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.

An early call for plate tectonics: The composition of continental crust far back in Earth's history gives us insight into when plate tectonics ramped up and has influenced ocean chemistry. Greber et al. looked at titanium isotopes in shales, which form from eroded continental crustal sediments, to estimate the composition 3.5 billion years ago, closer to the origins of Earth. They found a silica-rich composition, which indicates that plate tectonics was happening deep in our distant past. Other changes in crustal composition might be linked to changing ocean chemistry and major events such as the oxygenation of our atmosphere. Science , this issue p. 1271