Abstract. In the?resent period of quiescence, the Solfatara volcano, 1 km far from Pozzuoli,

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B8, PAGES 16,213-16,221, AUGUST 10, 2001 CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy G. Chiodini, 1 F. Frondini C. Cardellini, D. Granieri, 1 L. Marini, 3 and G. Ventura Abstract. In the?resent period of quiescence, the Solfatara volcano, 1 km far from Pozzuoli, releases 1500 t d- of hydrothermal CO2 through soil diffuse degassing from a relatively small area (0.5 krn2). This amount of gas is comparable to that released by crater plumemissions of many active volcanoes. On the basis of the CO2/H20 ratio measured in high-temperature fumaroles inside the degassing area, we computed a total thermal energy flux of 1.19 x 1013 J d -1(138 MW). Most of this energy is lost by shallow steam condensation and transferred to the atmosphere through the hot soil of the degassing area. The thermal energy released by diffuse degassing at Solfatara is by far the main way of energy release from the whole Campi Flegrei caldera. It is 1 order of magnitude higher than the conductive heat flux through the entire caldera, and, during the last 20 years, it was several times higher than the energy associated with seismic crises and ground deformation events. It is possible that changes in the energy flux from a magma body seated underneath Solfatara and/or argillification processes at relatively shallow depths determine pressurization events in the hydrothermal system and consequently ground deformation and shallow seismic swarms, as recorded during the recent episodes of volcanic unrest centered at Pozzuoli. 1. Introduction Solfatara volcano (Campi Flegrei, Italy) is a very suitable site to approach this problem: it is characterized by Most quiescent volcanoes dissipate important amounts of measurable soil degassing with monitored seismic activity energy through the direct expulsion of volcanic-hydrothermal fluids. Accurate energy estimates of this kind have been done in the favorable cases of hydrothermal-volcanic fluid input in crater lakes. The estimates of volcanic energy release in 14 different systems, based on mass and energy balances of the crater lakes, range from 4.7 to 33.3 x 1012 J d -1[Barberi et al., 1984; Brown et al., 1989; Shepherd and Sigurdsson, 1978; Hurst et al., 1991; Pasternack and Varekamp, 1997]. In other volcanoes without crater lakes (e.g., Etna [Allard et al., 1991a], Vulcano [Baubron et al., 1990; Chiodini et al., 1996a], Mammoth Mountain [Farrar et al., 1995]) as well as in many geothermal sites [Chiodini et al., 1998], large Copyright 2001 by the American Geophysical Union. Paper number 2001 JB /01/2001 JB and ground deformation episodes. At Solfatara we can study in detail the diffuse degassing process: (1) defining the size and the fracture pattern of the degassing structure, (2) measuring the total amount of expulsed gas through diffuse processes, (3) estimating the associated thermal energy flux, and (4) comparing this thermal energy with the energy released by conductive heat transfer, seismic activity, and ground deformation. 2. Geological Setting and Fracture Pattern of quantities of volcanic-hydrothermal CO2 are released through Solfatara soil diffuse emission. How large is the energy flux associated Solfatara volcano is a part of Campi Flegrei, a nested with such diffuse degassing processes? At Vulcano [Chiodini caldera resulting from two large collapses related to the et al., 1996a] and Nisyros [Brombach, 2000; Brombach et al., Campanian Ignimbrite (36 ka) and to the Neapolitan Yellow 2001] substantial amounts of thermal energy (1-6x1012 J d -1) Tuff (12 ka) eruptions (Figure 1 [Rosi and Sbrana, 1987; Orsi are released through the expulsion of volcanic-hydrothermal et al., 1996]). The post-12 ka activity has been characterized gases from relatively small areas. However, no information by several magmatic and hydromagmatic eruptions that exists on how thermal energy released by diffuse degassing occurred from vents located inside the Neapolitan Yellow compares to other ways of energy release (i.e., conductive Tuff caldera. The Campi Flegrei magmatic system is still heat transfer, seismic activity, ground deformation) from active: the last eruption occurred in 1538 A.D. at Monte quiescent volcanoes. Nuovo. Faults affecting the Campi Flegrei caldera follow two preferred strikes, NW-SE and NE-SW, which are the same of the faults affecting the Campanian Plain and the inner sectors 10sservatorio Vesuviano, Napoli, Italy. of the Apennine belt [Hyppolite et al., 1994; Orsi et al., 2 Dipartimento di Scienze della Terra, Universith di Perugia, 1996]. Perugia, Italy. The deformation history of Campi Flegrei has been 3 Dipartimento per lo Studio del Territorio e delle sue Risorse, characterized by phases of uplift and subsidence. In recent Universith di Genova, Genova, Italy. 16,213 times, bradyseismic crises occurred in and in During the episode a net uplift of 1.8 m centered on Pozzuoli was measured. It was accompained by more than 16,000 earthquakes (0<depth<4 km). Most of seismicity was concentrated in the Pozzuoli-

2 :... _..:: : :..:::..: 16,214 CHIODINI ET AL.: CO2 AND ENERGY RELEASE AT SOLFATARA VOLCANO / Campanian/gnimbdte (36 km caldera rim Neapolitan *gellow Tuff (I 2 ka} caldera rim 250 m I.Eruptive center yo'un: er than I2 ka / Fault ' 0.1m.221 ] m uplit phase Recent and present alluvium and Astmni vesiculated deposits: tuff pisolit/c (3.8 ash m Solfatara lapilli -stratified deposits: piso.lit'ic levels ash Olibano dome lavas Agnano and ves culated deposits: tuff ptsolitic (4.4 ash Fault Solhmra crater rim Fracture Cramr tim Figure 1. Structural sketch map of the (top) Campi Flegrei caldera (data from Rosi and Sbrana [1987] and Orsi et al. [ 1996] and (bottom) geological map of the Solfatara volcano. Solfatara area (Figure 1). In the Pozzuoli area, the seismic activity was characterized by low-energy swarms, while the Solfatara area was the epicentral zone for the most energetic earthquakes (3<Md<4 [Vilardo et al., 1991]). According to Bonafede and Mazz. anti [1998], the inflation episodes that occurred in and are the result of fluid pressure variations within the underlying geothermal system, possibly driven by the exsolution of high-temperature volatiles from the active magma chamber, which is located under the geothermal system (see below). At present, fumaroles and thermal springs occur in different sectors of the Campi Flegrei caldera. In particular, fumarolic activity occurs along the coast south of Pozzuoli and in the Mofete area and concentrates in the Solfatara area. Solfatara is a tuff cone (180 m above sea level) that consists of an hydrothermally altered breccia on which lies a sequence rich in accretionary lapilli of dune-bedded ash to lapilli beds, which cover an area of about 0.8 km 2 [Rosi and Sbrana, 1987]. Juvenile fragments are alkalitrachytes. Since the Solfatara products lie on the Olibano dome and on the 4.1 ka old Agnano-Monte Spina tephra and are-covered by the 3.8 ka old Astroni pyroclastics (Figure 1), it can be inferred that Solfatara activity developed between 3.8 and 4.1 ka. Phreatic (hydrothermal) eruptions from the Solfatara occurred in the XII century [Rosi and Santacroce, 1984]. At presenthe Solfatara tuff cone is carved by a km crater characterized by roughly subrectilinearims. The products of the tuff cone are strongly hydrothermally alterated and are affected by intense, diffuse degassing. The subvertical (dip>70 ø) inner walls of the Solfatara crater are affected by gravitative instability phenomena (rock falls). The Solfatara craterims are cut by two main faults striking NW-SE. Inside the crater area, both the NW-SE and NE-SW striking fracture systems occur. A 0.7 m wide NE-SW fracture opened inside the crater during the uplift phase [Rosi and Sbrana, 1987]. Outside the crater area, two NW-SE striking faults cut the eastern sector of the tuff cone. A mesostructural study was carried out on the Solfatara tuff cone in order to characterize the deformation pattern (Figure 2). Dip and strike of fault planes and fractures were measured both inside and outside the hydrothermal area. Where possible, slip sense of faults was determined following the criteria of Petit [1987]. A procedure of data selection was applied with the purpose to exclude the structures related to local topographic (gravity) effects [Mc Tigue and Mei, 1981]. Stress tensor ((51>(52>(53) and stress ratio R=((52-c 3)/(c 1-c 3) were computed through the direct inversion method [Sperner et al., 1993] using fault slip data. In sites where only minor fractures outcrop, the direction of extension was deduced following the criteria proposed by Hancock [1985] and Hancock and Engelder [1989]. Fault slip data show that the NW-SE striking faults affecting the Solfatara tuff cone are characterized by normal slips (75ø<pitch<90ø), with a maximum vertical offset of 40 m. Fault slip analysis indicates that these faults move in response to a normal stress field characterized by a sub-

3 CHIODINI ET AL.: CO2 AND ENERGY RELEASE AT SOLFATARA VOLCANO 16,215 vertical ol and a NE-SW striking subhorizontal 03. The calculated R value is Stress field analysis of focal mechanisms of earthquakes occurred in the Solfatara area during the crisis are also consistent with a prevailing normal stress regime characterized by subvertical ol and NE-SW striking subhorizontal o3 with R-0.2 [Zuppetta and Sava, 1993]. R values calculated from both structural and seismic data indicate o2eo3. [1995]. This observation suggests that in the hydrothermal areas the remote stress field responsible for the formation of the main NW-SE striking faults and fractures is perturbed by the superimposition of a local stress. 3. Fumarolic Activity and Conceptual Geochemical Model of the Hydrothermal System Outside the hydrothermal area, fractures associated with the faults show a preferred NW-SE strike and are arranged in conjugate sets dipping NE and SW (Figure 2). This At Solfatara the Bocca Grande (BG) and Bocca Nuova arrangement indicates that the fractures formed in response (BN) fumaroles (Figure 4) have the highest outlet to a NE-SW striking o3 and subvertical ol, a stress temperatures (145ø-165øC), while discharge temperatures of configuration which is consistent with that deduced by the other fumarolic vents are close to 100øC. Fumarolic effluents fault slip analysis. The average acute angle between the conjugate sets is 55 ø, a value consistent with hybrid (shear-extension) fractures [Hancock, 1985; Hancock and Engelder, 1989]. Inside the hydrothermal area, both within and outside the Solfatara crater, fractures show a NW-SE preferred strike and a second-order ENE-WSW strike (Figure 2). Plunges range between 60 ø and 90 ø. The preferred strikes of the nodal plane of the earthquakes that occurred in the Solfatara area coincide with those of the fractures affecting the have similar chemistry, with H20 as main component, followed by CO2 and H2S. Based on their stable isotopes, fumarolic fluids were interpreted as magmatic fluids variably contaminated by metamorphic and meteoric components [Panichi and Volpi, 1999; Tedesco and Scarsi, 1999; Cortecci et al., 1978; Allard et al., 1991b]. Helium isotopes (2.5-3 R/Ra) suggesthe involvement of a primary mantle source contaminated by crustal material [Tedesco et al., 1990]. However, since there is no detectable SO2, HC1, and HF in Solfatara fumarolic effluents, magmatic fluids are evidently Solfatara hydrothermalized zone. The cross-cutting buffered by a large hydrothermal system. relationships between the NW-SE and ENE-WSW Solfatara fractures are complex. In some places, the NW-SE striking fracture set cuts the ENE-WSW set, but the opposite was also observed. The cross-cutting relationships between the two fracture sets indicate a swapping between the two horizontal stresses, in accordance with the dynamic model of Caputo A conceptual geochemical model of the hydrothermal system, which explains the chemical composition of fumarolic fluids, was first proposed by Cioni et al. [ 1984] and then refined by Chiodini et al. [1992, 1996b] and Chiodini and Marini [1998]. The main elements of the model (Figure 3) are: (1) a heat source which is made up of a relatively Fault-slip data Solfatara crater rim Outside the hydrothermal area (station 2, 3, 5, 9, 11 and 12) strike % z50 m hydrothermal area 9 measurement station Inside the hydrothermal area (station 1, 4, 6a, 6b, 7, 8 and 10) N N _';o1 - rike Figure 2. (top) Results of mesostructural measurements can'ied out on the NW-SE striking faults affecting the Solafatara volcano. Variables O1, 02, and 03 are the principal stress axes computed from the inversion of faultslip data. Results of mesostructural measurements carried outside and inside the Solfatara hydrothermal area (inset).(bottom) Solid arrows indicate the direction of extension deduced from the orientation of fractures.

4 16,216 CHIODINI ET AL.: CO 2 AND ENERGY RELEASE AT SOLFATARA VOLCANO 4. Soil CO2 Diffuse Degassing Solfatara DDS does not coincide with the crater but covers a Solfatara DDS larger area to the east of the crater itself. The highest (Pco2 values overlap with faults and fractures, suggesting that the degassing process is strictly related to tectonic structures. Starting from the homogeneously distributed gas flux measurements, the total degassing output from the surveyed [,,,,...,.,,., condensed steam _= 3300 t/d ".. area was estimated by means of three different techniques, I '= " energy flux -- 1;21013 J/d - ="' I that is, arithmetic averaging, ordinary kriging (OK), and the I mass flux 4800 t/d I graphic-statistical approach (GSA) described by Chiodini et al. [1998]. Since the results can be affected by possible errors due to uncertainties at the borders of the surveyed area, the total CO2 output Qco2 was computed for a central square of 1 km 2, which comprises 333 measurements and completely delimits the Solfatara DDS (Figure 4). The arithmetic average of the 333 soil (Pco2 values is 1520 g m -2 d-1 which, integrated over the 1 km 2 surface, corresponds to a Qco2 of 1520 t d -. The OK gives an estimation of Q½o2 of 1280 t d -. Because of the log MAGMA BODY distribution of the (Pco2 values, neither the arithmetic Figure 3. Conceptual geochemical model of the Solfatara averaging nor the OK allow an estimation of the error which fumarolic field [Cioni et al., 1984; Chiodini et al., 1996b; is possible with the GSA. For this reason, in the following Chiodini and Marini, 1998]. The values of mass and energy discussions we will consider only the results of the GSA fluxes have been computed in this study (sections 4 and 5). method, which consists of two steps. In the first step the (P½o2 data are partitioned into homogeneous lognormal populations, using the graphic procedure of Sinclair [1974] (Figure 5b). The proportions ) and the statistical parameters of each shallow (few kilometers deep) magma chamber; (2) one or population are also computed. In the second step the mean some aquifers located over the magma chamber. The soil (Pco2 value (Mi) and the 95% confidence interval of the degassing magma supplies fluids and heat to the overlying mean are computed for each population by means of the aquifer(s) which dissipate the heat through boiling; and (3) an intensely fractured zone, sited above the uppermost aquifer and occupied by a pure vapor phase ("superheated" vapor zone), which is produced through boiling of the underlying aquifer(s). Gas equilibria in the CO2-CO-CH4-H20-H2 system indicate that temperature and Pmo conditions in the "superheated" vapor zone varied from 240øC, 30 bar during the crisis, to 210ø-220øC, 3-7 bar at present [Chiodini and Marini, 1998; G. Chiodini, Osservatorio Vesuviano, unpublishe data, 1999]. To estimate the total amount of gases released through diffuse degassing at Solfatarand surrounding zones, several surveys of soil CO2 fluxes ((Pco2) were carded out. Discussion is focussed here on the results of the 406 measurements performed from December 16 to 18, 1998, over an area of 1.16xl.06 km. The (Pco2 values, measured with the accumulation chamber method [Chiodini et al., 1996a; Farrar et al., 1995], range from 2 to 52,000 g m -2 d -1, with an average of 1270 g m -2 d -. The effects of meteorological conditions (Pco2 values were studied by recording (Pco2, barometric pressure, rain, soil and air temperature and Measuring point Log q:)co2,zt cate dn Fumarole (gm-2d- ) humidity every 2 hours with an automatic station installed at Solfatara well ' ' ' 2.8 Fault Solfatara crater. As a result of the stable meteorological Hotel conditions, only negligible changes in (Pco2 were recorded. Pisciarelli Tennis spdngs wells '1.8/ Fracture The contour map of log (Pco2(Figure 4) shows an area of about 0.5 km 2 of high (Pco2 values, representing an important Figure 4. Map of log q)co2, contoured every 0.2 log units. diffuse degassing structure (DDS), crossed by a NW-SE band The map was drawn by ordinary kriging. The diffuse of low fluxes. This DDS occurs in correspondence with the degassing structure (DDS), where CO2 fluxes are fed by products of the Solfatara volcano, while the NW-SE band of hydrothermal sources, are highlighted by light gray (log(pco2 > 1.8) and dark gray_ (log (Pco2 > 2.8). The data inside a low fluxes roughly coincides with the overlying impervious, central square of 1 km 2 (dashed square) were used to compute distal ashes erupted from the nearby Astroni volcano. The the total CO2 output (Qco2) from the Solfatara DDS.

5 o 7o 8O CHIODINI ET AL.' CO2 AND ENERGY RELEASE AT SOLFATARA VOLCANO 16,217 [Chiodini et al., 1998; Brombach, 2000; Brombach et al., 2001; G. Chiodini, Osservatorio Vesuviano, unpublishedata, 2000]. Since the mean (Pco2 value of population H is 3 orders of magnitude higher than background values, it is concluded that population H is representative of the CO2 diffuse degassing from the hydrothermal system. The total CO2 outputs from both the hydrothermal and background sources resulted to be Qco2,H = 1524 t d -1( t d -1) and Qco2,B = 18 t d -] (16-28 t d-i), respectively. 98 The Solfatara diffuse, hydrothermal CO2 output is more t3 and 40!.A H...,. J than the double than the mean CO2 flux, 648 t d -1, computed from the data of 101 nonerupting volcanoes crater plumes o>, ' 1Q Backgrou...,,J " t : q jbackground + Hydrothermal j reported by Williams et al. [1992] and is slightly higher than the CO2 flux computed by averaging the data of the crater plumes of 20 active volcanoesited in different geological settings, 1436 t d -1[Allard, 1992; Brantley and Koepenick, Log co2 1995], although the four major CO2-producer volcanoes, that is, Etna (Italy), Nyiragongo, Nyamuragira, Oldoinyo Lengai Figure 5, (a) Histogram and (b) probability plot of (three alkalic volcanoes of the East African Rift) were log co2. The data show a cle bimodal distribution excluded from this computation. These comparisons point out consistent with the p dal overlapping of a back ound population (B) and the population representative of the the significance of the diffuse degassing process affecting hydrothermal degassing (H). Solfatara and warn about possible underestimations of CO2 emissions from volcanoes when diffuse degassing is not measured. For example, Allard [1992] gives for Solfatara volcano a total CO2 flux of only 170 t d 'l because at thatime Sichel's t estimator [David, 1977]. Then the total soil CO2 the soil degassing contribution was not quantified. output (Qco2) for each population is calculated by multiplying Mi times the area covered by each population (which is given by the product) S, where S is the surveyed area). 5. Thermal Energy Release The histogram and the probability plot of the considered 333 data (Figure 5) show a clear bimodal distribution For estimating the flux of steam and thereby the heat flux consistent with the partial overlapping of the two lognormal involved in the diffuse degassing process, CO2 was used as a populations named B and H. On the basis of the Sinclair tracer of hydrothermal fluids, assuming that the H20/CO 2 ratio [1974] technique, mean log (Pco2 values of 1.25 and 3.21, of hydrothermal fluids, before steam condensation, is standar deviations of 0.44 and 0.57, and relative proportions recorded by fumarolic effluents. of 0.6 and 0.4, were computed for populations B and H, Out of the many fumarolic vents, with temperatures respectively. The mean (Pco2(Mi) and the 95% confidence ranging from the boiling temperature to 160øC, which are interval of the mean [David, 1977] resulted to be MB = 30 g present within the Solfatara DDS, the high-temperature m -2 d-1 (26-36 g m-2 d-1 ) and M. = 3811 g m-2 (3011 fumaroles BG and BN were sampled and analyzed in g m '2 d-i). December Three other fumarolic vents (i.e., Soll, Tom, The relatively low MB value suggests that the population B and Pi), located in other sectors of the Solfatara DDS (Figure represents the background fluxes due to soil biological 4), were also sampled, together with BG and BN in March activity. Similar background values, ranging from 10 to 40 g Analytical results are reported in Table 1. Despite these m -2 d-i, were measured in many other volcanic-geothermal fumaroles differ in outlet temperature and in the contents of areas, such as Vulcano Island, Colli Albani, and Vesuvio the most reactive gas species, that is, H2S, H2, and CO, they volcano (Italy), Nea Kameni and Nisyros (Greece), have similar contents of H20, CO2 and of the unreactive gases Yangbajain (China), and Mammoth Mountain (United States) Ar, N2, and He. It is unlikely that these similarities are Table 1. Composition of Solfatara Fumaroles Sample Date T, H20, CO2, N2, Ar, H2S, CH4, H2, CO, He, øc #mol/mol # mol/mol # mol/mol # mol/mol # mol/mol # mol/mol # mol/mol # mol/mol #mol/mol H20/CO2 BG Dec. 16, BN Dec. 16, BG Mar. 23, BN Mar. 23, Sol #1 Mar. 17, Pisciarelli Mar. 17, Tom Mar. 17,

6 16,218 CHIODINI ET AL.' CO2 AND ENERGY RELEASE AT SOLFATARA VOLCANO fortuitous, suggesting that (1) the entire Solfatara DDS is fed The results of 406 soil temperature measurements, at 10 by a unique source of hydrothermal fluids and (2) secondary cm depth, carried out concurrently with the December 1998 processes (i.e., condensation in the case of low-temperature (Pco2 campaign (Figure 6), showed that soil temperatures fumaroles and recycling of steam condensate in the case of superheated vents) does not affect significantly the original ratios between H20, CO2, and the unreactive gases. In particular, the measured H20/CO2 ratios span a narrow range, between 2.1 and 2.3 by weight. Therefore, the average H20/CO2 ratio of 2.2 by weight, ranged from 15 ø to 97øC inside the Solfatara DDS, while they were close to the mean air temperature of the season (11 øc) in the areas of background CO2 fluxes. The presence of this large energy flux is further proven by the results of the survey performed in May 1999, when both soil (Pco2 and soil thermal gradients (obtained from measured in fumarolic effluents in December 1998 and in temperature measurements at 10.5, 7.5, and 4 cm depths) March 1999, was assumed to be representative of the original fluids (i.e., before steam condensation) which fed the diffuse degassing process at Solfatara, at the time of the (Pco2 survey. were measured in 200 stations regularly distributed at 20 m distance. The investigated area, a small fraction of the entire Solfatara DDS, was affected, at that time, by an average CO2 Consequently, we compute that 3353 t d 'l of steam are flux of 2500 g m '2 d -1 which corresponds to an H20 flux of condensed to produce the 1524 t d 'l of CO2 expulsed through 5875 g m '2 d -1. The heat flux released through condensation of diffuse degassing. this amount of steam is 153 W m '2, from which it is inferred Neglecting the heat transferred by minor gas species, the that the thermal gradient through the soil is 153øC m -1, thermal energy released through diffuse degassing results to assuming an average thermal conductivity of 1 W øc 'l m 'l for be 1.19 x 1013 J d -1(138 MW). This value has been computed the soil itself (this is a reasonable value for a sediment with adding the following contributions. 1. The heat released by H20 gas moving from the hydrothermal reservoir to the steam condensation zone, that is, 1.90 x 1012 J d -1(Appendix A). Temperature, PH2o, and Pco2 in the hydrothermal reservoir were estimated referring to the BG and BN fumarolic effluents sampled during (G. Chiodini, Osservatorio Vesuviano, unpublishe data, 2000). For this period, gas equilibria in the CO2-CO-CH 4- H20-H2 system [Chiodini and Marini, 1998] indicate the following average values: T = 215øC, Pmo = 3.9 bar, and Pco2 = 0.74 bar. The condensation of the fluids has been assumed to occur at near surface conditions (T = 100øC, PH20 = 0.84 bar). 2. The heat given off by CO2 passing from the hydrothermal reservoir (T = 215øC and Pco2 = 0.74 bar) to atmospheric conditions (T = 11 øc, Pco2 = bar), that is, 1.16 x 1012 J d 'l (Appendix A). 3. The enthalpy of steam condensation at 100øC (7.57 x 10 2 J d-i), which is given by the product of the total amount of steam condensed in 1 day (3.353 x 109g) times the enthalpy of evaporation at 100øC, 2257 J g'l [Keenan et al., 1969]. 4. The heat lost by liquid water on cooling from 100øC to the average seasonal value of 11øC (1.25 x 1012 J d-l), which is given by the product of the enthalpy lost by 1 g of water (ZlH = J g-l) times the mass of water (3.353 x 109 g). Approximately 90% of the total thermal energy is transported by H20 (1.07 x 1013 J d -1) and most of it, 7.57 x 1012 J d -1, is dissipated through steam condensation. Assuming that the heat of condensation is totally transferred to the atmosphere by conductive transfer through the soil, an average heat flux of 224 W m -2 is computed for the Solfatara DDS in December The occurrence of such a large process of diffuse underground steam condensation and thermal energy release is qualitatively confirmed both by the presence of thermal waters outflows, within and around Solfatara DDS, and by soil temperature measurements. Evidence of underground circulation of condensed steam inside the Solfatara crater is provided by bubbling mud pools (Fangaia) and by a shallow well (Solfatara well) containing boiling condensates at 6 m depth (Figure 4). Around the Solfatara cone are present many thermal water outflows, some of which (i.e., Pisciarelli springs and Hotel Tennis wells, Figure 4) were interpreted as mixtures of evaporated meteoric waters and condensed steam [Valentino et al., 1999]. high porosity [Clauser and Huenges, 1995]). The arithmetic mean of the measured thermal gradients is 160øC m -1, which is very similar to the value obtained from the CO2 flux measurements. 6. Evidences of Pressurization Phenomena of the Hydrothermal System Strong variations in the concentrations of both main and minor gas species of BG and BN fumarolic effluents were observed before and during the last bradyseismic crisis of , as well as during the two minor episodes of and of 1994 [Allard et al., 1991b; Cioni et al., 1984, 1989; Martini, 1986; Martini et al., 1991; Tedesco, 1994; Tedesco and Scarsi, 1999]. In particular, each of these recent deformation episodes and seismic swarms was preceded by an increase in the heat Soil temperature (oc),.--,, " -, Solfatara, Crater dm C. ', DDS, Fault ß Measudng point ',,,... / Fracture Figure 6. Map of soil temperatures, at 10 cm depth, drawn by ordinary kriging.

7 CHIODINI ET AL.: CO2 AND ENERGY RELEASE AT SOLFATARA VOLCANO 16,219 transported by fumarolic fluids, as indicated by the growth in the H20/CO2 ratio (Figure 7). This behavior can be interpreted as an attempt of the hydrothermal system to dissipate the excess of energy deriving from its pressurization. In addition, the chemistry of fumarolic effluents suggests that after the bradyseismic crisis, the hydrothermal system was affected by a pressure decrease [Chiodini et al., 1992, 1996b; Chiodini and Marini, 1998]. An independent evidence of pressurization episodes of the hydrothermal system is provided by the azimuthal distribution of fractures inside and outside the Solfatara DDS (Figure 2). Outside the DDS the fractures show a NW-SE preferred strike, which is consistent with a stress field characterized by a NE-SW extension. Inside the DDS, the fracture strike shows two maxima oriented NW-SE and NE-SW. The presence of these two main fracture sets inside the DDS indicates that the stress field is perturbed by the superimposition of a local stress. According to numerical models, experimental models and field observations [Sibson, 1996; Zhang and Sanderson, 1997; Bianco et al., 1998], an increment of fluid pressure in a pre-existing fracture (i.e., the NW-SE fractures at Solfatara DDS) causes a compression in the surrounding rocks and the formation of new fractures, whose orientation (NE-SW) is orthogonal to that of the pre-existing discontinuities. This model is consistent with the data collected in the Solfatara area and is able to explain the cross-cutting relationships between the NW-SE and ENE-WSW fractures. In addition, it is remarkable that the preferred strikes of the nodal plane of the earthquakes occurred in the Solfatara area [Orsi et al., 1999] coincide with those of the fractures affecting the DDS, and that coexisting NW-SE and ENE- WSW fractures formed during the seismic crisis [Rosi and Sbrana, 1987; Dvorak and Gasparini, 1991]. These observations indicate, in agreement with the results of gravity models on the uplift episode [Bonafede and Mazzanti, 1998], that pressurization episodes of the Solfatara hydrothermal system play an important role in the fracturation pattern of the Solfatara DDS. In particular, one of these pressurization episodes led to the occurrence of a phreatic (hydrothermal) eruption in A.D [Rosi and Santacroce, 1984]. looo, 5o0-3,0 : t11. Ground deformation : '., ', ' l' Fumarote composition '2.5 L"eo : t:... ß... :: " "'.' 79 8! i me (y),2.t3 Figure 7. Phenomenological chronogram. An increase in the H20/CO2 ratio of the Solfatara fumaroles, and consequently in the heat transported by fumarolic fluids, preceded each of the recent uplift episodes and seismic swarms that occurred in Campi Flegrei. 't,5 DDS is a minimum estimate of the total convective hydrothermal flux at Campi Flegrei. The complete energy balance of the study area is beyond the objectives of this paper, and further investigations are needed for a better definition of the energy flows from the magma chamber toward the surface. Nevertheless, the estimated value is much higher than other known fluxes of energy released within the caldera structure during this noneruptive period, that is, mainly through thermal conduction, seismic events, and ground deformation. The conductive heat flux over the entire caldera (1 x x 10 2 J d - [Corrado et al., 1998]) is 1 order of magnitude lower than the heat flux associated with diffuse degassing at Solfatara. The energy dissipated by the Solfatara DDS in 1 day of activity is 10 times higher than the total elastic energy released during the seismic crisis, 10 2 J [Orsi et al., 1999]. Again, the energy released by the DDS during the last 20 years (assuming steady state), totals J, which is sufficient to cause an elevation of 0.76 m (approximately the average uplift of the entire caldera during the crisis) of 5800 km 3 of rocks with a density of 2000 kg m -3. This volume of rocks is 1 order of magnitude higher than the maximum volume of rocks displaced during the last bradyseismic episode (<400 km3). Based on these considerations, we suggesthat variations in these large energy fluxes could have played a pivotal role in the recent crises which affected the area. Both an increase in magma degassing and/or a large argillification process in the upper parts of the Solfatara DDS could have caused overpressurization of the hydrothermal system and, as a direct consequence, groun deformation and fracturing. The results obtained at Solfatara volcano show that the direct expulsion of hydrothermal gases from the relatively small DDS area, and related phenomena, is the main process through which the Campi Flegrei magmatic system dissipates its energy. The same process affects many other quiescent volcanoes as suggested by the numerous studies of crater lakes. The amount of gas and energy released by diffuse degassing structures can be quantified through CO2 flux measurements. Periodical surveys can constitute a powerful tool to monitor the activity of Campi Flegrei and of many 7. Discussion and Conclusions other volcanoes in the world characterized by the presence of similar DDS. Such data are needed for a better comprehension The computed value of 1.19 x10 3 J d ' for the thermal of volcanic dynamics and for the mitigation of volcanic risks. energy associated with diffuse soil degassing at Solfatara Appendix A: Changes in the Heat Content of a COz-HzO Gas Mixture Induced by Variations in Temperature and Pressure When a gas experiences changes in P,T conditions, its enthalpy, H, varies as expressed by the differential equation [Anderson and Crerar, 1993]: dh = }H dt + P T dp. (A1) The first term on the right side of (A1) can be rewritten as follows: I TT I P dt = C?dT. (A2) The dependence of the heat capacity at constant pressure Cp on temperature is satisfactorily expressed by the Maier Kelley equation:

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