Soil and fumarole gases of Mount Etna: geochemistry and relations with volcanic activity

Abstract

Between 1993 and 1996, gas samples were collected on Mt. Etna volcano from both high- and low-temperature fumaroles and from soils in areas of anomalous degassing located on the flanks of the volcanic edifice. The chemical composition of all the emitted gases is consistent with their magmatic origin. During their transport to the surface, they would undergo shallow mixing processes, mostly with air. Locally, as in the case of a point in the lower southwestern flank of the volcano (P39), very high contents of He and CH 4 indicate that deep gases mainly interact with ground waters and also with a hydrocarbon reservoir. The isotopic composition of carbon in CO 2 from these gases also suggests its primary magmatic origin, with inferred δ 13 C values of `magmatic' CO 2 in the range −2 to −1‰ vs. PDB. Such values, significantly more positive than those measured in MORBs, are in the typical range measured at the other Mediterranean volcanoes (−5 to 0 δ‰). More negative values can be explained by shallow interactions between magmatic and organic CO 2 and/or between the former and cold ( T<120°C) ground waters, whereas more positive values are thought to be mainly the result of interactions between magmatic CO 2 and hot ( T>120°C) ground waters. Both chemical and isotopic gas geothermometers agree in indicating calculated temperatures ranging from 180° to 270°C for the soil gases collected from low temperature fumaroles near the summit and temperatures around 150°C for the cold gases from P39, thus confirming the presence of a hydrothermal aquifer beneath this point. A chemical geothermometer applied on the gases collected from the high temperature fumaroles near the summit craters shows equilibrium temperatures ranging from 385° to 690°C. Lastly, many of the studied parameters showed anomalous variations in time during the studied period. The most important of these concerned the flux of CO 2 through the soil and the concentration of soil H 2, which were closely correlated with strong increases in the volcanic activity of Etna.