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Nature. 2015 Jan 15;517(7534):356-9. doi: 10.1038/nature14062.

The terrestrial uranium isotope cycle.

Author information

1
1] Bristol Isotope Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK [2] Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland.
2
Bristol Isotope Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK.
3
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-2000, USA.
4
Department of Earth Sciences, Durham University, Durham DH1 3LE, UK.
5
Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882-1197, USA.

Abstract

Changing conditions on the Earth's surface can have a remarkable influence on the composition of its overwhelmingly more massive interior. The global distribution of uranium is a notable example. In early Earth history, the continental crust was enriched in uranium. Yet after the initial rise in atmospheric oxygen, about 2.4 billion years ago, the aqueous mobility of oxidized uranium resulted in its significant transport to the oceans and, ultimately, by means of subduction, back to the mantle. Here we explore the isotopic characteristics of this global uranium cycle. We show that the subducted flux of uranium is isotopically distinct, with high (238)U/(235)U ratios, as a result of alteration processes at the bottom of an oxic ocean. We also find that mid-ocean-ridge basalts (MORBs) have (238)U/(235)U ratios higher than does the bulk Earth, confirming the widespread pollution of the upper mantle with this recycled uranium. Although many ocean island basalts (OIBs) are argued to contain a recycled component, their uranium isotopic compositions do not differ from those of the bulk Earth. Because subducted uranium was probably isotopically unfractionated before full oceanic oxidation, about 600 million years ago, this observation reflects the greater antiquity of OIB sources. Elemental and isotope systematics of uranium in OIBs are strikingly consistent with previous OIB lead model ages, indicating that these mantle reservoirs formed between 2.4 and 1.8 billion years ago. In contrast, the uranium isotopic composition of MORB requires the convective stirring of recycled uranium throughout the upper mantle within the past 600 million years.

PMID:
25592542
DOI:
10.1038/nature14062

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