The near-liquidus crystallization of a high-K basalt (PST-9 golden pumice, 49•4 wt % SiO 2 , 1•85 wt % K 2 O, 7•96 wt % MgO) from the present-day activity of Stromboli (Aeolian Islands, Italy) has been experimentally investigated between 1050 and 1175°C, at pressures from 50 to 400 MPa, for melt H 2 O concentrations between 1•2 and 5•5 wt % and NNO ranging from-0•07 to +2•32. A drop-quench device was systematically used. AuPd alloys were used as containers in most cases, resulting in an average Fe loss of 13% for the 34 charges studied. Major crystallizing phases include clinopyroxene, olivine and plagioclase. Fe-Ti oxide was encountered in a few charges. Clinopyroxene is the liquidus phase at 400 MPa down to at least 200 MPa, followed by olivine and plagioclase. The compositions of all major phases and glass vary systematically with the proportion of crystals. Ca in clinopyroxene sensitively depends on the H 2 O concentration of the coexisting melt, and clinopyroxene Mg-number shows a weak negative correlation with NNO. The experimental data allow the liquidus surface of PST-9 to be defined. When used in combination with melt inclusion data, a consistent set of pre-eruptive pressures (100-270 MPa), temperatures (1140-1160°C) and melt H 2 O concentrations is obtained. Near-liquidus phase equilibria and clinopyroxene Ca contents require melt H 2 O concentrations <2•7-3•6 and 3 ± 1 wt %, respectively, overlapping with the maximum frequency of glass inclusion data (2•5-2•7 wt % H 2 O). For olivine to crystallize close to the liquidus, pressures close to 200 MPa are needed. Redox conditions around NNO = +0•5 are inferred from clinopyroxene compositions. The determined preeruptive parameters refer to the storage region of golden pumice melts, which is located at a depth of around 7•5 km, within the metamorphic arc crust. Golden pumice melts ascending from their storage zone along an adiabat will not experience crystallization on their way to the surface.

We present experimental phase equilibria carried out on a pantelleritic bulk-rock composition with a peralkalinity index [PI INTRODUCTIONPantelleria island is the type-locality of pantellerite, an iron-and sodium-rich peralkaline rhyolite. Pantellerites, by far the most abundant rock-type at Pantelleria, occur as pyroclastic flows, pumice fall deposits, or as obsidianaceous lavas (Mahood & Hildreth, 1986). After the Plinian eruption of the Green Tuff (45 ka BP), and the related caldera collapse, resurgent felsic magmatism was mainly centered inside the caldera, producing either low-energy explosive eruptions or lava flows.Recent experimental investigations (Scaillet & Macdonald, 2001 and melt inclusion studies (Kovalenko et al., 1988;Webster et al., 1993;Wilding et al Barclay et al., 1996;Gioncada & Landi, 2010) have shown that peralkaline melts may be rather H 2 O-rich, in contrast to previous inferences. These studies have also demonstrated the role of pressure, water, oxygen fugacity, and silica and sodium disilicate activity (aSiO 2 , aNds) on the phase equilibria of felsic peralkaline magmas from the east Kenyan rift zone (e.g. Scaillet & Macdonald, 2001Caricchi et al., 2006). In this study we present the hydrous phase equilibria of a pantellerite with a peralkalinity index PI [ ¼ molar (Na 2 O þ K 2 O)/Al 2 O 3 ] ¼ 1·4, established within the pressure range 25^150 MPa, for temperatures between 680 and 8008C, with oxygen fugacities below the Ni^NiO solid buffer (NNO), and melt water contents (H 2 O melt ) up to 6 wt %. We apply our experimentally derived P^TĤ 2 O melt grid to natural pantellerites to assess the pre-eruptive conditions (T, P,H 2 O melt )ofthemostpowerful event of the post-caldera period, the Fastuca eruption (Orsi et al.,1989). Knowledge of the pre-eruptive conditions sheds light on the nature of the magmatic plumbing system associated with recent felsic volcanism at Pantelleria, with several obvious implications for the eruptive style of pantellerite magmas. The determination of magma storage conditions of pantelleritic magmas can also provide constraints on the more general problem of the origin of silica-oversaturated peralkaline magmas, which has been extensively debated (see Bohrson & Reid, 1997;Civetta et al., 1998;Marshall et al., 1998;Marshall et al. 2009;Scaillet & Macdonald, 2001;Nekvasil et al., 2004;Macdonald et al., 2008). Much of the controversy has arisen because of the lack of modern phase equilibrium constraints for peralkaline magmas, in contrast to their metaluminous or peraluminous counterparts (e.g. Dall' Agnol et al., 1999;Klimm et al., 2003;Bogaerts et al., 2006). GEOLOGICAL AND PETROLOGICAL BACKGROUNDPantelleria (Italy) is an entirely volcanic island ( Fig. 1) located within the Sicily Channel continental rift system, in the northern part of the African plate (e.g. Rotolo et al., 2006). Pantelleria is well known in the petrological literature because its eruptive products represent a typical bimodal suite: a mafic end-member, which consists of mildly alkali basalts, a...

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To testmechanisms of basalticmagma degassing,\ud continuous decompressions of volatile-bearing (2.7–3.8 wt%\ud H2O, 600–1,300 ppm CO2) Stromboli melts were performed\ud from 250–200 to 50–25 MPa at 1,180–1,140 C.Ascent rates\ud were varied from 0.25 to *1.5 m/s. Glasses after decompression\ud show a wide range of textures, from totally bubblefree\ud to bubble-rich, the latter with bubble number densities\ud from 104 to 106 cm-3, similar to Stromboli pumices. Vesicularities\ud range from 0 to *20 vol%. Final melt H2O concentrations\ud are homogeneous and always close to solubilities.\ud In contrast, the rate of vesiculation controls the finalmelt CO2\ud concentration. High vesicularity charges have glass CO2\ud concentrations that follow theoretical equilibrium degassing\ud paths, whereas glasses from low vesicularity charges show\ud marked deviations from equilibrium, with CO2 concentrations\ud up to one order of magnitude higher than solubilities.\ud FTIR profiles and maps reveal glass CO2 concentration gradients\ud near the gas–melt interface. Our results stress the\ud importance of bubble nucleation and growth, and of volatile\ud diffusivities, for basaltic melt degassing. Two characteristic\ud distances, the gas interface distance (distance either between\ud bubbles or to gas–melt interfaces) and the volatile diffusion\ud distance, control the degassing process. Melts containing\ud numerous and large bubbles have gas interface distances\ud shorter than volatile diffusion distances, and degassing proceeds\ud by equilibrium partitioning of CO2 and H2O between\ud melt and gas bubbles. For melts where either bubble nucleation\ud is inhibited or bubble growth is limited, gas interface\ud distances are longer than volatile diffusion distances.\ud Degassing proceeds by diffusive volatile transfer at the gas–\ud melt interface and is kinetically limited by the diffusivities of\ud volatiles in the melt. Our experiments show that CO2-oversaturated\ud melts can be generated as a result of magma\ud decompression. They provide a new explanation for the\ud occurrence of CO2-rich natural basaltic glasses and open new\ud perspectives for understanding explosive basaltic volcanism

International audienceA new, pre-Green Tuff (46 ka) volcanic stratigraphy is presented for the peralkaline Pantelleria Volcano, Italy. New 40Ar/39Ar and paleomagnetic data are combined with detailed field studies to develop a comprehensive stratigraphic reconstruction of the island. We find that the pre-46 ka succession is characterised by eight silica-rich peralkaline (trachyte to pantellerite) ignimbrites, many of which blanketed the entire island. The ignimbrites are typically welded to rheomorphic, and are often associated with lithic breccias and/or pumice deposits. They record sustained radial pyroclastic density currents fed by low pyroclastic fountains. The onset of ignimbrite emplacement is typically preceded (more rarely followed) by pumice fallout with limited dispersal, and some eruptions lack any associated pumice fall deposit, suggesting the absence of tall eruption columns. Particular attention is given to the correlation of well-developed lithic breccias in the ignimbrites, interpreted as probable tracers of caldera collapses. They record as many as five caldera collapse events, in contrast to the two events reported to date. Inter-ignimbrite periods are characterised by explosive and effusive eruptions with limited dispersal, such as small pumice cones, as well as pedogenesis. These periods have similar characteristics as the current post-Green Tuff activity on the island, and, while not imminent, it is reasonable to postulate the occurrence of another ignimbrite-forming eruption sometime in the future

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The island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major calderaforming eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution Ar data capture a long-term (N 15 ka) decline in eruption frequency with a shift in eruptive pace from 3.5 ka −1 to 0.8 ka −1 associated with a prominent paleosol horizon marking the only recognizable volcanic stasis around 12-14 ka. This shift in extraction frequency occurs without major changes in eruptive style, and is paralleled by a subtle trend of decreasing melt differentiation index. We speculate that this decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steadystate compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal evidence for a short-term silicic eruption.

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