Recent investigations revealed systematic nucleosynthetic Mo isotope anomalies in meteorites, affording clear evidence for variable excesses in p- and r process nuclides, and hence deficits in s process nuclides. These anomalies were interpreted as reflecting selective destruction/removal of unstable presolar components, within the framework of thermal processing models that also take into account data for other elements (e.g., Ru, Zr, Os). To test such models, this study has undertaken extensive measurements of Mo and Pt isotopes in iron meteorites, providing the most precise data for the broadest range of samples analysed to date.
The data presented here are in agreement with previous studies, with all groups analysed (except the IAB/IIICD complex) displaying deficits in s process Mo nuclides, with the extent varying between groups. This unique dataset allows, for the first time, resolution of decoupled p process and r process isotope effects, providing the basis for an updated thermal processing model. Mass-independent Pt isotope anomalies were also observed, but these are interpreted as entirely cosmogenic in origin, resulting from exposure of the meteoroids to galactic cosmic rays. No nucleosynthetic Pt isotope anomalies are resolvable, in accord with predictions from the updated thermal processing model.
Systematic variations in the stable isotope compositions of Mo (δ98Mo) and Pt (δ198Pt) within iron meteorite groups were found, reflecting internal processes within the parent bodies. In detail, these result from isotope fractionation during metal–sulphide partitioning of Mo, and solid–liquid metal partitioning of Pt, respectively. Significantly, a previously undetected correlation between the magnitude of the nucleosynthetic Mo isotope anomalies and δ98Mo values of iron meteorite parent bodies provides novel support for the thermal processing model. However, no signatures of elemental processing in the solar nebula are resolvable in δ198Pt, as any such effects were overprinted by the isotopic fractionation that accompanied partitioning of Pt between solid and liquid metal.