Experimental study of the partitioning of some platinum group elements (Pd and Ir) between orthopyroxene and silicate melt

• Published in: Geochimica et cosmochimica acta 2022-12, Vol.339 (C), p.127-138
• Main Authors: Zhang, Xueni, Maciag, Bryan J., Brenan, James M., Mungall, James E.
• Format: Article
• Language: English
• Subjects: Geochemistry & Geophysics; Iridium; Orthopyroxene; Palladium; Platinum group element; XANES
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2022.10.038

Past experiments and observations on natural samples have largely focused on the roles of olivine and chromite in controlling the behaviour of the platinum-group elements (PGE) during melting and solidification, whereas other phases, such as pyroxene, have gone largely uncharacterized. To address this, experiments have been done to measure the partitioning of Pd (with a subset of results for Ir), between orthopyroxene and silicate melt at 1340 °C, 0.1 MPa and log fO2 of FMQ − 1 to FMQ + 6 (FMQ = Fayalite-Magnetite-Quartz). The X-ray Absorption Near-Edge Structure (XANES) was measured in a subset of experiment glasses. Glass concentrations of Pd (corrected to unit Pd activity) increase from ∼6 to ∼650 ug/g over the fO2 range of experiments. The slope of the solubility-fO2 relation is consistent with Pd1+ as the dominant oxidation state, with evidence for Pd0 and Pd2+ at the lowest and highest experiment fO2, respectively. Consistent with this result, the XANES reveal spectral features similar to Pd0 and Pd2+ spectral reference materials (specRM) at the most reduced and oxidized synthesis conditions, respectively. Other lines of evidence require the presence of a third melt species, here interpreted to be Pd1+. Values of orthopyroxene/melt partition coefficients for Pd (DPdOpx/melt) are 0.0051 (+/−0.006) at log fO2