MELT VISCOSITIES IN THE SYSTEM NA-FE-SI-O-F-CL - CONTRASTING EFFECTS OF F AND CL IN ALKALINE MELTS

The shear viscosities of melts in the system Na-Fe-Si-O-F-Cl were dete rmined over a wide range of temperatures (400-1200 degrees C) at 1 atm pressure in air. The compositions are based on the addition of Fe2O3, FeCl3, and FeF3 to a base melt composition corresponding to sodium di silicate (Na2Si2O5). Viscosities were determined using concentric cyli nder and micropenetration methods and measurements span the range of 1 0(0.5) to 10(11) Pa.s. The chemical compositions of these melts were a nalyzed after the viscometry determinations. The iron is fully oxidize d under the conditions of the viscometry. Although F and especially Cl are volatile elements in silicate melts, levels of Cl and F up to ove r 3 and 4 wt%, respectively, were stabilized in these melts, assisted presumably by the presence of Fe3+. Although some volatilization occur red during the original synthesis of these samples, none occurred duri ng viscometry. The anionic substitutions Cl2O-1 and F2O-1 have very di fferent influences on the viscosity. The F2O-1 substitution causes a d rastic decrease in viscosity over the entire investigated range wherea s the Cl2O-1 substitution causes a much smaller decrease in viscosity in the high viscosity range and a slight increase in viscosity in the low viscosity range. As a consequence, minor to major element abundanc e of Cl in strongly peralkaline undersaturated volcanic rocks are not likely to significantly influence melt viscosity.