Chemical (He, H-2, CH4, Ne, Ar, Na) and isotopic (He, Ne, Ar, C) variations at the Solfatara crater (southern Italy): mixing of different sources in relation to seismic activity

Here we describe temporal variations of noble gas (He, Ne and Ar), carbon isotopes and chemical data of fluids collected at the Bocca Grande fumarole (Solfatara crater) from 1990 to 1994, before the occurrence of a seismic swarm. After more than seven years of quiescence, a sudden vertical ground deformation and a seismic swarm of 100 quakes with an M(max) = 1.5 with epicenter located beneath the Solfatara crater, was recorded from August 23 to 25 1994. Isotopic composition of He, Ne, Ar and C (from CO2) and abundances of He, Ne, Ar, H2, CH4 and N2 have been analysed from fumarolic fluids collected at the Solfatara crater. To date these isotopic ratios have not been used as precursors of earthquakes, although several cases have been reported in which the helium isotopic ratio was used to interpret volcanic unrest. All the analyzed isotopic ratios, chemical species and a calculated equilibrium temperature show a significant change a few months before the occurrence of the seismic activity and ground deformation, followed by a sudden return to values recorded prior the swarm. Migration towards the surface of deep (and hotter) gas phase related to crustal and magmatic sources with the following decrease of the surficial atmospheric component is the cause of such wide variations.