Condensation time of the solar nebula from extinct I in primitive meteorites.

Mineral separates from five carbonaceous chondrites were dated by extinct 16 million year (129)I, in an attempt to establish the condensation time of the solar nebula. Two Fe(3)O(4) or Fe(3)O(4)-FeS samples from the Murchison and Orgueil meteorites are older than any other material dated thus far, and apparently formed within 2 x 10(5) years of each other. The great age, close isochronism, and primitive nature of the samples suggest that the event recorded was the condensation stage of the solar nebula. It provides a suitable zero point for the chronology of the early solar system. The (129)I/(127)I ratio during condensation of the nebula was (1.46 +/- 0.04) x 10(-4). The recrystallized C4 chondrite Karoonda began to retain (129)Xe 1.8 +/- 0.5 million years after the above event. This short cooling time implies rapid aceretion (</=1 million years) and a shallow origin (</=10 km) below the surface of its parent body.