Ion probe measurements of Os, Ir, Pt, and Au in individual phases of iron meteorites

We have developed a technique to measure abundances of Os, Ir, Pt, and Au in situ in Fe-Ni metal on a microscale using secondary-ion mass spectrometry. A Cs+ beam is used to generate negative secondary ions, and interferences from molecular ions are eliminated by a combination of 40 eV of energy filtering and a mass resolution of similar to 1900. Ion yields for Fe, Co, Ni, and the PGEs vary considerably from spot to spot in meteoritic metal samples. Ion yield variations for Os and Ir correlate strongly with the Fe ion yield, that for Pt is weakly correlated, and that for Au is essentially uncorrelated. From ion yields for meteorite standards and the correlations with Fe ion yield, it is possible to obtain concentrations of Os, Ir, Pt, and Au in meteorite samples. Using this technique, a Cs+ beam current of 10 nA gives a spatial resolution of 10-20 mu m and detection limits of less than 1 ppm for Os, less than 0.1 ppm for Ir, and 10-20 ppb for Pt and Au. With refinement, it should be possible to measure Rh, Pd, and Ag and to improve the spatial resolution. However, Re, Ru, W, and Hf can only be measured as positive secondary ions. We have measured the abundances of Os, Ir, Pt, and Au in kamacite, taenite, and/or plessite in five iron meteorites: Canyon Diablo (IA), Cape York (Agpalilik) (IIIA), Colomera (TIE), Cruz del Aire (Anom), and Wallapai (IID). Our measurements show that Os, Ir, Pt, and Au partition preferentially into taenite relative to kamacite during slow cooling. Measured abundance ratios (taenite/kamacite) range from similar to 1.3 to similar to 2.1 for Os, Ir, and Pt, and from similar to 2 to similar to 6 for Gu. These ratios are consistent with those determined recently by laser-ablation ICPMS, but differ significantly from those determined by earlier workers. Low-temperature taenite/kamacite distribution coefficients inferred from our data are similar to 2.1 for Os, similar to 1.6 for Ir, similar to 1.9 for Pt, and similar to 6 for Au. PGEs are not enriched in taenite as much as Ni. Partitioning is controlled by the interplay of the size and electron configuration of Os, Ir, Pt, and Au and the crystal structures of kamacite and taenite. The ion probe can be used to further investigate the distribution and subsolidus redistribution of PGEs and Au in iron meteorites and in the metal in chondrites and stony irons. Copyright (C) 2000 Elsevier Science Ltd.