RAPID CHANGES IN THE NATURE OF THE H-CHONDRITES FALLING TO EARTH

We have previously identified a subgroup of Antarctic H chondrites tha t are significantly different from H chondrites among the modern falls in terms of induced thermoluminescence (TL), metallographic cooling r ate, and cosmogenic inert gas contents. Here we examine their terrestr ial and thermal history as apparent in their natural TL and radioactiv e cosmogenic isotope abundances. These meteorites have a tendency towa rds high Al-26 activities and fairly short C-14 and Cl-36 terrestrial ages (generally <100 ka). They also sometimes exhibit unusually high n atural TL levels, which we have previously interpreted as indicating o rbital evolution from perihelia >1.2 AU to similar to 1 AU within the last <10(5) years. We suggest that the nature of the meteorites fallin g to Earth is not independent of time but depends on stochastic events , such as the breakup of parent bodies and recent variations in orbit.