Lithium isotopic and concentration gradients in plagioclase - implications for pre- and syn-eruptive magmatic processes

The rapid diffusion of lithium (Li) in feldspar at magmatic temperatures allows timescales to be estimated for processes which would otherwise remain obscured.

Plagioclase phenocrysts from the Mesa Falls Tuff (MFT) rhyolite, a 1.30 Ma explosive, caldera-forming eruption in Yellowstone, show significant depletion in Li contents towards their rims (by a factor of four). The lack of correlation between Li and any other major/trace element precludes the low-Li rims to result from a late-stage magma mixing event. Rather, these rims record degassing prior to, or during, the eruption as the plagioclase rims equilibrate with the melt that is losing Li to the vapour phase. The volatility of Li in silicic magmas is supported by Li concentrations measured in groundmass glass of MFT that are a factor of five lower than the concentration of Li in quartz-hosted melt inclusions.

Plagioclase crystals from MFT have bulk Li isotopic compositions ranging from -2.3‰ to -1.4‰ (relative to LSVEC), reflecting the average and volume-integrated δ⁷Li. Based on the concentration gradients ( 20 ppm in the core to 5 ppm in the rim) in the plagioclase grains there is a potential for larger Li intra-mineral isotopic variability due to kinetic fractionation of Li isotopes within the plagioclase crystals as a result of the degassing processes. To address this, femtosecond-laser ablation-MC-ICP-MS was applied to measure δ⁷Li in situ with a spatial resolution of 50 µm for the first time. Guided by Li concentration maps generated by LA-ICP-MS, Li isotope profiles were measured across the plagioclase grains. These profiles reveal that the rims are on average six permil heavier (average δ⁷Li = +2.5‰) than the cores (average δ⁷Li = -3.5‰). The lowest average δ⁷Li of -7.5‰ are observed in an intermediate zone. Modelling these diffusion profiles allows determining the timescale of the degassing event. Estimating these timescales will provide useful constraints for monitoring volcanoes and could provide a better understanding of the pre/syn-eruptive behaviour of the vapour phase.