Dating Collisional Events: 36Cl–36Ar Exposure Ages of H-Chondritic Metals
✍ Scribed by Th. Graf; M.W. Caffee; K. Marti; K. Nishiizumi; K.V. Ponganis
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 81 KB
- Volume
- 150
- Category
- Article
- ISSN
- 0019-1035
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✦ Synopsis
A large fraction of the H chondrites (∼45%) have exposure ages around 7 Ma, presumably the time at which a large collision produced numerous fragments that were subsequently placed in Earthcrossing 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 36 Cl-36 Ar 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 ∼7-Ma event, this selection was expected to give a maximum resolution.
36 Cl-36 Ar exposure ages of H4 p.m. falls show a cluster at ∼7.6 Ma, while a wider cluster at ∼7.1 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 3 He/ 38 Ar ratios is bimodal with two clusters at ∼15 and ∼9, the latter apparently due to solar heating effects and a quasi-continuous 3 H diffusion loss from metal. 3 H 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.