Precision of a single measurement in the low signal intensity area is better with a combination of a 1013 Ω amplifier and a SEM.

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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Understanding the timescales of magma evolution and ascent is essential for interpreting geophysical monitoring signals from active volcanoes. In this study, we explore the potential of diffusion-driven Li concentration and isotope zoning profiles recorded by magmatic olivine crystals to unravel time scales of magma evolution processes. Lithium is a fast-diffusing element and may provide the opportunity to investigate changes in magma composition during magma ascent, shortly before eruption. Lithium chemical and isotopic profiles were determined in olivines from two localities in the Massif Central volcanic region (France) that have previously been investigated for their Fe-Mg isotope systematics. The combined investigation of isotopic and chemical profiles makes it possible to distinguish between crystal growth and diffusion events. Extremely low δ 7 Livalues down to −30.7 (relative to the commonly used Li isotope standard IRMM-16) in the crystal core regions and elevated values at crystal rims (δ 7 Li ∼8 to 10 ), along with increasing concentrations from cores (∼3 to 1 µg/g) toward rims (12 to 6 µg/g) were found. The shape and orientation of both the chemical and isotopic profiles indicate that they were dominantly generated by Li diffusion into and within the olivine grains during magmatic differentiation. While Mg-Fe isotope and major element profiles have been modeled by a single diffusion event (Oeser et al., 2015), concentration and isotope profiles of Li indicate that a second diffusion event took place, that was not recorded by the Mg-Fe exchange diffusion couple. The first diffusion event was interpreted as reflecting the residence of the olivine crystals in a magma chamber. As diffusion coefficients for Fe-Mg exchange diffusion are very well determined, the time scales of this event are likely best quantified by Mg-Fe isotopic exchange diffusion modeling (Oeser et al., 2015). This event probably also generated the low δ 7 Li observed in olivine cores. Comparing the length of the Mg-Fe and Li profiles could thus be used to determine the less well-known diffusion coefficients of Li in the studied olivine crystals. The findings of this study indicate that Li diffusion at low Li concentration levels, as typically observed in natural olivine, may be not as fast as previously thought. The second diffusion event might represent a short-lived event, such as degassing, related to the ascent of the magma and/or magma cooling after emplacement of the lava. Such