The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates

Abstract

We measured the isotopic compositions of U and Pb in eight Ca-Al-rich inclusions from the Allende chondrite, in whitlockite from the St. Séverin chondrite, and in a whitlockite and a whole rock sample from the Angra dos Reis achondrite (ADOR). The total range in 235U/ 238U in the acid-soluble phases of the Allende inclusions and meteoritic whitlockites is from 1/137.6 to 1/138.3, and to within the error limits (3–20‰) indistinguishable from normal terrestrial U. The 235U/ 238U ratios for whitlockites from St. Séverin and ADOR are the same as that in the bulk meteorites and are normal. The Pb isotopic compositions of the acid-soluble phases of the Allende coarse-grained inclusions (except C-1) give a mean 207Pb/ 206Pb model age of 4.559 ± 0.015AE which is consistent with the observation of normal U isotopic composition. The Pb isotopic compositions of coarse-grained (FUN) inclusion, C-1, and of two fine-grained inclusions from Allende indicate that the U-Pb systems of these samples have evolved differently from the rest of the meteorite. The 207Pb/ 206Pb model ages for the whitlockites and for bulk ADOR are the same as those for St. Séverin whitlockite, 4.551 ± 0.004AE. The Pb results from St. Séverin indirectly constrain the value of the U isotopic abundance to be within 7.4‰ of normal. Both the directly measured 235U/ 238U and the Pb data on St. Séverin are in complete disagreement with claims of large 235U excess reported by other workers on the same meteorite. The Th/U ratios of some acid-soluble phases of Allende inclusions and of meteoritic whitlockite show varying degrees of enrichment of Th relative to U by factors of 2 to 25 relative to the bulk meteorites. Previous studies have shown that 244Pu/ 238U ratios in whitlockite are also fractionated relative to the bulk meteorites. The observation of no isotopic effects in U reported here does not support the claims of either fractionation of 247Cm/ 238U or of a large amount of live 247Cm at the early stage of formation of the solar system. Using the present data we obtain a limit of 247Cm/ 235U≤ 4 × 10 −3 at the time of meteorite formation or differentiation. This implies that the last major “r” process contribution to the material in the protosolar nebula was ∼10 8 years prior to 26Al formation and injection.