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The effect of sodium on the solubilities of metals in silicate melts

¹ Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry Russian Academy of Sciences, Staromonetny 35, 109017 Moscow, Russia
² J.W. Goethe-Universität, Institut für Mineralogie (Abt. Petrologie und Geochemie), Senckenberganlage 28, 60054 Frankfurt, Germany
³ Universität zu Köln, Institut für Geologie und Mineralogie, Zülpicher Strasse 49b, 50674 Köln, Germany

Correspondence: ^* E-mail: aborisov{at}uni-koeln.de

We have studied experimentally the effects of variable Na₂O concentrations on the solubility of four metals (Fe, Co, Ni, and Pd) in silicate melts of anorthitediopside eutectic composition. Experiments were done at fixed temperatures of 1300 °C for Pd and 1400 °C for Ni, Co, and Fe. In the Fe solubility experiments, Na losses were severe. Therefore, a new experimental device (crucible-supported loop technique or CSLT) was designed to prevent Na losses of experimental charges primarily for experiments at low oxygen fugacities. The CSLT was tested extensively, and it was demonstrated that high Na partial pressures could be kept in a semi-closed crucible for at least 20 h. In experiments on Co solubilities, the CSLT was tested with reversed experiments.

The Fe and Co solubilities clearly decrease with increasing Na₂O contents. The Ni solubility is independent of Na₂O contents up to 4.5 wt%. At oxidizing conditions, a small decrease in Ni solubility with increasing Na content was found. Palladium shows a mixed behavior, decreasing solubility with increasing Na₂O, up to about 4 wt%. At higher Na contents, Pd solubility is independent of Na content.

The increase of FeO activity coefficients with increasing Na content found in this study may provide an explanation for decreasing FeO with decreasing degrees of partial melting in experimentally produced mantle melts (Hirschmann et al. 1998), as melts at low melting degrees are Na-rich.

Key Words: Melt properties • new technique • thermodynamics • experimental petrology