Quantitative determination of the shock stage of L6 ordinary chondrites using X-ray diffraction

Abstract

The shock stages of 14 L6 ordinary chondrites are estimated using the random X-ray diffraction patterns of polished thin section samples and the in-plane rotation method. The mean lattice strains and grain size factors for olivine and orthopyroxene are determined from the analyses based on the Williamson–Hall plots, which depict the tangent Bragg angle and integral breadth β. The lattice strain in olivine, eOl, is distributed from ~0.05% to ~0.25%, while that in orthopyroxene, eOpx, is distributed from ~0.1 to ~0.4%, where we selected the isolated peaks of olivine and orthopyroxene. The olivine peaks have Miller indices of (130), (211), (222), and (322), while the orthopyroxene peaks have Miller indices of (610), (511), (421), (631), and (12.1.2). The intercept for integral breadth β0O1 and β0Opx for the Williamson–Hall plots correlates with the grain size of the constituent minerals. The grain size is proportional to the inverse of β0 since the β intercept increases with the shock stage. Introducing a new parameter, –ε/log β0 for olivine (0.04–0.16) and orthopyroxene (0.07–0.32) reveals a clear relationship between them: –εOpx/log β0Opx = –0.01+ 2.0 (–εOl/log β0O1) (R > 0.9). In addition, the isolated peak of plagioclase (201) systematically changes as the shock stage increases, suggesting the progress of amorphization (maskelynitization). Another parameter, (I/FWHM)Pl(201) reveals additional relationships: –εOl/log β0O1 = 0.14(±0.01) − 5.2 × 10−5 (±5.7 × 10−6) × (I/FWHM)Pl(201), and –εOpx/log β0Opx = 0.25(±0.04) − 8.9 × 10−5 (±2.6 × 10−5) × 10–5 × (I/FWHM)Pl(201). These three parameters systematically change with the shock stage, suggesting that they are suitable shock barometers. The present method is useful to evaluate the shock stage of L6 chondrites and potentially enables quantitative shock stage classification for stony meteorites.