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.
