The origin of the fumaroles of La Solfatara (Campi Flegrei, South Italy)

The analysis of gaseous compositions from Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearly
reveals a double thermobarometric signature. A first signature at temperatures of about 360 C was inferred by methanebased
chemical–isotopic geoindicators and by the H2/Ar geothermometer. These high temperatures, close to the critical point
of water, are representative of a deep zone where magmatic gases flash the hydrothermal liquid, forming a gas plume. A second
signature was found to be at around 200–240 C. At these temperatures, the kinetically fast reactive species (H2 and CO)
re-equilibrate in a pure vapor phase during the rise of the plume. A combination of these observations with an original interpretation
of the oxygen isotopic composition of the two dominant species, i.e. H2O and CO2, shed light on the origin of fumarolic
fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated at
about 360 C by the vaporization of hydrothermal liquids. A typical ‘andesitic’ water type (dD 20&, d18O 10&) and
a CO2-rich composition ðXCO2 0:4Þ has been inferred for the magmatic fluids, while for the hydrothermal component a
meteoric origin and a CO2 fugacity fixed by fluid-rock reaction at high temperatures have been estimated. In the time the fraction
of magmatic fluids in the fumaroles increased (up to 0.5) at each seismic and ground uplift crisis (bradyseism) which
occurred at Campi Flegrei, suggesting that bradyseismic crises are triggered by periodic injections of CO2-rich magmatic fluids at the bottom of the hydrothermal system

Allard P., Baubron J. V .C., Le Bronec J., Luongo G., Maurenas J.
M., Pece R., Robe M. C., Tedesco D., and Zettwoog P. (1988)
Geochemical survey of volcanic gas soil emanations and
eruption forecasting: The Vesuvius case, Italy. In Proceedings
of the Kagoshima International Conference on Volcanoes.
Allard P., Maiorani A., Tedesco D., Cortecci G., and Turi B.
(1991) Isotopic Study of the Origin of Sulfur and Carbon in
Solfatara Fumaroles, Campi Flegrei Caldera. J. Volcanol.
Geotherm. Res. 48(1-2), 139–159.
Allard P., Jean-Baptiste P., D’Alessandro W., Parello F., Parisi B.,
and Flehoc C. (1997) Mantle-derived helium and carbon in
groundwaters and gases of Mount Etna, Italy. Earth Planet.
Sci. Lett. 148(3–4), 501–516.
Baldi P., Ferrara G. C., and Panichi C. (1975) Geothermal research
in western Campania (southern Italy): chemical and isotopic
studies of thermal fluids in the Campi Flegrei. In Proceedings of
the 2nd U.N. Symp. on Development and Use of Geothermal
Resources, San Francisco, pp. 687–697.
Balducci S., and Chelini W. (1992) Hydrothermal equilibria in the
active Mofete geothermal system, Phlegrean Fields, Naples
Italy. Acta Vulcanol. 2, 17–34.
Barberi F., Corrado G., Innocenti F., and Luongo G. (1984)
Phlegrean Fields 1982– 1984: Brief chronicle of a volcano
emergency in a densely populated area. Bull. Volcanol. 47, 175–
185.
Bolognesi L., Noto P., and Nuti S. (1986) Studio chimico ed
isotopico della solfatara di Pozzuoli: ipotesi sull’origine e sulle
temperature profonde dei fluidi. Rend. Soc. It. Mineral. Petrol.
41(2), 281–295.
Bonafede M., and Mazzanti M. (1998) Modelling gravity
variations consistent with ground deformation in the Campi
Flegrei caldera (Italy). J. Volcanol. Geotherm. Res. 81(1–2),
137–157.
Brombach T., Caliro S., Chiodini G., Fiebig J., Hunziker J. C., and
Raco B. (2003) Geochemical evidence for mixing of magmatic
fluids with seawater, Nisyros hydrothermal system, Greece.
Bull. Volcanol.(65), 505–516. doi:10.1007/s00445-003-0278-.
Caliro S., Chiodini G., Avino R., Cardellini C., and Frondini F.
(2005) Volcanic degassing at Somma-Vesuvio (Italy) inferred by
chemical and isotopic signatures of ground water. Appl.
Geochem. 20(6), 1060–1076.
Cassano E., Guglielminetti M., and Verdiani G. (1986) Phlegrean
Fields area; first results of geothermal research. Geologia
Applicata e Idrogeologia 21(1), 207–221.
Chiodini G., and Marini L. (1998) Hydrothermal gas equilibria; the
H2O–H2 –CO2 –CO–CH4 system. Geochim. Cosmochim. Acta
62(15), 2673–2687.
Chiodini G., Cioni R., Guidi M., Marini L., Raco B., and
Taddeucci G. (1992) Gas geobarometry in boiling hydrothermal
/systems: A possible tool to evaluate the hazard of
hydrothermal explosions. Acta Vulcanol. 2, 99–107.
Chiodini G., Cioni R., and Marini L. (1993) Reactions governing
the chemistry of crater fumaroles from Vulcano Island, Italy,
and implications for volcanic surveillance. Appl. Geochem. 8(4),
357–371.
Chiodini G., Cioni R., Magro G., Marini L., Panichi C., Raco B.,
and Russo M. (1996) Chemical and isotopic variations of Bocca
Grande fumarole (Solfatara volcano, Phlegrean Fields). Acta
Vulcanol. 8, 129–138.
Chiodini G., Cioni R., Guidi M., Magro G., Marini L., Panichi C.,
Raco B., and Russo M. (2000a) Vesuvius and Phlegrean Fields;
Gas geochemistry; Geochemical monitoring of the Phlegrean
Fields and Vesuvius (Italy) in 1996. Acta Vulcanol. 12(1–2),
117–119.
Lemmon E. W., McLinden M. O., and Friend D. G. (2005)
Thermophysical properties of fluid systems. In NIST Chemistry
WebBook, NIST Standard Reference Database Number 69, (eds.
J. P. J. Linstrom and W. G. Mallard). National Institute of
Standards and Technology, Gaithersburg, MD (http://
webbook.nist.gov).
Lyon G. L., and Hulston J. R. (1984) Carbon and hydrogen
isotopic compositions of New Zealand geothermal gases.
Geochim. Cosmochim. Acta 48, 1161–1171.
Martelli M., Nuccio P. M., Stuart F. M., Burgess R., Ellam R. M.,
and Italiano F. (2004) Helium strontium isotopic constrains on
mantle evolution beneath the Roman Comagmatic Province,
Italy. Earth Planet. Sci. Lett. 224, 295–308.
Marty B., Trull T., Lussiez P., Basile I., and Tanguy J. C. (1994)
He, Ar, O, Sr and Nd isotopeconstraints on the origin and
evolution of the Mount Etna magmatism. Earth Planet. Sci.
Lett. 126, 23–39.
Melluso L., Morra V., Perrotta A., Scarpati C., and Adabbo M.
(1995) The eruption of Breccia Museo (Campi Flegrei, Italy):
Fractional crystallization processes in a shallow, zoned magma
chamber and implications for the eruptive dynamics. J.
Volcanol. Geotherm. Res. 68, 325–339.
Panichi C., and Volpi G. (1999) Hydrogen, oxygen and carbon
isotope ratios of Solfatara fumaroles (Phlegrean Fields, Italy):
further insight into source processes. J. Volcanol. Geotherm.
Res. 91(2–4), 321–328.
Penta F. (1954) Ricerche e studi sui fenomeni esalativo idrotermali
ed il problema delle forze endogene. Ann. Geofis. 8(3), 1–94.
Press W. H., Teukolsky S. A., Vetterling W. T., and Flannery B. P.
(2001) Numerical recipes in Fortran 77: the art of scientific
computing. Cambridge Univ. Press, Cambridge.
Pruess K. (1991) TOUGH2 – A general purpose numerical
simulator for multiphase fluid and heat flow, LBL Report
29400, Lawrence Berkeley Lab.
Richet P., Bottinga Y., and Javoy M. (1977) A review of H, C, N,
O, S and Cl stable isotope fractionation among gaseous
molecules. Ann.Rev. Earth Planet. Sci. Lett. 5, 65–110.
Saccorotti G., Bianco F., Castellano M., and Del Pezzo E. (2001)
The July–August 2000 seismic swarms at Campi Flegrei
volcanic complex, Italy. Geophys. Res. Lett. 28(13), 2525–2528.
Sano Y., Wakita H., Italiano F., and Nuccio P. M. (1989) Helium
isotopes and tectonics in southern Italy. Geophys. Res. Lett. 16,
511–514.
Stull D. R., Westrum E. F., and Sinke G. G. (1969) The Chemical
Thermodynamics of Organic Compounds. Wiley.
Taran Y. A., Pokrovsky B. G., and Esikov A. D. (1989) Deuterium
and oxygen-18 in fumarolic steam and amphiboles from some
Kamchatka volcanoes:’andesitic waters’. Dokl. Akad. Nauk
SSSR 304, 440–443.
Tedesco D. (1996) Chemical and isotopic investigations of fumarolic
gases from Ischia Island (southern Italy); evidences of
magmatic and crustal contribution. J. Volcanol. Geotherm. Res.
74(3–4), 233–242.
Tedesco D. (1997) Systematic variations in the 3He/4He ratio and
carbon of fumarolic fluids from active volcanic areas in Italy
isotopic composition of deep-seated carbon. Geochim. Cosmochim.
Acta 37, 1709–1733.
Dellino P., Isaia R., La, Volpe L., and Orsi G. (2001) Statistical
analysis of textural data from complex pyroclastic sequences;
implications for fragmentation processes of the Agnano-Monte
Spina Tephra (4.1 ka), Phlegraean Fields, southern Italy. Bull.
Volcanol. 63(7), 443–461.
Epstein S., and Mayeda T. (1953) Variation of 18O content
of waters from natural sources. Geochim. Cosmochim. Acta 4,
213–224.
Fulignati P., Marianelli P., Proto M., and Sbrana A. (2004)
Evidences for disruption of a crystallizing front in a magma
chamber during caldera collapse: an example from the Breccia
Museo unit (Campanian Ignimbrite eruption, Italy) J. Volcanol.
Geotherm. Res. 133, 141–155.
Giggenbach W. F. (1975) A simple method for the collection and
analysis of volcanic gas samples. Bull. Volcanol. 39(1), Geochemistry
of Volcanic Gases (first part), 132–145.
Giggenbach W. F. (1980) Geothermal gas equilibria. Geochim.
Cosmochim. Acta 44(12), 2021–2032.
Giggenbach W. F. (1987) Redox processes governing the chemistry
of fumarolic gas discharges from White Island, New Zealand.
Appl. Geochem. 2(2), 143–161.
Giggenbach W. F. (1988) Geothermal solute equilibria, derivation
of Na–K–Mg–Ca geoindicators. Geochim. Cosmochim. Acta
52(12), 2749–2765.
Giggenbach W. F. (1991) Chemical techniques in geothermal
exploration. In Application of Geochemistry in Geothermal
Reservoir Development (ed. F. D’Amore). UNITAR, New
York, pp. 119–144.
Giggenbach W. F. (1992a) The composition of gases in geothermal
and volcanic systems as a function of tectonic setting.
Proceedings – International Symposium on Water–Rock Interaction
7, 873–878.
Giggenbach W. F. (1992b) Isotopic shifts in waters from geothermal
and volcanic systems along convergent plate boundaries
and their origin. Earth Planet. Sci. Lett. 113(4), 495–510.
Giggenbach W. F. (1997) The origin and evolution of fluids in
magmatic–hydrothermal systems. In Geochemistry of Hydrothermal
Ore Deposits (ed. H. Barnes). Wiley, New York, pp.
737–796.
Giggenbach W. F., and Gouguel R. L. (1989) Collection and
analysis of geothermal volcanic water and gas discharges.
Chemistry Division, DSIR, New Zealand. Report no. CD 2401.
Giggenbach W. F., and Matsuo S. (1991) Evaluation of results
from Second and Third IAVCEI field workshops on Volcanic
gases, Mt. Usu, Japan, and White Island, New Zealand. Appl.
Geochem. 6(2), 125–141.
Giggenbach W. F., and Poreda R. J. (1993) Helium isotopic and
chemical composition of gases from volcanic-hydrothermal