| Summary: | International audience How the solubility of volatile elements varies with magma composition, pressure and temperature controls theiroutgassing at active volcanoes. In general, the relative solubility of major volatile species in basalt increases fromCO2, H2O, S, Cl to F. The first three species are known to form a gas phase both before and during volcaniceruptions, whereas the last two principally outgas during and after an eruption. Filter-pack gas samples collectedfrom the main crater of the Holuhraun eruption (29 August 2014 to 27 February 2015) a few days after theeruption ended have been compared with those of the main eruptive plume, confirming the degassing order of themajor gaseous elements. The eruption was characterized by high lava discharge rate and significant gas emissionwith approximately 10 Tg, SO2. Primary magmatic degassing had elevated S/Cl, infinitive S/F due to almostnegligible F degassing and high trace metal fluxes to the atmosphere (Gauthier et al., 2016; Gislason et al., 2015)causing a significant environmental impact (Ilyinskaya et al., 2017; Stefansson et al., 2017). Nevertheless, snowprecipitated around the lava field and collected during the eruption had significant F concentrations despite itsvery low abundance in the main gas plume.The post-eruptive diluted gas emissions of Holuhraun is characterized by concentrations of major gases withSO2, HCl and HF ranging from 98-327, 55-107, 28-49, respectively (mg/m3), with much lower S/Cl (1.4 0.4),Cl/F (2.0 0.2) and hence elevated F/S (0.4 0.1) compared to the syn-eruptive gas (46 8, 11 5, 0,respectively). Low S/Cl mass ratios (0.9-1.6) correspond to a factor of four decrease in S degassing and a factorof seven increase in Cl degassing compared to the primary magmatic degassing. High F/Cl suggests that 50 timesmore F is released during the secondary degassing event. Outgassing of the lave field thus explains the elevated Fin the environmental samples such as the snow and rivers flowing to the lowland. The lava composition ...
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