HF-W INTERNAL ISOCHRONS FOR ORDINARY CHONDRITES AND THE INITIAL 182HF/180HF OF THE SOLAR SYSTEM

Abstract

Three ordinary chondrites, Forest Vale (H4), Ste. Marguerite (H4) and Richardton (H5) have been studied with 182Hf-182W system (t1/2=9 million years [m.y.]) in order to provide constraints on the initial 182Hf/180Hf for the chondritic parent body and the solar system. All three samples display positive correlations between 182W/184W and 180Hf/184W, providing confirmation that live 182Hf 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 182Hf/180Hf of (1.87+/-0.16)x10-4 and an initial 182W/184W 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 182Hf/180Hf is estimated to be (2.75+/-0.24)x10-4, comparable to previous estimates based on the differences in W isotopic compositions between iron meteorites and the mean of carbonaceous chondrites. The solar system 182W/184W initial of 0.86457+/-4 (#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 182Hf/180Hf 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 129I (t1/2=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 182Hf is synthesized in the same r-process supernovae events that produced the actinides over most of galactic history.