Ages of mare basalts on the lunar nearside

The chronology of lunar volcanism is based on radiometric ages determined f rom Apollo and Luna landing site samples, regional stratigraphic relationsh ips, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on n ew crater size-frequency distribution data for 139 spectrally and mor pholo gically defined basalt units which are exposed in six nearside impact basin s (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbr ium). Crater size-frequency distribution measurements are a statistically r obust and accurate method to derive absolute model ages of unsampled region s of the Moon. Compared to crater degradation ages, crater size-frequency a ges, 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 durin g the late Im brian Period at about 3.6-3.8 b.y., (3) significantly fewer b asalts were emplaced during the Eratosthenian Period, (4) basalts of Copern ican 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 for mation 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 Tranquillitat is) as well as in the Humboldtianum basin, which is of Nectarian age, the d istribution of surface ages is clearly dominated by only a single peak in t he number of erupted units at 3.63.8 b.y., (2) in the younger basins (Humor um, 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.