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

A large fraction of the H chondrites (similar to 45%) have exposure ages around 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 is 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 chondrites with reported bulk exposure ages between 4 and 10 Ma; all the H4's selected 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, falls, However, when we increase the database by including recent precise Ne ages, 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 cosmic-ray-produced He-3/Ar-38 ratios is bimodal with two clusters at similar 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 significantly 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.