Combined U–Pb geochronology and Sr–Nd isotope analysis of the Ice River perovskite standard, with implications for kimberlite and alkaline rock petrogenesis

Abstract

Growing interest in the mineral perovskite (CaTiO3) as a U–Pb chronometer and archive of near-primary Sr and Nd isotope compositions in magmatic systems highlights the need for well-characterized mineral standards. Based on a conventionally determined, high-precision Sr and Nd isotope data set, we propose perovskite from the alkaline Ice River intrusion as a natural reference material for Sr and Nd isotope ratio determinations. Ice River perovskite has mean present-day 87Sr/86Sr (TIMS) and 143Nd/144Nd (ID-MC-ICP-MS) ratios of 0.702838±51 and 0.512581±32, respectively (2-sigma uncertainties). The TIMS 206Pb/238U age of 361.7±1.0Ma (2-sigma), determined on the same crystal fragments as the Sr and Nd isotope compositions, falls within the accepted range of 355–372Ma for the Ice River intrusion. Although there is a ~2.2% difference between the ages of Ice River perovskite (~355–363Ma) reported in various U–Pb studies (determined by TIMS, SIMS, and LA-MC-ICP-MS methods), the material investigated here has the currently best-calibrated U–Pb systematics for perovskite. We therefore recommend the 206Pb/238U age of 361.7±1.0Ma as a reference value for use in geochronology studies.Perovskite is a prime target for petrogenetic studies of kimberlites and related alkaline rocks, because it preserves primary magmatic signatures. The ability to combine important petrogenetic information with high-resolution U–Pb emplacement ages at mineral scale will be instrumental for an improved understanding of deep magmatism beneath cratons and areas of rifted continental lithosphere.