Effect of oxygen fugacity on the glass transition, viscosity and structure of silica- and iron-rich magmatic melts.

Iron oxidation state affects the physical properties and structure of natural melts and, ultimately, magmatic and volcanic processes such as magma transport and eruptive style. Here, we study the glass transition temperature and melt viscosity of an iron-rich peralkaline rhyolite as a function of the iron oxidation state. Glass transition temperature is determined using a differential scanning calorimetry up to 680 °C. High-temperature viscosity is measured at 1385 °C using the concentric cylinder technique. Both the glass transition temperature and viscosity increase with oxygen fugacity conditions. Although significant changes in structure with temperature are expected, we observe a direct relationship between the effect of oxygen fugacity on bulk properties at high and low temperature and polymerization of glass structure as inferred by Raman spectroscopy. Our results show that viscosity models should consider the effect of oxygen fugacity on the melt viscosity at eruptive temperature together with the effect of water and crystal content.