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

Comparative 40Ar/39Ar, (U-Th)/Pb, and Pb-Pb Dating of Planetary Impactite Samples

Paper number: 125-3
Program: Technical Sessions
Time: 2:05 PM – 2:20 PM

Authors

Abstract

In the early years of lunar geochronology, the 40Ar/39Ar dating method played an outsized role in influencing our view of the impact history of the Moon. More recently, the majority of published planetary geochronology studies of returned or meteoritic impactite samples have focused on dating accessory minerals using (U-Th)/Pb chronometers (including Pb-Pb). In published cases where 40Ar/39Ar and (U-Th)/Pb or Pb-Pb data are available for the same sample, the 40Ar/39Ar chronometer usually yields substantially younger dates, sometimes hundreds of millions of years younger. A common interpretation of this observation has been that the 40Ar/39Ar data are incorrect, always indicating substantial radiogenic Ar loss, while the (U-Th)/Pb data are intrinsically robust and point to the true formation age of the impactite. We argue that properly explaining the discrepancy requires a more nuanced assessment of the data. We have applied the 40Ar/39Ar and (U-Th)/Pb methods to samples from a variety of terrestrial impact sites, finding that they are largely consistent, though the Ar dates are usually less precise. Extraterrestrial samples yield more complex results. Here we present high-spatial-resolution laser ablation 40Ar/39Ar dates for lunar impactite minerals and melt phases showing that some impact melt breccias yield dates probably reflecting the age of the melt matrix. In other breccias, melt matrix 40Ar/39Ar dates instead represent the timing of one or more post-melting thermal events related to later, nearby impacts. (U-Th)/Pb and Pb-Pb dates of accessory minerals in these rocks are also complicated to interpret unambiguously in most cases; some have Pb isotopic systematics largely unaffected by the impact event (yielding ages that are too old), some may yield the impact age, and some may also be partially reset during subsequent impacts. We urge that future studies employ the application of multiple chronometers as standard practice in order to establish the formation and long-term thermal evolution of planetary impactites.

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

doi: 10.1130/abs/2025AM-412863

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