Multi-day, real-time measurements of CO2 concentration, CO2/SO2 ratios, and d13C of CO2 in volcanic plume

Abstract:

New real-time measurements of CO₂ /SO₂ ratios, the CO₂ concentration and δ¹³C of CO₂ were acquired from July 16 to 20, 2014 in the active plume about 1 km away from the source at the Central Craters of Mount Etna volcano, Italy. During this innovative study we measured d¹³C in plume gases at a frequency of 0.1 Hz over 5 days of measurements. Assuming two member mixing processes, the extrapolated carbon-isotope composition of the 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 surprisingly larger variations over the 5-day study period. The range of extrapolated d¹³C values mostly overlaps with that obtained by discrete sampling and using isotope-ratio mass spectrometry technique (–2.5‰ < d¹³C < –0.5‰). However, the particular conditions of volcanic activity during the campaign lead us to propose that the carbon-isotope composition of CO₂ degassed from magma can reach values (up to +1.5‰), higher than previously reported.

Simultaneous measurements of the CO₂ and SO₂ concentrations using the MultiGAS technique were also performed. The volcanic d¹³C 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.

New questions about the variability of this geochemical tracer arise from the observed variations and the highest d¹³C values measured at Mount Etna during this campaign. The comparisons with the CO₂/SO₂ ratio also confirm that monitoring d¹³C in plume gases in real time, coupled to other geochemical tracers, is important for elucidating the magma dynamics at depth.