The determination of strontium isotope ratios by means of quadrupole-based ICP-mass spectrometry: a geochronological case study

Abstract

Quadrupole-based ICP-mass spectrometry (ICP-QMS) was used for the determination of ⁸⁷Sr/⁸⁶Sr isotope ratios in digests of rock samples originating from two magmatic silicate rock formations of the Vosges (the Kagenfels granite and the Nideck rhyolite). The rock formations studied are geographically close to one another and, overall, they show a similar chemical composition. In a preliminary study, the effect of various data acquisition parameters on the isotope ratio precision was systematically studied, permitting optimum conditions to be selected. Rh was used as an internal standard, allowing the blank correction to be made accurately. Cation exchange chromatography was used to avoid isobaric overlap of ⁸⁷Rb⁺ and ⁸⁷Sr⁺ ion signals to the largest possible extent, while mathematical correction was applied to correct for the remaining interference. The accuracy of the method developed was evaluated by means of isotopic analysis of an oceanic gabbro sample, for which the ⁸⁷Sr/⁸⁶Sr isotope ratio was previously characterized by means of thermal ionisation mass spectrometry (TIMS). An excellent agreement between the ICP-QMS and TIMS values was established. On the basis of the isochrons, constructed using the ⁸⁷Sr/⁸⁶Sr isotope ratio results and the contents of Rb and Sr (determined by energy-dispersive X-ray fluorescence spectrometry) obtained for the Kagenfels and Nideck samples, it could be concluded that for both rock formations, secondary processes (e.g., recrystallisation, high- and low-temperature alteration) have disturbed the Rb–Sr isotopic system. As a consequence, the uncertainties on (i) the initial ⁸⁷Sr/⁸⁶Sr isotope ratios and (ii) the ages thus determined are large. Nevertheless, the estimated ages appear to be geologically relevant and provide information on the timing of geological events that affected the rocks several tens of million years after the initial formation. Overall, this case study shows the merits of ICP-QMS for exploratory studies of Sr isotope systematics and geochronology in cases with sufficient variation in the ⁸⁷Sr/⁸⁶Sr isotope ratios.