Journal Article

The terrestrial uranium isotope cycle

Kelley, Katherine A., Niu, Yaoling, Freymuth, Heye, Sims, Kenneth W. W., Elliott, Tim, Andersen, Morten B.

2015

Overview

Examination of the global uranium cycle — whereby uranium from the Earth’s crust is first transported to the oceans and then returned, by subduction, to the mantle — shows that the subducted uranium is isotopically distinct from the Earth as a whole and that this signature has been stirred throughout upper mantle, arguably within the past 600 million years. Terrestrial uranium isotope cycle revealed Morten Andersen et al . show that uranium subducted into the mantle should be isotopically distinct, with a high 238 U/ 235 U ratio, as a result of alteration processes at the bottom of an oxic ocean, and mid-ocean-ridge basalts do indeed have a higher 238 U/ 235 U ratio than the bulk Earth, confirming the widespread recycling of uranium in the upper mantle. Whilst it has been argued that many ocean island basalts also contain a recycled component, the authors find that their uranium isotopic compositions are unperturbed from that of the bulk Earth. This suggests that that the subducted uranium in ocean island basalts was isotopically unfractionated prior to full oceanic oxidation at around 600 million years ago, reflecting the greater antiquity of the ocean island basalt source. In contrast, the uranium isotopic composition of mid-ocean-ridge basalts requires convective stirring of recycled uranium throughout the upper mantle within the past 600 million years. 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 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . 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 9 , 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 10 , 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/2151/209 / 704/2151/210 / Basalt / Chemical properties / Chemical research / Continental crust / Earth / Geochemical cycles / Geological research / Humanities and Social Sciences / Isotopes / letter / Magma / Meteors & meteorites / multidisciplinary / Oceans / Science / Systematics / Upper mantle / Uranium