Heat capacity and viscosity of basaltic melts with H2O ± F ± CO2

Abstract

We determined the viscosity and heat capacity of a series of two basaltic liquids containing H2O, F, H2O+CO2, H2O+F, and H2O+CO2+F. One was a natural calc-alkaline basalt from Fuego volcano, Guatemala, and the other was an Fe-free synthetic analog. The viscosity measurements were performed in the low-temperature, high-viscosity range (~109–1012Pas) just above the glass transition, where the kinetics of volatile exsolution are slow. Differential scanning calorimetry measurements were performed at atmospheric pressure from room temperature up to ~100K above the glass transition. The water contents ranged from nominally anhydrous to 3wt.% H2O, with F contents up to 2wt.%, and CO2 contents up to 0.2wt.%. Volatiles do not noticeably affect the heat capacity of glasses. The glass transition temperatures obtained from calorimetry and viscometry are in good agreement. Water has a strong viscosity-reducing effect on basaltic melts. F has a measurable viscosity-reducing effect in basaltic melts, but it is significantly smaller than that of water. The combined effects of H2O and F on viscosity appear to be additive on a wt.% basis. Both the effects of H2O and F on basaltic melts are smaller than those for more polymerized melts. Small quantities of CO2 do not measurably affect basaltic melt viscosity, at least in the presence of >1wt.% water. Future viscosity models incorporating fluorine need to account for the compositional dependence of its effects on dry and hydrous melts.