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| Titel |
The geochemical “crises” at La Fossa crater fumaroles of Vulcano Island (Italy): inferences on the dynamics, structure, and compositions of the magmatic system |
| VerfasserIn |
Antonio Paonita, Cinzia Federico, Piero Bonfanti, Giorgio Capasso, Salvatore Inguaggiato, Franco Italiano, Paolo Madonia, Guendalina Pecoraino, Francesco Sortino |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078843
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| Zusammenfassung |
| As frequently observed in closed-conduit volcanoes, the fumarolic field of La Fossa crater at
Vulcano Islands experiences episodes of rapid and remarkable changes in its geochemical
features (referred as “crises”) that normally last no more than a few months. Well
documented episodes in literature occurred in 1988 and 1996, characterized by increase in the
outlet temperature and steam output from fumarolic vents, marked variations in thermal
groundwaters and diffuse CO2 emissions from soils, nevertheless their meaning remains
widely debated.
Here we report the chemical and isotopic (C, H, O, and He) compositions of the fumarolic
fluids from La Fossa crater in the period 1999-2010. Consistent with records above, our data
show that the geochemical features of the fumarole system have experienced several
“crises” occurred from November 2004 to January 2005, from October 2005 to
January 2006, and from October to November 2009, each lasting no more than a few
months. Typical signatures of these short-term anomalies are large increments in
CO2, N2, and He concentrations, coupled to increased 13C/12C isotopic ratios.
Within a model of fumarolic fluids based on mixing between hydrothermal and
magmatic endmembers, we have developed a novel approach to constrain chemical
(He/CO2 and N2/He) and isotopic (13C/12C, D/H, and 3He/4He) ratios of the magmatic
endmember during the short-term anomalies. Although much of the geochemical
variability in fumaroles results from changes in mixing proportions, the magmatic fluid
unquestionably shows significant variations in time. The magmatic He/CO2, N2/He,
13C/12C, and 3He/4He values throughout 1988–1996 differed from those feeding the
anomaly at the end of 2004. Early clues of the new magmatic fluid appeared in
1998-1999, far from any short-term anomaly, whereas new and old magmatic fluids
coexisted after 2004. We quantitatively prove that the detected geochemical changes are
consistent with the degassing path of a magma having a latitic composition, and
suggest the presence of two magma ponding levels at slightly different pressures,
where bubble-melt decoupling can occur. The different He-isotope compositions at
these levels suggest low hydraulic connectivity typical of a complex reservoir with
dike and sill structures. In this framework, the so-called crises at the fumaroles
are probably due to the evolving conditions in the magmatic system, such as gas
buildup at the top of magma batches followed by massive discharge, activation of new
degassing levels due to reorganization of the magma system, and its interplay with the
stress field. These processes probably start years before a crisis. Far from crises,
geochemical variations with specific signatures can suggest the onset of changes and
reorganization in the magma system, and hence these phases that are apparently “not
anomalous” should be evaluated for their implications in volcanic surveillance. Such
a scenario explains the observed increases in both fumarole output and shallow
high-frequency seismicity (due to increased pore pressure) during the anomalies, while being
consistent with the concomitant absence of any deep seismicity or ground deformation. |
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