Primordial129Xe in meteorites

Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurböle (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurböle. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of¹²⁹Xe, where we infer ¹²⁹/Xe¹³²Xe= 0.56 ? 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of¹²⁹Xe from decay of¹²⁹I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low¹²⁹Xe seems to be to suppose that before decay to¹²⁹Xe r-process production at mass 129 condensed into dust as¹²⁹I, and that Arapahoe's parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped¹²⁹Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.