untitled 1 ANNALS OF GEOPHYSICS, VOL. 47, N. 4, August 2004 Key words melt inclusion – volatile – basaltic ex- plosive eruptions – Etna 1. Introduction Etna, located on the eastern coast of Sicily, Italy, is an active basaltic volcano character- ized by quasi-persistent effusive and explosive activity. Explosive activity has usually been considered very subordinate with respect to the lava flow eruptions, but the last decade ac- tivity and some recent studies of Holocene py- roclastic deposits (Coltelli et al., 1998a, 2000) indicate that explosive eruptions are quite fre- quent and can be very powerful. Since 1990, Etna has exhibited an extraordinarily high number of violent explosive events, including more than 150 fire fountain episodes charac- terized by the formation of eruptive columns from 2 to 12 km high, tephra volumes from 104 to 107 m3 and a magnitude which ranges from intense strombolian to subplinian (Calvari et al., 1991; Coltelli et al., 1998b). Holocene py- roclastic succession includes many subplinian eruptions and one plinian basaltic eruption (Coltelli et al., 1998a, 2000, 2003). The tephra composition is generally basaltic, ranging from picritic basalt to basic mugearite, similar to the lava flows which erupted during the same period. It is well known that magmatic volatiles play a key role in explosive volcanism. They control vesicularity and fragmentation process- es of the magma and consequently the style of the eruptions. For this reason we started to in- Mailing address: Dr. Paola Del Carlo, Istituto Nazio- nale di Geofisica e Vulcanologia, Sezione di Catania, Piaz- za Roma 2, 95123 Catania; e-mail: delcarlo@ct.ingv.it The relationship between volatile content and the eruptive style of basaltic magma: the Etna case Paola Del Carlo and Massimo Pompilio Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Italy Abstract Fourier Transform Infrared (FT-IR) spectroscopic analyses of melt inclusions from four explosive eruptions of Etna (Italy) were conducted to determine pre-eruptive dissolved volatile concentrations. The studied eruptions include the 3930 BP subplinian, the 122 B.C. plinian, and the 4 January 1990 and the 23 December 1995 foun- tain fire eruptions. Preliminary results indicate that H2O varies between 3.13 and 1.02 wt% and CO2 between 1404 and 200 ppm. The most basic products (3930 BP tephra) contain the highest concentrations of CO2 (1404 ppm), whereas fire fountain hawaiitic tephra present the lowest values (< 200 ppm) indicating a continuous de- gassing process during the differentiation and rising of the magma. Generally, similar behavior has been found for water, characterized by a decreasing content during the differentiation that is mainly found in the 3930 BP eruption, 1990 and 1995 fire fountain products. Considering the relevance of volatile content and behaviour in determining the eruptive style, we made some inferences on the eruptive mechanisms based on the initial high volatile content and the degassing dynamics inside the plumbing system. These two factors suggest the cause of the high explosive activity in this basaltic volcano. CHECKING COPY 2 Paola Del Carlo and Massimo Pompilio vestigate the amount of dissolved volatiles based on Melt Inclusion (MI) studies in some Etnean tephra. Here we report preliminary re- sults of dissolved volatile concentrations deter- mined by the FT-IR (Fourier Transform In- frared) spectroscopic technique and classical microanalytic methods from four explosive eruptions: the 3930 BP subplinian eruption, the 122 B.C. plinian eruption, the 4 January 1990 and the 23 December 1995 fire fountain erup- tions. The 3930 BP subplinian eruption and the 122 B.C. plinian eruption represent two limit- ing cases in the Holocene history of Etna be- cause the former produced the most basic mag- ma ever erupted at Etna (picritic) whereas the latter is one of the few documented basaltic plinian eruptions in the literature. The two fire fountain episodes were typical of others which occurred in the last 13 years, and were very en- ergetic forming eruptive columns several kilo- meters high. The actual activity of Etna repre- sents a favorable case to study since we can in- tegrate direct observations of explosive erup- tions, immediate investigation and sampling of the deposits, with quantitative geophysical data (e.g., seismic tremor and ground deformation) recorded by networks monitoring the volcano. Volatiles in Etna magmas have been previously investigated by various authors (Kamenetskiy et al., 1986; Metrich and Clocchiatti, 1989; Metrich et al., 1993; Allard et al., 1994). Most of these studies focused on chlorine, sulfur and fluorine and use, with the exception of Metrich et al. (1993), the «difference method» (100- EMP analysis total) as an indirect estimation of H2O and CO2 content. However, these volatile species, being the most abundant in magma (Johnson et al., 1994), have a major control over crystallization and vesiculation, density, rheology and thus eruption style (Metrich and Rutherford, 1998; Ochs and Lange; 1999; Gior- dano and Dingwell, 2003). Fig. 1. Bulk rock and MI compositions in Total Alkali Silica diagram (Le Maitre, 1989). Filled triangle: 3930 B.P. scoria bulk rock; filled diamonds: 122 B.C. scoria bulk rock; filled point: 4 January 1990 scoria bulk rock; filled squares: 23 December 1995 scoria bulk rock; triangle: 3930 BP MI; diamonds: 122 B.C. MI; point: 4 Jan- uary 1990 MI; squares: 23 December 1995 MI. N O N C O R R IS P O N D E A L L A F O T O S T A M P A T A D E G L I O R IG IN A L I Utente NON CORRISPONDE Utente ALLA FOTO STAMPATA Utente ORIGINALI Utente DEGLI Utente Fig. 1. Utente Prego verificare. Grazie. Utente Prego verificare. Grazie. 3 The relationship between volatile content and the eruptive style of basaltic magma: the Etna case 2. Volcanological and compositional features of the studied eruptions 2.1. The 122 B.C. plinian eruption Tephra produced by the 122 B.C. eruption covered the entire southeastern flank of Etna, destroying the ancient town of Catania. The sequence comprises pyroclastic fall and flow deposits divided into 7 units, whose deposi- tional characteristics, dispersal and physical parameters are reported in Coltelli et al. (1998a). The thickest fallout units, that repre- sent a guide-horizon for the late Holocene stratigraphy, are C and E (from 2 to 0.3 m) formed by scoria lapilli and lithic clasts (about 20 wt%). Juvenile clasts are dense hawaiitic scoria (fig. 1), characterized by abundant, large tabular phenocrysts of plagioclase, mi- nor olivine and rare clinopyroxene (table I); the same minerals form the groundmass with scarce, interstitial brown glass. Very rare am- phibole (kaersutite) microlites are present in the whole sequence. 2.2. The 3930 BP subplinian eruption The 3930 BP. subplinian eruption produced a scoria fallout with the dispersal axis toward the East (Coltelli et al., 2000). The thickest de- posit found (110 cm) crops out 14 km from the summit. The deposit shows a peculiar stratifica- tion due to the alternation of fine and coarse lapilli beds also marked by the change in the su- perficial color (brown, gray, red and purple). The deposit presents features of a distal strom- bolian fall layer but the thickness and dispersal indicate a greater energy of the eruption. The deposit is composed of highly vesicular, olivine-rich scoria lapilli and very rare lava lithics. The phenocryst assemblage consists of scarce (< 8%) olivine (Fo90) and minor (< 3%) Cr-diopsidic clinopyroxene. Olivine, clinopy- roxene and skeletal plagioclase form microlites in hyalopilitic-cryptocrystalline groundmass (table I). The bulk rock is a picritic-basalt ac- cording to the TAS classification (La Maitre, 1989; fig. 1). The groundmass is slightly more evolved, but its composition falls on the primi- tive side of the basalt field. They are the most basic products ever found among Etnàs alkaline suite (Pompilio et al., 1995). 2.3. The 4 January 1990 and 23 December 1995 fire fountain eruptions The 4 January 1990 fire fountain eruption from South East Crater was a very powerful paroxysmal episode. It produced a copious sco- ria fall on the northwestern flank of the volcano reaching a deposit thickness of 60 cm about 5 km from the vent along the dispersal axis (Cal- vari et al., 1991). Scoria are porphyritic hawai- ites (fig. 1) with phenocrysts of plagioclase, clinopyroxene, olivine and microphenocrysts of Ti-magnetite (table I). The 23 December 1995 fire fountain erup- tion from North East Crater produced a 9.5 km long eruptive column (Coltelli et al., 1998b). The good weather conditions allowed for clear observation of the eruptive column expansion. Table I. Mineral chemistry of the studied tephra. Microl.: microlites. 3930 BP rim/microl. 3930 BP core 122 B.C. rim/microl. 122 B.C. core 4 January 1990 4 January 1990 23 Dec. 1995 23 Dec. 1995 rim/microl. core rim/microl. core Plg An63-73 – An24-85 An60-80 An86-49 An71-81 An55-86 An64-87 Ol Fo84-88 Fo88-91 Fo70 Fo73-76 Fo70-71 Fo77-81 Fo68 – Cpx X(di) 0.76-0.79 0.82-0.78 0.22 0.38-0.43 0.41-0.57 – 0.47-0.57 0.53-0.54 Usp% – – 30-48 – – – – – 4 Paola Del Carlo and Massimo Pompilio The column was bent approximately 40° downwind toward the NNE and abundant lapilli and ash covered a wide area of the NE flank down to the coast producing damage to fruit plantations, cars and buildings. The de- posit thickness along the main dispersal axis was 6-7 cm at 6 km from the vent and 1-2 cm at 20 km close to the Ionian coast. Lapilli and bombs were highly vesicular and glassy. Their composition is hawaiitic (fig. 1) and the min- eral assemblage consists of phenocrysts of plagioclase, clinopyroxene, olivine and mi- crophenocrysts of Ti-magnetite (table I) in a groundmass which varies from hyalopilitic to intersertal. 3. Melt inclusion analyses 3.1. Experimental procedures We used Fourier Transform infrared spec- troscopy (FT-IR) to determine H2O and CO2 concentrations in the MI’s hosted in olivine phenocrysts of tephra except for the 4 Janu- ary 1990 fire fountain, which was measured in a clinopyroxene phenocryst. 122 B.C. eruption carbon dioxide and water were ana- lyzed at DST-University of Pisa in double- sided, polished MI by transmission IR spec- troscopy using a Nicolet Magna-IR 560, equipped with an EverGlo source, a laser Ge- KBr beam splitter and a DTGS detector, cou- pled with an IR NicPlan microscope. The number of scans varied between 512 and 2048, and spectral data were collected utiliz- ing OMNIC software. Carbon dioxide and water in 3930 BP, 4 January 1990 and 23 De- cember 1995 eruptions were analyzed at GPS-California Institute of Technology labo- ratory using double-sided polished MI’s and a Nicolet 60SX IR spectrometer equipped with an MCT detector, a Globar source, a KBr beam splitter, coupled to a NicPlan micro- scope. The number of scans varied between 1024 and 4096. Three spectra were collected on each MI, and the arithmetic mean of the resulting species concentration was calculat- ed. Absorbance was measured from peak heights, and after subtraction of the back- ground signal were extrapolated with a flexi- ble drawing curve. Concentrations (C) were calculated according the Beer-Lambert law: C = 100 AM/(ερe), where A is the molar ab- sorbance, M the molar mass (g/mol), ε the molar absorptivity (L/mol cm), ρ the glass density, and e the thickness in cm. Sample thickness was measured using a digital mi- crometer with a precision of ± 1µm and checked with calibrated video analyses sys- tem. Thicknesses of each measured MI are listed in table II and vary between 10 and 82 µm. Glass density values of MI’s of 2800 g/L was used for hawaiitic compositions and 2950 g/L for the basaltic one. Water is dissolved as molecular water (H2Omol.) and hydroxyl group (OH-) in the MI. The concentrations of total water (H2Omol. + + OH–) in MI were determined using the broad band at 3535 cm−1 of the total water. The values for molar absorptivity ε3535 used is 63 ± 3 L/mol cm determined for basaltic alkaline glass accord- ing to Dixon et al. (1995). Carbon dissolved in MI is present only as CO3−2 which gives the dou- blet absorption band at 1350-1430 and 1450- 1570 cm–1. The value for molar absorptivity ε1350 used is 375 ± 20 L/mol cm according to Fine and Stolper (1986). Minerals and glass composition were an- alyzed using an Energy Dispersive Analyti- cal (LINK eXl) spectrometer linked to the Cambridge Stereoscan 360 SEM, and quanti- tative analyses were obtained by ZAF cor- rection using natural standard for the calibra- tion. Analytical conditions were 15 keV of acceleration tension, 500 pA of probe cur- rent. Glass was analyzed using a raster of about 10 × 10 micron area in order to mini- mize Na loss. Replicate analyses of internal natural standards (mineral and glasses) as- sured an analytical precision of less than 1% for SiO2 and Al2O3, about 2% for FeO, MgO and CaO, and from 3-5% for remaining ele- ments. Due to the lower value (< 0.5%), chlorine and sulfur concentration obtained from SEM analyses must be considered only semi-quantitative and was not used for the discussion. Table II lists the major element compositions and dissolved volatile concen- trations of the studied MI. 5 The relationship between volatile content and the eruptive style of basaltic magma: the Etna case 3.2. MI features Abundant MI of variable size were found in olivine crystals from 122 B.C. tephra. Those chosen for this study are about 100 µm in max- imum diameter. Many others were observed but were too small to be prepared and measured by FT-IR spectroscopy. They are generally glassy, although some show limited crystallization of olivine and clinopyroxene around the border. Inclusions in olivine crystals of 3930 BP tephra are generally rare and very small. Quite often they appear empty as original trapped fluid or vapor and leaked. In a few favorable cases, where glass was trapped, the inclusions show negative crystal shapes and contain shrinkage bubbles. In 23 December 1995 olivine crystal MI’s are more abundant and show quite often negative crystal or a sub-rounded shapes ac- companied by shrinkage bubbles of variable sizes. No mineral inclusions are present in these MI. In the sample from the 4 January 1990 tephra, some large «hourglass» inclusions (An- derson, 1991) with small trapped crystals, were found in diopside crystals. 3.3. MI composition A MI analyzed from the 3930 BP subplinian eruption was trapped in high forsteritic olivine (Fo90) and has basaltic composition (fig. 1). This inclusion contains 2.45 wt% H2O and 1404 ppm CO2. Partitioning of Fe and Mg between olivine and MI (Kd 0.33) is close to equilibrium value for crystallization at depth (Mysen, 1975). The most complete set of measurements was carried out on products of the 122 B.C. Table II. Melt inclusion compositions. H2O and CO2 from FT-IR analyses. Bdl: below detection limit. S de- tection limit is ±100 ppm, *=Mg# in host clinopyroxene crystal. Eruption 3930 BP 122 B.C. 4 Jan. 1990 23 December 1995 Inclusion 294 362a 362b 362ca 362cb 362cc 363da 363a 363b 363c 363d 367a se66 231295b 231295 Thickness (micron) 17 37 20 10 10 10 20 13 31 33 20 31 22 65 82 SiO2 45.62 49.91 51.26 50.25 50.20 50.07 49.87 49.97 50.50 52.54 49.90 51.15 46.56 47.27 51.41 TiO2 1.00 1.76 1.78 1.66 1.64 1.69 1.70 1.75 1.70 1.81 1.72 1.72 2.14 1.31 1.52 Al2O3 10.18 15.94 16.52 16.65 16.29 16.08 17.47 16.11 16.94 16.37 16.14 16.57 15.64 18.43 15.39 FeOtot 7.36 8.89 8.04 7.54 7.84 7.77 8.75 8.06 7.39 7.50 8.44 7.58 9.69 9.25 9.93 MgO 11.20 3.98 3.64 3.71 3.85 3.84 4.08 4.11 3.72 3.52 4.01 3.68 3.69 3.74 2.94 CaO 13.28 7.05 6.94 7.77 7.10 7.18 6.97 6.89 6.44 6.41 6.86 6.97 8.46 8.90 6.35 Na2O 1.60 4.94 5.40 5.34 5.48 5.37 5.30 6.48 5.86 3.88 5.25 5.62 4.08 4.58 5.34 K2O 0.74 2.39 2.53 2.07 2.04 2.08 2.23 2.39 2.18 2.22 2.27 2.38 2.65 3.01 2.71 P2O5 0.49 1.09 1.18 0.75 0.68 0.79 0.80 0.84 0.95 1.19 0.94 0.94 0.82 0.86 1.04 S 0.40 0.18 0.16 0.20 0.16 0.18 0.19 0.15 0.20 bdl 0.20 0.18 bdl 0.24 bdl Cl 0.29 0.35 0.27 0.35 0.34 0.32 0.32 0.28 0.29 0.29 0.30 0.33 0.15 0.35 0.25 H2O 2.45 2.22 1.02 2.25 2.86 3.13 1.26 1.56 2.63 3.04 2.55 1.94 1.93 1.85 1.63 CO2 ppm 1404 272 283 503 880 576 289 451 266 207 615 221 3 210 bdl Fo (%) 90 73 73 75 74 75 74 75 75 74 74 74 0.73* 75 68 6 Paola Del Carlo and Massimo Pompilio plinian eruption (Del Carlo, 2001). MI have mugearite composition (fig. 1). The measured H2O concentrations range between 1.02 and 3.13 wt% and CO2 between 207 and 880 ppm. The host olivine has a homogeneous composi- tion (Fo72-75). Partitioning of Fe and Mg (Kd 0.17-0.25) between olivine and trapped glass indicates that the inclusions suffered only limit- ed post-entrapment crystallization (< 6%). MI studied in 1990 fire fountain tephra is hosted in diopside crystal. The melt has a hawai- itic/basic mugearitic composition (fig. 1) and contains 1.93% H2O and CO2 below detection. MI’s measured in 1995 fire fountain tephra was trapped in olivine crystals with Fo68-75. One MI has a mugearitic composition (fig. 1) and contains 1.63% H2O. The other has a lower alka- li and silica content but a higher water concentra- tion of 1.85 wt%. An appreciable CO2 content (210 ppm) was measured only in MI within olivine with Fo75. Partitioning of Fe and Mg be- tween olivine and MI (Kd 0.27-0.28) indicates, as in 122 B.C. MI, equilibrium between host miner- als and trapped liquid and excludes significant post-entrapment crystallization of host olivine. On the whole, for the studied MI the occur- rence of extensive post-entrapment crystalliza- tion can be ruled out, therefore the measured volatile concentrations represent the volatile content of the magma at the time of entrapment. Figure 3 shows the relationships between volatiles and single element or element ratios, that change during the differentiation process. Taking into account the whole set of measure- Fig. 2a-d. Photo of a) MI in 3930 BP tephra; b) photo of MI in 122 B.C. tephra; c) photo of MI in 4 January 1990 tephra; d) photo of MI in 23 December 1995 tephra. a b c d 7 The relationship between volatile content and the eruptive style of basaltic magma: the Etna case Fig. 3. Correlations between CO2 and H2O versus K2O composition, CaO/Al2O3 ratio and FeO/MgO ratio in MI from the studied tephra. Symbols as in fig. 1. Stars: data from Metrich et al. (1993). 8 Paola Del Carlo and Massimo Pompilio ments that span in composition from basalt to hawaiite, or the more restricted compositional range observed in the 122 B.C. MI’s, a general inverse correlation exists between CO2 and K2O and FeO-MgO ratio (fig. 3). K2O is con- sidered to be incompatible in the magma, and is expected to increase during the differentiation. Likewise the FeO/MgO ratio is expected to in- crease in the melt due to crystallization of maf- ic phases, while pyroxene and plagioclase pre- cipitation would diminish the CaO/Al2O3 ratio in the melt. In the CO2 versus K2O and FeO/MgO diagrams (fig. 3), the scatter is limit- ed and all the inclusions plot along a single cor- relation curve. This pattern is compatible with the concomitant occurrence of differentiation and CO2 loss from the magma, occurring dur- ing the ascent and cooling of a magma saturat- ed in CO2. A similar general relationship is ob- served using H2O as volatile species in these di- agrams. On the whole, the sign of the correla- tion does not change, but the scatter is large and not a single correlation curve can be drawn. In particular, in 122 B.C. MI’s, we observe a fast decrease in water content coupled with a small changes in compositional parameter re- lated to differentiation, indicating that they are poorly dependent of composition. This is com- patible with a rapid decompression and water exsolution accompanied by limited cooling and crystallization processes for the magma feeding the 122 B.C. eruption. A different trend, char- acterized by a decrease of water during the dif- ferentiation, is depicted by 3930 BP, 1990 and 1995 MI’s. The high-K MI’s compositions of Metrich et al. (1993), that correspond to recent Etna products, fall in the same trend. Instead, the low-K MI’s of Metrich et al. (1993) define another trend with a comparable slope but shift- ed toward lower water content (fig. 3). Noteworthy is that the inclusion with the most primitive composition does not have the higher water content. This implies that the first (probably deep) differentiation stage that pro- duced the less evolved melts between the 122 B.C. MI, from a primitive magma, such as that feeding the 3930 BP eruption, must have devel- oped in conditions of water undersaturation. Assuming that the measured volatile content represents the maximum amount dissolved in magma, a minimum pressure of entrapment can be calculated following Dixon and Stolper (1995) and a H2O and CO2 solubility model (fig. 4). In fig. 4, the highest pressure (≈ 260 Mpa) is associ- ated with the inclusion in the 3930 BP tephra, which is the most basic magma, while the lowest one (< 50 Mpa) corresponds to the 23 December 1995 fire fountain products. Inclusions in crystals of 122 B.C. eruption show a large variation in volatile content that corresponds to a trapping pressure ranging from 210 to 50 Mpa. 4. Discussion Volatile contents (H2O and CO2) found in Et- na tephra are on average higher than those found in other intra-plate basaltic magmas known in lit- erature, such as Kilauea (Hawaii) with 0.1-1 wt% of H2O and comparable CO2 content. Pre- vious work on dissolved volatiles from Etnean magmas reported similar water concentrations. For example, Metrich et al. (1993) found for the 1989-90 fire fountain tephra 1.1 wt% of water and in the 1892 eruption 2.3 wt% of water. Thus, Fig. 4. H2O (wt%) versus CO2 (ppm) diagram. Sat- uration curves at different pressures are represented according to Dixon and Stolper (1995) model for al- kaline magmas. Symbols as in fig. 1. 9 The relationship between volatile content and the eruptive style of basaltic magma: the Etna case the initial high volatile content and the degassing dynamics inside the plumbing system may have a major role in controlling the eruptive style of Etna suggesting the cause for the high explosive activity in this basaltic volcano. Inferences on the eruptive mechanisms can be proposed on the basis of the inverse correla- tion between volatiles and incompatible ele- ments (e.g., K2O) or indexes of differentiation (e.g., FeO/MgO). These relationships suggest: i) the devolatilisation processes take place while the differentiation is going on; ii) mag- mas feeding the explosive eruptions are saturat- ed in both (H2O and CO2) investigated volatile species. This pattern even with a distinct rate is visible also in the two evolution trends (high-K and low-K) found by Metrich et al. (1993) in MI’s analysed in recent Etnean volcanics. The trend in the H2O-CO2 diagram (fig. 4) suggests a devolatilisation in a closed system more than an open system. In addition, a large pressure range would exclude the entrapment of MI forming in shallow magma chamber (fig. 4), and further indicate that devolatilisation and de- gassing occurred in non-equilibrium conditions. These effects are more visible in the 122 B.C. eruption, where a large variation of volatile con- tent is not accompanied by significant changes in MI composition, suggesting limited cooling, crystallization and differentiation processes as- sociated with a fast uprising. Normally, the rheological properties and the volatile content of basaltic magmas do not allow plinian eruptions, but the sudden decompression of a basaltic magma may cause the nucleation of a large number of bubbles that rapidly increase bulk viscosity and reduce magma density allow- ing rapid ascent in the conduit. Disruption of low density vesicular magma results in a pyroclastic mixture leading to the formation of a plinian eruptive column. This is the case of the 122 B.C. eruption, for which a sudden decompression model of the large magma body was proposed in Coltelli et al. (1998a) to form a plinian eruption. The whole 122 B.C. magma was volatile saturat- ed with an incipient vesicularity. In this condi- tion, even a small decompression of the system could have caused the gas to be exsolved, al- though the pre-eruptive water content was quite low (about 1 wt%). At low magma ascent rates, gas separates from low-viscosity melt and bub- bles coalesce during the ascent producing the ex- plosion of large bubbles forming strombolian ac- tivity. Conversely, rapid ascent rates in basaltic magmas prevent bubble coalescence and can pro- duce fire fountains and, if the volume is greater, subplinian and plinian eruptions. So, different paths of decompression of initially similar mag- mas may deeply influence the eruptive style. 5. Conclusions The explosive activity of Etna represents a continuous hazard for lands and villages located on its flanks. The most recent eruption, which started on 27 October 2002, gives an excellent example having produced a continuous ash emission for more than two months and causing damage to the eastern Sicily economy mainly related to the prolonged closure of Catania air- port. So, understanding of the triggering mecha- nisms of high explosive basaltic eruptions is of fundamental importance in order to assess the probability of future, large explosive events. This study on MI in minerals provided use- ful information on differentiation and degassing processes that occur during the storage and as- cent of magma before the eruption, in particular the relationships between volatile content and composition. This preliminary study outlines that initial volatile content at Etna is higher than in other basaltic volcanoes indicating their im- portant role in controlling the eruptive style. We made also some inferences on the eruptive mechanisms based on the initial high volatile content and the degassing dynamics inside the shallow plumbing system, indicating these two factors as the cause of the high explosive activ- ity in this basaltic volcano. Acknowledgements Authors are grateful to R. Cioni and S. New- mann for their kind assistance in FT-IR meas- urements, and to H.E. Belkin and an anonymous referee for the review that greatly improved the manuscript. Research supported by Gruppo Nazionale per la Vulcanologia (Italy). 10 Paola Del Carlo and Massimo Pompilio REFERENCES ALLARD, P., J. CARBONNELLE, N. METRICH, H. LOYER and P. ZETTWOOG (1994): Sulfur output and magma degassing budget of Stromboli Volcano, Nature, 368 (6469), 326- 330. ANDERSON, A.T. 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