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

Three ordinary chondrites, Forest Vale (H4), Ste. Marguerite (H4) and Richa rdton (H5) have been studied with Hf-182-W-182 system (t(1/2) = 9 million y ears [m.y.]) in order to provide constraints on the initial Hf-182/Hf-180 f or the chondritic parent body and the solar system. All three samples displ ay positive correlations between W-182/W-184 and Hf-180/W-184, providing co nfirmation that live Hf-182 was present in these samples. The best-fit line s 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 resul ts with an apparent initial Hf-182/Hf-180 of (1.87 +/- 0.16) X 10(-4) and a n initial W-182/W-184 of 0.864710 +/- 15. If the Pb-Pb phosphate age of For est Vale (similar to 5 m.y. younger than the Pb-Pb age of Allende CAI) is a formation age, the bulk solar system initial Hf-182/Hf-180 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 W-182/W-184 initial of 0.8645 7 +/- 4 (epsilon(W) = - 4.99 +/- 0.46) is similar to the least radiogenic W reported for iron meteorites, suggesting an extremely early origin of thei r parent bodies and the segregation of metallic cores from planetesimals. T he solar system Hf-182/Hf-180 initial obtained in this is high relative to the initial abundances of other short-lived nuclides, including those with longer half-lives such I-129 (t(1/2) = 16 m.y.). However, this result is ex plicable 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. Ast rophys. J. 466, L109-113) who have proposed that Hf-182 is synthesized in t he same r-process supernovae events that produced the actinides over most o f galactic history. (C) 2000 Elsevier Science B.V. All rights reserved.