A record of impacts preserved in the lunar regolith

The absolute chronology of meteoroid impacts on the Moon is largely quantified by only a few 40Ar/39Ar “plateau ages” of rocks thought to be associated with specific impact events (Stöffler et al., 2006). We demonstrate a more broadly applicable approach by using high-resolution 40Ar/39Ar thermochronometry to investigate the physical conditions responsible for partially reset K-Ar systems in lunar rocks. Seven rocks from Apollo 16 regolith sample 63503 have plateau ages of either 3.9 billion yr (Ga) or 4.2 Ga and all experienced varying degrees of partial resetting. Concordance between diffusion kinetics and the degree of resetting among all samples shows that these observations are best explained by a heating event 3.3 Ga ago that lasted between ∼ 103 s (at ∼ 600 °C) and ∼ 20 yr (at ∼ 300 °C). We conclude that partial resetting of the K-Ar systems in these samples record an impact event ∼ 3.3 Ga ago that mixed several preexisting ejecta units in the Cayley Plains. If partially reset 40Ar/39Ar ages of other lunar highland samples also constrain the timing of late-stage reheating associated with impact events, they constitute an additional record of impacts preserved in the lunar regolith. A review of existing datasets from this perspective reveals that episodic pulses in the impactor flux in the inner system are common, and most likely related to dynamical events in the asteroid belt or outer Solar System.