Standardizing Re–Os geochronology: A new molybdenite Reference Material (Henderson, USA) and the stoichiometry of Os salts

Increasing use of Re–Os geochronology, especially applied to molybdenite, requires a standard for cross-laboratory calibration. A new Reference Material (RM) for Re–Os geochronology, a molybdenite powder acquired from the Henderson mine and mill in Colorado, has been characterized by two independent laboratories using isotope dilution and NTIMS. Following cleaning, the powder was split and bottled by NIST for distribution. Twelve bottles were randomly selected for this study. The AIRIE Program at Colorado State University and the Radiogenic Isotope Facility at the University of Alberta each produced replicate analyses from six of the bottles using two different Os spikes (one enriched in both 188Os and 190Os and one of normal isotopic composition), for a total of 48 analyses comprising four data sets. The age of the new RM is thus defined as 27.656 ± 0.022 Ma (95% conf.).We also address the long-standing issue of the stoichiometry of Os gravimetric standards, produced from ammonium and potassium Os salts. We advocate the use of (NH4)2OsCl6 as the starting material. The actual Os content of this salt can be precisely determined by ignition under an atmosphere of 2% H2/98% N2. Reduction of the salt using a gas of higher H2 content can lead to erroneous determinations of Os content, perhaps because of the formation of osmium hydrides. Furthermore, we show that it is not always correct to assume ideal stoichiometry for K2OsCl6.