High‐resolution microstructural and compositional analyses of shock deformed apatite from the peak ring of the Chicxulub impact crater

Abstract

The mineral apatite, Ca 5 (PO 4) 3 (F,Cl,OH), is a ubiquitous accessory mineral, with its volatile content and isotopic compositions used to interpret the evolution of H 2 O on planetary bodies. During hypervelocity impact, extreme pressures shock target rocks resulting in deformation of minerals; however, relatively few microstructural studies of apatite have been undertaken. Given its widespread distribution in the solar system, it is important to understand how apatite responds to progressive shock metamorphism. Here, we present detailed microstructural analyses of shock deformation in~560 apatite grains throughout~550 m of shocked granitoid rock from the peak ring of the Chicxulub impact structure, Mexico. A combination of high-resolution backscattered electron (BSE) imaging, electron backscatter diffraction mapping, transmission Kikuchi diffraction mapping, and transmission electron microscopy is used to characterize deformation within apatite grains. Systematic, crystallographically controlled deformation bands are present within apatite, consistent with tilt boundaries that contain the <c> (axis) and result from slip in <10 10> (direction) on f1 120g (plane) during shock deformation. Deformation bands contain complex subgrain domains, isolated dislocations, and low-angle boundaries of~1°to 2°. Planar fractures within apatite form conjugate sets that are oriented within either { 2110g, {2 1 10g, { 1 120g, or 11 20 È É. Complementary electron microprobe analyses (EPMA) of a subset of recrystallized and partially recrystallized apatite grains show that there is an apparent change in MgO content in shock-recrystallized apatite compositions. This study shows that the response of apatite to shock deformation can be highly variable, and that application of a combined microstructural and chemical analysis workflow can reveal complex deformation histories in apatite grains, some of which result in changes to crystal structure and composition, which are important for understanding the genesis of apatite in both terrestrial and extraterrestrial environments.

Characterization of shocked quartz grains from Chicxulub peak ring granites and shock pressure estimates

“…2019; Cox et al. 2020). Kinkbanding is common in biotite, muscovite, and chlorite, and is also observed, to a lesser extent, in plagioclase and quartz.…”

Impact Earth: A review of the terrestrial impact record

“…All rock types; tectosilicates (quartz, feldspars), nesosilicates (garnet, titanite), zircon, apatite. (Cox et al, 2020;L. Ferrière and Osinski, 2012;French, 2004;French and Koeberl, 2010;Stöffler and Langenhorst, 1994;Timms et al, 2019) In many cases, the original isotropic material has annealed over time and the PDFs are discerned as parallel lines of microscopic inclusions and are described a "decorated PDFs" (Fig.…”

Asymmetric shock deformation at the Spider impact structure, Western Australia

“…2017; Cox et al. 2020). Microstructures such as planar fractures and planar deformation bands have been described in zircon grains from many impact structures, but to date, have not been demonstrated to represent diagnostic evidence of impact (e.g., Kovaleva et al.…”