Abstract
Partitioning of Mg and Fe 2+ 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 fo 2 ∼1 log unit below the quartz-fayalite-magnetite buffer. The partition coefficient [K D = (Fe 2+/Mg) ol/(Fe 2+/Mg) melt] increases from 0.25 to 0.34 with increasing depolymerization of melt (NBO/T of melt from 0.25–1.2), and then decreases with further depolymerization of melt (NBO/T from 1.2–2.8). These variations are similar to those observed in natural basalt-peridotite systems. In particular, the variation in NBO/T ranges 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 Fe 2+ partitioning between olivine and melt must depend primarily on the composition or structure of melt. Such variations of K D may depend on the relative proportions of four-, five-, and six-coordinated Mg 2+ and Fe 2+ in melt as a function of degree of NBO/T.
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