Helium and Carbon isotopic characterization of the dissolved gases in Mt. Etna groundwater (Italy).

Abstract

We report the chemistry and He and C isotopic composition of dissolved gases in groundwaters of Mt. Etna for 17 sampling sites (wells, springs, artificial drainage) for the period February 2006-January 2007. A conceptual model on degassing and gas-water interaction processes, including the 2006 eruptive period of Mt. Etna is proposed. For all sites, CO2 is the most abundant dissolved gas phase, reaching concentrations up to 700 cc/l(w). Helium varies from 3,83x10-5 to 1.24x10-3 cc/l. The 13CCO2 (recalculated from 13CTDC) varies from -13.4 to -2.3 ‰ vs. V-PDB; the lower values result from mixing with organic carbon, while the higher values are identical to the C isotopic composition for Mediterranean volcanism. The 3He/4He ratios (R/RA with RA the 3He/4He ratio for air of 1.39x10-6) for dissolved gases in Etna groundwater ranges from 1.3 to 6.4 RA, depending on mixing proportions between air (1RA) and the Etnaean magmatic end-member (3He/4He = 7.1 RA). The groundwater at one site shows a clear trend towards mixing with a different magmatic end-member (3He/4He = 5.3 RA, western periphery), probably the result of the contribution of a more degassed magma. This is confirmed by the fact that this site does not show any temporal variations, as the others do during the 2006 eruption. The values for log(Ct/3He) are generally above 11, and can be as high as >13, clearly higher than log(Ct/3He) values for magmatic systems (~9.6). This is not surprising for dissolved gases, as CO2 is ~30 times more soluble in water than He wish is outgassed more easily.