Determination of ¹⁹⁰Os/¹⁸⁴Os ratio in the carbon-rich residues of stone meteorites by neutron activation method combined with fire-assay technique

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Using RNAA method plus fire-assay technique, the ¹⁹⁰Os/¹⁸⁴Os ratio has been measured in the carbon-rich (CR) residues of four stone meteorites (prepared by HF-HCl demineralization). The residues percent-weight-recovery is between 3–4 in Allende, Plainview and Rangala and 0.80 in Ambapur Nagla. After neutron activation and Hg distillation (vacuum), the aliquots of residues were fired (at temperature ∼1200°C) separately with the flux containing Ni powder and sulphur to incorporate osmium in Ni-NiS beads. The beads were cleaned, decomposed with H₂SO₄ in the presence of CrO₃ and Os was distilled with conc. HNO₃ as OsO₄ in thiourea solution (5% in 6M HCl). The CR-residues though contain isotopically anomalous Hg yet the collected Os from these show normal isotopic ratio within ± 10% variations. The Os concentrations are 1 to 2 order of magnitude higher than the bulk values.