Matrix effects during in situ U-Pb dating of perovskite with variable crystal structure: Evidence from the Tazheran Massif, Russia

Abstract

U-Pb geochronology of perovskite is one of the most reliable means of dating notoriously difficult to date and economically important kimberlites, carbonatites, and skarns. Matrix effects—isotopic variations due to chemical composition and crystal structure—are known to affect in situ U-Pb dating, but have not been documented in perovskite. Perovskite from the Tazheran Massif, Russia, has become a commonly used standard for U-Pb dating, so rigorously testing its general applicability is therefore critical. In this study, comparison between U-Pb ages generated by precise isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and in situ dating methods including both secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) for a series of perovskite crystals from the Tazheran Massif (Tazh-1, Tazh-2, Tazh-3, Tazh-7Bl, and Tazh-7Br) suggests that the in situ ages of some perovskite crystals (Tazh-1, Tazh-7Bl, and Tazh-7Br) are systematically offset from their corresponding ID-TIMS ages. Hence, not all Tazheran perovskite grains are suitable as standards and some previously published work may suffer from unrecognized inaccuracies. Various factors are investigated in an attempt to identify the origin(s) of the matrix effect biasing the in situ measurements. Although there are so far only a limited number of perovskite grains analyzed by all SIMS, LA-ICPMS, and TIMS methods, grains Tazh-1, Tazh-7Bl, and Tazh-7Br all have extremely low FeO, Nb, and LREE contents compared with the other Tazh perovskite and well characterized reference materials (AFK-5 and Ice River), indicating that such chemical variability may cause analytical differences. Furthermore, and perhaps more importantly, the Tazh-1, Tazh-7Bl, and Tazh-7Br perovskite grains are cubic and thus have different crystal structure from most Tazh perovskite and their standards that are orthorhombic. Therefore, crystal structure could be the principal reason for the matrix effect, with secondary matrix effects due to differences in chemical composition, that need to be taken into account in future in situ U-Pb dating of perovskite.