In situ Re-Os geochronology of Re-rich Palaeogene molybdenite by LA-ICP-MS/MS

In situ Re–Os geochronology by LA-ICP-MS/MS was previously demonstrated by reacting Os with CH₄ or N₂O reaction gasses. However, for both reactions, a minor proportion of the Re parent isotope also reacts, potentially leading to significant isobaric interferences of ¹⁸⁷Re on ¹⁸⁷Os, especially for young samples with little radiogenic in-growth. Here we present an interlaboratory comparison and compare three reaction gas mixtures (CH₄ + H₂ + He, N₂O and N₂O + He) with the aim to robustly date Palaeogene (66–23 Ma) molybdenite from the Bingham Canyon and Henderson deposits. CH₄ mixed with H₂ gas gives the highest sensitivity, while N₂O and He gas buffer Re reaction. On balance, the analytical method involving N₂O + He reaction gas is most suitable for dating Palaeogene molybdenite, resulting in age precision of 2.6% for Bingham and 5.8% for Henderson. For older, >1 Ga molybdenite, CH₄ + H₂ + He may give comparatively better age precision.