Iridium, ruthenium and rhodium in komatiites: Evidence for iridium alloy saturation

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    Abstract

    A large body of new data (over 400 samples) on platinum-group element (PGE) concentrations in Yilgarn Craton komatiites is combined with all available high-quality literature data, to reveal trends in a spectrum of komatiite lithologies ranging from spinifex-textured rocks to pure olivine and olivine–chromite adcumulates. The data set is restricted to Carius Tube-isotope dilution analyses, and high-precision nickel sulfide fire-assay-ICP-MS analyses, and we demonstrate good agreement between the two methods. Only S-poor samples are considered, and the data set is filtered to eliminate the effects of addition of cumulus chromite, and addition or segregation of magmatic sulfide liquid.The residual trends indicate that Ir, Ru and Rh are decoupled from one another and from Pt and Pd. Rhodium behaves more like Pt and Pd, whereas Ru is intermediate between Ir and Rh. Rhodium and Ru both show indications of olivine control with partition coefficients less than unity. The maximum Ir whole-rock content is a roughly linear positive function of MgO, but the minimum is constant, sub-1 ppb and independent of MgO. Ruthenium shows some evidence for limited control by chromite, but anomalous Ru enrichment is also found in some olivine cumulate samples with no evidence for accumulated chromite or sulfide. Neither Rh nor Ir shows evidence for any detectable control by chromite. The distribution of Ir, and to a lesser degree Ru and Rh, cannot be adequately explained by olivine control, and a major influence of an Ir-rich phase is strongly indicated.The pattern of Ir variance in relation to MgO bears some important similarities to that of Cr. This similarity arises from the fact that both elements are controlled by a temperature-dependent solubility limit: the chromite saturation surface in the case of Cr, and saturation in Ir (or Ir–Os) alloy in the case of Ir. Combining this effect with moderate compatibility in olivine allows us to successfully model the entire range of Ir data. The upper limit of Ir concentration in olivine cumulates, in an exact analogy to that for Cr, is interpreted as the cotectic ratio of olivine to alloy, as determined by the average slope of the tangent to the alloy solubility curve.Sulfide-undersaturated mantle melts of a wide range of compositions are likely to be Ir alloy saturated. Experimental predictions of very low Ir solubilities in magmas are borne out, with the further implication that true thermodynamic sulfide–silicate liquid partition coefficients, at least for Ir and by inference for the other PGEs, are of the order of 107 or even higher.
    Original languageEnglish
    Pages (from-to)44-58
    JournalChemical Geology
    Volume257
    Issue number1-2
    DOIs
    Publication statusPublished - 2008

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