Laboratory thermal emission spectroscopy of shocked basalt from Lonar Crater, India, and implications for Mars orbital and sample data

Abstract

Whereas the thermal infrared (TIR) spectra of specific minerals and/or igneous and metamorphic rocks have been compared to data of the Martian surface, TIR data of the impactite products of the shock metamorphism of basalt have not been examined in detail. The effects of shock on the thermal infrared spectrum of Deccan basalt are described here. Sample collection at Lonar Crater, India, yielded four classes of shocked basalt: classes 1, 2, 4, and 5. Spectral features attributed to labradorite in TIR spectrum of unshocked Deccan basalt are absent in the spectrum of class 2 shocked basalt. Petrography confirms that labradorite has been replaced by diaplectic glass (maskelynite). The TIR spectrum of class 2 shocked basalt is nearly identical to that of the Los Angeles shergottite. The addition of experimentally shocked plagioclase feldspars as TIR end-members improves fits and lowers RMS errors for the deconvolution of the TIR spectrum of the maskelynite-bearing impactite, and the correct mineralogy is chosen to within 5%. Class 4 shocked basalt contains vesiculated plagioclase glass (due to more heat) and highly fractured augites. Two class 5 spectral types have primary Si-O stretching vibrations at lower wave numbers than Si and Si-K glass end-members commonly used in analyses of TIR data, agreeing with lower SiO2 abundances (∼50%) determined from two techniques. At least three differences in spectral features exist between the two class 5 spectral types that we attribute to incomplete melting in the class 5A samples based on comparison to the class 4 spectrum. These samples and their TIR spectra represent excellent analogs for Martian shocked basalt and new lithologic end-members for use in spectral libraries used to analyze TIR data from Mars.