The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates
- Creators
- Chen, J. H.
-
Wasserburg, G. J.
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 ^(235)U/^(238)U 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 ^(235)U/^(238)U 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 ^(207)Pb/^(206)Pb model age of4.559 ± 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 ^(207)Pb/^(206)Pb 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 measured235U/238U and the Pb data on St. Séverin are in complete disagreement with claims of large ^(235)U 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 ^(244)Pu/^(238)U 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 ^(247)Cm/^(238)U or of a large amount of live ^(247)Cm at the early stage of formation of the solar system. Using the present data we obtain a limit of ^(247)Cm/^(235) U≤ 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 ^(26)Al formation and injection.
Additional Information
© 1981 Elsevier Scientific Publishing Company. Received August 25, 1980. Revised version received October 16, 1980. During this work we enjoyed extensive discussion with D A Papanastassiou and profited from his critical comments on the manuscript. We also wish to thank P. Pellas who provided a substantial and accurate critique. The preparation of the Allende samples and separation of the phosphates were carefully done by J Brown and G Meeker. We are grateful to C B Moore for providing the Eggs, to B Mason for 3529-40, and to P Pellas for the St Séverin sample. These results are submitted with some discomfort following the announcement of another extinct nuclide. It is our hope that these lengthy and difficult measurements which have yielded a null result are of some value in understanding nucleosynthetic time scales and processes. This work was supported by NSF grant PHY 76-7923638 and NASA grant NGL 05-002-188 Contributor Number 3453 (352) of the Division of Geological and Planetary Sciences.Additional details
- Eprint ID
- 42096
- DOI
- 10.1016/0012-821X(81)90202-8
- Resolver ID
- CaltechAUTHORS:20131028-114857404
- PHY 76-7923638
- NSF
- NGL 05-002-188
- NASA
- Created
-
2013-10-28Created from EPrint's datestamp field
- Updated
-
2021-11-10Created from EPrint's last_modified field
- Other Numbering System Name
- Lunatic Asylum Lab
- Other Numbering System Identifier
- 352