TY - JOUR
T1 - 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
AU - Ossa Ossa, Frantz
AU - Bekker, Andrey
AU - Hofmann, Axel
AU - Poulton, Simon W.
AU - Ballouard, Christophe
AU - Schoenberg, Ronny
N1 - Funding Information:
FOO and RS acknowledge financial support from the University of Tuebingen and the German Research Foundation DFG (Grant SCHO1071/11-1 ). The National Research Foundation of South Africa is thanked by AH (Grant 75892 ). Participation by AB was supported by Discovery and Accelerator Grants from the Natural Sciences and Engineering Research Council of Canada (NSERC). SWP acknowledges support from a Royal Society Wolfson Research Merit Award.
Publisher Copyright:
© 2021 Elsevier B.V.
PY - 2021/9/20
Y1 - 2021/9/20
N2 - 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.
AB - 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.
KW - Deoxygenated ocean
KW - Francevillian
KW - Natural nuclear reactors
KW - Oklo
KW - Uranium deposits
KW - Vanadium
UR - http://www.scopus.com/inward/record.url?scp=85105817707&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2021.120315
DO - 10.1016/j.chemgeo.2021.120315
M3 - Article
AN - SCOPUS:85105817707
SN - 0009-2541
VL - 578
JO - Chemical Geology
JF - Chemical Geology
M1 - 120315
ER -