CHEMICAL-PATTERNS OF THERMAL AQUIFERS IN THE VOLCANIC ISLANDS OF THE AEGEAN ARC, GREECE

Thermal spring waters and associated gases discharging on several volc anic islands and on mainland Greece along the Hellenic Volcanic Are (H VA) in the south Aegean sea have been investigated. The chemical chara cteristics of most of the spring waters suggest that the water in thei r feeding aquifers is largely derived from seawater that enters the hy drological circuits and mixes with local neutral low-salinity bicarbon ate waters. There are however discrepancies between the simple theoret ical mixing line between local meteoric waters and seawater, and the a ctual mixing line. This mixing is accompanied by partial Mg and SO4 pr ecipitation to secondary minerals and by the addition of K, Ca, HCO3, B, NH4, and SiO2 to the solutions as a result of water-rock interactio n processes. At Miles and Nisyros, where active hydrothermal systems a re present at shallow depth, the silica content of the thermal springs is much higher than that of springs on the other islands and mainland Greece. This suggests higher thermal gradients at depth there, and th e ascent of geothermal fluids to the surface along fractures prevalent ly located in the fumarolic fields within the latest eruptive centers and calderas. The fact that the springs are anomalously enriched in si lica only at Miles and Nisyros suggests indirectly that, apart from Sa ntorini island, where steam vents are present at the surface, the flui ds from the active high-enthalpy geothermal systems do not rise to the surface anywhere else along the HVA. If present, they are well confin ed under thick and impermeable covers. (C) 1997 CNR. Published by Else vier Science Ltd.