Helium isotope measurements performed between 1983 and 1988 on subaerial and submarine fumaroles of the Campi Flegrei caldera demonstrate the presence of a mantle component in both types of volcanic gases. The 3He/4He ratios plot in a narrow range from 2.0 to 3.2 times the air ratio (Ra = 1.4 × 10-6), despite large differences in helium concentrations. Their spatial variation suggests a greater leakage of mantle He in the central part than on the margins of the caldera, consistent with the central distribution of ground deformation and seismicity during the 1982-1984 bradyseismic crisis. In a simple mixing model between upper mantle He ( R Ra ≈ 8) and crustal He ( R Ra <- 0.1), the proportion of mantle-derived helium in these fluids would amount to ca. 40% at most. Compared to one existing analysis in 1978 (POLYAK and Tolstikhin, 1980), the results on the hottest fumaroles (Solfatara crater) show no evidence of increasing 3He/4He during the 1982-1984 events, suggesting no additional input of mantle-magmatic He. However, it is possible that the underlying magma chamber has a low 3He/4He ratio, close to the highest ratio measured in the fumaroles, as a result of deep crustal contamination, radioactive aging, and/or past contamination of the local mantle by U- and Th-rich (subducted?) crustal material. All three possibilities are consistent with geochemical data on the local volcanics. If so, the constant 3He/4He ratio of Campi Flegrei fumaroles during the 1982-1984 crisis could not be taken as indicative of a non-magmatic origin of the events, which would have important implications with regard to potential eruptive hazards in this area. Future He isotopic investigations on the solid products of Campanian volcanoes should help in settling this question. © 1990.
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