Shock metamorphism in plagioclase and selective amorphization.

Lidia Pittarello,Martin R Lee,Annemarie E Pickersgill,Luke Daly,Ludovic Ferrière

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.

Highlights

Plagioclase feldspar is the most abundant rockforming mineral in the crust of solid planetary bodies, including Earth, and represents up to 70% of the Moon’s crust (e.g., Papike et al 1991)
For pressures in excess of the Hugoniot Elastic Limit for plagioclase (3.5–4.5 GPa), the following shock features have been observed as pressure increases (i) fracturing and mosaicism, (ii) formation of planar deformation features (PDFs) that manifest as thin (
Optical microscope observations reveal that overall, the sample appears moderately shocked, displaying intense fracturing of garnet, kinking of biotite, and the widespread presence of planar microstructures in plagioclase and quartz. These shock features are homogeneously distributed across the thin section, and there is no gradient in the abundance of shock features from the surface of the shatter cone inward

Summary

Introduction

Plagioclase feldspar is the most abundant rockforming mineral in the crust of solid planetary bodies, including Earth, and represents up to 70% of the Moon’s crust (e.g., Papike et al 1991). The disparity in the number of studies is likely due to the relatively complex crystal structure of plagioclase in comparison to quartz, its variable response to shock depending on the amount of Ca versus Na in its structure For pressures in excess of the Hugoniot Elastic Limit for plagioclase (3.5–4.5 GPa), the following shock features have been observed as pressure increases (i) fracturing and mosaicism, (ii) formation of planar deformation features (PDFs) that manifest as thin (

Methods

Results

Discussion

Conclusion