The application of U–Pb geochronology to mafic, ultramafic and alkaline rocks: An evaluation of three mineral standards

Abstract

A key to proving the accuracy of U–Pb age determinations by the various techniques available is routine analysis of an internal, well-characterized matrix-matched mineral standard of known age. In general, there are very few well-studied natural mineral standards available to the geological community. In this study, the nature, geochemistry and U–Pb systematics of baddeleyite, titanite and perovskite from three potential mineral standards are evaluated. A compilation of concordant to near-concordant analyses of a single Phalaborwa baddeleyite crystal yields a precise U–Pb age of 2059.60 ± 0.35 Ma. A large variation in uranium (51–2124 ppm), thorium (0.5–73 ppm) and lead (19–782 ppm) concentrations and the presence of some U-bearing mineral inclusions (zircon, monazite) and baddeleyite/zircon intergrowths require some caution when using aliquots of this Phalaborwa baddeleyite crystal as a mineral standard. The Khan titanite crystal investigated here overall has a relatively uniform chemical composition (e.g. U = 584 ± 95 ppm; Th = 473 ± 73 ppm; Pb = 57 ± 8 ppm; Th/U = 0.81 ± 0.05) but parts of the crystal contain abundant mineral inclusions. The model 1 upper intercept age of 522.2 ± 2.2 Ma for nine Khan titanite analyses is considered the current best estimate for its crystallization age. Some variability in the U–Pb systematics with 207Pb/ 206Pb ages that vary between 508 and 560 Ma was observed and could be related to the presence of tiny U-bearing mineral inclusions, such as rutile or allanite, so careful selection of material from this titanite crystal is also required. A single perovskite crystal from the Ice River alkaline intrusion has a relatively uniform chemical composition: uranium (141 ± 25 ppm), thorium (1954 ± 270 ppm), lead (41 ± 5 ppm) and Th/U (14.5 ± 1.3). The U–Pb results for eight perovskite fractions yield a weighted average 206Pb/ 238U age of 356.5 ± 1.0 Ma. Although there are always some limitations to using natural mineral crystals as U–Pb standards, such as the presence of mineral inclusions, alteration, and geochemical heterogeneity, the three crystals investigated here are considered suitable to be used as internal U–Pb standards for any of the currently used U–Pb dating techniques.