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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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[1] Significant changes occurred in the chemistry of gases discharged from Nisyros fumaroles in the years, 1997 -2001, following a strong seismic crisis. Increasing H 2 S/CO 2 ratios and decreasing CH 4 /CO 2 ratios are attributed to an increased contribution of magmatic fluids to the hydrothermal system. Some fumaroles showed concurrent increases in H 2 and CO contents implying increases in temperature and pressure in the upper parts of the hydrothermal system. These changes reinforce the possibility of an increased hazard of hydrothermal eruptions at present with respect to the '90's. Moreover, both the present events and the historical hydrothermal eruptions may represent precursory signals of a new period of volcanic unrest at Nisyros.

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