Thermodynamic properties of the Pt-Fe system

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., γ^(alloy)_Fe <0.02 for X^(alloy)_Fe <0.3). Our data are consistent with an asymmetric regular solution of the form: RTln 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: log_(10)f_(O_2) = log{exp[lna^(liq)_(Fe2SiO4) - lna^(liq)_Si)2 - 2 lna^(alloy)_(Fe) -(-ΔG^(0)_(r)/RT)]} where ΔG^(0)_(r) is the standard state free energy for the reaction 2Fe^(alloy) + O_2 + SiO^(liq)_2 = Fe_(2)SiO^(liq)_4. We obtained values of a^(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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