An equation of state for silicate melts. III. Analysis of stoichiometric liquids at elevated pressure: shock compression data, molecular dynamics simulations and mineral fusion curves

Abstract

Experimental data and molecular dynamics simulations that constrain the densities of liquids of “mineral-like” stoichiometry at elevated pressure are evaluated using the liquid Equation of State (EOS) of Ghiorso (2004a). Phase equilibrium constraints on melt density are developed from experimental brackets on mineral fusion curves, specifically congruent melting of albite, anorthite, cristobalite/quartz/stishovite, diopside, enstatite, fayalite, forsterite, jadeite, nepheline, pyrope, sanidine, and titanite. A self consistent thermodynamic analysis of shock compression is applied to experimentally determined brackets on the Hugoniot for the liquid compositions CaMgSi₂O₆, CaAl₂Si₂O₈, Fe₂SiO₄, and (CaMgSi₂O₆)_0.64(CaAl₂Si₂O₈)_0.36. Molecular dynamics (MD) simulations of both melt density and melt structure are analyzed utilizing the liquid EOS of Ghiorso (2004a) and an ideal associated solution model that accounts explicitly for the effect of pressure and temperature on the oxygen coordination environment of silicon and aluminum. MD data on Mg₂Si₂O₆, CaMgSi₂O₆, SiO₂, Na₂Si₄O₉, CaAl₂Si₂O₈, NaAlSiO₄, NaAlSi₂O₆, and NaAlSi₃O₈ liquids are considered in conjunction with the fusion curve and shock compression experimental data sets. An internally consistent assessment of liquid EOS parameters from all available data sources is attempted for each liquid composition considered. Shock compression experiments on more chemically complex liquids of komatiite and MORB bulk composition are also examined. In support of the analysis of mineral fusion curves, volumetric properties as a function of temperature and pressure of diopside, enstatite (both <i>Pbca</i> and <i>C2/c</i>), fayalite, forsterite, and pyrope are evaluated from literature data. Experimental observations are parameterized to a Universal EOS (Vinet and others, 1986, 1987, 1989). It is found that the reference pressure properties derived by Ghiorso and Kress (2004) are internally consistent with the majority of high-pressure constraints on melt density. The notable exceptions are molten SiO₂ and melts in the system MgO-SiO₂. For the latter system, the calibration of Ghiorso and Kress (2004) fails to recover reference pressure volumes. The effect of configurational collapse (the volumetric response associated with changes in melt structure, such as changing oxygen coordination number about a cation) is systematically assessed from molecular dynamics simulation data. Fully depolymerized melts exhibit an ∼11 percent density increase associated with Si and Al transforming from IV- to V-fold coordination with respect to oxygen. Alkali aluminosilicate melts demonstrate ∼27 percent increase in melt density and SiO₂ and Na₂Si₄O₉ liquids show a density increase of ∼17 percent.