Ossa Ossa, F, Bekker, A, Hofmann, A et al. (3 more authors) (2021) Limited expression of the Paleoproterozoic Oklo natural nuclear reactor phenomenon in the aftermath of a widespread deoxygenation event ~2.11–2.06 billion years ago. Chemical Geology, 578. 120315. ISSN 0009-2541
Abstract
The only known case of natural fission reactors is hosted by high-grade uranium (U) deposits at Oklo-Okelobondo and Bangombé in sandstones of the ~2.1 Ga Francevillian Group, Gabon. However, the geochemical influence of the depositional environment on this unique natural nuclear phenomenon has not been clearly established. Localized, unusually high vanadium (V) enrichments are thought to have prevented such natural nuclear fission reactors from occurring in other Francevillian U deposits (e.g., Mounana, Boyindzi, and Mikouloungou). However, while U-bearing detrital monazite derived from Archean rocks surrounding the Francevillian basin is viewed as the main source of U, the source of V remains poorly constrained. Here, we combine petrographic and whole-rock geochemical data for the Francevillian Group sedimentary rocks, coupled with previously documented geochemical data for the Archean basement. These data suggest that, although ultramafic to mafic igneous rocks of the Mesoarchean Bélinga Group, and to some extent Archean granitoids, were likely important sources of V to the Francevillian U deposits, they were not the only source of the abnormally high V concentrations in the U deposits that did not produce natural nuclear reactors. Instead, hydrocarbon migration from V-rich black shales of the Upper Francevillian Group, deposited during widespread and protracted deoxygenation of the Paleoproterozoic ocean at the end of the Lomagundi Carbon Isotope Excursion (LE) at ~2.11–2.06 Ga, resulted in a redox front that precipitated the U deposits. These migrated V-rich hydrocarbons likely account for the high V concentrations, which ultimately prevented natural fission reactions from occurring in these U deposits. Similarities with other pyrobitumen-bearing Paleoproterozoic U deposits worldwide suggest that organic-rich source rocks, which deposited in open-marine settings under widespread hyper-euxinic conditions in the aftermath of the LE, played a key role in preventing the Oklo natural nuclear reactor phenomenon from reaching a larger extent.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2021 Elsevier B.V. All rights reserved. This is an author produced version of an article published in Chemical Geology. Uploaded in accordance with the publisher's self-archiving policy. |
Keywords: | Uranium deposits; Natural nuclear reactors; Oklo; Francevillian; Vanadium; Deoxygenated ocean |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Earth Surface Science Institute (ESSI) (Leeds) |
Funding Information: | Funder Grant number Royal Society WM150108 |
Depositing User: | Symplectic Publications |
Date Deposited: | 07 May 2021 13:08 |
Last Modified: | 01 Mar 2023 12:30 |
Status: | Published |
Publisher: | Elsevier |
Identification Number: | 10.1016/j.chemgeo.2021.120315 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:173744 |
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