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

The viscosities of fluorineand 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 12000( ` the vi~osity of the fluorine-bearing melt (albite + 5.8 wt.C~ fluorine substituted for oxygen, denoted AbF, O n) 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 13(X) _+ 200 P at 7.5 kbar to 430 + 65 P at 22.5 kbar. At 14000( . the viscosity of albite 4 2.79 wt.% ta te r (denoted AbFI20) decreases from 650 + 100 P at 7.5 kbar to 400 + 60 P at 22.5 kbar. Fluorine (as F20_ i ) and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures. "f'he ratio of the effects of 5.8 wt.~ fluorine [F / (F + O)molar = 0.10] and 2.79 wt.% watcr [OH/ IOH + O)molar = 0.10] on the log of melt viscosity IA Iog r t (AbF20 . l ) /A Iog rl(AbH20) ] 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 meh indicates that both F,O u and H2() maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, Abl-~O. i and AbH,O, may be explained in terms of the relatively depolymerized structures of AbF20 i and AbH20 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 2 0 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 midto 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.