Micro-X-ray diffraction assessment of shock stage in enstatite chondrites

A new method for assessing the shock stage of enstatite chondrites has been developed, using in situ micro-X-ray diffraction (mu XRD) to measure the full width at half maximum (FWHM chi) of peak intensity distributed along the direction of the Debye rings, or chi angle (chi), corresponding to individual lattice reflections in two-dimensional XRD patterns. This mu XRD technique differs from previous XRD shock characterization methods: it does not require single crystals or powders. In situ mu XRD has been applied to polished thin sections and whole-rock meteorite samples. Three frequently observed orthoenstatite reflections were measured: (020), (610), and (131); these were selected as they did not overlap with diffraction lines from other phases. Enstatite chondrites are commonly fine grained, stained or darkened by weathering, shock-induced oxidation, and metal/sulfide inclusions; furthermore, most E chondrites have little olivine or plagioclase. These characteristics inhibit transmitted-light petrography, nevertheless, shock stages have been assigned MacAlpine Hills (MAC) 02837 (EL3) S3, Pecora Escarpment (PCA) 91020 (EL3) S5, MAC 02747 (EL4) S4, Thiel Mountains (TIL) 91714 (EL5) S2, Allan Hills (ALHA) 81021 (EL6) S2, Elephant Moraine (EET) 87746 (EH3) S3, Meteorite Hills (MET) 00783 (EH4) S4, EET 96135 (EH4-5) S2, Lewis Cliff (LEW) 88180 (EH5) S2, Queen Alexandra Range (QUE) 94204 (EH7) S2, LaPaz Icefield (LAP) 02225 (EH impact melt) S1; for the six with published shock stages, there is agreement with the published classification. FWHM chi plotted against petrographic shock stage demonstrates positive linear correlation. FWHM chi ranges corresponding to shock stages were assigned as follows: S1 < 0.7 degrees, S2 = 0.7-1.2 degrees, S3 = 1.2-2.3 degrees, S4 = 2.3-3.5 degrees, S5 > 3.5 degrees, S6-not measured. Slabs of Abee (EH impact-melt breccia), and Northwest Africa (NWA) 2212 (EL6) were examined using mu XRD alone; FWHM chi values place both in the S2 range, consistent with literature values. Micro-XRD analysis may be applicable to other shocked orthopyroxene-bearing rocks.