Shock metamorphism in plagioclase and selective amorphization

Pittarello, L., Daly, L. , Pickersgill, A. E. , Ferrière, L. and Lee, M. R. (2020) Shock metamorphism in plagioclase and selective amorphization. Meteoritics and Planetary Science, 55(5), pp. 1103-1115. (doi: 10.1111/maps.13494)

Abstract

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.

• Item Type: Articles
• Additional Information: This work and L.P. are supported by the FWF Project V505-N29 “Shock metamorphism in plagioclase.”
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Pickersgill, Dr Annemarie and Lee, Professor Martin and Daly, Dr Luke
• Authors: Pittarello, L., Daly, L., Pickersgill, A. E., Ferrière, L., and Lee, M. R.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Journal Name: Meteoritics and Planetary Science
• Publisher: Wiley
• ISSN: 1086-9379
• ISSN (Online): 1945-5100
• Published Online: 30 May 2020
• Copyright Holders: Copyright © 2020 The Authors
• First Published: First published in Meteoritics and Planetary Science 55(5): 1103-1115
• Publisher Policy: Reproduced under a Creative Commons license