U-PB ages of volcanic and impact glass beads from Apollo 11 soil

Introduction: Impact and volcanic glasses from the lunar regolith carry unique information about magmatic and impact processes on the Moon and contain small quantities of U (100s of ppb to few ppm), allowing to utilize U-Pb isotope systematics to constrain the timing of eruptions and impact events on the Moon. To demonstrate this capability, new ages have been determined for volcanic and impact glass spherules from Apollo 11 sample 10084 using Secondary Ion Mass Spectrometry (SIMS). The improved precision of these new age determinations of volcanic glasses is comparable to that obtained by modern Pb-Pb dating of mare basalts [1], enabling comparison of timing of formation of basaltic flows and violent eruptions on the Moon. Dating of impact glasses helps to identify numerous periods of apparent increased frequency of impact events on the Moon.

Volcanic glasses: The studied volcanic glasses (174 spherules) are mostly represented by high-Ti (8-13%), lowAl (5-6%) and mostly low-K compositions (0.05-0.07%; and only three spherules exceeding 0.2%) if mare basalts
classification [2] is applied. Four spherules belong to very low TiO2 (VLT) group (TiO2 0.38 to 0.56%). All high-Ti glasses show similar behavior of their U-Pb systems and define a single U-Pb age of 3666 ± 11 Ma (MSWD=1.01, probability=0.45). Precision of age determined for VLT glasses is lower, due to small number of glasses (analyses), but demonstrates that VLT glasses are distinctly younger at 3235 ± 66 Ma (MSWD=1.4, probability=0.18).

Three chemical sub-groups of high-Ti, low-Al, low-K mare basalts at the Apollo 11 landing site identified by previous work [2] have range of ages from ~3855 Ma (sub-group B2), to ~3740 Ma (sub-group B3) to ~3690 Ma (subgroup B1) [1]. Investigated glasses appear to be 10-20 Ma younger than the latter mare basalts group. The youngest group of mare basalt samples from Apollo 11 site is represented by high-Ti, low-Al, high-K samples (group A), which show ages ranging from ~3580 to ~3530 Ma [1]. Investigated VLT glasses with the age between 3.3 and 3.2 Ga are significantly younger, suggesting that volcanism around Apollo 11 the landing site continued for 200-300 Ma past the latest volcanic episode revealed by the available mare basalts samples. Small number of VLT glasses identified in the sample, may indicate declining volume of erupting material.

Impact glasses: Investigated impact glass spherules can be split into highlands and mare glasses based on their chemical composition (e.g., utilizing CaO/ Al2O3-MgO/ Al2O3 diagram [3]). The latter comprise the majority of impact glasses population in the investigated sample (nearly 200 beads from 230 studied glasses), which reflects geological setting of Apollo 11 landing site surrounded by basaltic flows and suggests that most, if not all, mare impact glasses are of local origin. As a result, chemical variations observed in this group of glasses reflect composition of the basalts and overlying regolith, which appears to contain ~66% of mare basalt material, ~20% of feldspathic highlands material and ~8% of KREEP [4]. Presence of highlands material in the soils can explain their elevated Al2O3 content (~14%, as compared to less than 11% in Apollo 11 mare basalts) but may not account for Al2O3 concentrations in mare impact glasses, with approximately half showing concentrations between 15 and 21%. The increase in Al2O3 is accompanied by decreasing SiO2, Na2O and K2O indicating volatile loss experienced by significant proportion of mare impact glasses [5]. This is also supported by elevated Th/U ranging between 6 and 11 in about 60% of the glasses and reflecting volatile behavior of U during impacts. Highlands glasses show markedly high Al2O3 and SiO2 content, which is difficult to explain by either Apollo 11 basalts or local regolith as targets. Consequently, it is likely that these glasses originated outside of basaltic surroundings of Apollo 11 landing site.

Comparison of U-Pb age distribution patterns of Apollo 11 mare glasses and published U-Pb ages of glasses from Apollo 14 and Chang’e 5 soils [5], [6], indicate different proportions of glasses of different ages present in soil samples from different landing sites. Both Apollo 11 and Chang’e 5 beads populations appear to be dominated by glasses with ages younger than 1 Ga, while Apollo 14 soil contains significant proportion of glasses in 3.7-3.6 Ga time interval. Nevertheless, peaks in age distribution patterns around 5-8 Ma, 40-60 Ma, 100-130 Ma, 400-500 Ma and 1000-1100 Ma appear to be common in impact glass populations from all landing sites.

References: [1] Snape J. F. et al. (2019) Geochimica et Cosmochimica Acta 266:29–53. [2] Neal C. R. and Taylor L. A. (1992) Geochimica et Cosmochimica Acta 56:2177–2211. [3] Zeigler R. A. et al. (2006) Meteoritics & Planetary Science 41:263–284. [4] Korotev R. L. and Gillis J. J. (2001) Journal of Geophysical Research: Planets 106:12339–12353. [5] Nemchin A. A. et al. (2022) Geochimica et Cosmochimica Acta 321:206–243. [6] Long T. et al. (2022) Science advances 8:eabq2542.