On the calculation of cosmic-ray exposure ages of stone meteorites

Abstract

Abundances of cosmic ray-produced noble gases and 26Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 10 6 yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic 3He, 21Ne, and 38Ar in chondrites to an adopted ‘average’ shielding: 22Ne 21Ne = 1.114. The measured unsaturated 26Al concentrations and the calculated equilibrium 26Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10 −8 cm 3/g 10 6 yr) are: 3 He = 2.45, 21 Ne = 0.47, and 38 Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.