My collaboration with the TRR 170 focuses on finding the sample evidence for the ancient bombardment history on the Moon. The lunar crater chronology is best established for the period 4–3 billion years ago (Ga), due to the availability of Apollo impact breccias. However, it remains poorly constrained for ages >3.9 Ga. Nevertheless, the growing modelling, remote sensing, and geochronological arguments advocate for increased impacting rate on the Moon much before ~3.8–3.9 Ga, casting into doubt the onset and duration of the classical Late Heavy Bombardment (LHB). The recent focus in determining precise impact ages has shifted to in situ dating of accessory minerals (phosphates, baddeleyite, zircon) which yielded a common occurrence of ~4.2–4.3 Ga ages across different Apollo landing sites, in addition to 3.91–3.94 Ga events, suggesting that several basin forming impacts occurred >4 Ga. In our further attempts, we will focus on refining the timing of these ancient basin-forming impacts by coordinated microstructural analyses and in situ U-Pb/Pb-Pb systematics of accessory minerals in Apollo 16 and 17 breccias. This approach has been demonstrated to yield robust data in support of the contextualized interpretation of the complex ages commonly overprinted by younger events (e.g., by Imbrium).
Chronometric investigations