Determinations of osmium isotope ratios in iron meteorites and iridosmines by ICP-MS

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Osmium isotopes in three Os reagents, four terrestiral Os-Ir alloys (iridosmine) and two iron meteorites have been measured on an inductively coupled plasma mass spectrometer (ICP-MS), with the precision of around 1% for ¹⁸⁷Os/¹⁹²Os. Comparisons are made of our isotopic ratios with the previous ones. Apart from the variation of ¹⁸⁷Os abundance as naturally expected, there seems to be a slight systematic deviation for ¹⁸⁹Os. Also some discussions are given to ¹⁸⁴Os. The Os isotopic ratios for three iridosmine samples from Hokkaido, Japan, have been found quite similar to each other. The values for the Hokkaido iridosmines are 3–4% higher relative to the straight ¹⁸⁷Os growth line obtained by Allegre and Luck (1980). Finally, the isotopic “standard” abundances (%) for Os are presented by us: ¹⁸⁴Os, 0.019; ¹⁸⁶Os, 1.589; ¹⁸⁷Os, 1.519; ¹⁸⁸Os, 13.24; ¹⁸⁹Os, 16.37; ¹⁹⁰Os, 26.30; ¹⁹²Os, 40.96. These give us the Os atomic weight = 190.24.