U-Pb zircon and titanite systematics of the Fish Canyon Tuff: an assessment of high-precision U-Pb geochronology and its application to young volcanic rocks

摘要

A large data set of single and multi-grain zircon and titanite analyses from a sample of the Oligocene Fish Canyon Tuff (FCT), a voluminous ash flow from the San Juan Mountains of Colorado and widely used ~(40)Ar/~(39)Ar geochronological standard, has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution U-Pb zircon method. Twenty-three single zircon grains and seven small multi-grain fractions of the FCT yield an inverse-variance weighted mean ~(206)Pb/~(238)U date of 28.402 ± 0.023 Ma (2σ; MSWD 0.93) and a slightly olderweighted mean ~(207)Pb/~(235)U date of 28.529 ± 0.030 Ma (MSWD 0.74), which are insensitive to common Pb corrections. Initial ~(230)Th disequilibria calculated from a newly measured Th/U = 2.2 for FCT pumice shards indicate Th-deficiency for these zircons; its correction brings the weighted mean ~(206)Pb/~(238)U date to 28.478 ± 0.024 (MSWD 0.97), minimizing the discordance of the FCT zircon analyses. These calculations culminate in Concordia ages (Ludwig, 1998) for the crystallization of the FCT zircons of 28.476 ± 0.029 Ma (MSWD 1.50) without propagating decay constant errors, or 28.498 ± 0.035 Ma (MSWD 1.03) with propagated decay constant errors. These results are in close agreement with a previous single-grain ~(206)Pb/~(238)U zircon date of 28.41 ± 0.05 Ma for the FCT (Oberli et al., 1990), however they do not corroborate the discordia lower intercept date of 27.52 ± 0.09 put forth by Lanphere and Baadsgard (2001). The 28.476 ± 0.064 Ma Concordia age for FCT zircons (including systematic decay constant errors and uncertainty in Pb/U tracer calibration) is substantially older than all published ~(40)Ar/~(39)Ar sanidine ages, which range from 27.5 to 28.05 Ma (Lanphere and Baadsgaard, 1997; Renne et al. 1998). A lengthy period (～400 k.y.) of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy. However, FCT titanite, which may remin open to Pb diffusive exchange until quenching upon eruption, yield a minimum ~(230)Th-disequilibrium corrected weighted mean ~(206)Pb/~(238)U age of 28.395 ± 0.049 Ma (± 0.078 with tracer uncertainty), weakening residence time arguments, and supporting recent calls for re-evaluation of the ~(40)K decay constant and the accepted ages for commonly used ~(40)Ar/~(39)Ar mineral standards. The FCT zircon Concordia age also provides an empirical constraint on the decay constants of the U isotopes, indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of Jaffey et al. (1971).