The terrestrial uranium isotope cycle

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. Hereweexplorethe isotopiccharacteristics of this global uranium cycle. We show that the subducted flux of uranium is isotopically distinct,with high ²³⁸U/²³⁵Uratios, as a result of alteration processes at the bottom of an oxic ocean. We also find that mid-ocean-ridge basalts(MORBs) have ²³⁸U/²³⁵Uratios 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 component9, their uranium isotopic compositions do not differ fromthose of the bulk Earth. Because subducted uranium was probably isotopically unfractionated before full oceanic oxidation, about 600million years ago, this observation reflects the greater antiquity of OIB sources. Elemental and isotope systematics of uraniuminOIBs are strikingly consistent with previous OIBleadmodel 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 recycleduraniumthroughout the uppermantle within the past 600 million years.