In Situ Re-Os Dating of Molybdenite Using La-Icp-Ms/Ms. Analytical protocol and standardization.

Abstract

Dating with the Re-Os isotopic system provides several advantages compared to conventional chronometers such as Rb-Sr. Because of the sidero- and chalcophilic nature of Re and Os, those can be found as trace elements in sulfides, meaning that ages can be obtained for sulfide deposit, giving valuable information of ore genesis. To ensure the quality of the analytical measurements needed in isotopic dating, a reference material with known concentrations of Re and Os is necessary. The aim of this study is to develop a method to produce a homogenous tablet of the sulfide molybdenite, and validate its use as a reference material for calibration of in situ Re-Os dating using LA-ICP-MS/MS. Molybdenite from Kingsgate, Australia, was used as a test material, molybdenite from Moly Hill, Canada, as the reference material for the tablet, and molybdenite from Ytterby, Sweden, as a validation of the produced tablet. The molybdenite was milled by hand with ethanol in an agate mortar and pressed into a tablet in a tablet die set by applying 2.5 t (0.19 × 106 kPa) for 2 min. Molybdenite proved to be hard to mill due to its layered structure and milling in a high power planetary ball mill was therefore unsuccessful in producing fine material. The optimum pressure and corresponding time depended on several factors, such as the tablet die set and material, and will affect for example numbers of cracks in the tablet. The tablet was analyzed with LA-ICP-MS/MS, an analytical method that was chosen because it provides the opportunity to target specific ions, created after a reaction with a reaction gas, in this case CH4. This is a tool used to chemically separate the parent and daughter isotopes and overcome the isobaric overlap. Isotopes of Re and Os, as well as potential elements in inclusion, was measured. Despite signs of inclusions in the final tablets, the Re and Os signals seemed unaffected by this. Ratios for 187Re/187Os was calculated for the tablet of molybdenite from Moly Hill, and for Ytterby molybdenite. The literature Re-Os age for Moly Hill, 2646 Ma, was used to obtain a corresponding 187Re/187Os ratio, which in turn could be used to calculate a correction factor. This was applied to the Ytterby molybdenite to obtain corrected ratios, then calculating an age of 1823 ± 28 Ma, which overlaps the literature age for Ytterby, 1795 ± 2 Ma. The results validate the quality of the tablet produced of molybdenite from Moly Hill and confirms that it can be used for calibration of in situ Re-Os dating with LA-ICP-MS/MS, even if several improvements of the methods can be done in the future, both in the milling process and the analytical process.