Shock Metamorphism in Feldspar from the Chicxulub Impact Structure

Planar microstructures are a pre-shock characteristic of most feldspar group minerals, therefore great care must be taken to distinguish shock-related microstructures from those formed prior to impact such as exsolution lamellae, veins, cleavages, and twin planes [e.g. 2]. We investigated the microstructures of Or-rich alkali feldspar phenocrysts from granitoid rocks recovered from the peak ring of the Chicxulub impact structure during IODP-ICDP Expedition 364 [3]. Phenocrysts display microtextures that are characteristic of Or-rich alkali feldspars from undeformed igneous rocks (i.e., albite exsolution lamellae, veins of patch perthite) [4]. Also present are semi-planar microstructures, sub-micrometer in width and frequently developed in more than one orientation. Optically, the two main sets appear to be parallel to exsolution and twin lamellae in some grains, and sub-parallel to cleavage planes in others. TEM work revealed that planar microstructures comprise sub-micrometer wide subgrains with low angle semicoherent boundaries that are parallel to the trace of {110}. Subgrains are absent from veins of patch perthite, which may mean that the fluid-mineral interaction that was responsible for forming the patch perthite postdated shock metamorphism. The planar microstructures resemble strain-induced semicoherent twins, but they are too closely spaced and too narrow to be twins. They likewise show no chemical variation and are therefore not exsolution lamellae. As they appear to be different to microstructures that characterise unshocked feldspars, we suggest that these very narrow subgrains were likely generated by the impact event, but are distinctive from PDFs as they are not straight and preserve no evidence of amorphous material. Work is continuing to determine the microstructure and crystallographic orientation of the other sets of planar elements within these samples