CONSORTIUM STUDY OF THE UNUSUAL H-CHONDRITE REGOLITH BRECCIA, NOBLESVILLE

The Noblesville meteorite is a genomict, regolith breccia (H6 clasts in H4 matrix). Mossbauer analysis confirms that Noblesville is unusually fresh, not surprising in view of its recovery immediately after its fall. It resembles ''normal'' H4-6 chondrites in its chemical composition and induced thermoluminescence (TL) levels. Thus, at lest in its contents of volatile trace elements, Noblesville differs from other H chondrite, class A regolith breccias. Noblesville's small pre-atmospheric mass and fall near Solar maximum and/or its peculiar orbit (with perihelion <0.8 AU as shown by natural TL intensity) may partly explain its levels of cosmogenic radionuclides. Its cosmic ray exposure age of approximately 44 Ma, is long, is equalled or exceeded by <3% of all H chondrites, and also differs from the 33 +/- 3 Ma mean exposure age peak of other H chondrite regolith breccias. One whole-rock aliquot has a high, but not unmatched, Xe-129/Xe-132 of 1.88. While Noblesville is now among the chrondritic regolithic breccias richest in solar gases, elemental ratios indicate some loss, especially of He, perhaps by impacts in the regolith that heated individual grains. While general shock-loading levels in Noblesville did not exceed 4 GPa, individual clasts record shock levels of 5-10 GPa, doubtless acquired prior to lithification of the whole-rock meteoroid.