Ages of mare basalts on the lunar nearside

The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size-frequency distribution data for 139 spectrally and mor phologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size-frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size-frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5-2.0 b.y., starting at about 3.9-4.0 b.y. and ceasing at similar to2.0 b.y., (2) most basalts erupted during the late Im brian Period at about 3.6-3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin-filling volcanism probably started within similar to 100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre-Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.63.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3-3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin-filling volcanism (1.5-1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even similar to1.0 b.y. longer than in Tranquillitatis and Humboldtianum.