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...