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\xb0C the viscosity of the fluorine-bearing melt (albite + 5.8 wt.% fluorine substituted for oxygen, denoted AbF2O\u22121) decreases from5000 \xb1 750P at7.5kbar to1600 \xb1 240P at22.5kbar. At 1400\xb0C the viscosity of this melt decreases from1300 \xb1 200P at7.5kbar to430 \xb1 65P at22.5kbar. At 1400\xb0C the viscosity of albite + 2.79 wt.% water (denoted AbH2O) decreases from650 \xb1 100P at7.5kbar to400 \xb1 60P at22.5kbar. Fluorine (as F2O\u22121) 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 [\u0394 log \u03b7(AbF2O\u22121)/\u0394 log \u03b7(AbH2O)] equals0.90 \xb1 0.05, 0.84 \xb1 0.05and0.97 \xb1 0.05at7.5, 15and22.5kbar, respectively.\n\nComparison with available data on the high-pressure viscosity of albite melt indicates that both F2O\u22121 and H2O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF2O\u22121 and AbH2O, may be explained in terms of the relatively depolymerized structures of AbF2O\u22121 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\u22121 substitution results from the formation of non-bridging oxygens associated with network-modifying aluminum cations that are formed upon fluorine solution.\n\nThe 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.