Ages of eight recently fallen meteorites

Abstract

Radioactivities and He, Ne, and Ar stable isotopes were measured in eight recently fallen meteorites, and H3-He3, Na22-Ne22, Al26-Ne21, and Ar39-Ar38 cosmic-ray exposure ages were obtained. Except for Malakal and Nejo, the exposure ages from different pairs of isotopes were concordant. The H3-He3 age for Malakal, 2.4 ± 0.3 m.y., is 50% lower than its Na22-Ne22 age of 3.8 ± 0.4 m.y., and the H3-He3 age for Nejo, 6.4 ± 0.8 m.y., is 50% lower than its Na22-Ne22 age of 9.4 ± 1.3 m.y. Malakal and Nejo also have very low amounts of radiogenic He4 relative to radiogenic Ar40; essentially all the radiogenic He4 was lost from Malakal. The K-Ar40 gas-retention ages ranged from (0.60 ± 0.13) × 109 and (1.0 ± 0.4) × 109 years for Malakal and Wethersfield to greater than 4.0 × 109 years for the other meteorites. The exposure ages of all the meteorites ranged from 2.4 ± 0.3 m.y. for Malakal to 36 ± 3 m.y. for Denver. Havero, a ureilite, had an H3-He3 exposure age of 30 ± 4 m.y. and was rich in primordial Ar36 (planetary-type gas). Three of the meteorites, Dwaleni, Kabo, and Wethersfield, had light-dark structure; however, only Dwaleni was rich in primordial He4 and Ne20 (solar-type gas). The two meteorites with large primordial gas contents had unusual Ar radioactivities in their metal, indicating unusual orbits. The aphelion distance of Havero's orbit was at least 4.3 AU; on the other hand, Dwaleni had a nearly circular orbit.