ION MICROPROBE ²⁰⁶PB/²³⁸U AND ²³⁰TH/²³⁸U ZIRCON AGES FROM EXTRACALDERA RHYOLITES AT YELLOWSTONE: CONSTRAINTS ON MAGMA RESIDENCE TIMES AND EVOLUTION

Rhyolites erupted at 526-106 ka north of the Yellowstone caldera record the most recent migration of the Yellowstone melting anomaly. Ten of twelve domes/flows define a spatially (Norris-Mammoth Corridor), temporally (326-106 ka) and geochemically coherent group. These rhyolites are characterized by significant early magma mingling/mixing events (326-263 ka) followed by evolution of high-silica rhyolite magmas consistent with fractional crystallization (FXL) and magma chamber recharge.

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.