Molybdenite Reference Materials for In Situ LA-ICP-MS/MS Re-Os Geochronology

Abstract

Re-Os isotope-dilution geochronology has been widely used to date the timing of molybdenite, pyrite and chalcopyrite formation across a variety of geological settings. However, in situ methods have been impeded by the isobaric interference of ¹⁸⁷Re on ¹⁸⁷Os. In situ Re-Os geochronology using LA-ICP-MS/MS has been shown to be a useful technique to chemically separate Os from Re, as Os reacts with CH₄ to create higher-mass reaction products, which can then be measured with minimised interference of ¹⁸⁷Re. However, application of the method requires matrix-matched primary reference materials, e.g., age-homogenous molybdenite amenable to laser ablation. Here, we characterise and present two new molybdenite mineral reference materials for in situ Re-Os geochronology by LA-ICP-MS/MS, verified by ID-TIMS Re-Os measurements. We also present case studies from molybdenite samples with varying Re mass fractions and Re-Os age mapping. The method provides accurate and precise age data, with excellent precision for high Re samples. The benefits of the LA-ICP-MS/MS approach include: (1) simple sample preparation, (2) rapid data acquisition, (3) targeting of specific textural domains including growth zones and (4) the ability to simultaneously collect trace elements used to link the timing and conditions of ore-formation.