Low-δ18O rhyolites from Yellowstone:: Magmatic evolution based on analyses of zircons and individual phenocrysts

The Yellowstone Plateau volcanic field is one of the largest centers of rhyolitic magmatism on Earth. Major caldera-forming arefollowed by unusual low-6`0 rkyolites. A ea, awgen I. sotope, petrologic and geochemical data from rhyolites belonging to the 2.0 my eruptive history of Yellowstone are presented, with emphasis on the genesis of low-delta O-18 magmas erupted after the Huckleberry Ridge-Tuff (2.0 Ma, 2500 km) and Lava Geek Tuff (0.6 Ma, 1000 km(3)). Analyses of individual quartz and sanidine phenocrysts, obsidian samples and bulk zircons from low-delta O-18 lavas reveal: (1) oxygen isotope variation of 1 2 parts per thousand between individual quartz phenocrysts: (2) correlation of zircon crystal size and delta O-18; () extreme (up to 5 parts per thousand) zoning within single zircons; zircon cores have higher delta O-18; (4) Delta O-18 disequilibria between quartz, zircon and homogeneous unaltered host glass where zircon cores and some quartz phenocrysts have higher delta O-18 values. These features are present only in low-delta O-18 intra-caldera lavas that erupted shortly after caldera forming eruptions. We propose that older hydrothermally altered, O-18-depleted (delta O-18 similar to 0 parts per thousand), but otherwise chemically similar; rhyolites in the down-dropped block were brought nearer the hot interior of the magma chamber These rhyolite) were remelted, promoting formation of almost totally molten pockets of low-delta O-18 melt that erupted in different parts of the caldera (is separate low-delta O-18 lava flows. Alteration-resistant quartz and zircon in the roof rock survived earl), hydrothermal alteration and later melting to become normal delta O-18 xenocrystals (retaining their precaldera delta O-18 values) in the low-delta O-18 magma that formed by melting of hydrothermally delta O-18-depleted volcanic groundmass and feldspars. Zircon and quartz xenocrystals exchanged oxygen with new formed melt through diffusion and overgrowth mechanisms leading to partial or complete isotopic re-equilibration. Modeling of the diffusive exchange of zircon and quartz during residence in low delta O-18 magma explains delta O-18 and Delta (Qz-Zrc) disequilibria. Die exchange time to form zoned zircons is between a few hundred and a few thousand years, which reflects the residence time of low-delta O-18 magmas after formation and before eruption.