U-PB DATING OF PEROVSKITE BY LA-ICP-MS: AN EXAMPLE FROM THE OKA CARBONATITE, QUEBEC, CANADA

Abstract

The potential for U-Pb dating of the mineral perovskite, using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), has been evaluated. Perovskite can have relatively elevated U and Th contents, making it a potentially useful geochronometer. Kimberlite, lamprophyre and carbonatite intrusions are prone to alteration and typically contain abundant xenocrysts, thus standard techniques for dating the magmatic emplacement of these rocks becomes highly problematic. Perovskite, however, is commonly present as a primary magmatic mineral in these rocks and has been used to constrain the timing of kimberlite and lamprophyre emplacement. The main limitations to the production of accurate LA-ICP-MS U-Pb ages for perovskite are the lack of suitable standards coupled with the large common-Pb correction required.204Hg can produce large isobaric interference on the non-radiogenic204Pb isotope and methods based on208Pb/206Pb and U/Th ratios are hampered by the enhanced Th content present in most perovskite crystals. After careful calibration using zircon reference materials, common-Pb corrected206Pb/238U ages can be determined on each individual LA-ICP-MS analysis by using the measured207Pb/206Pb ratio which is dominated by the common-Pb proportion in phanerozoic rocks. This method assumes that the perovskite crystals analyzed are cogenetic and have the same initial-Pb ratios. The LA-ICP-MS method has been applied to data collected on two large, single perovskite crystals from the Oka carbonatite complex, Quebec, Canada. The weighted mean206Pb/238U age of 131 + /-7 Ma (N = 25, MSWD = 0.42) is consistent with published geochronology data from the region and suggests that LA-ICP-MS dating of perovskite could be a useful, low-cost alternative to compliment existing techniques. © 2006 Elsevier B.V. All rights reserved.