PU-244 FISSION XENON IN THE MOST PRIMITIVE METEORITES

The plutonium-244/xenon-136 ages of the Murchison, Murray and Orgueil meteorites have been calculated from the existing xenon isotope data a nd the uranium contents. The CI carbonaceous chondrite Orgueil, which is considered to be among the most primitive - in the sense of the lea st altered - sample of the solar system known to man, appears to have started to retain its xenon more than 5,000 million years ago, when th e ratio of Pu-244 to U-238 in the solar system was as high as (0.5 +/- 0.1) (atom/atom) and the CM carbonaceous chondrites Murchison and Mur ray started to retain their xenon about 4,940 million years ago, when the Pu-244 to U-238 ratio was about 0.17 (atom/atom).