Meteoritic, asteroidal, and theoretical constraints on the 500 Ma disruption of the L chondrite parent body

The high abundance of heavily shocked and degassed L chondrites with Ar-Ar ages around 500 Myr shows that the L chondrite parent body suffered a major impact 500 Myr ago. We infer from constraints on the thermal evolution of impact heated rocks after the 500-Myr event and the high abundance of shocked L chondrites that the parent body was catastrophically disrupted. The slow cooling rates of some shocked and degassed L chondrites (0.01-1 degrees C year(-1)) show that they were derived from kilometer-sized impact-heated fragments or rubble piles that were ejected from near the impact point. We suggest that the catastrophic dispersion of the parent body provided some fragments with sufficiently high velocities to put them into resonances and that this initiated the orbital evolution which resulted in the high flux of L chondrite meteoroids impacting the Earth at present. It appears likely that this was a ''slow-track'' type of dynamical evolution, with most objects avoiding drastic resonant changes of orbital eccentricity, and undergoing a slow random walk in orbital element space, driven by a sequence of random encounters with Mars and, later on, with the Earth. The nu(6) secular resonance provides a plausible mechanism to start this evolution, since fragments inserted into it in the main belt frequently have their eccentricities raised to values of about 0.4, sufficient for Mars-crossing but not for Earth-crossing orbits; on the other hand, recent numerical work has shown that the 3 : 1 mean motion resonance leads almost always to a fast-track evolution, ending up with a solar collision or a hyperbolic ejection within a few Myr. (C) 1996 Academic Press, Inc