Strontium isotope analysis of kimberlitic groundmass perovskite via LA-MC-ICP-MS

Strontium isotopic composition is a potentially powerful tracer in studies of kimberlitic rocks but the results from even the most carefully collected and stringently prepared bulk-rock samples are still hampered by contamination and alteration effects. Here we describe a LA-MC-ICP-MS technique which can obtain accurate, high precision Sr _i ratios from 50-150 μm kimberlitic groundmass perovskite without requiring time-consuming mineral separation procedures. Since perovskite is a robust magmatic phase with an extremely low Rb/Sr ratio, the effects of late-stage crustal contamination, post-emplacement alteration and age correction are minimised and results are more representative of primary melt compositions, while additionally preserving powerful grain-scale spatial and textural information. We demonstrate that the adopted protocol overcomes isobaric interferences from Kr ⁺, Rb ⁺, Er ²⁺ and Yb ²⁺, and that Ca dimers and Ca argides do not detectably affect the quality of ⁸⁷Sr/ ⁸⁶Sr ratios produced. To illustrate the utility of the technique, contrasting bulk-rock and in situ perovskite results from eleven Proterozoic kimberlites are documented.