Real-time measurements of δ13C, CO2 concentration, and CO2/SO2 in volcanic plume gases at Mount Etna, Italy, over 5 consecutive days

Abstract

We present new real-time measurements of the CO2 concentration and δ13C made from July 16 to 20, 2014 in diluted gases of the active plume emitted by the Central Craters at Mount Etna volcano, Italy. This innovative study involved measuring 13C in plume gases at a very high frequency over 5 days of measurements. The carbon-isotope composition calculated for volcanic CO2 ranged from –1.3‰ to +1.5‰, with uncertainties in the repeated single measurements (i.e., made over periods from 4 to 20 min) that were generally <0.7‰, and yet surprisingly varied by larger amounts over the 5-day study period. The range of calculated 13C values mostly overlaps with that indicated for the plume of the Central Craters obtained by discrete sampling and using the isotope-ratio mass spectrometry technique (–2.5‰ < 13C < –0.5‰). However, we propose that during particular conditions of volcanic activity, the carbon-isotope composition of CO2 degassed from magma can reach values (up to +1.5‰) that are higher than those reported previously. During this campaign we also made simultaneous measurements of the CO2 and SO2 concentrations using the MultiGAS technique. The volcanic 13C and CO2/SO2 ratios exhibited similar trends over the 5 days of measurements, with the ratios of both tracers peaking on July 16, possibly as a result of the early degassing of CO2 while an eruption was ongoing at Mount Etna. The observed variations and the highest 13C values measured at Mount Etna during this campaign lead to new questions about the variability of this geochemical tracer. The comparisons with the CO2/SO2 ratio also confirm that monitoring 13C in plume gases in real time, coupled to other geochemical tracers, is important for elucidating the magma dynamics at depth.