Compositions and ages of mare and highlands impact glasses in lunar soil 10084

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

Methods: 223 glass spherules from the 75-250 μm size fraction of Apollo 11 soil 10084 were identified as impact based on major element compositions measured by electron microscopy. Trace element compositions were measured by laser ablation inductively-coupled plasma mass spectrometry. These data were compared with the compositions of pristine Mg-suite rocks (Apollo 15 and 17), ferroan anorthositic suite rocks (Apollo 15 and 16), and KREEP-rich rocks (Apollo 15 and 17). U-Th-Pb isotopic compositions were measured by Secondary Ion Mass Spectrometry (SIMS).

Results and Discussion: Chemical compositions of the spherules were classified as mare or highlands based on their CaO/Al2O3 and MgO/Al2O3 ratios [1]. 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 that are typical for Apollo 11 basalts [2]. Most of the mare glasses have compositions corresponding to mixtures of basaltic compositions with variable proportions of highlands material and KREEP (e.g., 11-19 wt.% Al2O3, 3-10 wt.% TiO2, 11-20 wt.% FeO, 0.7-4.1 ppm Th) and form 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 [3].

Highlands glasses appear to contain predominantly Mg-suite troctolitic and noritic components, although petrologic characteristics of the immediate source rocks are undetermined, e.g., they could be impact breccias rather than pristine igneous rocks. Many highlands glasses with Mg-suite affinities form a trend towards basaltic components with higher TiO2 and FeO contents relative to pristine rock compositions (0.2-3.5 wt.% TiO2, 3-12 wt.% FeO). Minor ferroan anorthositic and KREEP-rich compositions are also present. These compositions are difficult to explain by either Apollo 11 basalts or local regolith as targets. Therefore, it is likely that the highlands glasses originated either from outside of the basaltic terrane proximal to the Apollo 11 landing site or crust underlying the basaltic flows [3].

Both groups show clear evidence of moderately volatile lithophile element depletion in their systematic trends to lower 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 ranging from 5 to 14 in mare glasses and from 8 to 14 in highlands glasses, possibly reflecting U volatility during impacts. The High-Ti, low-Al mare glasses that most closely correspond to the local basalts do not have volatile depleted compositions, suggesting a local provenance and delivery by relatively modest size impacts. Si-depletion in the 75-250 μm fraction studied here overlaps but does not extend in the extreme
compositions observed in the <20 μm of 10084, where the most depleted highlands glass bead has ~62 wt.% Al2O3 and ~4 wt.% SiO2 [4] compared to 32 and 38 wt.%, respectively, in our study. This might imply that finer glass
spherules form in higher-energy areas of the impact plume or greater diffusive loss from the finer-grained beads.

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

References: [1] Zeigler R. A. et al. (2006) Geochimica et Cosmochimica Acta 70:6050–5067. [2] Neal C. R. and Taylor L. A. (1992) Geochimica et Cosmochimica Acta 56:2177–2211. [3] Korotev R. L. and Gillis J. J. (2001) Journal of Geophysical Research 106:12339–12353. [4] Keller, L. P. and McKay, D. S. (1992) LPSC XXIII:673-674. [5] Long T. et al. (2022) Science advances 8:eabq2542.