Comparison of two different infrared spectral analysis based evaluation of impact induced shock events of the Chelyabinsk meteorite

Abstract

Shock metamorphic processes in minerals were observed in the recently fallen Chelyabinsk meteorite and compared with two infrared laboratory methods: DRIFT and ATR based spectral analysis types, attached to a Fourier Transformational Infrared Spectrometer (FTIR). Both of ATR and DRIFT methods have advantages and disadvantages for shock stage identification. However, while the ATR method has wide literature background, the DRIFT method was not used in shock metamorphic research yet, hence this study links ATR spectra with DRIFT spectra to obtain reference for such IR methods in the analysis of shock metamorphism. The results show that shock-based spectral changes could be better followed by the decreasing number of peaks with DRIFT than with ATR data; while the situation is opposite for FWHM values, which better characterizes the shock consequences from ATR data. The DRIFT analyses made from bulk meteorite material. Hence more mineral phases were identified than by separate measurements of ATR. The ATR spectra includes rather major vibration, But with DRIFT the minor bands could be also measured. Hence the other shock indicator, the disappearance of minor bands with increasing shock stage could be better followed by DRIFT. The important shock metamorphic indicator, the FWHM could be identified rather from ATR spectra, as the DRIFT spectra includes small sized peaks, and the FWHM values are near to the spectral resolution of DRIFT spectra. Both of ATR and DRIFT methods show shift of IR bands to higher wavenumber with increasing shock stage. Moreover, the DRIFT method shows the split of some minor bands in case of feldspar and pyroxene due to dimerization of silicate structure.With increasing shock stage one band disappeared and a new one appeared in case of feldspar DRIFT data, while feldspar did not emerge using ATR observations. In case of olivine the major bands shifted by +2… +5 cm−1 using DRIFT and ATR spectra also. In case of pyroxene one band disappeared and new forbidden bands emerged. These results provide starting points to develop shock estimation using DRIFT method too, what is a much more available at various laboratories. However, it is worth mentioning that DRIFT method usually identifies more mineral phases than ATR because of the difference in spatial resolution.