Vol48/04/2005def 755 ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005 Key words Aeolian Islands – Panarea – submarine fumaroles – gas chemistry – geochemical monitoring 1. Introduction On November 3 2002 at least three large and whitish plumes suddenly appeared at the sea surface about 3 km eastward of Panarea Is- land (Aeolian Archipelago, Southern Italy), in an area characterized by a relatively shallow sea (10-15 m deep) and by the presence of emerging reefs of different size, roughly form- ing an elliptical shape (fig. 1). The phenomenon proved to be the surface expression of huge gas columns emerging from the sea floor as a mix- The November 2002 degassing event at Panarea Island (Italy): five months of geochemical monitoring Bruno Capaccioni (1) (6), Franco Tassi (2)(4), Orlando Vaselli (2)(4), Dario Tedesco (3)(5) and Piermaria Luigi Rossi (6) (1) Istituto di Vulcanologia e Geochimica, Università degli Studi di Urbino, Italy (2) Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Italy (3) Dipartimento di Scienze Ambientali, Seconda Università degli Studi di Napoli, Caserta, Italy (4) Istituto di Geoscienze e Georisorse, CNR, Firenze, Italy (5) Istituto di Geologia Ambientale e Geo-Ingegneria, CNR, Roma, Italy (6) Dipartimento di Scienze della Terra e Geologico-Ambientali, Università degli Studi di Bologna, Italy Abstract On 3rd November 2002, at about 3 km off-shore of Panarea Island (Aeolian Islands, Southern Italy), a series of gas vents suddenly and violently opened from the seafloor at the depth of 10-15 m, with an unusually high gas flux and superimposing on the already existing submarine fumarolic field. Starting from the 12th November 2002 a discontinuous geochemical monitoring program was carried out. The emissions consisted in an emulsion whose liquid phase derived from condensation of an uprising vapor phase occurring close to the fluid outlets without significant contamination by seawater. The whole composition of the fluids was basically H2O- and CO2-dominated, with minor amounts of typical «hydrothermal» components (such as H2S, H2, CO and light hy- drocarbons), atmospheric-related compounds, and characterized by the occurrence of a significant magmatic gas fraction (mostly represented by SO2, HCl and HF). According to the observed temporal variability of the fluid compositions, between November and December 2002 the hydrothermal feeding system was controlled by ox- idizing conditions due to the input of magmatic gases. The magmatic degassing phenomena showed a transient nature, as testified by the almost complete disappearance of the magmatic markers in a couple of months and by the restoration, since January 2003, of the chemical features of the existing hydrothermal system. The most striking feature of the evolution of the «Panarea degassing event» was the relatively rapid restoration of the typ- ical reducing conditions of a stationary hydrothermal system, in which the FeO/Fe1.5O redox pair of the rock mineral phases has turned to be the dominating redox controlling system. Mailing address: Dr. Bruno Capaccioni, Istituto di Vul- canologia e Geochimica, Università degli Studi di Urbino, Loc. Crocicchia, 61029 Urbino, Italy; e-mail: b.capaccio- ni@uniurb.it 756 Bruno Capaccioni, Franco Tassi, Orlando Vaselli, Dario Tedesco and Piermaria Luigi Rossi ture of gas, fine-grained sediments and col- loidal sulphur, in an area previously character- ized by a gentle bubbling sub-marine fumarolic field (Gabbianelli et al., 1990; Italiano and Nuccio, 1991; Calanchi et al., 1995). A particu- larly large plume occurred a few tens of meters SE of Bottaro reef (hereafter named Bottaro 1). According to the size of the «crater» depression produced (about 20×14 m wide and 10 m deep), the event was probably the result of a sort of submarine «gas blast». In order to define the nature of these new gas emissions and their temporal evolution, a first gas sampling campaign was carried out starting on November 12 and followed by three samplings, almost on a monthly basis, until March 2003. 2. Geological and volcanological setting Panarea is the smallest island (3.3 km2) of the Aeolian Archipelago (i.e. the subaereal por- tion of the Aeolian Volcanic District – AVD). The AVD is a ring-like volcanic arc consisting of 7 islands and 10 seamounts, and it consti- tutes about 200 km of the inner side of the Peloritanian-Calabrian orogenic belt (Boccalet- ti and Manetti, 1978; Beccaluva et al., 1982, 1985; Gabbianelli et al., 1990; Calanchi et al., 2002). The dynamics of this arc (which is locat- ed along regional NS-, EW- and NE-SW-orient- ed fault systems) is determined by tectonics through some faults that are still active (Gas- parini et al., 1982; Lanzafame and Rossi, 1984). The AVD is characterized by a subduc- tion-related magmatism that ranges from calc- alkaline to shoshonite products. The volcanic activity has taken place almost entirely since the Quaternary, 400 Kyr up to the present (Calanchi et al., 2002). The morphology of the sea floor, as defined by the 100 m isobaths, resembles that of a vol- canic complex characterized by a multi-phase evolution (Gabbianelli et al., 1986, 1990). After the growth of a central apparatus, centered in the island and in the submarine surroundings, volcanic activity occurred within the eastern area through a NE-SW fissure system. According to radiometric ages, the subaere- al portion grew in a short period of time (150- 100 Kyr). After 50 Kyr of quiescence, the activ- ity resumed with the emplacement of the en- dogenous dome of Basiluzzo, located NE of Panarea, along the Panarea-Stromboli tectonic line (NE-SW fault system). Between Basiluzzo and Panarea, some small islets (Dattilo, Bot- taro, Lisca Bianca, Panarelli and Lisca Nera) (Calanchi et al., 1999a,b) are arranged along a circular rim about 1 km in diameter. The area defined by the islets has a crater-like shape, with a maximum depth of 30 m, and is charac- terized by an important gravimetric anomaly Fig. 1. Map of Panarea Island with the locations of Calcara, Bottaro 1, Frattura, Bottaro 2 and Bottaro 3 gas discharges. 757 The November 2002 degassing event at Panarea Island (Italy): five months of geochemical monitoring T ab le I. C he m ic al c om po si ti on o f su bm ar in e an d su ba er ea l (C al ca ra ) ga se s. G as c om po un d co nt en ts a re i n µ m ol /m ol . S am pl e D at e T °C C O 2 H C l H F S O 2 H 2S S H 2O N 2 C H 4 A r O 2 N e H 2 H e C O C al ca ra N ov em be r 20 02 10 1 60 60 4 61 .0 1. 94 1. 72 26 1 0. 05 93 8 22 5 39 5 31 1 3. 00 51 .5 0. 00 3 82 .9 0. 18 0. 00 9 C al ca ra D ec em be r 20 02 99 48 24 5 24 .8 0. 74 0. 58 21 3 0. 05 95 1 23 9 10 8 12 2 0. 66 4. 41 0. 00 1 41 .6 0. 09 0. 00 4 C al ca ra Ja nu ar y 20 03 98 52 78 3 6. 66 < 0. 01 < 0. 01 25 4 < 0. 01 94 6 47 6 25 2 12 6 2. 17 46 .8 0. 00 1 53 .2 0. 06 0. 00 7 C al ca ra M ar ch 2 00 3 99 41 80 6 7. 48 < 0. 01 < 0. 01 18 6 < 0. 01 95 7 80 6 96 .7 66 .4 1. 44 2. 73 0. 00 1 27 .5 0. 04 0. 00 2 B ot ta ro 1 N ov em be r 20 02 47 28 0 04 5 6. 23 1 25 .0 25 6 1. 60 9 2. 11 70 9 99 8 1. 29 1 1. 11 30 .3 29 5 0. 02 9 21 7 1. 52 1. 12 2 B ot ta ro 1 D ec em be r 20 02 46 33 6 49 4 23 6 0. 47 24 .2 4. 05 8 1. 18 65 7 32 0 1. 17 7 1. 62 24 .2 32 0 0. 02 2 33 5 2. 08 1. 73 7 B ot ta ro 1 D ec em be r 20 02 46 38 0 33 6 63 0 0. 58 86 .4 5. 19 4 1. 25 61 0 14 4 2. 51 8 4. 35 42 .2 50 1 0. 04 0 52 8 4. 74 3. 13 0 B ot ta ro 1 D ec em be r 20 02 46 32 9 21 5 23 2 0. 50 25 .5 5. 81 7 1. 43 66 3 25 5 92 1 1. 62 18 .7 17 2 0. 01 6 33 2 1. 21 1. 72 1 B ot ta ro 1 Ja nu ar y 20 03 37 46 2 00 6 15 0 < 0. 01 6. 89 4. 69 7 0. 18 53 1 57 9 1. 02 6 0. 87 16 .0 53 .5 0. 01 4 46 2 1. 64 1. 67 4 B ot ta ro 1 Ja nu ar y 20 03 37 44 4 83 3 16 3 < 0. 01 9. 25 4. 78 9 0. 19 54 7 89 4 1. 54 7 1. 36 25 .1 36 0 0. 02 1 37 4 1. 22 2. 23 7 B ot ta ro 1 M ar ch 2 00 3 35 74 0 79 1 34 0 < 0. 01 10 .7 18 .5 48 0. 39 23 6 00 2 2. 79 4 3. 22 46 .0 34 5 0. 03 9 1. 11 3 2. 27 4. 16 4 B ot ta ro N ov em be r 20 02 n. d. 29 4 56 9 12 .9 78 16 9 89 9 1. 87 3 0. 95 68 7 35 8 1. 54 0 4. 41 19 .2 30 3 0. 01 7 28 3 2. 13 1. 67 5 F ra tt ur a B ot ta ro D ec em be r 20 02 n. d. 34 9 36 9 36 3 0. 58 38 .5 2. 61 4 1. 25 64 5 72 5 1. 42 4 7. 83 20 .8 32 2 0. 01 7 10 6 1. 64 1. 13 2 F ra tt ur a B ot ta ro Ja nu ar y 20 03 n. d. 44 4 82 7 22 6 < 0. 01 8. 48 4. 81 5 0. 29 54 6 73 2 2. 18 8 4. 97 37 .6 51 3 0. 03 0 64 1 2. 26 2. 77 0 F ra tt ur a B ot ta ro M ar ch 2 00 3 n. d. 52 4 36 9 18 9 < 0. 01 4. 43 6. 64 1 0. 16 46 5 45 9 2. 09 7 2. 73 34 .5 55 8 0. 03 0 63 8 2. 29 4. 32 2 F ra tt ur a B ot ta ro 2 N ov em be r 20 02 n. d. 64 8 28 3 2. 03 1 20 .2 11 5 4. 54 5 5. 12 34 1 37 6 3. 06 2 0. 76 28 .8 45 7 0. 02 5 76 .5 4. 72 0. 20 3 B ot ta ro 2 D ec em be r 20 02 51 38 1 13 3 2. 62 2 22 .4 10 5 5. 40 7 4. 02 60 8 30 3 1. 83 5 0. 91 24 .3 46 2 0. 01 9 77 .1 3. 48 0. 09 1 B ot ta ro 2 Ja nu ar y 20 03 50 49 2 94 0 14 3 < 0. 01 6. 09 7. 19 7 0. 22 49 3 53 8 4. 98 7 0. 86 74 .7 1. 10 7 0. 06 2 3. 95 1. 78 0. 15 4 B ot ta ro 2 Ja nu ar y 20 03 50 46 2 50 9 20 1 < 0. 01 7. 13 6. 88 7 0. 22 52 6 87 8 2. 57 4 0. 98 40 .6 89 8 0. 03 5 1. 24 2. 23 0. 26 7 B ot ta ro 2 M ar ch 2 00 3 49 42 3 57 8 11 2 < 0. 01 < 0. 01 8. 67 0 0. 17 56 6 32 7 1. 28 6 0. 37 18 .2 1. 46 0. 01 4 4. 63 1. 88 0. 03 3 B ot ta ro 2 M ar ch 2 00 3 49 39 8 03 4 14 9 < 0. 01 < 0. 01 7. 81 0 0. 16 59 2 66 3 1. 16 9 0. 31 17 .1 15 5 0. 01 3 1. 19 1. 66 0. 06 8 B ot ta ro 3 N ov em be r 20 02 n. d. 37 4 64 4 3. 95 2 19 .2 22 2 1. 73 0 1. 55 61 0 31 0 6. 09 3 2. 75 81 .9 1. 60 3 0. 07 3 1. 33 2 4. 54 5. 55 1 B ot ta ro 3 D ec em be r 20 02 n. d. 24 2 40 9 31 7 0. 63 20 .6 3. 52 2 1. 13 75 2 49 5 90 7 3. 76 12 .9 16 2 0. 01 0 14 1 0. 75 1. 14 7 B ot ta ro 3 D ec em be r 20 02 n. d. 42 9 96 3 32 2 0. 65 23 .0 3. 55 7 1. 18 56 3 73 3 1. 70 3 5. 68 23 .5 33 8 0. 01 9 32 1 1. 39 2. 37 9 B ot ta ro 3 Ja nu ar y 20 03 18 52 0 93 9 98 .4 < 0. 01 3. 50 7. 55 4 0. 21 46 7 42 1 3. 18 0 2. 47 47 .2 74 9 0. 04 0 1. 81 2. 37 0. 37 8 B ot ta ro 3 Ja nu ar y 20 03 18 52 1 06 0 91 .0 < 0. 01 3. 62 7. 68 9 0. 22 46 8 97 1 1. 70 6 1. 11 26 .5 44 7 0. 02 4 2. 78 1. 99 0. 18 5 B ot ta ro 3 M ar ch 2 00 3 n. d. 27 7 90 4 56 .6 < 0. 01 < 0. 01 2. 44 4 0. 12 71 6 95 7 2. 35 5 0. 87 35 .0 24 3 0. 02 9 2. 45 1. 33 0. 04 0 B ot ta ro 3 M ar ch 2 00 3 n. d. 30 2 60 4 12 4 < 0. 01 < 0. 01 2. 54 6 0. 30 69 3 13 0 1. 42 2 1. 08 21 .1 14 7 0. 01 7 2. 03 1. 45 0. 13 5 758 Bruno Capaccioni, Franco Tassi, Orlando Vaselli, Dario Tedesco and Piermaria Luigi Rossi (Bonasia et al., 1973; Calanchi et al., 1999b), whose interpretation is still a matter of debate. This area is also affected by a strong exhalative and hydrothermal activity known since the Ro- man Age. The oldest detailed descriptions of such activities date back to the 18th century (De Dolomieu, 1783). On the other hand, the first modern descriptions of this hydrothermal sys- tem were given by Gabbianelli et al. (1990), Italiano and Nuccio (1991) and Calanchi et al. (1995). 3. Sampling and analytical methods Fluids were collected four times between November 2002 and March 2003 (table I) from four different sites (fig. 1): a) Bottaro 1, which includes both Bottaro 1 and Bottaro Frattura samples, located a few tens of meters from the western margin of the Bottaro islet and consist- ing of a central huge 15 m deep emission dis- charging from a crater-shaped depression on the seafloor and minor diffuse bubbling all around; b) Bottaro 2 is a group of gas discharges close to the NW corner of Bottaro islet at a depth of 7- 12 m; c) Bottaro 3 consists of diffuse gas bub- bling with relatively low flux and discharging SE of Bottaro islet in an area of 100-200 m2; d) Calcara is a pre-existing, weak fumarolic dis- charge located at about 10 m inland close to the eastern shore of Panarea Island. The gas emissions from Bottaro 1 vent have the largest flux of the entire fumarolic field, al- though a correct estimation of the total flux rate (not carried out in the present study), signifi- cantly decreased since December 2002, is diffi- cult to computed. Samples were collected in pre-weighted and pre-evacuated 50-ml thorion-tapped glass tubes, partially filled with 20 ml of a solution 0.15M Cd(OH)2 and 4M NaOH, connected to a plastic funnel positioned up-side-down over the uprising bubbles. To avoid contamination by seawater, the silicon tube between the funnel and the collecting glass tube was filled with Milli-Q water and isolated from seawater by a plastic plug (fig. 2). The plug was removed on- ly after the complete evacuation of the seawater contained in the funnel by gases. Acidic gases (CO2, SO2, H2S, HCl, HF) and water were trapped in the alkaline solution. Dur- ing sampling, elemental sulphur precipitates, SO2 dissolves in the alkaline solution, whereas H2S reacts with Cd2+ to form insoluble CdS. Inert gases (N2, O2, CO, H2, He, Ar, Ne, CH4), collected in the head-space, were ana- lyzed with a gas-chromatograph (Shimadzu 15a), equipped with a Thermal Conductivity Detector. During the analysis, separation of H2, He and Ne peaks was obtained by using a 9 m long molecular sieve column, at a temperature of 30°C. To allow a complete separation of Ar- O2 peaks, the temperature was lowered to 0°C by means of cryogenic equipment (Shimadzu CRG-15) fed by liquid CO2. After analysis of the inert gases, the solution was separated by centrifugation from the solid precipitates and oxidized with H2O2 to convert SO2 to SO42− that was then analyzed using a Dionex DX100 ion-chromatograph equipped with an Ionpac AS9-HC column. CdS in the solid phase was dissolved and oxidized with H2O2 and then analyzed by ion-chromatogra- Fig. 2. HCl/10-HF∗10-SO2 ternary diagram. Black circles: Bottaro 1 and Frattura gas discharges; open squares: Calcara gas discharge; open circles: Bottaro 2 gas discharges; open triangles: Bottaro 3 gas dis- charges. 759 The November 2002 degassing event at Panarea Island (Italy): five months of geochemical monitoring phy as SO42−. S0 was extracted from residual precipitate with CCl4 and oxidized to S2I2 with the addition of KI. Finally, sulphur of S2I2 was oxidized to SO42− by KBrO3 and analyzed by ion chromatography (Montegrossi et al., 2001). CO2 in the caustic solution was analyzed by acidimetric titration with 0.5N HCl solution. F− and Cl−, in the alkaline solution, were also ana- lyzed by ion-chromatography with the Ionpac AS9-HC column. To permit the complete sepa- ration of F− and OH− peaks a 1 µmol Na2CO3 solution was used as eluent phase instead of the 10 µmol Na2CO3 solution, the latter being com- monly utilized for ion-chromatographic analy- sis of anions. Analytical precision was <1% for major gas components and <5% for minor and trace com- pounds. 4. Volatile acid compounds or contamination by seawater? Some critical considerations Before providing any interpretation of the collected data it is useful to discuss what they really represent. As will be discussed below, the most impressive features of the gases collected on November and December 2002 are the pres- ence of chloride, fluoride, and sulphite, and the relatively high water contents, exceeding what would be expected in fluids at 50°C and 2.5 bar (i.e. the values of temperature and pressure dur- ing sampling). A possible persistence of acid species (SO2, HF and HCl; table I) in gases dis- charging from the sea bottom clearly conflicts with the high solubility in seawater of these compounds. This would imply a severe internal disequilibrium of the emerging gases that can only be explained by the huge gas flux, partic- ularly at Bottaro 1 vent. Despite the specific sampling procedures adopted to avoid seawater contamination, the problem of introducing sea- water droplets or micro-droplets has been con- sidered and discussed accordingly. The meas- ured SO2/HCl ratios of the collected gas sam- ples range from 0.08 to 0.19 and from 0.06 to 0.37 on November 2002 and December 2002, respectively. Since the SO2/HCl ratio of normal seawater is 0.14, the reasonable suspicion of the occurrence of seawater contamination instead of the introduction of volatile acid compounds into the sampling device cannot be ruled out. Nevertheless, volcanic gases with SO2/HCl ra- tios < 1 are not so uncommon. Some crater emissions at El Chichon volcano (Tassi et al., 2003) have SO2/HCl ratios in the range of 0.07- 0.64, whereas up to 0.007 SO2/HCl ratios have been measured at Usu volcano (Symonds et al., 1996). Moreover, the observed significant pos- itive correlation between the two gas com- pounds is not a critical factor since it can be due to either seawater contamination or input of acid species into the sampling device. Howev- er, according to the relatively high Br− content in seawater (65 mg/L), in the case of seawater contamination the collected NaOH solutions of Bottaro 1 samples collected on December 2002 should have at least 1 mg/L of Br−, well above its detection limit (0.01 mg/L; table I). In addi- tion, together with the absolute absence of Br−, it is noteworthy to point out the occurrence of high F− contents in spite of its relatively low content in seawater (1 mg/L). A further critical factor is provided by sulphur speciation analyt- ically observed in the NaOH solution of gas samples. Besides the H2S fraction, precipitated as CdS, and the elemental sulphur, collected as residual precipitate (see above), SO32−, that can only be explained by a direct collection of gaseous SO2, is the prevailing sulphur com- pound before the addition of H2O2 to the NaOH solution. This seems to definitively exclude contamination by seawater. The dramatic drop of seawater pH (see below), the corrosion effect on shells of foraminifers in the surrounding ar- eas (Panieri et al., 2003), other than skin irrita- tion reported by the scuba-divers represent con- vincing clues of the presence of highly acidic species in thermal fluid discharges. The excess of water clearly conflicts with the expected «instantaneous» condensation of water vapour at 50°C and 2.5 bar. It seems rea- sonable to assume that water vapour condensed close to the sampling point and the condensa- tion could have been transported by the gas flux as micro-droplets into the sampling device, to- gether with acid species partially or totally dis- solved in it (as suggested by Chiodini, pers. comm.). Thus, the detected chloride, fluoride and sulphite contents could derive, at least par- 760 Bruno Capaccioni, Franco Tassi, Orlando Vaselli, Dario Tedesco and Piermaria Luigi Rossi tially, from the condensed phase collected dur- ing sampling instead of directly from the gas phase. Hence, we conclude that the acid gas species, characterizing the chemical composition of sub- marine gases discharged close to Panarea Island on November and December 2002, were mostly dissolved in the condensed phase before the in- troduction into the sampling device, without any significant contamination by normal or chemical- ly modified seawater. 5. The November 2002 gas composition Following the above mentioned considera- tions, the presence of variable amounts of wa- ter-soluble acid species (HF, HCl and SO2) in sampled gas discharges requires prevailing dry conditions of the fluids up to shallow depths, other than a severe disequilibrium between the submarine gas discharges and seawater. As al- ready stated, this can only be explained by the observed huge gas flux, able to minimize chem- ical and physical interactions between the emerging fluids and the surrounding environ- ment. Analytical data for major, minor and trace gas compounds are listed in table I. The highest outlet temperature, measured as close as possi- ble to the gas vents and probably representing a weighted mean between seawater and fluid temperature, was 50°C, with pH values ranging between 5.0 and 5.5 (three units lower with re- spect to normal seawater; table I). Simultane- ous measurements at La Calcara gas discharge (fig. 1) revealed a temperature range of 95- 99°C, in agreement with what was observed by Calanchi et al. (1995). On November 2002, besides of the occur- rence of water soluble acids, the gas compo- sition of the submarine gas discharges was char- acterized by the prevalence of H2O (up to 710000 µmol/mol), CO2 (up to 650000 µmol/mol), N2 and H2S (up to 6100 and 4500 µmol/mol, respec- tively). H2 and CO contents have a large compo- sitional variability in terms of space, ranging from 0.0076 to 0.13 µmol/mol and from 0.2 to 5.6 µmol/mol, respectively. Concerning the solu- ble acids, the already mentioned prevalence of HCl over SO2 with respect to the typical compo- sitions of the acid-bearing subaereal gases from Vulcano crater (fig. 2) could be related to the «stripping» of HCl from an acidified chlorine- rich seawater (Symonds et al., 2001). Simple thermodynamic calculations based on the follow- ing equilibrium reactions: and allow us to obtain a PHCl in the range of those measured (4-12 mbar) by stripping HCl from seawater at 100°C and pH = 2, conditions becom- ing less severe if we consider halite saturated so- lutions and/or temperature in excess of 100°C. The relative contents of O2-Ar-Ne (fig. 3) clearly point to an atmospheric origin for these compounds through degassing of air saturated and heated seawater. Moreover, the trend to- HCl HClg aq= HCl Cl Haq = + - + Fig. 3. Ar-O2/10-Ne*1000 ternary diagram. Air and Air Saturated Water (ASW) compositions are report- ed. Black circles: Bottaro 1 and Frattura gas dis- charges; open squares: Calcara gas discharge; open circles: Bottaro 2 gas discharges; open triangles: Bottaro 3 gas discharges. 761 The November 2002 degassing event at Panarea Island (Italy): five months of geochemical monitoring wards the Ar-Ne side suggests a variable O2 consumption that seems to increase from No- vember 2002 to March 2003. 6. Temporal variations Between November 2002 and March 2003 the monitored submarine gas exhalations dis- played a complex combination of temporal and spatial changes of their chemical compositions. The temporal variations of H2O and CO2 contents measured at Bottaro 1 and the H2O and CO2 val- ues calculated according to the method of Chio- dini and Cioni (1989) are reported in fig 4. The H2O/CO2 ratios progressively decreased from November 2002 to March 2003: after January 2003 the gas mixture became CO2-dominated. The comparison between calculated and meas- ured contents of H2O and CO2 suggests that in November and, partly, in December 2002, the rel- ative amounts of collected water was not so dif- ferent from those calculated at the re-equilibra- tion zone, while a significant removal of the con- densed phase seems have occurred from January 2003. Between November and December 2002, water-soluble acid species (HCl, HF and SO2) al- so underwent a sharp decrease (fig. 5), closely approaching the analytical detection limits. Such a behaviour recorded at Bottaro 1 area is in agree- ment with the expected trends for a progressive increase in the seawater/gas ratio along the whole fracture feeding system, as suggested by Sy- monds et al. (2001). The progressive disappear- ance of water-soluble acid species and collected water correspond to an increase in vapour phase condensation, possibly related to with the percep- tible decrease of the gas flux at Bottaro 1 vent. Binary diagrams in fig. 6a,b report the expected trends (calculated according to the method sug- gested by Chiodini et al., 1996) for H2 and SO2 versus He at various degrees of gas dissolution in water, starting from the mean composition of the Bottaro 1 gas sample collected in November 2002. Both H2 and He (fig. 6a), due to their very low solubility on seawater, should display an in- Fig. 4. Temporal variation of CO2 and H2O con- tents of Bottaro 1 and Frattura gas discharges (mean values) between November 2002 and March 2003. Calculated CO2 and H2O concentrations (Chiodini and Cioni, 1989) are reported. Fig. 5. Temporal variation of HCl, HF and SO2 contents of Bottaro 1 and Frattura gas discharges (mean values) between November 2002 and March 2003. 762 Bruno Capaccioni, Franco Tassi, Orlando Vaselli, Dario Tedesco and Piermaria Luigi Rossi crease as dissolution proceeds. Although the mean compositions of Bottaro 1 fluid discharge (closed circles; fig. 6a) sampled in November and December 2002 actually followed the expected dissolution trend, samples collected after January 2003 dramatically shifted towards an H2-rich and He-poor end-member. Similarly, SO2 versus He contents (fig. 6b) at Bottaro 1 seem to be con- trolled by dissolution and liquid separation only in November and December 2002, while, in the following months, the trend moved towards an apparent SO2-poor end-member. Moreover, the composition of the whole gas discharges collect- ed in December 2002 (open circles; fig. 6a), able to describe the spatial evolution of the chemical characteristics of the system, followed the same temporal pattern shown by Bottaro 1 in the peri- od January-March 2003. Thus, the distribution of samples in fig. 6a,b seems to define a two-com- ponent mixing process consisting of: i) a H2-rich/ /He-poor (hydrothermal) and ii) a H2-poor/ /He-rich (residual magmatic?) end-member. This seems to represent the main controlling mecha- nism also in terms of spatial variability. Figure 7 provides further insights into the temporal evolution of low water-soluble, redox- and temperature-dependent chemicals at Bottaro 1 in the period November 2002-March 2003. In March 2003, H2 content was almost an order of magnitude higher with respect to the values measured in November and December 2002. Al- so CO showed a concomitant increase in its con- tents (1-2 to 4-5 µmol/mol; table I). These com- positional changes cannot only be explained in terms of a simple balance between the two differ- ent end-members. Temporal changes in the Fig. 6a,b. H2 versus He (a) and SO2 versus He (b) diagrams. Expected trends for simple solubilization process are reported. Black circles: mean values of gas samples collected in November 2002, December 2002, January 2003 and March 2003 at Bottaro 1 and Frattura gas discharges; open circles: gases sampled in December 2002 at Bottaro 1, Frattura, Bottaro 2 and Bottaro 3 gas discharges. a b Fig. 7. Temporal variation of H2 and CO contents of Bottaro 1 and Frattura gas discharges (mean val- ues) within the period November 2002-March 2003. 763 The November 2002 degassing event at Panarea Island (Italy): five months of geochemical monitoring chemical-physical properties of the hydrothermal reservoir feeding the submarine discharge sys- tem, such as an increase in temperature and/or re- ducing conditions, must be invoked. Figure 8 plots CO and H2 concentrations versus H2S/Ar ratios. H2S is a temperature- and redox-depend- ent chemical species that forms a redox pair with SO2 and equilibrates with the H2/H2O pair typi- cally at magmatic conditions (Giggenbach, 1987, 1996; Giggenbach and Glover, 1992). Due to both the chemical inertia and the very low water solubility of Ar, the abrupt increase in H2S/Ar ra- tios measured in March 2003 cannot be ex- plained by simple passive enrichment during dis- solution processes. Since H2S is favoured by de- creasing temperatures and increasing reducing conditions, its positive correlation with CO and H2 (favoured by the increase of both temperature and reducing conditions of the system) suggests that redox conditions between January and March 2003 had a fundamental role to control the chemical variations recorded at the surface. 7. Concluding remarks The November 2002 degassing event occur- ring close to Panarea Island was supplied by an uprising SO2-bearing fluid of possible magmatic origin, variously mixed with a «transient», rapid- ly evolving hydrothermal fluid. In November and partly in December 2002, both fluids were chemically recognizable. The whole emitted flu- id was possibly a sort of emulsion, made up of a mixture of gas and micro-droplets of condensa- tion containing ions from acid species dissolu- tion, dragged along the ascending conduits by the extremely high gas flux. According to the tempo- ral variation of the fluid compositions recorded between November 2002 and March 2003 two main steps on the evolution of the feeding system can be recognized. The first step, occurring be- tween November and December 2003, was dom- inated by a selective dissolution of the chemical species, mostly affecting the «magmatic» com- ponents, leading to a passive enrichment of the less soluble compounds. The second step, that took place after January 2003, appears to be al- most completely dominated by the hydrothermal Fig. 8. H2 versus H2S/Ar and CO versus H2S/Ar di- agrams of Bottaro 1 and Frattura gas discharges (mean values). Fig. 9. Log(CO/CO2) versus log(H2/H2O) diagram of Bottaro 1 and Frattura gas discharges. Rock (HM, FM and GT) and gas (SO2/H2S) redox buffers are re- ported. 764 Bruno Capaccioni, Franco Tassi, Orlando Vaselli, Dario Tedesco and Piermaria Luigi Rossi component which undergoes a rapid chemical evolution of its feeding system. In the Bottaro 1 area this chemical evolution can be well summa- rized by the H2/H2O versus CO/CO2 binary dia- gram (fig. 9). The samples appear to be arranged along a trend which from November 2002 to March 2003 moved towards a lower temperature (from 300 to 200°C) and towards increasing re- ducing conditions. As previously discussed, in the period between November and December 2002 the partial condensation of water vapour did not significantly modify the gas/water ratios observed, while it played a crucial role after Jan- uary 2003. 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