Phase equilibrium constraints on pre-eruptive conditions of recent felsic explosive volcanism at Pantelleria Island, Italy.

We present experimental phase equilibria carried out on a pantelleriticbulk-rock composition with a peralkalinity index [PI¼molar(Na2OþK2O)/Al2O3]¼1·4, which is representative of the mostenergetic pumice fall eruption of the resurgent post-caldera volcanismon Pantelleria island. For the explored conditions (P¼25^150MPa; T¼680^8008C; H2Omelt up to 6 wt %; fO2 NNO,where NNO is nickel^nickel oxide buffer) clinopyroxene is the liquidusphase followed by alkali feldspar and then quartz.The crystallizationof amphibole is limited to temperatures below 7008C.Aenigmatite crystallizes near the liquidus for P 100MPa.Whenclinopyroxene is the sole liquidus phase, the coexisting melt is characterizedby a decrease in peralkalinity index to 1·2, if compared withthe starting material, approaching comendite composition. In contrast,when alkali feldspar joins the liquidus phase assemblage, residualmelts become extremely peralkaline (PI¼3·2), with FeOcontents in excess of 14 wt %. Comparison of our experiments withnatural phase abundances and compositions constrains the depth ofthe Fastuca pantellerite reservoir to be around 5 1km (orP¼120 20MPa) at T¼730 108C, with H2Omelt around4wt %, and fO2 1·5 log unit below NNO. Such conditions correspondto the dominant phase assemblage of feldspar, clinopyroxeneand aenigmatite. Amphibole occurrence requires both higher meltwater contents and lower temperatures (up to 6·5 wt % at 6808C),and may reflect provenance from a deeper and wetter portion of thetapped reservoir. In contrast, quartz is more probably related to crystallizationduring magma ascent, which explains the lower meltwater content of its melt inclusions. Overall, the results confirmthat pantellerites may be both water-rich and reduced magmas.These characteristics imply that any coexisting fluid phase in the reservoirwill be dominated by water and hydrogen species, the latteramounting to a few mole per cent. Magma unrest at Pantelleriamay thus be heralded by emissions of hydrous and reduced gases(i.e. CO2-poor and H2-rich), if the present-day reservoir is felsic incomposition.