Induced thermoluminescence (TL) measurements have identified a subset of Antarctic (H) chondrites which have significantly higher induced TL peak temperatures than other Antarctic (H) chondrites or modern falls. This group was not produced by weathering or shock, but appears to have been produced by differences in thermal history. This group also consists of meteorites with (\sim 8-\mathrm{Myr}) cosmic ray exposure ages and relatively large ({ }^{3} \mathrm{He}) / ({ }^{22} \mathrm{Ne}) and ({ }^{21} \mathrm{Ne} /{ }^{22} \mathrm{Ne}) ratios suggestive of small degrees of shielding compared to other Antarctic and non-Antarctic (H)-chondrites. Metallographic cooling rate determinations confirm the unusual thermal history of this subset, H5 chondrites in this subset having cooling rates of (\sim 100{ }^{\circ} \mathrm{C} / \mathrm{Myr}), compared to (\sim 10-20^{\circ} \mathrm{C} / \mathrm{Myr}) for other (\mathrm{H} 5) chondrites.
Possible origins for this unusual subset include formation in a near-surface layer on a (H)-chondrite parent body, formation on a more rapidly cooling parent body which was not directly related to the parent body currently dominating the (H)-chondrite meteorite flux, or by impact processing during the (8-\mathrm{Myr}) event. Thermoluminescence data from meteorites from six Antarctic collection sites indicate that this subset dominated the (H)-chondrite flux (\sim 300,000) years ago, but ceased to be represented in the flux (\sim 20,000) years ago. The generally smaller size of these meteoroid bodies may have allowed them to evolve more rapidly to Earthcrossing orbits but also resulted in their rapid destruction in space. These data show that the (\boldsymbol{H})-chondrite flux has changed over a relatively short period of time in terms of average meteoroid size and thermal history. 1993 Academic Press, Inc.