Origin of the fumarolic fluids of Vulcano Island,Italy and implications for volcanic surveillance |
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| Authors: | G Chiodini R Cioni L Marini C Panichi |
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| Institution: | (1) Dipartimento di Scienze della Terra, Università di Perugia, Piazza dell'Università, 06100 Perugia, Italy;(2) Istituto di Geocronologia e Geochimica Isotopica, CNR, Via Cardinale Pietro Maffi 36, 56100 Pisa, Italy;(3) ChemGeo, via Basilicata, 4, 56100 Pisa, Italy;(4) Istituto Internazionale per le Ricerche Geotermiche, CNR, Piazza Solferino 2, 56100 Pisa, Italy |
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| Abstract: | Variations in  D and 18O values with H2O contents and outlet temperatures indicate that the fumaroles of La Fossa crater have discharged mixtures of magmatic water and marine hydrothermal water, since 1979. The contribution of meteoric water was low in the period 1979–1982 and very low afterwards. The 18O values of the marine-hydrothermal component of +5 to +7.2 are due to isotopic exchange with the 18O-rich silicates of the rocks under high-temperature and low-permeability conditions. The 18O value of the magmatic end-member is generally +3.5 to +4.3, although values as high as +5.5 to +6.5 were reached in the summer of 1988, when magma degassing appears to have extended into the core of the magma body. The D values of the end-member were close to -20, typical of andesitic waters. Both the isotopic values and chemical data strongly support a  dry model, consisting of a central magmatic gas column and a surrounding hydrothermal envelope, in which marine hydrothermal brines move along limited fracture zones to undergo total evaporation on approaching the conduits of magmatic fluids. The vents at the eastern and western boundaries of the fumarolic field are fed by fluids whose pressure is governed by the coexistence of vapor, liquid and halite, giving rise to a high risk of phreato magmatic explosions, should magma penetrate into these wet environments. Most La Fossa eruptions were triggered by an initial hydrothermal blast and continued with a series of phreatomagmatic explosions. The fluids discharged by the Forgia Vecchia fumaroles are mixed with meteoric water, which is largely evaporated, although subordinate loss of condensed steam may be responsible for scrubbing most of the acidic gas species. The temperatures and pressures, and the risk of a sudden pressure increase, are low. A boiling hydrothermal aquifer at 230° C is present underneath the Baia di Levante beach. This area has a minor risk of hydrothermal explosions. |
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| Keywords: | Fumaroles Fluid chemistry  D value" target="_blank">gif" alt="delta" align="BASELINE" BORDER="0">D value 18O value" target="_blank">gif" alt="delta" align="BASELINE" BORDER="0">18O value Volcanic surveillance Vulcano Island |
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