High-Precision Mass-Dependent Molybdenum Isotope Variations in Magmatic Rocks Determined by Double-Spike MC-ICP-MS

摘要

Small mass-dependent variations of molybdenum isotope ratios in oceanic and island arc rocks are expected as a result of recycling altered oceanic crust and sediments into the mantle at convergent plate margins over geological timescales. However, the determination of molybdenum isotope data precise and accurate enough to identify these subtle isotopic differences remains challenging. Large sample sizes – in excess of 200 mg – need to be chemically processed to isolate enough molybdenum in order to allow sufficiently high-precision isotope analyses using double-spike MC-ICP-MS techniques. Established methods are either unable to process such large amounts of silicate material or require several distinct chemical processing steps, making the analyses very time-consuming. Here, we present a new and efficient single-pass chromatographic exchange technique for the chemical isolation of molybdenum from silicate and metal matrices. To test our new method, we analysed USGS reference materials BHVO-2 and BIR-1. Our new data are consistent with those derived from more involved and time-consuming methods for these two reference materials previously published. We also provide the first molybdenum isotope data for USGS reference materials AGV-2, the GSJ reference material JB-2 as well as metal NIST SRM 361.