Quantitative shock measurement of olivine in ureilite meteorites

Ureilites are the second most abundant achondrite meteorite group, yet they are from an unknown source. They are ultramafic in composition and have undergone thermal metamorphism as well as shock deformation since their formation. In this work, olivine grains from six monomict ureilites, Northwest Africa 7059, Elephant Moraine 96042, Shisr 007 (all similar to shock level S3), Northwest Africa 2221 (S3-S4), Larkman Nunatak 04315 (S5), and Allan Hills A81101 (similar to S6) and one polymict ureilite, Elephant Moraine 87720 (S5), were examined by in situ micro-X-ray diffraction (XRD). It is observed in this study that with increasing shock, the samples develop more complicated 2-D XRD patterns from diffracted spots, to streaks, to asterism, and "spotty" Debye rings, and these patterns correspond to shock metamorphic effects observed by optical microscopy from undulatory extinction to mosaicism and annealing or recrystallization textures. These 2-D XRD patterns, representing olivine strain-related mosaicity (SRM), as documented by given lattice planes, were each measured as the sum of the full-width-half-maximum along the Debye ring or chi dimension ( n-ary sumation [FWHM chi]) to quantify the shock-related deformation recorded in the olivine grains. In each meteorite, all olivine lattice planes show similar measured n-ary sumation (FWHM chi) values for the diffraction streaks. This work demonstrates an increasing trend of average n-ary sumation (FWHM chi) in degrees of arc, 4.0 +/- 1.7 degrees, 4.1 +/- 1.4 degrees, 3.7 +/- 1.8 degrees (all S3), 4.9 +/- 2.1 degrees (S3-S4), 7.0 +/- 3.5 degrees (S5), 13.4 +/- 5.7 degrees (S6), and 7.5 +/- 3.3 degrees (polymict, S5), which agrees well with petrographic observations. This quantitative method contributes to a more comprehensive shock classification system specifically for ureilites, complementing recently published shock classification systems for olivine-bearing rocks.