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

Quadrupole-based ICP-mass spectrometry (ICP-QMS) was used for the determina tion of Sr-87/Sr-86 isotope ratios in digests of rock samples originating f rom two magmatic silicate rock formations of the Vosges (the Kagenfels gran ite and the Nideck rhyolite). The rock formations studied are geographicall y close to one another and, overall, they show a similar chemical compositi on. In a preliminary study, the effect of various data acquisition paramete rs on the isotope ratio precision was systematically studied, permitting op timum conditions to be selected. Rh was used as an internal standard, allow ing the blank correction to be made accurately. Cation exchange chromatogra phy was used to avoid isobaric overlap of Rb-87(+) and Sr-87(+) ion signals to the largest possible extent, while mathematical correction was applied to correct for the remaining interference. The accuracy of the method devel oped was evaluated by means of isotopic analysis of an oceanic gabbro sampl e, for which the Sr-87/Sr-86 isotope ratio was previously characterized by means of thermal ionisation mass spectrometry (TIMS). An excellent agreemen t between the ICP-QMS and TIMS values was established. On the basis of the isochrons, constructed using the Sr-87/Sr-86 isotope ratio results and the contents of Rb and Sr (determined by energy-dispersive X-ray fluorescence s pectrometry) obtained for the Kagenfels and Nideck samples, it could be con cluded that for both rock formations, secondary processes (e.g., recrystall isation, high- and low-temperature alteration) have disturbed the Rb-Sr iso topic system. As a consequence, the uncertainties on (i) the initial Sr-87/ Sr-86 isotope ratios and (ii) the ages thus determined are large. Neverthel ess, the estimated ages appear to be geologically relevant and provide info rmation 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 systemat ics and geochronology in cases with sufficient variation in the Sr-87/Sr-86 isotope ratios.