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

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 across 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. In addition to rapid analysis, a main advantage of in-situ K–Ca dating is the ability to avoid Ca- and Sr-bearing inclusions, which are a limiting factor for conventional bulk mineral dating via TIMS. Both Sr and Ca are highly reactive with both SF ₆ and N ₂ O to form M-F, M-O or M-OH reaction products. In contrast, K and Rb are non-reactive with either gas enabling efficient separation of the parent-daughter isotope pairs, ⁴⁰ K– ⁴⁰ Ca and ⁸⁷ Rb– ⁸⁷ Sr. Additionally, mixing a small amount of H ₂ with SF ₆ or N ₂ O efficiently eliminates ⁴⁰ Ar based interferences and reduces the background generated by the high ion load in the reaction cell when measuring mass to charge ratio (m/z) 40. This study compares the accuracy, precision, and product ion sensitivity of four reaction gas combinations: SF ₆ only, SF ₆ plus 2 mL min ⁻¹  H ₂ , N ₂ O plus 7 mL min ⁻¹  H ₂ , and N ₂ O plus 10 mL min ⁻¹  H ₂ , by analysing a range of micas and feldspars with previously constrained dates: MDC and Kola phlogopites, Högsbo and Robins Folly muscovites, G71560 polylithionite, and F-KN and Bohus K-feldspars. Using these gas mixtures, coupled Rb–Sr and K–Ca dates were obtained from a single ablation spot in low Ca-bearing (5–300 ppm) micas and feldspars, with age uncertainties of 2 % and 5 %, 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 SF ₆ plus 2 mL min ⁻¹  H ₂ was found to be the most effective for coupled K–Ca and Rb–Sr dating, producing the highest sensitivity for reacted species while minimising the background on reacted ⁴⁰ Ca. 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.