Isotopic and elemental analysis of uraninite concentrates using inductively coupled plasma-mass spectrometry (ICP-MS)

Inductively coupled plasma-mass spectrometry (ICP-MS) is capable of measuring isotopic and elemental abundances in geologic materials easily and rapidly. Although the precision of isotope ratio data obtained by ICP-MS is inferior to that by thermal ionization mass spectrometry, it is adequate for application to a number of geochemical exploration problems.National Institute of Standards and Technology (NIST) Pb metal standard reference material 981 (NBS981), was used as the isotopic standard to correct the measured isotope intensities for mass discrimination. The mean relative standard deviation (RSD) of the determinations of the abundances of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb in the two other NIST Pb isotope reference materials, NBS981 and NBS982, was better than 0.3%, whereas the RSD for the determination of the less abundant ²⁰⁴Pb was 0.4%. Accuracy was demonstrated by repeated analysis of NBS981, NBS982, and NBS983. The Student t-statistic ranged between −1.75 and 2.04 for the abundances of the 4 Pb isotopes in the three NIST materials.Data from a suite of 13 uraninite-rich samples from Labrador demonstrate the ability of ICP-MS to determine age and geochemical information sufficient for regional interpretations. The determined radiogenic ²⁰⁷Pb/²⁰⁶Pb ratios of 12 of the samples give ages between 1697 and 1805 Ma with average uncertainties (one standard deviation) of 4 Ma, whereas one of the samples has an age of 495 ± 4 Ma. The average age of the 12 samples was 1752 ± 27 Ma. Along with the Pb isotope intensities, ²³²Th and ²³⁸U were measured and the U-Pb age determined from a fit of the ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U for 9 of the samples. The concordia intercept age of 1740 Ma for the best-fit line is in good agreement with the mean ²⁰⁷Pb/²⁰⁶Pb age of 1752 Ma.