SHOCK DEFORMATION STAGE CLASSIFICATION IN ORDINARY CHONDRITES USING IN SITU 2D XRD SIMPLE DIFFRACTION PEAK WIDTH MEASUREMENT OF OLIVINE

Ordinary chondrites are the most common meteorites delivered to the Earth, carrying a mineralogical record of the early solar system and shock deformation history of its small bodies. We seek to place the shock classification of ordinary chondrites in a quantitative frame, using 2D micro X-Ray Diffraction (µXRD) data from olivine grains in eleven L and LL chondrites that represent petrographic shock stages S1 – S5 (NWA 2385, NWA 2398 [S1]; NWA 4560, NWA 2040 [S2]; Shelburne, JaH 026 [S3]; Bruderheim, NWA 3087, Lake Labyrinth [S4]; Etter, NWA 1779 [S5]). Diffraction data were obtained using a Bruker D8 Discover XRD with Co Kα radiation (λ=1.78897Ǻ), and beam diameter of 300 µm, done in situ on 8 to 27 targets in each hand sample or thin section. Olivine strain-related mosaicity is observed petrographically to increase with greater shock deformation, and manifests in 2D XRD as increased diffraction peak streak length, with complex multi-peak shapes more apparent at high shock. Simple peaks become more scarce as shock stage increases, with NWA 4560 (S2) having 62% simple peaks (171/288 reflections), whereas Etter (S5) has only 25% simple peaks (20/71 reflections). Here, the full width at half maximum (FWHMᵪ) of the simple peak shapes was measured along the chi (χ) angle of the 2D XRD Debye rings for all available olivine lattice reflections. Where tested, there does not appear to be any significant difference in FWHMᵪ values between the olivine (130), (112), and (131) lattice planes. Using all reflections, the average FWHMᵪ streak length values increase with shock stage: S1 (0.44°±0.04°), S2 (0.58°±0.08°), S3 (0.67°±0.05°), S4 (0.76°±0.05°), and S5 (0.86°±0.04°). Using just the top 25% of FWHMᵪ measurements, lower shock stages are more clearly separated, but S4 and S5 overlap at ~1.05°, suggesting a mechanical upper limit of deformation recorded by simple diffraction peaks in olivine. Simple XRD streak lengthening provides a quantitative basis for shock classification.