Radiation ages and gas-retention ages of carbonaceous chondrites and unequilibrated ordinary chondrites

Helium, Ne, and Ar were determined by mass spectrometry in 25 carbonaceous chondrites, and in 12 unequilibrated ordinary chondrites. The occurrence of primordial inert gases, and the possibility of severe diffusion losses in certain carbonaceous chondrites, leads to the following considerations: (a) U, Th-He ages cannot be estimated for the C1 and C2 types of carbonaceous chondrites; (b) only 'minimum' 3Hec ages can be estimated for the C1 and C2 types of carbonaceous chondrites; and (c) 38Arc ages cannot be estimated for any of the meteorites, except Karoonda. The radiation ages of the carbonaceous chondrites and of the UOC are not grossly different from those of the ordinary chondrites. However, the following trends can be noted: the ages of the C1 and C2 carbonaceous chondrites are, on average, shorter than those of the ordinary chondrites - none of the former is greater than 15 m.y.; and four, perhaps five C2 carbonaceous chondrites, Boriskino, Cold Bokkeveld, Haripura, Nawapali, and Nogoya, may come from a single collision 0.1 - 0.6 m.y, ago, with the most likely date of 0.2 ± 0.1 m.y. There is a rough correlation between the types of carbonaceous chondrites and K-Ar ages - C1 meteorites give ages of 1 - 1.5 x 109 yr, C2 meteorites of 0.6 - 4.6 x 109 yr, and (C3 + C4) meteorites of 2 - 4 x 109 yr. The great majority of the UOC seem to have cooled to below 200-300 deg. C some 2 . 5 - 4.5 x 109 yrs ago. Among the L3 chondrites there is a rough anti-correlation between retention age and degree of recrystallization. This seems to imply that the more highly equilibrated chondrites come from greater depth inside the parent object than the UOC. (author)