The peculiarity of the quiescent La Fossa volcano is the occurrence of “crises” characterized by strong increases of fumarole T and output and by chemical changes indicative of an increasing input of magmatic fluids. Several surveys carried out during a new “crisis” began in November 2004 indicate that the total diffuse CO2 emission for the crater area increases by one order of magnitude during crises (up to 1600 ton·d−1 in December 2005). Concern exists on the possibility that these crises be related to an unrest process leading to eruption. The repetition along decades of the same gas compositional variations during crises, their temporal coincidence with increases of the local shallow seismicity, and the lack of any significant ground motion, rather suggest that they correspond to moments of increasing volatile release from a stationary magma system.

[1] In order to test the potentiality of soil CO 2 diffuse degassing measurements for the study of underground mass and heat transfer in geothermal systems detailed surveys were performed at Latera caldera, which is an excellent test site, because of the abundant available subsurface data. Over 2500 measurements of soil CO 2 flux revealed that endogenous CO 2 at Latera caldera concentrates on a NE-SW band coinciding with a structural high of fractured Mesozoic limestones hosting a water-dominated high-enthalpy geothermal reservoir. The total hydrothermal CO 2 degassing from the structural high has been evaluated at 350 t dÀ1 from an area of 3.1 km 2 . It has been estimated that such a CO 2 release would imply a geothermal liquid flux of 263 kg s À1, with a heat release of 239 MW. The chemical and isotopic composition of the gas indicates a provenance from the geothermal reservoir and that CO 2 is partly originated by thermal metamorphic decarbonation in the hottest deepest parts of the system and partly has a likely mantle origin. The ratios of CO 2 , H 2 , CH 4 , and CO to Ar were used to estimate the T-P conditions of the reservoir. Results cluster at T $ 200-300°C and P CO2 $ 100-200 bars, close to the actual well measurements. Finally, the approach proved to be an excellent tool to investigate the presence of an active geothermal reservoir at depth and that the H 2 -CO 2 -CH 4 -CO-Ar gas composition is a useful T-P geochemical indicator for such CO 2 rich geothermal systems.

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