Provenance and Pb isotopic ages of lunar volcanic and impact glasses from the Apollo 17 landing site

Abstract

Glassy spherules found in the lunar regolith can be used to investigate the volcanic and impact history of the Moon. Here we report the first study combining petrography with microbeam geochemical data for major-element, trace-element and Pb isotopic compositions of individual spherules that were separated from a sample of regolith collected at the Taurus-Littrow Valley on the Moon. Petrographic characteristics of 216 spherules were established by optical and electron microscopy. About half of these spherules show obvious evidence of formation by meteorite impacts into the lunar regolith, such as relict clasts, schlieren, vesicles, FeNi metal blebs, irregular shapes and heterogeneous nucleation textures. The remaining spherules have textures indicating rapid quenching of melt droplets, including clast-free glass and microcrystalline to vitrophyric textures. These were produced by either impact melting or volcanic fire fountaining. Chemical compositions of clast-poor spherules were determined by electron microprobe and laser ablation ICPMS, and the provenance of each spherule was classified as either feldspathic highlands, mare or mixed highlands-mare. Spherules with feldspathic highlands compositions are droplets of impact melt that provide a regional-scale sampling of regolith and rock compositions in the vicinity of the landing site. A group of picritic, high-TiO₂ mare spherules has a limited range of major and trace-element compositions similar to the famous Apollo 17 orange glass; these are likely to be volcanic in origin. A small number of mare spherules with very low Ti (VLT) and unusually aluminous compositions were also found. We tentatively conclude that the VLT spherules reflect igneous compositions, whereas the aluminous mare spherules are likely to be impact mixtures. We also conducted a reconnaissance LA-ICPMS study of Pb isotopic compositions and U–Th–Pb concentrations to investigate the feasibility of dating individual spherules. ²⁰⁷Pb/²⁰⁶Pb model ages and U–Th–Pb chemical ages of the picritic mare spherules are similar to those of the 3.8–3.5 Ga Apollo 17 mare basalts, consistent with an igneous origin for the glasses. In contrast, impact spherules have systematically younger chemical ages (≤2 Ga). About half of the impact spherules appear to be ≤500 Ma, consistent with the recent discovery of a young spike in the spherule age populations at other landing sites. The abundance of relatively young ages with a predominantly local provenance suggests that most of the impact spherules were produced by small impact events, perhaps related to craters ≤1 km in diameter.

Key Words:

• lunar spherules
• impact melt
• mare basalts
• lunar crust
• radiometric dating
• Apollo 17

Acknowledgements

The data presented here were collected as part of K. Adena's honours research at the ANU. Frank Brink, Bob Rapp and Charlotte Allen provided expert assistance with electron microscopy and laser ablation ICPMS. John Mya prepared an initial separation of the spherules from the bulk regolith. We thank NASA for loaning us the sample of 78481 for this research. Journal reviews by Alexander Nemchin and Ryan Zeigler are greatly appreciated. RSES funding for lab costs is acknowledged.

Additional information

Notes on contributors

M. D. Norman

†Present address: Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. ‡Present address: Centre for Advanced Microscopy, R. N. Robertson Building 46, Sullivans Creek Road, Australian National University, Canberra, Australia, 0200.