Deriving accurate eruption ages from complex zircon populations: insights from zircon trace element chemistry and intercalibration with astronomical time

Recent technical developments in U-Pb geochronology by isotope dilution thermal ionization mass spectrometry allow dating of high uranium accessory minerals at permil precision and accuracy. Such high temporal resolution may result in complex zircon age populations reflecting prolonged crystallization of autocrystic zircon as well as incorporation of antecrystic and xenocrystic zircon. While permil analytical precision allows to track evolution of Cenozoic magmatic systems at unprecedented temporal resolution (e.g. Schaltegger et al., 2009), aforementioned complexities may compromise deriving accurate deposition ages of ash beds from high precision zircon U-Pb dates. Schoene et al. (2010) suggest that the youngest closed system zircon (i.e. the youngest zircon inferred to be unaffected by lead loss) of complex populations best approximates the deposition age of ash beds. We present results of two studies testing our ability to derive eruption ages from complex zircon populations. We first present high precision zircon U-Pb dates from ash beds intercalated with astronomically tuned Miocene sediments. All ash beds yield complex zircon age populations recording prolonged crystallization at the 10-100 ka scale. However, in all cases, the youngest closed system zircons yield 206Pb/238U dates indistinguishable from the astronomical age and thus accurately date ash bed deposition. As analytical uncertainties approach the duration of precession cycles (i.e. ± 10 ka), zircon U-Pb derived ash bed deposition ages are potentially excellent tie points for astronomical tuning and to test the accuracy of existing astrochronologies. The second approach combines high precision U-Pb geochronology with trace element analysis on the same volume of zircon (referred to as U-Pb TIMS-TEA by Schoene et al., in press). We present new zircon U-Pb dates from the Fish Canyon Tuff that record ~400 ka of crystallization. Trace element compositions of dated zircons are used to model equilibrium melt composition employing published zircon-melt partition coefficients. This allows us to evaluate whether the youngest dated zircons crystallized in chemical equilibrium with their host melt, represented by interstitial glass quenched during eruption and thus accurately date eruption. We further compare our zircon U-Pb dates with results of recent studies reevaluating the age of the Fish Canyon Tuff based on intercalibration with astronomical time (Kuiper et al., 2008) and revisited 40K decay constants (Renne et al., 2010). Kuiper et al., 2008, Science, v. 320, p. 500-504; Renne et al., 2010, GCA, v. 74, p. 5349-5367; Schaltegger et al., 2009, EPSL, v. 286, p. 208-218; Schoene et al., 2010, Geology, v. 38, p. 387-390; Schoene et al., in press, GCA. Funded by European Community’s 7th Framework programme (GA no. 215458)