Use of FePt alloys to eliminate the iron loss problem in 1 atmosphere gas mixing experiments: Theoretical and practical considerations

Abstract

Theoretical and practical considerations are combined to place limits on the iron content of an FePt alloy that is in equilibrium with silicate melt, olivine and a gas phase of known $f_{{\text{O}}_{\text{2}} }$. Equilibrium constants are calculated for the reactions:

$$2{\text{Fe}}^{\text{o}} + {\text{SiO}}_{\text{2}} + {\text{O}}_{\text{2}} \rightleftharpoons {\text{Fe}}_{\text{2}} {\text{SiO}}_{\text{4}}$$

((1))

$${\text{Fe}}^{\text{o}} + \frac{1}{2}{\text{O}}_{\text{2}} \rightleftharpoons {\text{FeO}}$$

((2))

.

These equilibria may be used to choose an appropriate iron activity for the FePt alloy of an experiment. The temperature dependence of the equilibrium constants is calculated from experimental data. The Gibbs free energy of reaction (1) obtained using thermochemical data is in close agreement with ΔG_rxn calculated from the experimental data. Reaction (1) has the advantage that it is independent of the Fe²⁺/Fe³⁺ ratio of the melt, but is limited to applications where olivine is a crystallizing phase and requires a formulation for $a_{{\text{SiO}}_{\text{2}} }^{{\text{liq}}}$. Reaction (2) uses an empirical approximation for the FeO/Fe₂O₃ ratio of the liquid, and is independent of olivine saturation. However, it requires a formulation for a ^liq_FeO . Either equilibrium constant may be used to calculate the appropriate FePt alloy in equilibrium with a silicate melt.

If experiments are conducted at an $f_{{\text{O}}_{\text{2}} }$ parallel that of a buffer assemblage, a small range of FePt alloys may be used over a large temperature interval. For example, an alloy containing from 6 % to 9 % Fe by weight is in equilibrium with olivine-saturated tholeiites and komatiites at the quartzfayalite-magnetite buffer over the temperature interval 1,400° C to 1,100° C. Lunar basalt liquids in equilibrium with olivine at 1/2 log unit below the iron-wüstite buffer require an FePt alloy that contains 30–50 wt. % iron over a similar temperature interval.