Hf–W internal isochrons for ordinary chondrites and the initial [formula omitted] of the solar system

Abstract

Three ordinary chondrites, Forest Vale (H4), Ste. Marguerite (H4) and Richardton (H5) have been studied with 182 Hf– 182 W system ( t 1/2=9 million years [m.y.]) in order to provide constraints on the initial 182 Hf/ 180 Hf for the chondritic parent body and the solar system. All three samples display positive correlations between 182 W/ 184 W and 180 Hf/ 184 W , providing confirmation that live 182 Hf was present in these samples. The best-fit lines define comparable slopes, passing through, or close to, the data for the bulk carbonaceous chondrites Allende and Murchison. At the current level of resolution this indicates a common and well-mixed chondritic reservoir in terms of the Hf–W isotope system. Forest Vale yields the most precise results with an apparent initial 182 Hf/ 180 Hf of (1.87±0.16)×10 −4 and an initial 182 W/ 184 W of 0.864710±15. If the Pb–Pb phosphate age of Forest Vale (∼5 m.y. younger than the Pb–Pb age of Allende CAI) is a formation age, the bulk solar system initial 182 Hf/ 180 Hf is estimated to be (2.75±0.24)×10 −4, comparable to previous estimates based on the differences in W isotopic compositions between iron meteorites and the mean of carbonaceous chondrites. The solar system 182 W/ 184 W initial of 0.86457±4 ( ε W=−4.99±0.46) is similar to the least radiogenic W reported for iron meteorites, suggesting an extremely early origin of their parent bodies and the segregation of metallic cores from planetesimals. The solar system 182 Hf/ 180 Hf initial obtained in this study is high relative to the initial abundances of other short-lived nuclides, including those with longer half-lives such as 129 I ( t 1/2=16 m.y.). However, this result is explicable with the model of Wasserburg et al. (Wasserburg, G.J., Busso, M., Gallino, R., 1996. Abundances of actinides and short-lived nonactinides in the interstellar medium: diverse supernova sources for the r-processes. Astrophys. J. 466, L109–113) who have proposed that 182 Hf is synthesized in the same r-process supernovae events that produced the actinides over most of galactic history.