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

Maartje F Hamers,Gill M Pennock,Marco Herwegh,Martyn R Drury

doi:10.1111/maps.12711

Maartje F Hamers, Gill M Pennock + Show 2 more

Open Access

https://doi.org/10.1111/maps.12711

Copy DOI

Abstract

AbstractPlanar 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.

Highlights

The presence of planar deformation features (PDFs) in quartz is one of the most reliable forms of evidence for the impact origin of suspected geological structures or stratigraphic layers
The combination of color scanning electron microscope cathodoluminescence (SEM-CL) imaging and transmission electron microscopy (TEM) analysis of shocked quartz grains confirms the great potential of color scanning electron microscope (SEM)-CL imaging as a method to identify PDFs in quartz, and illustrates the complexities that are involved in interpreting CL emissions, especially in images
A very useful result of this study for crater research is that color SEM-CL imaging can distinguish between fresh and healed PDFs or basal Brazil twins: nonluminescent PDFs are amorphous, while healed PDFs and basal

Summary

Introduction

The presence of planar deformation features (PDFs) in quartz is one of the most reliable forms of evidence for the impact origin of suspected geological structures or stratigraphic layers. PDFs are planar microstructures that form in quartz and other minerals (e.g., feldspar) as a result of a shock wave passing through the rocks (e.g., papers in French and Short 1968; von Engelhardt and Bertsch 1969; Sto€ffler and Langenhorst 1994; Grieve et al 1996) and can be observed in standard thin sections in a standard petrographic microscope. Transmission electron microscopy (TEM) observations on the submicron scale structure of the lamellae are often necessary to distinguish between shock and nonshock-related structures and to confirm the shock origin of planar microstructures in quartz (e.g., Sto€ffler and Langenhorst 1994; Grieve et al 1996; French and Koeberl 2010). TEM studies involve difficult and timeconsuming sample preparation and often require extensive training

Objectives

Methods

Results

Discussion

Conclusion