COSMIC-RAY EXPOSURE HISTORY OF ORDINARY CHONDRITES

Te histograms are dominated by stochastic events and the continuous supply of asteroidal material can account only for a minor background of the Te histograms. It is difficult, therefore, to infer collisional or dynamical half-lives for chondrite populations from measured Te histograms. An attempt has been made to identify major collisional events among the major classes of ordinary chondrites in order to estimate the frequency of stochastic events. The H-chondrites recorded two major events, ~7 Ma and ~33 Ma ago; among the L-chondrites, events at ~28 Ma and ~40 Ma ago were identified, and the LL-chondrites suffered a collision ~15 Ma ago. Orbital maturity in the inner solar system is documented by the p.m./total fall ratio among observed meteorite falls. All chondrite classes exhibit a uniform ratio of 2/3 , except type H5 chondrites which reveal a ≤0.5 p.m./total fall ratio. The evidence for stochastic fragmentation and delivery of meteoroids to the Earth may suggest that the present predominance of chondritic samples among the meteorite falls may represent an excursion from a more typical population distribution. The records on the thermal history of parent asteroids retained in the form of radiogenic components and 39Ar-40Ar systematics reveal that in some cases major gas losses did occur during the last 0.1-1 Ga. -from Authors