Micro‐FTIR Spectroscopy of Lunar Pyroclastic and Impact Glasses as a New Diagnostic Tool to Discern Them

Abstract

Lunar glasses are characterized by complex origin (i.e., volcanism or impact), compositional diversity (e.g., picritic, basaltic, or feldspathic), and visible/near‐infrared spectra similarity. This makes it is problematic to distinguish different types of lunar glasses in laboratory and remote data. In this study, we present micro‐FTIR (Fourier Transform Infrared) spectroscopy spectra (550–1450 cm–1) of a suite of lunar volcanic origin glasses (i.e., pyroclastic glasses) and impact‐generated glasses (i.e., mare and highland impact glasses), identified in lunar breccia meteorite Northwest Africa 7948. The results show that lunar pyroclastic glasses, mare impact glasses, and highland impact glasses exhibit different FTIR spectral characteristics: (1) the Christiansen Feature positions of pyroclastic glasses are generally at a longer wavelength (i.e., >~8.3 μm equivalent to <~1,205 cm–1) than the spectra of mare and highland impact‐generated glasses (i.e., <~8.3 μm equivalent to >~1,205 cm–1); (2) a relatively strong minor peak was distinctly observed at longer wavelength (~13.5–16.5 μm equivalent to ~600–750 cm–1) for highland impact glasses. Therefore, new supplementary FTIR diagnostic criteria were proposed to discern different types of lunar glasses. Our studies demonstrated that midinfrared spectra could provide an effective tool to non‐destructively and quickly distinguish lunar glasses in laboratory (e.g., for the future Chang'E‐5 returned soils).

Discerning Lunar Pyroclastic and Impact Glasses via Raman Spectroscopy

“…BSE images lunar glasses in NWA 7948 were taken from Zeng et al. (2019). The minerals were labeled: Px, pyroxene; Pl, plagioclase.…”

“…It just needs about 0.5–1 min for the measurement of one spectrum of lunar glasses. (4) Raman spectroscopy have the spatial resolution of ∼1 μm, which is lower than that of micro‐FTIR (∼5–20 μm; Morlok et al., 2020; Zeng et al., 2019).…”

“…In this case, the Raman criteria may not be so diagnostic. Actually, none of the criteria (i.e., textural criteria, geochemical criteria, and micro‐FTIR criteria) are completely diagnostic for distinguishing lunar pyroclastic and impact glasses, when these methods were used individually (Delano, 1986; Zeng et al., 2019). However, previous works indicated that lunar glass can be distinguished when multiple diagnostic criteria are satisfied (e.g., Delano, 1986).…”

“…The analyzed pyroclastic glasses (PGs), mare impact glasses (MIGs), and highland impact glasses (HIGs) are marked in these images. BSE image of NWA 7948 was taken from Zeng et al (2019).…”

“…A wide range of impact-induced mafic and feldspathic glass areas were selected for Raman spectra measurements. The same BSE image of melt pocket-1 in NWA 7948 was taken fromZeng et al (2019). results show that: (1) 30 PG fragments were recognized in breccia meteorites NWA 7948, NWA 10480, and NWA 12384(Figure 2and S1), with high MgO/Al 2 O 3 ratios of >1.25; (2) 26 MIG fragments were recognized in lunar breccias NWA 7948 and NWA 10480(Figures 2 and S2), having relatively low MgO/ Al 2 O 3 ratios of <1.25 and high CaO/Al 2 O 3 ratios of >0.75; (3) 32 HIG fragments and glassy areas were identified in NWA 7948(Figures 2, 3, and S3), which are characterized by low MgO/Al 2 O 3 (<1.25) and CaO/Al 2 O 3 (<0.75) ratios.…”