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 i sotopes and chemical data of fluids collected at the Bocca Grande fumarole (Solfatara crater) from 1990 to 1994, before the occurrence of a seismic sw arm. After more than seven years of quiescence, a sudden vertical ground de formation and a seismic swarm of 100 quakes with an M-max = 1.5 with epicen ter located beneath the Solfatara crater, was recorded from August 23 to 25 1993. Isotopic composition of He, Ne, Ar and C (from CO2) and abundances o f He, Ne, Ar, H-2, CH4 and N-2 have been analysed from fumarolic fluids col lected at the Solfatara crater. To date these isotopic ratios have not been used as precursors of earthquakes, although several cases have been report ed in which the helium isotopic ratio was used to interpret Volcanic unrest . All the analyzed isotopic ratios, chemical species and a calculated equil ibrium temperature show a significant change a few months before the occurr ence of the seismic activity and ground deformation, followed by a sudden r eturn to values recorded prior the swarm. Migration towards the surface of deep (and hotter) gas phase related to crustal and magmatic sources with th e following decrease of the surficial atmospheric component is the cause of such wide variations. (C) 1999 Elsevier Science B.V. All lights reserved.