ISOTOPIC STUDY OF THE ORIGIN OF SULFUR AND CARBON IN SOLFATARA FUMAROLES, CAMPI FLEGREI CALDERA

Isotopic study of the origin of sulfur and carbon in the hottest (Solfatara) fumaroles of Campi Flegrei caldera, Southern Italy, was carried out on gas samples collected between 1983 and 1988, i.e. during and after the 1982-1984 seismo-volcanic crisis. The results for sulfur (H2S), the first ever reported on these gases, indicate a mean ∂34S of -0.3±0.3‰ (range: -0.7 to +0.1‰) versus Canyon Diablo Troilite standard, consistent with an igneous derivation of this element, from either active magma degassing or/and leaching of reduced sulfur-bearing minerals in the volcanic layers. The lack of peculiar ∂34S variation during and after the crisis suggests that the chemical variation of H2S and S/C ratio in the fumaroles (increase and then decrease by a factor 3) were not due to a changing origin of sulfur. The mean ∂13C of carbon (CO2) over the period of survey, -1.6±0.2‰ (range: -1.9 to -1.3‰) versus PDB standard, is similar to the values obtained before the crisis (since 1970). Such an isotopic constancy requires a large and stable source of carbon feeding the fumaroles. The measured ∂13C values are much higher than those typical of primary mantle-magmatic carbon (-6±2‰) and plot within the ∂13C range for marine carbonates (0±2‰). Such high values may reflect either (a) 13C-fractionation during degassing of CO2 from the underlying (≤5 km depth) magma chamber or (b) the contribution of heavy CO2 of sedimentary origin, derived from either thermometamorphism of Mesozoic limestone series embedding the magma chamber or, possibly, past contamination of the local mantle by subducted sediments. Various arguments, among which volcanological evidence of an isolated and cooling magma reservoir (which would have been extensively degassed and, so, depleted in 13C along with time), the low 3He/4He ratios and the broad 13C-enrichment of volcanic fluids in the region, and geochemical evidence of crust-magma fluid interactions, suggest that a considerable fraction (≥60%) of CO2 in Solfatara fumaroles derives from carbonate sediments in the basement. The contribution of magma-derived CO2 may be higher within the central part of the caldera (including Solfatara crater) than toward its western margin, where fumarolic and geothermal well gases exhibit lower 3He/4He ratios and still higher ∂13C values. Such a geochemical pattern is consistent with the central distribution of ground deformation and seismicity during the 1982-1984 crisis and with the idea of a residual magma body, confined beneath the central part of the structure. Alternatively, higher ∂13C and lower 3He/4He ratios toward the western margin may result from dilution of Solfatara-type gas during progressively deeper water boiling. Finally, accepting that Solfatara CO2 derives from simple crustal mixing between magmatic and sedimentary carbon, its constant isotopic composition (together with the constant He isotope ratio) would restrict the possibility of magma intrusion and/or higher magmatic gas input as mechanisms responsible for the 1982-1984 events. However, this conclusion would no more hold true if the magma itself, or even its mantle source, were previously contaminated by crustal carbon. © 1991.