A model for Ischia hydrothermal system: Evidences from the chemistry of thermal groundwaters

Ischia volcano, in Central Italy, has long been known for its copious surface hydrothermal manifestations,
signs of a pervasive circulation of hot fluids in the subsurface. Because of the significant chemical
heterogeneity of fumarolic gas discharges and hot spring discharges, evidences of a complex hydrothermal
setting, a definite model of fluid circulation at depth is currently unavailable, in spite of the several previous
efforts. Here, we report on the chemical and isotopic composition of 120 groundwater samples, collected
during several sampling surveys from 2002 to 2007. The acquired data suggest that the composition of
surface manifestations reflect contributions from meteoric water, sea water, and thermal fluids rising from
two distinct hydrothermal reservoir, with equilibrium temperatures of respectively ~150 °C and ~270 °C, and
depths of 150–300 m and N300 m (but possibly N1000 m). We also make use of an isotopic characterization
of the dissolved gas phase in thermal waters to demonstrate that the Ischia hydrothermal system is currently
supplied by a deep-rising gas component (DGC), characterized by CO2 ~97.7±1.2 vol.% (on a water-free
basis), δ13CCO2=−3.51±0.9‰, and helium isotopic ratio of about 3.5 Ra (3He/4He ratio normalized to the
air ratio, Ra), likely magmatic in origin. An assessment of the thermal budget for Ischia hydrothermal system
is also presented, in the attempt to derive a first estimate of the size and rate of degassing of the magmatic
reservoir feeding the gas emissions. We calculate that a heat flow of about 153–222 MW presently drives
hydrothermal circulation on the island, which we suggest is supplied in convective form (e.g., by the ascent
of a high-T magmatic vapour phase) by complete degassing of 2.2–3.3 107 m3 yr−1 of trachytic magma (with
~2.1 wt.% dissolved H2O content). If extrapolated to entire period of quiescence lasting since the Arso
eruption in 1302 A.D., this volume corresponds to 1.6–2.3 1010 m3 of magma degassed in about 700 years of
quiescent activity.