Non-nucleosynthetic heterogeneity in non-radiogenic stable Hf isotopes: Implications for early solar system chronology

Nucleosynthetic heterogeneity and secondary neutron capture reactions may have important implications for 176Lu-176Hf chronology and modelling of early planetary evolution. So far, the relevance of these phenomena for the Lu-Hf system has not been explored. We therefore have analyzed the non-radiogenic stable Hf-isotope composition (177Hf, 178Hf, 179Hf, and 180Hf) of meteorites, meteorite components, and terrestrial rock samples to identify nucleosynthetic or neutron capture-induced variations. All analyzed chondrites have uniform 178Hf/177Hf and 180Hf/177Hf values that cannot be resolved from the average terrestrial composition. Thus, there is no evidence for nucleosynthetic heterogeneity in chondrites or Earth and these data support the use of a chondritic reference value for the Hf-isotope composition of the Bulk Silicate Earth. This homogeneity contrasts with nucleosynthetic heterogeneities found in lighter elements and provides evidence for a separate synthesis of light and heavy r-process nuclei.Various mesosiderite samples and one lunar meteorite display coupled 178Hf/177Hf and 180Hf/177Hf anomalies that are associated with neutron capture-induced deviations in 149Sm/154Sm and 150Sm/154Sm. However, the analyzed chondrites and an aubrite show only Sm-isotope anomalies, and these are the result of neutron capture. The Hf-isotope anomalies require substantial capture of epithermal neutrons, whereas Sm anomalies result primarily from thermal neutron capture. The non-radiogenic stable isotope composition of Hf is thus a suitable monitor for epithermal neutron capture reactions. The data reveal distinct neutron energy spectra: mesosiderites are characterized by high epithermal-to-thermal neutron fluence ratios, whereas the remaining samples show low epithermal-to-thermal ratios.Secondary neutron capture may significantly increase the measured 176Hf/177Hf in whole-rock meteorite samples without causing a resolvable shift in 176Lu/177Hf. Thus it could potentially induce scatter in Lu-Hf whole-rock isochrons and produce spurious initial 176Hf/177Hf values. However, the slopes of internal (i.e., mineral) isochrons cannot be increased significantly by secondary neutron capture. This process therefore cannot account for the unrealistically old 176Lu-176Hf 'ages' (e.g., ∼ 4.75 Ga) of some meteorites.