Determination of Osmium Concentration and Isotope Composition at Ultra-low Level in Polar Ice and Snow

Here we use two chemical separation procedures to determine exceptionally low Os concentrations (∼10^-15 g g^-1) and Os isotopic composition in polar snow/ice. Approximately 50 g of meltwater is spiked with ¹⁹⁰Os tracer solution and frozen at -20 °C in quartz-glass ampules. A mixture of H₂O₂ and HNO₃ is then added, and the sample is heated to 300 °C at 100 bar. This allows tracer Os to be equilibrated with the sample as all Os species are oxidized to OsO₄. The resulting OsO₄ is separated using either distillation (Method-I) or solvent-extraction (Method-II), purified, and measured using negative thermal ionization mass spectrometry (N-TIMS). A new technique is presented that minimizes Re and Os blanks of the Pt filaments used in N-TIMS. We analyze snow collected from Summit, Greenland during 2009, 2014, and 2017. We find that the average Os concentration of the snow is 0.459 ± 0.018 (95% C.I.) fg g^-1 corresponding to an Os flux of 0.0579 ± 0.0023 (95% C.I.) fmol cm^-2 yr^-1. The average R(¹⁸⁷Os/¹⁸⁸Os) ratio of the Summit snow is 0.264 ± 0.026 (95% C.I.). Assuming that the volcanic source is negligible, the average ratio indicates that about 0.0518 ± 0.0040 (95% C.I.) fmol cm^-2 yr^-1 of Os is of cosmic derivation, corresponding to an accretion rate of extra-terrestrial Os to the Earth of 264 ± 21 mol yr^-1.