Experimental constraints on the partitioning of Ru, Rh, Ir, Pt and Pd between chromite and silicate melt: The importance of ferric iron

James M. Brenan, Craig F. Finnigan, William F. McDonough, Veronika Homolova

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92 Citations (Scopus)

Abstract

We have performed partitioning experiments to assess the role of chromium-rich spinel in controlling the behavior of the platinum-group elements (PGEs) during igneous differentiation. Spinels were equilibrated with natural and synthetic iron-bearing basalt at 0.1MPa and 2GPa at 1400-1900°C over an fO 2 range of IW+1.6 to IW+7.Results from relatively reduced, graphite-encapsulated experiments done at 2. GPa indicate that Ru is compatible in Cr-spinel (mineral/melt partition coefficient, D, of ~. 4), followed by Rh and Ir, which are moderately incompatible (D range of 0.04 to ~. 1), with Pt and Pd the most incompatible (D. <. 0.2). Partition coefficients for Ir, Ru and Rh measured at more oxidizing conditions in this and previous studies are 10 to 1000 times higher than results from experiments using graphite capsules.We account for the variation in spinel-melt partitioning with a model which considers both the affinity of the PGE cation for a particular spinel lattice site, and the change in site occupancy accompanying the increase in ferric iron component with fO 2. Assuming that Ir and Rh are present as divalent species, with a strong affinity for VI-fold coordination, D Ir and D Rh are predicted to rise rapidly with the ferric iron component, explaining the large D-values for magnetite-rich spinels. Model results indicate that D Ir≤20 and D Rh are ≤100 for ferric-iron poor, Cr-rich compositions, as would crystallize in komatiites, some layered intrusions, and ophiolites. The overall compatibility of Ru for chromite is consistent with the predominance of Ru 3+ at experiment conditions and the similarity in the size of Ru 3+ to Cr 3+ and Fe 3+. The increase in D Ru with the ferric iron content of the spinel likely involves a strong effect of mineral composition superimposed on a change in melt speciation (Ru 2+ to Ru 3+) with increased fO 2. The effect of mineral composition is a consequence of the difference in octahedral site preference energy (OSPE) between Ru 3+, Fe 3+ and Cr 3+, with stronger partitioning of Ru into Fe 3+-rich compositions due to the enhanced reduction in energy gained by the Ru 3+ substitution. Ru partition coefficients for ferric-iron poor spinel are expected to be ~30, which is somewhat lower than values estimated from natural samples obtained from in situ chromite analyses.Results indicate that the ferric iron content of chromite exerts a strong control on the partitioning of some PGEs which should be taken into account in both future experimental work and in models of igneous differentiation.

Original languageEnglish
Pages (from-to)16-32
Number of pages17
JournalChemical Geology
Volume302-303
DOIs
Publication statusPublished - 2012 Apr 2
Externally publishedYes

Keywords

  • Chromite
  • Ferric iron
  • Mineral/melt partitioning
  • Platinum-group elements
  • Spinel

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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