Chemical and isotopic constraints on hydrological processes in Unzen volcanic geothermal system

Abstract

Geothermal systems developed in volcanic regions release magmatic volatiles that may forecast volcanic eruptions. These volatiles can be emitted directly in gaseous form to the atmosphere or can be absorbed by condensed geothermal fluid and groundwater that discharges at surface thermal features. The subsurface flow dynamics of these fluids are therefore crucial for the interpretation of their chemical and isotopic compositions. Furthermore, since geothermal fluids are globally used as energy and cultural resources, such information is also important for their sustainable management. In this context, we investigated the subsurface residence times of fluids from three geothermal areas in Shimabara peninsula (Unzen, Shimabara and Obama) by measuring their tritium and 36Cl activities, along with their chemical and stable isotopic compositions. At Shimabara, the trace tritium activities of the geothermal fluids indicate that magmatic volatiles were transported by pre-nuclear (residence time >60 years) groundwaters. Tritium and δD-δ18O data indicate that the steam feeding the Unzen geothermal field is also derived from pre-nuclear meteoric water, and contributing about a quarter of the water budget. The 36Cl/Cl ratio of the geothermal fluids in Obama exceeds that of seawater, indicating subsurface addition of nucleogenic chloride during prolonged water-rock interaction.