Other
Scientific paper
Mar 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icar..150..181g&link_type=abstract
Icarus, Volume 150, Issue 1, pp. 181-188 (2001).
Other
7
Scientific paper
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 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 36Cl-36Ar 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. 36Cl-36Ar 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 3He/38Ar ratios is bimodal with two clusters at ~15 and ~9, the latter apparently due to solar heating effects and a quasi-continuous 3H diffusion loss from metal. 3H 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.
Caffee Marc W.
Graf Th.
Marti Kurt
Nishiizumi Kuni
Ponganis K. V.
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