Cosmic-ray-produced stable isotopes in iron meteorites

Abstract

The isotopic composition of vanadium, calcium, and potassium extracted from several iron meteorites was measured. The isotopes V50, Ca46, Ca43, and K40 were found to be strongly enriched by cosmic-ray-induced spallation reactions. The concentrations of these isotopes were determined by isotopic dilution techniques. The results are compared with data on cosmogenic Sc45 and rare gas isotopes in order to determine the distribution of stable spallation products. The concentration C(A,Z) of a stable spallation product can be given as a function of the total mass loss ΔA = 56 − A by the equation where γ = 1 for cumulative and 0<γ<1 for noncumulative isotopes. This equation is experimentally verified for a wide range of ΔA. The constants k1 and k2 were calculated for each meteorite. Especially from the constants k2 we obtain information concerning the location of the samples inside of the preatmospheric meteorite bodies. Exposure ages were estimated by comparing the concentrations of K40 with those of stable isotopes. Comparison with Cl36-Ar36 exposure ages indicates an essentially constant cosmic-ray bombardment during the time of exposure.