Hf-W internal isochrons for ordinary chondrites and the initial 182Hf/180Hf Of the solar system

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) X 10(-4) and an initial (182)W/(184)W of 0.864710 +/- 15. If the Pb-Pb phosphate age of Forest Vale (similar to 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)X 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 (epsilon(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 is high relative to the initial abundances of other short-lived nuclides, including those with longer half-lives such (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. (C) 2000 Elsevier Science B.V. All rights reserved.