Effect of H2O, and combined effects of H2O + F, H2O + CO2, and H2O + F + CO2 on the viscosity of a natural basalt from Fuego volcano, Guatemala

We measured the viscosity of 5 series of remelted natural basalt from Fuego volcano, Guatemala. These series include single and multiple volatile species: H₂O, F, H₂O-F, H₂O-CO₂, and H₂O-CO₂-F. The hydrous glasses were synthesized at 3 kbar and 1250°C in Internally Heated Pressure Vessels. The multiple volatile series were synthesized at 5 kbar and 1250°C. CO₂ was added as Ag₂C₂O₄, F as AlF₃, and H₂O as distilled water. The anhydrous, F-bearing series was synthesized at 1 atm by simply remelting the Fuego basalt and adding F as CaF₂.The natural, dry, remelted Fuego basalt has an NBO/T of 0.64. The following comparisons are based on parallel-plate viscosity measurements in the range ~10⁸ to 10¹² Pa s. The temperature at which the viscosity is 10¹² Pa s (T₁₂) is taken to be the viscosimetric glass transition temperature (T_g). The addition of 2 wt.% H₂O results in a decrease of T₁₂ of ~150°C for basalt. Fluorine on its own has a measurable, but much smaller effect, than the equivalent amount of water. Indeed, ~2 wt.% F results in a T₁₂ depression of only ~30°C. When H₂O and F are both present, their effects are approximately additive. For example, the viscosity of a basalt with 1.44 wt.% H₂O is very similar to the viscosity of a basalt with ~1 wt.% H₂O and ~1.25 wt.% F, and the viscosities of a basalt with 2.29 wt.% H₂O and a basalt with ~1.65 wt.% H₂O and ~1.3 wt.% F are also very similar. The effect of CO₂ is somewhat ambiguous. The viscosity of a basalt with ~1.7 wt.% H₂O, ~1.3 wt.% F and ~0.2 wt.% CO₂ is essentially the same as the viscosity of a basalt with 2.29 wt.% H₂O, so CO₂ seems to have a negligible or even viscosity-increasing effect when F and H₂O are also present. However, a basalt with ~0.84 wt.% H₂O and ~0.09 wt.% CO₂ has about the same viscosity as a basalt with 1.34 wt.% H₂O, which could suggest a strong (viscosity-decreasing) effect of very small amounts of CO₂. These results suggest that the effects on viscosity of F in basaltic systems are small, and that the effects of combined F and H₂O are approximately additive. CO₂, when present in significant amounts (~1000s of ppm) seems to have a negligible or even a slight viscosity-increasing effect, which may partially counteract the effects of dissolved H₂O or H₂O + F. However, small amounts of CO₂ (a few 100 ppm) may further decrease the viscosity of hydrous basalt. Because the viscosity-reducing effect of added H₂O is strongest at low water contents, degassed basaltic lavas containing only small amounts of H₂O, CO₂ and F may still be substantially more fluid than truly volatile-free liquids.