Experimental Crystallization of a High-K Arc Basalt: the Golden Pumice, Stromboli Volcano (Italy)

Abstract

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.

Carbon dioxide in silica-undersaturated melt. Part I: The effect of mixed alkalis (K and Na) on CO2 solubility and speciation

“…Reported P, T measurements are accurate to within ± 20 bars and ± 5ºC, respectively. All runs were ended by drop-quenching the sample holder into the cold part of the IHPV, which resulted in quench rates of 150°C/s for that kind of apparatus (Di Carlo et al, 2006;Iacono-Marziano et al, 2008;Morizet et al, 2010). In all cases, opening of the capsules produced a hissing sound suggesting that an excess fluid phase was present in the capsule during the experiment.…”

A model of degassing for Stromboli volcano

“…An intermediate magma ponding zone at Stromboli is also supported by geodetic data (Bonaccorso et al, 2008); (ii) secondly, for the magma to become extensively de-hydrated, it is required that gas bubbles escape from this ponding zone, a fact which might be favoured by the presence of a geological discontinuity (the interface between volcanic rocks and the basement lies at about 2.4-3.5 km depth; Di Roberto et al, 2008), and/or promoted by vesicularity of the magma reaching a critical threshold for gas percolation (and permeable gas flow) (Burton et al, 2007a). Whatever the cause, magma dehydrated thus implies gas-melt separation (and thus transition to open-system degassing regime) at 2-4 km bsv depth; (iii) finally, de-hydration of the stored magmas raises their liquidus temperatures, hence promoting extensive crystallization (Métrich et al, , 2010Di Carlo et al, 2006), and ultimately leading to transition from the LP to the H 2 O poor (b1.5 wt.%; Fig. 7a) and crystal-rich (30-50%) HP magma (Fig.…”

Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5-ka story inferred from in situ Sr-isotope and trace element data

“…On the contrary, plagioclase is the dominant mineral phase of the shallow degassed HP-magmas where it crystallizes in equilibrium with clinopyroxene and olivine (e.g., Francalanci et al, 2004;Landi et al, 2004;Di Carlo et al, 2006;Bertagnini et al, 2008). scoria-pumice mingling) because only in those cases they arrive directly to the surface without mixing with the HP-magmas at shallow levels (Francalanci et al, 1999Armienti et al, 2007;Fornaciai et al, 2009).…”