The importance of high precision in the evaluation of U-Pb zircon age spectra

Abstract

We present high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon data from the Carboniferous Punteglias granodiorite from the Central Alpine basement in the Aar massif in Switzerland. These analyses yield complex age spectra consisting of both concordant and normally discordant dates, which form a linear array indicative of mixing of two age components within individual zircon crystals. While the upper intercept age reflects zircon growth during Variscan magmatism, the lower intercept of 24 ± 11 Ma is coincident with the Alpine orogeny. There is poor evidence from cathodo-luminescence imagery that magmatic zircon may have younger overgrowths, therefore, we pretreated an aliquot of zircon with physical abrasion followed by chemical abrasion to remove any potential later zircon overgrowths, and to directly date the initial magmatic domains of the zircon. This pretreatment significantly reduced age dispersion and yielded an age of 335.479 ± 0.041 Ma for the crystallization of zircon in the Punteglias granodiorite. Due to the effectiveness of this pretreatment, we interpret that the magmatic zircon was overgrown with younger domains during the Alpine orogeny. Detangling the observed complications in the Punteglias granodiorite zircons was largely possible due to the particularly high analytical precision of the ID-TIMS U-Pb dates that made it possible to detect discordance between the 206Pb/238U and 207Pb/235U systems. Lower-precision U-Pb ID-TIMS dates would not allow identification of such a two-component mixture and lead to analytically concordant points spreading over 105 to 106 years, and potentially leading to erroneous scientific interpretation. As an example, we model two hypothetical situations of simple mixing of two age components in the U-Pb concordia space to demonstrate the effect of precision on the ability to accurately interpret geological models from complicated age spectra. Combining these new high-precision data and modeling results highlights the importance of choosing the optimal, physical and/or chemical sample pre-treatment and the challenge of accurately interpreting complicated U-Pb data in terms of crystallization age.