The near-liquidus crystallization of a high-K basalt (PST-9 golden pumice, 49·4 wt % SiO₂, 1·85 wt % K₂O, 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 H₂O concentrations between 1·2 and 5·5 wt % and ΔNNO 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 H₂O concentration of the coexisting melt, and clinopyroxene Mg-number shows a weak negative correlation with ΔNNO. 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 H₂O concentrations is obtained. Near-liquidus phase equilibria and clinopyroxene Ca contents require melt H₂O concentrations <2·7–3·6 and 3 ± 1 wt %, respectively, overlapping with the maximum frequency of glass inclusion data (2·5–2·7 wt % H₂O). For olivine to crystallize close to the liquidus, pressures close to 200 MPa are needed. Redox conditions around ΔNNO = +0·5 are inferred from clinopyroxene compositions. The determined pre-eruptive 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 = molar (Na₂O + K₂O)/Al₂O₃] = 1·4, which is representative of the most energetic pumice fall eruption of the resurgent post-caldera volcanism on Pantelleria island. For the explored conditions (P = 25–150 MPa; T = 680–800°C; H₂O_melt up to 6 wt %; fO₂ ≤ NNO, where NNO is nickel–nickel oxide buffer) clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz. The crystallization of amphibole is limited to temperatures below 700°C. Aenigmatite crystallizes near the liquidus for P ≥ 100 MPa. When clinopyroxene is the sole liquidus phase, the coexisting melt is characterized by a decrease in peralkalinity index to 1·2, if compared with the starting material, approaching comendite composition. In contrast, when alkali feldspar joins the liquidus phase assemblage, residual melts become extremely peralkaline (PI = 3·2), with FeO contents in excess of 14 wt %. Comparison of our experiments with natural phase abundances and compositions constrains the depth of the Fastuca pantellerite reservoir to be around 5 ± 1 km (or P = 120 ± 20 MPa) at T = 730 ± 10°C, with H₂O_melt around 4 wt %, and fO₂ 1·5 log unit below NNO. Such conditions correspond to the dominant phase assemblage of feldspar, clinopyroxene and aenigmatite. Amphibole occurrence requires both higher melt water contents and lower temperatures (up to 6·5 wt % at 680°C), and may reflect provenance from a deeper and wetter portion of the tapped reservoir. In contrast, quartz is more probably related to crystallization during magma ascent, which explains the lower melt water content of its melt inclusions. Overall, the results confirm that pantellerites may be both water-rich and reduced magmas. These characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species, the latter amounting to a few mole per cent. Magma unrest at Pantelleria may thus be heralded by emissions of hydrous and reduced gases (i.e. CO₂-poor and H₂-rich), if the present-day reservoir is felsic in composition.

New experiments have been performed on a high-K basalt (PST-9) from Stromboli volcano, Italy, to constrain the physical conditions of golden pumice magmas at their storage level and discuss their petrogenesis. Fluid-present, H₂O- and CO₂-bearing, near-liquidus experiments were performed at 1150°C between 100 and 400 MPa and under oxidizing conditions. Glasses were analyzed by Fourier transform IR spectroscopy and their H₂O and CO₂ concentrations compared with those in glass inclusions. Most glass inclusions cluster near the 200 MPa isobar, suggesting entrapment at a depth of ∼8 km. Golden pumice magmas have viscosities of 7–9 Pa s and densities of 2·48–2·57 g/cm³. Compositions of experimental fluids coexisting with melts along the liquidus have been estimated by mass balance. They range from CO₂-rich (XH₂O ∼ 0·2) at 400 MPa to H₂O-rich (XH₂O ∼ 0·8) at 100 MPa. The free fluid phase present at the reservoir level has an XH₂O of ∼0·6, consistent with equilibration with a near-liquidus golden pumice magma below 300 MPa. Clinopyroxene is the liquidus phase in all high-pressure experiments, either fluid-absent (H₂O) or fluid-present (H₂O–CO₂). In contrast, at 0·1 MPa, cpx and ol appear together on the liquidus and olivines are more Fo-rich (up to 89·1) than those crystallizing at high pressures (up to 87·3). The composition of cotectic liquids multiply saturated in cpx and ol has been experimentally determined. Most pumices and a majority of melt inclusions have compositions of cotectic melts. In contrast, PST-9 plots in the cpx field and is representative of less evolved ankaramitic magmas parental to golden pumices. Melt inclusions trapped in Fo >87 olivines form a group of ultra-calcic compositions plotting in the cpx field, interpreted as boundary layer melts locally generated by cpx dissolution in the deep reservoir. Ankaramitic melts in the Stromboli feeding system are proposed to derive from primitive mantle melts by combined crystallization, mixing, wall-rock interaction and assimilation.

Submarine magmatism in the Sicily Channel Rift began in the early Pliocene and lasted until almost 200 yr ago. We present here petrological and geochemical data on volcanic rocks dredged from Graham and Nameless banks and Pantelleria seamounts in the Pelagian sector of the Sicily Channel Rift. Petrological evidence suggests that the ascent of magmas to the surface was relatively rapid, probably through channels superimposed over the major tectonic discontinuities of the Rift. Major and trace element data indicate an ocean island basalt affinity for Graham and Nameless bank alkaline lavas and a depleted tholeiitic signature for one Pantelleria seamount, which had a shallower mantle source. Sr-Nd-Pb isotopic compositions suggest a heterogeneous mantle source involving both focus zone ( FOZO) and high- mu( HIMU) components. Lead isotopic compositions are slightly less radiogenic in the tholeiitic basalt and more radiogenic ( closer to HIMU) in the alkaline lavas. We hypothesize that a decompressional melting mechanism affected the mantle beneath the Sicily Channel at different levels. Our data fit in a regional scenario characterized by the presence of HIMU or FOZO components in alkaline and calc-alkaline volcanics all around Sicily. The explanation for the common isotopic composition could lie in a ( fossil) plume head that variably contributed to magmatogenesis in response to variable lithospheric stretching