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, Shişr 007 (all ˜shock level S3), Northwest Africa 2221 (S3-S4), Larkman Nunatak 04315 (S5), and Allan Hills A81101 (˜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 χ dimension (∑[FWHMχ]) to quantify the shock-related deformation recorded in the olivine grains. In each meteorite, all olivine lattice planes show similar measured ∑(FWHMχ) values for the diffraction streaks. This work demonstrates an increasing trend of average ∑(FWHMχ) in degrees of arc, 4.0 ± 1.7°, 4.1 ± 1.4°, 3.7 ± 1.8° (all S3), 4.9 ± 2.1° (S3–S4), 7.0 ± 3.5° (S5), 13.4 ± 5.7° (S6), and 7.5 ± 3.3° (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.

中文翻译：

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橄榄石陨石中橄榄石的定量冲击测量

Ureilites 是第二丰富的无球粒陨石群，但它们的来源不明。它们的成分为超镁铁质，自形成以来经历了热变质和冲击变形。在这项工作中，来自 6 个单体 ureilites 的橄榄石粒，西北非 7059，象碛 96042，Shişr 007（全部 ~ 震级 S3），西北非 2221（S3-S4），Larkman Nunatak 04315（S5）和 Allan Hills 8 〜S6）和一种多聚脲石，大象冰碛87720（S5），通过原位显微X射线衍射（XRD）进行检查。在这项研究中观察到，随着冲击的增加，样品从衍射点、条纹、星状和“斑点”德拜环形成更复杂的二维 XRD 图案，这些图案对应于通过光学显微镜观察到的从波动消光到镶嵌现象和退火或再结晶纹理的冲击变质效应。这些代表橄榄石应变相关镶嵌性 (SRM) 的 2-D XRD 图案，如给定晶格平面所记录的，每个都被测量为沿德拜环或 χ 维度的全宽半最大值之和（∑[FWHMx ]) 来量化橄榄石颗粒中记录的与冲击相关的变形。在每个陨石中，所有橄榄石晶格平面都显示出类似的衍射条纹测量 ∑(FWHMx) 值。这项工作展示了平均 ∑(FWHMχ) 的增加趋势，以弧度为单位，4.0 ± 1.7°、4.1 ± 1.4°、3.7 ± 1.8°（所有 S3）、4.9 ± 2.1°（S3–S4）、7.0 ± 3.5° (S5)、13.4 ± 5.7° (S6) 和 7.5 ± 3.3° (polymict, S5)，这与岩相观察非常吻合。