Giovanni Lanzo

Ida Di Carlo

Michel Pichavant

Silvio G Rotolo

Bruno Scaillet

Bruno Scaillet Highlights

Origin of primitive ultra-calcic arc melts at crustal conditions -Experimental evidence on the La Sommata basalt, Vulcano, Aeolian Islands

Keywords: Primitive arc magmas, Ultra-calcic, Experiments, Phase equilibria, Vulcano, Aeolian arc

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Magmas with primitive geochemical characteristics are rare in arc settings. Nevertheless, they occur in virtually every island arc and are of great petrological significance as they constitute a valuable source of information on the nature of source rocks, conditions of partial melting and compositions of primary melts at convergent plate margins. Since different potential source rocks are present in arcs, and conditions of partial melting can be variable, a spectrum of primary magma compositions is expected. [START_REF] Tatsumi | Subduction Zone Magmatism[END_REF] defined up to 5 types of primary magmas in subduction zones, shoshonitic, high-, medium-and low-K, and high-Mg andesites (for the latter see [START_REF] Wood | Origin of primitive high-Mg andesite: constraints from natural examples and experiments[END_REF]. [START_REF] Drummond | A model for trondhjemite-tonalite-dacite genesis and crustal growth via slab melting: archean to modern comparisons[END_REF] proposed additional primary magma types (adakites) generated by melting of the subducted oceanic crust. Arc magmas also include a group of primitive ultra-calcic compositions that are interpreted to be generated from specific but still poorly constrained mantle source rocks [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF][START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF][START_REF] Médard | Melting of amphibole-bearing wehrlites: an experimental study on the origin of ultra-calcic nepheline-normative melts[END_REF][START_REF] Sorbadere | Constraints on the origin of nephelinenormative primitive magmas in island arcs inferred from olivine-hosted melt inclusion compositions[END_REF].

On a global basis, arc magmas are compositionally evolved because protracted differentiation takes place to produce the typical terms of the arc sequence, from basalt to rhyolite. These differentiation processes tend to mask the geochemical characteristics of the primary magmas. Distinguishing between partial melting (mantle-related) and early differentiation (crust-related) processes is especially difficult for arc magmas (e.g., [START_REF] Takahashi | Primary magmas at the volcanic front of the NE Japan arc: coeval eruption of crustal low-K tholeiitic and mantle-derived medium-K calc-alkaline basalts at Azuma volcano[END_REF]). Yet, for practical petrological and volcanological purposes, it is important to establish the nature of primary magmas, as well as the composition of the most primitive magmas from which the others are derived (e.g., [START_REF] Smith | Primitive magmas in arctype volcanic associations: examples from the southwest Pacific[END_REF].

In this paper, the above approach is applied to the Aeolian arc, south of the Tyrrhenian Sea. This arc hosts several active reference stratovolcanoes such as Vulcano and Stromboli (Fig. 1). A wide range of compositions, from basalt to rhyolite, erupts along the arc [START_REF] Peccerillo | Compositional variations of magmas in the Aeolian arc: implications for petrogenesis and geodynamics[END_REF]. Primitive magma compositions are very rare and include calcalkaline basalts at Alicudi and ultra-calcic compositions at Vulcano and also Stromboli. Glass inclusions from the La Sommata cone on Vulcano are classic examples of primitive ultra-calcic melts in an island arc setting [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF]. These glass inclusions are also reference samples of natural hydrous basanitic melts [START_REF] Schiano | Transcrystalline melt migration and Earth's mantle[END_REF][START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF]. The La Sommata inclusions occur in olivine and clinopyroxene phenocrysts in a basalt which also displays primitive ultra-calcic characteristics. For our understanding of the magmatic evolution in the Aeolian arc, the petrogenetic significance of such ultracalcic compositions needs to be evaluated. It is also important to test whether the La Sommata basalt can be a viable parent for the more evolved magmas erupted at Vulcano [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. To achieve these goals, a combined experimental and theoretical approach based on the La Sommata basalt has been undertaken.

Primitive ultra-calcic melt compositions have recently attracted much attention, starting with [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF]. Ultra-calcic compositions from arc settings are silica-poor (SiO 2 as low as 44 wt.%), alkalirich (Na 2 O + K 2 O N 3 and up to 7 wt.% for the potassic varieties) and ne-normative. They have very elevated CaO contents (up to 18 wt.%) and very high CaO/Al 2 O 3 ratios (N 1.0 and up to 1.6) at moderately high MgO concentrations (8-10 wt.%; compositional data from [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF][START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF][START_REF] Médard | Melting of amphibole-bearing wehrlites: an experimental study on the origin of ultra-calcic nepheline-normative melts[END_REF][START_REF] Schiano | Transcrystalline melt migration and Earth's mantle[END_REF][START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF][START_REF] Sonzogni | Transcrystalline melt migration in clinopyroxene[END_REF][START_REF] Rose-Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF][START_REF] Chen | Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions[END_REF]Fig 2). Liquids with characteristics as above cannot be produced by partial melting of common lherzolitic mantle and their origin is the subject of active research (e.g., [START_REF] Kogiso | Experimental study of clinopyroxenite partial melting and the origin of ultra-calcic melt inclusions[END_REF][START_REF] Green | Island-arc ankaramites: primitive melts from fluxed refractory lherzolitic mantle[END_REF][START_REF] Schmidt | Ultra-calcic magmas generated from Ca-depleted mantle: an experimental study on the origin of ankaramites[END_REF][START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF][START_REF] Médard | Melting of amphibole-bearing wehrlites: an experimental study on the origin of ultra-calcic nepheline-normative melts[END_REF][START_REF] Georgiev | Origin of nepheline-normative high-K ankaramites and the evolution of eastern Srednogorie arc in SE Europe[END_REF][START_REF] Marchev | High-K ankaramitic melt inclusions and lavas in the Upper Cretaceous Eastern Srednogorie continental arc, Bulgaria: implications for the genesis of arc shoshonites[END_REF][START_REF] Sorbadere | Constraints on the origin of nephelinenormative primitive magmas in island arcs inferred from olivine-hosted melt inclusion compositions[END_REF]. At present, the origin of ultra-calcic arc compositions is only constrained from experimental data obtained on a volatile-free synthetic mixture [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Experimental data on natural ultra-calcic arc rocks and in presence of volatiles are lacking. Yet, the primitive basalt from La Sommata has a bulk composition close to the synthetic mixture investigated by [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]; in particular, CaO/Al 2 O 3 and wt.% MgO are almost identical (Fig. 2). Therefore, this composition offers the opportunity to document the phase relations of a representative ultracalcic arc basalt. The experimental results presented in this paper contribute to a better experimental characterization of ultra-calcic arc melts in general and enable their origin to be further discussed. 2. Regional, petrological and volcanological background and starting material

Primitive compositions in the Aeolian arc

The Aeolian archipelago (Fig. 1) represents a Pleistocene volcanic arc whose origin is related to the westward subduction of the Ionian plate below the Calabrian-Peloritan thrust belt, in the context of the southeastern propagation of the Tyrrhenian basin, following the rollback of the Ionian slab (e.g., [START_REF] Ventura | Kinematics of the Aeolian volcanism (southern Tyrrhenian Sea) from geophysical and geological data[END_REF]. The Aeolian arc consists of seven subaerial volcanic edifices (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea and Stromboli) and several volcanic seamounts surrounding the Marsili basin (Fig. 1). The volcanic activity of the Aeolian Islands started about 1.3 My ago [START_REF] Ventura | Kinematics of the Aeolian volcanism (southern Tyrrhenian Sea) from geophysical and geological data[END_REF] and is still active on three of the seven major islands, Lipari (last eruption in 1230 AD; [START_REF] Arrighi | Eruptions of the last 2200 years at Vulcano and Vulcanello (Aeolian Islands, Italy) dated by high-accuracy paleomagnetism[END_REF], Stromboli (persistently active), and Vulcano (last eruption in 1888-1890). Rock compositions range from basalt to rhyolite and belong dominantly to the calc-alkaline, high-K calc-alkaline and shoshonitic series, more rarely to the potassic series (see for instance [START_REF] Peccerillo | Compositional variations of magmas in the Aeolian arc: implications for petrogenesis and geodynamics[END_REF]. In detail, the arc is segmented in three main sectors and magma compositions vary both at the local and regional scales, and also with time at a given volcano. There is a general agreement that the erupted mafic compositions originate from primary magmas produced by partial melting of an heterogeneous mantle source, and follow a complex polybaric evolution in the arc crust (Peccerillo et al., 2013 and references therein). Primitive magma compositions rarely occur. They include products mainly from Alicudi and Vulcano islands (Fig. 1). The calc-alkaline basalts from Alicudi have the most primitive whole-rock major and trace element characteristics of the entire Aeolian arc, with Mg# up to 73, Ni up to 150 ppm and Cr up to 800 ppm [START_REF] Peccerillo | Sr-Nd-Pb-O isotopic evidence for decreasing crustal contamination with ongoing magma evolution at Alicudi volcano (Aeolian Arc, Italy): implication for style of magma-crust interaction and mantle source compositions[END_REF]. These basalts contain 50.4-51.7 wt.% SiO 2 , 0.93-1.38 wt.% K 2 O and 8.78-9.61 wt.% MgO, have CaO/Al 2 O 3 ranging from 0.64 to 0.71, are hy-normative and plot in the field of primary melts from lherzolitic mantle (Fig. 2). Rocks with similar characteristics (48.6-50.5 wt.% SiO 2 , 0.84-1.26 wt.% K 2 O, 8.43-8.48 wt.% MgO, 70-297 ppm Cr, 68-109 ppm Ni, CaO/Al 2 O 3 b 0.5) have been dredged in the lowermost slope north of Alicudi and Filicudi [START_REF] Beccaluva | Petrology and K/Ar ages of volcanics dredged from the Eolian seamounts: implications for geodynamic evolution of the southern Tyrrhenian basin[END_REF].

At Vulcano, the La Sommata basalt is the most primitive eruptive product [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. The scoria sample used in this study (Som-1) is a shoshonitic basalt with 49.55 wt.% SiO 2 , 2.28 wt.% K 2 O and 8.78 wt.% MgO when normalized to 100% (Table 1). It is ne-normative (5% ne), has an elevated CaO/Al 2 O 3 (1.03) and plots in the ultra-calcic field [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF]Fig 2), as the La Sommata glass inclusions (MI). Numerous studies have been performed on the La Sommata MI, mostly on inclusions in olivine phenocrysts [START_REF] Métrich | Sulfur abundance and its speciation in oxidized alkaline melts[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF][START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF][START_REF] Schiano | Transcrystalline melt migration and Earth's mantle[END_REF][START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF][START_REF] Rose-Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF][START_REF] Chen | Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions[END_REF], but also in clinopyroxene [START_REF] Sonzogni | Transcrystalline melt migration in clinopyroxene[END_REF]. The La Sommata MI (Table 1) have 46.5-50.5 wt.% SiO 2 , 1.34-2.83 wt.% K 2 O and 6.08-14.30 wt.% MgO and are characterized by high CaO/Al 2 O 3 ratios between 1.0 and 1.6, typical of the ultra-calcic signature (Schiano et al., 2000, Fig. 2). These MI are strongly ne-normative (ne up to 10%) and hydrous. FTIR analyses have yielded a range of dissolved H 2 O concentrations between 1.8 and 3.8 wt.% [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. More recently, Mercier et al. (2010) analyzed 3.8-4.5 wt.% H 2 O in olivine-hosted inclusions by Raman microspectroscopy. The latest measurements have yielded 0.6-4.9 wt.% H 2 O by SIMS [START_REF] Rose-Koga | Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions[END_REF] and 1.0-5.3 wt.% H 2 O with a new Raman-FTIR calibration [START_REF] Chen | Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions[END_REF].

Primitive compositions also occur at Stromboli. Basaltic rocks there cluster near 6-7 wt.% MgO for 10-11 wt.% CaO, and a few samples have N7.5 wt.% MgO (see [START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF] for a recent compilation). The high-MgO group includes, in the recent period of activity, one golden pumice erupted during a paroxysm, and, in the older periods, one sample from the Younger Series and two dyke samples (see [START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF] and references therein, Fig. 2). The golden pumice (PST-9) has 7.71 wt.% MgO, 239 ppm Cr and 67 ppm Ni [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF][START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF]. It displays a clear calcic signature with 12.48 wt.% CaO and CaO/Al 2 O 3 = 0.80 [START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF]. In comparison, the other primitive Stromboli samples have lower CaO and CaO/Al 2 O 3 (Fig. 2). Fo-rich olivines in golden pumices host glass inclusions with ~49 wt.% SiO 2 , ~2 wt.% K 2 O and MgO up to N 8 wt.% (corrected for postentrapment crystallization). They have CaO/Al 2 O 3 ranging from 0.67 up to 1.03 (Fig. 2) and are ne-normative (~5% ne, [START_REF] Bertagnini | Stromboli volcano (Aeolian Archipelago, Italy): an open window on the deep-feeding system of a steady state basaltic volcano[END_REF][START_REF] Métrich | Conditions of magma storage, degassing and ascent at Stromboli: new insights into the volcanic plumbing system with inferences on the eruptive dynamics[END_REF][START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF], and display a strong ultra-calcic affinity [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF][START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF].

We conclude to the occurrence of two main groups of primitive magma compositions in the Aeolian arc. Primitive calc-alkaline magmas are found on Alicudi [START_REF] Peccerillo | Sr-Nd-Pb-O isotopic evidence for decreasing crustal contamination with ongoing magma evolution at Alicudi volcano (Aeolian Arc, Italy): implication for style of magma-crust interaction and mantle source compositions[END_REF], possibly also among rocks dredged on seamounts [START_REF] Beccaluva | Petrology and K/Ar ages of volcanics dredged from the Eolian seamounts: implications for geodynamic evolution of the southern Tyrrhenian basin[END_REF]. Both primitive ultracalcic rocks and MI are well represented on Vulcano and they also occur at Stromboli, although with a less marked ultra-calcic imprint. The presence of ultra-calcic characteristics both at Vulcano and Stromboli suggests that this specific geochemical signature is widespread in the Aeolian arc. It is worth mentioning that high-Ca inclusions in high Fo olivines are also found in south Italy but outside the Aeolian arc (e.g., Mt. Maletto, [START_REF] Kamenetsky | Primitive magmatism of Mt. Etna: insights from mineralogy and melt inclusions[END_REF].

The island of Vulcano: volcanological background

Vulcano is the southernmost island of the Aeolian archipelago (Fig. 1). Its geological evolution encompasses the last 130 ka (to the last 1888-1890 eruption) in seven main eruptive epochs marked by major unconformities, caldera collapses and quiescence periods (see De [START_REF] De Astis | The Aeolian volcanic district: volcanism and magmatism[END_REF]Astis et al., , 2013, for details), for details). Erupted magmas at Vulcano cover the widest compositional range of the whole Aeolian archipelago, from silica-undersaturated potassic rocks to rhyolites, and spread the high potassium calc-alkaline (HKCA) and shoshonite (SHO) suites [START_REF] Keller | Petrology of some volcanic rock series of the Aeolian Arc, southern Tyrrhenian Sea: calc-alkaline and shoshonitic associations[END_REF][START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF], 2000), with a general increase of potassium content with time. The older products (120-30 ka) are characterized by a silica content b56 wt.%, whilst the younger products (b 30 ka) include more silicic and potassic rocks [START_REF] De Astis | Geology, volcanic history and petrology of Vulcano (central Aeolian archipelago[END_REF]. The pre-30 ka activity erupted the most primitive products.

In detail, the most mafic compositions (MgO = 6.1-8.8 wt.%, De [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF] were mainly erupted between 80 and 30 ka and are related to the Il Piano Synthem, within the Piano Caldera [START_REF] De Astis | The Aeolian volcanic district: volcanism and magmatism[END_REF]. These products are represented by the Passo del Piano lava (Mg# = 0.57, CaO/Al 2 O 3 = 0.65, De [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF] and the Monte Rosso trachybasalt (Mg# = 0.52, CaO/Al 2 O 3 = 0.64-0.73; De [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF], all with lower Mg#, wt.% MgO and CaO/Al 2 O 3 than the La Sommata basalt (ca. 41 ka B.P., C.A. Tranne, personal communication, 2014). Previous analyses of this basalt (Mg# = 0.57-0.59, MgO = 8.07-8.64 wt.%, CaO/Al 2 O 3 = 0.90-1.0, De [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] are consistent with our own results (Table 1). They confirm the primitive characteristics of the La Sommata basalt and stress its specific ultra-calcic composition. [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Source of data, La Sommata: WR (Som-1) from Table 1, MI (data corrected for post-entrapment crystallization and normalized to 100%) from [START_REF] Métrich | Sulfur abundance and its speciation in oxidized alkaline melts[END_REF]; [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]; [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF][START_REF] Schiano | Transcrystalline melt migration and Earth's mantle[END_REF] 2011), MI data from [START_REF] Bertagnini | Stromboli volcano (Aeolian Archipelago, Italy): an open window on the deep-feeding system of a steady state basaltic volcano[END_REF]. Source of data, Alicudi: WR from [START_REF] Peccerillo | Plio-Quaternary volcanism in Italy[END_REF]. The curve limiting the compositional domain of lherzolite partial melts is taken from [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. The horizontal dashed line separates the ultra-calcic (CaO/Al 2 O 3 N 1) and non-ultra-calcic (CaO/ Al 2 O 3 b 1) fields. (2010). Asterisk labels the composition of the host crystals. Prim-1 is the theoretical composition of a primitive end-member magma; see text for details.

La Sommata is a scoria cone built close to the southern rim of the Caldera del Piano (Fig. 3). Its products consist of vesiculated scoriaceous lapilli, dense scoria bombs and subordinate centimetric lava lithic blocks that originated from a fluctuating strombolian-type activity. Primitive major and trace element characteristics and low 87 Sr/ 86 Sr = 0.7042 [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF] have led [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] to propose the La Sommata basalt as representative of parental magmas that fed the pre-Lentia caldera activity (older than 15 ka B.P.). However, the magmatic evolution at Vulcano is more complex than differentiation of a single parental magma and the importance of open-system and magma mixing processes has been emphasized in several studies [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF][START_REF] Ventura | Kinematics of the Aeolian volcanism (southern Tyrrhenian Sea) from geophysical and geological data[END_REF][START_REF] Del Moro | Sr, Nd, and Pb isotope evidence for open system evolution at Vulcano, Aeolian Arc, Italy[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF][START_REF] Clocchiatti | Assessment of a shallow magmatic system: the 1888-90 eruption, Vulcano Island, Italy[END_REF][START_REF] Piochi | Constraining the recent plumbing system of Vulcano (Aeolian Arc, Italy) by textural, petrological, and fractal analysis: the 1739 A.D. Pietre Cotte lava flow[END_REF].

The La Sommata basalt: petrography and mineralogy

We selected for our purposes only vesiculated scoriaceous lapilli, distinctly less porphyritic than scoriaceous bombs. Point counting showed the presence of ca. 10 vol.% phenocrysts on a vesicle-free basis (Table 1). The hybrid nature of the phenocryst population has been recognized previously and [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] estimated that the La Sommata basalt represents a product of mixing between a primitive and a more evolved shoshonitic magma in a 80:20 proportion. Accordingly, the phenocryst assemblage is divided into two groups:

(1) a basaltic assemblage, which comprises clinopyroxene (Cpx), olivine (Ol) and Cr-spinel (Sp). Diopsidic clinopyroxene (En 48-49 Fs 5-7 , X Mg = molar MgO/(MgO + FeO tot ) = 0.87-0.92) is the most abundant phase (Table 1). Olivine is Fo-rich (Fo 87-91 ). Spinel is a chromite with Cr 2 O 3 up to 52 wt.% and Cr# (at. 100 * Cr/(Cr + Al)) up to 82. Both clinopyroxene and spinel can be found included in olivine. Petrographic observations suggest cotectic crystallization of olivine and clinopyroxene phenocrysts (Fig. 4).

(2) a shoshonitic assemblage, which is characterized by phenocrysts and microphenocrysts of salitic clinopyroxene (En 36-44 Fs 10-16 ), olivine (Fo 68-84 ) and rare plagioclase (An 55-66 ).

The groundmass (bulk composition SiO 2 = 53.1 wt.%, K 2 O/Na 2 O = 1.35, Table 1) is extensively crystallized (Fig. 4). Plagioclase is very abundant and Ti-magnetite is also present.

Clinopyroxene-liquid and olivine-liquid equilibria were checked by calculating Fe-Mg mineral-liquid partition coefficients with the bulk Som-1 rock taken as the liquid composition (Som-1 glass, Table 1). A range of fO 2 between NNO + 0.5 to NNO + 2 was considered to compute the liquid FeO concentration [START_REF] Kress | The compressibility of silicate liquids containing Fe 2 O 3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox state[END_REF], yet the conclusions of the calculations are almost independent of the choice of fO 2 . The most Fs-poor Cpx and Fo-rich Ol phenocrysts of the basaltic assemblage are out of equilibrium with Som-1. Only evolved clinopyroxenes (Fs ≥ 7, X Mg ≤ 0.87) and olivines (Fo b 88) approach equilibrium with the bulk rock. So, the most primitive phenocrysts (Fs 5 , Fo 91 ) are in fact xenocrysts, interpreted to have crystallized from a liquid more primitive than Som-1. This crystallization stage presumably occurred prior to mixing with the shoshonitic magma [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. Therefore, Som-1 does not correspond exactly to an end-member primitive liquid. However, the La Sommata basalt is almost identical to typical ultra-calcic arc compositions such as the synthetic mixture CaNe experimentally investigated by [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF], being also close to compositions of MI in La Sommata clinopyroxene (Fig 2). Therefore, Som-1 can be viewed as a reasonable natural approximation of an ultracalcic arc melt.

Experimental and analytical techniques

Experimental strategy

The choice of experimental T-P-H 2 O conditions was guided by previous experimental studies on similar compositions [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF][START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF] and also by the range of water contents of the glass inclusions [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF][START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF][START_REF] Chen | Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions[END_REF]. Accordingly, experiments were carried out at pressures between 150 and 50 MPa and temperatures between 1150 and 1050 °C. An isobaric section at 150 MPa, corresponding to the highest water concentrations measured in the glass inclusions [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF][START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF][START_REF] Chen | Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions[END_REF], was investigated in detail. An isothermal section at 1150 °C was also studied, the latter being a reasonable crystallization temperature for phenocrysts in La Sommata basalt on the basis of homogenization temperatures of glass inclusions [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] and of calculations using the MELTS software [START_REF] Chen | Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions[END_REF].

Experimental redox conditions above the NNO buffer were imposed to match the oxidizing fO 2 of the La Sommata rock. Olivine-spinel oxybarometry [START_REF] Ballhaus | High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle[END_REF] yields a ΔNNO = +1.8 using mineral compositions from Table 1. In comparison, a ΔNNO = +0.72 was determined by [START_REF] Métrich | Sulfur abundance and its speciation in oxidized alkaline melts[END_REF] from S 6+ /S tot ratios in La Sommata melt inclusions, so less oxidizing but in agreement with the fO 2 range implied by olivine-spinel oxybarometry (NNNO).

Preparation of the starting material

A batch of 20 g of Som-1 scoriae was finely crushed (down to 30-50 μm size) then melted twice in a platinum crucible at 1400 °C for 3 h in air, with grinding in between. A few glass chips were taken and ground to produce the starting powdered dry glass material. Depending on experimental temperatures, capsules were made of either Au, Au 90 Pd 10 or Au 80 Pd 20 tubing (15 mm length, 2.5 mm internal diameter, 0.2 mm wall thickness). All experiments were performed under fluidpresent conditions. The initial composition of the fluid phase ranged from pure H 2 O to pure CO 2 . CO 2 was added only to reduce experimental aH 2 O and so to control the melt H 2 O content since glass inclusions in La Sommata basalt contain no detectable CO 2 [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. Distilled water was first introduced with a microsyringe, then CO 2 (as silver oxalate), and finally 30 mg of the powdered dry glass (Table 1). The capsules were then arc welded in a liquid nitrogen bath in order to prevent water loss. After welding, the capsules were weighed and then left in an oven at 110 °C for 2 h to homogenize the water distribution in the charge. For each experiment at given P-T conditions, 3 to 5 capsules were run together, each with different starting XH 2 O [= molar H 2 O/ (H 2 O + CO 2 )] ranging from 1 to 0.

Experimental equipment

All experiments were carried out in an Internally Heated Pressure Vessel (IHPV) working vertically, pressurized with H 2 -Ar gas sequentially loaded at room temperature [START_REF] Scaillet | Improvements of the Shaw membrane technique for measurement and control of ƒH 2 at high temperature and pressures[END_REF][START_REF] Scaillet | Experimental crystallization of leucogranite magmas[END_REF] and equipped with a fast quench device modified after [START_REF] Roux | A fast quench device for HIPV[END_REF]. The experimental charges were loaded into an alumina sample holder hung in the hot zone of a furnace made of a double parallel Mo winding. Pressure was recorded by a transducer calibrated against a Heise Bourdon tube gauge with an uncertainty of ± 20 bars. Two Stype thermocouples allowed a continuous control of the temperature in the top and the bottom of the hot-spot (i.e. for a length of 5 cm). The temperature gradient in the hot-spot zone was always b 5 °C. At the end of the experiment, the Pt wire hanging the sample holder was fused electrically allowing the experimental charges to fall in the cold part of the vessel in a few seconds [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF].

Control and monitoring of oxygen fugacity

The use of a H 2 -Ar gas mixture of fixed composition imposes a given partial pressure of hydrogen (ƒH 2 ) in each experiment, which in turn allows the ƒO 2 to be determined for each charge (e.g., [START_REF] Scaillet | Improvements of the Shaw membrane technique for measurement and control of ƒH 2 at high temperature and pressures[END_REF][START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF]. For a given experiment (hence fixed ƒH 2 ), the ƒO 2 of a given charge is dependent on the melt H 2 O concentration (H 2 O melt ). The ƒO 2 for each capsule was calculated from the water dissociation equilibrium (K w = fH 2 O/(fH 2 * fO 2 0.5 ) with K w taken from Robie et al.

(1979) and ƒH 2 O calculated from H 2 O melt using the model of [START_REF] Burnham | The importance of volatile constituents[END_REF]. The experimental ƒH 2 was determined by using an empirical calibration between the H 2 pressure initially loaded in the vessel and the ƒH 2 . The latter was obtained from other experimental runs performed at the same P-T conditions using the same vessel and procedure than in the present study, and where experimental ƒH 2 were determined with Ni-Pd sensors (e.g., Di [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF].

Analytical techniques

Experimental charges were mounted in epoxy resin, polished and examined with a scanning electron microscope (SEM-EDS) for the identification of mineral phases, textural observations, and to check for quench crystallization. An electron microprobe (EPMA) was used for the determination of phase compositions. Analytical conditions were as follows: (i) SEM-EDS (Oxford LEO 440 at University of Palermo): acceleration voltage 20 kV, beam current 600 pA; (ii) EMP (Cameca SX50 and SX Five at University of Orléans: acceleration voltage 15 kV, sample current 6 nA). A focused beam was used for crystals whilst for glasses a defocused beam of 10 μm was adopted to minimize alkali migration.

The water contents of the experimental glasses were determined using the "by-difference" method [START_REF] Devine | Comparison of microanalytical methods for estimating H 2 O contents of silicic volcanic glasses[END_REF]. For each analytical session, a calibration curve was established using a set of secondary basaltic glass standards of known H 2 O and alkali contents (PST-9, Di Carlo et al., 2006). This method yields an uncertainty typically from ± 0.5 to ± 1.0 wt.% H 2 O depending on glass analytical totals. These estimates were checked against H 2 O concentration measurements by FTIR on experimental glasses from 4 near-liquidus charges. The FTIR absorption spectra were acquired on a Hyperion 2000 Bruker FTIR (at University of Palermo). Measurements were performed on doublypolished experimental glasses employing a Globar source, a KBr beam splitter and an HgCdTe detector. 256 scans were accumulated for each spectrum with a resolution of 2 cm -1 . The water content was derived from the Beer-Lambert law using the band at 3550 cm -1 (H 2 O tot ) and a molar extinction coefficient (ε) of 43.96 L mol -1 cm -1 [START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF]. The density of the starting volatile-free glass (2718 ± 11 kg m -3 ) was measured by using a Mettler Toledo Density kit and a high precision balance at University of Orléans. For the volatilebearing experimental glasses, densities were calculated using an iterative method [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF].

Experimental results

The experiments (24 charges in total) are divided into two groups: (i) isobaric experiments at 150 MPa and (ii) isothermal experiments at 1150 °C. The data used for the construction of the phase diagrams are detailed in Table 2. In this study, melt H 2 O contents were varied from 0.7 to 3.5 wt.% (Table 2). The FTIR data are in good agreement with the H 2 O concentrations estimated with the "by difference" method (Table 2), which validates our "by-difference" estimations of the glass H 2 O content. The oxygen fugacity is in the range NNO + 0.2 to + 1.9 (Table 2), encompassing the range of fO 2 estimated for the La Sommata basalt. There is no systematic difference in fO 2 between the isobaric and isothermal experiments.

Attainment of equilibrium, quench crystallization and iron loss

Run duration ranged between 20 to 26 h, which is considered long enough to achieve crystal-melt equilibrium in basaltic compositions under our P-T conditions (e.g. Di [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF][START_REF] Barclay | A hornblende basalt from western Mexico: watersaturated phase relations constrain a pressure-temperature window of eruptibility[END_REF]. This is witnessed by a number of textural and chemical features, among which: (i) the shapes of crystals: clinopyroxene, olivine and plagioclase crystals have euhedral and equant shapes suggesting growth at small degrees of undercooling [START_REF] Muncill | Crystal-growth kinetics of plagioclase in igneous systems: one-atmosphere experiments and application of a simplified growth model[END_REF]; (ii) uniform crystal distributions in experimental products (neither crystal settling nor seriate crystal sizes were observed); (iii) homogeneity of glass composition throughout each charge; (iv) consistent changes of the melt composition with T, P, H 2 O melt ; (v) low residuals of the mass balance calculations (Table 2); (vi) nearequilibrium mineral-melt partition coefficients. The Fe-Mg crystalliquid distribution coefficient (K d Fe-Mg cpx-liq, Table 2) is 0.29 ± 0.05 for clinopyroxene-liquid (calculated using all Fe as FeO tot in both crystal and melt), close to values in recent crystallization experiments on basaltic compositions (0.25, [START_REF] Grove | Magmatic processes that generated the rhyolite of Glass Mountain[END_REF]0.31, Di Carlo et al., 2006). For olivine-liquid, the distribution coefficient (K d Fe-Mg ol-liq ) is 0.25 ± 0.03 using melt FeO calculated from [START_REF] Kress | The compressibility of silicate liquids containing Fe 2 O 3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox state[END_REF], which is slightly lower (but close) than the expected range for olivine in basaltic liquids (0.30, [START_REF] Grove | Magmatic processes that generated the rhyolite of Glass Mountain[END_REF]0.33, Pichavant et al., 2002;0.28, Barclay and Carmichael, 2004;0.36, Di Carlo et al., 2006;0.32, Pichavant and Macdonald, 2007). For plagioclase, the Na-Ca crystalliquid distribution coefficient determined for charge #5-5 (Table 2, K d Na-Ca plag-liq = 1.5) is in agreement with experimental data under similar P-T conditions and H 2 O melt [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF].

The use of the fast quench device dramatically reduced the importance of quench crystallization [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF]. Nevertheless, SEM observations revealed the presence of quench phases growing on Fe-oxides in some charges (runs #3-1, #3-2, #3-3, #5-1, #52, Table 2). Glass in charge #5-1 is anomalously low in MgO and K 2 O, and the other charges with quench crystals are characterized by K d Fe-Mg cpx-liq and K d Fe-Mg ol-liq slightly lower than average (Table 2). The amount of Fe loss was obtained from the mass balance calculations and is expressed as the relative difference between the FeO tot of the starting glass and that calculated by summing FeO tot concentrations for all phases present in the charge (ΔFeO, Table 2). The resulting average value is -3.8%, whilst the range is -0.3 to -11.5%. Six on a total of 24 charges have N5% Fe loss and one N10%. We conclude that Fe loss was kept to a low level in our experiments.

Mineral phases

Mineral phases encountered include a Fe-oxide, clinopyroxene, olivine and plagioclase. Crystal sizes rarely exceeded 20 μm. Olivine reached sizes ~100 μm in near-liquidus charges, but is generally smaller when the proportion of crystals increases (Fig. 5). Exceptionally large plagioclase crystals (~100 μm) were found in one charge (#5-5, Table 2). Feoxides have very small sizes (less than 10 μm and as small as 1-2 μm) which make their identification difficult, especially in charges with low melt H 2 O content (#2-4, 3-5, 6-4; Table 2; Fig. 5).

Phase relationships at 150 MPa

The isobaric phase equilibrium diagram is shown in Fig. 6. Solid lines represent mineral saturation curves constructed using the data set from Table 2. All saturation curves have strongly negative slopes, as found in previous studies [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF]. Over all the investigated T-H 2 O Fig. 5. Back-scattered electron microphotograph of experimental charge #2-4 illustrating the crystallization of a cotectic (Cpx + Ol co-saturated) assemblage. Experimental conditions in Table 2. Clinopyroxene (cpx) is Fs 9 and olivine (ol) Fo 89 (data in Table 3). Notice the presence of small Fe-oxide crystals included in olivine and the lack of quench crystals. section, Fe-oxide is the liquidus phase, followed by clinopyroxene, olivine, and plagioclase. However, Fe-oxide was not positively detected in all charges because of its very small size. There are also differences in fO 2 between charges that could influence Fe-oxide stability. Clinopyroxene and olivine crystallize very close to each other (Fig. 6); only one charge (#2-3, Table 2) demonstrates that clinopyroxene precedes olivine in the crystallization sequence. The H 2 O content of cotectic (olivine-and clinopyroxene-saturated) liquids increases with decreasing temperature from 2.7 wt.% (1150 °C), 2.9 wt.% (1120 °C) to 3.3 wt.% (1080 °C). The melt H 2 O content at plagioclase saturation decreases from 2.8 wt.% (1050 °C) to about 1.2 wt.% (1120 °C). Conditions of H 2 O-saturation are defined by the CO 2 -free charges #2-1, 5-1, and 8-1. For these charges, the melt H 2 O concentrations (some being duplicated by FTIR measurements) range between 3.3 and 3.6 wt.% H 2 O at 1150 °C (Table 2). These values are in good agreement with H 2 O solubilities calculated for the specific melt compositions with the model of [START_REF] Burnham | The importance of volatile constituents[END_REF].

Phase relationships at 1150 °C

The isothermal phase diagram is shown in Fig. 7. Three pressure conditions were explored at 1150 °C: 150, ~100 and ~50 MPa. In this representation, saturation curves have positive slopes. Our data do not reveal any change in the sequence of crystallization with pressure but the clinopyroxene and olivine saturation curves become closer to each other with decreasing pressure (see below). With decreasing melt H 2 O content, Fe-oxide is followed by clinopyroxene, olivine and plagioclase. Clinopyroxene and olivine saturation curves have steep slopes and are very close to each other. The H 2 O content of cotectic liquids decreases with decreasing pressure from about 2.4 wt.% (150 MPa), 2.2 wt.% (100 MPa) to 1.6 wt.% (50 MPa). It is worth emphasizing that plagioclase was not encountered at 1150 °C. Therefore, the plagioclase saturation curve (Fig 7) is estimated. The vapor-saturation curve is defined by the charges #2-1, 3-1 and 61 which have melt H 2 O concentrations in close agreement with solubilities calculated with the model of [START_REF] Burnham | The importance of volatile constituents[END_REF].

Proportions and compositions of crystals

Proportions of crystalline phases range from ~0 (several charges are above the liquidus, Table 2) to 54% (charge #8-3, Table 2), generally increasing with decreasing temperature, pressure and melt H 2 O content. Clinopyroxene is the most abundant mineral phase (up to 50 wt.%, in charge #8-3), followed by Fe-oxide (up to 5 wt.%) and olivine (up to 3 wt.%). Plagioclase is never very abundant, being present only at low levels (b1 wt.%).

The Fe-oxide proportion tends to decrease with increasing crystallinity. Because of its small size, Fe-oxide could not be analyzed nor unequivocally detected in all charges. This phase is a Ti-, Al-, Mg-, and Cr-bearing magnetite (Tables 3,4). TiO 2 and Al 2 O 3 , which are positively correlated, range from 0.58 to 3.08 and 4.39 to 10.92 wt.%, respectively, both slightly increasing with increasing crystallinity. However, there is also an influence of fO 2 : increasing fO 2 decreases the TiO 2 and Al 2 O 3 concentration in Fe-oxide. The MgO concentration ranges from 12.4 wt.% at near-liquidus conditions (T = 1150 °C) to 7.23 wt.% at lower temperatures (T = 1050 °C), and is positively correlated with ƒO 2 . Fe-oxide is relatively Cr-rich (6.88 wt.% Cr 2 O 3 ) at near-liquidus conditions, but the Cr concentration drops to below detection at lower temperatures. It is emphasized that chromite compositions typical of the basaltic phenocryst assemblage have not been found in experimental products due to the low Cr concentration of the starting material (~250 ppm). This is consistent with our previous conclusion that Som-1 bulk rock does not correspond exactly to an end-member primitive liquid despite nearprimary compositions (olivine, chromite, clinopyroxene, melt inclusions) being present.

Clinopyroxene composition (Tables 3,4) ranges from diopside (Wo 47 En 46 Fs 7 , X Mg = 0.87) to augite (Wo 44 En 40 Fs 16 , X Mg = 0.72). TiO 2 , Al 2 O 3 and Cr 2 O 3 contents range from 0.22 to 0.73, 2.72 to 7.17 and from 0 to 0.71 wt.%, respectively. With progressive crystallization, X Mg decreases, Wo slightly decreases, TiO 2 and Al 2 O 3 increase and Cr 2 O 3 drops to below detection. The Ca (apfu) in pyroxene shows a weak positive correlation with the melt H 2 O content.

Olivine has Fo between 78 and 92, 0.49 ± 0.16 wt.% CaO and 0.3 ± 0.08 wt.% MnO. The Fo content of experimental olivine is governed by two parameters, fO 2 (or ΔNNO) and the crystallinity of the charge (or the MgO content of the melt). Fo increases with increasing fO 2 and decreases with the proportion of crystals, this latter variable being itself determined by the experimental parameters (P, T, H 2 O melt ). The most Fo-rich olivines correspond to the most oxidized charges (ΔNNO ~1.5) with crystallinities between 10 and 20% (charges #5-2, #3-2, Table 2). Crystal-poor (b10%) charges with ΔNNO between 2. Squares give conditions of experimental charges and types of phase assemblages encountered. Open square: phase is absent; filled square: phase is present. Note that Fe-oxide occurs in most charges although its presence could not be confirmed in all because of its small size. L: liquid; Cpx: clinopyroxene; Ol: olivine; Plag: plagioclase. All charges include a fluid phase (see text). Pure H 2 O vapour is designated as V and the L -L + V phase boundary is the H 2 O solubility curve. 2. Squares give conditions of experimental charges and types of phase assemblages encountered. Open square: phase is absent; filled square: phase is present. Note that Fe-oxide occurs in most charges although its presence could not be confirmed in all because of its small size. L: liquid; Cpx: clinopyroxene; Ol: olivine; Plag: plagioclase. All charges include a fluid phase (see text). Pure H 2 O vapour is designated as V and the L -L + V phase boundary is the H 2 O solubility curve. The location of the plagioclase saturation curve (dashed) is estimated. + 0.5 and + 1 yield Fo ~90 (charge #6-3, Table 2) and olivines with Fo b 85 all come from charges with ΔNNO b +1 and proportions of crystals between 21 and 54%.

Plagioclase was satisfactorily analyzed only in charge #5-5 (An 67 , Table 3).

Experimental glass compositions

Upon progressive crystallization, experimental glasses shift from Som-1 towards more evolved compositions. Taking alternatively MgO or CaO/Al 2 O 3 as a differentiation index, SiO 2 , Al 2 O 3 , Na 2 O, K 2 O and P 2 O 5 increase with differentiation whereas CaO and MgO decrease, the latter from 8.8 to 3.2 wt.% (Tables 34). FeO tot and TiO 2 show little variations. In the most evolved glasses, SiO 2 contents reach ~55 wt.%, Al 2 O 3 ~18 wt.% and K 2 O ~5.5 wt.%, in the range of latitic whole-rock compositions at Vulcano [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF], and CaO/Al 2 O 3 drops from ~1 to 0.36 (Tables 34). Glass in charge #2-3 (L + Cpx phase assemblage) plots in the range of glasses in L + Cpx + Ol charges, the latter covering most of the glass chemical range, from 8.2 to 3.5 wt.% MgO. Glasses in L + Cpx + Ol + Plag charges generally plot at the evolved ends (highest SiO 2 , Al 2 O 3 and K 2 O) of compositional trends.

Cotectic (Cpx + Ol co-saturated) glasses at 1150 °C have CaO decreasing with the MgO concentration (Fig. 8). At a given MgO, the CaO concentration systematically increases with decreasing pressure from 154, 88 to 44 MPa, in agreement with previous results [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF]. The Som-1 composition plots on extensions of the 44 and 88 MPa cotectics, suggesting that, in this range of pressures and for a temperature of 1150 °C, clinopyroxene and olivine could appear together on the Som-1 liquidus. Therefore, the clinopyroxene and olivine saturation curves (Fig. 7) have been drawn closer to each other with decreasing pressure. Relative to the extension of the 154 MPa cotectic, Som-1 plots in the Cpx field, consistent with the determined phase relations (clinopyroxene slightly earlier than olivine at 150 MPa, Fig. 6).

Magmatic evolution at Vulcano

Below, experimental results on Som-1 are used to constrain the magmatic evolution at Vulcano. To do so, an inverse approach will be applied, i.e., the Vulcano differentiation sequence will be examined "uphill", beginning with relatively evolved compositions to progressively constrain the characteristics of primitive, possibly primary, melts, and discuss their origin as well as the nature of the early differentiation processes.

Magma differentiation

First, experimental glasses are compared with the La Sommata MI compositions (Fig. 8). The 44 MPa cotectic experimental glasses and the glass inclusions in clinopyroxene overlap. This suggests that small degrees of crystallization of a Som-1 type melt at low pressures can generate liquid compositions similar to MI trapped in clinopyroxene. Thus, MI in clinopyroxene simply represent melts generated during the initial stages of crystallization-differentiation of Som-1 like shoshonitic basaltic liquids. It is worth emphasizing here that MI in clinopyroxene have a less marked ultra-calcic character (lower CaO and CaO/Al 2 O 3 ) than MI in olivine (Figs. 2,8). The latter have CaO and CaO/Al 2 O 3 markedly higher than all experimental glasses and they follow a slightly negative trend in the CaO vs. MgO diagram (Fig. 8) that contrasts with the positive trend for cotectic glasses and clinopyroxene MI. In other diagrams (e.g., CaO/Al 2 O 3 vs. SiO 2 , Al 2 O 3 or K 2 O, not shown), MI in olivine plot opposite to experimental differentiation trends, extending to SiO 2 , Al 2 O 3 and K 2 O lower than in Som-1. Clearly, glass inclusions in olivine differ from melts generated from the crystallization of Som-1 type melts.

Second, experimental glasses are compared with whole-rock compositions erupted at Vulcano. To do so, products from the Caldera del Piano [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF] and from the opening phases of the Caldera della Fossa [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF], both broadly contemporaneous with the La Sommata eruption, have been selected. In the CaO/Al 2 O 3 vs. MgO plot (Fig. 9), as well as in other diagrams (not shown), experimental glasses and whole-rock compositions closely overlap. This indicates that progressive crystallization of Som-1 type magmas can generate differentiated melts (Fig. 9), in the range of latitic whole-rocks on Vulcano (i.e., N 56 wt.% SiO 2 , b 2 wt.% MgO, De Astis et al., 1997). We conclude that primitive melts such as the La Sommata basalt can be parental both to melts trapped in clinopyroxene and to the primitive part (i.e., 48-55 wt.% SiO 2 ) of the shoshonitic sequence at Vulcano.

Composition of the primitive end-member magma

Conclusions above about magma differentiation leave open the questions of (1) the composition of the primitive end-member magma at Vulcano, (2) the mechanism of generation of melts such as the olivine MI and (3) the mutual relationships between MI in olivine, 4, normalized to 100 wt.%) are distinguished by pressure (44, 88, 154 MPa, Table 2). Melt inclusion data as in Fig. 1. The compositions of the La Sommata basalt (Som-1) and of the theoretical primitive end-member magma (Prim-1) are shown (see Table 1). [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. The compositions of the La Sommata basalt (Som-1) and of the theoretical primitive endmember magma (Prim-1) are shown (see Table 1).

on the one hand, and MI in clinopyroxene, on the other hand. These are important issues since MI in La Sommata olivine are being used as reference samples for a variety of purposes [START_REF] Schiano | Transcrystalline melt migration and Earth's mantle[END_REF][START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF] and, therefore, their compositional significance needs to be established.

The theoretical composition of the primitive end-member basaltic melt at Vulcano has been estimated starting from the suggestion [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] that the La Sommata basalt represents a mixture between a primitive and a shoshonitic magma in a 80:20 proportion. Accordingly, 20% shoshonite has been subtracted from the Som-1 composition (Table 1) to generate the composition of the primitive endmember magma. To do so, different shoshonitic end-members were tested [START_REF] De Astis | Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)[END_REF][START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] and different endmember magma compositions calculated. However, the results are not strongly dependent on the particular choice of the shoshonitic endmember and a representative composition of the primitive endmember basaltic melt (Prim-1) is given in Table 1. It has 47.7 wt.% SiO 2 , 1.90 wt.% K 2 O and 10.25 wt.% MgO which would classify it as a shoshonitic basalt. Compared to Som-1, Prim-1 is shifted towards lower SiO 2 , Al 2 O 3 and K 2 O, and higher CaO and MgO. Fo 90.8 olivine is calculated to be at equilibrium with Prim-1 (calculations performed at NNO + 1 and with K d Fe-Mg ol-liq = 0.25, see above), in the range of the most Fo-rich crystals in the La Sommata basalt. Prim-1 has a very elevated CaO/Al 2 O 3 (1.30), is strongly ne-normative (10% ne) and plots in the middle of the olivine MI group (Figs. 8,9). We conclude that the theoretical composition of the primitive end-member is virtually identical to the average of MI in La Sommata olivines. This strongly suggests that those MI are representative of end-member primitive basaltic liquids present at Vulcano, in particular during the pre-Lentia caldera activity. MI in La Sommata olivine thus represent remnants of those primitive parental melts which underwent mixing with more differentiated products to eventually produce Som-1 type magmas. Upon subsequent differentiation, compositions such as the clinopyroxene MI and the least fractionated terms of the shoshonitic series were generated (Fig. 10).

Origin of the ultra-calcic arc melt signature

We stress that the end-member primitive basaltic melt defined above has a marked ultra-calcic affinity (SiO 2 b 48 wt.%, Na 2 O + K 2 O N 3 wt.%, MgO = 10.25 wt.%, CaO/Al 2 O 3 ≫ 1, 10% normative ne, [START_REF] Schiano | Primitive CaO-rich, silicaundersaturated melts in island arcs: evidence for the involvement of clinopyroxene-rich lithologies in the petrogenesis of arc magmas[END_REF]. Therefore, the origin of such primitive compositions needs to be clarified. Below, we combine our experimental results on Som-1 with evidence from Aeolian arc volcanoes to propose a revised interpretation for the origin of ultra-calcic arc melt compositions.

6.1. Liquidus phase equilibria of the La Sommata basalt Som-1

In our experiments on hydrous Som-1 melt, clinopyroxene, olivine and plagioclase occur as the major crystallizing phases. Three experimental assemblages have been found: L + Cpx, L + Cpx + Ol and L + Cpx + Ol + Plag. Except above the liquidus, clinopyroxene-free assemblages are absent. Note that Fe-oxide should be added to the three assemblages above, although its presence has not been confirmed in all charges. Orthopyroxene (Opx) has not been encountered, despite experiments being performed well below the liquidus and residual melts reaching SiO 2 contents ~55 wt.% (compare with other phase equilibrium studies on primitive basalts, e.g., [START_REF] Pichavant | Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent, Lesser Antilles arc[END_REF].

One characteristic feature of experimental phase equilibria for Som-1 is that clinopyroxene is the liquidus phase at 150 MPa, an unexpectedly low pressure range on the basis of previous experimental studies on basalts (e.g. [START_REF] Gust | Phase relations of a high-Mg basalt from the Aleutian island arc: implications for primary island arc basalts and high-Al basalts[END_REF][START_REF] Falloon | Refractory magmas in back-arc basin settingsexperimental constraints on the petrogenesis of a Lau Basin example[END_REF][START_REF] Pichavant | Source and H 2 O content of high-MgO magmas in island arc settings: an experimental study of a primitive calc-alkaline basalt from St. Vincent, Lesser Antilles arc[END_REF]. Clinopyroxene (and also orthopyroxene for primary melts from a lherzolite source) is expected to saturate at relatively high pressures on the liquidus of primary basalts. Olivine would saturate at lower pressures and, for primary basaltic melts, multiple saturation points are usually found in the range 1-1.2 GPa, or higher if the system is hydrous (e.g. [START_REF] Pichavant | Source and H 2 O content of high-MgO magmas in island arc settings: an experimental study of a primitive calc-alkaline basalt from St. Vincent, Lesser Antilles arc[END_REF]. In the case of Som-1, co-saturation of olivine and clinopyroxene on the liquidus occurs at very low pressures. At 150 MPa, Cpx and Ol are very close to appear together on the liquidus, only one charge demonstrating that Cpx crystallizes earlier than Ol (Fig. 6). At 1150 °C, Ol would join Cpx on the hydrous Som-1 liquidus at a pressure lower than 150 MPa, most probably between 44 and 88 MPa since the Som-1 bulk composition lies in between the extension of the 44 and 88 MPa cotectics (Figs. 7,8). Such a very low pressure range for Ol and Cpx co-saturation, and the total absence of Opx on the liquidus, are very unusual features for a relatively primitive basalt.

To check whether our experimental results are influenced by the particular choice of Som-1 as starting material, MELTS calculations have been performed on the Prim-1 composition. Near liquidus phase equilibria were computed [START_REF] Ghiorso | Chemical mass transfer in magmatic processes. IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures[END_REF] on Prim-1 and also for consistency on Som-1. The calculations were performed for different pressures, constant H 2 O melt = 3.5 wt.% and a fO 2 set at NNO + 1. For Som-1 at 150 MPa, MELTS finds Ol to appear first on the liquidus at 1178 °C; Cpx comes in second at 1145 °C. For Prim-1, Ol is also the liquidus phase, crystallizing at 1194 °C and Cpx at 1160 °C so that Cpx Interaction between a primary calk-alkaline (?) melt and wehrlite or gabbroic wall-rock lithologies in the deep part of the plumbing system generates ultra-calcic melts. The ultra-calcic melts then ascend to the upper accumulation zone where it mixes with differentiated magmas (shoshonite-latite) to produce the La Sommata basalt. See text for details.

appears either 33 (Som-1) or 34 °C (Prim-1) below the 150 MPa liquidus. Upon increasing pressure, Cpx joins Ol on the liquidus, at either 520 MPa (Som-1) or 560 MPa (Prim-1). Therefore, although MELTS fails to reproduce our experimental results, pressures ≪ 1 GPa are confirmed for Cpx + Ol co-saturation, both for Prim-1 and Som-1. Most importantly, MELTS calculations do not support a major difference in phase relations between Som-1 and Prim-1, which indicates that our experimental results are representative and applicable to discuss the origin of primitive ultra-calcic magmas at Vulcano. 6.2. Liquidus phase equilibria of ultra-calcic arc melts: comparison between synthetic and natural compositions [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF] carried out near-liquidus experiments on a synthetic composition (CaNe) representative of ultra-calcic arc compositions. Cpx was found as the liquidus phase at 500 and 700 MPa, and Ol as the liquidus phase at 0.1 MPa. A pressure of ~200 MPa was determined for Cpx + Ol co-saturation on the CaNe liquidus at a temperature of 1220 °C. The third phase to appear in the crystallization sequence was plagioclase and orthopyroxene was not encountered [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Therefore, phase relations determined in this study for the La Sommata basalt are identical in critical aspects to those obtained on the synthetic CaNe ultra-calcic composition [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Liquidus temperatures higher than in this work were found in [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF], a consequence of the use of nominally anhydrous compositions in the latter study. We conclude that experiments on both natural (Som-1) and synthetic (CaNe) arc ultra-calcic compositions yield phase equilibrium results that are essentially identical.

Origin of ultra-calcic arc melts

Ultra-calcic arc melts have been initially interpreted as primary compositions generated by melting of specific mantle sources. Experimental results have provided constraints on the nature of possible source rocks (e.g., [START_REF] Kogiso | Experimental study of clinopyroxenite partial melting and the origin of ultra-calcic melt inclusions[END_REF][START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF][START_REF] Médard | Melting of amphibole-bearing wehrlites: an experimental study on the origin of ultra-calcic nepheline-normative melts[END_REF][START_REF] Sorbadere | Constraints on the origin of nephelinenormative primitive magmas in island arcs inferred from olivine-hosted melt inclusion compositions[END_REF]. For their synthetic CaNe ultra-calcic arc composition, [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF] found either Cpx or Ol crystallizing on the liquidus, consistent with an origin by partial melting of a source rock leaving either clinopyroxene or olivine as residual phases. They noted the very low pressure (~200 MPa) of Cpx + Ol co-saturation on the CaNe liquidus. Such an abnormally low pressure range was explained by olivine fractionation, the CaNe composition investigated being olivine-depleted, yet a maximum pressure of 1 GPa was inferred for CaNe to be at equilibrium with a wehrlitic residual assemblage [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Therefore, it was concluded that ultra-calcic melts must be generated in the arc crust [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. This interpretation is clearly strengthened by our experimental results. Cosaturation of Cpx + Ol on the liquidus of Som-1 occurs at very low pressures, being located between 44 and 88 MPa at 1150 °C (see above). Although Som-1 is not exactly representative of the end-member ultra-calcic melt at Vulcano (because of mixing with the shoshonitic magma, see above), MELTS calculations for Som-1 and Prim-1 yield Cpx + Ol co-saturation pressures that are within 50 MPa of each other. Therefore, an end-member primitive ultra-calcic arc melt such as Prim-1 presumably would be co-saturated with a wehrlite mineral assemblage at pressures not much higher than Som-1, i.e., ≪ 1 GPa. The inverse dependence of the CaO content of cotectic melts with pressure (Fig. 8; see also [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF] provides a simple yet powerful indication for ultra-calcic melts being generated at very low pressures: for a composition such as Som-1 to be cosaturated with a wehrlitic phase assemblage, pressures ≤100 MPa are necessary.

If ultra-calcic arc melts indeed represent primary compositions, then both the results of [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF] and of this study require another mineral phase to be involved in the partial melting process. Amphibole was proposed by [START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF] to be an ideal candidate because it would impart to the partial melt the ne-normative, alkali-rich, character typical of the ultra-calcic signature. The interpretation originally proposed for the origin of ultra-calcic arc melts involves sequential melting of an amphibole wehrlite source [START_REF] Médard | Melting of amphibole-bearing wehrlites: an experimental study on the origin of ultra-calcic nepheline-normative melts[END_REF][START_REF] Sorbadere | Constraints on the origin of nephelinenormative primitive magmas in island arcs inferred from olivine-hosted melt inclusion compositions[END_REF]. Amphibole would be the first phase to be consumed during the melting process, leaving a refractory wehrlitic residual mineralogy, consistent with the liquidus phase equilibria determined for both CaNe and Som-1, and yielding a melt whose composition is controlled by the proportion of amphibole in the source and the peak melting temperature [START_REF] Kogiso | Experimental study of clinopyroxenite partial melting and the origin of ultra-calcic melt inclusions[END_REF].

However, ultra-calcic compositions might also represent derivative (i.e., non-primary) melts. [START_REF] Danyushevsky | Melt inclusions in primitive olivine phenocrysts: the role of localized reaction processes in the origin of anomalous compositions[END_REF] have shown that ultra-calcic glass inclusion compositions can originate during complex localized dissolution-reaction-mixing processes within magmatic plumbing systems. This type of process occurs at the margins of primitive magma bodies, where newly injected, hotter, primitive magma comes into contact with previously solidified, cooler, wall-rocks, either cumulates or crystal mushes. Dissolution of mineral phases from the mush, mixing of reaction products with the primitive melt, and coupled crystallization of olivine from the primitive magma, may lead to the trapping of the generated ultra-calcic melts as MI.

One way to distinguish between the two types of models (primary vs. non-primary) is to constrain the pressure range to generate compositions such as Som-1 and Prim-1. From our experimental results, Som-1 can be generated at equilibrium with a wehrlite assemblage at ≤100 MPa and Prim-1 at a pressure slightly higher. We note, however, that the data also make it possible that Som-1 (and Prim-1) are generated at equilibrium with a clinopyroxenite assemblage which would imply higher pressures. Nevertheless, in this case, clinopyroxene would be much earlier than olivine in the crystallization sequence, which is observed neither in the experiments (Figs. 6,7) nor in the rocks (compositions of the most primitive Ol and Cpx are broadly in equilibrium and textural evidence suggests cotectic crystallization). Therefore, the only possibility remaining is that the ultra-calcic melt is generated at equilibrium with a cotectic (wehrlite) assemblage, implying that the pressure of crystallization of the cotectic assemblage (for appropriate values of temperature and melt H 2 O concentration) gives the pressure of melt generation. Considering the pressure range implied by our results (for Som-1 ≤ 100 MPa at 1150 °C and 2.5 wt.% H 2 O, Fig. 7), it seems difficult to consider ultra-calcic arc compositions as primary melts generated at mantle depths. Rather, we propose that these specific compositions record the interaction between primitive, presumably peridotite-derived melts and clinopyroxene-rich wallrocks, cumulates or mushes, that are present in the arc crust, most probably as products of the crystallization of older magma batches. A similar two-component model was proposed by [START_REF] Sorbadere | Constraints on the origin of nephelinenormative primitive magmas in island arcs inferred from olivine-hosted melt inclusion compositions[END_REF] who showed that ultra-calcic arc melts are products of mixing between partial melts from peridotite and from amphibole-bearing clinopyroxenite.

Evidence from the Aeolian arc

Both Vulcano and Stromboli offer the opportunity to apply and refine the general model above. Concerning Vulcano, our experimental results are combined with data on La Sommata MI and results of MELTS calculations to infer the pressure of crystallization of the cotectic clinopyroxene + olivine assemblage in Prim-1 which, from above, determines the depth of the melt-wall-rock interaction process that generates the ultra-calcic liquids. [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] reported a range of H 2 O concentrations of 1.8-3.8% in La Sommata olivine MI and determined MI homogenization temperatures (T h ). The recent results of [START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF] imply higher H 2 O melt , 3.8-4.5 wt.%. [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] reported systematically higher T h for olivine than for clinopyroxene MI. However, the high T h (N1250 °C) found for MI in olivine most probably reflect loss of volatiles during heating [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] since clinopyroxene and olivine crystallize very close to each other in Som-1 (Figs. 4,6,7). The gaussian T h distribution (centered at 1160 °C) for MI in clinopyroxene suggests that those MI record temperatures the closer to real trapping values [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF].

Conditions of crystallization of the cotectic assemblage can be constrained from experimental phase equilibria. It is readily apparent from the isothermal section (Fig. 7) that co-precipitation of clinopyroxene + olivine at 1150 °C requires a pressure of 150 MPa minimum for H 2 O melt = 2.7 wt.%, the cosaturation pressure increasing with H 2 O melt . At 150 MPa, a temperature beyond 1150 °C would be necessary for the co-precipitation of clinopyroxene + olivine if H 2 O melt is b 3 wt.% (i.e., 1180-1200 °C for 2.5 wt.% H 2 O, Fig. 6). Assuming 1160 °C (the median of T h for clinopyroxene MI, [START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF] as the temperature of co-precipitation of clinopyroxene and olivine and taking a range of H 2 O melt between 3.5 and 4.0 wt.% [START_REF] Mercier | Spectroscopic analysis (FTIR, RAMAN) of water in mafic and intermediate glasses and glass inclusions[END_REF], pressures N150 MPa are required for the crystallization of the cotectic assemblage in Prim-1. Using a P-T-H 2 O melt grid constructed from our experimental data (see Di [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF]their Figure 11) and incorporating temperature corrections from our MELTS calculations to account for the compositional difference between Som-1 and Prim-1, a pressure range between 250 and 350 MPa is inferred for Cpx + Ol co-saturation in Prim-1. In the context of the Aeolian Arc, this corresponds to middle crustal pressures (i.e., to 9-13 km with a crustal density of 2700 kg m -3 ) since the Moho beneath Vulcano is located at ~20 km depth [START_REF] De Astis | Geology, volcanic history and petrology of Vulcano (central Aeolian archipelago[END_REF].

The model proposed for La Sommata involves interaction between a primary melt generated in a presumably peridotitic mantle wedge and wehrlitic (or gabbroic) rocks residing in the middle to lower parts of the Aeolian arc crust. Evidence for wehrlite is missing at Vulcano although wehrlitic and clinopyroxenitic enclaves are known on several other Aeolian islands [START_REF] Peccerillo | Sr-Nd-Pb-O isotopic evidence for decreasing crustal contamination with ongoing magma evolution at Alicudi volcano (Aeolian Arc, Italy): implication for style of magma-crust interaction and mantle source compositions[END_REF][START_REF] Laiolo | Mafic and ultramafic xenoliths in San Bartolo lava field: new insights on the ascent and storage of Stromboli magmas[END_REF]. Gabbroic bodies occur both at Vulcano [START_REF] Faraone | The monzogabbroic intrusion in the Island of Vulcano, Aeolian Archipelago, Italy[END_REF] and on other islands [START_REF] Salvioli-Mariani | Silicate melt inclusions in the cumulate minerals of gabbroic nodules from Stromboli volcano (Aeolian Islands, Italy): main components of the fluid phase and crystallization temperatures[END_REF][START_REF] Mattioli | Sub-volcanic infiltration and syneruptive quenching of liquids in cumulate wall-rocks: the example of the gabbroic nodules of Stromboli (Aeolian islands, Italy)[END_REF][START_REF] Laiolo | Mafic and ultramafic xenoliths in San Bartolo lava field: new insights on the ascent and storage of Stromboli magmas[END_REF]. The melt-wall-rock interaction process would produce a three component mixture between a primary (unreacted) melt (not observed at Vulcano), an ultra-calcic melt locally present within the melt-mush reaction zone and an assemblage of newly formed clinopyroxene and olivine crystals. From the pressure range estimated above, the interaction process would take place in the lower part of the plumbing system, most probably in the lower magma accumulation zone, located at 190-350 MPa [START_REF] Clocchiatti | Assessment of a shallow magmatic system: the 1888-90 eruption, Vulcano Island, Italy[END_REF], 250-500 MPa [START_REF] Zanon | Magmatic feeding system and crustal magma accumulation beneath Vulcano Island (Italy): evidence from CO2 fluid inclusions in quartz xenoliths[END_REF] or 300-500 MPa [START_REF] De Astis | Geology, volcanic history and petrology of Vulcano (central Aeolian archipelago[END_REF] beneath Vulcano. Previous work has suggested that this zone is the main site for magma recharge, differentiation, AFC and mixing [START_REF] Zanon | Magmatic feeding system and crustal magma accumulation beneath Vulcano Island (Italy): evidence from CO2 fluid inclusions in quartz xenoliths[END_REF][START_REF] De Astis | Geology, volcanic history and petrology of Vulcano (central Aeolian archipelago[END_REF]. Prim-1 type magmas, following their generation in this deep reservoir, may later ascend and mix with more differentiated products to generate magmas represented by the La Sommata basalt (Fig. 10).

This general model bears strong similarities with the deep magmatic evolution at Stromboli. The golden pumice PST-9 has clinopyroxene preceding olivine in the crystallization sequence at 400 MPa, and is cosaturated with Cpx + Ol at ~150 MPa (1100 °C, H 2 O melt = 3.5 wt.%, Di [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the Golden Pumice, Stromboli volcano (Italy)[END_REF], in a similar pressure range as other ultra-calcic compositions such as Som-1 and CaNe. Barometric estimates from H 2 O-CO 2 systematics and experimental results demonstrate that the ultra-calcic MI in Stromboli olivine (Fig. 2) were trapped at crustal pressures (between 150 and 250 MPa, [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF][START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF][START_REF] Métrich | Conditions of magma storage, degassing and ascent at Stromboli: new insights into the volcanic plumbing system with inferences on the eruptive dynamics[END_REF]. Those MI plot in the Cpx primary field yet they are found trapped in olivine crystals implying that they represent local melts [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF]. Assimilation of dunite and wehrlite lithologies is necessary to explain the trace element geochemistry of golden pumices [START_REF] Schiavi | Degassing, crystallization and eruption dynamics at Stromboli: trace element and lithium isotopic evidence from 2003 ashes[END_REF]. Therefore, in an analogous way to Prim-1 at Vulcano, golden pumice magmas at Stromboli are the result of the interaction between primitive, mantle-derived, melts and clinopyroxene-rich lithologies in the plumbing system [START_REF] Pichavant | Experimental constraints on the deep magma feeding system at Stromboli Volcano, Italy[END_REF][START_REF] Pichavant | Petrography, mineralogy and geochemistry of a primitive pumice from Stromboli: implications for the deep feeding system[END_REF].

Summary and conclusions

(1) Two main groups of primitive compositions occur in the Aeolian arc: calc-alkaline (Alicudi basalts, possibly some seamount basalts) and ultra-calcic (Vulcano basalts, Vulcano and Stromboli MI). The ultra-calcic signature may be a common feature in the Aeolian arc. (2) On Vulcano, the La Sommata basalt is a reference primitive nenormative arc basalt with a strong ultra-calcic affinity. Although it does not correspond exactly to an end-member primitive liquid, the La Sommata basalt is a natural proxy of an ultra-calcic arc melt and, as such, it offers the opportunity to document the phase relations of a natural ultra-calcic arc composition. (3) Experimental phase equilibria for Som-1 (determined between 44 and 154 MPa, for 0.7-3.5 wt.% H 2 O and fO 2 between NNO + 0.2 and +1.9) show clinopyroxene slightly preceding olivine at 150 MPa and olivine joining clinopyroxene on the liquidus between 44 and 88 MPa. Plagioclase is the third phase in the crystallization sequence. Orthopyroxene is lacking. (4) Cotectic experimental glasses and natural glass inclusions in clinopyroxene overlap. Upon progressive crystallization, experimental glasses shift from Som-1 towards more evolved compositions similar to latitic whole-rocks erupted at Vulcano. Melts such as the La Sommata basalt can be parental both to melts trapped in clinopyroxene and to the least fractionated part (i.e., 48-55 wt.% SiO 2 ) of the shoshonitic sequence at Vulcano, erupted in the same period. (5) The calculated composition of the end-member primitive melt is virtually identical to the average of MI in La Sommata olivines. Therefore, MI in olivine are representative of end-member primitive basaltic liquids at Vulcano. Such primitive melts mixed with more differentiated products to eventually produce the La Sommata basalt. (6) The experimental results on the La Sommata basalt are identical in critical aspects to those obtained previously on a synthetic ultracalcic arc composition. Phase relations for the end-member primitive melt, calculated with MELTS, are not significantly different from those determined on the La Sommata basalt. All experimental results suggest that generation of primitive ultra-calcic melts leaves a clinopyroxene-rich residual source, either wehrlite at low pressures or clinopyroxenite at higher pressures. (7) A wehrlitic residual source is implied by the mineralogical and petrographical characteristics of the La Sommata products. Thus, ultra-calcic arc compositions at Vulcano are generated at crustal pressures. They represent non-primary melts produced by the interaction between primary, presumably peridotite-derived melts, and clinopyroxene-rich wall-rocks in the Aeolian arc crust. (8) The La Sommata basalt and MI record a process of interaction between a primary melt generated in the mantle wedge (not observed at Vulcano) and wehrlitic or gabbroic rocks present in the lower part of the plumbing system. The interaction process is inferred to take place between 250 and 350 MPa, most probably in the lower magma accumulation zone beneath Vulcano. (9) The model proposed for Vulcano has close similarities with the deep magmatic evolution at Stromboli. Primitive ultra-calcic magmas in the Aeolian arc are generated at crustal instead of mantellic conditions.

Fig. 1 .

 1 Fig. 1. Sketch map of the Tyrrhenian Sea basin showing the location of the Aeolian Arc and of the seven subaerial volcanic edifices and seamounts discussed in text. The inset shows the general geodynamic context including the Africa-Eurasia boundary, the Sicily channel extension system and the location of the Aeolian domain (square).

Fig. 2 .

 2 Fig. 2. CaO/Al 2 O 3 vs. MgO plot showing primitive compositions in the Aeolian arc. Compositions from Alicudi, Stromboli and Vulcano are distinguished by different colours, respectively brown, blue and black. Whole-rock (WR) compositions are plotted as large filled squares, melt inclusions (MI) in olivine as filled circles and MI in clinopyroxene as small filled squares. The red cross is the ultra-calcic arc synthetic mixture CaNe[START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Source of data, La Sommata: WR (Som-1) from Table1, MI (data corrected for post-entrapment crystallization and normalized to 100%) from[START_REF] Métrich | Sulfur abundance and its speciation in oxidized alkaline melts[END_REF]; Gioncada et al. (1998); Schiano et al. (2000, 2006); Mercier et al. (2010); Sonzogni et al. (2010); Rose-Koga et al. (2012) and Chen et al. (2013). Note that Som-1 and CaNe are almost coincident. Source of data, Stromboli: WR from Pichavant et al. (2011), MI data from[START_REF] Bertagnini | Stromboli volcano (Aeolian Archipelago, Italy): an open window on the deep-feeding system of a steady state basaltic volcano[END_REF]. Source of data, Alicudi: WR from[START_REF] Peccerillo | Plio-Quaternary volcanism in Italy[END_REF]. The curve limiting the compositional domain of lherzolite partial melts is taken from[START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. The horizontal dashed line separates the ultra-calcic (CaO/Al 2 O 3 N 1) and non-ultra-calcic (CaO/ Al 2 O 3 b 1) fields.

  Fig. 2. CaO/Al 2 O 3 vs. MgO plot showing primitive compositions in the Aeolian arc. Compositions from Alicudi, Stromboli and Vulcano are distinguished by different colours, respectively brown, blue and black. Whole-rock (WR) compositions are plotted as large filled squares, melt inclusions (MI) in olivine as filled circles and MI in clinopyroxene as small filled squares. The red cross is the ultra-calcic arc synthetic mixture CaNe[START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. Source of data, La Sommata: WR (Som-1) from Table1, MI (data corrected for post-entrapment crystallization and normalized to 100%) from[START_REF] Métrich | Sulfur abundance and its speciation in oxidized alkaline melts[END_REF]; Gioncada et al. (1998); Schiano et al. (2000, 2006); Mercier et al. (2010); Sonzogni et al. (2010); Rose-Koga et al. (2012) and Chen et al. (2013). Note that Som-1 and CaNe are almost coincident. Source of data, Stromboli: WR from Pichavant et al. (2011), MI data from[START_REF] Bertagnini | Stromboli volcano (Aeolian Archipelago, Italy): an open window on the deep-feeding system of a steady state basaltic volcano[END_REF]. Source of data, Alicudi: WR from[START_REF] Peccerillo | Plio-Quaternary volcanism in Italy[END_REF]. The curve limiting the compositional domain of lherzolite partial melts is taken from[START_REF] Médard | Liquidus surface of ultra-calcic primitive melts: formation conditions and sources[END_REF]. The horizontal dashed line separates the ultra-calcic (CaO/Al 2 O 3 N 1) and non-ultra-calcic (CaO/ Al 2 O 3 b 1) fields.

Fig. 3 .

 3 Fig. 3. Geological sketch map of Vulcano island, showing the distribution of the volcanic units erupted during the five different periods of activity (modified from De Astis et al., 2006). The La Sommata cone is located in the southern portion of the island, within the Caldera del Piano.

Fig. 4 .

 4 Fig. 4. Back-scattered electron microphotographs of the La Sommata basalt sample showing olivine (ol) and clinopyroxene (cpx) phenocrysts of the basaltic assemblage and their textural relations consistent with cotectic crystallization. Notice the extensively crystallized groundmass.

Fig. 6 .

 6 Fig. 6. Experimentally determined T-H 2 O melt phase diagram for Som-1 at P = 150 MPa. Experimental data in Table2. Squares give conditions of experimental charges and types of phase assemblages encountered. Open square: phase is absent; filled square: phase is present. Note that Fe-oxide occurs in most charges although its presence could not be confirmed in all because of its small size. L: liquid; Cpx: clinopyroxene; Ol: olivine; Plag: plagioclase. All charges include a fluid phase (see text). Pure H 2 O vapour is designated as V and the L -L + V phase boundary is the H 2 O solubility curve.

Fig. 7 .

 7 Fig. 7. Experimentally determined P-H 2 O melt phase diagram for Som-1 at T = 1150°C. Experimental data in Table2. Squares give conditions of experimental charges and types of phase assemblages encountered. Open square: phase is absent; filled square: phase is present. Note that Fe-oxide occurs in most charges although its presence could not be confirmed in all because of its small size. L: liquid; Cpx: clinopyroxene; Ol: olivine; Plag: plagioclase. All charges include a fluid phase (see text). Pure H 2 O vapour is designated as V and the L -L + V phase boundary is the H 2 O solubility curve. The location of the plagioclase saturation curve (dashed) is estimated.

Fig. 8 .

 8 Fig. 8. CaO vs. MgO plot showing compositions of cotectic (Cpx + Ol co-saturated) experimental glasses at 1150 °C and of glass inclusions in the La Sommata basalt. Data for the experimental glasses (Table 4, normalized to 100 wt.%) are distinguished by pressure (44, 88, 154 MPa, Table2). Melt inclusion data as in Fig.1. The compositions of the La Sommata basalt (Som-1) and of the theoretical primitive end-member magma (Prim-1) are shown (see Table1).

Fig. 9 .

 9 Fig. 9. CaO/Al 2 O 3 vs. MgO plot showing compositions of experimental glasses from this study, of melt inclusions in the La Sommata basalt and of selected eruption products from Vulcano (see text). Melt inclusion data as in Fig. 1. For the experimental glasses, the data (Tables 3-4, normalized to 100 wt.%) are distinguished by phase assemblage. Whole-rock compositions from De Astis et al. (1997) and[START_REF] Gioncada | A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feedings system[END_REF]. The compositions of the La Sommata basalt (Som-1) and of the theoretical primitive endmember magma (Prim-1) are shown (see Table1).

Fig. 10 .

 10 Fig. 10. Schematic representation of the Vulcano plumbing system (after De Astis et al., 2013) showing the mantle-crust boundary and the lower and upper magma accumulation zones.Interaction between a primary calk-alkaline (?) melt and wehrlite or gabbroic wall-rock lithologies in the deep part of the plumbing system generates ultra-calcic melts. The ultra-calcic melts then ascend to the upper accumulation zone where it mixes with differentiated magmas (shoshonite-latite) to produce the La Sommata basalt. See text for details.

Table 1

 1 Compositional and modal data for the La Sommata basalt sample used in the experiments.

		SiO 2	TiO 2	Al 2 O 3	FeO t	MnO	MgO	CaO	Na 2 O	K 2 O	P 2 O 5	Cr 2 O 5	Total	CaO/Al 2 O 3		Mg# Phenocryst abundance
																(vol.%)
	Som-1
	Glass (n = 179)	49.55 (0.3) 0.74 (0.0) 12.66 (0.2) 10.23 (0.3) 0.21 (0.1) 8.78 (0.1)	12.97 (0.2) 2.18 (0.1) 2.28 (0.1) 0.40 (0.0) -	98.02	1.03		60
	Groundmass
	(n = 20)	53.12 (1.0) 0.64 (0.1) 19.95 (1.4) 7.33 (1.0)	0.19 (0.1) 1.18 (0.5)	4.54 (1.7)	4.97 (0.4) 6.73 (1.0) 0.77 (0.2) -	97.74	0.23		20
	Clinopyroxene													En	Fs	7
	Phenoxtal	54.69 (0.7) 0.2 (0.1)	1.88 (0.1)	3.18 (0.5)	0.13 (0.0) 16.73 (0.8) 22.87 (1.1) 0.56 (0.0) 0.0 (0.0)	0.0 (0.0)	-	98.45	48	5	90
	MicPheno	53.47 (1.6) 0.43 (0.1) 3.15 (1.3)	4.52 (1.2)	0.14 (0.0) 16.04 (1.1) 21.88 (0.4) 0.52 (0.1) 0.0 (0.0)	0.04 (0.0) -	99.07	47	7	86
	MicLit	50.72 (2.0) 0.70 (0.3) 5.30 (2.2)	6.88 (1.2)	0.12 (0.0) 13.94 (1.8) 21.78 (0.7) 0.62 (0.2) 0.08 (0.1) 0.0 (0.0)	-	98.06	42		78
	Olivine													Fo		3
	Phenoxtal	41.49 (0.2) 0.03 (0.0) 0.05 (0.0)	8.93 (0.3)	0.18 (0.1) 48.37 (0.2) 0.39 (0.0)	0.71 (0.1) 0.04 (0.0) 0.0 (0.0)	-	99.42	91
	MicPheno	41.86 (0.8) 0.04 (0.0) 0.05 (0.0)	9.4 (2.3)	0.24 (0.1) 47.84 (1.8) 0.34 (0.0)	0.74 (0.1) 0.03 (0.0) 0.06 (0.1) -	99.35	90
	MicLit	40.30 (1.0) 0.03 (0.1) 0.05 (0.1)	15.65 (6.1) 0.41 (0.2) 42.33 (5.4) 0.48 (0.2)	0.85 (0.2) 0.0 (0.0)	0.03 (0.1) -	97.58	83
	Plagioclase													An	Ab	0.3
	Phenoxtal	51.74	0.10	30.71	1.09	0.0	0.02	11.64	3.96	0.82	0.0	-	98.91	59
	MicPheno	51.36 (0.7) 0.1 (0.1)	31.84 (0.2) 0.92 (0.1)	0.04 (0.0) 0.01 (0.0)	11.85 (0.6) 3.79 (0.1) 0.58 (0.0) 0.0 (0.0)	-	98.67	61
	MicLit	51.26 (1.0) 0.15 (0.1) 31.02 (1.3) 1.26 (0.6)	0.07 (0.1) 0.11 (0.2)	11.62 (0.8) 3.77 (0.4) 0.96 (0.6) 0.01 (0.0) -	98.40	59
	Spinel													Cr#	Usp	b.1
	MicPheno	0.11 (0.1)	0.56 (0.1) 8.87 (0.4)	26.03 (0.8) 0.24 (0.2) 11.52 (0.3) 0.07 (0.1)	0.66 (0.2) 0.0 (0.0)	0.01 (0.0) 52.00 (0.4) 92.35	80	-
	MicLit	1.32 (1.4)	6.96 (1.8) 7.41 (1.5)	78.65 (2.2) 0.47 (0.2) 3.30 (0.6)	0.45 (0.2)	1.07 (0.2) 0.17 (0.3) 0.06 (0.1) 0.59 (0.4)	86.46	-
	Glass inclusions
	in olivine not recalculated 45.28	0.54	11.06	7.85	0.18	8.53	14.45	2.03	1.94	0.28	-	92.14	1.31		*Fo 90
	in olivine recalculated	48.8	0.7	12	9.6	0.2	9.9	14.1	2.2	2.1	0.33	-	-	1.18		*Fo 88
	in clinopyroxene	46.89	0.99	11.8	11.69	0.25	7.71	12.45	1.94	1.28	0.24	-	95.24	1.05		*Fs 6
	Prim-1
	calculated	47.73	0.74	11.25	10.76	0.22	10.25	14.60	2.13	1.90	0.42	-	100.00 1.30
	Som-1 starting glass (after melting of whole rock) average and standard deviations (in brackets) were calculated on 179 analyses. En = 100 * Mg/(Mg + Fe + Ca), Fs = 100 * Fe/(Mg + Fe + Ca), An = 100 * Ca/(Ca + Na + K), Ab = 100 * Na/(Ca +
	Na + K). Mg# = Mg/(Mg + Fe); Cr# = Cr/(Cr + Al). All Fe as FeO t . All analyses are reported on a 100% basis, the original totals being given. Modal analyses (void-free) by point counting on 3600 points. Representative analyses of olivine-hosted glass
	inclusions: not recalculated for post-entrapment crystallization from Gioncada et al. (1998), recalculated for post-entrapment crystallization from Rose-Koga et al. (2012); glass inclusion in clinopyroxene (not recalculated) from Sonzogni et al.

Table 2

 2 Experimental conditions. MPa, 21.5 h, ƒH 2 = 0.132 MPa, Au 90 Pd 10 .Water activity (aH 2 O) calculated from H 2 O melt using the[START_REF] Burnham | The importance of volatile constituents[END_REF] model and melt compositions from Tables3 and 4; ƒH 2 experimentally determined as explained in text; logƒO 2 calculated from ƒH 2 and experimental ƒH 2 O (see text); ΔNNO = logƒO 2 -logƒO 2 of the NNO buffer calculated at the given T; tr: occurrence in trace amounts (b0.5 wt.%); (?): presence of oxide not confirmed due to its very small size. Phase proportions are calculated by mass balance; gl, glass; cpx, clinopyroxene; ol, olivine; pl, plagioclase; ox, Fe-oxide; qc, quench crystals. Iron loss (ΔFeO) is calculated as 100 • (FeO calc -FeO st. glass )/FeO st. glass . FeO calc and ∑R 2 are obtained from the mass balance calculations. H 2 O concentrations in glass estimated with the "bydifference" method[START_REF] Devine | Comparison of microanalytical methods for estimating H 2 O contents of silicic volcanic glasses[END_REF]; numbers in brackets indicate water concentrations measured by FTIR. K d

	Charge	H 2 O melt (wt.%)	aH 2 O	log ƒO 2	ΔNNO	Phase assemblage	∑R 2	ΔFeO (%)	K d Fe-Mg	cpxliq	K d Fe-Mg	ol-liq	K d Ca-Na	pl-liq
	Som-1
	Run 2, 1150 °C, 153.9 MPa, 23 h, ƒH 2 = 0.163 MPa, Au 80 Pd 20 .
	(2-1)	3.52 (3.45)	0.994	-6.46	1.83	gl(100) + ox(tr)	0.2	-1.58
	(2-2)	3.15 (3.13)	0.873	-6.58	1.72	gl(100) + ox(tr)	0.1	-5.62
	(2-3)	2.69	0.662	-6.82	1.48	gl(90) + cpx(10) + ox(tr)	0.1	-2.29	0.262
	(2-4)	1.74	0.305	-7.49	0.80	gl(79) + cpx(20) + ol(1) + ox(?)	0.0	-11.46	0.228		0.235
	(2-5)	1.20	0.158	-8.06	0.23	gl(74) + cpx(24) + ol(2) + ox(tr)	0.0	-4.81	0.288		0.277
	Run 5, 1120 °C, 140.84 MPa, 20 h, ƒH 2 = 0.129 MPa, Au 80 Pd 20 .
	(5-1)qc	3.45	0.923	-6.79	1.87	gl(100) + ox(tr)
	(5-2)qc	2.84	0.629	-7.12	1.54	gl(79) + cpx(17) + ol(2) + ox(2)	0.3	-0.73	0.192		0.170
	(5-3)	2.55	0.484	-7.35	1.31	gl(75) + cpx(21) + ol(2) + ox(2)	0.1	-0.81	0.287		0.216
	(5-4)	1.45	0.202	-8.11	0.55	gl(64) + cpx(33) + ol(0.4) + ox(3)	0.3	-1.89	0.252		0.245
	(5-5)	1.21	0.145	-8.39	0.27	gl(63) + cpx(34) + ol(3) + pl(tr) + ox(tr)	0.1	-7.11	0.216		0.209		1.49
	Run 8, 1080 °C, 146.3 (8-1) 3.34	0.709	-7.60	1.58	gl(65) + cpx(31) + ol(tr) + ox(3)	0.8	-0.28	0.291
	(8-2)	2.01	0.311	-8.31	0.86	gl(51) + cpx(45) + ol(0.4) + ox(3.6)	0.9	-3.31	0.314		0.265
	(8-3)	1.41	0.186	-8.76	0.41	gl(46) + cpx(50) + ol(1) + pl(tr) + ox(3)	0.0	-1.08	0.303		0.285
	Run 7, 1050 °C, 159.43 MPa, 25.7 h, ƒH 2 = 0.0.131 MPa, Au.
	(7-5)	3.37	0.637	-8.08	1.50	gl(61) + cpx(33) + ol(2) + ox(4)	1.0	-3.19	0.397		0.247
	(7-6)	2.20	0.327	-8.66	0.92	gl(50) + cpx(45) + ol(0.4) + pl(tr) + ox(4.6)	0.1	-0.72	0.315		0.299
	Run 3, 1150 °C, 87.5 MPa, 21 h, ƒH 2 = 0.120 MPa, Au 80 Pd 20 .
	(3-1)qc	2.43	0.870	-6.86	1.44	gl(100)
	(3-2)qc	2.17	0.720	-7.02	1.28	gl(91) + cpx(9) + ol(tr) + ox(tr)	0.1	-0.72	0.244		0.176
	(3-3)qc	1.93	0.590	-7.20	1.11	gl(87) + cpx(13) + ol(0.2) + ox(tr)	0.4	-5.95	0.212		0.175
	(3-4)	1.55	0.370	-7.60	0.70	gl(79) + cpx(19) + ol(2) + ox(tr)	0.2	-3.10	0.295		0.254
	(3-5)	1.25	0.240	-7.98	0.33	gl(74) + cpx(23) + ol(3) + ox(?)	0.1	-7.80	0.302		0.284
	Run 6, 1150 °C, 44.07 MPa, 20.1 h, ƒH 2 = 0.122 MPa, Au 80 Pd 20 .
	(6-1)	1.95 (2.17)	0.990	-7.29	1.02	gl(100) + ox(tr)	0.2	-4.84
	(6-2)	1.89 (1.94)	0.940	-7.34	0.97	gl(100) + ox(tr)	0.4	-3.32
	(6-3)	1.62	0.720	-7.57	0.74	gl(93) + cpx(6) + ol(1) + ox(tr)	0.3	-3.73	0.232		0.226
	(6-4)	0.70	0.170	-8.82	0.51	gl(87) + cpx(11) + ol(2) + ox(tr)	0.1	-9.88	0.292		0.222
						Fe-Mg cpx-liq = (Fe/Mg) cpx /(Fe/Mg) liq ; K d Fe-Mg ol-liq = (Fe/Mg) ol /(Fe/Mg) liq ; K d Ca-Na pl-liq = (Ca/Na) pl /(Ca/Na) liq . For cpx, the Kd is cal-
	culated with FeO = FeO tot .

Table 3

 3 SOM-1 experimental compositions at 150 MPa.

	Charge Phase	SiO 2	TiO 2	Al 2 O 3	FeO t	MnO	MgO	CaO	Na 2 O	K 2 O	P 2 O 5	Cr 2 O 3	Total	Fo	En Fs	An Mg# Usp CaO/Al 2 O 3
	SOM-1 Startgl(n = 179) 48.56(0.3) 0.72(0.1)	12.41(0.2) 10.03(0.1) 0.21(0.3)	8.61(0.1) 12.72(0.2) 2.14(0.1) 2.24(0.1)	0.39(0.1) -	98.02	-	-	-	-	0.60	-	1.03
	2-1	gl(n = 8)	46.75(0.2) 0.71(0.1)	12.21(0.1)	9.81(0.3) 0.20(0.1)	8.31(0.1) 11.99(0.2) 2.07(0.1) 2.04(0.1)	0.42(0.2) 0.00(0.0) 94.51	-	-	-	-	0.60	-	0.98
	2-2	gl(n = 5)	47.19(0.1) 0.69(0.1)	12.22(0.1)	9.20(0.3) 0.19(0.0)	8.30(0.2)	12.2(0.2) 2.07(0.1) 2.13(0.1)	0.46(0.0) 0.00(0.0) 94.66	-	-	-	-	0.62	-	1.00
	2-3	gl(n = 5)	47.26(0.3) 0.78(0.1)	13.18(0.2) 10.00(0.2) 0.16(0.1)	7.86(0.1) 11.42(0.3) 2.29(0.1) 2.33(0.1)	0.48(0.1) 0.00(0.0) 95.76	-	-	-	-	0.58	-	0.87
		cpx(n = 3)	51.75(0.1) 0.22(0.1)	2.67(0.0)	5.59(0.1) 0.00(0.0) 16.77(0.1) 21.87(0.2) 0.20(0.0) 0.01(0.0)	0.10(0.0) 0.00(0.0) 99.18	-	47 9	-	0.84	-	-
	2-4	gl(n = 11)	48.51(0.3) 0.85(0.1)	14.91(0.3)	9.91(0.3)	0.2(0.1)	6.38(0.3) 10.39(0.4) 2.66(0.1) 2.77(0.1)	0.46(0.2) 0.00(0.0) 97.04	-	-	-	-	0.53	-	0.70
		cpx(n = 3)	50.16(0.2) 0.41(0.0)	4.60(0.2)	5.36(0.2) 0.17(0.1) 15.15(0.3) 21.91(0.3) 0.32(0.0) 0.13(0.0)	0.21(0.0) 0.00(0.0) 98.42	-	45 9	-	0.83	-	-
		ol(n = 3)	40.39(0.2) 0.00(0.1)	0.59(0.1) 10.37(0.1) 0.23(0.0)	45.1(0.3)	0.87(0.3) 0.09(0.0) 0.14(0.0)	0.01(0.0) 0.00(0.0) 97.80		-	-	-	-	-	-
	2-5	gl(n = 8)	48.49(0.4) 0.86(0.1)	15.48(0.1) 10.08(0.3) 0.22(0.1)	5.68(0.1) 10.01(0.2) 2.75(0.1) 2.85(0.1)	0.58(0.1) 0.00(0.0) 97.00	-	-	-	-	0.50	-	0.65
		cpx(n = 3)	50.07(0.2) 0.46(0.0)	4.95(0.2)	7.79(0.2) 0.05(0.0) 15.22(0.3) 21.53(0.2) 0.32(0.1) 0.14(0.0)	0.00(0.0) 0.00(0.0) 100.52 -	44 12 -	0.78	-	-
		ol(n = 2)	39.49(0.1) 0.00(0.0)	0.06(0.0) 17.00(0.2) 0.25(0.2) 44.74(0.3)	0.48(0.3) 0.00(0.0) 0.02(0.0)	0.00(0.0) 0.00(0.0) 102.04 -	-	-	-	-	-
	5-1	gl(n = 6)	47.39(1.4) 0.73(0.0)	13.19(0.3)	8.90(0.6) 0.14(0.1)	6.59(1.8) 12.45(1.8) 2.19(0.2) 1.54(1.0)	0.48(0.0) 0.00(0.0) 93.59	-	-	-	-	0.57	-	0.94
		ox(n = 2)	0.13(0.0) 0.58(0.1)	4.39(0.2) 70.44(0.5) 0.00(0.0)	8.41(0.3)	0.58(0.2) 0.99(0.1) 0.00(0.0)	0.00(0.0) 3.05(0.2) 88.56	-	-	-	-	-	1	-
	5-2	gl(n = 3)	48.55(0.2) 0.77(0.1)	15.01(0.2)	8.67(0.3) 0.19(0.1)	5.83(0.1) 11.21(0.4) 2.55(0.2) 2.38(0.1)	0.49(0.1) 0.00(0.0) 95.65	-	-	-	-	0.55	-	0.75
		cpx(n = 3)	50.40(1.0) 0.23(0.0)	3.16(0.5)	4.61(0.1) 0.12(0.0) 16.16(0.3) 22.76(0.1) 0.19(0.0) 0.04(0.0)	0.31(0.1) 0.00(0.0) 98.12	-	46 7	-	0.86	-	-
		ol(n = 2)	40.28(0.1) 0.00(0.01)	0.00(0.0)	8.08(0.0) 0.27(0.1) 50.02(0.2)	0.37(0.1) 0.00(0.0) 0.02(0.0)	0.10(0.0) 0.01(0.0) 99.25		-	-	-	-	-	-
		ox(n = 2)	0.27(0.0) 1.17(0.1)	5.11(0.3) 70.07(0.3) 0.64(0.1)	7.61(0.6)	0.49(0.0) 1.04(0.1) 0.04(0.0)	0.00(0.0) 2.75(0.2) 89.19	-	-	-	-	-	3	-
	5-3	gl(n = 5)	49.50(1.0) 0.75(0.0)	15.05(0.7)	8.90(0.3) 0.20(0.1)	5.70(0.5) 11.02(1.0) 2.65(0.3) 2.55(0.4)	0.48(0.1) 0.00(0.0) 94.29	-	-	-	-	0.51	-	0.73
		cpx(n = 5)	49.06(0.5) 0.47(0.1)	5.88(1.1)	7.47(0.4) 0.15(0.1)	14.9(0.8) 21.32(0.9) 0.39(0.3) 0.35(0.2)	0.40(0.0) 0.06(0.0) 100.44 -	44 10 -	0.82	-	-
		ol(n = 3)	40.15(0.4) 0.03(0.1)	0.02(0.0)	9.01(0.1) 0.35(0.1) 47.88(0.7)	0.40(0.1) 0.00(0.0) 0.02(0.0)	0.11(0.1) 0.01(0.0) 97.97		-	-	-	-	-	-
		ox(n = 2)	0.01(0.1) 1.60(0.2)	6.16(0.2) 70.05(0.6) 0.56(0.1)	7.85(0.3)	0.42(0.0) 0.20(0.0) 0.08(0.0)	0.30(0.1) 0.99(0.1) 88.23	-	-	-	-	-	4	-
	5-4	gl(n = 4)	50.08(0.4) 0.75(0.1)	16.68(0.2)	8.93(0.3) 0.19(0.1)	5.21(0.1)	8.64(0.2) 3.14(0.2) 3.26(0.1)	0.62(0.0) 0.00(0.0) 97.5	-	-	-	-	0.51	-	0.52
		cpx(n = 3)	48.12(0.8) 0.53(0.8)	5.60(0.6)	6.38(0.6) 0.21(0.1)	14.8(0.2) 21.81(0.2) 0.25(0.0) 0.07(0.0)	0.40(0.0) 0.05(0.0) 98.21	-	44 10 -	0.81	-	-
		ol(n = 4)	40.41(0.3) 0.02(0.0)	0.01(0.0) 14.68(0.2) 0.35(0.1) 46.63(0.3)	0.34(0.0) 0.00(0.0) 0.02(0.0)	0.00(0.0) 0.00(0.0) 102.45 -	-	-	-	-	-
		ox(n = 2)	0.16(0.0) 2.36(0.1)	7.38(0.2) 69.89(0.5) 0.31(0.1)	7.23(0.3)	0.43(0.1) 0.00(0.0) 0.14(0.1)	0.30(0.1) 1.00(0.1) 89.2	-	-	-	-	-	6	-
	5-5	gl(n = 3)	49.19(0.1) 0.90(0.1)	16.79(0.3) 10.04(0.4) 0.23(0.1)	4.48(0.1)	8.58(0.2) 3.21(0.1) 3.27(0.2)	0.51(0.1) 0.00(0.0) 97.19	-	-	-	-	0.44	-	0.51
		cpx(n = 5)	49.72(0.1) 0.60(0.1)	6.25(0.0)	6.80(0.1) 0.25(0.0) 14.06(0.1) 21.17(0.2) 0.52(0.2) 0.42(0.2)	0.44(0.0) 0.10(0.0) 98.53	-	42 12 -	0.79	-	-
		ol(n = 3)	38.91(0.2) 0.07(0.0)	0.00(0.0) 15.27(0.0) 0.47(0.1) 42.03(0.4)	0.49(0.0) 0.00(0.0) 0.05(0.0)	0.17(0.0) 0.03(0.0) 97.48		-	-	-	-	-	-
		pl(n = 3)	49.94(0.2) 0.02(0.0)	33.08(0.4)	0.78(0.0) 0.00(0.0)	0.00(0.0) 12.59(0.2) 9.00(0.3) 0.48(0.1)	0.00(0.0) 0.00(0.0) 100.05 -	-	-	67	-	-	-
	8-1	gl(n = 5)	48.32(3.8) 0.66(0.1)	14.78(1.4)	6.76(2.0) 0.10(0.1)	4.22(3.7)	9.10(2.5) 2.34(0.7) 2.63(1.64) 0.51(0.2) 0.00(0.0) 89.43	-	-	-	-	0.53	-	0.62
		cpx(n = 3)	49.9(0.7) 0.57(0.1)	5.38(0.7)	6.95(0.5) 0.13(0.1) 14.93(0.7) 21.89(0.6) 0.39(0.1) 0.22(0.1)	-	-	-	-	43 11 -	0.79	-	-
		ox(n = 2)	0.01(0.1) 1.60(0.2)	6.16(0.2) 70.05(0.6) 0.56(0.1)	7.85(0.3)	0.42(0.0) 0.20(0.0) 0.08(0.0)	0.3(0.1)	0.99(0.1) 88.23	-	-	-	-	-	4	-
	8-2	gl(n = 3)	52.56(0.4) 0.69(0.0)	16.78(0.1)	6.45(0.5) 0.16(0.1)	3.31(0.1)	6.42(0.1) 3.68(0.1) 4.14(0.2)	0.67(0.1) 0.00(0.0) 94.87	-	-	-	-	0.48	-	0.38
		cpx(n = 3)	49.22(0.6) 0.70(0.1)	6.65(0.7)	8.45(0.3) 0.13(0.1) 13.83(0.5) 20.51(0.4) 0.53(0.1) 0.30(0.1)	0.13(0.0) 0.00(0.0) 100.44		42 14 -	-	-	-
		ol(n = 3)	39.8(0.6) 0.07(0.1)	0.57(0.1) 16.17(0.1) 0.46(0.0)	44.4(0.4)	0.71(0.3) 0.06(0.0) 0.08(0.1)	0.10(0.0) 0.00(0.0) 102.42 -	-	-	-	-	-
		ox(n = 2)	1.33(0.1) 1.29(0.1)	6.42(0.2) 77.99(0.4) 0.40(0.0)	9.02(0.3)	1.19(0.5) 1.21(0.5) 0.00(0.0)	0.43(0.1) 0.01(0.0) -	-	-	-	-	-	2	-
	8-3	gl(n = 3)	52.17(0.1) 0.74(0.1)	16.46(0.2)	7.16(0.2) 0.17(0.1)	3.06(0.5)	5.93(0.3) 3.93(0.1) 5.13(0.3)	0.83(0.1) 0.00(0.0) 95.57	-	-	-	-	0.43	-	0.36
		cpx(n = 4)	49.05(0.5) 0.67(0.0)	7.16(0.5)	8.99(0.5) 0.23(0.1) 12.69(0.3) 19.55(0.3) 0.83(0.1) 0.64(0.1)	0.10(0.0) 0.00(0.0) 99.93	-	40 16 -	0.72	-	-
		ol(n = 3)	40.08(0.2)	0.1(0.1)	0.67(0.3) 19.11(0.3) 0.23(0.1)	38.6(0.1)	0.42(0.2) 0.98(0.0) 0.04(0.0)	0.00(0.0) 0.00(0.0) 100.22 -	-	-	-	-	-
		ox(n = 2)	1.67(0.2) 3.08(0.3)	10.92(0.4) 73.49(0.7) 0.66(0.0)	7.99(0.5)	0.65(0.0) 1.30(0.1) 0.26(0.0)	0.00(0.0) 0.00(0.0) -	-	-	-	-	-	9	-
	7-5	gl(n = 5)	50.13(0.4) 0.56(0.1)	15.74(0.5)	5.53(0.5) 0.05(0.1)	4.17(0.4)	9.18(0.9)	1.8(0.4) 2.89(0.2)	0.53(0.1) 0.00(0.0) 90.57	-	-	-	-	0.57	-	0.58
		cpx(n = 3)	49.52(0.5) 0.73(0.1)	6.99(1.0)	7.3(0.5) 0.15(0.1) 13.87(0.9) 21.22(1.0) 0.53(0.2) 0.36(0.2)	0.00(0.0) 0.00(0.0) 100.67 -	42 12 -	0.77	-	-
		ol(n = 3)	41.09(0.4) 0.02(0.0)	0.02(0.1) 10.66(0.4) 0.35(0.1) 50.32(0.7)	0.22(0.0) 0.00(0.0) 0.02(0.0)	0.00(0.0) 0.00(0.0) 102.7		-	-	-	-	-	-
		ox(n = 2)	7.07(0.5) 2.05(0.7)	7.01(0.8) 70.01(0.9)	0.6(0.0)	8.05(0.8)	2.85(0.7) 1.51(0.5)	0.3(0.1)	0.09(0.0) 0.00(0.0) -	-	-	-	-	-	6	-
	7-6	gl(n = 6)	52.18(0.3) 0.64(0.1)	17.09(0.3)	6.01(0.3) 0.19(0.1)	3.39(0.1)	6.55(0.3) 3.62(0.2) 4.01(0.2)	0.66(0.1) 0.00(0.0) 94.33	-	-	-	-	0.50	-	0.38
		cpx(n = 3)	48.92(0.3) 0.63(0.1)	7.05(0.3)	7.61(0.3) 0.15(0.1) 13.63(0.3) 20.33(0.4) 0.62(0.1) 0.39(0.1)	0.00(0.0) 0.00(0.0) 99.33	-	42 13 -	0.76	-	-
		ol(n = 3)	39.47(0.2) 0.09(0.0)	0.53(0.4) 16.34(0.7) 0.39(0.1) 44.18(0.8)	0.63(0.2) 0.04(0.1) 0.07(0.1)	0.00(0.0) 0.00(0.0) 101.74 -	-	-	-	-	-
		ox(n = 2)	5.94(0.6) 1.53(0.3)	7.05(0.8) 73.37(0.9)	0.3(0.0)	7.94(0.5)	1.97(0.4) 1.44(0.3) 0.33(0.1)	0.00(0.0) 0.00(0.0) -	-	-	-	-	-	4	-

Compositions of experimental glasses and minerals analyzed at EPMA. SOM-1, starting glass; other abbreviations as in Table

2

. CaO/Al 2 O 3 is calculated for the glass phase. In brackets standard deviation with n = number of analyses. All Fe as FeO tot . Wo = 100 * Ca/(Mg + Fe + Ca), En = 100 * Mg/(Mg + Fe + Ca) and Fs = 100 * Fe/(Mg + Fe + Ca) with Fe = FeO t ; An = 100 * Ca/(Ca + Na + K); Ab = 100 * Na/(Ca + Na + K).

G. Lanzo et al. / Journal of Volcanology and Geothermal Research 321 (2016) 85-101

Compositions of experimental glasses and minerals analyzed at EPMA. SOM-1, starting glass; other abbreviations as in Table3. CaO/Al 2 O 3 is calculated for the glass phase. In brackets standard deviation with n = number of analyses.

Acknowledgements

This work is based on the PhD thesis of the first author which was supported both by the University of Palermo and the University of Orléans. Funding by an FFR grant of the University of Palermo to SGR and by an ESF grant (in the framework of the MeMoVolc program) to GL is acknowledged. The final version has benefited from the constructive comments of an anonymous reviewer and of the Editor.