Shock metamorphism in plagioclase and selective amorphization

1,2Lidia Pittarello,3,4,5Luke Daly,3Annemarie E. Pickersgil,1Ludovic Ferrière,3Martin R. Lee
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13494]
1Department of Mineralogy and Petrography, Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
2Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A‐1090 Vienna, Austria
3School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ UK
4Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO Box U 1987, Perth, Western Australia, 6845 Australia
5Australian Centre for Microscopy and Microanalysis, University of Sydney, Sydney, 2006 New South Wales, Australia
Published by arrangement with John Wiley & Sons

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.

 

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