Multi-Stage Magma Evolution in Intra-Plate Volcanoes: Insights From Combined in situ Li and Mg–Fe Chemical and Isotopic Diffusion Profiles in Olivine

Abstract

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

Time scales of syneruptive volatile loss in silicic magmas quantified by Li isotopes

“…The decreased Li concentration and elevated δ 7 Li toward crystal edges can be explained by scavenging of Li from the melt into the vapor phase during magma ascent and degassing (e.g., Genareau et al, 2009;Giuffrida et al, 2018;Steinmann et al, 2020). 6 Li preferentially moves into the vapor phase during vesiculation (Watson, 2017), resulting in the depletion of 6 Li along with Li concentrations in the melt, inducing a disequilibrium between plagioclase and melt (Fig.…”

“…The extent to which diffusion can homogenize these records reflects the time between onset and termination of diffusion upon quenching. Here, we use a novel technique combining Li concentration and Li stable-isotope ratios (Steinmann et al, 2020) in plagioclase phenocrysts from the MFT and applying diffusion modeling to constrain time scales of Li loss from the melt. Concentration profiles through crystals confirm high-Li cores, from 12 to 33 ppm depending on the crystal (mean 22 ppm; n = 1440), decreasing to 19 to ~5 ppm at the very edge of the crystals (mean 12 ppm; n = 208).…”

Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts

“…A combination of Li concentration gradients and diffusive Li isotope fractionation in plagioclase was employed by Neukampf et al (2021) to estimate the time span between volatile degassing and quenching during the eruption of a magma. Steinmann et al (2020) unravelled multi-stage magmatic diffusion processes in olivine that could not be resolved by major elements (Fe-Mg interdiffusion) alone.…”

Degassing from magma reservoir to eruption in silicic systems: The Li elemental and isotopic record from rhyolitic melt inclusions and host quartz in a Yellowstone rhyolite

“…The high and variable diffusivity of Li isotopes can cause large variations of Li concentration along chemical potential gradients and result in up to 40‰ kinetic fractionation of 7 Li/ 6 Li (Richter et al, 2003). The δ 7 Li variations of this range can be observed within individual mineral phases at all scales, from a few microns (e.g., Jeffcoate et al, 2007;Parkinson et al, 2007;Lynn et al, 2018;Cisneros de León and Schmitt, 2019;Steinmann et al, 2020) up to several tens of meters (e.g., Lundstrom et al, 2005;Marschall et al, 2007;John et al, 2012;Beinlich et al, 2020). To provide further complexity to kinetic Li isotope behaviour, a recent study by Neukampf et al (2021) on a flow pumice sample from the Mesa Falls Tuff (MFT; Fig.…”