Experimental constraints on pre-eruptive conditions of a chemically-zoned peralkaline ignimbrite: the Green Tuff eruption at Pantelleria Island (Italy)

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Abstract

Pantelleria island is the type locality for pantellerite, aniron and alkali-rich rhyolite. The eruptive products outcroppingin the island fall in a mafic end member (mildly alkalinebasalt) and a felsic end member (metaluminous trachytes andpantellerites). A key event in the volcanological history of theisland is the Green Tuff eruption the sole ignimbrite atPantelleria compositionally zoned from crystal-poorpantellerite at the base to crystal-rich trachyte at the top.We experimentally investigated the phase relations of thepantelleritic end member AI = 1.8 (agpaitic index = molar ratioNa2O+K2O/Al2O3) and trachyte end member AI = 1.05 of theGreen Tuff eruption. The intensive variables were investigatedby performing phase equilibrium experiments at 0.5, 1 and1.5 kbar, 750-950°C, fluid saturation conditions with XH2O=(H2O/H2O+CO2) between 0 and 1, and redox conditions closeto FMQ (fayalite-magnetite-quartz buffer).Preliminary results show that at 850°C pantelleritic chargeswere above the liquidus regardless their water content. Below800°C clinopyroxene is the liquidus phase followed byamphibole and alkali feldspar. Aenigmatite and quartzcrystallized at 750°C and XH2O lower than 0.8. Consideringthe other end-member, trachytic charges at 850°C are stronglycrystallized. The liquidus phase is clinopyroxene crystallizingat 950°C and XH2O<0.8 followed by iron-rich olivine andalkali feldspar. Iron-bearing minerals record the effect of bothH2O and fO2, showing progressive iron enrichment when XH2Odecreases. Alkali felspar becomes the most abundant mineralphase when XH2O<0.8 at 900°C or XH2O<1 at 850°C both at1 and 1.5 kbar. Experiments reproduce well the mineralassemblages of the natural rocks, giving information onmagma storage conditions and showing that compositionalzoning in magma chamber is related to gradients intemperature and volatile contents. A still open question is theparent-daugther relationship between trachyte and pantellerite.Even though pantelleritic magma evolution has been wellreproduced experimentally more experiments are needed toclarify this issue.
Lingua originaleEnglish
Numero di pagine1
Stato di pubblicazionePublished - 2015

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