Precision measurement of lead isotopes ratios: preliminary analyses from the U.S. mine, Bingham Canyon, Utah

A gas-source mass spectrometer has been constructed for the precise measurement of lead isotope ratios. Sixteen analyses on 4 different preparations of the same galena made over a period of 2 months gave 95% confidence limits (per analysis) of ²⁰⁶Pb/²⁰⁴Pb= 0.080%, ²⁰⁷Pb/²⁰⁶Pb= 0.042%and²⁰⁸Pb/²⁰⁶Pb= 0.046%.

Eight samples from the U.S. mine in the Bingham district have a linear relationship over the 1% range of their²⁰⁶Pb/²⁰⁴Pb ratios. The simplest model fitting these data suggests that the lead was separated from a primary system (μ=8.98) 1630±150m.y. ago and subsequently mixed with a radiogenic lead of similar age; biotite K-Ar dates for altered intrusive rocks associated with the ores provide an apparent age of mineralization and suggest that isotopic evolution of the lead was terminated about 36 m.y. ago.