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Abstract Hydration fronts penetrate 50–135 μm into glassy rhyolite embayments hosted in quartz crystals from the Mesa Falls Tuff in the Yellowstone Plateau volcanic field. The hydration fronts occur as steep enrichments that reach 2.4 ± 0.6 wt% H2O at the embayment opening, representing much higher values than interior concentrations of 0.9 ± 0.2 wt% H2O. Molecular water accounts for most of the water enrichment. Water speciation indicates the hydration fronts comprise absorbed meteoric water that modified the original magmatic composition of the rhyolitic glass. We used finite difference diffusion models to demonstrate that glass rehydration was likely produced over a few decades as the ignimbrite cooled. Such temperatures and time scales are consistent with rare firsthand observations of decadal hydrothermal systems associated with cooling ignimbrites at Mount Pinatubo (Philippines) and the Valley of Ten Thousand Smokes (Alaska). 

The faces of volcanic phenocrysts may be marked by imperfections occurring as holes that penetrate the crystal interior. When filled with glass these features, called embayments or reentrants, have been used to petrologically constrain magmatic ascent rate. Embayment ascent speedometry relies on the record of disequilibrium preserved as diffusion-limited volatile concentration gradients in the embayment glass. Clear, glassy embayments are carefully selected for speedometry studies. The use and subsequent descriptions of pristine embayments overrepresent their actual abundance. Here, we provide a textural analysis of the number, morphology, and filling characteristics of quartz-hosted embayments. We target a collection of large (i.e., >20 km³erupted volume) silicic eruptions, including the Bishop Tuff, Tuff of Bluff Point, Bandelier Tuff, Mesa Falls Tuff, and Huckleberry Ridge Tuff in the United States, Oruanui Tuff in New Zealand, Younger Toba Tuff in Indonesia, the Kos Plateau Tuff in Greece, and the Giant Pumice from La Primavera caldera in Mexico. For each unit, hundreds of quartz crystals were picked and the total number of embayment-hosting crystals were counted and categorized into classifications based on the vesicularity and morphology. We observed significant variability in embayment abundance, form, and vesicularity across different eruptions. Simple, cylindrical forms are the most common, as are dense glassy embayments. Increasingly complex shapes and a range of bubble textures are also common. Embayments may crosscut or deflect prominent internal cathodoluminescence banding in the host quartz, indicating that embayments form by both dissolution and growth. We propose potential additional timescales recorded by embayment disequilibrium textures, namely, faceting, bubbles, and the lack thereof. Embayment formation likely occurs tens to hundreds of years before eruption because embayment surfaces are rounded instead of faceted. Bubble textures in embayments are far from those predicted by equilibrium solubility. Homogenous nucleation conditions likely allow preservation of pressures much greater than magmastatic inside embayments. Our textural observations lend insight into embayment occurrence and formation and guide further embayment studies.