Dating collisional events: Cl-36-Ar-36 exposure ages of H-chondritic metals

A large fraction of the H chondrites (similar to 45%) have exposure ages ar ound 7 Ma, presumably the time at which a large collision produced numerous fragments that were subsequently placed in Earth-crossing orbits, Although this exposure age peak is observed for all petrographic types, the maxima observed among subtypes H3-H6 differ. The fall time ratios of H5 chondrites do not show the excess of afternoon falls relative to morning falls that i s typical for all other ordinary chondrites, indicating that a subgroup of the H5's experienced a distinct orbital evolution. We report results of Cl- 36- Ar-36 exposure ages obtained for metal separates of 6 H4 and 10 H5 chon drites with reported bulk exposure ages between 4 and 10 Ma; all the H4's s elected are afternoon falls, while the H5's are morning falls. If there is structure in the similar to7-Ma event, this selection was expected to give a maximum resolution. Cl-36-Ar-36 exposure ages of H4 p.m. falls show a cluster at similar to7.6 Ma, while a wider cluster at similar to7.1 Ma is found for the H5 a,m, fall s, However, when we increase the database by including recent precise Ne ag es, the H4 p,m. falls define a peak at 7.6 +/- 0.2 Ma, while a shifted peak for the H5 a,m. falls is observed at 7.0 +/- 0.2 Ma. The distribution of c osmic-ray-produced He-3/Ar-38 ratios is bimodal with two clusters at simila r to 15 and similar to9, the latter apparently due to solar heating effects and a quasi-continuous H-3 diffusion loss from metal, H-3 loss is signific antly more frequent among H5 a,m. falls, consistent with the interpretation that a subgroup of H5 chondrites experienced a distinct orbital evolution, The "anomalous" H5 group may represent a small surviving tail of fragments that were directly injected into a resonance after a collision 7.0 Ma ago. (C) 2001 Academic Press.