Precise in situ87Rb–87Sr dating of terrestrial and extraterrestrial samples with a Nu Sapphire XD LA-CRC-MC-ICP-MS/MS

Abstract

In this study, we report for the first time mass shifted Sr isotope analyses using a double Wien mass filter in combination with SF₆ reaction gas in the collision/reaction cell (CRC) of a Nu Sapphire XD CRC-MC-ICP-MS/MS. Measuring Sr⁺ isotopes as SrF⁺ allows avoiding the interference of ⁸⁷Rb⁺ on ⁸⁷Sr⁺ as, unlike Sr isotopes, ⁸⁷Rb⁺ does not react with SF₆ and thus is measured at mass 87. The performance of the mass filter on the overall transmission was first tested without the CRC, with doping experiments using a NIST SRM 987 solution doped with a multi-element solution containing Mo, Pd, Ru, and Cd among others. These doping tests return a mass spectrum where only ions with a mass-to-charge (m/z) between 78 and 100 are transmitted through the mass window, and notably no ions with a m/z > 100 are transmitted, providing a clean mass spectrum area where SrF⁺ isotopes can be measured interference-free. This demonstrates that no analytical artefacts from the mass filter are transmitted through the mass window. In the second step of the method validation, doping experiments in solution mode with the CRC show no significant effect of cell-based oxide formation (notably no ³²S¹⁹F₃¹⁶O⁺) which could induce matrix effects, with ⁸⁷Sr/⁸⁶Sr measured in the doped NIST SRM 987 solution in agreement with the value measured for the undoped NIST SRM 987 solution. Ultimately, in situ Rb–Sr measurements in a series of reference materials including synthetic glasses NIST SRM 610 and NIST SRM 612 and basaltic glasses BCR-2G, BHVO-2G and BIR-1G return precise and accurate ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr values. Our new method was successfully applied to the dating of the Dartmoor granite and the Erg Chech 002 achondrite, spanning a range of ages from ∼300 to 4565 Ma old, with the need for a second normalisation to a matrix-matched calibrant (Shap granite and NWA 6950 lunar meteorite, respectively) for accurate ⁸⁷Rb/⁸⁶Sr determination. Thus, this study demonstrates the capability of the Nu Sapphire XD to perform in situ ⁸⁷Rb–⁸⁷Sr dating in geological samples. This minimally destructive method also allows preservation of petrographic context, and is thus very well suited for the analyses of the small and precious samples which will be brought back to Earth by future sample return missions.