BREAKUP AND STRUCTURE OF AN H-CHONDRITE PARENT BODY - THE H-CHONDRITE FLUX OVER THE LAST MILLION YEARS

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 ∼8-Myr cosmic ray exposure ages and relatively large 3 He/22Ne and 21Ne/22Ne 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 ∼100 °C/Myr, compared to ∼10-20 °C/Myr for other H5 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-Myr event. Thermoluminescence data from meteorites from six Antarctic collection sites indicate that this subset dominated the H -chondrite flux ∼300,000 years ago, but ceased to be represented in the flux ∼20,000 years ago. The generally smaller size of these meteoroid bodies may have allowed them to evolve more rapidly to Earth-crossing orbits but also resulted in their rapid destruction in space. These data show that the H -chondrite flux has changed over a relatively short period of time in terms of average meteoroid size and thermal history. © 1993 by Academic Press, Inc.