Activity Coefficients of Silicon in Iron-nickel Alloys: Experimental Determination and Relevance for Planetary Differentiation

Introduction: Metal segregation und subsequent formation of an FeNi core is the major differentiation process in large planets such as the earth, but metal separation and aggregation will also occur in smaller planetsimals such as asteroids. Core formation implies partial or full equilibration of core forming FeNi with liquid silicates, implying partial or complete redistribution of siderophile elements between metal and silicates. This also occurs for the major components of molten silicates such as SiO2 which will be partly reduced and partition into core forming metal. Most groups of iron meteorites are thought to be derived from metal cores or metal aggregations that formed during melting of planetesimals. The Si content of iron meteorites should thus record the conditions that prevailed during metal-silicate equilibration. The quantification of this process requires a better understanding of the partitioning of Si between metal and silicate which depends on temperature, oxygen fugacity, silicate composition and nickel content of the alloy. One additional parameter that is not well known is the activity coefficient of Si in FeNi-alloys. Literature data [1] only exist for very high Si concentrations, much larger than expected for iron meteorites [2]. We therefore performed experiments at lower Siconcentrations and at various temperatures and alloy compositions to determine the activity coefficients of silicon in iron-nickel alloys. Experimental: Silicates consisting of an eutectic, quartz saturated anorthite-diopside-quartz-mixture were equilibrated with FeNi-loops in one atmosphere furnaces with controlled oxygen fugacity. The alloy composition was varied from pure iron to pure nickel. Temperature ranged from 1300 to 1400°C. After equilibration times of 2 to 3 days, samples were quenched to glass by quick removal from the furnace, polished sections were prepared and metal and silicates were analyzed with the EMP. The oxygen fugacity was controlled with CO/CO2 gas mixing and measured with an yttrium-oxide-stabilized zirconiumoxide sensor. The f(O2) was also determined by measuring the FeO-content in the silicate glass and calculating the f(O2) according to the reaction: Fe + 0.5 O2 → FeO (eq. 1) In the calculation activity of FeO in silicate melts an activity coefficient of 1.7 was used according to [3]. Both methods for the determinations of f(O2) agreed to within 0.1 log units at 1400°C and 1350°C, and to within 0.4 log units at 1300°C. Due to the low silicon content in metal, high probe currents of approximately 1.5 μA, voltages from 10 to 15 kV and beam diameters from 10 to 20 μm were used. Silicon concentrations in metal varied from 140 ppm to 7600 ppm. In all cases Si profiles through the metal loop (0.1 mm metal foil thickness) were measured. Only experiments without zoning of Si in metal were considered. Results: From the mass fraction of silicon in the alloy measured by EMP γSi was calculated by the following equation: