Compositions and ages of mare and highlands impact glasses in Apollo 11 soil

Abstract
Impact glasses in the lunar regolith carry unique information about surface processes, terranes, and element fractionation during impacts, complementing information about magmatic processes derived from volcanic glass beads and rock samples. Here we present major and trace element data, and U-Th-Pb isotopic compositions of impact glasses that define diverse source rocks sampled by impact events, accompanying element fractionation, and the timing of impacts on the Moon.

Chemical compositions of the 223 glass spherules (75-250 μm) were classified as mare or highlands based on their major element ratios (Zeigler et al., 2006). All of these are interpreted as of impact origin. Mare glasses comprise ~86% of the studied particles reflecting the geological setting of the Apollo 11 landing site. However, only 10 of these have the high-TiO2 (>6 wt.%) low-Al2O3 (<11 wt.%) compositions typical for Apollo 11 basalts (Neal and Taylor, 1992). Most of the mare glasses have compositions corresponding to mixtures of basaltic, highlands and K-, REE-, P-rich (KREEP) material forming a continuous trend from basaltic towards highlands compositions. This is consistent with a predominantly regolith source considering that previous studies have found that the Apollo 11 regolith represents a mixture of 66% mare basalt, 20% feldspathic highlands material and 8% of a KREEP-rich component (Korotev and Gillis, 2001).

Both groups exhibit a systematic depletion in moderately volatile lithophile elements (Na, K, and Si and trace elements, e.g., Rb), which is probably related to impact conditions. Many of the depleted glasses also have elevated Th/U ratios (5-14 in mare glasses and 8-14 in highlands glasses), possibly reflecting U volatility during impacts. The high-Ti, low-Al mare glasses with local basalt affinities are not volatile-depleted, suggesting provenance from modest-size local impacts.

The volatile-depleted glasses are characterized by consistently lower U-Pb ages with 60% of them having ages <150 Ma and 19% of them having ages between 150 and 500 Ma. Young U-Pb ages (< 1 Ga) were also reported for glasses mostly formed in impact craters of 1-5 km in diameter from Chang’e 5 soil (Long et al., 2022). This may suggest that even modest impacts can produce significant depletions of moderately volatile elements, producing a clearer geochronologic record of the impact history of the Moon.

References
Korotev R.L. & Gillis J.J., 2001. A new look at the Apollo 11 regolith and KREEP Journal of Geophysical Research 106, 12339–12353.
Long, T. et al., 2022. Constraining the formation and transport of lunar impact glasses using the ages and chemical compositions of Chang’e-5 glass beads. Science advances 8, eabq2542.
Neal C.R. & Taylor L.A., 1992. Petrogenesis of mare basalts: A record of lunar volcanism. Geochimica et Cosmochimica Acta 56, 2177–2211.
Zeigler et al., 2006. The geochemistry and provenance of Apollo 16 mafic glasses. Geochimica et Cosmochimica Acta 70, 6050–5067.