the origin of the fumaroles of la solfatara (campi flegrei, south italy)
TRANSCRIPT
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Geochimica et Cosmochimica Acta 71 (2007) 3040–3055
The origin of the fumaroles of La Solfatara(Campi Flegrei, South Italy)
S. Caliro a,*, G. Chiodini a, R. Moretti a, R. Avino a, D. Granieri a,M. Russo a, J. Fiebig b
a Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italyb Geologisch–Palaontologisches Institut, J.W. Goethe Universitat, Senckenberganlage 32, 60325 Frankfurt, Germany
Received 5 December 2006; accepted in revised form 6 April 2007; available online 14 April 2007
Abstract
The analysis of gaseous compositions from Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearlyreveals a double thermobarometric signature. A first signature at temperatures of about 360 �C was inferred by methane-based chemical–isotopic geoindicators and by the H2/Ar geothermometer. These high temperatures, close to the critical pointof water, are representative of a deep zone where magmatic gases flash the hydrothermal liquid, forming a gas plume. A sec-ond 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 inter-pretation of the oxygen isotopic composition of the two dominant species, i.e. H2O and CO2, shed light on the origin of fuma-rolic fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated atabout 360 �C by the vaporization of hydrothermal liquids. A typical ‘andesitic’ water type (dD � �20&, d18O �10&) anda CO2-rich composition ðX CO2
� 0:4Þ has been inferred for the magmatic fluids, while for the hydrothermal component ameteoric origin and a CO2 fugacity fixed by fluid-rock reaction at high temperatures have been estimated. In the time the frac-tion of magmatic fluids in the fumaroles increased (up to �0.5) at each seismic and ground uplift crisis (bradyseism) whichoccurred at Campi Flegrei, suggesting that bradyseismic crises are triggered by periodic injections of CO2-rich magmatic fluidsat the bottom of the hydrothermal system.� 2007 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
Located in the Campanian region (South Italy), thePhlegraean Volcanic District (PVD) is a densely populatedactive volcanic area, including the Campi Flegrei (CF) cal-dera, the islands of Procida and Ischia, plus a number of sub-merged volcanoes (Fig. 1a). Volcanological, geophysical andgeochemical evidences (De Vita et al., 1998; De Vita et al.,1999; Dellino et al., 2001; Zimanowski et al., 2003) supportthe hypothesis that remnants of the magma source feedingthe two large eruptive events of Campanian Ignimbrite (37
0016-7037/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.gca.2007.04.007
* Corresponding author. Fax: +39 0816108466.E-mail address: [email protected] (S. Caliro).
ky BP) and Neapolitan Yellow Tuff (14.9 ky BP) are involvedin more recent volcanic episodes (e.g. Agnano-Monte Spinaeruption, 4 ky BP).
Recurrent seismic activity and ground deformation epi-sodes (bradyseismic crises) occurred at CF at least over thelast 30 years. Bradyseismic crises are characterized byphases of sharp ground uplift and seismic activity followedby slow aseismic periods of subsidence. Two main upliftphases were recorded: in 1968–1972 and in 1982–1984. Ineach case, the deformation was confined within a radiusof 6 km, with maximum values of about 2 m for both peri-ods at the caldera center. The town of Pozzuoli was par-tially evacuated during the 1984 crisis (Barberi et al.,1984), when more than 16,000 earthquakes (depth < 4 km)occurred with the most energetic ones (3 < Md < 4) having
Fig. 1. (a) Sketch of the Campi Flegrei caldera. Location ofSolfatara crater, Ischia and Procida islands are indicated; (b) Close-up view of Solfatara crater; main fumaroles, mud pools (fangaia),and the area of diffuse degassing are indicated.
The origin of the fumaroles of La Solfatara 3041
their epicenters in the Solfatara of Pozzuoli (Vilardo et al.,1991), a volcanic crater generated about 4 ky BP (Fig. 1a;hereafter Solfatara). Subsequent minor uplifts interruptedthe period of slow subsidence that began in 1985 (Troiseet al., 2007).
According to Bonafede and Mazzanti (1998) and Chio-dini et al. (2003), inflation episodes are related to fluid pres-sure variations within the underlying geothermal system,the most evident manifestation of which is the fumarolicfield of Solfatara. A conceptual geochemical model of sucha geothermal system was first proposed by Cioni et al.(1984) and then refined by Cioni et al. (1989), Chiodiniet al. (1992, 1996), Chiodini and Marini (1998) and Chiodi-ni et al. (2000a, 2001a). In line with this geochemical modelthe heat source of the hydrothermal system is a magmachamber located at depth that supplies heat to the overlyingaquifer(s), causing boiling and separation of a gas phase atconditions of maximum enthalpy for saturated steam(236 �C, 31 bar; Cioni et al., 1984). This gas phase occupiesthe shallower part of the system (‘‘vapor zone’’), for whichgas equilibria in the CO2–CO–CH4–H2O–H2 system indi-cate temperatures from 200 to 240 �C and P H2O from 1 to
20 bar (Chiodini and Marini, 1998; Chiodini et al.,2001a). The involvement of magmatic fluids, variably con-taminated by metamorphic and meteoric components,was supported by stable isotopes studies (Cortecci et al.,1978; Allard et al., 1991; Panichi and Volpi, 1999; Tedescoand Scarsi, 1999).
Measurements of diffuse soil CO2 fluxes in the Solfataraarea (�1.4 km2) were carried out from 1998 to 2000 andgave a daily amount of deeply derived CO2 discharged atSolfatara of about 1500 tons, whereas the correspondingheat loss emitted by the geothermal system was estimatedto be �100 MW. Such estimates of energy and mass fluxesindicate that the degassing process of fumarolic fluids atSolfatara is by far the most energetic process occurring atCF (Chiodini et al., 2001a, Chiodini et al., 2005). Phys-ico-numerical modeling of the degassing process, couplingthe transport of heat and multi-phase (steam and liquid)multi-component (water and carbon dioxide) fluids, sug-gested that the bradyseismic crises are caused by major epi-sodes of magma degassing (Chiodini et al., 2003; Todescoet al., 2003).
In this paper, we present a more comprehensive geo-chemical model of the Solfatara hydrothermal system inlight of the numerous new isotopic and chemical composi-tional data of the fumarolic fluids collected during the lastseven years. A species by species check of the attainment ofchemical equilibrium of the various gas species under differ-ent T–P-redox conditions, typical of hydrothermal and ofvolcanic environments, is for the first time presented forthe Solfatara fumaroles. This geothermometric approachis complemented by a new application of the H2–Ar gasgeoindicator and by use of the carbon exchange betweenCH4 and CO2 as isotopic geothermometer. Additionally,a new methodological approach based on the inversion ofthe chemical and isotopic data collected at surface isadopted for the assessment of the H2O–CO2 compositionof the magmatic and hydrothermal components feedingthe fumarolic emissions.
2. ADOPTED DATA SETS AND ANALYTICAL
METHODS
This work is based on the compositional data (408chemical analyses; 203 oxygen isotopic compositions and178 hydrogen compositions of steam condensate) of thetwo main fumarolic vents at Solfatara, Bocca Grande(BG) and Bocca Nuova (BN), and those of other fumarolescollected during the period 1983–2006 (Fig. 1b). A selectedset of analysis is reported in Table 1 while the entire data setis available in Electronic annex. BN and BG vents have thehighest outlet temperatures (145–161 �C) of the whole CFarea, while most of other vents discharge at temperatureslower than 100 �C. Samples collected from 1983 to 1999were analyzed in the laboratories of IGGI and IIRGCNR (Pisa) for chemical composition of gases and isotopiccomposition of steam condensate (analytical errors dD±1&, d18O ±0.1&) and were already published (e.g. Cioniet al., 1984, 1989; Chiodini et al., 1992, 1996; Chiodini andMarini, 1998; Panichi and Volpi, 1999; Chiodini et al.,2000a, 2001a). Since 1999 Solfatara fumarolic fluids have
Table 1Chemical and isotopic composition of Solfatara fumaroles since 1999
Sample data T �C H2O CO2 H2S Ar N2 CH4 H2 He CO d18O dD d13CCO2d13CCH4
BG 03/06/1999 162.0 848000 149000 1448 0.444 506 33.1 405 1.18 0.551 �0.68 �26.5BG 03/08/1999 150.0 835000 161000 1487 0.580 536 30.8 424 1.26 0.566 �0.95 �26.2BG 24/03/2000 151.0 853000 144000 1147 0.479 444 18.3 342 1.15 0.403 �0.95 �27.3 �1.66BG 22/08/2000 160.5 847000 150000 1244 0.392 460 19.0 254 1.28 0.396 �1.15 �26.0 �1.38BG 07/03/2001 160.9 840000 157000 1218 0.423 409 14.6 304 1.17 0.424 �0.78 �28.1 �0.75BG 29/11/2001 159.8 838000 159000 1243 0.390 418 22.7 334 1.20 0.406 �1.08 �25.9 �1.53BG 14/03/2002 159.3 836000 161000 1389 0.421 424 20.0 372 1.25 0.551 �1.14 �28.7 �0.73BG 15/07/2002 160.6 831000 166000 1464 0.469 456 18.4 400 1.47 0.574 �1.16 �1.31 �19.2BG 14/04/2003 161.8 818000 179000 1441 0.697 563 25.0 452 1.89 0.619 �0.88 �23.8 �1.22BG 06/08/2003 161.8 827000 170000 1390 0.610 501 19.3 436 1.78 0.669 �0.77 �22.3 �1.87BG 28/11/2003 163.5 821000 175000 1400 0.549 465 16.0 407 1.78 0.651 �1.07 �26.2BG 06/02/2004 162.3 817000 180000 1360 0.520 483 17.6 408 1.87 0.461 �1.09 �26.8 �1.70BG 21/10/2004 163.2 817000 179000 1337 0.420 505 20.4 412 1.81 0.542 �1.00 �23.3BG 22/03/2005 159.8 812000 184000 1378 0.698 524 22.5 455 2.00 0.591 �1.27 �25.1BG 02/08/2005 159.9 810000 186000 1470 1.012 534 19.9 476 1.95 0.627 �1.47 �18.8BG 09/01/2006 159.1 809100 188400 1462 0.443 485 17.7 453 1.94 0.658 �1.10 �26.2 �1.40BG 08/03/2006 162.0 796600 198400 1488 17.9 484 2.13 0.685 �1.20 �25.0 �1.30 �18.5BN 29/04/1999 145.2 852000 145000 1075 0.545 569 31.4 430 1.42 0.500 �1.08 �30.3BN 16/09/1999 150.9 852000 145000 1081 0.450 509 24.7 404 1.21 0.535 �1.08 �24.8BN 03/01/2000 149.4 854000 143000 1102 0.719 528 20.1 423 1.34 0.427 �0.96 �28.3BN 26/08/2000 144.7 841000 156000 1043 0.424 445 16.1 326 1.23 0.501 �1.26 �28.3 �1.74BN 01/03/2001 146.3 862000 136000 837 0.419 387 12.2 293 1.11 0.413 �0.38 �22.9 �1.45BN 11/08/2001 146.5 839000 158000 991 0.439 460 17.7 350 1.33 0.463 �1.04 �27.0 �1.68BN 08/01/2002 148.0 840000 158000 1049 0.548 440 19.8 338 1.25 0.482 �0.95 �25.8 �1.68BN 15/07/2002 146.3 832000 165000 1073 0.444 446 14.7 388 1.43 0.642 �0.17 �24.1 �1.08 �20.6BN 16/01/2003 148.0 846000 151000 777 0.414 467 18.4 393 1.72 0.450 �0.86 �27.7 �1.30BN 14/04/2003 147.7 831000 166000 1043 0.404 472 17.9 362 1.54 0.531 �0.81 �27.3 �1.26BN 06/02/2004 147.2 821000 176000 847 0.441 461 13.8 386 1.68 0.513 �1.19 �27.5 �1.60BN 21/10/2004 147.6 809000 188000 1050 1.022 564 18.4 452 1.96 0.657 �1.19 �27.0BN 22/03/2005 142.3 826000 171000 1007 0.478 480 17.1 428 1.83 0.569 �1.27 �26.9BN 02/08/2005 147.1 820000 177000 1036 0.784 503 15.8 461 1.80 0.692 �1.58 �19.6BN 09/01/2006 145.0 825000 173000 1011 0.381 441 13.3 418 1.75 0.626 �1.10 �30.0 �1.50BN 08/03/2006 144.6 817400 180500 1036 0.580 489 14.0 450 1.87 0.729 �1.50 �29.8 �1.50 �19.4Pisciarelli 17/03/1999 95.6 845000 152000 675 0.574 469 15.8 252 1.46 0.037Pisciarelli 03/06/1999 96.9 869000 129000 604 0.564 507 19.0 256 1.40 0.063Pisciarelli 23/08/2000 96.0 849000 149000 599 0.509 443 15.0 166 1.28 0.078Pisciarelli 21/12/2000 96.2 809000 188000 696 0.491 518 17.5 294 1.53 0.048
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Fig. 2. Theoretical values of logðX H2O=X H2Þ þ logðX CO=X CO2
) vs.3 logðX CO=X CO2
Þ þ logðX CO=X CH4Þ for a single saturated vapor
phase and a single saturated liquid phase are shown as vapor lineand liquid line, respectively. Equilibrium gas contents in a singlesaturated liquid phase are computed using the vapor–liquiddistribution coefficient, Bi, defined as Bi ¼ ðX i=X H2OÞvap=
ðX i=X H2OÞliq. The composition of vapors separated in a single-stepat varying temperatures (T0) (single-step vapor separation lines orSSVS lines) and the compositions of superheated vapors equili-brated at different T –P H2O values (solid lines) are also reported. Theequilibrium condition for the high-T fumaroles of Solfatara cratercan be inferred for a superheated vapor phase at temperaturesbetween 200 and 240 �C. The possible occurrence of H2 and CO re-equilibration upon cooling of the gas phase (see text), has also beeninvestigated by drawing the single gas-phase theoretical re-equil-ibration line: an original liquid at 360 �C (point A) vaporizesgenerating a vapor with composition represented by point B andre-equilibrates at 150 �C 6 T 6 360 �C and redox condition con-trolled by the Campanian Volcanoes relation (CV). Solfataracompositions plot along the re-equilibration pathway supportingthe occurrence of such process.
The origin of the fumaroles of La Solfatara 3043
been routinely analyzed for chemical composition at theGeochemistry Laboratory of INGV-Osservatorio Vesuvi-ano. For the determination of major gas species, fumarolicgases were collected in under-vacuum flasks containing a 4-N NaOH solution (Giggenbach, 1975; Giggenbach andGouguel, 1989). Condensates of water vapor and non-con-densable gases were sampled by flowing the fumarolic gasesthrough a condenser cooled at �20–30 �C by water. Gasconstituents were analyzed following the methods by Cioniand Corazza (1981), modified for the analysis of He, Ar,O2, N2, H2 and CH4. The chemical composition of thesenon-absorbed gases, mainly present in the headspace overthe NaOH solution, was in fact measured by gas chroma-tography through a unique injection on two molecular sievecolumns (MS 5 A capillary, 30 m · 0.53 mm · 50 lm; Heand Ar as carrier gases) using TCD detectors. Carbon diox-ide and sulfur species absorbed in the alkaline solution wereanalyzed after oxidation via H2O2, by acid–base titrationand ion chromatography, respectively (analytical error
3044 S. Caliro et al. / Geochimica et Cosmochimica Acta 71 (2007) 3040–3055
±3%). Because of reaction in alkaline solution to formCOOH� (Giggenbach and Matsuo, 1991), CO was ana-lyzed on dry gas samples by means of gas chromatographicseparation with a MS 5 A 1/8 · 50 in column (He as carriergas) coupled with a high-sensitivity Reduced Gas Detector(HgO).
Most of the isotopic analyses (data from 1999 to 2002)of oxygen and hydrogen (water) and carbon (CO2) wereperformed at the Geochemistry Laboratory of INGV-Pa-lermo using a Finnigan Delta plus mass spectrometer. Oxy-gen isotopic compositions are measured after equilibrationwith CO2 at 25 �C (Epstein and Mayeda, 1953), H afterreduction with Zn (Coleman et al., 1982) and C after cryo-genic purification of CO2 (analytical errors are: dD ±1&,d18O ±0.1& and d13C ±0.1&). Isotopic data since 2003were performed at the Geochemistry Laboratory ofINGV-Osservatorio Vesuviano using a Finnigan DeltaplusXP continuous flow mass spectrometer coupled withGasbenchII device (analytical errors are: dD ±1&, d18O±0.08& and d13C ±0.06&). Carbon isotopic analyses ofCH4 were performed at the Laboratory of G.P.I. Universityof Frankfurt (analytical error d13C ±0.2&) following themethods by Lyon and Hulston (1984).
Fig. 3. logðX CH4=X H2OÞ vs: logðX CO2
=X H2OÞ diagrams. The figuresshow the theoretical values for pure vapor and liquid phases, atredox conditions controlled by (a) CV, (b) DP and (c) FE redoxbuffers (Eqs. (12), (11) and (10), respectively). Dashed line of theliquid phase above 340 �C, the upper limit of vapor–liquiddistribution coefficients Giggenbach (1980), is extrapolated assum-ing that the vapor/liquid concentration ratio of each species is unityat the critical temperature. The measured H2O–CO2–CH4 relativecompositions of Solfatara fumaroles plot along the liquid phaseline of a, suggesting the presences of a high temperature liquid(360 �C) at redox conditions typical of the hydrothermal systems ofCampanian Volcanoes (CV).
3. RESULTS
3.1. Chemical composition of fumaroles and gas equilibria
Solfatara fumarolic effluents have similar chemistry,with H2O as the main component, followed by CO2 andminor amounts of H2S, N2, H2, CH4, He, Ar and CO.Fumarolic gases do not show any detectable SO2, HCl,and HF, due to the scrubbing of magmatic gases withinthe hydrothermal system (Cioni et al., 1984; Chiodiniet al., 2001a).
3.1.1. H2–CO2–CO–CH4–H2O system
Hydrothermal gas equilibria in the H2–CO2–CO–CH4–H2O system have been reviewed by Chiodini and Marini(1998), who compared analytical data from numerousworldwide hydrothermal systems with theoretical composi-tions of both equilibrium vapor and equilibrium liquidphases, and compositions of the vapor phases separatedin a single-step from the liquids at various temperatures.In this approach fH2O, the fugacity of water, is controlledby the coexistence of vapor and liquid, assumed as purewater, and expressed by the T –fH2O relation (Giggenbach,1980):
Logf H2O ¼ 5:51� 2048=T ðKÞ ð1Þ
In the present study, we chose pure water as the liquidproxy for the hydrothermal system of Solfatara. This choiceis supported by the relatively low salinity generally encoun-tered by deep drillings at CF (Cassano et al., 1986; Balducciand Chelini, 1992). In particular the CF23 well, a 1800-mdeep geothermal well drilled in 1953–1954 in the area ofPisciarelli (Fig. 1b), encountered 0.2–0.4 m NaCl geother-mal liquids at a temperature of about 300 �C at the bottomof well (Penta, 1954). According to Chiodini et al. (2001b),who reconsidered gas equilibria in the presence of NaCl
brines at different salinities (1, 2, 3 m and NaCl saturatedbrines), these low salinities should not imply major effectsin the estimations based on the used gas geoindicators.
Dissociation reactions of H2O and CO2 and the forma-tion reaction of CH4 by reduction of CO2, are combinedobtaining the following two reactions which are indepen-dent of the redox conditions:
CO2 þH2�COþH2O ð2Þ3CO2 þ CH4� 4COþ 2H2O ð3Þ
Assuming that the ratios of fugacity coefficients CH2/CH2O,
CCO/CCO2, and CCH4
/CCO2do not deviate significantly from
unity in the typical P–T range of hydrothermal systems, theequilibrium constants of reactions (2) and (3) can be ex-pressed as (Chiodini and Marini, 1998):
logðX CO=X CO2Þ � logðX H2=X H2OÞ ¼ �2248=T ðKÞ þ 2:485
ð4Þ
The origin of the fumaroles of La Solfatara 3045
3 logðX CO=X CO2Þ þ log ðX CO=X CH4
Þ ¼ �17813=T ðKÞþ 19:605 ð5Þ
Expressions (4) and (5) hold for the equilibrated vaporphase (vapor line in Fig. 2), whereas the liquid composition(liquid line in Fig. 2) has been computed assuming that gasspecies distribute between vapor and liquid phases accord-ing to the distribution coefficients, Bi (Giggenbach, 1980;Chiodini and Marini, 1998). The liquid and the vapor linesdelimitate the field of vapor produced by boiling processes.In particular the SSVS lines in Fig. 2 represent the compo-sition of the vapor generated by single-step vapor separa-tion (SSVS) processes from liquids at different originaltemperatures T0. The SSVS compositions have been com-puted from Bi values assuming isenthalpic boiling (for moredetails see Chiodini and Marini, 1998). The upper left partof the diagram, i.e. above the vapor line, is representative of‘superheated’ vapor, i.e. of a vapor phase characterized byfH2O lower than that corresponding to the saturated vapor.The ‘superheated’ vapor field is reported in Fig. 2 as aT –P H2O grid with P H2O � fH2O. All Solfatara data fall inthe field of the ‘‘superheated vapor’’: the main vents of Sol-fatara (BG and BN) yield temperatures generally from 200to 240 �C and P H2O from 1 to 20 bar while the other fuma-roles point to lower temperatures (130–190 �C) and lowerP H2O values. It is important to note that such estimates
Fig. 4. log ðX CH4=X CO2
Þ vs: 1000=T ðKÞ diagram. Analytical ratiosfor the Solfatara fumaroles are plotted against the equilibriumtemperatures computed through CH4–CO2 isotopic geothermom-eter. Theoretical ratios in a single saturated vapor phase and in asingle saturated liquid phase, under redox conditions controlled bythe hydrothermal redox buffers of Campanian Volcanoes (CV), areshown for reference. Theoretical ratios expected for varying waterfugacities and redox conditions fixed by the magmatic SO2–H2Sbuffer (Giggenbach, 1987) are also reported. Solfatara fumarolesplot close to the equilibrium conditions supporting that methane ispossibly formed by CO2 reduction in the hydrothermalenvironment.
are based on the assumption that the individual ratiosXCO/X CO2
, X H2=X H2O and X CH4
/X CO2equilibrate at the
same temperature, pressure and redox potential.In an alternative scenario the gas composition could be
affected by re-equilibration processes during the ascent andcooling of the fluids. Because CH4 reacts slower than H2
and CO upon changes in temperature, pressure or redoxpotential (Giggenbach, 1987, 1991), the equilibrium condi-tions inferred in Fig. 2 might be not valid. In order to verifythe above estimates and test this latter hypothesis, a speciesby-species check of attainment of equilibria conditions wasperformed.
3.1.2. H2O–CO2–CH4 gas system
Water, CO2 and CH4 most probably would not be af-fected (or only to a very minor extent) by the temperaturedecrease during the ascent of the gas. Water and CO2,which are the main species, do not change significantly inconcentration during re-equilibration and CH4 is one ofthe kinetically slowest species to equilibrate in the hydro-thermal environment (Giggenbach, 1991). Consequentlythese species can be used to investigate the deepest and hot-test temperatures of the hydrothermal system.
Taking the ‘‘full equilibrium’’ reaction of Giggenbach(1988):
Ca–Al–silicateþK-feldsparþ CO2�K-micaþ calcite
ð6Þ
as a thermodynamic proxy for water–rock reactions in thehydrothermal reservoir, fCO2
is fixed at any temperatureby the following relation (Giggenbach, 1988):
log f CO2¼ 0:0168 T ðKÞ � 8:369 ð7Þ
Fig. 5. Geothermometric diagram of logðX H2=X H2OÞvs: log
ðX CO=X CO2Þ. Equilibrium values for the vapor phase at different
temperatures and redox condition typical of hydrothermal environ-ments (FE, DP, CV) and volcanic gases (H2S–SO2, Giggenbach,1987) are reported. The analytical gas ratios of the fumaroles ofSolfatara plot between the CV and DP redox conditions attemperatures of 200–240 �C for BG and BN fumaroles and at lovertemperatures (130–190 �C) for the other fumaroles.
Fig. 6. Geothermometric diagram of log ðX H2=X H2OÞ vs:
logðX Ar=X H2OÞ. The reported theoretical compositions of theliquid (ASW), the vapor in equilibrium with ASW and the vaporproduced by a single step separation process at different temper-atures are computed for the redox conditions of CampanianVolcanoes (CV). The analytical values of Solfatara fumarolessuggest an almost complete vaporization of a liquid phase at hightemperatures (>360 �C) and a subsequent re-equilibration of the H2
during the cooling of the gas (see text). For comparison the averagevalues for the fumaroles of Vesuvio (Chiodini et al., 2001b) andIschia (Chiodini et al., 2004b) hydrothermal systems are alsoreported, showing high temperature conditions in agreement withthose estimated through H2–CO2–CO–CH4–H2O system equilibria.
3046 S. Caliro et al. / Geochimica et Cosmochimica Acta 71 (2007) 3040–3055
Assuming that CH4 is produced within the hydrothermalsystem by reduction of CO2, in line with the reaction:
CO2 þ 2H2O�CH4 þ 2O2 ð8Þ
fCH4is a function of temperature and redox conditions
according to:
log f CH4¼ log KCH4
� 2 log f O2þ 2 log f H2O þ log f CO2
ð9Þ
where KCH4is the thermodynamic constant of the reaction
Eq. (8) log KCH4¼ �42007=T ðKÞ þ 0:527, Chiodini and
Marini, 1998). Possible redox conditions in hydrothermalenvironments are approximated satisfactorily by either the(FeO)–(FeO1.5) buffer of Giggenbach (1987) (FE):
log f O2¼ 10:736� 25414=T ðKÞ ð10Þ
or the empirical relationship of D’Amore and Panichi(1980) (DP):
log f O2¼ 8:20� 23643=T ðKÞ ð11Þ
In the case of the Campanian hydrothermal systems(Vesuvio, Campi Flegrei and Ischia), the redox conditionswere found to be better described by the following relation-ship (CV):
log f O2¼ 7:75� 23169=T ðKÞ ð12Þ
that suggests an hydrothermal environment more oxidizingthan that described by the classical FE and DP buffers(Chiodini and Marini, 1998; Chiodini et al., 2004b).
The relations reported above for fH2O (Eq. (1)), fCO2(Eq.
(7)) and fCH4(Eq. (9)) have been used to derive the H2O–
CO2–CH4 geoindicator reported in the log X CH4=X H2O vs.
log X CO2=X H2O diagrams of Fig. 3a–c which refer to CV,
DP and FE redox conditions respectively. In Fig. 3a–cthe measured H2O–CO2–CH4 relative compositions of Sol-fatara fumaroles are compared with the theoretical valuesfor pure vapor and liquid phases, and for SSVS composi-tions. Analytical data plot close to the theoretical line rep-resenting the equilibrium composition of the liquid phaseunder redox condition well approximated by the CV rela-tion (Fig. 3a). This agreement suggests the presence of ahydrothermal liquid at high temperature, about 360 �C,close to the critical point of water, from which the fluidsdischarged by Solfatara fumaroles are generated. The betteragreement of analytical data with the compositional grid ofFig. 3a relative to Fig. 3b and c, again suggests that the Sol-fatara hydrothermal system is more oxidizing than thosedescribed by the typical FE and DP buffers.
An independent estimate of the CH4 equilibration tem-perature can be done on the basis of the carbon isotopic com-positions of fumarolic CH4 and CO2. By using fractionationfactors from Horita (2001), the available carbon isotope data(Table 1) indicate equilibration temperatures of 360–436 �C.In the logCH4/CO2 vs. 1000/T (K) diagram of Fig. 4,the carbon isotopic temperatures and the measuredlogCH4/CO2 values of Solfatara plot close to the equilibriumfor CV redox conditions. This supports again our hypothesisthat methane is possibly formed by CO2 reduction (Eq. (8))in the hottest parts of the hydrothermal environment.
3.1.3. H2O–H2–CO–CO2 gas system
Unlike methane-based ratios, X CO=X CO2and X H2
=X H2O
ratios should readjust their values according to the lowertemperatures encountered by the ascending vapor phase.Carbon monoxide and molecular hydrogen were in fact rec-ognized to be gas species that react quickly in response totemperature changes both in high temperature volcanicfumaroles (Giggenbach, 1987; Chiodini et al., 1993) andhydrothermal systems (Chiodini et al., 2002).
The X CO=X CO2and X H2
=X H2O ratios of the vapor phasecan be computed at any temperature from the dissociationconstants of H2O and CO2 and from the redox conditionsof the system:
logðfH2=fH2OÞ ¼ log KH2
� 1=2 log fO2
ffi logðX H2=X H2OÞ ð13Þ
logðfCO=fCO2Þ ¼ log KCO � 1=2 log fO2
ffi logðX CO=X CO2Þ ð14Þ
where KH2½log KH2
¼ 2:548� 12707=T ðKÞ] and KCO
[logKCO = 5.033 � 14955/T (K)] are the equilibrium con-stants of water and carbon dioxide dissociations (data fromStull et al., 1969). Eqs. (13) and (14) have been used to drawthe vapor-phase geothermometric grid of Fig. 5. In this dia-gram, Solfatara data are compared with the equilibriumvalues computed along both hydrothermal (FE, DP andCV) and volcanic redox buffer (H2S–SO2, Giggenbach,1987). The fumaroles fall in a position intermediate betweenCV and DP redox relations: the main vents BG and BN
The origin of the fumaroles of La Solfatara 3047
indicate temperatures from 200 to 240 �C while the otherfumaroles show lower temperatures (130–190 �C). Thesetemperatures are significantly lower than those estimatedby CH4 based geoindicators suggesting that the assumptionof equilibration among all the gas species involved in thesystem is no longer valid. Nevertheless, it is interesting tonote that the temperatures estimated in Fig. 5 are close tothe temperatures estimated in Fig. 2. This coincidence isnot surprising because the weight of CH4 on the combinedgeothermometric functions used in Fig. 2 is relatively small.In this new and more realistic interpretation of Solfatarafumaroles, the P H2O grid in the superheated vapor field ofFig. 2, and the low pressure estimates of the previous con-ceptual model, are not anymore valid. The position of thepoints depends in fact on re-equilibration processes whichaffect H2 and CO but not CH4. The pattern that followsduring re-equilibration of H2 and CO at decreasing temper-ature is illustrated in Fig. 2. A liquid originally at 360 �C(point A in Fig. 2) vaporizes generating a vapor with com-position represented by point B. The re-equilibration pat-tern of the gas from position B to the measuredcompositions is highlighted in Fig. 2 by the line labeled‘‘CO and H2 re-equilibration in a vapor phase from 360to 150 �C’’ that has been drawn for CV redox conditions.
3.1.4. H2–Ar geothermometer
Another gas geoindicator used for hydrothermal systemsis the H2–Ar geothermometer (Giggenbach, 1991), which isbased on the strong dependence of fH2
on temperature andon the assumption that the Ar content of hydrothermal li-quid phase is equal to that of air saturated water (ASW),i.e. log Ar/H2O = �6.52 (Giggenbach, 1991; Chiodini
Fig. 7. Diagram of log (XH2S/XH2O) vs. 1000/T(K). The vapor andthe liquid phases are computed considering redox conditions fixedby the Campanian Volcanoes buffer (CV, Eq. (12)). Analyticalratios of Solfatara fumaroles are reported versus both CO–H2
temperatures (Fig. 2) and H2–Ar temperatures (Fig. 6). The smalldependence of vapor composition on temperature does not allowany geothermometric estimation, however the analytical data arecompatible with a re-equilibration in the vapor phase also forsulfur species.
et al., 2001b). The assumption of an Ar-content typical ofASW seems to hold also for magmatic vapors as suggestedby Giggenbach (1987). The general expressions of the H2–Ar geothermometer for the vapor and liquid phases are:
logðX H2=X ArÞV ¼ logðX H2
=X H2OÞV � logðX Ar=X H2OÞV¼ Aþ B=T ðKÞ � 1=2 log fO2
� log BAr
� logðX Ar=X H2OÞL ð15Þ
and
logðX H2=X ArÞL ¼ logðX H2
=X H2OÞL � logðX Ar=X H2OÞL¼ Aþ B=T ðKÞ � 1=2 log fO2
� log BH2
� logðX Ar=X H2OÞL ð16Þ
In Eqs. (15) and (16) A and B are the coefficients of theVan’t Hoff equation describing the dependence of KH2
ontemperature, logBAr and log BH2
are the log-form of thevapor–liquid distribution constant of Ar and H2 for purewater (logBAr = 2.1083 � 0.016 T (K) and log BH2
=2.401 � 0.014 T (K), Giggenbach, 1980; Chiodini et al.,2001b); logðX Ar=X H2OÞL is the log-form of the concentra-tion of Ar in the liquid phase assumed equal to that ofair–saturated water (ASW). A graphic representation ofthis geothermometer is given by the logðX H2
=X H2OÞ vs:logðX Ar=X H2OÞ plot in Fig. 6, where the analytical valuesof Solfatara fumaroles are reported together with the theo-retical compositions computed (i) for a liquid (ASW), (ii)the vapor in equilibrium with ASW and (iii) the vapor pro-duced by a single-step separation process at temperatures of150, 200, 250, 300 and 350 �C. Most of the experimentalpoints of Solfatara plot at temperatures higher than360 �C, close to the critical point of pure water. A possiblescenario which well explains measured data is the almost
Fig. 8. Ternary plot of relative N2, He, and Ar contents ofSolfatara fumaroles. The potential end-member components, thevolcanic-hydrothermal gases from different systems in the world(Giggenbach, 1997) and other volcanic-hydrothermal systems ofthe region (i.e. Ischia and Vesuvio) are reported for comparison.The increase of N2 relative contents from Ischia, toward Solfataraand Vesuvio suggest a mixing process between the end-members formantle and arc-type derived volatiles.
3048 S. Caliro et al. / Geochimica et Cosmochimica Acta 71 (2007) 3040–3055
complete vaporization of a liquid phase at high tempera-tures (360–370 �C) and the subsequent re-equilibration ofthe H2 during the cooling of the gas which causes the de-crease of logðX H2
=X H2OÞ. Under these conditions, the effectsof H2 re-equilibration would cause only a minor underesti-mation of the H2–Ar temperatures. However, if a completevaporization of the liquid would occur at low temperatures,the H2 re-equilibration in the vapor phase would cause anincrease of the log ðX H2
=X H2OÞ and a significant overestima-tion of the H2–Ar temperature. Summarizing, the H2–Argeoindicator indicates that the fumarolic effluents of Solfa-tara are most probably generated by a process of almostcomplete vaporization of a liquid characterized by high-temperatures (360–370 �C) that are consistent with thoseestimated from chemical equilibria in the CH4–CO2–H2Osystem and from the isotopic geothermometer based onthe CH4–CO2 couple (Figs. 3a and 4).
3.1.5. H2S content
According to Giggenbach (1980) the H2S fugacity inhydrothermal environments is controlled by pyrite coexis-ting with an unspecified aluminum-silicate expressed bythe following empirical reaction (modified after Giggen-bach, 1980):
FeS2 þH2Oþ ðH2OÞ� ðFeOÞ þ 1=2O2 þ 2H2S ð17Þ
where (FeO) represents two-valent iron incorporated intothe lattice of this unidentified aluminum silicate and(H2O) the iron-free aluminum silicate in its protonatedform. From reaction (17), we derived the graphic represen-tation of the log X H2S=X H2O vs: 1000=T ðKÞ for the vaporand the liquid phases, considering redox conditions fixedalong the CV buffer (Fig. 7). The figure shows that for anequilibrated vapor phase in hydrothermal environmentsthere is a small dependence of log X H2S=X H2O on tempera-ture: from 100 to 400 �C and for the adopted redox func-tion log X H2S=X H2O varies only of 0.3 log units (from�2.9 to �3.2).
Fig. 9. Diagram of X CO2vs: X N2
of fumarolic gases. The correla-tions of CO2 with respect to N2 highlight the mixing between themagmatic and the hydrothermal components and can be employedto infer the X CO2
of the hydrothermal component, considering theX N2
of hydrothermal component as equal to the ASW.
This small dependence of H2S/H2O ratio on temperaturedoes not permit the derivation of a reliable geoindicator ofdeep temperatures. Consequently, values measured at Sol-fatara vents during the last 20 years of monitoring(�2.87 ± 0.12 as average value) can be compatible eitherwith an equilibration in the vapor phase at temperaturesof 150–240 �C (indicating a behavior similar to that of fastre-equilibrating H2 and CO species) or with the vaporiza-tion of hydrothermal liquid at temperatures close to thecritical point of the water, as inferred by the H2–Ar geoin-dicator (Fig. 7). In any case, analytical data suggest thatsulfur species in the hydrothermal system are mainly con-trolled by interactions of gases with the host rocks.
3.2. Origin of the CO2, N2, He and Ar
The inert gas constituents of fumarolic effluents He, Ar,and N2 can be used as indicator of the primary source of thefluids. Such species were used by Giggenbach and Gouguel(1989), Giggenbach (1992a), and Giggenbach and Poreda(1993) to characterize the origin of volatile components inrelation to the tectonic setting.
The relative N2, He, and Ar contents of Solfatara fuma-roles are reported in Fig. 8 together with those of (i) poten-tial end-member components, (ii) volcanic-hydrothermalgases from different systems of the world (Giggenbach,1997) and (iii) other volcanic-hydrothermal systems of theCampanian region, i.e. Ischia (Chiodini et al., 2004b) andVesuvio (Chiodini et al., 2001b). Solfatara gases, as wellas those of Ischia and Vesuvio, plot between the end-mem-bers for mantle and arc-type volatiles (Giggenbach, 1997),showing an increase of N2 relative contents from Ischia to-ward Solfatara and Vesuvio. The increase in N2 contents isaccompanied by a decrease in the maximum values of3He/4He ratios measured in the fumaroles which vary from3.7 to 4.6 R/Ra (where Ra is the helium isotopic ratio in theatmosphere, 1.39 · 10�6) at Ischia (Marty et al., 1994;Tedesco, 1996; Inguaggiato et al., 2000), from 2.6 to 3.4R/Ra at Solfatara (Sano et al., 1989; Tedesco et al., 1990;Marty et al., 1994) and from 2.7 to 2.8 R/Ra at Vesuvio(Allard et al., 1988; Tedesco et al., 1991). These values indi-cate a significant contribution of mantle derived primordial3He variably contaminated by He of crustal origin (Allardet al., 1997; Tedesco, 1997). Interestingly 3He/4He ratiosof the fumaroles practically overlap the values measuredin the olivine and pyroxenes of the volcanic products ofPVD (2.5–5.2 R/Ra, Martelli et al., 2004). According toMartelli et al. (2004) these relatively low 3He/4He ratiosdo not result from shallow magma contamination, ratherthey reflect the composition of the mantle beneath theregion, metasomatized by the addition of crustal fluids fromthe subducted plate.
Carbon isotopic compositions of the fumarolic CO2 atSolfatara (d13C �1.4 ± 0.4&, Table 1), Vesuvio (d13C from0 to 0.5&; Allard et al., 1988; Chiodini et al., 2001b; Caliroet al., 2005) and Ischia (d13C from �3.0 to �5.2&; Tedesco,1996; Inguaggiato et al., 2000), are generally more positivethan those typical of primary mantle carbon (d13C�6 ± 2&, Deines and Gold, 1973; Kyser, 1986), but morenegative than those expected from the CO2 entirely derived
Fig. 10. Diagram of oxygen isotopic composition of the (H2O + CO2) gas system d18OH2OþCO2versus the relative oxygen atom fraction of CO2
(vCO2). In the upper axis the corresponding CO2 molar fraction (X CO2) is reported. The isotopic variation on vCO2 of the computed magmatic
fluids is also reported as the ‘‘theoretical composition of magmatic fluids’’ line. Solfatara fumaroles show a trend representative of a mixingprocess between magmatic and hydrothermal components. The inferred composition for the magmatic component is X CO2
= 0.38 ± 0.06,d18O = 10.5 ± 0.3& and for the hydrothermal components is X CO2
= 0.11 ± 3 · 10�3, d18O = 4.7 ± 0.5&. Error bars of estimatedcompositions are also reported. See text for further details.
The origin of the fumaroles of La Solfatara 3049
by decarbonation of marine limestone. These values wereinterpreted as due to mixing between a mantle source andcarbon derived from limestone (Chiodini et al., 2004a).Considering He isotope data and the regional extensionof the CO2 degassing processes which affects the Campaniaregion (Chiodini et al., 2004a), we suggest that this mixingof different carbon sources occurs at depths, resulting in a’not typical’ local mantle contaminated by subducted car-bonate-rich sediments. The same process would be respon-sible of the peculiar He–N2–Ar compositions thatcharacterize the three Campanian volcanoes.
In order to extrapolate the CO2 content of the hydrother-mal component, we considered the diagram X CO2
vs. X N2of
Fig. 9. The linear correlation between CO2 and N2 suggestsa mixing between a magmatic-component rich in CO2 andN2 and a hydrothermal component poor in N2. Such a cor-relation has been used to compute a CO2 molar fraction fora hydrothermal component characterized by X N2
� 10�5,consistent with that of Air Saturated Water (ASW). Becauseof the scatter of data, in order to evaluate the uncertaintieson the estimate we have adopted the Monte Carlo method(Press et al., 2001). Considering the quantitative uncertain-ties in our experimental measurements (see Section 2),1000 realizations of synthetic data sets have been done in or-der to estimate the errors associated to the regression lineparameters. The results for the regression line areX CO2
¼ 96� 5� X N2þ 0:11� 3� 10�3 and a X CO2
valueof 0.11 ± 3 · 10�3 has been estimated for the hydrothermalcomponent ðX N2
� 10�5Þ considering error propagation ofparameters. It is worth noting that X CO2
of about 0.11 forthe component B, as inferred in Fig. 9, implies that thehydrothermal liquid approaches a ‘‘full equilibrated’’ water(X CO2
� 0:10 at 350–360 �C; Giggenbach, 1988) as indepen-dently suggested by gas equilibria.
3.3. Origin of the Water
The origin of the water component of Solfatara fuma-roles is here investigated on the basis of a large data setof H and O isotopic compositions of fumarolic conden-sates (see Electronic annex). With respect to previous datainterpretations (Chiodini et al., 1996; Panichi and Volpi,1999), we have also considered the role of oxygen isotopeexchange between H2O and CO2 in the gas phase. Chio-dini et al. (2000b) have in fact shown that the measuredd18OCO2
and d18OH2O values on six fumarolic samples col-lected at Solfatara are very close to the theoretical frac-tionation (Richet et al., 1977) expected for oxygenexchange between the two gaseous molecules at the dis-charge temperatures (T ranging from 97.4 to 162 �C).On the basis of these data and of similar results fromother 5 worldwide fumarolic systems, Chiodini et al.(2000b) also demonstrated that oxygen exchange betweenCO2 and steam in natural gas phases is fast enough to al-low rapid isotopic re-equilibration within a wide tempera-ture range (100–1000 �C). Upon cooling of the gas, theincreasing oxygen isotopic fractionation between CO2
and H2O progressively enriches the steam of the lighterisotopic component, whereas the d18O composition ofthe whole H2O+CO2 gas system, i.e. d18OH2OþCO2
, willremain constant regardless of temperature, unless oxygenaddition or loss is present. Because d18OCO2
is not avail-able for most of the samples, the d18OH2OþCO2
values ofSolfatara fumaroles have been computed from (Chiodiniet al., 2000b):
d18OH2OþCO2¼ vCO2
� d18OCO2þ ð1� vCO2
Þ � d18OH2O
¼ d18OH2O þ vCO21000 ln aðT Þ ð18Þ
Fig. 11. Isotopic composition of Solfatara fumarolic steam. d18OH2O (‘analytical values’) and the computed values of water at conditions ofthe deep reservoir (360 �C) are reported together with the local meteoric water, the local seawater (Baldi et al., 1975) and the ’andesitic water’field (Taran et al., 1989; Giggenbach, 1992b). The computed composition of the hydrothermal component at reservoir condition and thecomputed compositions of the magmatic water upon cooling of gas phase from 900 �C to reservoir condition are also reported. The computedfumarolic values have been calculated at the inferred deep temperature of 360 �C starting from the analytical values of the steam condensatesand from the measured H2O–CO2 compositions. These values show a unique cluster of points, with respect to the analytical ones, supportingthe occurrence of oxygen isotopic re-equilibration between H2O–CO2 at different discharge temperatures of an original fluid generated by amixing process between magmatic fluids and local hydrothermal component.
3050 S. Caliro et al. / Geochimica et Cosmochimica Acta 71 (2007) 3040–3055
where vCO2is an oxygen atom fraction of CO2 in the CO2–
H2O system, i.e. vCO2¼ 2X CO2
=ð1þ X CO2Þ, where X is the
molar fraction, d18OH2O is the analytical value of the sam-pled fumarolic fluid and a(T) is the CO2–H2O oxygen frac-tionation factor computed at the discharge temperatureusing the equation of Richet et al. (1977).
In the d18OH2OþCO2vs: vCO2
diagram (Fig. 10), Solfatarafumarolic fluids describe a trend between CO2-rich compo-sitions, characterized by a ‘‘heavy’’ oxygen (component A),and a CO2-poor component displaying lower d18O values(component B). In the framework of the hydrothermal sys-tem of CF it is reasonable to assume that component A is amagmatic component while component B is representativeof hydrothermal fluids of shallower origin. The mixing be-tween these two end-members, which originate the lineartrend in Fig. 10, can be expressed by the following isotopemass balance:
d18OH2OþCO2 ;f ¼ ½vCO2 ;h � d18OCO2 ;h þ ð1� vCO2 ;hÞ � d
18OH2O;h� ð1� Y Þ þ ½vCO2 ;m � d
18OCO2 ;m
þ ð1� vCO2 ;mÞ � d18OH2O;mY ð19Þ
where the subscripts f, h and m refer, respectively, to thefumarolic, hydrothermal and magmatic fluids and Y isthe fraction of magmatic fluids present in the fumaroles.The oxygen isotopic composition of magmatic fluids at Sol-fatara can be computed assuming the isotopic equilibriumbetween the magmatic fluids and the isotopic compositionof volcanic products of CF, i.e. d18O = 8.20&, correspond-ing to the most common trachytic rocks (Turi and Taylor,1976; Turi et al., 1991). Considering the isotopic fractiona-tions (Zheng, 1993; Zhao and Zheng, 2003) between
magma, CO2 and water, values of d18OH2O;m ¼ 8:69& andof d18OCO2 ;m ¼ 11:88& are obtained for the local magmaticcomponent at 900 �C. Such a temperature represents anaverage from estimates based on the petrogeny’s residuasystem (Ne–Ks–SiO2) as well as from feldspar-hosted fluidinclusions (Melluso et al., 1995; Fulignati et al., 2004)found in some trachytic products of CF. In Fig. 10, the linelabeled ‘Theoretical composition of magmatic fluids’ showsthe dependence of d18OH2OþCO2 ;m on vCO2
.Finally, the linear correlation between d18OH2OþCO2 ;f and
vCO2of fumarolic samples (Fig. 10) has been used in order
to compute the compositions of the two end-members in-volved in the mixing (i.e. magmatic and hydrothermal com-ponents). In order to estimate errors associated to the linearregression, the Monte Carlo method (Press et al., 2001 , seeSection 3.2) has been applied to d18OH2OþCO2 ;f and vCO2
data, yielding d18O ¼ 16:3� 1:4� vCO2þ 1:5� 0:4. The
values of d18OH2OþCO2 ;m ¼ 10:5� 0:3& and vCO2¼
0:55 � 0:07 ðX CO2¼ 0:38� 0:06Þ for the magmatic com-
ponent have been estimated, considering error propagation,at the intersection of the regression line with the ‘Theoreti-cal composition of magmatic fluids’ line (Fig. 10). Similarly,the values of d18OH2OþCO2 ;h ¼ 4:7� 0:5& and vCO2
¼0:2� 6� 10�3 (X CO2
¼ 0:11� 3� 10�3Þ have been esti-mated for the hydrothermal component at the intersectionof the regression line with vCO2
¼ 0:2 (i.e. at the CO2 con-centration of the hydrothermal component, previously esti-mated in Fig. 9). By adopting these average values in Eq.(19), we find that the fraction of the magmatic componentin the fumarolic effluents of Solfatara ranges from 0.06 to0.45, with an average value of 0.26 all over the whole tem-poral record of geochemical data. An independent check
Fig. 12. Revisited geochemical conceptual model of Solfatara, reporting the zones of deep mixing and H2, CO and H2S re-equilibration in thevapor phase. Gas/liquid mass fractions and low-T isotherms are reported, as returned by the physico-numerical simulations described in detailin Chiodini et al. (2003) and Todesco et al. (2003). The TOUGH2 geothermal simulator (Pruess, 1991) was used to model the effects of theinjection at the bottom of the hydrothermal system of a hot mixture of steam and carbon dioxide representative of discharged fumarolic fluids.Computations were constrained by a CO2 flux rate and gas/steam ratio equivalent to those measured at Solfatara.
The origin of the fumaroles of La Solfatara 3051
for the ‘‘average’’ mixing process can be done by means of asimple energy balance, assuming that all the heat carried bymagmatic gases ðHm ¼ 0:62Hm
H2O þ 0:38HmCO2Þ plus the heat
of the hydrothermal liquid ðHhl ¼ 0:89HhlH2O þ 0:11Hhl
CO2Þ is
equal or higher than the heat demanded for the vaporiza-tion processes, which is the heat content of the formed
gas phase ðHf ¼ X H2OHfH2O þ X CO2
HfCO2
, where X H2O and
X CO2are the measured fumarolic molar fractions). The heat
balance can be expressed in terms of the magmatic fractionY as:
Y Hm þ ð1� Y ÞHhl P Hf ð20Þ
By using enthalpy data for pure species (Johnson et al.,1991; Lemmon et al., 2005), expression (20) can be true onlyfor high temperatures of the hydrothermal liquid. In detail,the mixing and the vaporization process can occur only forhydrothermal liquid at temperatures >350 �C assuming (i)an average Y value of 0.26, (ii) that the maximum heatcontent of the magmatic component is 673 kJ/mol. (i.e.temperatures 6900 �C) and (iii) that the temperature ofthe outgoing ideal gaseous mixture is equal to that of thehydrothermal liquid, with P H2O (>165 bar) fixed along theliquid–vapor water boundary and the corresponding P CO2
(�40 bar, for Y = 0.26) given by Raoult’s law. It is worthnoting that these high temperatures of the hydrothermal li-quid are in substantial agreement with the temperaturesindependently returned by the adopted gas geoindicators.
The deuterium content of the magmatic component can-not be directly derived by analytical data of the rocks fromCF, therefore it is reasonable to assume for magmatic fluidsthe reference value for convergent plate boundaries of thePacific plate, generally characterized by dD values of�20 ± 10& and d18O values of +10 ± 3& (‘andesiticwater’, Taran et al., 1989; Giggenbach, 1992b). Similarcompositions were also inferred for the magmatic watersof volcanoes from the Mediterranean area (i.e. Vulcano,Chiodini et al., 2000b; Nisyros, Brombach et al., 2003).Starting from (i) the measured deuterium compositions ofthe fumaroles, (ii) the estimated fractions of magmaticfluids and iii) the reference magmatic value, we derive thedeuterium composition of the hydrothermal component.Computed dD values of �33& ± 4& practically overlapthe range of local meteoric water (��33&, Baldi et al.,1975; Bolognesi et al., 1986) supporting the meteoric originof the hydrothermal component.
Finally, Fig. 11 synthesizes the sequence of processesaffecting the H2O stable isotope composition of Solfatarafumaroles: the magmatic component cools and mixes withthe hydrothermal component of meteoric origin generatingvapors at 360 �C (‘computed values’ in Fig. 11); 18O-exchange between H2O and CO2, during the cooling ofthe vapor from 360 �C up to the discharge temperatures,causes a further isotopic shift which is larger in the100 �C fumarole Pisciarelli with respect to the 140–160 �Cfumaroles FC, BN and BG.
3052 S. Caliro et al. / Geochimica et Cosmochimica Acta 71 (2007) 3040–3055
4. DISCUSSION AND CONCLUSION
With a flux of deeply derived fluids of about 5000 tond�1 and an energetic release of �100 MW (Chiodiniet al., 2001a), Solfatara appears to be one of the largestfumarolic manifestations of the world. The present studywas aimed at defining the geochemical process behind sucha large surface emission, investigating both the sources ofthe discharged fluids and the reactions governing the com-position of gas species. The results of a specie by speciecheck of the attainment of equilibrium of the various gasspecies under different T–P-redox conditions show thatthe kinetically fast reactive species CO and H2 equilibratein a pure vapor phase at temperatures of 200–240 �C inthe hydrothermal environment. On the other hand, theapplication of CH4–CO2–H2O and H2–Ar geoindicators,as well as temperature estimates based on C isotopic ex-change between CH4 and CO2, give higher temperatures(P360 �C), close to the critical point of pure water. Thesehigher temperatures are likely to represent the conditionsat the bottom of the hydrothermal system. On the basisof stable isotopes of water we infer that in this deep zonemagmatic fluids rich in CO2 and with isotopic compositionsimilar to an ‘andesitic’ water type, mix with hydrothermalliquids having a deuterium composition similar to localmeteoric waters. In particular, by solving the d18OCO2þH2O
mass balance, oxygen isotope data of all the samples col-lected in the 1983–2006 period indicate that the Solfatarafumaroles discharge, on a molar average, a mixture of�0.26 magmatic gases and �0.74 hot hydrothermal liquid,which should completely vaporize during the process. Forthis mixing zone we estimate a temperature of about360 �C and a pressure of 200–250 bar, hence a depth of2000–2500 m by assuming near hydrostatic conditions.Since this depth is consistent with those from hypocenterlocation of the fluid-triggered seismic swarms (i.e. earth-quakes of August 2000, Saccorotti et al., 2001), our studysuggests a possible pivotal role played by fluids close to crit-ical–supercritical transition in the generation of part of theseismicity recorded at Solfatara. This zone, where mag-matic and meteoric component mix, splits the Solfatara sys-tem in two parts. Above the mixing zone hydrothermalconditions dominate favoring the formation of reducedgas species such as CH4 and H2S and the disappearing ofthe acid oxidant magmatic species (i.e. SO2, HCl, HF). Be-
Fig. 13. Chronogram of compositions of BG fumarole expressed as X CO2
compositions and periods of enhanced fluid injection (grey areas) are als
low this zone, more typical magmatic conditions, character-ized by higher temperatures and by the presence of theoxidized acid species, should prevail.
In Fig. 12 this conceptual model of the Solfatara hydro-thermal system is compared with the results of previousstudies based on physico-numerical simulations (Chiodiniet al., 2003; Todesco et al., 2003). In particular Fig. 12shows the expected temperatures and vapor–liquid ratiosas returned by the physical approach at steady conditions,i.e. after simulating 2000 years of injection of a 350 �Chot mixture of steam and carbon dioxide with compositionsimilar to that of the fumarolic fluids before the bradyseis-mic crisis of 1982–84, and with a flux rate similar to thatmeasured at Solfatara (Chiodini et al., 2003). Accordinglyto the geochemical model, the numerical simulations pre-dicts in the whole central column of ascending fluids, abovethe injection zone, the presence of a separated vapor phase,either as a single phase gas zone (at 1500–1400 m and at300–100 m depths, see Fig. 12) or in a two phase (gas–li-quid) zone. In this central zone of the plume, the fluid flowsessentially as a gas phase that moves from the high temper-ature area, near the injection zone, to a shallow single-phase gas zone (spgz) at temperatures ranging from 190to 230 �C, in agreement with the ‘vapor zone’ (T 200–240 �C) indicated by geochemical analyses.
An important issue for discussion is the implication ofthis new geochemical interpretation of Solfatara fumarolesfor the volcanic surveillance of CF. During the last 24 yearsof geochemical monitoring of fumarolic compositions, theX CO2
=X H2O ratio showed three evident peaks in 1985,1990 and 1995, which followed of few months ground upliftperiods (Fig. 13). In our model these peaks reflect fumaroliccompositions rich in the magmatic component (up to 0.45magmatic fraction in 1985, Fig. 13), possibly because ofthe occurrence at depth of episodes of magma degassingduring ground uplift periods. The process was simulatedby the physico-numerical approach reproducing the shapeof the geochemical signal by injecting for relatively shortperiods large amounts of hot CO2 rich gases into the hydro-thermal system (Fig. 13; Chiodini et al., 2003). Further-more, other physico-numerical simulations showed thatperiods of intense degassing of CO2-rich hot fluids can ex-plain other relevant features of the 1984, 1990 and 1995 un-rest crises at CF, such as ground deformation andgravimetric anomalies (Todesco et al., 2004; Todesco and
=X H2O and the computed fraction of magmatic fluids. Simulated gaso reported (modified from Chiodini et al., 2003).
The origin of the fumaroles of La Solfatara 3053
Berrino, 2005). After the year 2000, the fumarolicX CO2
=X H2O ratios have shown no peaks, but a slow increas-ing trend that is still ongoing at the time of writing. Thisdifferent behaviour of the fumarolic composition reflects,in our opinion, a change in the degassing style at depths.If this increasing trend represents the ascending portionof a CO2-rich fluids, then it is likely to be related to adegassing episode from a deeper portion of the magmaticsystem. Alternatively, this behaviour could be related tothe same magmatic source that now degases more continu-atively (or by more frequent smaller pulses) than in theprevious periods. Ground deformation patterns have alsochanged during the last years. In particular, a slow grounduplift phase started in 2004 is still ongoing, which is charac-terised by a longer duration and a slower deformation ratewith respect to the previous uplift episodes (Troise et al.,2007).
Different implications of this new geochemical modelshould be emphasized with respect to the previous interpre-tation of Solfatara fumaroles. Cioni et al. (1989) suggestedthat the X CO2
=X H2O ratio behaved as a true precursor of the1984 crisis, because such a chemical indicator was thoughtto be controlled by the boiling of a hydrothermal aquiferthermally close to the point of maximum enthalpy of satu-rated steam (236 �C). In this earlier interpretation the de-crease of the X CO2
=X H2O ratio observed before the 1984crises, and also before the smaller crises that followed,was thought to indicate an increasing boiling process anda pressurization of the aquifer due to an increase of the heatflux from the magma body. On the contrary, we show thatthe X CO2
=X H2O ratio is controlled by the mixing of mag-matic and hydrothermal components. The decrease of theX CO2
=X H2O ratio corresponds to periods of low flux of themagmatic component and of depressurization of the hydro-thermal plume in agreement with the ground subsidencethat always accompanied periods of gas/steam decrease.
The presented results give a more comprehensive geo-chemical picture of the process in operation at Solfataraand provide new constraints for more realistic simulationsof the degassing process in the framework of the surveillanceof the hydrothermal-volcanic activity at Campi Flegrei.
ACKNOWLEDGMENTS
We wish to thank A. Caracausi and another anonymous re-viewer for their constructive comments to the manuscript. C.Minopoli is acknowledged for his support to fumarole samplingin the frame of surveillance activities. F. Grassa is thanked forhis support in the isotopic analyses. E. Del Pezzo and A. Costaare thanked for their helpful suggestions. This work was partiallyfunded by the Italian Dipartimento della Protezione Civile in theframe of the 2004–2006 Agreement with Istituto Nazionale diGeofisica e Vulcanologia – INGV and by European ProjectVOLUME.
APPENDIX A. SUPPLEMENTARY DATA
Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.gca.2007.04.007.
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