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

Olivine/melt partitioning of Sigma Fe, 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 (f(O2)) at 0.1 MPa pressure. Total iron oxide content of the starting materials was similar to 20 wt%. The f(O2) was to used to control the Fe3+/Sigma Fe (Sigma Fe: total iron) of the melts. The Fe3+/Sigma Fe and structural roles of Fe2+ and Fe3+ were determined with Fe-57 resonant absorption Mossbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of f(O2) was estimated from the M6ssbauer data and existing melt structure information. It varies by similar to 100% in melts coexisting with olivine in the FDA3 system and by about 300% in the FD system in the Fe3+/Sigma Fe range of the experiments (0.805-0.092). The partition coefficients (D-i(ol-melt) = wt% in olivine/wt% in melt) are systematic functions of f(O2) and, therefore, NBO/T of the melt. There is a D-i(ol-melt)-minimum in the FDA3 system at NBO/T-values corresponding to intermediate Fe3+/Sigma Fe (0.34-0.44). In the Al-free system, FD, where the NBO/T values of melts range between similar to 1 and similar to 2.9, the partition coefficients are positively correlated with NBO/T (decreasing Fe3+/Sigma Fe). These relationships are explained by consideration of solution behavior in the melts governed by Q''-unit distribution and structural changes of the divalent cations in the melts (coordination number, complexing with Fe3+, and distortion of the polyhedra). (c) 2006 Elsevier Inc. All rights reserved.