Comparative zircon U-Pb geochronology of impact melt breccias from Apollo 12 and lunar meteorite SaU 169, and implications for the age of the Imbrium impact

The ages of zircons from high-Th impact-melt breccias (IMBs) from meteorite Sayh al Uhaymir (SaU) 169 and from rock fragments in soil samples from Apollo 12 have been determined using the SHRIMP-II ion microprobe. The IMBs are very similar to each other in chemistry, mineralogy and texture, and the zircons from the KREEP-rich (high-Th) crystalline impact melt have similar U and Th contents and identical ages, within uncertainties, of 3920±13 (2σ) Ma (SaU 169) and 3914±7 (2σ) Ma (Apollo 12). The age results support the idea that the high-Th IMBs (Apollo 12 and SaU 169) formed in the same impact event. The similarity of composition and age suggest that SaU 169 and the high-Th IMB fragments of Apollo 12 originated from the same area of the Procellarum KREEP Terrane. We interpret the age of zircon grains in the Apollo 12 high-Th IMB as a precise and direct determination of the age of the Imbrium impact. This age is significantly older than the commonly cited age of 3.85Ga but is similar to recent determinations from SIMS U-Pb dating of Apollo 14 apatite grains and with anticipated revision of ages by ⁴⁰Ar- ³⁹Ar and ⁸⁷Rb- ⁸⁶Sr. The present zircon ²⁰⁷Pb- ²⁰⁶Pb age is the first direct zircon age determination of the Imbrium impact event from an Apollo sample. Previous measurements of zircon ages of Apollo IMBs have recorded events pre-dating the Imbrium basin-forming event.