Variability of Chelyabinsk meteoroid stones studied by Mössbauer spectroscopy and X-ray diffraction.

The meter-scale variations of material properties of the 20-m sized Chelyabinsk meteoroid are critical for understanding why the meteoroid fragmented the way it did and caused the devastating airburst that sent over 1600 people to the hospital for treatment of glass cuts and minor injuries on February 15, 2013. From a range of differently looking unweathered meteorite fragments that were recovered shortly after the event, these material differences were probed by means of optical and scanning electron microscopy, X-ray diffraction (XRD), and the high velocity resolution Mössbauer spectroscopy. All main and some minor iron-bearing phases were identified on the basis of XRD data and Mössbauer spectra. The Fe2+ partitioning between the M1 and M2 sites in silicate phases was determined independently using XRD and Mössbauer data. Different meteorite fragments show a range of 570–1180 K in the temperature of the Fe2+ and Mg2+ equilibrium distribution between the M1 and M2 sites in olivine, while that in orthopyroxene falls in the range 870–1180 K (these ranges were estimated using both techniques). This fact points out a slightly different thermal history of these minerals before they accumulated in different parts of the Chelyabinsk meteoroid. The Chelyabinsk meteoroid is a fragmental breccia from materials formed at different depths in their parent body, or from materials that experienced different annealing temperatures in impacts. In addition, the fusion crust from two fragments, studied by XRD and Mössbauer spectroscopy, experienced a different thermal history during entry, suggesting that the fragment with mixed light and dark lithologies was located deeper inside the initial meteoroid than the fragment with only light lithology, or fragmented less readily. Mössbauer spectrum of Chelyabinsk LL5 fragment No 2 (left) and differences in the total relative areas for the Mössbauer spectral components corresponding to iron-bearing minerals (variations in the relative iron content in these minerals) for different fragments of Chelyabinsk LL5 meteorite: No 1 and No 1a with light lithology, No 2 and No 2a with mixed light and dark lithologies and No 3 with black lithology: Ol – olivine, OPy – orthopyroxene, CPy – clinopyroxene, Tr – troilite, Chr – chromite, Her – hercynite, Ilm – ilmenite, M – metallic iron alloy (right). Unlabelled Image • The internal differences on a meter-size scale in the 20-m sized Chelyabinsk meteoroid • Physical properties of different Chelyabinsk meteoroid fragments • Different thermal histories of silicates in different parts of the Chelyabinsk meteoroid [ABSTRACT FROM AUTHOR]