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., γ alloy Fe <0.02 for X alloy Fe <0.3). Our data are consistent with an asymmetric regular solution of the form: RT ln γ alloy Fe =[W G1 +2(W G2 -W G1 )X alloy Fe] (X alloy Pt ) 2 where W G1 = -138.0 ± 3.3 kJ/mol and W G2 = -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 alloy Fe (T 1 ) / ln γ alloy Fe (T 2 ) = T 2 / T 1 . The Pt-Fe alloy composition in equilibrium with a FeO-bearing silicate liquid can be obtained from: where ΔG 0 r is the standard state free energy for the reaction 2Fe alloy + O 2 gas +SiO 2 liq = Fe 2 SiO liq 4 . We obtained values of α alloy Fe from our model and used the program MELTS together with the thermodynamic properties of these elements to evaluate activities of SiO 2 and Fe 2 SiO 4 components in the liquid and ΔG 0 r . 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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Kessel et al. (2001) studied this question.