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To reflect magmatic conditions, volcanic rocks must retain their compositions through eruption and post-eruptive cooling. Mostly, this is the case. However, welded ignimbrites from the Yellowstone–Snake River Plain magmatic province reveal systematic modification of the lithium (Li) inventory by post-eruptive processes. Here we show that phenocrysts from slowly cooled microcrystalline ignimbrite interiors consistently have significantly more Li than their rapidly quenched, glassy, counterparts. The strong association with host lithology and the invariance of other trace elements indicate that Li remains mobile long after eruption and readily passes into phenocrysts via diffusion as groundmass crystallisation increases the Li contents of the last remaining melts. Li isotopic measurements reveal that this diffusion during cooling combined with efficient degassing on the surface may significantly affect the Li inventory and isotopic compositions of volcanic rocks. Utilisation of Li for petrogenetic studies is therefore crucially dependent on the ability to ‘see through’ such post-eruptive processes.

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Most explosive, silicic volcanoes spend thousands of years in repose between eruptive events. The timing of the switch from repose to eruption is key to interpreting monitoring signals and improving the safety of people living close to active volcanoes. We addressed this question using a novel technique based on lithium isotopic (δ7Li) and elemental concentration profiles within plagioclase crystals from the Mesa Falls Tuff of the Yellowstone volcanic system (Idaho and Wyoming, USA), constraining volatile degassing to occur on minimum time scales of tens of minutes prior to eruption. During this ephemeral time, Li abundances drop by a factor of four to 10 from crystal cores to rims, accompanied by an increase in δ7Li of as much as 10‰, reflecting diffusion-driven equilibration between plagioclase cores and outgassed, Li-poor melt. New times scales obtained in this study show the potential for rapid syneruptive changes in the volatile inventory of magmas.

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