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Temperature-pressure-composition model for melt viscosity in the Di-An-Ab system

University of British Columbia

A model for the viscosity (η) of melts in the system CaMgSi₂O₆-CaAl₂Si₂O₈-NaAlSi₃O₈ (Di-An-Ab) based on data compiled from the literature is presented. The model is calibrated on 560 measurements of melt viscosity at 1 bar pressure on 40 individual melt compositions and 303 measurements obtained at high pressures on 18 melt compositions. The model uses the Tammann-Vogel- Fulcher equation for temperature (T)- dependence and accounts for variations in melt composition (X) and pressure (P). At ambient pressures, the model spans 15 orders of magnitude of η, a T range of 933-2450 K, and reproduces the datasets well (RMSE = 0.26). The high-P model is calibrated over a T range of 970-2470 K and P range of 50 to 13100 MPa and reproduces the high-pressure data sets to within (larger) experimental uncertainties (RMSE = 0.35). The T-P-X model predicts the glass transition temperatures (Tg), melt fragilities (m) and activation energies (Ea) across the ternary with increasing P. Used in conjunction with chemical proxies for melt structural organization (i.e. SM and NBO/T), the model informs on changes in melt polymerization as functions of T, X, and P. Di-An-Ab melts with SM values > 25 have positive pressure coefficients and show a minimal to substantial increase in viscosity with increasing P. Melts with SM less than 25 show negative P-coefficients indicating a decrease in viscosity with increasing P. The P-effect might be correlated with the degree of polymerization and explained by the increasing coordination numbers of silicate and aluminum cations at elevated pressures. Model values of Tg for Di-An-Ab melts range between 930 K and 1130 K at ambient conditions; whilst at elevated pressures, Tg values are depressed for highly polymerized melts by as much as 150K at 5000 MPa but are higher for more depolymerized melts (i.e. SM > 25). A comparison of the liquidus surface topology for the ternary system to the corresponding isokoms of viscosity shows the relative effects of X and T on melt viscosity to be highly varied.

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2020-09-01

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/75812

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/