Identification of shocked quartz by scanning cathodoluminescence imaging

Abstract

Abstract-Quartz grains subjected to high-strain-rate shock waves owing to meteorite or cometary impact on Earth's surface commonly display shock lamellae. These lamellae appear as remarkably straight, thin, planar features (microstructures) in sets within which lamellae are essentially parallel to each other and spaced 1 2 0 p m apart. Two or more intersecting sets are typically present. Shock lamellae are commonly recognized and identified by optical methods, by use of the transmission electron microscope (TEM), and by etching polished sections and subsequent examination with a scanning electron microscope (SEM) operated in the secondary electron mode. We present here a method for observing planar microstructures in shocked quartz by using a cathodoluminescence (CL) detector attached to a SEM. The method relies on the fact that planar microstructures in quartz arising as a result of shock display no CL whatever; thus, they show up as distinct, thin, black lines on otherwise luminescent quartz grains. We used scanning CL imaging to study shocked quartz from the Ries Crater, Germany, a well-known impact crater of Miocene age. We demonstrate that shockproduced planar microstructures are clearly displayed in SEM-CL images and can be distinguished from microfractures generated by tectonism, and subsequently filled with quartz, and other similar features not related to impact events. The SEM-CL method provides a powerful supplement to other methods of identifying shocked quartz. It commonly provides better spatial resolution than does standard optical methods, and does not require etching of quartz grains. Further, it is easier and faster to use than are TEM methods, although it is not capable of the fine-scale defect analysis possible with TEM.

Non-luminescent nature of the planar deformation features in shocked quartz from the Ries impact structure, Germany: A new interpretation

“…5b). These results may corroborate the observations of Boggs et al (2001) and agree with the conclusions obtained from our Raman spectroscopic observations (Gucsik et al 2011). Alternatively, the non-luminescent nature of PDFs may be due to the absence of traps or recombination centers because of amorphization.…”

“…The Al center might disappear due to the modification of energy levels accompanying amorphization, or the breakdown of the SiO 4 tetrahedral structure by shock metamorphism (Boggs et al 2001). Therefore, the emission band at ~385 nm has potential for indicating the types of deformation affecting quartz in different shock ranges, in which no other crystallographic changes have been observed.…”

“…However, light emission in the UV region of the spectrum is based on a direct return from the CB to the VB (Yacobi and Holt 1990;Gaft et al 2005). According to Boggs et al (2001) and Nasdala et al (2003), quartz is an insulator mineral, in which the band gap between CB and VB is too large for thermal (at relatively low temperatures) excitation to promote electrons from lower-energy states to the higher-energy states.…”

“…The luminescence properties of shocked quartz and its shock-induced microstructures both in natural and experimental cases are poorly understood (e.g., Ramseyer et al 1992;Dypvik et al 1996;Seyedolali et al 1997;Boggs et al 2001;Gucsik et al 2003Gucsik et al , 2004Trepmann et al 2005;Götte 2009;Okumura et al 2009;Hamers and Drury 2011 and references therein). In this study our primary aim was to explain the CL behavior of shocked quartz, including the origin and characteristics of the CL signal in PDFs.…”

“…The CL spectroscopic properties of shocked quartz, as well as its PDFs, have been described from various shock metamorphic environments such as terrestrial impact structures (e.g., Ramseyer et al 1992;Dypvik et al 1996;Seyedolali et al 1997;Boggs et al 2001;Trepmann et al 2005;Götte 2009; Hamers and Drury 2011 and references therein) and shock recovery experiments (Gucsik et al 2003). The question of whether shock metamorphic effects can modify the luminescent nature of quartz under high pressure was first addressed in a pioneering study by Boggs et al (2001). However, these authors were not able to explain the CL behavior of PDFs, in particular, their non-radiative character shown in the CL images.…”