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Numerous measurements of CO 2 degassing from the soil, carried out with the accumulation chamber method, indicate that in the period April-July 1995 the upper part of the Fossa cone released a total output of 200 t d -1 of CO 2 , which corresponds to approximately 1000 t d -1 of steam. These large amounts of fluids are of the same order of magnitude as those released by the high temperature fumarolic field located inside the crater. The spatial distribution of soil gas fluxes shows that the main structures releasing CO 2 are the inner slopes of the crater and a NW-SE line, located NE of the crater rim, which correspond to the main direction of Vulcano Island active faults. The comparison of the wCO 2 maps with the soil temperature distribution, derived from both direct measurements and airborne infrared images, indicates the occurrence of extensive condensation of fumarolic steam within the upper part of the Fossa cone, whose total amount is comparable to the rainfall budget. Part of the condensate which originates from this process contributes to the recharge of the phreatic aquifer of Porto Plain, modifying the chemical and isotopic composition of the groundwater.

The chemical and isotopic compositions (dD H2O , d 18 O H2O , d 18 O CO2 , d 13 C CO2 , d 34 S, and He/N 2 and He/Ar ratios) of fumarolic gases from Nisyros, Greece, indicate that both arc-type magmatic water and local seawater feed the hydrothermal system. Isotopic composition of the deep fluid is estimated to be +4.9€0.5‰ for d 18 O and 11€5‰ for dD corresponding to a magmatic water fraction of 0.7. Interpretation of the stable water isotopes was based on liquid-vapor separation conditions obtained through gas geothermometry. The H 2 -Ar, H 2 -N 2 , and H 2 -H 2 O geothermometers suggest reservoir temperatures of 345€15 C, in agreement with temperatures measured in deep geothermal wells, whereas a vapor/liquid separation temperature of 260€30 C is indicated by gas equilibria in the H 2 O-H 2 -CO 2 -CO-CH 4 system. The largest magmatic inputs seem to occur below the Stephanos-Polybotes Micros crater, whereas the marginal fumarolic areas of Phlegeton-Polybotes Megalos craters receive a smaller contribution of magmatic gases.

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The total flux of sulfur dioxide from the Italian volcanoes Etna, Stromboli, and Vulcano was determined using the differential absorption lidar technique. The measurements were performed from an oceanographic research ship making traverses under the volcanic plumes with the lidar system sounding vertically. By combining the integrated gas concentration over the plume cross section with wind velocity data, it was possible to determine the total fluxes of SO2 from the three volcanoes, all measured within a 3‐day period in September 1992. We found total fluxes of about 25, 180, and 1300 t/d for Vulcano, Stromboli, and Etna, respectively. These data, collected with an active remote‐sensing technique, were compared with simultaneous recording with passive differential optical absorption spectroscopy (DOAS) using the sky radiation as the light source. Since the geometry of the light paths crossing the volcanic plume is not well defined in the passive measurements, a correction to the DOAS data is required. The SO2 results are also compared with previously available data from correlation spectroscopy measurements. Lidar measurements on atomic mercury were also made for the plumes from Stromboli and Vulcano, but the system sensitivity and range only allowed estimates of upper limits for the Hg fluxes.

Two geochemical surveys carried out in March 1991 and September 1992 revealed the existence of a hydrothermal system in the southern portion of Montserrat Island, below Soufrière Hills Volcano. This conclusion is supported by the presence of: (a) the thermal springs of Plymouth which are fed by deep Na-Cl waters (Cl concentration F25 000 mg/kg, temperature ca. 250 7C) mixed with shallow steam-heated waters; (b) the four fumarolic fields of Galway's Soufrière, Gages Upper Soufrière, Gages Lower Soufrière, and Tar River Soufrière, where acid to neutral, steam-heated waters are present together with several fumarolic vents, discharging vapors formed through boiling of hydrothermal aqueous solutions. Involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by: (a) the high 3 He/ 4 He ratios of fumarolic fluids, i.e., 8.2 R A at Galway's Soufrière and 5.9 R A at Gages Lower Soufrière; (b) the dD and d 18 O values of Na-Cl thermal springs and steam condensates, indicating the involvement of arc-type magmatic water in the formation of deep geothermal liquids; and (c) the CH 4 / CO 2 ratios of fumarolic fluids, which are lower than expected for equilibrium with the FeO-FeO 1.5 hydrothermal rock buffer, but being shifted towards the SO 2 -H 2 S magmatic gas buffer.

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