Ordinary chondrite shock stage quantification using in situ 2‐D X‐ray diffraction of olivine

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Alexandra N. Rupert, Phil J.A. McCausland, Roberta L. Flemming

Meteoritics & Planetary Science
Version of Record online:21 September 2020

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“Ordinary chondrites record shock metamorphism resulting from hypervelocity collisions on small bodies, and underpin the petrographic assessment of shock stage, a scale of progressive stages of shock metamorphism from S1 (unshocked) to S7 (shock melted). In this work, olivine grains in 11 L and LL chondrites (S1–S5) were investigated in thin section and hand sample using in situ two‐dimensional X‐ray diffraction (2‐D XRD). Olivine grains were measured under a 300 µm X‐ray beam for multiple lattice reflections, by measuring diffracted streak length along the chi (χ) dimension (Debye ring dimension), to examine their strain‐related mosaicity. Olivine strain‐related mosaicity was observed to increase with greater shock deformation, with more complex multi‐peak streaks apparent at higher shock levels. The full width at half maximum (FWHMχ) of the simple peak shapes along χ was measured to quantify petrographic shock stage for comparison with that described optically. The average FWHMχ values for simple peaks in olivine show an increase with increasing shock stage: S1 (0.44°± 0.06°), S2 (0.58°± 0.11°), S3 (0.67°± 0.15°), S4 (0.76°± 0.13°), and S5 (0.86°± 0.12°). This method complements optical petrographic methods and offers a ±1 shock stage accuracy in determining shock stage. In particular, 2‐D XRD analysis of strain‐related mosaicity allows quantitative analysis of shock stage in shock‐darkened samples that are difficult to work with petrographically, and for hand samples without need for thin section preparation.”

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