ORIGIN AND EVOLUTION OF SILICIC MAGMATISM AT YELLOWSTONE BASED ON THE ION MICROPROBE ANALYSIS OF ISOTOPICALLY-ZONED ZIRCONS

The present a high-resolution look at Yellowstone magmatism using ion microprobe analysis for &delta18O in zircons with precision of ±0.2‰ and a 10um spot size. We report 357 analyses of cores and rims, and isotope profiles of 142 zircons in 11 units that represent Yellowstone ignimbrites and post-caldera lavas that were earlier studied by laser fluorination, and previously dated in the same spots by SHRIMP. New observations are the following. 1) Most zircons from ~0.5 Ma post-caldera low-&delta18O Upper Geyser Basin lavas are zoned with higher-&delta18O and highly variable U-Pb ages in cores, that suggest inheritance from pre-caldera >0.64 Ma rocks. 2) Many higher-&delta18O zircon cores in these lavas have 0.64-0.50 Ma U-Pb ages that postdate the caldera formation, but predate the lava's eruption age by <20 ky, and represent inheritance of unexposed post-Lava Creek tuff intracaldera subvolcanic units, formed by cooling the LCT batholith. 3) These 0.5Ma zircons suggest that zircon crystallization, hydrothermal alteration, and zircon re-incorporation in later low-&delta18O magma happened quite rapidly, < 10 to 20 ky. 4) Voluminous 0.25-0.1 Ma intracaldera Central Plateau lavas contain zircons with both high-, and low-&delta18O cores. These youngest lavas were formed after LCT mush was crystallized, by remelting a variety of rocks from high-d18O pre-caldera and low-&delta18O post-caldera units followed by residence in a common intermediate-d18O melt prior to eruption. 5) 2Ma Huckleberry Ridge tuff contains zoned zircons with lower-&delta18O cores suggesting zircon inheritance from the low-&delta18O hydrothermally-altered carapace as an important process during formation of large magma bodies prior to caldera collapse. Large-volume silicic magmatism at Yellowstone reflects rapid, shallow-level remelting of earlier erupted and hydrothermally-altered Yellowstone source rocks. Pulses of basaltic magma provided the heat that drove postcaldera volcanism. Each post-caldera Yellowstone lava was variable in &delta18O, but inherited zircons that retained earlier the age and &delta18O values, and represents an independent magma batch that was generated rapidly by remelting of source rocks of various ages and &delta18O. The LCT magma was already an uneruptable mush by the time of the first post-LCT eruption, ca. 0.1 My after the climatic caldera forming eruption.