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).