Abstract
Lithium gradients in plagioclase are capable of recording extremely short-lived processes associated with gas loss from magmas prior to extrusion at the surface. We present SIMS profiles of the 7Li/30Si ion ratio in plagioclase crystals from products of the paroxysmal sequence that occurred in the period 2011–2013 at Mt. Etna (Italy) in an attempt to constrain the final ascent and degassing processes leading to these powerful eruptions involving basic magma. The observed Li concentrations reflect cycles of Li addition to the melt through gas flushing, and a syn-eruptive stage of magma degassing driven by decompression that finally produce significant Li depletion from the melt. Modeling the decreases in Li concentration in plagioclase by diffusion allowed determination of magma ascent timescales that are on the order of minutes or less. Knowledge of the storage depth beneath the volcano has led to the quantification of a mean magma ascent velocity of ~43 m/s for paroxysmal eruptions at Etna. The importance of these results relies on the application of methods, recently used exclusively for closed-system volcanoes producing violent eruptions, to open-conduit systems that have generally quiet eruptive periods of activity sometimes interrupted by sudden re-awakening and the production of anomalously energetic eruptions.
Highlights
Modes and timescales of magma ascent and degassing in the shallow portion of a volcano plumbing system are crucial to understanding the dynamics of an eruption
The rates of Li diffusion in plagioclase are 10−10-10−12 m2 s−1 for a temperature range of 200–850 °C, i.e. orders of magnitude faster than NaSi-CaAl interdiffusion[20,21,22]. This means that Li diffusion is sufficiently rapid to constrain short-lived magmatic processes, because any Li concentration gradient in plagioclase would be rapidly homogenised through diffusion to the equilibrium profile, unless arrested by sudden cooling of magma after emission[12]
We report here the results of Secondary Ion Mass Spectrometry (SIMS) investigations of Li in plagioclase crystals found in agglutinates and rheomorphic lava flows produced by Mt
Summary
Thin sections representative of agglutinates and rheomorphic lavas from various paroxysmal events occurred at Mt. We used a 5 nA 16O, 6–8 μm Ø primary beam to sputter secondary ions from micro-areas chosen along transects within the crystals and in the adjacent matrix glass. The SEM-EDS/WDS SiO2 (wt.%) values at each spot were adopted in the SIMS quantification of Li. in order to further reduce the residual matrix effects and take into account the different SiO2 (wt.%) values among the plagioclase crystals and the NIST glasses (~50 wt.% SiO2 vs 72.2 wt.% SiO2 in the NIST-series glasses), we applied a correction to the relative-to-Si ion yield for Li, i.e., I(Li)/I(Si) / Li(at)/Si(at) [where I(Li) and I(Si) represent the ionic signals for Li and Si, and Li(at) and Si(at), their respective atomic concentrations]25. The 30Si count rate was converted into 30Sii, which is proportional to the amount of plagioclase sputtered during the analysis. The 7Li/30Sii signal ratio represents a value proportional to the molar Li content within the plagioclase, i.e. proportional to the number of Li atoms for the tetrahedral site[17]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
