Preferred orientation distribution of shock‐induced planar microstructures in quartz and feldspar

Abstract

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselk€ a, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre-impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP-ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal-stage analyses performed on petrographic thin sections. Our results show that such shock-related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.

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

“…Shock features were observed in alkali‐feldspar and plagioclase (i.e., PFs filled with opaque minerals and also some possible PDFs; see Pittarello et al. 2020), titanite, and apatite (with different types of planar microstructures; Timms et al. 2019; Cox et al.…”

“…Other accessory minerals, including monazite, ilmenite, rutile, chalcopyrite, cobaltoan pyrite, stolzite/raspite, galena, uranothorite, and uranothorianite, were also detected during our SEM survey. Shock features were observed in alkali-feldspar and plagioclase (i.e., PFs filled with opaque minerals and also some possible PDFs; see Pittarello et al 2020), titanite, and apatite (with different types of planar microstructures; Timms et al 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.…”

Shock effects in feldspars: An overview

“…EBSD can also be used to determine a family of planes to which the trace of a plane most likely belongs. However, to complete the orientation measurement in 3D, a "dip" measurement is acquired using the U-stage (Pittarello and Koeberl, 2017;Pittarello et al, 2020aPittarello et al, , 2020c or by making a FIB trench perpendicular to the trace of the plane and measuring the dip off the resultant image (Pickersgill and Lee, 2015). Thus far, using the FIB technique for this purpose has been demonstrated only for quartz, however the same principle should work for feldspars.…”

Chicxulub impact structure, IODP‐ICDP Expedition 364 drill core: Geochemistry of the granite basement

“…Impact-induced shock metamorphic features are apparent in most rock-forming minerals, that is, multiple sets of PFs, feather features (FFs), in average 2.8 sets of planar deformation features (PDFs), undulose extinction, and occasional kinkbanding in quartz grains (for details, see Feignon et al 2020); in alkali-feldspar and plagioclase (i.e., PFs filled with opaque minerals and also some possible PDFs; see Pittarello et al 2020), titanite, and apatite (with different types of planar microstructures; Timms et al 2019;Cox et al 2020). Kinkbanding is common in biotite, muscovite, and chlorite and also observed, to a lesser extent, in plagioclase and in quartz (Figs.…”