Evidence of a recent input of magmatic gases into the quiescent volcanic edifice of Panarea, Aeolian Islands, Italy

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L07619, doi: /2003gl019359, 2004 Evidence of a recent input of magmatic gases into the quiescent volcanic edifice of Panarea, Aeolian Islands, Italy S. Caliro, 1 A. Caracausi, 2 G. Chiodini, 1 M. Ditta, 2 F. Italiano, 2 M. Longo, 2 C. Minopoli, 1 P. M. Nuccio, 3 A. Paonita, 2 and A. Rizzo 2 Received 23 December 2003; revised 18 March 2004; accepted 23 March 2004; published 14 April [1] On 2nd/3rd November 2002, a huge amount of gas, mainly composed of CO 2, was suddenly released from the sea bottom off the coast of Panarea, producing a crater 20 by 10 meters wide and 7 meters deep. The gas output was estimated to be 10 9 l/d, two orders of magnitude higher than that measured in the 1980s. The anomalous degassing rate lasted for some weeks, slowly decreasing to an almost constant rate of about l/d after two months. The geothermo-barometric estimations revealed an increase of both the temperature and pressure in the geothermal system feeding the sampled vents. The 3 He/ 4 He ratios were similar to those measured in nearby Stromboli. We have monitored the area for the last two decades, and based on our intensive and extensive geochemical measurements, have ascertained that the geothermal reservoir has lost its steady state. We maintain that a new magmatic input caused these phenomena. INDEX TERMS: 1010 Geochemistry: Chemical evolution; 1040 Geochemistry: Isotopic composition/ chemistry; 8424 Volcanology: Hydrothermal systems (8135). Citation: Caliro, S., A. Caracausi, G. Chiodini, M. Ditta, F. Italiano, M. Longo, C. Minopoli, P. M. Nuccio, A. Paonita, and A. Rizzo (2004), Evidence of a recent input of magmatic gases into the quiescent volcanic edifice of Panarea, Aeolian Islands, Italy, Geophys. Res. Lett., 31, L07619, doi: /2003gl Introduction [2] The marine sector surrounding the island of Panarea (Aeolian Islands, Italy) is affected by widespread submarine emissions of CO 2 rich gases and thermal water discharges. During the night between November 2nd and 3rd 2002, a sudden increase in the outgassing of hydrothermal fluids occurred between the islets of Bottaro and Lisca Bianca, probably in response to a submarine explosion (Figure 1). The event was accompanied by a seismic swarm of low intensity. In a matter of hours a large crater developed, that was meters wide and 7 meters deep. The degassing rate was so intense that it produced a large degassing bubble on the surface of the sea, the water of which was white due to the presence of clay and sand and changes in its ph (ph = ). The event triggered other five main areas of intense submarine degassing, located at depths between 8 and 30 meters. The smell of H 2 S was detectable on the main island half a mile away. After a few days, the degassing activity decreased, although it clearly remained higher than prior to the event, while only 3 vents still looked like boiling water on the surface of the sea. [3] The geochemical investigations carried out around the island of Panarea since the beginning of the 1980 s, have already revealed the presence of a large field of submarine gas emissions and thermal water discharge, the temperatures of which range between 30 and 100 C. The field is spread over an area of about 4 km 2 off the east coast of Panarea, along the main regional tectonic alignments: NE-SW, NW-SE and N-S (Figure 1). The emissions have been recognized as originating from a geothermal reservoir that is fed by magmatic fluids derived from a cooling magmatic body, which has probably intruded in recent times [Italiano and Nuccio, 1991]. The temperatures and pressures of the main reservoir level were estimated to be up to 240 C and 30 bar respectively [Italiano and Nuccio, 1991; Calanchi et al., 1995]. The total gas output was evaluated to be about l/day over the whole area [Italiano and Nuccio, 1991]. [4] Before the outgassing crisis, it was not possible to detect the presence of the hydrothermal activity from the surface, however, the appearance of the boiling water observed during the November 2002 event cannot be considered a unique phenomenon, as some historical reports refer to a bollitore, which was probably located in the same area around Panarea (Figure 1). [5] The surveys carried out as a consequence of the November 2002 crisis revealed a serious modification of both the gas composition and the degassing rate. These are mainly to be referred to the large vents that opened up near the above mentioned islets (Figure 1). [6] The results proposed here highlight how an apparently steady-state hydrothermal system can undergo unexpected changes in its activity. The data recorded lead to the hypothesis that an injection of magmatic gases into the dormant volcanic system of Panarea was the cause of the phenomena observed. 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Naples, Italy. 2 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy. 3 Dipartimento Chimica e Fisica della Terra ed Applicazioni, Palermo, Italy. Copyright 2004 by the American Geophysical Union /04/2003GL Gas Geochemistry [7] The fluids discharged from the main emissions (#1, #2 and #8; Figure 1), were collected from the sea floor at a depth of 8 15 m, and were analyzed in accordance with standard methods [Giggenbach, 1975; Cioni et al., 1984]. Several campaigns have been effected since November 5th 2002 with the aim of investigating the possible variations in the geochemical parameters over time. The gases are L of5

2 Figure 1. Rough map of the investigated area east of the island of Panarea (modified from Italiano and Nuccio [1991]). Tectonic lineaments are reported as well as the rim of the caldera structure (elliptical dashed line). The islets of Dattilo, Bottaro, Lisca Bianca and Lisca Nera have been recognized as being the remnants of a crater rim. mainly CO 2 ( vol. %) with minor amounts of H 2 S ( vol. %), H 2 ( vol. %), N 2 ( vol. %), He ( vol. %), Ar ( vol. %), CH 4 ( vol. %) and CO ( vol. %). [8] This composition is the result of the interaction between original deep gases and seawater, at a relative low temperature (from 30 to 100 C), at equilibrium with the atmosphere. The gas-seawater interaction causes both the condensation of steam and the loss of the highly soluble acid species (i.e., SO 2, HCl, HF) present in the original deep gases. Therefore, the analytical data reflect the composition of the non-condensable fraction of the original fluids. Furthermore, the relative composition of this non-condensable gas fraction is affected by the gas-sea water interaction for the following reasons: (a) fractionation of gases by partial dissolution in water (i.e., H 2 S and CO 2 are much more soluble than the remaining species) and (b) enrichment in atmospheric components dissolved in seawater, such as N 2,O 2, Ar. At Panarea, the first process (a) has minor effects on the measured composition, because the fraction of CO 2 dissolved by sea water is in any case less than 30%. The mixing between deep gaseous and dissolved atmospheric end-members (process b) is shown in the ternary He-Ar-N 2 diagram (Figure 2), in which the Panarea samples define an alignment joining the typical composition of deep gases associated with the volcanichydrothermal systems of Italy (i.e., Solfatara of Pozzuoli, Vesuvio and Stromboli) and the ASW composition (Air- Saturated Water). [9] The 3 He/ 4 He ratios, expressed as R/Ra (Ra is the helium isotopic ratio in the atmosphere, equal to ), span a narrow range from 4 to 4.3, showing a strong similarity with the dissolved gases extracted from the thermal waters at Stromboli before and during the eruption [Inguaggiato and Rizzo, 2004]. Even though these data suggest a primary magmatic origin of the gases, the unreactive gas species used in Figure 2 and the He isotopic composition do not discriminate the gases released directly by magma degassing or after storage in a hydrothermal system. Acid gases which are normally absent from hydrothermal fluids cannot be used as tracers of hydrothermal or volcanic origin, because they could be lost during the interaction with sea-water. [10] The absence of any significant correlation between the reactive gas species CO, H 2,CH 4 and He, suggests that dissolution in seawater is not the main process governing the relative amounts of CO, H 2 and CH 4. This implies that the effects of gas dissolution in water are limited (see above) and may be neglected as far as geothermometric applications are concerned. Figure 2. He-Ar-N 2 diagram. Compositions of the non reactive gas species found in the samples collected at Panarea are plotted along the mixing line between Air Saturated Water and a deep-component having similar compositions to those of other Italian volcanoes. Figure 3. Diagram of log H 2 /CH 4 versus log CO/CH 4 : a) Compositional data from 23 hydrothermal systems (given by Chiodini and Marini [1998]) and data from high-t volcanic fumaroles (data from Giggenbach [1987], Chiodini et al. [1993], and Giggenbach [1996]); b) Panarea samples. The theoretical grid for hydrothermal gases was drawn assuming the presence of pure liquid water. 2of5

3 [11] To gain an insight into the volcanic or hydrothermal origin of the Panarea gases, in Figure 3 (log H 2 /CH 4 versus log CO/CH 4 ) we investigated the H 2 -CO-CH 4 gas system. In this figure the Panarea gases are compared to typical compositions of hydrothermal and volcanic gases derived from a large published data set. We have also shown the expected composition of equilibrated gases at different temperature-pco 2 values and at redox conditions typical of the hydrothermal environment. These are represented by the empirical fo 2 -temperature relation of D Amore and Panichi [1980]. Volcanic gases are characterized by H 2 /CH 4 and CO/CH 4 ratios that are orders of magnitude higher than those pertinent to hydrothermal systems. It should be emphasized that all the experimental data on hydrothermal gases plot within the theoretical grid while all the high-t volcanic fumaroles fall outside the hydrothermal compositional field, thereby reflecting more oxidizing conditions. [12] The Panarea emissions collected in the eighties [Italiano and Nuccio, 1991] show compositions close to those from the hydrothermal environment, with lower H 2 /CH 4 ratios. This is likely to be due to H 2 removal caused by surface oxidative processes, which are favored by the interaction of gases with water at low temperatures [Chiodini, 1994]. Instead, the data collected after the November 3rd 2002 event lie outside the hydrothermal field and show relative contents of H 2,CH 4 and CO that are more typical of high-t volcanic fumaroles. In any case, the considerable compositional change observed for the same emissions after the November 3rd event suggests an increased input of magmatic gases into the hydrothermal system. Figure 4. CO-CO 2 couple versus 1/T diagram. Equilibrium lines refer to different redox buffers (i.e., FeO- FeO1.5) as well as to those reported by D Amore and Panichi [1980], that are typical of a hydrothermal environment. The H 2 S-SO 2 buffer is applied to volcanic gases [Giggenbach, 1987]. A temperature of C was estimated for the Panarea samples. Figure 5. Temporal variations of CO concentrations in the gas emitted at Bottaro (a) as well as the total gas output measured in the entire Panarea degassing area (b) are plotted. The vertical dashed line represents the onset of the crisis. Data are reported as month/year. The volume of emitted gas was corrected for hydrostatic pressure. [13] Thus, the new compositions are compatible with two conceptual models: (a) the gases represent the residual fraction, after condensation, of a volcanic fluid like that emitted from high-t fumaroles, or (b) the emissions are fed by an atypical hydrothermal system characterized by greater oxidizing redox conditions, in response to the increase of the input of magmatic fluids. [14] In the first hypothesis, methane is most probably not in equilibrium with the other species, whereas H 2 /H 2 O and CO/CO 2 ratios quickly re-equilibrate with temperature and pressure at redox conditions which are fixed by the simultaneous presence of significant amounts of H 2 S and SO 2 [Giggenbach, 1987, 1996; Chiodini et al., 1993]. The only geothermometric indication is given by the CO-CO 2 couple which would indicate temperatures between C at reasonable values of P H2O (Figure 4). It has to be pointed out that this model is not in contrast with Italiano and Nuccio [1991], who recognized the presence of a hydrothermal system below the investigated area. In fact, the dramatic input of magmatic fluids could easily have induced the total vaporization of the system, at least in the somewhat peripheral area near Bottaro islet. [15] In accordance with hypothesis b), assuming both equilibrium within the system H 2 O-H 2 -CO 2 -CO-CH 4 and that the gas is representative of an equilibrated vapor phase in equilibrium with a saline brine, for the main Bottaro emission we computed equilibrium temperatures from 285 to 340 C and steam pressures from 70 to 145 bar. Both temperature and pressure estimations are largely based on the CO content of the gases, which corresponds to the lowest T-P estimated in the samples with lowest CO contents. The time variation in the CO content of the Bottaro emission (Figure 5a), which shows an increase from November 2002 to May 2003, followed by a further 3of5

4 ongoing decrease, is probably linked to the temperaturepressure perturbation caused by the input of magmatic gases into the hydrothermal system. 3. CO 2 Flux Measurements [16] Since 1985, flux measurements have been carried out yearly following the direct method reported by Italiano and Nuccio [1991]. An almost constant degassing rate, of the order of l/day of CO 2 (Figure 5b), was found, which was reduced by about half in Measurement campaigns effected in 1993, 1996 and 1999 confirmed the constant rate of gas output ( l/day). [17] On 13 November, after the beginning of the crisis, detailed measurements were performed in the vents of the Lisca Bianca area, and a value around l/day was estimated. However, it was not possible to measure the gas output inside the submarine crater near Bottaro because the strong water flux, drew the divers towards the exhalative area and then thrust them upwards toward the surface, putting their safety at serious risk. New measurements were carried out in the Lisca Bianca area on 20th January, and gave values almost one order of magnitude lower than that of November 2002 ( l/day). The main Bottaro vent was not measured again as the divers encountered similar problems as previously described. However, it is worthwhile noting that the average diameter of the degassing area of the Bottaro vent was estimated to be about twelve meters on 13 November, but only about 5 meters in January. The degassing area decreased by almost a factor of 10 thereby implying that, at constant flux velocity, the gas output decreased by almost one order of magnitude. [18] Detailed flux measurements were performed over the entire exhalative area in May 2003 (Figure 5b). The gas output from Lisca Bianca was just slightly lower ( l/day) than that of January. The measurements inside the crater near Bottaro yielded degassing rates around l/day at the main emission point, whereas the other gas emissions only contributed l/day. As a result, the total gas output from the entire degassing area was in the range of l/day. The last campaign, performed in July 2003, gave values of about half the previous total gas output ( l/day), while the different emission points maintained similar proportions of degassing rates as observed in the previous campaign. With the aim of estimating the order of magnitude of the main emission near Bottaro in November 2002 and January 2003, when direct measurements were impossible, a constant ratio between the Lisca Bianca and Bottaro outgassing rates, as measured in May and July, was assumed. The value obtained for November was almost one order of magnitude higher than that of January, in good agreement with our previous conclusions based on the size of the main emission area. These values, added to those of Lisca Bianca s, provided a fairly accurate estimation of the total gas output on 13 November and 20 January ( l/day and l/day, respectively; Figure 5b). [19] In view of the latter result, we can reasonably state that, ten days from the onset of the crisis, the total gas output was at least one order of magnitude higher than the total gas output measured before the crisis. A simple visual evaluation suggests that the gas output was even higher during the first two or three days of the crisis. Based on the drastic drop of the gas output by January and on the almost steady subsequent values (Figure 5b), we conclude that the climax of the crisis lasted less than three months. 4. Conclusions [20] The strong compositional change in the samples collected after the onset of the November 3rd 2002 event, accompanied by an increase in the output of CO 2, was compared with the composition of the samples collected in the 1980s. This suggests that there has been an increase in the input of magmatic fluids into the hydrothermal system feeding the exhaling field since the 1980s. [21] Two models could explain the recognized variation: [22] a) the gases emitted represent residual volcanic gases which lost acidic gases due to condensation and interaction with seawater. In this case the temperatures ranging between 350 and 450 C have been estimated only roughly, however they probably refer to the upper zone of the system. Furthermore, we assume that the previously existing hydrothermal system was either partially or even totally vaporized. [23] b) submarine emissions are fed by a hydrothermal system with atypical redox conditions that are more oxidizing than that expected for hydrothermal environments. We estimated temperatures up to 340 C and steam pressures of about 140 bars. [24] Although the two models have different implications as far as hazard assessment and risk management are concerned, especially as regards the possible evolution of the phenomenon, both scenarios imply a strong involvement of magmatic fluids in the triggering of the November 2002 degassing event. The results also show that, at present, temperature and fluid pressure at depth are higher than during the 1980s, even though they are still decreasing. In view of this, further geochemical monitoring of the entire degassing area would be extremely important and recommendable, also taking into consideration the fact that the steady state of the hydrothermal system could change quite quickly. [25] Acknowledgments. The authors wish to thank Mrs. P. Dyer for English suggestions which improved the clarity of the paper. References Calanchi, N., B. Capaccioni, M. Martini, F. Tassi, and L. Valentini (1995), Submarine gas-emission from Panarea Island (Aeolian Archipelago): Distribution of inorganic and organic compounds and inferences about source conditions, Acta Vulcanol., 7, Chiodini, G. (1994), Temperature, pressure and redox conditions governing the composition of the cold CO 2 gases discharged in north Latium (central Italy), Appl. Geochem., 9, Chiodini, G., and L. Marini (1998), Hydrothermal gas equilibria: The H 2 O- H 2 -CO 2 -CO-CH 4 system, Geochim. Cosmochim. Acta, 62, Chiodini, G., R. Cioni, and L. Marini (1993), Reactions governing the chemistry of crater fumaroles from Vulcano Island, Italy, and implications for volcanic surveillance, Appl. Geochem., 8, Cioni, R., E. Corazza, and L. Marini (1984), The gas/steam ratio as indicator of heat transfer at the Solfatara fumaroles, Phlegraean Fields (Italy), Bull. Volcanol., 47, D Amore, F., and C. Panichi (1980), Evaluation of deep temperature of hydrothermal systems by a new gas-geothermometer, Geochim. 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5 Giggenbach, W. F. (1975), A simple method for the collection and analysis of volcanic gas samples, Bull. Volcanol., 39, Giggenbach, W. F. (1987), Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zealand, Appl. Geochem., 2, Giggenbach, W. F. (1996), Chemical composition of volcanic gases, in Monitoring and Mitigation of Volcanic Hazards, edited by R. Scarpa and R. I. Tilling, pp , Springer-Verlag, New York. Inguaggiato, S., and A. Rizzo (2004), Dissolved helium isotope ratios in ground-waters: A new technique based on gas-water re-equilibration and its application to a volcanic area, Appl. Geochem., 19, Italiano, F., and P. M. Nuccio (1991), Geochemical investigations of submarine exhalations to the east of Panarea, Aeolian Islands, Italy, J. Volcanol. Geotherm. Res., 46, S. Caliro, G. Chiodini, and C. Minopoli, Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, via Diocleziano 398, I Napoli, Italy. A. Caracausi, M. Ditta, F. Italiano, M. Longo, A. Paonita, and A. Rizzo, Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, via Ugo La Malfa 153, I Palermo, Italy. (andrearizzo@pa.ingv.it) P. M. Nuccio, Dipartimento Chimica e Fisica della Terra ed Applicazioni, Via Archirafi 36, I Palermo, Italy. 5of5