Mg-Fe partitioning between olivine and mafic-ultramafic melts

The partitioning of Mg and Fe between olivine and mafic-ultramafic melts has been examined based on previous and new experimental data. We find that the exchange partition coefficient, K-D, changes primarily as a function of melt composition, expressed either as MgO+FeO content or degree of polymerization, NBO/T, rather than temperature or pressure. K-D increases from about 0.23 +/- 0.02 at an NBO/T of similar to 0 to a maximum of 0.35 +/-0.02 at an NBO/T of similar to 2, and then decreases to 0.23 +/-0.02 for the pure olivine system (NBO/T=4). We suggest that the following empirical relation yields a reliable estimate of KD for mantle derived melts: K-D = 0.235 + 0.120.(NBO/T) - 0.031.(NBO/T)(2). The change of K-D shown here should be taken into account when calculating the composition of olivine coexisting with basaltic to ultramafic melts.