Low-δ18O Rhyolites from Yellowstone: Magmatic Evolution Based on Analyses of Zircons and Individual Phenocrysts

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Abstract
The Yellowstone Plateau volcanic field is one of the largest centers of rhyolitic magmatism on Earth. Major caldera-forming eruptions are followed by unusual low-δ18O rhyolites. New oxygen isotope, 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-δ18O magmas erupted after the Huckleberry Ridge Tuff (2·0 Ma, 2500 km3) and Lava Creek Tuff (0·6 Ma,1000 km3). Analyses of individual quartz and sanidine phenocrysts, obsidian samples and bulk zircons from low-δ18O lavas reveal: (1) oxygen isotope variation of 1–2‰ between individual quartz phenocrysts; (2) correlation of zircon crystal size and δ18O; (3) extreme (up to 5‰) zoning within single zircons; zircon cores have higher δ18O; (4) Δ18O disequilibria between quartz, zircon and homogeneous unaltered host glass where zircon cores and some quartz phenocrysts have higher δ18O values. These features are present only in low-δ18O intra-caldera lavas that erupted shortly after caldera-forming eruptions. We propose that older, hydrothermally altered, 18O-depleted (δ18O ∼0‰), but otherwise chemically similar, rhyolites in the down-dropped block were brought nearer the hot interior of the magma chamber. These rhyolites were remelted, promoting formation of almost totally molten pockets of low-δ18O melt that erupted in different parts of the caldera as separate low-δ18O lava flows. Alteration-resistant quartz and zircon in the roof rock survived early hydrothermal alteration and later melting to become normal δ18O xenocrysts (retaining their pre-caldera δ18O values) in the low-δ18O magma that formed by melting of hydrothermally 18O-depleted volcanic groundmass and feldspars. Zircon and quartz xenocrysts exchanged oxygen with newly 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-δ18O magma explains δ18O and Δ(Qz–Zrc) disequilibria. The exchange time to form zoned zircons is between a few hundred and a few thousand years, which reflects the residence time of low-δ18O magmas after formation and before eruption.

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