Phase Equilibrium Constraints on Pre-eruptive Conditions of Recent Felsic Explosive Volcanism at Pantelleria Island, Italy

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Abstract
We present experimental phase equilibria carried out on a pantelleritic bulk-rock composition with a peralkalinity index [PI = molar (Na2O + K2O)/Al2O3] = 1·4, which is representative of the most energetic pumice fall eruption of the resurgent post-caldera volcanism on Pantelleria island. For the explored conditions (P = 25–150 MPa; T = 680–800°C; H2Omelt up to 6 wt %; fO2 ≤ NNO, where NNO is nickel–nickel oxide buffer) clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz. The crystallization of amphibole is limited to temperatures below 700°C. Aenigmatite crystallizes near the liquidus for P ≥ 100 MPa. When clinopyroxene is the sole liquidus phase, the coexisting melt is characterized by a decrease in peralkalinity index to 1·2, if compared with the starting material, approaching comendite composition. In contrast, when alkali feldspar joins the liquidus phase assemblage, residual melts become extremely peralkaline (PI = 3·2), with FeO contents in excess of 14 wt %. Comparison of our experiments with natural phase abundances and compositions constrains the depth of the Fastuca pantellerite reservoir to be around 5 ± 1 km (or P = 120 ± 20 MPa) at T = 730 ± 10°C, with H2Omelt around 4 wt %, and fO2 1·5 log unit below NNO. Such conditions correspond to the dominant phase assemblage of feldspar, clinopyroxene and aenigmatite. Amphibole occurrence requires both higher melt water contents and lower temperatures (up to 6·5 wt % at 680°C), and may reflect provenance from a deeper and wetter portion of the tapped reservoir. In contrast, quartz is more probably related to crystallization during magma ascent, which explains the lower melt water content of its melt inclusions. Overall, the results confirm that pantellerites may be both water-rich and reduced magmas. These characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species, the latter amounting to a few mole per cent. Magma unrest at Pantelleria may thus be heralded by emissions of hydrous and reduced gases (i.e. CO2-poor and H2-rich), if the present-day reservoir is felsic in composition.

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