PALLADIUM, PLATINUM AND RUTHENIUM IN IRON-METEORITES AND THEIR TAXONOMIC SIGNIFICANCE

Pd, Pt and Ru have been quantified in 117 iron meteorites by procedures involving a combination of extraction into organic solvents and analysis of the extracts by graphite furnace atomic absorption spectrometry. Our data for these three metals, particularly for Pd, have enabled us to assemble a sufficiently large pool of information from which to draw conclusions concerning their taxonomic potential based on their distribution pattern in iron meteorites. Unlike Ru and Pt, the Pd concentrations give a positive slope when plotted against Ni contents and their mass abundance ratios are very close to the cosmic ratio. Pd and Ni are probably enriched in the later solid during fractionation. Distribution coefficients (solid/liquid concentration ratios) for Ir, Pt, Ru and Pd as determined from the Rayleigh equation are 4.0, 1.4, 1.1 and 0.8, respectively. This sequence is some measure of the usefulness of the elements for taxonomic classification of iron meteorites and is the same sequence as obtained elsewhere by statistical interpretation of abundance data. The positive correlation between the Pt and Ru abundances is extremely close and there is also a good.negative correlation between concentrations of Pd and each of Pt and Ru. It is proposed that low-Ni IIICD meteorites belong more appropriately to group IAB since we have been unable to find intermediate values for Ru, Pd, or Pt linking the low-Ni and high-Ni subgroups of IIICD.