Effects of water and fluorine on the viscosity of albite melt at high pressure: a preliminary investigation

The viscosities of fluorine- and water-bearing melts based on albite composition have been determined at 7.5, 15 and 22.5 kbar by the falling-sphere method. All melt viscosities decrease isothermally with increasing pressure. At 1200°C the viscosity of the fluorine-bearing melt (albite + 5.8 wt.% fluorine substituted for oxygen, denoted AbF₂O_-1) decreases from 5000 ± 750 P at 7.5 kbar to 1600 ± 240 P at 22.5 kbar. At 1400°C the viscosity of this melt decreases from 1300 ± 200 P at 7.5 kbar to 430 ± 65 P at 22.5 kbar. At 1400°C the viscosity of albite + 2.79 wt.% water (denoted AbH₂O) decreases from 650 ± 100 P at 7.5 kbar to 400 ± 60 P at 22.5 kbar. Fluorine (as F₂O_-1) and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures. The ratio of the effects of 5.8 wt.% fluorine [ F (F + O) molar = 0.10] and 2.79 wt.% water [ OH (OH + O) molar = 0.10] on the log of melt viscosity [ Δ log η(AbF₂O_-1) Δ log η(AbH₂O)] equals 0.90 ± 0.05, 0.84 ± 0.05 and 0.97 ± 0.05 at 7.5, 15 and 22.5 kbar, respectively. Comparison with available data on the high-pressure viscosity of albite melt indicates that both F₂O_-1 and H₂O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF₂O_-1 and AbH₂O, may be explained in terms of the relatively depolymerized structures of AbF₂O_-1 and AbH₂O melts. The depolymerization of albite melt by the addition of water results from the formation of SiOH bonds. The depolymerization of albite melt by F₂O_-1 substitution results from the formation of non-bridging oxygens associated with network-modifying aluminum cations that are formed upon fluorine solution. The strong viscosity-reducing effects of water and fluorine in albite melt at pressures corresponding to the mid- to lower continental crust indicate that these two components will strongly influence the dynamic behavior of anatectic melts during initial magma coalescence and restite-melt segregation.