Anat Shahar (UCLA), Jan. 10 2007

Title: High temperature iron isotopic fractionation - an experimental approach

Abstract: A recent study (Williams et al. 2004) presented iron isotope data for a suite of mantle minerals in an attempt to determine whether iron isotopic fractionation could be used as a tracer for changes in mantle oxidation state. These workers found lower ⁵⁷Fe/⁵⁴Fe for mantle spinels with higher oxygen fugacity and an inverse correlation between ⁵⁷Fe/⁵⁴Fe and Fe³⁺ concentration in spinel. These findings contradict theory, which suggests that the highly charged Fe³⁺ will result in stiffer bonds and therefore greater ⁵⁷Fe/⁵⁴Fe. Since it is unknown whether iron isotopes at high temperature fractionate as a function of the local oxidation state, it is difficult to untangle the effects seen in these natural samples. Crystal chemistry imposes restrictions on the relationship between oxidation state and oxygen fugacity, as some minerals incorporate

Fe³⁺ into their structures (spinel) while others will not even under oxidizing conditions (olivine). In view of the lack of experimental data on the effect of oxidation on high temperature stable isotope fractionation, and in order to corroborate the Fe fractionation factors predicted by theory, we have begun high-temperature experiments designed to show how ⁵⁷Fe/⁵⁴Fe (and ⁵⁶Fe/⁵⁴Fe) changes in spinel. Piston cylinder experiments were performed using 50 mg of a mixture of fayalite (Fe₂SiO₄) and quartz (SiO₂),10 mg of hematite (Fe₂O₃), and 10 mg of water in a gold capsule ~15 mm in length. Gold is used as the capsule material due to its low tendency to alloy with iron. The hematite has been spiked with a known amount of ⁵⁴Fe to test for equilibrium. The experiments were performed at 10 kbar and 800 C. The goal is to form magnetite, fayalite, and quartz according to the reaction

Fe₂SiO₄ + 3Fe₂O₃ + SiO₂ + H₂O + H₂ 4 Fe₂SiO₄ +2Fe₃O₄ + SiO₂ + 2H₂O

We spike the reactant hematite with ⁵⁴Fe in order to trace the progress of the reaction and to make use of the three-isotope technique as a means for bracketing equilibrium compositions.