Chemical features and isotopic composition of gaseous manifestations on Vulcano Island, Aeolian Islands, Italy: An interpretative model of fluid circulation

Abstract

Between 1987 and 1995, gases were sampled from various manifestations on Vulcano Island including various fumaroles at La Fossa crater and Baia di Levante and diffuse soil gas emissions in several areas which are characterized by high C02 fluxes. Since the end of 1987, volcanic activity has increased considerably, showing a marked rise in temperature (300°C in 1987 to 670°C in December, 1992) and in output of steam from crater fumaroles. In addition, variations have been observed in the chemical and isotopic compositions of gases and thermal waters, which can be attributed to an increased inflow of deeper fluids. The isotopic composition of fumarolec steam seems to be the result of mixing processes between a positive endmember of deep origin (δD = +10‰ ∼ +15‰ δ 18O= +6‰ ∼ +8‰) and two endmembers of meteoric origin with different oxygen-shifts. Variations observed in the C0 2 isotopic composition suggest that all sampled manifestations are fed by a single deep source which may be magmatic. Such a source would be characterized by the 6 13C CO 2 values of high-temperature fumaroles (δ 13C CO 2 ≅0‰) these values are noticeably more positive than those generally attributed to magmatic gases (δ 13C CO 2 = −5‰ ∼ −8‰) implying crustal contamination processes of the magma. The observed decrease in δ 18O CO 2 values of the crater fumaroles (from +20‰ in 1978 to between +12‰ and +15‰ in 1993–1994) was probably caused by two different processes. The first is a reduction of the enrichment factor between C0 2 and H 2O which is related to the increased temperature of the gases, and the second is a mixing with more negative waters of meteoric origin. Finally, C0 2 addition or removal processes caused by interactions between deep gases and shallow hydrothermal waters are likely to be responsible for the different chemical and isotopic compositions of gaseous emissions in the Vulcano Porto area and the crater fumaroles.