SIMS Constraints on the Near-Eruption Crystallization of Accessory Minerals from Intracontinental Rhyolites of the Western United States

A strength of secondary ion mass spectrometry (SIMS) is micron-scale spatial resolution, in particular in the vertical dimension, with typical sampling depths of 1-5 micrometers for U-Th disequilibrium and U-Pb geochronology. This shallow sampling depth can be leveraged to date the final few micrometers of crystallization preserved by single crystals of zircon and other accessory minerals that have been embedded into a mount of soft metal. When applied to zircons from the Quaternary rhyolites of the Yellowstone Plateau, this sampling approach has yielded 238U-230Th and 206Pb/238U dates that effectively constrain eruption ages. SIMS analyses of the crystal faces of zircons from the ca. 630 ka Lava Creek Tuff eruption yield 206Pb/238U dates that are within error of their eruption age derived from other radioisotopic techniques and astrochronology. Rhyolite lavas from the youngest episode of volcanism at Yellowstone caldera yield 238U-230Th dates between ca. 70 and 120 ka, in each case within millennia of their respective eruption ages from 40Ar/39Ar dating of their sanidines. However, the interiors of these same zircons yield crystallization ages that may be tens of thousands of years older than their rims, suggesting a near-eruption recycling of antecrysts from intrusions or a long-lived magma mush. The crystal faces of zircons and allanites from other intracontinental rhyolites in the western United States, including those erupted at Mono Craters in eastern California, similarly yield 238U-230Th dates that closely match their eruption ages. A notable exception is the coarsely porphyritic and megacryst-rich rhyolite of Inyo Domes, California, whose zircons yield crystal face dates that range up to 200 k.y. before eruption. However, zircons from a crystal-poor facies of the Inyo Domes yield dates that are indistinguishable from the ca. 1 ka eruption age, suggesting that the coarsely porphyritic portion represents a mush of antecrysts that was quickly mobilized. The observation of near-eruption dates for the crystal faces of accessory minerals from multiple intracontinental rhyolites of the western United States indicates that this SIMS approach is a promising analytical technique for dating many of the silicic tephras that are preserved in Quaternary sediments of the Basin and Range Province.