A possible effect of melt structure on the Mg-Fe partitioning between olivine and melt |
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Authors: | Ikuo Kushiro Bjorn O Mysen |
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Institution: | a Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015-1305, USA |
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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 ∼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 0.25-1.2), 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 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 a function of degree of NBO/T. |
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