The I-Xe chronometer

Xe-129, from the decay of the now-extinct 16.7 Ma I-129, accumulates in iod ine-bearing sites and since most iodine host phases are secondary, the I-Xe system is typically a chronometer for post-formational processes. The vali dity of the I-Xe chronometer is confirmed by comparison with Pb-Pb ages on phosphate and feldspar separates from twelve meteorites. Phosphate separate s are found to be concordant with Pb-Pb for all six samples in which useful I-Xe data were obtained. Feldspar is a better iodine host than apatite in H chondrites, typically providing good I-Xe isochrons. These too are concor dant with the Pb-Pb ages of the corresponding phosphates for five out of si x feldspar separates. The exception is Allegan whose feldspar yields one of the oldest I-Xe ages observed, similar to those for CI and CM magnetites. We attribute this to a more primary mineralization, predating the secondary phosphate from which the comparison Pb-Pb age was obtained. Absolute I-Xe ages, found using the reported Pb-Pb age of Acapulco phosphate provide an a bsolute I-Xe age of 4.586 +/- 0.002 Ga for both Shallowater and Bjurbole ir radiation standards. This allows relative I-Xe ages to be interpreted in th e context of absolute ages.