The chemical and isotopic compositions (deltaD(H2O), delta(18)O(H2O),
delta(18)O(CO2), delta(13)C(CO2), delta(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+/-0.5parts per thousand for delta(18)O and -11+/-5parts per
thousand for deltaD 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-H2O geothermometers suggest reservoir temperatures of
345+/-15 degreesC, in agreement with temperatures measured in deep
geothermal wells, whereas a vapor/liquid separation temperature of
260+/-30 degreesC is indicated by gas equilibria in the
H2O-H-2-CO2-CO-CH4 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.