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| Titel |
Geochemical processes governing the compositional features of the crater fumarolic field at Mt. Etna |
| VerfasserIn |
Marcello Liotta, Antonio Paonita, Antonio Caracausi, Mauro Martelli, Andrea Rizzo, Rocco Favara |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036076
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| Zusammenfassung |
| Mt Etna is one of the most-active volcanoes in the world. It is characterized by major
eruptions, frequent Strombolian activity, and ash emissions. The volcano summit consists of
the central crater of Voragine surrounded by the three active cones of the North-East Crater,
Bocca Nuova, and the South-East Crater. They are characterized by very fractured and
unstable edges. Under these conditions most of the fractures represent preferential degassing
pathways for volcanic fluids, so that the main fumarolic fields develop in such fractured areas.
The geochemistry of the fumaroles at the summit area of Mt. Etna was investigated.
Fumarolic samples were collected between June 2008 and August 2009. Gas samples
were usually collected as “dry gas” and analyzed for the concentrations of He,
H2, O2, N2, CO, CH4, and CO2. Fumarolic gases were also sampled a few times
using the classical Giggenbach bottles and Giggenbach-type bottles filled with
ammonia and silver nitrate in order to determine the SO2/H2S ratio. In addition a novel
method was employed in order to sample fumaroles characterized by high content
of atmospheric gases. Two types of fumaroles were identified: low-temperature
fumaroles, which are dominated by CO2 with minor amounts of SO2 and H2S, and
negligible halogen contents, and high-temperature fumaroles, which are strongly
air-contaminated and characterized by appreciable amounts of volcanogenic carbon,
sulfur, and chlorine. Our data clearly indicate that secondary processes modify
the composition of the fluids once they leave the magma body. A model based on
thermodynamic data is proposed to explore such postmagmatic processes. We computed
the equilibrium composition of magmatic gases that cool starting from magmatic
temperatures under several pressure conditions. The model, which uses Etnean plume
geochemistry as starting composition of fluids exsolved from magma, shows that
SO2 and H2S control the redox conditions of the gas mixture during the cooling,
until the reactions involving CO/CO2 and H2/H2O ratios are fully quenched. The
scrubbing processes occurring subsequent to condensation and gas–liquid water
interaction allow total removal of HCl and partial removal of sulfur species. During the
ascent toward the surface, the concentration of CH4 increases in all fumaroles due
to a modest contribution from hydrothermal fluid. A geochemical model for the
interaction of pristine magmatic fluids with shallower systems is proposed. The
model explains geochemical changes at the crater fumaroles in terms of variable
hydrothermal and magmatic contributions, and modified thermodynamic conditions. |
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