Coupled K–Ca and Rb–Sr dating by LA-ICP-MS/MS – reaction gas optimisation and geological applications

Abstract. Both the Rb-Sr and K-Ca β-decay isotopic systems can be used to date a range of mica and feldspar group minerals and have the potential to unravel cooling and alteration processes in a wide range of geological settings. The development of LA-ICP-MS/MS has enabled direct in-situ analysis of these β-decay geochronometers via chemical separation with reactive gases within the mass-spectrometer. As well as rapid analysis, the main advantage of in-situ K–Ca dating is that Ca- and Sr-bearing inclusions can be avoided, which are a limiting factor for conventional bulk mineral dating via TIMS. Both Sr and Ca are highly reactive with both SF₆ and N₂O to form M-F, M-O or M-OH reaction products, while K and Rb are unreactive with either gas, enabling efficient separation of the parent-daughter ⁴⁰K–⁴⁰Ca and ⁸⁷Rb–⁸⁷Sr isotope pairs. Additionally, mixing a small amount of H₂ with SF₆ or N₂O efficiently eliminates ⁴⁰Ar based interferences and reduces the background generated by the high ion load in the reaction cell when measuring mass/charge ratio (m/z) 40. This study compares the accuracy, precision and product ion sensitivity between four reaction gas combinations: SF₆ only, SF₆ plus 2 ml min^-1 H₂, N₂O plus 7 ml min^-1 H₂, and N₂O plus 10 ml min^-1 H₂, by analysing a range of micas and feldspars with previously constrained dates: MDC & Kola phlogopites, Högsbo and Robins Folly muscovites, G71560 polylithionite, and F-KN and Bohus K-feldspars. Using these gas mixtures we present coupled Rb–Sr and K–Ca dates from a single ablation spot in low Ca-bearing (5–300 ppm) micas and feldspars to within 2 and 5 % age uncertainty, respectively. The direct coupling of Rb–Sr and K–Ca dates within the same ablation volume allows assessment of isotopic disturbances at high spatial resolution. The gas combination of SF₆ plus 2 ml min^-1 H₂ was found to be most effective for coupled K–Ca and Rb–Sr dating in generating the highest sensitivity of reacted species and in reducing the background for reacted ⁴⁰Ca. Analysis of the FK-N feldspar from Madras, India shows the potential for the two isotopic systems to reveal decoupled dates, with the 515 ± 22 Ma Rb–Sr date representing the crystallisation of the granite and the 437 ± 38 Ma K–Ca date indicating late-stage hydrothermal activity.