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

Abstract

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 AbF 2O −1) decreases from5000 ± 750P at7.5kbar to1600 ± 240P at22.5kbar. At 1400°C the viscosity of this melt decreases from1300 ± 200P at7.5kbar to430 ± 65P at22.5kbar. At 1400°C the viscosity of albite + 2.79 wt.% water (denoted AbH 2O) decreases from650 ± 100P at7.5kbar to400 ± 60P at22.5kbar. Fluorine (as F 2O −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(AbF 2O −1)/Δ log η(AbH 2O)] equals0.90 ± 0.05, 0.84 ± 0.05and0.97 ± 0.05at7.5, 15and22.5kbar, respectively. Comparison with available data on the high-pressure viscosity of albite melt indicates that both F 2O −1 and H 2O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF 2O −1 and AbH 2O, may be explained in terms of the relatively depolymerized structures of AbF 2O −1 and AbH 2O melts. The depolymerization of albite melt by the addition of water results from the formation of Si OH bonds. The depolymerization of albite melt by F 2O −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.