Argon-39 and tritium in meteorites

Abstract

The radioactive isotopes argon-39 and tritium are measured in a number of iron and stone meteorites. The stable isotope helium-3 is also measured and the argon-38 content is estimated from the helium-3 content. The cosmic ray exposure ages obtained from the argon-39 and argon-38 are: 4·2 × 10 8 years for the Sikhote-Alin iron meteorite, 4·5 × 10 8 years for the Treysa iron meteorite, 0·3 × 10 8 years for the Pitts iron meteorite, 5 × 10 8 years for the Norton County achondrite, and 0·8 × 10 8 years for the St. Michel chondrite. The ratio of tritium to argon-39 radioactivities at the time of fall is 140 ± 20 for the Norton County meteorite, 56 ± 15 for the St. Michel meteorite, 6 ± 1 for the Treysa meteorite, and less than 1 2 for the Sikhote-Alin meteorite. For the stone meteorites this ratio is consistent with the production probabilities and the hypothesis of the constancy of cosmic rays. For the iron meteorites this is lower than expected from the production probabilities. Either the cosmic ray bombardment is not constant or tritium loss from diffusion occurs for the iron meteorites. Also reported is the lack of argon-39 and tritium in a number of undated iron meteorite falls, which indicates that they fell more than 1500 years ago. Only one undated iron meteorite fall, Washington County, gave a small trace of argon-39, indicating that it fell about 1000 years ago. Possible interpretations of the cosmic ray exposure ages are discussed.