Phosphate ages in Apollo 14 breccias: Resolving multiple impact events with high precision U-Pb SIMS analyses

The U-Pb systems of apatite and merrillite grains within four separate Apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry. No systematic difference was identified between the ²⁰⁷Pb/²⁰⁶Pb ages of the apatites and merrillites. A combined ²⁰⁷Pb/²⁰⁶Pb age of 3927±2Ma (95% conf.) is determined for three of these samples (14305,103: 3926±4Ma; 14306,150: 3926±6Ma; 14314,13: 3929±4Ma). By combining these data with the ages previously obtained for zircons in Apollo 12 impact melt breccia fragments and the lunar meteorite SaU 169, a weighted average age of 3926±2Ma (95% conf.) is obtained, which is attributed to the formation of the Imbrium basin. An age of 3943±5Ma is determined for the fourth breccia (14321,134), which is similar to ages of 3946±15Ma and 3958±19Ma, obtained from several older phosphates in 14305,103 and 14314,13. The weighted average of these three older ages is 3944±4Ma (95% conf.). This is indistinguishable to the age (3938±4Ma; 2σ) obtained for a different Apollo 14 impact melt breccia in a previous study. After investigating likely sources for this older ~3940Ma age, we conclude that the Humorum or Serenitatis basin forming events are likely candidates. The potential identification of two large impact events within ~15Myrs has important implications for the rate of lunar bombardment around 3.95-3.92Ga. This study demonstrates the importance of high-precision age determinations for interpreting the impact record of the Moon, as documented in lunar samples.