U-Pb systematics of uranium-rich apatite from Adirondacks: Inferences about regional geological and geochemical evolution, and evaluation of apatite reference materials for in situ dating

Abstract

High‑uranium apatite samples from iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains, New York, are evaluated as potential standards for in situ U-Pb dating of high-U apatite. Age data for these minerals also have implications for better understanding the geological evolution in the region. Secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses of these samples show that they have high and variable uranium contents of 5–485 ppm. High-precision isotope-dilution thermal ionization mass spectrometry (ID-TIMS) dating yielded U–Pb errorchron ages of 907 ± 14 Ma (MSWD = 20) and 924 ± 13 Ma (MSWD = 547), showed disturbance in the U–Pb systems, and revealed radiogenic compositions of initial Pb (206Pb/204Pb ratios of 339 ± 48 and 162 ± 39). Nominal age calculations using assumed initial Pb isotopic compositions close to average terrestrial crustal Pb yield biased and older ages despite the radiogenic Pb isotopic composition of the apatites (206Pb/204Pb from 1300 to >4000). The apatite ages calculated using the measured initial Pb isotopic compositions are younger than the U–Pb ages of 1008.0 ± 3.2 Ma and 992.4 ± 7.7 Ma determined by SIMS for zircon cores and rims, respectively, separated from one of these ore samples. It indicates protracted fluid activity in the deposit that lasted for ~100 Ma after the ore formation, or later disturbance of the apatite. The time of U enrichment that we inferred from radiogenic composition of initial Pb is within uncertainties with primary ore crystallization, indicating that high 206Pb/204Pb ratios of initial Pb were caused by dissolution-reprecipitation of primary apatite.Despite the variability in U contents and disturbance of the U–Pb system in the apatites, our SIMS and LA-ICPMS analyses show that these samples can be used, with due caution, as reference materials (RMs) for dating apatite with similarly high-U concentration. They potentially provide more precise and accurate results compared to the usage of better behaved but lower-U apatite RMs due to improved counting statistics and reduced matrix effects, especially important for LA-ICPMS with less sensitive quadrupole mass analysers.The McClure Mountain apatite, a widely used reference for in situ dating, was used in the present study as a matrix-matched reference material for in situ LA-ICPMS analyses. We also analysed it by ID-TIMS and obtained more radiogenic Pb-isotopic data and more precise ages that are in agreement within uncertainties with the previously published result (Schoene and Bowring, 2006); combined data yielded a U-Pb errorchron age of 525.3 ± 1.7 Ma (MSWD = 70). The data corrected for initial Pb using the total U/Pb isochron yield the single Concordia age of 525.10 ± 0.75 Ma (MSWD = 1.1). Two independent ID-TIMS U–Pb studies support the use of this apatite as a primary RM for dating of low-U apatite with a normal composition of initial Pb. However, its small grain size and low U concentration (11–29 ppm) can cause elevated uncertainty in LA-ICPMS age determinations for unknowns. Better precision in our LA-ICPMS analyses was achieved using Madagascar (MAD1 and MAD2) apatite RM with U contents in the range of 28–58 ppm.