Abstract

We determined activity-composition relationships for the Pt-Fe system by equilibrating Fe-oxides with Pt-Fe alloys at temperatures in the range of 1200–1400 °C and oxygen fugacities from 1.6 to 7.7 log units above the iron-wüstite (IW) buffer. The system is characterized by strong negative deviations from ideality throughout the investigated temperature range (e.g., γFealloy<0.02 for XFealloy <0.3). Our data are consistent with an asymmetric regular solution of the form:

 
\[R\mathit{T}\ ln\ {\gamma}^{alloy}_{Fe}\ =\ [\mathit{W}_{\mathit{G}_{1}}\ +\ 2(\mathit{W}_{\mathit{G}_{2}}\ {-}\ \mathit{W}_{\mathit{G}_{1}})\ \mathit{X}_{Fe}^{alloy}](\mathit{X}_{P\mathit{t}}^{alloy})^{2}\]

where WG1 = −138.0 ± 3.3 kJ/mol and WG2 = −90.8 ± 24.0 kJ/mol (1σ). Based on experiments at 1200–1400 °C, variations in the activity coefficients at a given composition are consistent with ln γ Fealloy(T1) / ln γFealloy(T2) = T2 / T1.

The Pt-Fe alloy composition in equilibrium with a FeO-bearing silicate liquid can be obtained from:

 
\[log_{10}\mathit{f}_{O_{2}}\ =\ log\ \{exp[ln\ \mathit{a}^{liq}_{Fe_{2}SiO_{4}}\ {-}\ ln\ \mathit{a}^{liq}_{SiO_{2}}\ {-}\ 2\ ln\ \mathit{a}^{alloy}_{Fe}\ {-}\ (\frac{{-}{\Delta}\mathit{G}^{0}_{r}}{R\mathit{T}})]\}\]

where ΔG0r is the standard state free energy for the reaction 2Fealloy + O2gas+ SiO2liq = Fe2SiO4liq. We obtained values of aFealloy from our model and used the program MELTS together with the thermodynamic properties of these elements to evaluate activities of SiO2 and Fe2SiO4 components in the liquid and ΔG0r. We provide sample calculations showing how to predict the optimum Fe concentrations for pre-saturation of Pt-bearing containers to reduce Fe loss from the charge during experiments on magmatic liquids at high temperatures and pressures from 1 atm to 40 kbar.

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