Distinction between amorphous and healed planar deformation features in shocked quartz using composite color scanning electron microscope cathodoluminescence (SEM‐CL) imaging

Abstract

Abstract-Planar deformation features (PDFs) in quartz are one of the most reliable and most widely used forms of evidence for hypervelocity impact. PDFs can be identified in scanning electron microscope cathodoluminescence (SEM-CL) images, but not all PDFs show the same CL behavior: there are nonluminescent and red luminescent PDFs. This study aims to explain the origin of the different CL emissions in PDFs. Focused ion beam (FIB) thin foils were prepared of specific sample locations selected in composite color SEM-CL images and were analyzed in a transmission electron microscope (TEM). The FIB preparation technique allowed a direct, often one-to-one correlation between the CL images and the defect structure observed in TEM. This correlation shows that composite color SEM-CL imaging allows distinction between amorphous PDFs on one hand and healed PDFs and basal Brazil twins on the other: nonluminescent PDFs are amorphous, while healed PDFs and basal Brazil twins are red luminescent, with a dominant emission peak at 650 nm. We suggest that the red luminescence is the result of preferential beam damage along dislocations, fluid inclusions, and twin boundaries. Furthermore, a high-pressure phase (possibly stishovite) in PDFs can be detected in color SEM-CL images by its blue luminescence.

Scanning electron microscope cathodoluminescence imaging of subgrain boundaries, twins and planar deformation features in quartz

“…However, TEM analysis of the Rochechouart sample (Fig. 2, Hamers et al 2016) shows sections of Dauphiné twins exist between healed PDFs that are, indeed, free of dislocations and subgrains, so it is feasible that the SEM-CL from the Dauphiné twin indicated by the white arrow in Fig. 5c, d also arises solely from the twin boundary.…”

“…The Cap de Creus quartzite (sample CML-2) is highly deformed and recrystallised, with most grains showing well developed subgrains. The Rochechouart lithic breccia sample (RO2, grain 17) contains quartz grains with healed PDFs, Dauphiné twins and subgrains, while the Vredefort granite gneiss sample (sample 55, grain 3) contains quartz grains with basal Brazil twin type PDFs, Dauphiné twins and subgrains (Hamers 2013;Hamers et al 2016).…”

“…These bright lines were identified using TEM as Brazil twins (Fig. 3b, c in Hamers et al 2016): the dislocation density in the Brazil twin plane varied, with parts of the twin plane decorated with dislocations and most parts free of dislocations. Dauphiné and Brazil twin boundaries mostly coincide so any differences in CL intensity are difficult to distinguish: however, one short section of Dauphiné twin boundary (white arrow in Fig.…”

“…In particular, composite colour CL imaging in the scanning electron microscope (SEM), in which red, green and blue filters are used, is useful in distinguishing two types of PDFs, a nonluminescent type and a red to infrared CL type (Hamers and Drury 2011;Hamers et al 2016). A one-to-one correlation of SEM-CL and TEM showed that the non-luminescent shock features were amorphous PDFs, whereas the red CL type was healed PDFs or shock induced Brazil twins (Hamers et al 2016). Tectonic deformation lamellae show red to blue CL (Hamers and Drury 2011).…”

“…Cathodoluminescence (CL) is good for examining defects arising from differences in impurities associated with growth zones (Götze et al 1999(Götze et al , 2001Götze 2009) and deformation microstructures, such as fractures, deformation lamellae, PDFs and mechanical Brazil twins found in shocked quartz (Seyedolali et al 1997;Boggs et al 2001;Cavosie et al 2010;Hamers and Drury 2011;Hamers 2013;Hamers et al 2016). In particular, composite colour CL imaging in the scanning electron microscope (SEM), in which red, green and blue filters are used, is useful in distinguishing two types of PDFs, a nonluminescent type and a red to infrared CL type (Hamers and Drury 2011;Hamers et al 2016). A one-to-one correlation of SEM-CL and TEM showed that the non-luminescent shock features were amorphous PDFs, whereas the red CL type was healed PDFs or shock induced Brazil twins (Hamers et al 2016).…”