A model for silicate melt viscosity in the system CaMgSi2O6-CaAl2Si2O8-NaAlSi3O8

摘要

Five hundred eighty-five viscosity measurements on 40 melt compositions from the ternary system CaMgSi2O6 (Di)-CaAl2Si2O8 (An)-NaAlSi3O8 (Ab) have been compiled to create an experimental database spanning a wide range of temperatures (660-2175 degrees C). The melts within this ternary system show near-Arrhenian to strongly non-Arrhenian properties, and in this regard are comparable to natural melts. The database is used to produce a chemical model for the compositional and temperature dependence of melt viscosity in the Di-An-Ab system. We use the Vogel-Fulcher-Tammann equation (VFT: log eta = A + B/(T - C)) to account for the temperature dependence of melt viscosity. We also assume that all silicate melts converge to a common viscosity at high temperature. Thus, A is independent of composition, and all compositional dependence resides in the parameters B and C. The best estimate for A is -5.06, which implies a high-temperature limit to Viscosity Of 10(-5.06) Pa s. The compositional dependence of B and C is expressed by 12 coefficients (b(i=1,2.6,) C-j=1,C-2..6) representing linear (e.g., b(i=1:3)) and higher order, nonlinear (e.g., b(i=4:6)) contributions. Our results suggest a near-linear compositional dependence for B (< 10% nonlinear) and C (< 7% nonlinear). We use the model to predict model VFT functions and to demonstrate the systematic variations in viscosity due to changes in melt composition. Despite the near linear compositional dependence of B and C, the model reproduces the pronounced nonlinearities shown by the original data, including the crossing of VFT functions for different melt compositions. We also calculate values of T-g for melts across the Di-An-Ab ternary system and show that intermediate melt compositions have T-g values that are depressed by up to 100 degrees C relative to the end-members Di-An-Ab. Our non-Arrhenian viscosity model accurately reproduces the original database, allows for continuous variations in theological properties, and has a demonstrated capacity for extrapolation beyond the original data. Copyright (c) 2005 Elsevier Ltd.