207Pb‐206Pb isochron and age of chondrites

Abstract

The isotopic composition of lead was measured in 16 chondrites and 1 achondrite, and the lead contents were determined by isotope dilution. Lead was isolated by volatilization in vacuo and purified by anion exchange procedures. Total procedural blanks (∼10 ng) are small relative to quantities of lead analyzed (≥ 1 μg). The best least squares isochron for the chondrites is (207Pb/204Pb) = (4.718 ± 0.001) + (0.5991 ± 0.0032) (206Pb/204Pb), corresponding to a mean formation age of 4.505 ± 0.008 Gy (using the most recent half‐life values). Individual chondrites yield distinctly different model ages, with a range of ∼50 My about the mean, but this may be due in part to variations in initial lead. The least squares 208Pb/204Pb‐206Pb/204Pb correlation line indicates a mean 232Th/238U atomic ratio of 3.54 for the suite of chondrites. The Norton County achondrite point does not lie on the isochron for chondrites and yields an age of 4.550 ± 0.003 Gy, confirming Rb‐Sr indications that achondrites may have been formed ∼50 My earlier than the chondrites. The chondrite isochron does not pass through the presently accepted value for primordial lead, based on Canyon Diablo troilite, indicating the possible necessity for a more complicated evolutionary model, with the chondrites forming ∼50 My later than that iron meteorite. Volatilization of radiogenic lead during chondrule formation might introduce variations in the initial lead of meteorites formed at about the same time.