Redox equilibria of iron and silicate melt structure: Implications for olivine/melt element partitioning

Olivine/melt partitioning of ΣFe, Fe2+, Mg2+, Ca2+, Mn2+, Co2+, and Ni2+ has been determined in the systems CaO–MgO–FeO–Fe2O3–SiO2 (FD) and CaO–MgO–FeO–Fe2O3–Al2O3–SiO2 (FDA3) as a function of oxygen fugacity (fO2fO2) at 0.1 MPa pressure. Total iron oxide content of the starting materials was ∼20 wt%. The fO2fO2 was to used to control the Fe3+/ΣFe (ΣFe: total iron) of the melts. The Fe3+/ΣFe and structural roles of Fe2+ and Fe3+ were determined with 57Fe resonant absorption Mössbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of fO2fO2 was estimated from the Mössbauer data and existing melt structure information. It varies by ∼100% in melts coexisting with olivine in the FDA3 system and by about 300% in the FD system in the Fe3+/ΣFe range of the experiments (0.805–0.092). The partition coefficients (Diol-melt=wt% in olivine/wt% in melt) are systematic functions of fO2fO2 and, therefore, NBO/T of the melt. There is a Diol-melt-minimum in the FDA3 system at NBO/T-values corresponding to intermediate Fe3+/ΣFe (0.34–0.44). In the Al-free system, FD, where the NBO/T values of melts range between ∼1 and ∼2.9, the partition coefficients are positively correlated with NBO/T (decreasing Fe3+/ΣFe). These relationships are explained by consideration of solution behavior in the melts governed by Qn-unit distribution and structural changes of the divalent cations in the melts (coordination number, complexing with Fe3+, and distortion of the polyhedra).