Influence of Fe2+/Fetot on the viscosity of melts from the Colli Albani Volcanic District (Italy) – foidite to phonolite

Abstract

Viscosity of a melt strongly depends on melt composition and the ratio of network modifiers to network formers. In silicate melts, Fe exists as Fe2+ and Fe3+, both affecting the viscosity in different manners: Fe2+ acts as a network modifier and lowers the viscosity whereas Fe3+ behaves as a network former and increases the viscosity. The Fe speciation strongly depends on the oxygen fugacity fO2.In this study, the viscosities of Fe-rich synthetic tephri-phonolitic, foiditic and tephritic melts from the Colli Albani Volcanic District (Italy) are investigated as a function of Fe2+/Fetot in the glass transition range. Base glasses made in air at 1600 °C at log10fO2 = −0.68 are re-equilibrated at log10fO2 = −4 and − 7 in a gas mixing furnace at 1250 °C. Melts made in air and at log10fO2 = −4 have nearly identical Fe2+/Fetot due to the different experimental temperatures. At 700 °C, the viscosity of these melts drops by ~2, ~1.5 and ~ 0.4 log10 Pa s with increasing Fe2+/Fetot from ~0.45, ~0.39 and ~ 0.44 to ~0.76 for the tephri-phonolite, the foidite and the tephrite, respectively. These melts have H2O concentrations up to 0.03 wt%. It is shown that the addition of ~0.23 wt% H2O reduces the viscosity by the same amount as the increase in Fe2+/Fetot.In addition, the viscosities of a phonolite from Mt. Vesuvius are studied over a range of Fe2+/Fetot values (~0.41 to 0.87). In contrast to the other melts of this study, viscosity appears to increase with increasing Fe2+/Fetot. However, this change in viscosity results from a depletion of ~14 mol% in network modifying alkali and alkaline earths elements and a loss of approx. 50 atom% Fe both affecting the viscosity of this melt more effectively than the increase in Fe2+/Fetot.