40 Ar/ 39 Ar ages of impacts involving ordinary chondrite meteorites

Abstract

Abstract Impact ages have now been determined for c. 100 ordinary chondrites, providing information about impacts in the asteroid belt throughout the history of the Solar System. Most notably, there is evidence for (a) impact events in the first 100 Ma of Solar System history, during the accretion of material; (b) an increase in impact ages produced between 3500 and 4100 Ma, presumably related to the suspected ‘lunar cataclysm’; (c) many L chondrites being degassed during the disruption of their parent body slightly less than 500 Ma ago; and (d) at least one younger event in the H chondrites. There are no 40 Ar/ 39 Ar ages that clearly match the cosmic ray exposure ages of any meteorites, which means that the collisions that liberated the fragments we now have as meteorites did not produce time–temperature histories capable of degassing radiogenic 40 Ar. Since there is no way to know a priori whether an ordinary chondrite impact melt is the result of a recent or ancient event, much of the progress will continue to come from reconnaissance studies of shocked ordinary chondrites.