Shock effects in plagioclase feldspar from the Mistastin Lake impact structure, Canada

Abstract

-Shock metamorphism, caused by hypervelocity impact, is a poorly understood process in feldspar due to the complexity of the crystal structure, the relative ease of weathering, and chemical variations, making optical studies of shocked feldspars challenging. Understanding shock metamorphism in feldspars, and plagioclase in particular, is vital for understanding the history of Earth's moon, Mars, and many other planetary bodies. We present here a comprehensive study of shock effects in andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada. Samples from a range of different settings were studied, from in situ central uplift materials to clasts from various breccias and impact melt rocks. Evidence of shock metamorphism includes undulose extinction, offset twins, kinked twins, alternate twin deformation, and partial to complete transformation to diaplectic plagioclase glass. In some cases, isotropization of alternating twin lamellae was observed. Planar deformation features (PDFs) are notably absent in the plagioclase, even when present in neighboring quartz grains. It is notable that various microlites, twin planes, and compositionally different lamellae could easily be mistaken for PDFs and so care must be taken. A pseudomorphous zeolite phase (levyne-Ca) was identified as a replacement mineral of diaplectic feldspar glass in some samples, which could, in some instances, also be potentially mistaken for PDFs. We suggest that the lack of PDFs in plagioclase could be due to a combination of structural controls relating to the crystal structure of different feldspars and/or the presence of existing planes of weakness in the form of twin and cleavage planes.

A Study of Shock-Metamorphic Features of Feldspars from the Xiuyan Impact Crater

“…The moderately shocked feldspar in this study shows that alternate twin lamellae were converted to diaplectic glass without flow texture or chemical composition changes, which is consistent with the characteristics of solid-state phase transformation. Diaplectic feldspar glass formation in alternate twin lamellae has been reported previously in Ries crater [4], Vredefort [30] and Mistastin Lake impact structures [5], and was explained to be a result of an unsymmetrical orientation of the twinning-plane to the normal of the shock front [4,5].…”

“…Shock features of feldspar have been observed in impactites from impact craters on Earth [4,5], ordinary chondrites [6,7], as well as Vesta [8], Lunar [9] and Martian [10] meteorites. The currently known shock-metamorphic features in natural feldspar include irregular fractures and planar fractures [5], undulatory extinction and mosaicism [11], planar deformation features (PDFs) [12], diaplectic glass (sometimes called maskelynite) [4,13], vesicular glass [14], lingunite [15], and decomposition [16]. The shock-metamorphic features of feldspar can provide clues for the search for impact craters on Earth and can also enrich the understanding of the behavior of feldspar under high temperatures and high pressures.…”

Volume 364: Chicxulub: Drilling the K-Pg Impact Crater

“…High-resolution 40 Ar/ 39 Ar analyses and electron microscopy on shocked and melted impactites, as well as U/Pb dating of zircon and other geo-and thermochronometers, will be used to study their pressure-temperature-time and deformational history and for highprecision dating of the Chicxulub impact. Shock metamorphism of the feldspathic components will be used to investigate how impact processes affect argon retention (Pickersgill et al, 2015). Shock metamorphism and pyrometamorphic indicators for rock-forming minerals will help constrain peak shock pressure and temperature regimes (Grieve et al, 1996;Tomioka et al, 2007;Wittmann et al, 2009;Huber et al, 2011;Rae et al, 2015).…”

Microspectroscopic and Petrographic Comparison of Experimentally Shocked Albite, Andesine, and Bytownite

“…The optical petrographic effects of shock in feldspars are progressive (Jaret et al, 2014;Kieffer et al, 1976;Pickersgill et al, 2015;Stöffler, 1971): the appearance of PDFs, the formation of diaplectic glass, and finally, whole-scale melting. Based on the trend of shock textures in quartz, several classification schemes for assessing shock level in natural samples were developed (Chao, 1968;Singleton et al, 2011;Stöffler, 1971;von Engelhardt & Stoffler, 1968).…”