COSMOGENIC NUCLIDES AND NUCLEAR TRACKS IN THE CHONDRITE KNYAHINYA

Cosmic-ray produced He, Ne, Ar as well as 10Be, 26Al, 53Mn and nuclear tracks were determined in samples from known positions within the L5 chondrite Knyahinya. Our results show that Knyahinya experienced a single-stage exposure history as a meteoroid of approximately spherical shape. The inferred preatmospheric mass was about 1300-1400 kg, corresponding to a mean radius of 45 cm. Good agreement is observed between the preatmospheric shapes derived from cosmic-ray track densities and from 22Ne 21Ne ratios. The exposure age of Knyahinya is 40.5 Ma. The 10Be 21Ne ratios are constant, although the concentrations of both nuclides increase by more than 20% from positions of lowest to highest shielding. The constancy of this ratio allows refined shielding corrections in computations of exposure ages. High 26Al activities of up to 77 dpm/kg indicate that this nuclide is more efficiently produced by low energy particles than is 21Ne. © 1990.