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

Arnold Gucsik,Ildikó Gyollai,Péter Rózsa,Kiyotaka Ninagawa,Natalie Deseta,Hirotsugu Nishido,Tasuku Okumura

doi:10.1556/24.58.2015.3.2

Abstract

Quartz grains from the Ries impact structure containing shock-induced microstructures were investigated using Scanning Electron Microscopy in cathodoluminescence (SEM-CL), secondary electron (SEM-SE) and back-scattered electron (SEM-BSE) modes as well as Mott–Seitz analysis. The purpose of this study is to evaluate the mechanism by which CL detects Planar Deformation Features (PDFs) in quartz, which is one of the most important indicators of shock metamorphism in rock-forming minerals. PDFs are micron-scale features not easily identified using optical microscopy or scanning electron microscopy. The CL spectrum of PDFs in quartz that has suffered relatively high shock pressure shows no or a relatively weak emission band at around 385 nm, whereas an emission band with a maximum near 650 nm is observed independent of shock pressure. Thus, the ~385 nm intensity in shocked quartz demonstrates a tendency to decrease with increasing shock metamorphic stage, whereas the 650 nm band remains fairly constant. The result indicates that the emission band at 385 nm is related to the deformed structure of quartz as PDFs.

Highlights

Planar Deformation Features (PDFs) of rock-forming minerals, especially in quartz, are one of the most important mineralogical indicators of shock metamorphism, which is considered evidence for the impact origin of certain geological structures (French and Short 1968; Stöffler 1974; Hörz 1982; Grieve 1987, 1991; Grieve et al 1996; Koeberl 1997)
The CL spectra of these samples exhibit broad bands at ~385 nm in the near-ultraviolet to blue range, and at ~650 nm in the red range, which might be associated with Al centers and a non-bridging oxygen hole center (NBOHC), respectively
This indicates that recombination centers or traps for photon emission are more closely-spaced within the band gap (BG) (Fig. 4), which is probably caused by the partial amorphization of the quartz and/or an instantaneous increase in temperature up to 500 °C

Summary

Introduction

Planar Deformation Features (PDFs) of rock-forming minerals, especially in quartz, are one of the most important mineralogical indicators of shock metamorphism, which is considered evidence for the impact origin of certain geological structures (French and Short 1968; Stöffler 1974; Hörz 1982; Grieve 1987, 1991; Grieve et al 1996; Koeberl 1997). 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) These authors were not able to explain the CL behavior of PDFs, in particular, their non-radiative character shown in the CL images. The shock-induced amorphization in PDFs of quartz samples from the Ries impact structure (at the Polsingen sampling site) was analyzed by micro-Raman spectroscopy (Gucsik et al 2011). They found that Raman intensity exhibits a moderate increase in the range of 230 to 450 cm–1 indicating that PDFs are in-filled by an amorphous phase. They concluded that the SiO4 tetrahedral cluster-type structure is generated along the breaking of Si–O–Si bonds in the quartz structure by shock pressure (Gucsik et al 2011 and references therein)

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