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Science 3 November 2006:
Vol. 314. no. 5800, p. 721
DOI: 10.1126/science.314.5800.721g
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This Week in Science

The use of the long-lived neodymium (Nd)-samarium (Sm) radioactive decay system for understanding very early processes in our solar system and Earth's differentiation depends on knowing the initial solar isotopic ratios. However, there is wide variation among measured Nd isotope levels among meteorites and compared with terrestrial samples. Two reports indicate that the early solar nebula was not well mixed with respect to this dating system or barium isotopes (see the 6 October news story by Kerr). Variations in Nd isotopes seen between chondrites and earth samples led to the suggestion that some Nd isotopes were sequestered deep in the earth. Andreasen and Sharma (p. 806) have measured Nd and Sm isotopes in primitive carbonaceous chondrite meteorites and find that the variations among meteorites are real and are caused primarily by a p-process (photodissociation of nuclides) deficit in carbonaceous chondrites relative to ordinary chondrites, eucrites (from Vesta), and the terrestrial standard. Ranen and Jacobsen (p. 809) have measured barium isotopes in chondrites and found that they also exhibit variations among meteorite types, which they interpret as implying that the protosolar nebular was heterogeneous. Chondritic meteorites originated in a different place and were more enriched in supernova-derived material compared with Earth.




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