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Two geochemical surveys carried out in March 1991 and September 1992 revealed the existence of a hydrothermal system in the southern portion of Montserrat Island, below Soufrière Hills Volcano. This conclusion is supported by the presence of: (a) the thermal springs of Plymouth which are fed by deep Na-Cl waters (Cl concentration F25 000 mg/kg, temperature ca. 250 7C) mixed with shallow steam-heated waters; (b) the four fumarolic fields of Galway's Soufrière, Gages Upper Soufrière, Gages Lower Soufrière, and Tar River Soufrière, where acid to neutral, steam-heated waters are present together with several fumarolic vents, discharging vapors formed through boiling of hydrothermal aqueous solutions. Involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by: (a) the high 3 He/ 4 He ratios of fumarolic fluids, i.e., 8.2 R A at Galway's Soufrière and 5.9 R A at Gages Lower Soufrière; (b) the dD and d 18 O values of Na-Cl thermal springs and steam condensates, indicating the involvement of arc-type magmatic water in the formation of deep geothermal liquids; and (c) the CH 4 / CO 2 ratios of fumarolic fluids, which are lower than expected for equilibrium with the FeO-FeO 1.5 hydrothermal rock buffer, but being shifted towards the SO 2 -H 2 S magmatic gas buffer.

Both geochemical and geophysical evidence indicates that the activity of the fumarolic system of the Fossa crater can be divided into two periods. From 1978 to 1983, such activity has been mainly controlled by two competing processes which affect the permeability of the fumarolic system at deep levels: (1) rock fracturing induced by faulting and (2) mineral deposition‐alteration. The latter, which causes a slow decrease of the permeability at deep levels through clogging of fractures and voids, prevailed at the end of this period, determining a fluid pressure increment in the deep parts of the system. From 1984 to 1989, fluid pressure at deep levels remained persistently high, triggering local microseismic swarms. Furthermore, PT conditions much higher than before were attained in the zones where hydrothermal fluids seep into the conduits from lateral aquifers, owing to an uprising of the isotherms, a deepening of these seeping zones, or both.

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