A NEW MOLYBDENITE REFERENCE MATERIAL (RM) FROM HENDERSON, COLORADO FOR INTER-LABORATORY COMPARISON AND GLOBAL STANDARDIZATION

Increasing use of Re-Os geochronology, especially applied to molybdenite (MoS₂), requires a standard for international 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. Henderson molybdenite was chosen for the RM because: (1) the ore is nearly mono-mineralic, (2) the Henderson mill processes only ore from the Henderson mine, (3) the porphyry-style Henderson ore body was emplaced relatively rapidly, and (4) the deposit is young and the MoS₂ has a relatively low Re concentration allowing low level facilities access to the RM. 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 bottles using two different Os spikes (one enriched in both ¹⁸⁸Os and ¹⁹⁰Os and one of normal Os isotopic composition), for a total of 48 analyses comprising four data sets. The age of the new RM is defined as 27.656 ± 0.022 Ma (95% C.I.; Markey et al., in press, Chemical Geology).

Os salt needed to create a gravimetric Os standard solution and the normal Os spike became a concern during characterization of the RM. Standard solutions underpin spike calibrations or are used as spike solutions, and thus, both the isotopic composition and concentration of the solution must be accurately and precisely known. After much experimentation, it was found that (NH₄)₂OsCl₆ reduced to metallic Os in a 2% H₂/ 98% N₂ reducing atmosphere produces the most accurate, reproducible standard (Selby and Creaser, 2001, Economic Geology).

The Henderson molybdenite RM provides the first high precision NTIMS data for a molybdenite RM allowing inter-laboratory Re-Os data comparisons. In addition to characterizing the molybdenite powder, issues with gravimetric Os standard solutions, spike creation and calibration, and inter-laboratory variability are demonstrated. The final result is a RM that will be made available through NIST for researchers worldwide.