COMPARATIVE MICRO-X-RAY DIFFRACTION STUDY OF SHOCK METAMORPHOSED OLIVINE AND PYROXENE IN UREILITE METEORITES

Ureilites are primitive achondrite meteorites that consist mostly of large olivine and pyroxene grains which typically show triple junction grain boundaries, similar to those seen in terrestrial metamorphic rocks. All ureilites show shock features that are familiar in meteorites, such as silicate darkening and undulatory extinction. Shock metamorphism is thought to be ubiquitous in ureilites, having played an essential role in modifying ureilite parent body composition and textures. In moderate to highly-shocked ureilites there is, however, a textural inconsistency between olivine and pyroxene, where olivine shows features attributed to high shock such as recrystallized or annealed textures, whereas pyroxene exhibits low to moderate shock textures as evidenced by undulatory extinction. The controversy is unresolved by the current shock classification scheme based in optical petrography. Micro-X-ray diffraction (µXRD) provides further information on crystallographic structures and their deformation. The quantitative µXRD analysis of 2D diffraction images allows for the measurement of full-width-half maximum on peaks integrated from XRD patterns along the chi direction (FWHM𝜒). Previous studies showed a positive correlation between FWHM𝜒 and shock degree. We have examined olivine and pyroxene in three ureilite samples, Northwest Africa 2221, Larkman Nunatak 04315, and Allan Hills A 81101, which represent moderate, moderate-to-high, and high degrees of shock, respectively. We have compared olivine and pyroxene’s textures optically as well as their 2D XRD patterns and conducted quantitative µXRD analysis. A minimum of 20 olivine and 15 pyroxene grains were measured in each sample. Olivine exhibited streaks, asterism and spotty Debye rings whereas pyroxene showed streaks and extremely elongated streaks. Despite the optical textural inconsistency, olivine and pyroxene showed consistent trends in FWHM𝜒 by quantitative µXRD analysis of the shock-disrupted grains. This study shows: 1) pyroxene and olivine form different textures upon shock loading, however, both minerals provide consistent results as individual shock recorders; 2) quantitative µXRD analysis is helpful to examine strain-related deformation as a complementary tool alongside the traditional optical petrographic approach.