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 CH 4 or N 2 O reaction gasses. However, for both reactions, a minor proportion of the Re parent isotope also reacts, potentially leading to significant isobaric interferences of 187 Re on 187 Os, especially for young samples with little radiogenic in-growth. Here we present an interlaboratory comparison and compare three reaction gas mixtures (CH 4 + H 2 + He, N 2 O and N 2 O + He) with the aim to robustly date Palaeogene (66–23 Ma) molybdenite from the Bingham Canyon and Henderson deposits. CH 4 mixed with H 2 gas gives the highest sensitivity, while N 2 O and He gas buffer Re reaction. On balance, the analytical method involving N 2 O + 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, CH 4 + H 2 + He may give comparatively better age precision.