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

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

Jul 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..74..266d&link_type=abstract

Earth and Planetary Science Letters, Volume 74, Issue 2-3, p. 266-274.

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Scientific paper

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 Si-OH 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.

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