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

摘要

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.