YOUNG AND OLD RIMS ON ACCESSORY MINERALS: INSIGHTS FROM THE NEAR-ERUPTION CRYSTALLIZATION OF ZIRCONS FROM THE CRYSTAL-POOR AND -RICH COMPONENTS OF THE ~0.7 KA ERUPTION OF INYO DOMES, CALIFORNIA

With vertical sampling depths of only ~2–6 μm, secondary ion mass spectrometry analyses allow ²³⁸U–²³⁰Th and ²⁰⁶Pb/²³⁸U dating of the final few μm of crystal rim growth on accessory minerals from Quaternary silicic magmas. Studies conducted at the Stanford-USGS SHRIMP-RG laboratory provide information about the extent to which near-eruption crystallization is recorded by accessory minerals from erupted silicic magmas. In most of our studied cases (e.g., Yellowstone, Coso, Mono Craters, La Primavera, Rotoiti), the outer rims on a majority population of zircons ± allanites yield U-Th or Pb/U dates that are within error of their eruption age derived from independent radioisotopic techniques (e.g., ⁴⁰Ar/³⁹Ar, U-Th/He). However, a minority of examples yield majority zircon populations that do not appear to record near-eruption crystallization and instead yield outer rim dates that are 10³–10⁵ years older than eruption (e.g., Toba, Chaos Crags, Glacier Peak). Zircons from the Inyo Domes, a series of rhyolitic domes with mixed textural facies erupted together at 0.7 ka, provide insight into the petrologic significance of these zircon populations lacking near-eruption rims. The coarsely porphyritic and megacryst-rich facies of the Inyo Domes, a petrographically complicated assemblage, contain zircons yielding outer rim U-Th and U-Pb dates ranging from ~20 to 200 kyr before eruption. In contrast, zircons from the petrographically simple and crystal-poor facies of the Inyo Domes yield dates within error of the eruption age, suggesting that the coarsely porphyritic facies represents an antecrystic cargo that was remobilized by intrusion of petrologically young rhyolite and erupted before zircon crystallization could recommence. These observations suggest that crystal-rich silicic magmas, in particular those dominated by antecrystic major phases and representing erupted mushes, are unlikely to yield zircons recording near-eruption crystallization. Whether zircons record near-eruption crystallization, or not, in silicic magmas from a single volcanic system (e.g., Lassen: Chaos Crags versus Rockland tephra) may be correlated with petrographic complexity and could provide insight into the roles of rejuvenation and eruption triggering in a volcanic system.