High-precision ID-TIMS U-Pb geochronology of perovskite (CaTiO3) from the Ice River Complex, southeastern British Columbia

Uranium‑lead perovskite in situ geochronology has become a cornerstone technique for determining the emplacement timing of alkaline, ultrapotassic, and silica-undersaturated igneous rocks, kimberlites, and carbonatites. Accurate in situ dates are dependent on the availability of matrix matched mineral reference materials which themselves are chemically well characterized and dated accurately to the highest possible precision. When dating perovskite to high precision, such as by isotope dilution thermal ionization mass spectrometry (ID-TIMS), appropriately accounting and correcting for the quantity and isotopic composition of Pb incorporated into the crystal upon crystallization (Pb_i) is a large source of uncertainty and potential inaccuracy. Unfortunately, although ultra-high precision perovskite dates are attainable with modern mass spectrometry techniques, the accuracy of applied Pb_i compositions, which can be a considerable percentage of total Pb in a crystal, has not kept pace, resulting in percent level inaccuracy on precisely measured isotopic ratios.

In an effort to characterize the age and initial Pb isotopic composition of a readily available, relatively pure perovskite endmember (93–98%) which will be useful as a matrix matched age reference material for in situ U-Pb geochronology, we date crystals isolated from three samples from the Ice River Complex (samples 81IR6, I90.3, and I92.30) by ID-TIMS and evaluate their suitability as known-age reference materials. We directly determine the isotopic composition of Pb_i in each sample by ID-TIMS measurement of cogenetic low ²³⁸U/²⁰⁴Pb (μ