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

Abstract

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.