Evidence of deep magma degassing and ascent by geochemistry of peripheral gas emissions at Mount Etna (Italy): Assessment of the magmatic reservoir pressure

Five gas discharges in the area of Mount Etna volcano (Italy) and in the near Hyblean
plateau have been monitored since 1996. All the emissions displayed low contributions from crustal fluids, whereas magmatic gases were the main component. Selective dissolution of these gases into hydrothermal aquifers has been recognized and modeled, allowing us to calculate the original composition of the magma-released gases. The inferred composition of the magmatic gases exhibits synchronous variations of He/Ne and
He/CO2 ratios, which are coherent with the magma degassing process. On the basis of numerical simulations of volatile degassing from Etnean basalts we have computed the initial and final pressures of the magma batches feeding the emissions. We thus can define the levels of the Etna plumbing system where magmas are stored. Pressure values were around 360 and 160 MPa for initial and final stages, respectively, meaning related depths of about 10 and 3 km below sea level, matching those obtained by geophysical
investigations for the deep and shallow magma reservoirs. In addition, we have been able
to recognize episodes of magma migration from the deeper reservoir toward the shallow one. An important magma injection into the shallow storage volume was detected during the onset of the 2001 eruption (17 July). No further injection had taken place during this period until September 2001, providing a possible reason for the quick exhaustion of the eruption. In view of this we suggest that the sampled emissions are a
powerful geochemical tool to investigate the Etna’s plumbing system and its magma dynamics, as well as the development of eruptive events.

Allard, P., Endogenous magma degassing and storage at Mt. Etna, Geophys. Res. Lett., 24, 2219–2222, 1997.
Barberi, F., F. Innocenti, G. Marinelli, and R. Mazzuoli, Vulcanesimo e tettonica a placche: Esempi nell’area mediterranea, Mem. Soc. Geol. It.,13, 327–358, 1974.
Barbier, E., F. Musmeci, and P. Sarocco, Banca Nazionale dati geotermici, CNR internal report, Ist. Int. Ric. Geotermiche, Pisa, Italy, 1998.
Bonaccorso, A., and D. Patanè, Shear response to an intrusive episode at Mt. Etna volcano (January 1998) inferred through seismic and tilt data,
Tectonophysics, 334, 61–75, 2001.
Bonaccorso, A., R. Velardita, and L. Villari, Ground deformation modelling of geodynamic activity associated with 1991– 1993 Etna eruption, Acta Vulcanol., 4, 87–96, 1994.
Bottinga, Y., and M. Javoy, MORB degassing: Bubble growth and ascent,Chem. Geol., 81, 255– 270, 1990.
Brimhall, G. H., and D. A. Crerar, Ore fluids: Magmatic to supergene, in Thermodynamic Modelling of Geologic Materials: Minerals, Fluids and
Melts, edited by I. S. E. Charmicael and H. P. Eugster, Rev. Mineral., 17,235–282, 1987.
Brusca, L., A. Aiuppa, W. D’Alessandro, F. Parello, P. Allard, and A. Michel, Geochemical mapping of magmatic gas-water-rock interactions
in the aquifer of Mount Etna volcano, J. Volcanol. Geotherm. Res.,108, 199–218, 2001.
Capasso, G., and S. Inguaggiato, A simple method for the determination of dissolved gases in natural waters: An application to thermal waters from Vulcano Island, Appl. Geochem., 13, 631– 642, 1998.
Caracausi, A., R. Favara, S. Giammanco, P. M. Nuccio, A. Paonita, G. Pecoraino, and A. Rizzo, Mount Etna: Geochemical signals of magma
ascent and unusually extensive plumbing system, Geophys. Res. Lett.,30(2), 1057, doi:10.1029/2002GL015463, 2003.
Cataldi, R., F. Mongelli, P. Squarci, L. Taffi, G. Zito, and C. Calore, Geothermal ranking of the Italian territory, Geothermics, 24, 115–129,
1995.
Chester, D. K., A. M. Duncan, J. E. Guest, and C. R. J. Kilburn, Mount Etna, the Anatomy of a Volcano, Chapman and Hall, New York, 1985.
Chiarabba, C., A. Amato, E. Boschi, and F. Barberi, Recent seismicity and tomographic modeling of the Mount Etna plumbing system, J. Geophys. Res., 105, 10,923–10,938, 2000.
Chiodini, G., W. D’Alessandro, and F. Parello, Geochemistry of gases and water discharged by the mud volcanoes at Paternò , Mt. Etna (Italy), Bull.
Volcanol., 58, 51–58, 1996.
Clocchiatti, R., J. Weisz, M. Mosbah, and J. C. Tanguy, Coexistence de ‘‘verres’’ alcalins et thoéliitiques saturés en CO2 dans les olivines des hyaloclastites d’Aci Castello (Etna, Sicilie, Italie): Arguments en faveur d’un manteau anormal et d’un réservoir profond, Acta Vulcanol., 2, 161–
173, 1992.
D’Alessandro, W., S. De Gregorio, G. Dongarra`, S. Guerrieri, F. Parello, and B. Parisi, Chemical and isotopic characterization of the gases of
Mount Etna (Italy), J. Volcanol. Geotherm. Res., 78, 65–76, 1997.
Favara, R., S. Giammanco, S. Inguaggiato, and G. Pecoraino, Preliminary estimate of CO2 output from Pantelleria Island volcano (Sicily, Italy):
Evidence of active mantle degassing, Appl. Geochem., 16, 883–894,2001.
Gardner, J. E., M. Hilton, and M. R. Carroll, Experimental constrains on degassing of magma: Isothermal bubble growth during continuous
decompression from high pressure, Earth Planet. Sci. Lett., 168, 201–218, 1999.
Gardner, J. E., M. Hilton, and M. R. Carroll, Bubble growth in highly viscous silicate melts during continuous decompression from high
pressure, Geochim. Cosmochim. Acta, 64, 1473–1483, 2000.
Giammanco, S., S. Inguaggiato, and M. Valenza, Soil and fumarole gases of Mount Etna: Geochemistry and relations with volcanic activity,
J. Volcanol. Geotherm. Res., 81, 297–310, 1998.
Giggenbach, W. F., Chemical composition of volcanic gases, in Monitoring and Mitigation of Volcanic Hazards, edited by R. Scarpa and R. I. Tilling,pp. 221–256, Springer-Verlag, New York, 1996.
Giggenbach, W. F., M. Martini, and E. Corazza, The effects of hydrothermal processes on the chemistry of some recent volcanic gas discharges,
Per. Mineral., 55, 15–28, 1988.
Hirn, A., R. Nicolich, J. Gallart, M. Laigle, L. Cernobori, and ETNASEIS Scientific Group, Roots of Etna volcano in faults of great earthquakes,
Earth Planet. Sci. Lett., 148, 171–191, 1997.
Johnson, J. W., E. H. Oelkers, and H. C. Helgeson, SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bars and 0 C to 1000 C, Comput. Geosci., 18, 899–947, 1992.
Javoy, M., and F. Pineau, The volatile record of a ‘‘popping’’ rock Mid-Atlantic-Ridge at 14 N: Chemical and isotopic composition of the gas
trapped in the vesicles, Earth Planet. Sci. Lett., 107, 598–611, 1991.
Kamenetsky, V., and R. Clocchiatti, Primitive magmatism of Mt. Etna: Insights from mineralogy and melt inclusions, Earth Planet. Sci. Lett.,
142, 553–572, 1996.
Kazahaya, K., I. Shinohara, and G. Saito, Excessive degassing of Izu-Oshima volcano: Magma convection in a conduit, Bull. Volcanol., 56,
207–216, 1994.
La Delfa, S., G. Patane`, R. Clocchiatti, J.-L. Joron, and J.-C. Tanguy,Activity of Mount Etna preceding the Febrary 1999 fissure eruption:
Inferred mechanism from seismological and geochemical data, J. Volcanol. Geotherm. Res., 105, 121–139, 2001.
Lanari, R., P. Lundgren, and E. Sansosti, Dynamic deformation of Etna volcano observed by satellite radar interferometry, Geophys. Res. Lett.,
25, 1541–1544, 1998.
Marty, B., T. Trull, P. Lussiez, I. Basile, and J.C. Tanguy, He, Ar, O, Sr and Nd isotope constraints on the origin and evolution of Mount Etna magmatism, Earth Planet. Sci. Lett., 126, 23– 39, 1994.
Massonet, D., P. Briole, and A. Arnaud, Deflation of Mount Etna monitored by spaceborne radar interferometry, Nature, 375, 567–570, 1995.
Metrich, N., and R. Clocchiatti, Melt inclusion investigation of the volatile behavior in historic alkali basaltic magmas of Etna, Bull. Volcanol., 51,185–198, 1989.
Metrich, N., and M. J. Rutherford, Low crystallization paths of H2O saturated
basaltic-hawaiitic melts from Mt Etna: Implications for opensystem degassing of basaltic volcanoes, Geochim. Cosmochim. Acta, 62,
1195–1205, 1998.
Metrich, N., R. Clocchiatti, M. Mosbah, and M. Chaussidon, The 1989–90 activity of Etna: Magma mingling and ascent of H2O-Cl-S-rich basaltic
magma: Evidence from melt inclusions, J. Volcanol. Geotherm. Res., 59, 131–144, 1993.
Murru, M., C. Montuori, M. Wyss, and E. Privitera, The locations of magma chambers at Mt. Etna, Italy, mapped by b-values, Geophys.
Res. Lett., 26, 2553–2556, 1999.
Nakai, S., H. Wakita, P. M. Nuccio, and F. Italiano, MORB-type neon in an enriched mantle beneath Etna, Sicily, Earth Planet. Sci. Lett., 153, 57–66, 1997.
Nuccio, P. M., and A. Paonita, Magmatic degassing of multicomponent vapors and assessment of magma depth: Application to Vulcano Island (Italy), Earth Planet. Sci. Lett., 93, 467–481, 2001.
Nunnari, G., and G. Puglisi, The global position system as a useful technique for measuring round deformation in volcanic area, J. Volcanol.
Geotherm. Res., 61, 267–280, 1994.
O’Nions, R. K., and E. R. Oxburgh, Helium, volatile fluxes and the development of the continental crust, Earth Planet. Sci. Lett., 90, 331–347, 1988.
Ozima, M., and F. A. Podosek, Noble Gas Geochemistry, Cambridge Univ. Press, New York, 1983.
Papale, P., Modeling of the solubility of a two component H2O + CO2 fluid in silicate liquids, Am. Mineral., 84-4, 477–492, 1999.
Proussevitch, A. A., and D. L. Sahagian, Dynamics of coupled diffusive and decompressive bubble growth in magmatic systems, J. Geophys.
Res., 101, 17,447–17,456, 1996.
Proussevitch, A. A., and D. L. Sahagian, Dynamics and energetics of bubble growth in magmas: Analytical formulation and numerical modeling,
J. Geophys. Res., 103, 18,223–18,251, 1998.
Tanguy, J.-C., M. Condomines, and G. Kieffer, Evolution of the Mount Etna magma: Constraints on the present feeding system and eruptive
mechanism, J. Volcanol. Geotherm. Res., 75, 221– 250, 1997.
Trigila, R., F. J. Spera, and C. Aurisicchio, The 1983 Mount Etna eruption: Thermochemical and dynamical inferences, Contrib. Mineral. Petrol.,
104, 594–608, 1990.