DENGLI H3.8 CHONDRITE NOBLE-GASES - EVIDE NCE OF THE REGOLITH NATURE OF BRECCIATION AND OF 2-STAGE COSMIC IRRADIATION

The noble gases He, Ne, Ar, Kr, and Xe were investigated in samples of the Dengli chondrite. Slightly fractionated gases of the solar wind ( He-4/Ne-20 to 414) were found to be present. The substantial variation s in the contents of frapped and spallogenic argon components may be a ttributed to the presence of fragments differing in solar gas concentr ations and in cosmic radiation exposures. The meteorite K/Ar age value s obtained (mean value 3.82 Ga) indicate there were no recent impact o r thermal events. The exposure age values for different meteorite samp les, calculated according to Ne-21(cosm) and Ar-38(cosm) vary from 6.4 to 22.0 Ma and correlate with the concentration of Ar-36(trap). Such a relationship in the case of the Dengli chondrite, which does not hav e a dark-light structure, could be indicative of a mixture of fragment s irradiated in the regolith layer of the parent body of the meteorite with nonirradiated material and of a brief exposure of the breccia af ter the break-up of the parent body. The data obtained for Kr and Xe, together with the data in the literature for Al-26, likewise testify t o a short (ca. 1 Ma) exposure age of the meteorite itself. It is concl uded that the Dengli chondrite breccia is of a regolith nature and tha t the breccia material has a two-stage cosmic exposure history.