Discerning Lunar Pyroclastic and Impact Glasses via Raman Spectroscopy

Abstract

AbstractIt is a challenge to distinguish the various lunar glasses, which are characterized by complex origin (volcanic eruption or impact melting) and large compositional variation (picritic, basaltic, or feldspathic). In this work, we present the Raman spectra (200–1,500 cm−1) of a series of lunar glasses (i.e., pyroclastic glasses [PGs], mare impact glasses [MIGs], and highland impact glasses [HIGs]), which were recognized in lunar breccia meteorites Northwest Africa 7948, 10480, 12384. As the Raman technique is sensitive to the composition of glasses, our results show that different types of lunar glasses exhibit various Raman peak positions (RPP) in 400–600 cm−1 and 800–1,200 cm−1 spectral ranges. Specifically, RPP for PGs commonly shift upward to high frequency in the 400–600 cm−1 spectral range (i.e., > 570 cm−1), compared with the RPP for MIGs and HIGs (i.e., < 570 cm−1). In addition, MIGs and HIGs can be distinguished by the relationship of their RPP in 400–600 and 800–1,200 cm−1 ranges (i.e., for MIG, RPP800–1,200 > 1,510 − RPP400–600; for HIG, RPP800–1,200 < 1,510 − RPP400−600). Based on these observations, new Raman diagnostic criteria were proposed to discern lunar pyroclastic and impact glasses (although there may be instances of lunar glasses with different compositions than those measured here). This method offers a nondestructive, quick, and low‐cost way for distinguishing different types of lunar glasses in laboratory (e.g., for the future returned‐soils sampled by Chang'E‐5).