(CO2), estimated to be around 25 Megatons/year [Allard et al., 1991] and one order of magnitude higher than the annual CO2 output from Kilauea. The ratio ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 2, 1057, doi:10.1029/2002GL015463, 2003
Mount Etna: Geochemical signals of magma ascent and unusually extensive plumbing system A. Caracausi, R. Favara, S. Giammanco, F. Italiano, A. Paonita, G. Pecoraino, and A. Rizzo Istituto Nazionale di Geofisica e Vulcanologia, I.N.G.V—Section of Palermo, Via Ugo La Malfa, Palermo, Italy
P. M. Nuccio Dipartimento di Chimica e Fisica della Terra ed Applicazioni (CFTA), University of Palermo, Palermo, Italy Received 10 May 2002; revised 2 September 2002; accepted 12 November 2002; published 23 January 2003.
[1] Five years of gas monitoring from selected sites suggest that Mt Etna’s plumbing system is much more extensive than previously reported. It extends at least 40 km SW from the volcano’s boundary along the NE-SW regional fault, where it discharges about 200 tons/day of gas, containing helium with mantle-type isotopic composition. Synchronous variations of 3He/4He isotopic ratios in gas sampled at sites located 60 kilometers apart have allowed us to detect pulses of ascending magma in the plumbing system, thus providing a powerful tool for eruption forecasting. Following summer 2001 eruption, the still increasing trend of the 3He/4He ratios indicates that magma storage is even now occurring at a shallow depth. Hence, the volcano maintains a high capacity to re-erupt within the next few I NDEX T ERMS : 1040 Geochemistry: Isotopic months. composition/chemistry; 8419 Volcanology: Eruption monitoring (7280); 8434 Volcanology: Magma migration; 8439 Volcanology: Physics and chemistry of magma bodies. Citation: Caracausi, A., R. Favara, S. Giammanco, F. Italiano, A. Paonita, G. Pecoraino, A. Rizzo, and P. M. Nuccio, Mount Etna: Geochemical signals of magma ascent and unusually extensive plumbing system, Geophys. Res. Lett., 30(2), 1057, doi:10.1029/2002GL015463, 2003.
1. Introduction [2] Even during quiescent periods, the Mount Etna volcano (Italy) releases a huge amount of carbon dioxide (CO2), estimated to be around 25 Megatons/year [Allard et al., 1991] and one order of magnitude higher than the annual CO2 output from Kilauea. The ratio between erupted magma and released gases highlights an excessive degassing probably due to convective magma movements in a complex plumbing system [Allard, 1997]. The structure and development of such a system seems to be closely linked to regional tectonics, having three main directions: NNW-SSE, E-W and NE-SW [La Delfa et al., 1999; Bonaccorso and Patane`, 2001]. [3] The plumbing system size of volcanoes critically affects the magma volumes involved in volatile degassing and eruptive activity [Kazahaya et al., 1994]. In extensive systems, convective magma overturns actually occur quite easily, and cause magma ascent and degassing accompanied
Copyright 2003 by the American Geophysical Union. 0094-8276/03/2002GL015463
by minor seismic signals with respect to magma intruding into the country rocks. [4] Aiming to acquire information on the size of Etnean plumbing system and magma movements inside, we have monitored the chemical and isotopic composition of some gaseous manifestations located in the Eastern and Southern parts of the volcano. We have also monitored a gas emission located 40 km southwest of Mount Etna along the NE-SW trending fault, on the northernmost area of Hyblean volcanics (Figure 1). Gases were normally sampled twice a month over a period of more than five years.
2. Gas Geochemistry [5] The collected gases are generally CO2-dominated, and display variable amounts of methane that becomes the main component only at the FS and, sometimes, VS sites. The nitrogen concentration is normally below 1 vol%, although it reaches 9 vol% at the FS site. Helium is present with concentrations up to 120 ppmv in CO2-dominated gases and almost one order of magnitude higher in CH4dominated gases. [6] The 3He/4He ratios are in the range of 6.5 ± 1, matching those of helium trapped in the olivine phenocrystals of Etnean lavas (6.7 ± 0.4 Ra; Marty et al. [1994]) and suggesting a magmatic genesis of helium. Such a signature, lower than a typical MORB-derived gas, is well explained by metasomatic contamination suffered by the Etna subcontinental mantle [Nakai et al., 1997]. The carbon isotope composition of methane, displaying d13C values between 36 and 53% (Stadio to Naftia, respectively), suggest an organic genesis of such a compound [Hoefs, 1987]. It implies contributions by crustal gases and, in view of that, CH4-dominated emissions at constant He concentrations and helium isotopic compositions in the range of the crustal value (0.04 Ra, O’Nions and Oxburgh [1988]) have been recognized in the Etnean area, both at the Gioitto site and the Bronte wells (see Figure 1). Helium isotopic and chemical balances may be attempted between a typical magmatic term and the Gioitto crustal endmember. As a result, the contamination of the monitored emissions by crustal fluids is extremely low and cannot account for the high CH4 content at the sites (Figure 2). The wide range of methane content (2.5 – 98 vol.%), together with the similar He/CH4 ratio among the monitored emissions, suggests an enrichment of both He and CH4 by selective dissolution of more soluble species (e.g., CO2) into the aquifers [D’Ales-
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[8] Etnean magmas start to exsolve a vapour phase at a depth of 15– 20 km [Kamenetsky and Clocchiatti, 1996]. During the exsolution, helium isotopes undergo kinetic mass fractionation, as the light mass (3He) diffuses at much higher velocity into the gas phase, where it is enriched [Ozima and Podosek, 1983, Nuccio and Valenza, 1998]. As a consequence, 3He is depleted in the residual melt. Such a condition can be modeled by an equation analogous to the one reported by Hoefs [1987] for Rayleigh distillation: R=R0 ¼ Fðk1Þ
ð1Þ
where R is the 3He/4He ratio in melt, F is the residual fraction of helium after a degassing event, subscript ‘‘0’’ refers to initial conditions (namely 3He/4He in melt before the degassing event) and k is the kinetic fractionation factor (k = [4/3]1/2). Equation (1) predicts that degassing causes a decrease in the 3He/4He ratio of the melt, which
Figure 1. Sketch map of the sampled sites (from Valensise and Pantosti [2001], modified]. Collected samples consisted of bubbling gases taken near the village of Paterno`, from Vallone Salato (VS site) and Salinelle (Stadio site) mud volcanoes, and from a water channel close to Fondachello village (FS site). Venting gases were collected from soil degassing at Contrada Pescheria (P39 site) and inside a factory for CO2 exploitation (drilled wells at Naftia site). All the samples were collected into pyrex containers with two vacuum valves. The main structural elements of Eastern Sicily are shown, highlighting that Naftia, VS, P39, Stadio and FS sampling sites fall close to the NE-SW faults system.
sandro et al., 1997; Giammanco et al., 1998], which does not affect helium isotopic ratios.
3. Pre-Eruptive Events of Magma Ascent [7] During the five years of gas monitoring, we observed a large variability over time in 3He/4He ratios, with slight differences between the average values at the various sites (Figure 3). Although some minor variations appear random, the widest modifications in the isotopic values took place almost simultaneously at all sites. Being much higher than the uncertainties of our measurements (one order of magnitude or even more, Figures 2 and 3), they cannot be due to any analytical errors. The correlations between the sites calculated throughout the whole period were always highly significant, with correlation coefficients ranging from 0.75 (P39 vs. FS) to 0.96 (Stadio vs. FS). Except for VS gases, the main variations in 3He/4He over time occur at an almost constant CH4 content (Figure 2), thus they cannot be explained by variable crustal contributions. The typical magmatic 3He/4He values and the occurrence of the major variations during periods of volcanic unrest allow us to infer that the process controlling the helium isotopic ratios of all the gas emissions takes place in the Etnean magmatic system.
Figure 2. Helium isotopic ratio (as R/Ra, namely 3He/4He sample versus 3He/4He atmospheric) plotted versus the CH4 content in the sampled gas emissions. Curved lines describe binary mixing processes between the Gioitto crustal endmember (0.05 Ra, 28 ppmv helium and 90 mol% CH4) and a typical magmatic term (6.7 Ra, CH4 < 0.5 vol%) having: line a) average helium concentration values of the site having the highest CO2 content (Naftia, 80 ppmv); line b) average helium concentration values of the site marked by the lowest helium content (Stadio, 32 ppmv). The line named ‘‘dissolution’’ describes the evolving path of a magmatic gas that partially dissolves in aquifers, whereas variations of 3He/4He ratio at a constant CH4 content are interpreted as due to magma degassing. Chemical analyses of He, O2, N2, CH4 and CO2 were carried out with a Perkin Elmer 8500 gas-chromatograph equipped with a 4 m Carbosieve 5A column and double detector (Flame Ionization Detector with methanizer and Hot Wire Detector). Analytical errors are below 5%. Helium isotopic analyses have been made by a split flight tube static vacuum mass spectrometer (VG 5400-VG Isotopes), modified to simultaneously detect 3He and 4He ion beams. The high performances of our mass spectrometer and the rigorous calibration procedure allow us to obtain very modest analytical error. Several repeatability tests have displayed that the overall uncertainties of our isotopic data are below 0.3%. He isotopic ratios have been corrected for small atmospheric contamination [Sano and Wakita, 1985].
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[11] Following the significant volcanic activity subsequent to the February 1999 eruption, all the sites have shown a long-term increasing trend in the 3He/4He ratio (Figure 4). The overall isotopic composition of the emitted helium is the result of the balance between He from less degassed magmas, marked by a high 3He/4He ratio, and 3He depleted gases coming from more degassed melt. This trend, together with the long-term progressive inflation of the entire volcanic edifice [La Volpe et al., 1999; Patane` et al., 2001], has indicated magma accumulation at shallow levels before the last July – August 2001 eruption. It follows that the Etnean volcanic system has undergone a progressive replenishment of new volatile-rich melts, which has been capable of making up for the 3He loss due to volatile degassing. Figure 3. Some examples of variations in the helium isotopic ratio over time. Sources of geophysical data are given in the text. generates a declining trend in the released gases. Negative peaks of the 3He/4He values can be interpreted as geochemical signals of magma migration towards the surface. Positive peaks of the 3He/4He ratio could be generated either by the release of kinetically-enriched 3He gas-bubbles during magma vesiculation [Nuccio and Valenza, 1998], or by the outgassing of new volatile-rich magma coming from depth. In this respect, Stadio and FS emissions would be fed by more degassed melts raised at shallower portions of the plumbing system compared to those melts feeding the P39 emissions. [9] Except for minor variations, co-variations of the 3 He/4He ratios have clearly been observed at all sites, suggesting the occurrence of episodes of magma ascent and outgassing involving most of the plumbing system. In agreement with our interpretation, such isotope variations occur just prior to or during important episodes of volcanic unrest, marked by fire fountains and/or lava flows. By using equation (1), the lowering of the 3He/4He ratios, observed during the 1996, 1999 and 2001 unrests (Figure 3), allow for the computing of helium losses from magma of about 45, 35 and 16%, respectively. Coherently with the recorded geochemical temporal variations, several geophysical signals [La Volpe et al., 1999; Privitera et al., 2001; Aloisi et al., 2001] linked to magma rise took place during the same periods (Figure 3). Deep and intermediate seismic swarms occurred just before the 3He/4He decrease due to magma ascent. Ground deformations occurred during or after the ascent, whereas the lower 3He/4He values preceded the eruptive episodes by a few weeks. [10] The July – August 2001 eruption, however, lacked deep earthquakes, whereas the isotopic signal of magma ascent occurred before very shallow seismic tremors and the final eruption. This suggests that the magma involved in the eruption had already intruded into shallower levels below the volcano during earlier migrations and subsequent episodes of ascent occurred by motion of fresh magma through degassed melt (see below). The significant variations of He isotope ratios occurring in the year 2000 corroborate this conclusion. In the three events we have studied, the geochemical (isotopic) signal of magma ascent lasted for about a month, giving information on the time-scale of magma movements through the crust.
4. Mt. Etna’s Plumbing System [12] The production of CO2 from the Naftia wells (170– 200 tons/day), the same quantity emitted naturally before the drilling [Ponte, 1934], is surprisingly high and an order of magnitude above that discharged from other volcanoes during periods of volcanic activity unrest (i.e. Vulcano Island, La Solfatara, Mommoth Mount; Favara et al. [2001]). It suggests the connection of this site with active magmatic intrusions in the local 30 km-thick crust [Barberi et al., 1974]. It is a point worth noting that gases collected from Naftia wells have the same helium isotopic signature as those released around the Mt Etna volcano and, above all, it undergoes the same temporal variations. Indeed, computed correlation coefficients range between 0.85 (vs. FS) and 0.91 (vs. VS), and are always highly significant, whereas the variations are much higher than the analytical uncertainties. Furthermore, a branch of high heat fluxes (up to 100 mW/m2), extending from Mt. Etna’s boundary to tens of kilometers SW of the Naftia site along the NE-SW trending fault system, has been reported [Cataldi et al., 1995; Barbier et al., 1998]. As a consequence, we can infer a direct link of the Naftia system with the Etnean magmatic system. Considering that the Naftia site is located 40 km southward of Mt. Etna’s boundary, the real Etnean plumbing system appears to be much wider than reported in literature [Sharp et al., 1980; Guest and Duncan, 1981; Hirn et al., 1997] and amongst the world’s most extensive continental volcanoes.
Figure 4. Five years of 3He/4He monitoring at P39 site. The thick line displays the increasing trend in the 3He/4He ratios since the end of 1999. Similar long-term increases occur at all the sites.
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[13] The extraordinarily large plumbing system accounts for the exceptionally huge amount of CO2 emitted by Mount Etna. In fact, large quantities of mantle melt may rise and degas, sink down into the magma column or in part escape laterally into the crust through the regional faults, permitting the replacement of new magma [Allard, 1997; Hirn et al., 1997]. Indeed, the coupling over time of Etna’s volcanic activity and the biggest earthquakes in Eastern Sicily [Hirn et al., 1997] highlights the direct link between volcanism and tectonism in the Etnean area. The magma motion inside the system can occur either as the ascent of volatile-rich magma batches through degassed melt, following the process proposed by Kazahaya et al. [1994], or magma intrusion into country rocks. In both cases, this will cause a similar isotopic signature in the released gases, although it is obvious that the consequences will be very different. Geophysical signals, such as seismicity, could in fact be lacking in the former case, making the geochemical signal the main tool for detecting magma movements.
5. Conclusions [14] The geochemical monitoring developed at Mt Etna over the last five years has enabled us to devise new tools aimed at giving advance warnings for forecasting volcanic eruptions. [15] The surprisingly synchronous variations recorded in the helium isotopic ratios over the sampling period, even at sites located 60km apart, appear to be due to an unusually extensive plumbing system. In relation to the seismicity and the ground deformations [Bonaccorso and Patane`, 2001], the continuous growth of the 3He/4He ratio, together with its discontinuous short-term variations, indicate transfers of large amounts of magma in pulses, through convective mechanisms within the plumbing system, similar to that proposed by Kazahaya et al. [1994]. [16] Accordingly, the ascent of volatile-rich magma batches through degassed melt would have provided the engine for the onset and the feeding of the July – August 2001 eruption. Following our interpretation, the increasing trends in 3He/4He ratios observed at all sites, as well as the ongoing inflation of the volcano, strongly support the conclusion that magma storage is at present occurring below Mount Etna, so the volcano keeps high its capacity to reerupt within the upcoming months.
[17] Acknowledgments. The isotope compositions of methane were taken from the INGV-Pa database. The comments of two anonymous reviewers improved the manuscript.
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A. Caracausi, R. Favara, S. Giammanco, F. Italiano, A. Paonita, G. Pecoraino, and A. Rizzo, Istituto Nazionale di Geofisica e Vulcanologia, I.N.G.V—Section of Palermo, Via Ugo La Malfa, 153, 90146 Palermo, Italy. P. M. Nuccio, Dipartimento di Chimica e Fisica della Terra ed Applicazioni (CFTA), University of Palermo, Via Archirafi 36, 90123 Palermo, Italy.
