Experimental and thermodynamic constraints on mineral equilibrium inpantelleritic magmas

Abstract

Crystallization experiments on two pantellerites from Pantelleria, Italy, provide new evidence for the relationships between mineral phases in pantelleritic rocks aswell as the influence of temperature and redox conditions on mineral assemblages. Experiments were performed at 1 kbar with temperature ranging between 750–900°C, and fluid saturation conditionswith XH2O (=H2O/H2O+CO2) between 0 and 1. Redox conditionswere fixed at, or slightly below, the FMQbuffer. Results showthat at temperature of 900 °C pantelleriticmagmas arewell above the liquidus regardless their water content; we also observed a decrease in liquidus temperature (800°C) with increasingly reducing conditions. Mineral assemblages of the natural rocks have been successfully reproduced, particularly the relationship between fayalite and aenigmatite, which appear to be strongly controlled by melt peralkalinity, temperature and redox conditions. This is the first time that fayalitic olivine have been synthetized in experimental studies on pantellerites, which was found to be stable only for temperatures ≥750°C while amphibole can be stable at temperatures as high as 800°C at high fF2. Experimental results have been compared with the composition of mineral phases as well as with the results obtained from mineral equilibrium, geothermometry, and oxygen barometry studies on pantelleritic lava and tuffs from Pantelleria (Italy), Eburru (Kenya) and Menengai (Kenya). Petrological characteristics appear similar at different locations worldwide, with typical assemblages of anorthoclase and sodian clinopyroxene with variable fayalite, aenigmatite, Fe-Ti oxides and amphibole.