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

- Effect of F is much smaller than effect of H2O on the viscosity of basaltic melts.
- Effects of F and H2O on viscosity and heat capacity are additive, on wt.% basis.
- Small concentrations of CO2 have negligible effect on viscosity and heat capacity.

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 (~ 109-1012 Pa s) 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 ~ 100 K above the glass transition. The water contents ranged from nominally anhydrous to 3 wt.% H2O, with F contents up to 2 wt.%, and CO2 contents up to 0.2 wt.%. 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 > 1 wt.% water. Future viscosity models incorporating fluorine need to account for the compositional dependence of its effects on dry and hydrous melts.