Rb-Sr-analyses of Apollo 16 melt rocks and a new age estimate for the Imbrium basin: Lunar basin chronology and the early heavy bombardment of the moon

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Rb-Sr-model ages on 7 impact “glass-bombs” and internal Rb-Sr isochrons for two crystalline impact melt rocks from the Apollo 16 collection have been determined. The post-Cayley “glass-bombs” with model ages between 4.75 ± 0.45 AE and 3.97 ± 0.08 AE can be classified according to their calculated single stage (87Rb86Sr)I-ratios: 67728, 67946, and 67627.8 point to a KREEP-free precursor terrain—the Descartes highlands; whereas 63566, 67567, 67627.10 and 67629 are derived from the more heterogeneous Cayley plains. The very feldspar-rich impact melt rock 65795, which is compositionally similar to the group of feldspathic microporphyritic melt breccias (FM-suite), yields a crystallization age of 3.81 ± 0.04 AE (2σ; λ87Rb = 1.42−11yr−) and ISr of .69929 ± 3. In the subophitic impact melt rock 60635.5 two ages were recorded: One isochron with 3.87 ± 0.02AE and ISr of 0.69920 ± 3 gives the crystallization age of a fine-grained internal clast. At 3.75 ± 0.03 AE this fragment was incorporated into the host lithology of 60635 which is a member of the Anorthositic Noritic Melt Rock (ANMR) group. This group has a crystallization age of about 3.75 AE. The ANMR rocks and several samples of Apollo 14 crystalline impact melt rocks belong to a suite of “young crystalline impact melt rocks” ranging in age from 3.71 to 3.81 AE. These ages are in conflict with the canonical age of the Imbrium event (3.85 AE). It is argued that the young Apollo 14 and 16 melt rocks cannot represent solid exotic fragments or melt splashes ejected from post-Imbrium local or distant craters. Instead, these rocks are derived from coherent impact melt sheets of pre-Imbrian craters with diameters larger than 5–7 km. They are interpreted as clasts of the Fra Mauro and Cayley formation breccia deposits, and therefore their ages are used as an upper limit for the age of the Imbrium event. We suggest that the Imbrium basin and the related Fra Mauro and Cayley formations were formed 3.77 ± 0.02 AE ago and could be even as young as 3.75 AE. As a consequence, we adopt 3.92 ± 0.03 AE, 3.87 ± 0.03 AE, and 3.84 ± 0.04 AE as ages for the Nectaris, Serenitatis, and Crisium basins, respectively, in agreement with the relative crater densities measured on the ejecta blankets of these basins (Wilhelms, 1984). The proposed age sequence leads to an average formation interval for the observed 12–13 Nectarian basins of 7 to 14 m.y. leaving ~30 pre-Nectarian basins of unknown age. These facts suggest that there is no “late terminal lunar cataclysm” in the sense of a culmination of the lunar impact rate at ~3.8 AE ago. Rather, the observations are compatible with a steeply and steadily decreasing flux of impactors (Hartmann, 1980) in the sense of an “early heavy bombardment” which started at the time of the moon's accretion and terminated around 3.75 AE ago.