Earthquake-induced variations in the composition of the water in the geothermal reservoir at Vulcano Island, Italy

During 1979–1989, variations were observed in the oxygen composition of the water contained in the geothermal reservoir at Vulcano Island, Italy.The reservoir water, that has a magmatic origin, showed an oxygen composition of +1.0±0.5‰ δ¹⁸O during periods without local tectonic earthquakes, and an oxygen composition of +3.4±0.5‰ δ¹⁸O after the highest-energy seismic activity that occurred recently near the island. A slight increase of the δ¹⁸O value in the reservoir water was also observed after a low-energy sequence of tectonic earthquakes that occurred at very shallow depth just beneath Vulcano Island. These ¹⁸O variations in the reservoir water are consistent with earthquake-induced increases in the contribution from high-temperature δ¹⁸O-rich magmatic condensate to the geothermal reservoir, and with subsequent decreases in the δ¹⁸O value due to ¹⁸O exchanges at the temporarily increased reservoir temperature during reactions between the highly reactive magmatic condensate and the local rocks.Only minor changes in the deuterium composition of the reservoir water occurred with time, as the δD value in the magmatic condensate released from the magma after major local earthquakes quickly approached the δD value of the water contained in the geothermal reservoir.Also the chloride concentration in the reservoir water appears to be linked to the contribution from the magmatic fluid. This chloride content seems not to have undergone major changes with time, as it may be buffered by temporary increases in the reservoir temperature up to values >300°C induced by major local earthquakes. This mechanism may possibly occur also in other magmatic–hydrothermal systems.