Measuring 87Sr/86Sr variations in minerals and groundmass from basalts using LA-MC-ICPMS

We outline a technique which uses laser ablation (LA) sampling and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) to obtain relatively accurate in situ Sr isotope ratios in a range of common geologic materials. We have undertaken a thorough evaluation of potential elemental and molecular interferences including Ca dimers and Ca argides, Fe dioxides, Ga and Zn oxides, doubly charged REEs and Hf, and singly charged Kr and Rb. Critical interferences include Kr, Rb, and doubly charged Er and Yb ions, while molecular species have only a limited impact on Sr isotope ratios. To demonstrate the accuracy of this technique, we have analyzed minerals, including marine carbonate, plagioclase, and clinopyroxene, which offer differing concentrations of interfering elements. Unlike most previous studies, we have chosen samples that are not abnormally enriched in Sr to show the potential utility of this technique. In addition, we offer the first in situ LA-MC-ICPMS analyses of fine-grained basaltic and basaltic andesite groundmass, which are critically dependent on accounting for interfering Rb and REEs. We also address potential complications and pitfalls associated with the technique and LA-MC-ICPMS in general. Current results are more accurate than any previously attained on a wider range of materials and will allow for detailed assessments of additional interferences, baseline inaccuracies, and potential pit-related or material-related fractionation. The accuracy and reproducibility of Sr isotope ratios obtained with this technique will allow for the identification of processes involved in magmagenesis, the determination of crystal residence ages, and aid in studies involving biological materials such as otoliths, bones and teeth.