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 $\mathrm{SiO}_{2}=65.2$ wt%, $\mathrm{Al}_{2}\mathrm{O}_{3}=15.2$ wt% and a peralkaline index $(\mathrm{P.I.} = \mathrm{molar}[(\mathrm{Na}_{2}\mathrm{O}+ \mathrm{K}_{2}\mathrm{O})/\mathrm{Al}_{2}\mathrm{O}_{3}]) \sim 1$ while the pantellerite has $\mathrm{SiO}_{2}=72.2$ wt%, $\mathrm{Al}_{2}\mathrm{O}_{3}=11$ wt% and a $\mathrm{P.I.}=1.3$. Solubility experiments were performed in the pressure range of 50–300 MPa at ${T}=950~\text{°C}$ for the trachyte and 50–200 MPa at ${T}=850~\text{°C}$ 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 ${\sim }2.5$ wt% at 50 MPa to ${\sim }5.8$ 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.

Keywords

• Alkaline magmas
• Trachyte
• Pantellerite
• Water solubility
• Experiments
• Alkaline magmas