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Pantelleria island is the type locality for pantellerite, an iron and alkali-rich rhyolite. The eruptive products outcropping in the island fall in a mafic end member (mildly alkaline basalt) and a felsic end member (metaluminous trachytes and pantellerites). A key event in the volcanological history of the island is the Green Tuff eruption the sole ignimbrite at Pantelleria compositionally zoned from crystal-poor pantellerite at the base to crystal-rich trachyte at the top. We experimentally investigated the phase relations of the pantelleritic end member AI = 1.8 (agpaitic index = molar ratio Na2O+K2O/Al2O3) and trachyte end member AI = 1.05 of the Green Tuff eruption. The intensive variables were investigated by performing phase equilibrium experiments at 0.5, 1 and 1.5 kbar, 750-950°C, fluid saturation conditions with XH2O= (H2O/H2O+CO2) between 0 and 1, and redox conditions close to FMQ (fayalite-magnetite-quartz buffer). Preliminary results show that at 850°C pantelleritic charges were above the liquidus regardless their water content. Below 800°C clinopyroxene is the liquidus phase followed by amphibole and alkali feldspar. Aenigmatite and quartz crystallized at 750°C and XH2O lower than 0.8. Considering the other end-member, trachytic charges at 850°C are strongly crystallized. The liquidus phase is clinopyroxene crystallizing at 950°C and XH2O

© 2015 Elsevier B.V. We present a comprehensive overview of the magnetic properties of the four known tektite fields and related fully melted impact glasses (Aouelloul, Belize, Darwin, Libyan desert and Wabar glasses, irghizites, and atacamaites), namely magnetic susceptibility and hysteresis properties as well as properties dependent on magnetic grain-size. Tektites appear to be characterized by pure Fe2+ paramagnetism, with ferromagnetic traces below 1 ppm. The different tektite fields yield mostly non-overlapping narrow susceptibility ranges. Belize and Darwin glasses share similar characteristics. On the other hand the other studied glasses have wider susceptibility ranges, with median close to paramagnetism (Fe2+ and Fe3+) but with a high-susceptibility population bearing variable amounts of magnetite. This signs a fundamental difference between tektites (plus Belize and Darwin glasses) and other studied glasses in terms of oxygen fugacity and heterogeneity during formation, thus bringing new light to the formation processes of these materials. It also appears that selecting the most magnetic glass samples allows to find impactor-rich material, opening new perspectives to identify the type of impactor responsible for the glass generation.

Partitioning experiments between a basaltic melt from Mt. Etna and a low-density hydrous fluid or vapor containing H2O or H2O–CO2 were performed at 1200–1260 °C, at pressures between 1 and 200 MPa, either near the nickel–nickel oxide (NNO) buffer or at two log units above it (NNO+2), and with different chloride concentrations. Most of the experiments were done at chloride-brine-undersaturated conditions, although at the highest Cl concentrations explored brine saturation might have been reached. The average partition coefficients (DCl fluid/melt) over the range of Cl concentrations were derived on a weight basis by plotting the calculated concentrations of Cl in the fluid phase versus the measured ones in the melt. For H2O–Cl experiments in which the Cl concentration in the melt was ≤0.4 wt.%, a negative dependence between DClf/m and pressure is observed. DClfluid/melt in H2O+Cl-bearing experiments ranges between 11–14 at 1 and 25 MPa to 6 at 200 MPa at NNO; and between 4 at 50 MPa and 13 at 100 MPa at ΔNNO≥2. Addition of CO2 at NNO yields lower partition coefficients than in CO2-free conditions over the pressure range investigated. The negative pressure dependence observed for H2O–Cl experiments disappears when CO2 is present in the system. Overall, once CO2 is introduced in the system, Cl fugacity in the silicate melt tends to increase, thus resulting in a decrease of DCl f/m. Application to Mt. Etna shows that the composition of the volcanic plume in terms of Cl records very shallow pressures of magma equilibration with its exsolved fluid.

Partitioning of halogens between gas and basaltic melts at 1200-1265°C and 10-2000 bar (Alletti, 2008) has been analyzed in detail by means of the CTSFG model (Moretti and Ottonello, 2005) in order to: 1) evaluate redox conditions consistent with related experimental information such as PH2 and S2−/S6+ ratio, 2) estimate the relative abundance of H2O, CO2, KCl, NaCl and HCl of the gas phase, 3) investigate the solubility mechanisms of chlorine and fluorine in (basaltic) melts. For chlorinebased runs, it was found that for input [Na+K]/Cl atom ratios > 0.45 the proportion of gaseous NaCl and KCl exceeds that of HCl. In terms of solubility mechanisms, halogen chemical reactivity can be assessed in terms of dissolution into the melt as chloride or fluoride ion, i.e, Cl− and F−. A complementary solubility mechanism can be invoked to improve the precision and explain somehow unexpected features, such as i) a P-independent background values of halogen solubility, and, for chlorine only, ii) a XCl,melt-squareddependence shown by Cl dissolution. In this additional mechanism, undissociated alkali-halogenides dissolve in the melt up to a solubility limit which depends on structural conditions, i.e. on melt composition for the same T. Although this could be a fascinating hypothesis, somehow recalling the concentration limits of Henry’s law behavior for trace element dissolution into minerals, it is then questioned whether this may reflect or not mixing of a molten salt-like component with the silicate melt. Partition coefficients derived from equilibrium constants of reactions involving Cl− and F− show well the role of water vapor as stripping agent of chlorine. Therefore, the more the CO2 in the system, the more the amount of chlorine getting dissolved into the melt phase.

Despite the importance of halogens (F, Cl) in volcanic degassing, solubility and fluid/melt partitioning of these elements have not been comprehensively studied in natural basaltic melts. Experimental determinations of halogen solubility in Mount Etna melts are lacking, despite this volcano being one of the most active and intensively monitored on Earth with an estimated output of thousands tonnes of halogens per day. In order to better understand halogen degassing, we present the results of a series of halogen partitioning experiments performed at different pressures (1-200 MPa), redox conditions (from Δ NNO = + 2 to Δ NNO = - 0.3) and fluid compositions. Experiments used a hawaiitic, glassy, alkaline basalt with Mg# = 0.59, sampled during the July 2001 eruption of Mount Etna. A series of experiments were conducted using H2O-NaCl or H2O-NaF solutions. The effect of CO2 in multi-component fluid H2O-CO2-NaCl or H2O-CO2-NaF was also investigated. The experimental run products were mostly glasses, but a few run products contained less than 10% crystals. The concentration of halogens in the fluid phase after the experiment was calculated from mass balance, and the partition coefficients for both Cl and F at the studied conditions determined. Using these measurements and thermodynamical models, the dependence of these partition coefficients on the fugacities of various gaseous species was investigated.