A comparison of geochronological methods commonly applied to kimberlites and related rocks: Three case studies from Finland

Abstract

Despite their economic and scientific significance, the tectonic drivers that lead to the genesis of kimberlites and related rocks remain enigmatic. The presence of any spatio-temporal relationship(s) between the rift, drift and collision history of continents and the emplacement of kimberlites can only be elucidated with accurate geochronological constraints. In this study, we evaluated the three most common radiogenic dating methods used to determine the emplacement ages of kimberlites and related rocks - Rb-Sr phlogopite, U-Pb perovskite and 40Ar/39Ar phlogopite. We selected minimally altered samples from the Kaavi-Kuopio kimberlites, ultramafic lamprophyres and kimberlites from Kuusamo, and olivine lamproites from the Lentiira-Kuhmo-Kostomuksha cluster, all located in Finland. Our 33 new age determinations indicate that the olivine lamproites and ultramafic lamprophyres were emplaced on the Karelian craton during the interval between ~1180–1210 Ma, the Kuusamo kimberlites between ~730–750 Ma, and the Kaavi-Kuopio kimberlites between ~585–620 Ma.Our results show that the Rb-Sr method regularly yields reproducible eruption ages from either magmatic or macrocrystic (xenocrystic) mica. In addition, the method can be employed on a single population of mineral separates using a sequential acid-leaching technique, without the need for an estimate of 87Sr/86Sr(i). The U-Pb perovskite technique typically requires significant correction for ‘common-Pb’ i.e. Pb incorporated at the time of mineral formation and unrelated to subsequent radiogenic ingrowth. Here, we utilised isotope dilution analyses of co-existing spinel to extend the ‘spread’ in U-Pb isochron space obtained by in situ LA-ICP-MS analyses of perovskite and enhance the accuracy and precision of intrusion ages. Our tests also show that two-point U-Pb isochrons based on solution analyses of perovskite and spinel generally provide anomalously older ages. Relative to Rb-Sr and U-Pb results from the same kimberlite, we show that apparently precise 40Ar/39Ar plateau-ages, determined from step-heating experiments, are often anomalously ‘old’, which is attributable to 39Ar recoil effects. In cases where the 40Ar/39Ar step-heating spectra are even marginally ‘disturbed’, 40Ar/39Ar total-gas ages compare favourably with ages from other geochronometers and appear to provide accurate estimates for the emplacement age of kimberlites and related rocks.Overall, there is good agreement between the three geochronometers using the above approaches, which indicates that Rb-Sr phlogopite, U-Pb perovskite and 40Ar/39Ar phlogopite dating can provide robust age constraints for the emplacement of kimberlites and related rocks. The geochronometer of choice will rely on the freshness, abundance and size of datable material. Wherever possible, the application of multiple geochronometers is recommended.