Investigation of near-infrared spectroscopic characteristics of plagioclase in the Martian crust, implications from Martian meteorites

The composition of Mars' crust is crucial for reconstructing the internal structure and geological evolution of the planet. Recent observations based on high-resolution near-infrared spectral data have identified plagioclase-bearing geological units on the Martian surface, appearing in multiple distant locations ^[1],[2]. The spectral characteristics imply extremely low content of basic minerals, indicating the potential lithology of ferroan anorthosites ^[1] or felsic rocks ^[2], challenging the classic view that the Martian crust is primarily basaltic. However, thermal infrared spectra suggest that the silica content of previously identified plagioclase outcrops does not match that of felsic rocks on Earth ^[3]. In addition, the characteristic absorption of plagioclase at ~1.25µm has been found in the bulk spectra of rocks containing 30-80 wt% plagioclase, corresponding to a range of feldspar-bearing lithologies ^[4]. The plagioclase-bearing lithology on Mars identified with the characteristic spectral absorption feature remains unclear.

In this study, we analyzed both the visible-near-infrared point spectra and hyperspectral images of a set of Martian meteorites, specifically the basaltic shergottites, which are so far the most representative samples from the Martian crust. An integrated BSE and EDX analysis (TIMA) which characterized the mineralogy, grainsize and texture was performed on the same sample set. We found that all the point spectra of basaltic shergottites contain the ~1.25 µm band, with the potential contribution from the electronic transition of iron in either plagioclase or olivine. Martian olivine, being more iron-rich, is expected to show stronger and wider bands at around 1 µm, with greater contribution from the 0.85 and 1.25 µm band due to Fe²⁺ in the M1 site [5], which overlaps with the distinctive absorption of Fe-bearing plagioclase. Based on the analysis of amplitude ratio and area ratio at 1 µm and 1.25 µm after Gaussian fitting, the olivine-phyric basaltic shergottites have systematically stronger ~1.25 µm band than those without olivine phenocrysts. Meanwhile, the abundances of plagioclase in the samples varying from 9.2% to 36.5% do not correlate with the strength of the ~1.25 µm band. We derived the distinct spectral characteristics of Martian ferroan plagioclase from the hyperspectral image cubes co-registered to the mineral phase maps. Our results suggest that the presence and abundance of iron-bearing plagioclase in the samples cannot be determined solely based on the absorption band centered at ~1.25 µm. Further investigation into the spectral variability of plagioclase would reveal its correlation with composition, grain size and crystallinity. The analysis can be used to reinterpret the orbital spectroscopy data of key areas and provide valuable references for future interpretations of Martian surface remote sensing data.

Reference：

[1] Carter J, Poulet F. (2013), Nature Geoscience, 6(12): 1008-1012;[2] Wray J J et al. (2013), Nature Geoscience, 6(12): 1013-1017;[3] Rogers et al. (2015), Geophysical Research Letters 42.8: 2619-2626;[4] Barthez M et al. (2023), Journal of Geophysical Research: Planets,128(8): e2022JE007680; [5] Isaacson Peter J. et al. (2014), American Mineralogist 99: 467 - 478.