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 similar 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 (similar to 30% glass) compared to the basalt sample (similar to 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 similar to 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. Plain Language Summary During formation of an impact crater, minerals in rocks experience very high pressures that cause damage to their crystal structures. The primary mineral most easily changed by pressure is plagioclase feldspar, a common mineral in volcanic rocks (basalts) that occur on planetary surfaces. We used two basalts compressed to high shock pressures in the laboratory to analyze how their minerals changed with increasing pressure. We created thin slices of the shocked rocks and viewed their infrared reflectance and Raman spectra under a microscope to document the onset of formation of less crystalline, glassier materials that appeared at high pressures. Similar experiments were conducted previously on plagioclase minerals to document their changes at high pressures. The basaltic rocks analyzed here showed slightly more subdued changes compared to nearly monominerallic plagioclase samples, but overall trends were comparable. This was mainly due to the differences in glass content in the starting samples, because glasses share similar spectral shapes to highly shocked plagioclase. This ambiguity between impact-shocked materials and preexisting glass in volcanic rocks may complicate efforts to distinguish between the two when analyzing rocks with robotic instruments on planetary surfaces, but the techniques shown here demonstrate that such efforts would be valuable.