Geochemical Assessment of the Thermal Fluids Emerging Along the Aegean Volcanic Arc (Greece)

Abstract

Aegean volcanic arc developed during Quaternary time as a result of a lithosphere subduction process. Starting from Soussaki-Loutraki and Methana thermal areas, located in the Peloponnese peninsula, the arc proceeds through Aegina, Milos and Santorini islands, to reach, in the south-eastern part, Kos and Nisyros islands. Volcanic activities are still well pronounced at Nisyros in form of seismic activity, of craters of very recent hydrothermal explosions, hot fumaroles and thermal springs, but these surface manifestations tend to decrease moving to the north-western part of the arc. Isotopic and chemical analyses were performed on water and gas samples collected in different areas of the volcanic arc and compared with the literature data, in order to attempt the assessment of the thermal fluids emerging in these volcanic structures. H, O and C stable isotopes, coupled with the major chemistry, have been used to evaluate the origin of the main components of the fluids such as H2O, CO2, H2, CH4, N2; reservoir temperatures were derived by solute concentrations, and by isotopic fractionations in the (H2-H2O), (CO2-CH4) and (H2-CH4) pairs. Chemical solute concentrations and isotopic composition of waters were interpreted in terms of mixing and subsurface boiling processes, involving cold meteoric waters as well as hydrothermal and seawaters. Gas geochemistry reveals that input of magmatic fluids must be taken into consideration in the highest temperatures environments. Methana, Aegina, Kos and, partially Soussaki-Loutraki areas likely represent geothermal systems in their waning stage, while surface fluids of Santorini, Milos and Nisyros reflect the existence of still active geothermal systems.