Uranium-lead abundances and isotopic studies in the chondrites Richardton and Farmington

Abstract

207Pb/ 204Pb- 206Pb/ 204Pb whole meteorite isochrons for Richardton (H5) and Farmington (L5) are presented and give Pb-Pb ages of 4.545 ± 0.010 and 4.620 ± 0.010 Ga respectively (errors ± 2σ). The Pb-Pb isochron for Farmington passes below the Can˜on Diablo troilite composition, which may therefore not be the initial Pb composition for this meteorite. All samples show an apparent excess radiogenic lead for single-stage (closed-system) evolution when Can˜on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess Pb cannot be explained by terrestrial contamination. There is no unique isotopic composition for initial lead that yields concordant ages at 4.55 Ga for all samples for either meteorite. The data likewise cannot be reconciled to Can˜on Diablo initial lead through any of the conventional two- and three-stage evolution models. The apparent excess Pb, with respect to a Can˜on Diablo troilite composition and a single-stage closed-system history, and the apparent inhomogeneous initial Pb isotopic compositions, appear to be real. This may be an indication that the U-Pb systems in these meteorites are disturbed, but this disturbance cannot be described consistently by any of the conventional episodic evolution models.