Planar lamellar substructures in quartz

Abstract-Planar deformation features (PDFs) in quartz are essential proof for the correct identification of meteorite impact structures and related ejecta layers, but can be confused with tectonic deformation lamellae. The only completely reliable method to demonstrate the shock origin of suspected (sub-) planar microstructures, transmission electron microscope (TEM) observations, is costly and time consuming. We have used a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM) to image both PDFs and tectonic deformation lamellae in quartz to demonstrate the potential of a simple method to identify PDFs and define characteristics that allow their distinction from tectonic deformation lamellae. In both limited wavelength grayscale and composite color SEM-CL images, PDFs are easily identified. They are straight, narrow, well-defined features, whereas tectonic deformation lamellae are thicker, slightly curved, and there is often no clear boundary between lamella and host quartz. Composite color images reveal two types of CL behavior in PDFs: either they emit a red to infrared CL signal or they are nonluminescent. The color of the CL signal emitted by tectonic deformation lamellae ranges from blue to red. For comparison, we also imaged several shocked quartz grains at cryogenic temperature. In most cases, the PDF characteristics in cryo-CL images do not differ significantly from those in images recorded at room temperature. We conclude that SEM-CL imaging, especially when color composites are used, provides a promising, practical, low cost, and nondestructive method to distinguish between PDFs and tectonic lamellae, even when the simplest CL techniques available are used.

Section: Discussionmentioning

confidence: 99%

Scanning electron microscope‐cathodoluminescence (SEM‐CL) imaging of planar deformation features and tectonic deformation lamellae in quartz

Hamers¹,

Drury

2011

“…Such conditions are clearly not fulfilled in volcanic events. For these reasons, it is incorrect and very confusing, when rare deformation lamellae in volcanic tuffs are called "shock lamellae" and to speak of "shock mosaic structure" (e.g., Carter et al, 1986Carter et al, , 1990Rice, 1987;Lyons et al, 1993). Engelhardt and Bertsch (1969) briefly reviewed the optical character of the, deformation and transformation of quartz by endogenic processes compared to shock effects resulting from impact.…”

Section: Comparison Of Shock-metamorphosed and Endogenically Deformedmentioning

confidence: 99%

Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience*

Grieve

Langenhorst

Stöffler

1996

350

440

Abstract— The occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies. Its utility is due to its almost ubiquitous occurrence in terrestrial rocks, its overall stability and the fact that a variety of shock metamorphic effects, occurring over a range of shock pressures, have been well documented. These shock effects have been generally duplicated in shock recovery experiments and, thus, serve as shock pressure barometers. After reviewing the general character of shock effects in quartz, the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored. The short pulse lengths in experiments may account for the difficulty in synthesizing the high‐pressure polymorphs, coesite and stishovite, compared to natural occurrences. In addition, post‐shock thermal effects are possible in natural events, which can affect shock altered physical properties, such as refractive index, and cause annealing of shock damage and recrystallization. The orientations of planar microstructures, however, are unaffected by post‐impact thermal events, except if quartz is recrystallized, and provide the best natural shock barometer in terms of utility and occurrence. The nature of planar microstructures, particularly planar deformation features (PDFs), is discussed in some detail and a scheme of variations in orientations with shock pressure is provided. The effect of post‐impact events on PDFs is generally limited to annealing of the original glass lamellae to produce decorated PDFs, resulting from the exsolution of dissolved water during recrystallization. Basal (0001) PDFs differ from other PDF orientations in that they are multiple, mechanical Brazil twins, which are difficult to detect if not partially annealed and decorated. The occurrence and significance of shock metamorphosed quartz and its other phases (namely, coesite, stishovite, diaplectic glass and lechatelierite) are discussed for terrestrial impact structures in both crystalline (non‐porous) and sedimentary (porous) targets. The bulk of past studies have dealt with crystalline targets, where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures. In sedimentary targets, the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature, which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets. This is discussed, along with examples of variations in the relative abundances of planar microstructures and their orientations, which are attributed to textural variations in sedimentary target rocks. Examples of the shock metamorphism of quartz in distal ejecta, such as at the K/T boundary, and from nuclear explosions are illustrated and are equivalent to that of known impact structures, except with ...

“…Although the resolution of the optical microscope does not allow to see the detailed defect microstructure, most of the listed characteristics can also be recognized at the optical scale. Therefore, TEM study is not necessarily demanded to identify the origin of planar features (Lyons et al 1993) but may be recommendable in cases of suspected impact structures with samples that experienced strong thermal overprint (Leroux et al 1994;Joreau et al 1997). A careful optical analysis of planar features, the precise measurement of orientations using the universal stage (Langenhorst 2002), the inspection of cogenetic minerals, and the geologic setting are usually sufficient criteria to distinguish PDFs from deformation lamellae (Reimold 1994).…”

Section: Distinction Between Deformation Lamellae and Planar Deformatmentioning

confidence: 99%

Experimental reproduction of tectonic deformation lamellae in quartz and comparison to shock‐induced planar deformation features

Vernooij

Langenhorst

2005

Abstract-Planar features can develop in quartz during comparatively slow tectonic deformation and during very fast dynamic shock metamorphism. Despite their very different structural nature, tectonically induced deformation lamellae have sometimes been mistaken as shock-induced planar deformation features (PDFs). To understand the formation of deformation lamellae and to address the substantial differences between them and PDFs, we have conducted deformation experiments on single crystals of quartz in a Griggs-type apparatus, at a temperature of 800 °C, a confining pressure of 12 kbar, and a strain rate of 0.7-1.1 ⋅ 10 −6 . The deformed samples were analyzed with transmission electron microscopy (TEM) and compared to natural PDFs from the Ries Crater, Germany. TEM revealed that tectonic deformation lamellae are associated with numerous sub-parallel curved subgrain walls, across which the orientation of the crystal changes slightly. The formation of deformation lamellae is due to glide-and climb-controlled deformation in the exponential creep regime. In contrast, the PDFs in shocked quartz from the Ries are perfectly planar, crystallographically controlled features that originally represented amorphous lamellae. Due to postshock annealing and hydrothermal activity they are recrystallized and decorated with fluid inclusions.