125 Lunar Science and Exploration in the Artemis Era

Current and anticipated science return in the areas of geology, composition, and interior of the Moon from current lunar-focused studies and upcoming payloads expected to land on the Moon in the next decade.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 214C

Searching for Concordancy between the U-Pb and Ar-Ar Radiometric Systems in Various Types of Apollo Impactites

Paper number: 125-4
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

One of the main findings of previous geochronologic studies of Apollo samples is that the Earth-Moon system might have experienced a spike of mass flux input at 3.75−3.95 Ga according to 40Ar-39Ar dating. Yet, due to the relatively limited geographic distribution of Apollo sampling sites, an alternative interpretation is that the clustering of conventional 40Ar-39Ar dates may simply represent the approximate age of the Imbrium impact event.

Imbrium is the largest, youngest impact basin on the near side. However, the Imbrium age is still poorly defined. At present, there is a ca. 50 Myr difference in estimates of Imbrium age based on 40Ar/39Ar dates of ~3870 Ma and 207Pb/206Pb dates of ~3920 Ma. This age difference could be explained by two possibilities: (1) the 40Ar/39Ar and 207Pb/206Pb date clusters represent two different events; (2) the two systems record the same event but behave differently.

In this study, we compare the behaviors of the Ar-Ar and Pb-Pb radiometric systems in various types of lunar impactites from different landing sites. The Apollo rocks we studied include basaltic impact melt rock 14276-48, micropoikilitic impact melt rocks 15357-9 and 62235-38, and shocked granulite 79215-68. The 40Ar/39Ar dates are from Michael et al. (2018), and we used secondary ion mass spectrometry to acquire 207Pb/206Pb dates of accessory minerals (baddeleyite = Bdy, phosphates = Phs).

The 40Ar/39Ar dates are consistent among three impactites. Apollo 14276 has consistent 40Ar/39Ar (3876 ± 17 Ma) and 207Pb/206Pb (3866 ± 3 Ma, Bdy + Phs) dates, while the 207Pb/206Pb dates of 15357 (3931 ± 4 Ma, Bdy + Phs), 62235 (3920 ± 9 Ma, Bdy + Phs) and 79215 (3918 ± 4 Ma, Phs) are 50 ± 10 Myr older than their 40Ar/39Ar dates. Our data show that there was a major impact event at ~3870 Ma, and the K-Ar and U-Pb systems both recorded this event. The 207Pb/206Pb dates of 15357, 62235 and 79215 represent another impact event at ~3920 Ma, most likely Imbrium. We tentatively conclude that the aforementioned 40Ar/39Ar and 207Pb/206Pb date clusters of Apollo samples represent two different events, and 40Ar/39Ar clusters may represent the Orientale event.

References: Michael G. et al. (2018) Icarus 302, 80-103.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413517

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