Raman and Infrared Microspectroscopy of Experimentally Shocked Basalts

Abstract

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.

A revised shock history for the youngest unbrecciated lunar basalt—Northwest Africa 032 and paired meteorites

“…This relationship between grain size and amorphization pressure in plagioclase has been demonstrated experimentally (Johnson et al. 2020).…”

“…Complete amorphization of plagioclase in NWA 032 and partial amorphization in paired meteorites may be reconciled by the fine-grained nature of NWA 032 groundmass, where conversion to glass is facilitated by fine grain sizes owing to the shorter length scale of dislocations. This relationship between grain size and amorphization pressure in plagioclase has been demonstrated experimentally (Johnson et al 2020).…”

Quantifying Shock Effects of Mars Sample via Micro‐FTIR Spectra of Plagioclase

“…Micro-FTIR (Fourier Transform Infrared Spectroscopy) is a powerful tool for investigating planetary materials (Jaret, Johnson, et al, 2018;Johnson et al, 2020;Kereszturi et al, 2015Kereszturi et al, , 2017Kereszturi et al, , 2023Morlok et al, 2010Morlok et al, , 2012Morlok et al, , 2019Morlok et al, , 2020Zeng et al, 2019). Previous studies have shown that the micro-FTIR spectra are sensitive to the structure of plagioclase, including unshocked plagioclase, partially shocked plagioclase, and high-pressure maskelynite Jaret et al, 2015;Martin et al, 2017;Zeng, Joy, et al, 2018).…”

Nano‐FTIR Investigation of the CM Chondrite Allan Hills 83100

“…Mid‐infrared (MIR) spectroscopy has been used to great success toward quantitatively determining the mineralogy of geologic samples (Clark & Roush, 1984; Hapke, 1981; Hunt & Salisbury, 1970; Johnson et al., 1983; King & Ridley, 1987; Mustard & Pieters, 1987). It has been employed in a variety of contexts from determining bulk composition of powdered samples (Friedlander et al., 2015; Lane et al., 2011; Shirley & Glotch, 2019) to spectroscopic imaging of rock sections via micro‐Fourier transform infrared (micro‐FTIR) (Farrand et al., 2016, 2018; Jaret et al., 2015, 2018; Johnson et al., 2020; Yesiltas et al., 2019; Yesiltas & Kebukawa, 2016).…”