Compositional dependence of the activity of nickel in silicate melts

The effect of melt composition on the solubility of Ni in haplobasaltic melts was investigated at 1 atm via additions of SiO₂ (Quartz), Mg₂SiO₄ (Forsterite), and Na₂SiO₃ (Na-metasilicate) to the 1 atm diopside-anorthite eutectic melt composition. The experimental method applied was the mechanically assisted equilibration technique of Dingwell et al. (1994). Quenched glass samples were analyzed for major elements and Ni by the electron microprobe, and Ni was also determined in all samples by ICP-AES techniques. The Ni solubility and hence activity of NiO is not influenced by the addition of either SiO₂ or Mg₂SiO₄ to the 1 atm anorthite-diopside eutectic composition. In contrast, addition of Na₂SiO₃ leads to a decrease in the Ni solubility up to 20 wt% of Na₂SiO₃ in the melt. Further addition results in a reversal of this trend, i.e., an increase of the Ni solubility up to about 45 wt% Na₂SiO₃. This observation is likely due to an exchange reaction of 2 Na for each Ca and/or Mg atom stabilizing aluminate tetrahedra in the melt structure. After all aluminate tetrahedra are stabilized by Na atoms, further addition of Na-metasilicate results in a network-modifying role of Na in the melt structure, creating new coordination possibilities for Ni. Activity coefficients for NiO in these melts have been calculated and compared with the literature. The present work leads to the conclusion that melt composition plays a minor role in determining the melt-olivine Ni partitioning. Other factors, amongst them temperature, are expected to play a much larger role.