Compositional and Geochronological Constraints on the Late Heavy Bombardment

Details Compositional and Geochronological Constraints on the Late Heavy Bombardment Compositional and Geochronological Constraints on the Late Heavy Bombardment

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.4603c&link_type=abstract

American Astronomical Society, DPS meeting #39, #46.03; Bulletin of the American Astronomical Society, Vol. 39, p.504

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Radiometric dating and compositional clustering of lunar impact-melt rocks form the backbone of the lunar cataclysm hypothesis. Precise age determinations of Apollo and Luna impact-melt rocks, lunar meteorite impact-melt clasts, lunar soil spherules, and lunar glass fragments define the classic formulation of the late heavy bombardment: a sharp peak at 3.9 Ga, a steep decline after 3.9 Ga perhaps only 20-200 Myr long, and few, if any, positively identified rocks of impact origin prior to 4.0 Ga. Geological associations between compositional groups of impact-melt rocks and specific impact basins imply that five large basins formed on the Moon within 200 Myr. A counterargument says that all, or nearly all, the 3.9-Ga impact melt rocks are products of the Imbrium basin-forming impact. It is still a matter of debate whether this hypothesis can accommodate the multiple groups of impact melt that are resolvable from each other in age and in trace-element composition. The 3.9 Ga age peak and subsequent steep decline are not well mirrored in meteorite data. Radiometric ages in ordinary chondrites and HED meteorites peak around 3.9 Ga but ages older and younger than 3.9 Ga are common. We have a single shock age for Martian meteorites: ALH 84001 crystallized at 4.50 Ga but was shocked at 3.92 ± 0.04 Ga. Differences in relative impact velocity, impact-melt production, and sampling rate could explain some of the differences between the meteorite and lunar records. One way to anchor the early end of the lunar flux would be to directly sample the impact-melt sheet of a large lunar basin distant from Imbrium, such as the South Pole-Aitken basin, where a large amount of melt probably still resides on the basin floor and could be directly sampled by a human or robotic mission. B.A.C. is supported by NASA Cosmochemistry grant #NNX07AI57G.

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