COLLISIONAL HISTORY OF H-CHONDRITES

The cosmic ray exposure histories of ordinary chondrites were recently reviewed (Marti and Graf, 1992). Here, we discuss in detail currently available cosmic ray exposure records and the adopted exposure ages of individual H chondrites. We further present analyses of the distinct histograms of petrographic subgroups and the implications for orbital evolution and collisional records. In addition to the stochastic events 7 to 8 m.y. ago, a peak at 33 Ma represents a statistically significant deviation from an exponential age distribution, and signals a major collisional event at this time. The H6 subgroup also recorded a similar event 24 m.y. ago. These records present significant arguments against a continuous supply of H chondrites from the asteroid belt. We show that the records of fall times for H5s are distinct from other subgroups and that a different orbit for the H5 parent is indicated in collisional events 7 to 8 m.y. ago. The systematics of noble gas losses in the H5 subgroup are consistent with this interpretation. The evidence for stochastic fragmentation and subsequent delivery to Earth of H chondrites suggests that their present predominance among falls may represent a deviation from a representative population distribution.