In situ Rb–Sr and K–Ca dating by LA-ICP-MS/MS: an evaluation of N2O and SF6 as reaction gases

Abstract

In situ dating of K-rich minerals, e.g. micas and K-feldspar, by the Rb–Sr isotopic system is a new development made possible by the ICP-MS/MS technique. Online chemical separation of Rb and Sr is possible in an O₂-filled reaction cell, wherein a portion of the Sr reacts to SrO⁺ while simultaneously no RbO⁺ is formed. O₂ reactions provide stable analytical conditions sufficient for precise and accurate determination of Rb/Sr and Sr/Sr isotopic ratios using 80 micron laser ablation spots. However, to date <10% of the Sr reacts with O₂ as reaction gas, leaving room for improvement using more potent reaction gases. With a more efficient reactive transfer, it should be possible to obtain similar results with a smaller laser spot size, hence gaining higher spatial resolution. In this study, we have evaluated N₂O and SF₆ as reaction gases since they have previously been shown to react strongly with Sr⁺, without affecting Rb⁺. Analytical conditions, including cell parameters and reaction gas flow rate were optimized while ablating NIST SRM 610. The main reaction product is SrO⁺ for N₂O reaction and SrF⁺ for SF₆ reaction. Both gases show significantly higher reaction product formation compared to O₂ with >85% of Sr reacting with N₂O and >70% Sr reacting with SF₆; Rb does not react with either gas. As a result, the sensitivity for Sr reaction products is ∼10 times higher with N₂O and ∼8 times higher with SF₆ compared to O₂. With these more reactive gases, the error of mica isochron ages, calibrated against a newly developed nano-particulate pressed powder tablet of mica–Mg, is ∼1% using a 50 μm laser spot. Our tests show that both N₂O and SF₆ form interfering reaction products, e.g., SrOH (N₂O), SiF₃ and TiF₃ (SF₆) that are difficult to handle using single mass spectrometer instruments, but which can be overcome using MS/MS. Using SF₆ combined with H₂, it is possible to measure ⁴⁰Ca⁺ as ⁴⁰Ca¹⁹F⁺, free from interference of ⁴⁰Ar⁺ and ⁴⁰K⁺. This facilitates the dating of micas by the K–Ca isotopic system; we present the first in situ K–Ca age determination.