: in PDFs that are considered amorphous (Ries) as well as in basal, Brazil twin type PDFs (Vredefort) and decorated PDFs (Rochechouart, Popigai). An explanation for this is not apparent from the CL images alone and will require further research into the exact nature of the defects, composition, or water content variations that might cause the typical red CL signal of PDFs.
Possible Causes of the CL Behavior of Planar Deformation Features and Difference between Red- and Nonluminescent PDFs
Several causes of red CL emission in quartz are mentioned in the literature. High water content or low Ti/Fe ratio can result in a red CL emission (Marshall 1988). Substitutional incorporation of Fe3+ into the quartz lattice probably gives rise to an emission band around 705 nm (red to infrared) (Götze et al. 2001). Fitting et al. (2001) observed a red peak (around 650 nm) in the CL spectrum of SiO2 glass, indicating that also amorphous SiO2 is capable of producing red CL (although, of course, CL color cannot prove anything on crystal structure). The most important red CL peak is the common 620–650 nm emission band in quartz. It consists of two overlapping components at 620 nm and at 650 nm, and can be caused by an oxygen vacancy or a nonbridging oxygen hole center (NBOHC, ≡Si-O, a dangling oxygen bond) (Stevens Kalceff and Phillips 1995; Stevens Kalceff et al. 2000; Götze et al. 2001). Different precursors for the NBOHC have been proposed that influence the band position. Among these precursors are hydrogen and sodium impurities, hydroxyl groups (≡Si-OH) (620 nm), peroxy linkages (≡Si-O-O-Si≡) (650 nm), and strained silicon-oxygen bonds (Si--O) (650 nm) (Stevens Kalceff and Phillips 1995; Götze et al. 2001).
The 650 nm emission increases during electron bombardment (Götze et al. 2001), which is an indication that this might be the emission of the red-luminescent PDFs, as they become brighter after repeated scanning with the electron beam. Furthermore, this emission band is commonly observed in shocked quartz grains (Ramseyer et al. 1992; Ramseyer and Mullis 2000; Trepmann et al. 2005; Götte 2009; Okumura et al. 2009; Kayama et al. 2010). Götte (2009) attributed the presence of a 630–650 nm emission band in shocked quartz from the Siljan and Araguainha craters to thermal breaking of OH groups, which are incorporated in the quartz. However, according to Stevens Kalceff et al. (2000), the emission related to the NBOHC with OH precursor is at 620 nm, and attenuates quickly under electron irradiation at room temperature. This does not fit our observation that the intensity of the emission in red-luminescent PDFs increases after repeated scanning.
Also, high water content or hydrogen impurities in the PDFs could explain the red luminescence, because H2O is much more soluble in the amorphous material within PDFs than in the adjacent crystalline quartz (Grieve et al. 1996). The occurrence of basal PDFs decorated with tiny fluid inclusions illustrates that along Brazil twin boundaries, water content might also be locally increased, giving rise to the same CL behavior for the two different types of PDF.
A final explanation for the red luminescence of PDFs might be strained silicon-oxygen bonds. It is possible that strained Si-O bonds are present in the SiO2 within PDFs (or were present before annealing), where the crystal structure is disordered or (partly) destroyed by the shock wave. These strained bonds could form the precursors for NBOHCs. Also, basal, Brazil twin PDFs might contain strained silicon-oxygen bonds as a result of the high differential stresses that form these structures during the shock event. Although no bonds need to be broken to form this type of twin, there will be some strain at the twin boundary (McLaren et al. 1970).
Of course, the above interpretations remain rather speculative and measurements of CL spectra of the emission from the PDFs could provide valuable extra information for the interpretation of the PDF characteristics in CL images.
The difference in CL emission between the red- and nonluminescent PDFs could be caused by structural differences of the material within the PDFs, as a result of increasing shock pressure and temperature. As mentioned previously, the occurrence of multiple sets of nonluminescent PDFs per grain, their close and regular spacing, and thickness and slightly wavy boundaries, all indicate formation under high shock pressures (Langenhorst 1994; Ferrière et al. 2008). According to the model presented by Langenhorst (1994), the transformation of crystalline quartz to diaplectic quartz glass is a process in which the number of dense, amorphous PDFs increases with increasing shock pressure and temperature, until the whole grain consists of diaplectic glass, with fluidal glass (lechatelierite) only occurring when the residual (postshock) temperature is sufficiently high. Three phases occur during this process: (1) when both shock and postshock temperatures are below the melting point of quartz, extremely narrow, straight PDFs, consisting of a superheated, dense, amorphous phase, form by solid-state transformation, to compensate for crystal lattice incompatibilities at the shock wave propagation front; (2) at higher shock pressure, the shock temperature increases to just above the quartz melting temperature, and the PDFs are at a sufficiently high temperature to melt a small region of the adjacent crystalline quartz, resulting in thicker PDFs with more wavy boundaries; and (3) when, at even higher shock pressure, the shock temperature is significantly higher than the quartz melting temperature, the crystalline regions between the PDFs melt completely and the whole grain transforms into diaplectic quartz, which is quenched before complete decompression.
In this model, the red-luminescent (rhombohedral) PDFs could form during the first stage, when no melt is formed, but an amorphous phase in which the quartz lattice is disordered, but retains some of its structure. The CL emission centers that form during this stage must either survive postshock annealing, or be a secondary feature, because the red-luminescent PDFs are also observed in altered impact structures, such as the Rochechouart structure. During the second stage, the thicker, nonluminescent PDFs form, filled with quartz melt or diaplectic glass, because of the higher temperature associated with higher shock pressure. The complete destruction of the quartz crystal structure might result in nonluminescence of the material within the PDFs.