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 A r-Ar ages around 500 Myr shows that the L chondrite parent body suffer ed a major impact 500 Myr ago. We infer from constraints on the therma l evolution of impact heated rocks after the 500-Myr event and the hig h abundance of shocked L chondrites that the parent body was catastrop hically 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 sufficientl y high velocities to put them into resonances and that this initiated the orbital evolution which resulted in the high flux of L chondrite m eteoroids impacting the Earth at present. It appears likely that this was a ''slow-track'' type of dynamical evolution, with most objects av oiding drastic resonant changes of orbital eccentricity, and undergoin g 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 evoluti on, since fragments inserted into it in the main belt frequently have their eccentricities raised to values of about 0.4, sufficient for Mar s-crossing but not for Earth-crossing orbits; on the other hand, recen t numerical work has shown that the 3 : 1 mean motion resonance leads almost always to a fast-track evolution, ending up with a solar collis ion or a hyperbolic ejection within a few Myr. (C) 1996 Academic Press , Inc