Tracking the evolution of silicic caldera systems through zircon geochronology

Quantification of timescales of magmatic evolution and dating of eruptions is critical for understanding large silicic caldera systems, which are capable of producing destructive supereruptions (erupting >1000 km³ of tephra). In-situ dating of both the outer rims and cores of zircon crystals provides a powerful means to investigate these timescales. The micron-scale spatial resolution and shallow sampling depth of secondary ion mass spectrometry (SIMS) for U-Th and U-Pb geochronology enables dating of the final few micrometers of crystallization preserved by single zircon crystals. SIMS (using the USGS-Stanford SHRIMP) analyses of zircon crystal faces from the Lava Creek Tuff (LCT) supereruption, Yellowstone (Wyoming), yield ²⁰⁶Pb/²³⁸U dates that match their eruption age derived from other radioisotopic techniques (⁴⁰Ar/³⁹Ar) as well as astrochronology, thus constraining the age of eruption at ca. 630 ka. Zircon geochronology results also show that both LCT ignimbrite members erupted over a geologically brief interval. A notable exception of zircon outer rim ages constraining eruption ages is the coarsely porphyritic and megacryst-rich rhyolite of Inyo Domes, Long Valley caldera (California), whose zircons yield crystal face dates that range up to 200 kyr 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 long-lived mush of antecrysts that were quickly mobilized. Dating of these mixed magmas reveals that the rhyolitic mush was episodically tapped over a period of hundreds of thousands of years. Periods of thermal rejuvenation elucidated by the zircon ages correspond with episodes of glacial unloading, suggesting a causal link between climate change and volcanism at Long Valley.