Origin and evolution of silicic magmatism at Yellowstone based on ion microprobe analysis of isotopically zoned zircons

The origin of large-volume Yellowstone ignimbrites and smaller-volume intra-caldera lavas requires shallow remelting of enormous volumes of variably O-18-depleted volcanic and sub-volcanic rocks altered by hydrothermal activity. Zircons provide probes of these processes as they preserve older ages and inherited delta O-18 Values. This study presents a high-resolution, oxygen isotope examination of volcanism at Yellowstone using ion microprobe analysis with an average precision of +/-0.2 parts per thousand and a 10 mu m spot size. We report 357 analyses of cores and rims of zircons, and isotope profiles of 142 single zircons in 11 units that represent major Yellowstone ignimbrites, and post-caldera lavas. Many zircons from these samples were previously dated in the same spots by sensitive high-resolution ion microprobe (SHRIMP), and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination. We additionally report oxygen isotope analyses of quartz crystals in three units. The results of this work provide the following new observations. (1) Most zircons from post-caldera low-delta O-18 lavas are zoned, with higher delta O-18 values and highly variable U-Pb ages in the cores that suggest inheritance from pre-caldera rocks exposed on the surface. (2) Many of the higher-delta O-18 zircon cores in these lavas have U-Pb zircon crystallization ages that postdate caldera formation, but pre-date the eruption age by 10-20 kyr, and represent inheritance of unexposed post-caldera sub-volcanic units that have delta O-18 similar to the Lava Creek Tuff. (3) Young and voluminous 0.25-0.1 Ma intra-caldera lavas, which represent the latest volcanic activity at Yellowstone, contain zircons with both high-delta O-18 and low-delta O-18 cores surrounded by an intermediate-delta O-18 rim. This implies inheritance of a variety of rocks from high-delta O-18 pre-caldera and low-delta O-18 post-caldera units, followed by residence in a common intermediate-delta O-18 melt prior to eruption. (4) Major ignimbrites of Huckleberry Ridge, and to a lesser extent the Lava Creek and Mesa Falls Tuffs, contain zoned zircons with lower-delta O-18 Zircon cores, suggesting that melting and zircon inheritance from the low-delta O-18 lydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse. (5) The delta O-18 zoning in the majority of zircon core-rim interfaces is step-like rather than smoothly inflected, suggesting that processes of solution-reprecipitation were more important than intracrystalline oxygen diffusion. Concave-downward zircon crystal size distributions support dissolution of the smaller crystals and growth Of rims on larger crystals. This study suggests that silicic magmatism at Yellowstone proceeded via rapid, shallow-level remelting of earlier erupted and hydrothermally altered Yellowstone source rocks and that pulses of basaltic magma provided the heat for melting. Each post-caldera Yellowstone lava represents an independent homogenized magma batch that was generated rapidly, by remelting of source rocks of various ages and delta O-18 values. The commonly held model of a single, large-volume, super-solidus, mushy-state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data. Rather, the source rocks for the Yellowstone volanism were cooled below the solidus, hydrothermally altered by heated meteoric waters that caused low delta O-18 values, and then remelted in distinct pockets by intrusion of basic magmas. Each packet of new melt inherited zircons that retained older age and delta O-18 values. This inter relation may have significance for interpreting seismic data for crustal low-velociy zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side. New basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas. We also suggest that the Lava Creek Tuff magma was already an uneruptable mush by the time of the first post-caldera eruption after 0.1 Myr of the climactic caldera-forming eruption.