REDOX EQUILIBRIA OF IRON AND SILICATE MELT STRUCTURE: IMPLICATIONS FOR OLIVINE/MELT ELEMENT PARTITIONING

Abstract

Olivine/melt partitioning of ?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-Al2 O3-SiO2 (FDA3) as a function of oxygen fugacity (fO2) at 0.1 MPa pressure. Total iron oxide content of the starting materials was ?20 wt%. The fO2 was to used to control the Fe3+/?Fe (?Fe: total iron) of the melts. The Fe3+/?Fe and structural roles of Fe2+ and Fe3+ were determined with 57Fe resonant absorption M?ssbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of fO2 was estimated from the M?ssbauer 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+/?Fe range of the experiments (0.805-0.092). The partition coefficients (Diol - melt = wt % in olivine/wt% in melt) are systematic functions of fO2 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+/?Fe (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+/?Fe). 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). ? 2006 Elsevier Inc. All rights reserved.