The near-liquidus crystallization of a high-K basalt (PST-9 goldenpumice, 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, atpressures from 50 to 400 MPa, for melt H2O concentrationsbetween 1 2 and 5 5 wt % and DNNO ranging from 0 07 toþ2 32. A drop-quench device was systematically used. AuPd alloyswere used as containers in most cases, resulting in an average Fe lossof 13% for the 34 charges studied. Major crystallizing phasesinclude clinopyroxene, olivine and plagioclase. Fe–Ti oxide wasencountered in a few charges. Clinopyroxene is the liquidus phaseat 400 MPa down to at least 200MPa, followed by olivine andplagioclase. The compositions of all major phases and glass varysystematically with the proportion of crystals. Ca in clinopyroxenesensitively depends on the H2O concentration of the coexisting melt,and clinopyroxene Mg-number shows a weak negative correlationwith DNNO. The experimental data allow the liquidus surface ofPST-9 to be defined. When used in combination with melt inclusiondata, a consistent set of pre-eruptive pressures (100–270MPa),temperatures (1140–1160 C) and melt H2O concentrations isobtained. Near-liquidus phase equilibria and clinopyroxene Cacontents require melt H2O concentrations <2 7–3 6 and 3 1 wt %, respectively, overlapping with the maximum frequency ofglass inclusion data (2 5–2 7 wt % H2O). For olivine to crystallizeclose to the liquidus, pressures close to 200MPa are needed.Redox conditions around DNNO ¼ þ0 5 are inferred from clinopyroxenecompositions. The determined pre-eruptive parametersrefer to the storage region of golden pumice melts, which is locatedat a depth of around 7 5 km, within the metamorphic arc crust.Golden pumice melts ascending from their storage zone along anadiabat will not experience crystallization on their way to the surface.
|Numero di pagine||27|
|Rivista||Journal of Petrology|
|Stato di pubblicazione||Published - 2006|
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology