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 2.4billion 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.
Highlights
The University of Rhode Island Faculty have made this article openly available
We have analysed fresh mid-ocean ridge basalts (MORB) glass from all three, major oceanic basins and ocean island basalts (OIB) that cover a large portion of mantle heterogeneity as gauged from radiogenic isotopic compositions
We 6 principally focus on determining the isotopic composition of U added by submarine 7 alteration to the igneous oceanic crust, which is the key flux in accounting for the low 8 Th/U of MORB6,8
Summary
1 precision measurements especially challenging, so we further analysed two eucrites. 2 These higher [U] samples should still provide a useful planetary datum given the 3 incompatible, lithophile and refractory nature of U. The data using time-integrated Th/U 3 form a tighter array, since any recent Th/U fractionations from source composition 4 during melting and melt migration to the surface are removed (see methods for 5 detailed discussion) Both plots independently document a similar 6 relationship with Th/U becoming increasingly sub-chondritic in OIB sources younger 7 than ~2.4 Ga. 8 9 The remarkable implication of the ideas presented above is that the two-stage rise in atmospheric oxygen, reconstructed from observations on the Earth’s surface, is reflected in the Th-U-Pb systematics of mantle-derived basalts. This value is reassuringly consistent with an estimate based on a markedly different approach using Pb isotopes30. 22
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