Completo_DEF.qxd 1409 ANNALS OF GEOPHYSICS, VOL. 47, N. 4, August 2004 Key words Aeolian Islands – fluid inclusions – crustal xenoliths – homogenisation temperature – magma plumbing system 1. Introduction Understanding the role of shallow level evolutionary processes in arc volcanoes has an important bearing on the way the various vol- canoes work, which is a necessary preliminary requirement to understand both the petrological (i.e. RACF processes) and volcanological evo- lution of magma. Modeling magma ascent rates, however, requires the determination of the residence depths of magma (i.e. location magmatic chambers) based on reliable geo- barometers. Fluid and melt inclusion studies in magmatic minerals and/or in xenoliths en- trained in volcanic rocks represent a valuable technique to reach this objective (e.g., Roedder, 1965; Clocchiatti et al., 1994; Andersen and Neumann, 2001). Many studies have shown that fluid inclusions may be abundant in lavas and xenoliths, reflecting composition, pressure and temperature conditions of the fluid phases trapped during magma ascent and degassing (see Andersen and Neumann, 2001). Mailing address: Dr. Rossana Bonelli, Dipartimento di Scienze della Terra, Università degli Studi di Siena, Via La- terina 8, I-53100 Siena, Italy; e-mail: bonelli5@unisi.it Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy): evidence from fluid and melt inclusions in quartz xenoliths Rossana Bonelli (1), Maria Luce Frezzotti (1,2), Vittorio Zanon (3) and Angelo Peccerillo (3) (1) Dipartimento di Scienze della Terra, Università degli Studi di Siena, Siena, Italy (2) Istituto di Geologia Ambientale e Geoingegneria (IGAG) - CNR, Roma, Italy (3) Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Perugia, Italy Abstract Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka) from Alicudi contain abundant melt and flu- id inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I) inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I) and 0.47-0.26 g/cm3 (Type II). Early Type I fluid inclusions in- dicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data sug- gest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km), site of fractional crystallization, mixing with source – derived magma, and various degrees of crustal assimila- tion. The second magma reservoir is located at shallow crustal depths (about 6 km), the site where magma rested for a short time before erupting. 1410 Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo The Aeolian arc (southern Tyrrenian Sea) comprises seven volcanic Islands, some of which are active volcanoes (Vulcano, Strom- boli, and, possibly, Lipari and Panarea) and six principal seamounts (Glauco, Sisifo, Enarete, Eolo, Lamatini and Alcione). Volcanic rocks range from mafic to acid and have calc-alkaline (CA), High-K Calc-Alkaline (HKCA), Shosho- nitic (SHO) and potassic alkaline (KS) compo- sitions, which are closely associated in time and space (see De Astis et al., 1997, 2000). The Alicudi island is a composite volcano made up of dominant lavas and minor pyroclas- tic rocks of calcalkaline affinity sited at the western margin of the Aeolian arc. In spite of the many volcanological and geochemical stud- ies carried out in recent decades, there is little or no information on the internal structure of Alicudi volcano and on the way the volcanic plumbing system works. This paper presents fluid and melt inclu- sions data which contribute to the development of a model for the plumbing system of Alicudi, and for its evolution through time. 2. Geological and volcanological setting The volcanism of the Aeolian archipelago initiated about one million years ago, generated by subduction of the Ionian plate under the Cal- abro-Peloritani continental margin (Barberi et al., 1973; Ellam et al., 1988). Alicudi is one of the youngest islands in the Aeolian arc (90 ka; Gillot, 1987) located on the western side of the archipelago. The island represents the summit part of a complex stratovolcano, extending to 675 m above sea level and about 2000 m below (fig. 1). The overall circular pattern of its base (an area of about 5 km2) and the almost perfect conical shape of this stratovolcano suggests a development due to a central activity, without any significant migration of the feeding conduit (Villari, 1980). According to several authors (Villari, 1980; Peccerillo and Wu, 1992; Peccerillo et al., 1993) Alicudi was built during three different subsequent volcanic cycles separated by caldera collapses (fig. 1). The first two cycles (Scoglio Galera, 90-60 ka and Dirittuso, 55 ka) were characterised by emission of calcalkaline basaltic and basalt-andesitic lavas and minor pyroclastic products. High-K andesitic lava flows and domes were emplaced during the third phase (Montagnole and Filo dell’Arpa, 28 ka) from the summit crater on the southern flank of the volcano. The volcanic products exhibit a restricted compositional range (from basalts to high-K andesite) and display the most primitive petro- logical and geochemical characteristics over the entire Aeolian arc (Peccerillo and Wu, 1992; Peccerillo et al., 1993). Fig. 1. Schematic geological map of Alicudi showing the volcanics belonging to the three different volcanic cycles, simplified from Manetti et al. (1989). A = first eruptive cycle - Scoglio Galera Formation (~ 90-60 ka), B = second eruptive cycle - Dirittuso Formation (~ 60 ka), C = third eruptive stage - Montagnola and Filo dell’Arpa Formation (~ 28 ka). Grey dots indicate sampling localities. 1411 Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy) 3. Analytical methods Sample preparation for fluid inclusion stud- ies was done by making doubly polished sec- tions of 100-150 µm thickness. Microthermom- etry analyses were performed at the University of Siena with the Linkam TH600 heating and cooling Stage. SYNFLINC® standard synthetic fluid inclusions were used to calibrate the stage, checking the temperature at the CO2 (– 56.6°C) and H2O (0°C) triple points. Accuracy at stan- dard points was estimated ± 0.1°C. Mac Flincor® software package (Brown, 1989) with the equation of state by Holloway (1981) for CO2 were used to calculate the isochores for fluid inclusions. Density values of CO2 were calculated from Angus et al. (1976). Density of mixed inclusions of CO2 and N2 was derived from Kerkhof and Thièry (2001). Fluid inclusions were further analysed with a confocal Labram multichannel spectrometer of Jobin-Yvon Ltd. in the laboratory of the Uni- versity of Siena. The excitation line at 514.5 nm was produced by an Ar+ laser. The Raman inten- sity was collected with a Poltier-cooled CCD detector. The beam was focused to a spot size of about 1-2 µm using an Olympus 100 × lens. The scattered light was analysed using a Notch holographic filter with a spectral resolution of 1.5 cm−1 and grating of 1800 grooves/mm. Electron – microprobe analyses of mineral phases were performed with a Cameca SX 50 (IGAG-CNR, Roma) operated at an acceleration voltage of 15 kV and a probe current of 15 nA. Mineral analyses were performed with a focused (1 µm) beam. Natural and synthetic silicates were used as standards for mineral analyses. Melt inclusions microthermometry of a sin- gle xenolith was performed at the Vrije Uni- versity of Amsterdam using a high temperature (up to 1600°C) heating/quenching stage (Sobolev and Kostyuk, 1975). Temperatures were measured with a Pt-Pt09Rh10 thermo- couple, calibrated with gold, silver and syn- thetic compounds. Experiments were per- formed at 1 atm He, purified by a 700°C Ti fil- ter. Heating times varied from 1 to 6 hours due to the slow kinetics of high-silica melts. Opti- mal heating rates of 2-5°C/min were used above 700-900°C and much lower rates (5- 30°C/hour) near homogenisation temperature. Measurement uncertainties were estimated to be ± 5°C. After quenching, host minerals were mounted on epoxy and polished until melt in- clusions were exposed to surface. EMP analy- ses of melt inclusions were performed using a four-WDS-spectrometer JEOL Ltd. JXA 8800M Superprobe at the Vrije University of Amsterdam using an acceleration voltage of 15 kV and a beam current of 25 nA. Spot sizes were 2 ÷ 10 µm, with single-element counting times of 25 ÷ 50 s on the peak and 10 ÷ 25 s on the bottom. 4. Petrography of lavas and xenoliths Studied lavas and xenoliths were collected in three distinctive localities in Alicudi, which are considered representative of the overall petrological and volcanological evolution of the island (fig. 1): 1st cycle – basaltic lavas at Scoglio Galera and Malopasso (Scoglio Galera activity: ∼ 90- 60 ka); 2nd cycle – basaltic-andesitic lavas at Bazzi- na (Dirittuso activity: ∼ 55 ka); 3rd cycle – andesitic lavas at the Alicudi Porto and Perciato (Montagnole and Filo del- l’Arpa activity: ∼ 28 ka). Xenoliths are particularly abundant in the basaltic lavas of the first two cycles, whereas they are scarce or absent in the third stage andesites. 4.1. Host lavas The rocks of Alicudi are basalts, basaltic an- desites and high-K andesites, which define a subalkaline trend showing transitional charac- ters between calcalkaline and high-K calcalka- line according to the classification proposed by Peccerillo and Taylor (1976). Lavas are por- phyritic, with plagioclase as the dominant phe- nocryst phase in all rock types, whereas olivine, although ubiquitous, is abundant (> 30%) only in basalts. Clinopyroxene phenocrysts occur in moderate amounts in all rocks. Representative mineral compositions of Alicudi lavas are re- ported in table Ia,b. 1412 Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo Basalts from the first cycle of activity (Scoglio Galera) display a holocrystalline tex- ture and a phenocryst content ranging from 30 to 50% of the total rock volume. Plagioclase makes up more than 50% of the total phenocryst abun- dance and consists of bytownite-labradorite (An% = 83-73; table Ia). Olivine is also abun- dant (≥ 30%) and occurs as phenocrysts (Fo79) and microphenocrysts. Clinopyroxene occurs both as phenocrysts and in the groundmass and can be classified as augite (Wo% = 40-43; En%= = 45-47; Fs% = 9-13; table Ia). Basaltic andesites from the second cycle of activity (Dirittuso) are porphyritic in texture and are similar to the basalts described above, but with less olivine (≥ 20%). Plagioclase is by far the most abundant phenocryst phase and has compositions similar to those in the basalts, with max An% = 81 (not shown). Olivine (Fo74) and clinopyroxene (Wo% = 40-43; En% = 45-47; Fs% = 9-13) have compositions similar to those in the basalt. These two last minerals represent the principal phases in the groundmass as well. High-K andesites from the third cycle of ac- tivity (Montagnole and Filo dell’Arpa) show porphyritic textures with dominant plagioclase phenocrysts of labradoritic composition (An%= = 65-58). The olivine (Fo78-76) content is less than 20% and clinopyroxene shows a composi- tion similar to that in basalts (Wo% = 40; En%= Table Ia. Major (wt %) elements of the studied phenocrysts in basaltic lavas of Alicudi. Samples SG7 TC5 SG7 SG7 SG7 SG7 SG7 SG7 SG7 mineral OL OL CPX CPX CPX PL PL PL PL note core rim core rim SiO2 38.69 36.98 52.31 51.42 52.52 47.53 48.46 47.88 49.76 TiO2 – – 0.48 0.41 0.54 – 0.09 0.04 – Al2O3 – – 2.48 3.86 2.73 33.06 31.80 32.95 31.17 FeO tot 19.07 23.42 8.38 5.91 7.02 0.66 0.59 0.58 0.70 MnO 0.28 0.44 0.18 0.07 0.14 – – – – MgO 41.52 38.36 16.26 16.32 16.33 – – – – CaO 0.18 0.25 19.94 20.88 21.03 17.06 16.18 16.99 14.91 K2O – – 0.01 – – 0.08 0.11 0.09 0.13 Na2O – – 0.29 0.22 0.22 1.83 231 1.96 2.91 Totale 99.74 99.44 100.33 99.09 100.53 100.22 99.54 100.48 99.57 Fo% 79.50 74.48 – – – – – – – Fa% 20.50 25.52 – – – – – – – Wo% – – 40.48 43.25 42.61 – – – – En % – – 45.94 47.07 46.05 – – – – Fs % – – 13.58 9.68 11.34 – – – – Mg* 0.80 0.74 0.77 0.83 0.80 – – – – An% – – – – – 83.34 78.91 82.29 73.34 Ab% – – – – – 16.17 20.42 17.19 25.87 Or% – – – – – 0.49 0.66 0.52 0.79 OL: olivine, CPX: clinopyroxene, PL: plagioclase; blank entries: not analysed; Mg* = Mg/(Mg+FeTOT). 1413 Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy) = 45; Fs% = 13; table Ib). The groundmass con- tains the same phases as the phenocrysts, ex- cept olivine, set in glassy matrix. 4.2. Quartz xenoliths The lavas of Alicudi commonly contain xeno- liths of both magmatic and metamorphic origin, similar to observations in the other Aeolian Is- lands (e.g., Frezzotti et al., 2003). Metamorphic xenoliths show variable compositions and tex- tures and can be classified as: i) quartz aggre- gates, ii) garnet-cordierite and garnet-silliman- ite gneisses, iii) vesuvianite-grossular-bearing skarns and iv) metapelites (Honnorez and Keller, 1968; Peccerillo and Wu, 1992). Magmatic lithologies are represented by gabbros and dior- ites consisting of plagioclase and clinopyroxene. Previous petrological and geochemical in- vestigations suggest that metamorphic xeno- liths represent residual material of partially melted gneiss and schists, thus constituting compelling field evidence of interaction be- tween magma and crustal rocks beneath Alicu- di volcano (Peccerillo and Wu, 1992). Study rocks consist of quartz xenoliths which contain abundant fluid and melt inclusions. Quartz xenoliths are composed mainly of quartz (> 95%), with accessory plagioclase, pyroxene, Table Ib. Major (wt %) elements of the studied phenocrysts in andesitic lavas of Alicudi. Samples ARP ARP ARP ARP ARP ARP ARP ARP ARP mineral OL OL CPX OPX OPX OPX PL PL PL note core rim SiO2 37.53 37.31 51.75 52.75 52.38 52.39 52.04 51.87 52.72 TiO2 0.01 0.03 0.45 0.22 0.30 0.20 0.06 0.01 0.06 Al2O3 0.04 0,03 1,78 0,97 0,80 1,45 30.33 30.47 29.00 FeO tot 20.51 21.74 8.53 18.02 21.37 18.15 0.56 0.55 0.73 MnO 0.40 0.42 0.36 0.66 0.95 0.83 0.02 - 0.03 MgO 41.63 39.75 16.10 24.85 21.73 25.57 0.07 0.08 0.09 CaO 0.18 0.25 20.37 2.02 1.90 1.32 13.20 13.11 11.83 K2O 0.02 0.01 0.02 0.02 0.10 0.01 0.20 0.19 0.32 Na2O – 0.03 0.28 0.06 0.06 0.02 3.84 3.76 4.45 Totale 100.32 99.57 99.64 99.57 99.59 99.94 100.32 100.04 99.23 Fo% 78.34 76.51 – – – – – – – Fa% 21.66 23.49 – – – – – – – Wo% 0.24 0.34 40.96 3.94 3.83 2.55 – – – En % 77.82 75.90 45.07 67.54 60.99 68.78 – – – Fs % 21.94 23.76 13.97 28.51 35.18 28.67 – – – Mg* 0.78 0.76 0.76 0.70 0.63 0.71 – – – An% – – – – – – 64.75 65.09 58.38 Ab% – – – – – – 34.09 33.78 39.74 Or% – – – – – – 1.17 1.12 1.88 OL: olivine, CPX: clinopyroxene, OPX: ortopyroxene, PL: plagioclase; blank entries: not analysed; Mg* = = Mg/(Mg+FeTOT). Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo apatite, zircon and opaque (table II). They gener- ally have angular shapes and sizes ranging from a few mm to a few dm. Quartz (400-µm – 2mm) shows variable microstructural characteristics and is present both as rounded grains often rimmed by cristobalite (i.e. melting; fig. 2a), and as recrys- tallised crystal aggregates (i.e. annealing; fig. 2b). Significant quantities of internal glass, present both as intergranular veins and as short trails or clusters of silicate-melt inclusions, are often ob- served within single rounded quartz grains. Such a glass does not represent infiltration by host lavas since it has a distinctive high-silica rhyolitic composition. 5. Melt inclusions Melt inclusions are present only in restitic quartz grains showing rounded morphology (fig. 2a). The dimensions of the inclusions vary according to their textural distribution. Their size is commonly about 30 µm, but inclusions of greater diameter were sometimes observed. Inclusions are commonly present isolated or in small clusters in most of the quartz grains (fig. 2c). Most melt inclusions contain glass and a bubble, ± locally one or more crystals (quartz, feldspars, ilmenite, clinopyroxene, sulphides or oxides) are present in the glass. In some inclu- Table II. Mineral compositions (wt %) of studied quartz-rich xenoliths. Samples TC3 TC3 TC5 TC5 TC5 TC3 TC3 TC3 Mineral CPX CPX OPX OPX OPX PL PL PL note core rim core rim SiO2 54.73 52.99 52.51 50.54 51.39 54.52 53.40 53.39 TiO2 – 0.76 0.11 – 0.02 – – – Al2O3 0.40 1.73 0.30 0.74 0.78 27.76 29.01 29.32 FeO tot 2.60 4.44 24.23 24.72 21.85 – 0.51 0.38 MnO – 0.36 1.72 1.63 1.54 – – – MgO 17.18 16.84 20.84 21.38 22.68 – – – CaO 24.61 22.77 0.97 0.22 0.40 11.08 12.42 12.56 Na2O 0.17 0.31 0.01 0.03 – 5.19 4.43 4.17 K2O – – – 0.02 0.02 0.37 0.34 0.31 Totale 99.69 100.20 100.69 99.28 98.68 98.92 100.12 100.12 Wo% 48.68 45.57 1.93 0.43 0.80 – – – En % 47.30 46.92 57.71 58.83 62.81 – – – Fs % 4.02 7.51 40.36 40.73 36.39 – – – Mg* 0.92 0.86 0.59 0.59 0.63 – – – An% – – – – – 52.96 59.60 61.37 Ab% – – – – – 44.93 38.47 36.83 Or% – – – – – 2.11 1.93 1.79 CPX: clinopyroxene, OPX: ortopyroxene, PL: plagioclase; blank entries: not analysed; Mg* = Mg/(Mg+ + FeTOT). 1414 1415 Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy) Fig. 2a-f. Photomicrographs of studied quartz xenoliths and fluid inclusions: a) and b) quartz grain textures in a quartz xenolith from a basaltic rock of the 1st cycle of Alicudi (Scoglio Galera): a) this type of quartz is trans- parent and generally has a rounded contour; b) this type of quartz is milky, highly fractured with cracks filled with trails secondary fluid inclusions; c) trail of texturally early Type I fluid inclusions in a quartz grain (Scoglio Galera). Type I fluid inclusions (black arrows) occur associated with silicate - melt inclusions (white arrows); d) cluster of early Type I inclusions in a quartz grain from Dirittuso; e) trail of late Type II fluid inclusions in a quartz grain from Dirittuso. These inclusions are clearly secondary origin and contain CO2; f) late trails of Type II CO2 inclusions distributed along two main directions in a quartz grain (Scoglio Galera). a c d e f µ µ µµ µ µ b 1416 Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo sions, a large bubble is present, often contain- ing immiscible CO2. The presence of CO2 in melt inclusions is associated with the CO2-rich fluid in host xenolith. Homogenisation temperatures of silicate-melt inclusions are between 1060 and 1120 ± 5°C. The glass consists of SiO2 (69-78 wt.%), Al2O3 (10-11 wt.%), K2O (3-5 wt.%), Na2O (1- 2 wt.%) and very little or no FeO, MgO (chem- ical data from four melt inclusions are reported in table III). 6. Fluid inclusions 6.1. Composition and distribution Fluid inclusions from studied xenoliths melt instantaneously in a narrow T interval (– 56.9 ÷ Table III. Results of electron – microprobe analy- ses of silicate – melt inclusion in a quartz xenolith of Alicudi. Samples ARP ARP ARP ARP SiO2 78.10 72.60 69.00 75.10 TiO2 0.18 0.11 0.87 0.02 Al2O3 10.30 11.60 11.20 11.60 Cr2O3 0.01 0.00 0.02 0.02 Fe2O3 0.23 0.98 1.78 0.34 FeO 0.47 1.97 3.56 0.69 MnO 0.03 0.09 0.08 0.05 MgO 0.20 0.72 1.68 0.16 CaO 0.48 2.14 3.89 0.69 Na2O 1.72 1.67 1.17 1.91 K2O 4.67 3.97 3.33 4.96 P2O5 0.00 0.00 0.29 0.00 SO3 0.00 0.03 0.00 0.00 F 0.03 0.04 0.01 0.10 Cl 0.23 0.48 0.00 0.28 Totale 96.65 96.40 96.88 95.92 7500 2000 2100 2200 Raman shift (cm–1) 2300 2400 2500 6500 5500 4500 3500 2500 2142 CO 2329 N2 In te n s it y Fig. 3. Histogram of melting temperatures (Tm) for studied fluid inclusions. Most CO2 melting occurs within a narrow temperature interval between – 56.9 and – 56.3°C (maximum at – 6.6°C). Fig. 4. Raman spectrum for a fluid inclusion in a quartz xenolith from Scoglio Galera. The inclusion contains nitrogen (10 mole%) and carbon monoxide (14 mole%) besides CO2. ÷ – 56.3; highest frequency Tm = – 56.6; fig. 3), suggesting that CO2 is the major phase present. Ra- man spectra (28 collected) reveal the presence of other species (table IV). Nitrogen (1-10 mole%) n n Tm 1417 Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy) Table IV. Chemical composition of fluid inclusions from Raman and microthermometric measurements. Samples ThL Thv Tm CO2 N2 CO (°C) (°C) (°C) (mole %) (mole %) (mole %) SG1 d)bis* – 16.5 – 57.9 90 2 8 SG1 d)bis – 17.0 – 58.0 92 1 7 SG1 d)bis – 15.0 – 58.1 82 4 14 SG1 d)bis – 17.6 – 57.9 92 2 6 SG10 d)2 5.0 – – 57.7 95 5 – SG10 d)2 – 24.0 – 57.0 97 3 – SG7 a)1 – 0.5 – 57.0 97 3 – SG7 f) 16.6 – – 57.5 96 4 – 2° Cycle of Dirittuso (55 ka) TC5a) 22.0 – – 57.2 97 3 – TC5c) – 2.5 – – 58.9 90 10 – TC5e) 8.8 – – 57.3 98 1 1 TC5f) 4.5 – – 58.1 94 6 – TC5f) 4.5 – – 58.5 94 6 – TC5f) 0.8 – – 58.7 93 7 – TC5f) 16.3 – – 57.8 96 4 – TC5f) 5.2 – – 58.9 95 5 – TC5f) 7.0 – – 58.7 95 5 – TC1-1a)1 20.2 – – 57.1 97 3 – TC1-1a)1 20.0 – – 57.1 97 3 – TC1-1a)2 – 23.5 – 57.0 97 3 – TC1-1b) – 25.5 – 57.0 97 3 – TC1-1c) 27.0 – – 57.1 98 2 – TC1-1c) – 29.0 – 57.1 98 2 – TC1-1c) – 23.0 – 57.2 97 3 – TC1-1c) – 23.5 – 57.2 97 3 – * Raman spectrum (fig. 4). 1° Cycle of Scoglio Galera (90-60 ka) ± carbon monoxide (1-14 mole%) are always pres- ent in fluid inclusions with melting below – 56.9°C and slow melting behaviour (fig. 4; table IV). Carbon dioxide is present in two different generations of inclusions: Type I - early inclusions are present in xeno- liths of the 1st and 2nd cycles (fig. 2c,d). They are isolated or in a small clusters with sizes ranging 3 ÷ 10 µm and shapes generally regu- lar. Partial decrepitation is visible (haloes of small inclusions around the inclusion cavity) (Anderson et al., 1984). Early inclusions are of- ten associated with silicate-melt inclusions sug- gesting concomitant trapping. Type II - late inclusions are present in all studied xenoliths and occur in variable shape and size. They are distributed along inter- and intragranular trails (fig. 2e,f). Only a few among late Type II inclusions show evidence of partial decrepitation. 1418 Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo Fig. 5. Histograms of homogenisation temperatures for fluid inclusions present in xenoliths of the studied vol- canic island (Alicudi). Histograms report homogenisation temperatures to the liquid phase (ThL) for early Type I fluid inclusions and homogenisation temperatures to the vapour phase (ThV) for late Type II fluid inclusions. F re q u en cy F re q u en cy 6.2. Homogenisation temperatures and fluid density calculations Quartz xenoliths from Scoglio Galera and Dirittuso contain high-density (fig. 2c,d) and low-density (fig. 2e,f) fluid inclusions – Type I and Type II inclusions, respectively. The dis- tribution of fluid inclusions from Filo del- l’Arpa is different. Here, inclusions are ex- tremely rare and all of Type II. Scoglio Galera (1st cycle of activity; ∼ 90 ka): Type I inclusions always ho- mogenise to liquid (ThL) in an interval of temperatures between – 0.5 and 29.5°C (fig. 5), with calculated density values between 0.93 and 0.61 g/cm3. Most ThL values, clus- tering around 28°C, represent re-equilibrat- ed fluid inclusions (decrepitation; Vityk and Bodnar, 1998). Type II inclusions homogenise to vapour (ThV) in the range of temperatures between – 6 and 31°C, with corresponding density val- ues between 0.08 and 0.47 g/cm3. The ThV his- togram shows a few data scattered in a wide range of re-equilibrated inclusions. Dirittuso (2nd cycle of activity; ∼ 55 ka): microthermometric data for early Type I inclu- sions show a distribution of temperatures be- tween – 2.5°C and 31.1°C, with resulting densi- ty values between 0.93 and 0.47 g/cm3 (the low- est temperatures around – 2.5°C to belong to a CO2 + N2 fluid inclusion, with density value of 0.83 g/cm3) (fig. 5). Values between 22 and 31°C belong to re-equilibrated inclusions. Type II vapour-rich inclusions show homogenisation temperatures between 0 and 31°C, (d = 0.10 – 0.47 g/cm3). Lowest ThV values, below 26°C, correspond to re-equilibrated fluid inclusions. Th 1419 Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy) Filo dell’Arpa (3rd cycle of activity; ∼ 28 ka): this cycle is represented by two low-densi- ty inclusions (ThV), with homogenisation tem- peratures between 6 and 18°C. Density values are between 0.12 and 0.18 g/cm3. 7. Discussion 7.1. Isochore calculation Isochoric lines representative for P and T conditions of fluid trapping in xenoliths dur- ing ascent of lavas for the different volcanic cycles are reported in fig. 6. The figure shows the presence of two isochoric bands, corresponding to the bimodal distribution of density values for the different inclusion types. The narrowest interval is between 0.47 and 0.26 g/cm3, corresponding to Type II low-density inclusions; the other interval (between 0.93 and 0.68 g/cm3) corresponds to Type I high-density fluid inclusions. Note that this last isochoric band is much wider due to the presence of density data from re- equilibrated inclusions. Not all of the fluid inclusion densities observed in xenoliths re- flect original densities at trapping condi- tions: most Type I fluid inclusions show tex- tural evidence for decrepitation and fluid density decrease. Decrepitation is confirmed by the frequent presence of small haloes of tiny bubbles and/or little fractures around the inclusions, and by the scattered Th distribu- tion. Reset density data for Type I inclusions will not be considered in the following dis- cussion. 7.2. Temperature and pressure estimates Before any meaningful geological interpre- tation can be proposed it is necessary: first to fully describe the temperature and pressure conditions under which Type I and Type II CO2 fluids are trapped, and second to verify the fluid-xenoliths evolution during ascent. The temperature conditions for the fluids at the time of trapping are based on homogenisation temperatures of silicate melt inclusions, since microstructural evidence clearly indicates con- comitant trapping of high-silica melt and (Type I) CO2 fluids in the inclusions. Homogenisation temperatures of silicate- melt inclusions show values between 1060 and 1120°C (± 5°C). For this reason, we assume 1100°C as a mean trapping temperature for flu- id inclusions present in xenoliths. At 1100°C, pressure estimates from fluid in- clusions form two distinct pressure intervals that are interpreted to reflect two different episodes of fluid entrapment. Both undisturbed early Type I inclusions from the 1st cycle of Scoglio Galera and from the 2nd cycle of Dirittuso show identical maximum pressure estimates at ≈ 6 kbar. Such a pressure corresponds to a depth of about 24 km, assuming 2.7 g/cm3 as the average Fig. 6. Isochoric bands, corresponding to the bi- modal distribution of densities for the first two mag- matic cycles of activity of the Alicudi island: the first at high pressure corresponding to early Type I fluid inclusions and the latter at shallower pressures corre- sponding to late Type II fluid inclusions, see text. T T P P 1420 Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo value of density for the stratigraphic sequence beneath Alicudi (Zanon, 2001). A second episode of fluid trapping in quartz xenoliths from lavas of both the 1st and 2nd cy- cles occurred at a later stage, and is represented by low-density Type II inclusions. Type II fluid inclusions indicate similar pressure values and between 1.7 and 0.2 kbar (about 6 km depth). For Filo dell’Arpa andesitic lavas (3rd cy- cle), low-density Type II inclusions form a dis- tinct pressure interval, between 0.5 and 0.3 kbar (about 1 km depth). 7.3. Polybaric evolution of Alicudi’s magmas The data obtained from the study of fluid and melt inclusions hosted in quartz-rich xeno- liths allow us to propose a schematic model for the plumbing system of Alicudi volcano, and to describe its modification through time, illustrat- ed in fig. 7a-c. Fluid inclusion barometry bear evidence that basaltic and basaltic andesitic lavas of the first and second cycles (Scoglio Galera, ∼ 90 ka and Dirittuso, ∼ 55 ka) under- went the same polybaric evolution in the crust. Type I fluid inclusion data for the first two vol- canic cycles indicate that a deep magma storage level has been present beneath the island, locat- ed at lower-crustal depths (about 24 km; fig. 7a,b), since the Moho beneath Alicudi island was proposed at 21-25 km (Falsaperla et al., 1984). Such a magma accumulation level prob- ably corresponds to a deep magma chamber lo- cated at the crust-mantle boundary, where man- tle magmas rested and underwent contamina- tion and very limited crystal fractionation. Barometric data from low-density Type II fluid inclusions indicate that a second magma storage level is present located at shallower depths (about 6 km). Shallow level magma chambers are fed by the deep magma chambers. The occurrence of quartz xenoliths, which con- tain both low-density (Type I) and high-density (Type II) fluids, the lack of extensive density re- setting for Type I deep fluids, and the preserva- tion of quartz xenoliths themselves, all suggest that the residence time of deep magmas and re- lated quartz xenoliths in the shallow chambers is very short, possibly a few days. Thus, we can conclude that a single volcanic plumbing system was active during the first two cycles, and from 90 to 55 ka, related to the eruption of basaltic and basaltic andesite lavas (fig. 7a,b). The above model implies that the large quantities of mafic magmas which characterise the first two cycles of the activity of Alicudi reflect tapping of deep level reservoirs, where dominant mixing processes do not allow magma differentiation toward silicic composition. According to fluid inclusion data, the over- all magma ascent evolution shows abrupt vari- ations during the third cycle. For late andesitic lavas of Filo dell’Arpa, inclusion evidence in- dicates that a deep magma chamber is conspic- uously absent (fig. 7c). A single magma accu- mulation level is indicated by Type II low den- sity fluid inclusions at about 1 km depth; that is more superficial than those present during the first two cycles (compare fig. 7a,b with fig. 7c). It would be inaccurate, however, to propose a model for andesite crustal evolution based only on fluid inclusion evidence. The absence of Type I inclusions and the limited amount of Type II in- clusions in quartz xenoliths contained in an- desitic lavas might be related to the scarcity of studied samples, since andesites do not contain abundant metamorphic rocks. Higher viscosity and lower temperatures of such lavas compared to previous basaltic ones may have prevented melting and incorporation of crustal rocks with- in the ascending magma. For these reasons, we cannot exclude that andesites formed in the same deep magma chambers where early stages basalts and basaltic andesites rested resulting a similar polybaric evolution. 8. Concluding remarks Present fluid inclusions data indicate that two principal levels of magma accumulation were present beneath the island during the first two cycles (Scoglio Galera and Dirittuso), indicating a polybaric evolution for Alicudìs magmas. A first deep accumulation level is present at depths close to the Moho (∼ 24 km). At these stages xenoliths were possibly trapped along with (Type I) CO2 ± N2 ± CO fluids in the basaltic lavas. 1421 Evolution of the volcanic plumbing system of Alicudi (Aeolian Islands – Italy) Fig. 7a-c. Schematic section modelling the magma plumbing system beneath Alicudi island, as inferred from fluid inclusion study. Lithological boundaries are from Falsaperla et al., 1984 (see text). a b c P P P h h h 1422 Rossana Bonelli, Maria Luce Frezzotti, Vittorio Zanon and Angelo Peccerillo A second magma storage level at upper crustal depths (about 6 km) has been active throughout the volcanic history of the island. Here magmas and xenoliths rested for a time span sufficient for new CO2 fluid trapping (Type II) and old Type I fluid inclusion density resetting. 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