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 CH₄ indicate that deep gases mainly interact with ground waters and also with a hydrocarbon reservoir. The isotopic composition of carbon in CO₂ from these gases also suggests its primary magmatic origin, with inferred δ${}^{13}\text{C}$ values of `magmatic' CO₂ 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 CO₂ 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 CO₂ 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 CO₂ through the soil and the concentration of soil H₂, which were closely correlated with strong increases in the volcanic activity of Etna.

Introduction

Mount Etna is the largest volcano in Europe (∼1200 km² of total surface with a maximum height of ∼3300 m above sea level) and is one of the most active in the world. The relatively easy access to its active areas makes it an ideal site for field volcanological studies. An enormous amount of data has so far been collected on Etna for a great number of volcanological parameters (e.g., Chester et al., 1985; GNV, 1990). However, remarkably few data are available on the chemistry of Etna's volcanic gases, especially those emitted from fumaroles (Le Guern, 1972; Huntingdon, 1973; Allard, 1983), mainly because of the absence of suitable sampling sites. Air contamination in the sampled gases is generally very high, due both to the high permeability of the loose pyroclastic materials which make up most of the soils of Etna's summit and to the low flux of the emitted gases as a consequence of their preferential drainage towards the nearby open summit craters. Therefore, in recent years the attention of geochemists has been focused on soil gases sampled on the lower flanks of the volcano. Many studies in the last ten years have shown that important soil degassing phenomena occur diffusely on peripheral areas of Etna along active tectonic structures, where CO₂ is the most abundant species emitted (Aubert and Baubron, 1988; Allard et al., 1991; Anzà et al., 1989, Anzà et al., 1993; D'Alessandro et al., 1992; Giammanco et al., 1995, Giammanco et al., 1997; Parello et al., 1995; Baubron, 1996). The temporal variations in the concentrations of some of these soil gases, and in particular in the diffuse flux of CO₂, have shown significant correlations with variations in volcanic activity (Giammanco et al., 1995; Parello et al., 1995; Giammanco and Inguaggiato, 1996).

The aims of the present study were to characterize the chemical and isotopic compositions of the gases emitted from the crater fumaroles and from the soils of Etna at various altitudes. Secondly, by repeating our sampling, we studied how the chemical and isotopic compositions of these gases can be affected by variations in volcanic activity.