Iron and titanium solution properties in peraluminous and peralkaline rhyolitic liquids

Abstract

The saturation surface of pseudobrookite (Fe2TiO5) was determined for melts in the system SiO2-Al2O3-K2O-FeO-Fe2O3-TiO2 at 1400° C and 1 atm. The variation in concentrations of Fe2O3, TiO2 and Fe2TiO5 in liquids can be used to infer relative changes in activity coefficients of these components with changing K2O/(K2O+Al2O3) of the melts. Saturation concentrations of these components are low and relatively constant in the peraluminous melts and increase with increasing K2O/(K2O+Al2O3) in peralkaline liquids. The activity coefficients of Fe2O3 and TiO2 and Fe2TiO5, therefore, are higher in peraluminous liquids than in peralkaline liquids in this system. In addition, the iron redox ratio was measured as a function of K2O/(K2O+Al2O3) for liquids just below the saturation surface; \(f_{{\text{O}}_{\text{2}} }\) was fixed so all variations in redox ratio are entirely due to changes in melt composition. The redox ratio from unsaturated liquids was applied to saturated liquids where redox analysis of the glass is impossible. The homogeneous equilibrium experiments indicate that the activity coefficient of Fe2O3 relative to that of FeO is significantly greater in peraluminous melts than peralkaline melts. Both the heterogeneous and homogeneous equilibria suggest that in peralkaline liquids K+in excess of that required to charge balance tetrahedral Al+3 is used to stabilize both Fe+3 and Ti+4. Calculations show that ferric iron and titanium compete equally effectively for charge-balancing potassium but neither can outcompete aluminum. The observed changes in solution properties of Fe2O3 and TiO2 in the synthetic melts are used to explain variations in Fe-Ti oxide stabilities in natural peraluminous and peralkaline rhyolites and granites. Since the activity coefficients of both ferric iron and titanium are significantly higher in peraluminous liquids than in peralkaline liquids, Fe-Ti oxides should occur earlier in the crystallization sequence in peraluminous rhyolites than in peralkaline rhyolites. In addition, iron will be reduced in peraluminous granites and rhyolites relative to peralkaline ones under comparable P, T, and \(f_{{\text{O}}_{\text{2}} }\). Finally, observed crystallization patterns for minerals containing highly charged cations other than ferric iron and titanium are evaluated in the context of this and other experimental studies.