GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.3 161 that CH 4 and H 2 S concentrations are way higher is some soil samples than in fumaroles, and that [H 2 ] is always positively correlated with T. ������������ �������� ���� ���� ���� � �������� �� Furthermore, sampling data show that a decrease of H 2 matches an increase of H 2 S and CH 4 due to T increasing with depth (Figs. 3a and 3b). On these evidences something has to be hypothesized causing either preferential enrichment of CH 4 , H 2 S or variations in gases equilibrium processes. One possible explanation is the preferential CO 2 removal in surface acid muddy water pools. Another could be that near surface gases condense with neutral pH causing CO 2 dissolution, and thus enrichment in other species. If this cannot be excluded, the quick re-equilibrium of some reactions involving the analysed chemical species seems more noteworthy. We consider, then, two possible reaction for CH 4 and H 2 S genesis and enrichment: CO 2 4H 2 CH 4 2H 2 O (1) Molar ratios in Eq. 1 justify the observed quick [H 2 ] decrease and the less significant (CH 4 ) increase with decreasing depth. The linear relation in Fig. 3a shows that reaction 1 occurs quickly and at shallow depth during gas arise, because we observed significative H 2 and CH 4 variations at different depths in the first meter of the soil. To justify H 2 S enrichment, gas equilibria in sulphur species cannot be considered because H 2 S is the only gas species in the Solfatara volcanic/hydrothermal gas (Chiodini et al. , 2001). As previously described, Giggenbach (1980) and Caliro et al. ������ ������ ���� (2007) stated that f H 2 S in hydrothermal system is mainly controlled by pyrite coexisting with unspecified allumo-silicate. We can similarly speculate H 2 S enrichment in the shallow Solfatara soil by hydrolysis with sulphide minerals: MeS 2 H 2 (H 2 O) MeO 2H 2 S (2) All our soil samples show temperature values lower than 100 C and inverse relationship with H 2 S concentrations; this could reflect a variation in thermodynamic conditions compared to the hot fumarolic fluids. In our vertical profile sampling, an increase in H 2 S is very marked and this evidence suggests that the process happens close the surface. Reaction 2 can be related with emission temperatures, gas flow, soil humidity (condensate) and f H 2 . In fact, when there are high flow/temperature conditions, Eq. 2 can be slowed or even inhibited by a “carrier effect” of the fumarolic gas which likely prevents gas-rock interaction of H 2 or doesn’t allow changes in redox conditions. As we have previously showed, variations in f H 2 modifies equilibria in Solfatara gas species. Soil (CO), analyzed only in vertical samplings, is affected by reactions in near surface levels too. In Fig. 3d (CO) is plotted as a function of temperature. Samples at T close to 90/100 °C show a CO concentration enrichment, relatively to fumarolic composition (up to 9 ppm against 3.2 ppm in Bocca Grande fumarole). Following Giggenbach (1987), we can linearly combine redox reactions (1) and (3): CO 2 H 2 CO H 2 O (3) To obtain Eq. 4 which explains [CO] increase in the soil: 3CO 2 CH 4 4CO 2H 2 O (4) The previously discussed (CH 4 ) increase, framed in a re-equilibration system of all gas species described by Eqs. 1, 2 and 4, leads to a contemporaneous increase in (CO). The observed CO decrease in some soil samples at T< 80 C is likely related to dilution into the soil with air, resulting also in a T decrease. Conclusions. Previous authors �������� � ���������� ����������� ����� ��� ��������� proposed a conceptual geochemical model for Solfatara system, which describes the mixing process between the magmatic component and the hydrothermal one at depth, ruling out the emitted fluid composition and variations into the crater.