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

作者：Sprung Peter^*; Scherer Erik E; Upadhyay Dewashish; Leya Ingo; Mezger Klaus

来源：Earth and Planetary Science Letters, 2010, 295(1-2): 1-11.

DOI：10.1016/j.epsl.2010.02.050

• non-radiogenic stable Hf isotopes
• nucleosynthesis
• secondary neutron capture
• Lu-Hf
• CHUR
• early Earth

Nucleosynthetic heterogeneity and secondary neutron capture reactions may have important implications for Lu-176-Hf-176 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 (Hf-177, Hf-178, Hf-179, and Hf-180) of meteorites, meteorite components, and terrestrial rock samples to identify nucleosynthetic or neutron capture-induced variations. All analyzed chondrites have uniform Hf-178/Hf-177 and Hf-180/Hf-177 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 Hf-178/Hf-177 and Hf-180/Hf-177 anomalies that are associated with neutron capture-induced deviations in Sm-149/Sm-154 and Sm-150/Sm-154. 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 Hf-176/Hf-177 in whole-rock meteorite samples without causing a resolvable shift in Lu-176/Hf-177. Thus it could potentially induce scatter in Lu-Hf whole-rock isochrons and produce spurious initial Hf-176/Hf-177 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 Lu-176L-Hf-176 'ages' (e.g., similar to 4.75 Ga) of some meteorites.

• 出版日期2010-6-15