Water solubility in trachytic and pantelleritic melts: An experimental study

Abstract

Solubility experiments were performed on a trachyte and a pantellerite from Pantelleria. The trachyte has SiO2 = 65.2 wt%, Al2O3 = 15.2 wt% and a peralkaline index (P.I. = molar[(Na2O + K2O)/Al2O3]) ∼ 1 while the pantellerite has SiO2 = 72.2 wt%, Al2O3 = 11 wt% and a P.I. = 1.3. Solubility experiments were performed in the pressure range of 50-300 MPa at T = 950 °C for the trachyte and 50-200 MPa at T = 850 °C for the pantellerite. The water content of experimental glasses was determined by Karl Fischer titration, elemental analyser and FT-IR spectroscopy. Water content appears similar in both compositions for analogous pressure conditions, varying from ∼2.5 wt% at 50 MPa to ∼5.8 at 200 MPa. Comparison of the experimental data with the thermodynamic models shows that the empirical model of Moore et al. [1998] better matches the experimental data for both compositions, while the thermodynamic models of Papale et al. [2006] and Ghiorso and Gualda [2015] tend to overestimate melt water content, probably due to the paucity of peralkaline melt compositions used for calibration. One inference of these new solubility data is on MI-derived depth of felsic reservoirs at Pantelleria which deepens from 2.4 to 3.5 km.