ION MICROPROBE 206PB/238U AND 230TH/238U ZIRCON AGES FROM EXTRACALDERA RHYOLITES AT YELLOWSTONE: CONSTRAINTS ON MAGMA RESIDENCE TIMES AND EVOLUTION
Disequilibrium corrected 206Pb/238U zircon ages of the oldest rhyolite, Willow Park Dome (40Ar/39Ar eruptive age 326 ± 2 ka, uncertainties 1σ), range from 280 to 403 ka, with a weighted mean age of 353 ± 23 ka (n=6, MSWD=0.5). Similarly, 206Pb/238U ages for a younger mingled rhyolite, the Gardner River flow (40Ar/39Ar age 301 ± 3 ka) are 300 to 437 ka, with a weighted mean age of 340 ± 17 ka (n=8, MSWD=1.4). Cathodoluminescence (CL) imaging reveals that most zircons exhibit simple magmatic oscillatory zoning; inherited cores are rare. These results indicate mean magma residence times of ~30-40 ka for early evolution of the system. Zircons from the Paintpot Hill dome (40Ar/39Ar age 208 ± 5 ka) yield 230Th/238U disequilibrium ages ranging from 205 to 225 ka, within uncertainty of the eruptive age, with four spots in apparent secular equilibrium (older than ~300 ka). 206Pb/238U ages of these spots yielded ages of 276 to 311 ka. 230Th/238U ages from Gibbon Hill dome (40Ar/39Ar age 134 ± 3 ka) range from 122 to 180 ka, with two spots in secular equilibrium; these did not produce reliable 206Pb/238U. Four of the ten 230Th/238U analyses are older outliers based on MSWD criteria, with the remainder defining a weighted mean age of 129 ± 3 ka, indistinguishable from the eruptive age. CL imaging identifies well defined magmatic zoning in all zircons, with no inherited cores.
These data suggest apparent magma residence times ranging up to ~100 ka. The spatial, temporal and geochemical data suggest that extracaldera rhyolites were derived from a magma system characterized by a >200 ka lifespan and are consistent with either a single magma chamber evolving by FXL and recharge, or crystallization and subsequent rejuvenation of the resulting crystal mush over this time period. Both models require an evolving, active magma system.