Abstract

Partitioning of Mg and Fe2+ between olivine and mafic melts has been determined experimentally for eight different synthetic compositions in the temperature range between 1335 and 1425°C at 0.1 MPa pressure and at fo2 approximately 1 log unit below the quartz-fayalite-magnetite buffer

The partition coefficient [KD=(Fe2+/Mg)ol/(Fe2+/Mg)melt] increases from 0.25 to 0.34 with increasing depolymerization of melt (NBO/T of melt from 0.25 to 1.2), and then decreases with further depolymerization (NBO/T from 1.2 to 2.8). These variations are similar to those observed in natural basalt-peridotite systems. In particular, the variation in the NBO/T range for basaltic-picritic melts (0.4-1.5) is nearly identical to that obtained in the present experiments. Because the present experiments were carried out at constant pressure (0.1 MPa) and in a relatively small temperature range (90°C), the observed variations of Mg and Fe2+ partitioning between olivine and melt must depend primarily on the composition or structure of melt. Such variations of KD may depend on the relative proportions of four-, five- and six-coordinated Mg2+ and Fe2+ in melt as function of degree of depolymerization of melt (NBO/T).