Determination of Specific Component Activities in Tholeiitic Melts: Principles and Technique

A method is presented for the determination of in situ FeO activities in polycomponent melts. The method utilizes the solubility of iron in platinum according to the equation: Fe$_{\text{Pt}}$ + $\frac{1}{2}$O$_{2}$ = FeO$_{(\text{melt})}$. The activities of FeO in the melt have been determined by using the following expression. a$_{\text{FeO(melt)}}$ = $\frac{p_{\text{O}_{2}}^{\frac{1}{2}}}{p_{\text{O}_{2}}^{0\frac{1}{2}}}$ a$_{\text{FeO}}^{0}$, where a$_{\text{FeO}}^{0}$ is the activity of stoichiometric FeO in the particular iron oxide and p$_{\text{O}_{2}}^{0\frac{1}{2}}$ is the oxygen pressure in equilibrium with solid oxide and a Pt-Fe alloy containing the same amount of Fe as determined in the melt experiment. Attention is drawn to the possible errors which may be incurred by extrapolating values of a$_{\text{FeO}}$ from simple binary or ternary systems to multicomponent magmatic liquids, due to the presence of component interaction effects.