New thermodynamic treatment of the partitioning of iron with different valence and spin states between Mg-perovskite and ferropericlase

Abstract:

The partitioning of iron between Mg-perovskite (Mg-Pv) and ferropericlase (Fp) is largely related to the phase relation and dynamics of the lower mantle. It has long been treated by the K_D value, (Fe/Mg)_Pv/(Fe/Mg)_Fp. However, this treatment is not correct for lower mantle Mg-Pv because iron in Mg-Pv in the lower mantle (assumed as pyrolite composition) involves both Fe²⁺ and Fe³⁺, and the partitioning of iron is largely affected by the amount of Al and the spin transition of iron with pressure. Here, we propose a new thermodynamic treatment of the partitioning of iron with different valence and spin states between Mg-Pv and Fp in the lower mantle. Experimental reports of the pyrolite composition indicate that Mg-Pv contains nearly equal amounts of Fe and Al, and Fe³⁺ is more dominant than Fe²⁺ and coupling with Al. Also, while Fe²⁺ remains as high spin (HS) at the A site, Fe³⁺ changes from HS to low spin (LS) and shifts from the A site to the B site with pressure in Al-bearing Mg-Pv. Therefore, Mg-Pv in the lower mantle can be modeled as the solid solution of 5 end-members, MgSiO₃, ^HSFe²⁺SiO₃, ^HSFe³⁺AlO₃, Al^LSFe³⁺O₃, and Al₂O₃, where the first and second cations indicate the A site and B site cations, respectively. Likewise, Fp can be modeled as the solid solution of 3 end-members, MgO, ^HSFe²⁺O, and ^LSFe²⁺O. In addition, Fe metal exists as an isolated phase. Using this model of coexisting Mg-Pv, Fp, and Fe-metal, the four partition coefficients of iron, involving ^HSFe²⁺, ^HSFe³⁺, and ^LSFe³⁺ in Pv, and ^HSFe²⁺ and ^LSFe²⁺ in Fp, are derived. It is now required to experimentally determine these four partition coefficients of iron at the various pressure-temperature conditions.