Low-ै 18 O Rhyolites from Yellowstone: Magmatic Evolution Based on Analyses of Zircons and Individual Phenocrysts
AbstractThe Yellowstone Plateau volcanic field is one of the largest centers of rhyolitic magmatism on Earth. Major caldera-forming eruptions are followed by unusual low-ै 18 O 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-ै 18 O magmas erupted after the Huckleberry Ridge Tuff (2·0 Ma, 2500 km 3 ) and Lava Creek Tuff (0·6 Ma,1000 km 3 ). Analyses of individual quartz and sanidine phenocrysts, obsidian samples and bulk zircons from low-ै 18 O lavas reveal: (1) oxygen isotope variation of 1–2‰ between individual quartz phenocrysts; (2) correlation of zircon crystal size and ै 18 O; (3) extreme (up to 5‰) zoning within single zircons; zircon cores have higher ै 18 O; (4) ख 18 O disequilibria between quartz, zircon and homogeneous unaltered host glass where zircon cores and some quartz phenocrysts have higher ै 18 O values. These features are present only in low-ै 18 O intra-caldera lavas that erupted shortly after caldera-forming eruptions. We propose that older, hydrothermally altered, 18 O-depleted (ै 18 O ∼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-ै 18 O melt that erupted in different parts of the caldera as separate low-ै 18 O lava flows. Alteration-resistant quartz and zircon in the roof rock survived early hydrothermal alteration and later melting to become normal ै 18 O xenocrysts (retaining their pre-caldera ै 18 O values) in the low-ै 18 O magma that formed by melting of hydrothermally 18 O-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-ै 18 O magma explains ै 18 O and ख(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-ै 18 O magmas after formation and before eruption.