Raman and Infrared Microspectroscopy of Experimentally Shocked Basalts

Petrographic, micro-Raman and micro-Fourier transform infrared spectral analyses of experimentally shocked basalt and basaltic andesite samples (up to ~63 GPa) indicated that progressive amorphization of plagioclase feldspar with increasing pressure resulted in detectable spectral variations that mimic those of the experimentally shocked, plagioclase-dominated rocks analyzed in earlier studies. However, variations in starting sample composition and/or shock propagation through the minerals and mesostasis within these basaltic rocks resulted in variable distribution of shock effects within the samples, as manifested in their infrared and Raman spectra. In particular, the presence of primary igneous glass within the starting samples subdued the observed effects because of the spectral similarity between the glass and shocked plagioclase (maskelynite and plagioclase glass). This caused ambiguity in distinguishing between amorphization resulting from shock effects and that associated with primary glass, particularly for the more hypocrystalline basaltic andesite (~30% glass) compared to the basalt sample (~10% glass). Nonetheless, the correlation between shock pressures and key spectral parameters (Raman peak ratios, infrared reflectance peak band heights) for plagioclase-bearing sample areas terminated in the ~25–30 GPa pressure range. This was consistent with the amorphization onset pressures of andesine and reflected the increasing presence of maskelynite at these higher pressures. Higher spatial resolution mapping using these techniques could provide better insight regarding the influence of grain boundaries and mineralogical variations on shock propagation in fine-grained, glassy, basaltic samples. This would help refine the nature and magnitude of shock propagation effects in naturally shocked basaltic samples analyzed during in situ investigations of planetary surfaces.