Analytical limits on high precision Os isotope analyses: Implications for Re-Pt-Os geochronology and early Earth's mantle

Osmium is one of only two elements (along with Pb) with more than one radiogenic isotope produced by long-lived radionuclide decay. These are 187Os, formed through β--decay of 187Re, and 186Os, formed by α-decay of 190Pt. Despite the widespread utility of the Re-Os system in geosciences, the long half-life and low abundance of 190Pt lead to extremely limited 186Os/188Os variability in most geological materials, with only 108 ppm variation in 186Os/188Os expected over 4.5 Gyrs of mantle evolution. Typical analytical precisions of ~25–30 ppm for 186Os/188Os therefore severely limit the utility of this system.

We analysed the DROsS isotope reference material repeatedly under conditions previous highlighted to optimise precision of Os isotope analyses, and using a 1012 Ω amplifier to analyse the low abundance isotope 184Os (Waterton et al., 2023). Despite producing the most precise analysis yet of terrestrial 184Os/188Os, we failed to improve on the precision of previous methods for all other Os isotope ratios. This is most likely due to uncertainties in measured amplifier gain factors, which are greater when using mixed arrays of 1011 and 1012 Ω resistors than when using 1011 Ω resistors alone, though Faraday Cup deterioration could also contribute. Multi-dynamic Os isotope measurements could largely eliminate both of these uncertainties.

During the same analytical campaign, we also analysed chromitite samples from the Ujaragssuit Intrusion, North Atlantic Craton, Greenland. Analyses of 186Os/188Os from this locality have previously been interpreted to reflect Hadean mantle depletion. Both our new data and a re-analysis of existing data reveals that no Hadean ages can be confidently identified. Instead, the combined Re-Pt-Os systems identify a concordant ~3.25 Ga age for Ujaragssuit intrusion, consistent with independent constraints. Both radiogenic and mass-independent stable Os isotopes indicate a chondritic source, reflecting the large-scale Os isotope homogeneity in the solar nebula observed from primitive meteorites.

Reference:

Waterton, P., Woodland, S., Pearson, D. G., Serre, S. H. & Szilas, K. (2023). Probing the 186Os/188Os precision barrier: New recommended values for the DROsS reference material and an assessment of mixed 1011 and 1012 Ω amplifier arrays. Geostandards and Geoanalytical Research 1–24. https://doi.org/https://doi.org/10.1111/ggr.12532.