Isotope and chemical studies for a geothermal assessment of the island of Nisyros (Greece)

Two distinct hydrothermal aquifers occur beneath the volcanic crater. The deeper one is characterised by temperatures above 330 degree C and a chemical and isotopic composition in agreement with a thermal modification of seawater by water/rock interaction and boiling processes at depth. The shallow aquifer has temperatures below 170 degree C and salt content and isotopic composition between those of seawater and local groundwaters. The steam produced at the caldera rim (Kaminakia group) has a delta D and delta super(18)O of -42 per mil and -6.1 per mil, respectively, whereas the steam produced inside the caldera (Stephanos and Polybotes group) exhibits average delta values of -11 per mil and + 2.5 per mil for D and super(18)O, respectively. Such values are indicative of different parent waters for the above two aquifers. The isotope geothermometers based on the pairs (CH sub(4)-H sub(2)), (H sub(2)O-H sub(2)) and (CH sub(4)-CO sub(2)) from the same fumaroles yield values consistently within the 100-350 degree C range.