H–O–C–S isotope and geochemical assessment of the geothermal area of Central Greece

Abstract

A geochemical survey on the thermal fluids released by the geothermal system of Sperchios basin and Aedipsos area was undertaken. This geothermal area is characterized by high salinity associated with a degassing of CO2. In order to investigate the origin of thermal water, the mixing processes between meteoric, magmatic and sea water involved in the deep geothermal systems and subsequently to evaluate the deep aquifer temperature, chemical and isotopic content analyses (major elements, B, Li, 18O, 2H, 13C, 34S) have been carried out upon 59 thermal and cold water samples from springs and boreholes.The chemical and isotopic data of the thermal Cl-rich water springs of Thermopylae–Psoroneria–Kamena Vourla and Aedipsos suggest that they are fed by deep “parent” geothermal liquid mixed with local groundwater and seawater respectively. The deep geothermal liquid is a mixture of local groundwater (~26.3%), seawater (~56%) and magmatic water (~17.7%). For Aedipsos thermal water the magmatic participation is between 5 and 10%, while in the case of Thermopylae, Psoroneria and Kamena Vourla magmatic contribution is between 1.5 and 3%.For the Hypati thermal water, the relationship between δ2H and δ18O values of water and Cl− content indicates significant δ18O enrichment with respect to local meteoric water, at least 2‰, caused by the water–rock isotope exchange at relatively high temperatures.Carbon isotope analysis also confirms the participation of deep CO2 in the geothermal system: carbon source coming from mixing between magmatic and shallow “crustal” limestone (thermal or metamorphic decarbonation of limestone).Assessments from chemical and isotopic geothermometers applied on the thermal spring water suggest the probable existence of a deep geothermal reservoir of medium and high enthalpy.