[1] Marked increases of CO 2 , H 2 and He dissolved in thermal waters and changes in the dissolved carbon isotopic composition, were observed at Stromboli before the 28 December 2002 eruption and before a violent explosive paroxysm occurred on 5 April 2003. High anomalous CO 2 flux values were recorded at the crater rim since a week before the eruption onset. The first anomalies in the thermal waters (dissolved CO 2 amount) appeared some months before the eruption, when magma column rose at a very high level in the conduit. High peaks of dissolved H 2 and He were recorded a few days before the paroxysm. Carbon isotopic composition indicates a magmatic origin of the dissolved CO 2 whose increase, together with those of H 2 and He, is attributed to an increasing output of deep gases likely produced by depressurization of a rising batch of a deep gas-rich magma, whose fragments have been emitted during the explosion.

No abstract

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.

No abstract

In 2011-2012, Santorini was characterized by seismic-geodetic-geochemical unrest, which was unprecedented since the most-recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He-Ne-Ar-isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570-1573 and 1925-1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The 3 He/ 4 He ratios (3.1-4.0 Ra) are significantly lower than the typical arc-volcano values (R/Ra $ 7-8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3 He/ 4 He ratio of parental magmas generated in the local metasomatized mantle. The 3 He/ 4 He values of enclaves (3.1-3.6 Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He-isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO 2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3 He/ 4 He ratios up to 4.0 Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more-primitive 3 He-rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption.

[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.

No abstract

No abstract