SHRIMP in situ isotopic analyses of REE,Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombé natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (?-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U–Pb systematics of highly altered zircons, the redistribution of U and Pb.     

Natural nuclear fission reactors: time constraints for occurrence,and their relation to uranium and manganese deposits and to the evolution of the atmosphere

Knowledge of the formation conditions of Francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 Ma ago. The model explaining the formation of the Oklo uranium deposits suggests that at the time of sediment deposition in the Franceville basin 2150 million years ago, the oxygen deficient atmosphere would have inhibited uranium dissolution. Dissolution of uranium was only possible during later diagenesis, approximately 1950 Ma. Reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to U⁴⁺, forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions. Such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a ‘transition phase’ some 2450–2100 Ma ago. The manganese deposits were formed before the uranium deposits, during the deposition of the black shales and very early diagenesis, and thus at a time when oxygen content in atmosphere was very low. Carbon isotopes data of organic matter show decrease of δ¹³C upward in the Francevillian series (−20 to −46% PDB) reflecting the high CH₄ and low O₂ contents in the atmosphere during sediment deposition. This favoured anoxic conditions during deposition of the basinal FB black shales and likewise the migration of Mn over long distances. The manganese precipitated first as Mn-oxides at the shallow edges of the Franceville basin, in photic zones, where photosynthetic organisms flourished. Mn-oxides were then reduced in the black shales forming Mn-carbonates when conditions became more reducing during transgression episodes and/or the first stages of burial. In the black shales, reducing conditions prevailed until recent weathering, allowing the good preservation of organic matter and the Mn deposits. The present-day alteration is responsible for the dissolution of Mn-carbonates and precipitation of Mn-oxides at the water table to form the high grade Mn ore (45–50% Mn). Development of photosynthesizing organisms, a volcanic source of the Mn, and favourable palaeogeography of the Francevillian basins are all important parameters for the formation of the Mn deposits. For the occurrence of the natural nuclear reactors, the age of 2.0 Ga is the main parameter that controls the abundance of fissile ²³⁵U and the critical mass. Before 2.0 Ga the ²³⁵U/²³⁸U ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved. Then, after 2.0 Ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the ²³⁵U/²³⁸U ratio was too low to support criticality.     

Uraninite recrystallization and Pb loss in the Oklo and Bangombé natural fission reactors, Gabon

The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO₂ in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombé, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones.The analyzed uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2σ). Thus, the first major Pb loss from uraninite occurred at both Oklo and Bangombé during regional extension and the intrusion of a dyke swarm in the Franceville basin, ∼860-890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the “chemical” ages of the uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that uraninite at both the Oklo and Bangombé natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the uraninite structure.     

Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Steady decline in the percentage of 235U in terrestrial uranium made natural fission impossible after about 1.8 Ga.Fission before 1.8 Ga disturbed the lead isotope system at various places worldwide,su...     

Migration and retention of elements at the Oklo natural reactor

The Oklo natural reactor, Gabon, permits study of fission-produced elemental behavior in a natural geologic environment. The uranium ore that sustained fission reactions formed about 2 billion years before present (BYBP), and the reactor was operative for about 5 × 10⁵ yrs between about 1.95 to 2 BYBP. The many tons of fission products can, for the most part, be studied for their abundance and distribution today. Since reactor shutdown, many fissiogenic elements have not migrated from host pitchblende, and several others have migrated only a few tens of meters from the reactor ore. Only Xe and Kr have apparently been largely removed from the reactor zones. An element by element assessment of the Oklo rocks' ability to retain the fission products, and actinides and radiogenic Pb and Bi as well, leads to the conclusion that no widespread migration of the elements occurred. This suggests that rocks with more favorable geologic characteristics are indeed well suited for consideration for the storage of radioactive waste.     

Characterization of a La-Ce-Sr-Ca aluminous hydroxy phosphate in nuclear zone 13 in the Oklo uranium deposit (Gabon)

A La-Ce-Sr-Ca aluminous hydroxy phosphate of the goyazite-florencite-crandallite series was observed in the nuclear reactor zone 13 in the Oklo uranium deposit (Gabon). It has a trigonal elementary cell with a?=?6.98 ± 0.03 Å and c?=?16.35?±?0.04?Å (space group P3m). It occurs in a U-rich sample depleted in ²³⁵U (²³⁵U/²³⁸U?=?0.0047). Its chemical composition, measured by ion microprobe analyses, shows that it contains fission products (Zr, LREE) whereas the isotopic composition of Ba and Sr is natural. Ion microprobe imaging indicates that the nearby uraninite crystals are depleted in fissiogenic LREE. This study shows that the migration of some fission products, probably related to a dolerite dike intrusion, is limited by the formation of aluminous hydroxy phosphates.     

Shale as a repository for radioactive waste: The evidence from Oklo

Study of the 1.8 billion-year-old “fossil nuclear reactor” zones at the Oklo Mine in the Republic of Gabon shows that many of the elements produced by fission have been almost completely retained, as evidenced by proper budgets of stable daughter elements. Plutonium, ruthenium, the rare earth elements, zirconium, and palladium have been effectively retained while most chalcophile elements exhibit some degree of remobilization. The alkali and alkaline earth elements have migrated to varying degrees but their presence in gangue affected by younger periods of alteration suggests redistribution not far removed from sites of formation. More important, such migration may not have started until some 25,000,000 years after the reactor shut down. The noble gases xenon and krypton escaped with apparent ease during the 500,000 years the reactor was operative, and iodine seems to have been mobile. The Oklo reactor ores, significantly, occur in shale infilled into a fracture system in organo-argillaceous sandstone. So many of the fission-produced elements retained in this shale along with evidence that most others may have been only locally redistributed lends support to considering shales in geologically stable areas for radioactive waste disposal.     

Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon)—inferences from U, REE, Zr, Mo and Se isotopes

Mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the Oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides. To address these issues, in situ isotopic analyses by SHRIMP and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products (exchangeable and non exchangeable) and their association with mineral phases. The distribution patterns of U, REE, Zr and Mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances, owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality. The results emphasize the mobility of Zr at Oklo, this element being often considered as “immobile” during water–rock interactions. The main output is the demonstration of the net effects of sorption and coprecipitation processes. Chlorite and to a lesser extent illite were found to have adsorbed significant amounts of U, REE, Zr (and probably Th) and less sorbing elements such as Mo. Coprecipitation of secondary UO₂ and P-rich coffinite within the alteration halo is also an important means of retardation. The concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour. No retention effect was found for Se in the immediate vicinity of the reactor and this element may have moved farther from its source of production. These results have interesting implications for the evaluation of long-term containment of radionuclides. They provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary U-minerals. This study also demonstrates the robustness of these retention processes over extremely long periods of time.     

深部天然核反应堆——一种可能的山脉隆升动力源

板块碰撞造山模式难以解释造山作用在时间上的“滞后”和在空间上的“差位”现象，大量研究表明山脉隆升的动力来自深部，因此，探索深部地质过程的物理和化学机制及其导致的能量再分配是认识山脉隆升动力学机制的关键，若干线索表明地幔中的核反应中可能在岩石圈动力学演化过程中起过不可忽视的作用，通过分析U，Th等元素的地球化学性质及其在深变质过程中的行为，认为伴随板块俯冲发生的陆壳物质向地幔的再循环有可能在办流圈顶部形成富含U，Th,K等放射性亲石元素和碳质（石墨）的地质体，通过理论上的论证和对Oklo天然核反应堆遗迹的反演表明这些地质体可能有一部分具备了形成石墨核反应堆的条件。这种反应堆在亚临界状态下的断续运行有可能为其上方岩石圈中的岩浆作用提供热源，进入超临界状态时将可能导致深部核爆炸，成为地震，火山爆发和岩石圈破裂的动力来源，据此建立的山脉隆升的动力学初步模式，可以解释造山带的形成，高原的隆升，深源地震及其前兆的成因和金刚石及其相关岩石地壳浅部置位的机制。     

Natural radioactivity in basement rocks and stream sediments, Sharm El Sheikh Area, South Sinai, Egypt: radiometric levels and their significant contributions

The present study examines the natural radioactivity in basements rocks including granites and associated dikes besides stream sediments in Sharm El Sheikh area. Two main rock units are concerned; granites pertaining to Younger Granites of Precambrian age and clastic sedimentary rocks related to Miocene and Pliocene ages. This area is traversed by two shear zones that were delineated as the master fractures trending NW and NNE and controlling uranium and thorium enrichment in granites and dikes. The field and laboratory radiometric measurements revealed radiometric anomalies, in particular, along shear zones. The results of radiometric analyses including concentrations of equivalent uranium (eU), equivalent thorium (eTh), Ra, and K radionuclides as well as the calculated ratios eTh/eU and eU/Ra for representative samples belonging to all rock varieties, revealed that the felsic dikes have the highest values of the average radionuclides potential followed by the alkali feldspar granites, while the mafic dikes display the lowest radioactivity potential. On the other hand, the trends defined in the variation of uranium and thorium reflects the amount of remobilization of uranium that has occurred within the plutons. The eTh and eU/eTh ratios shows a negative correlation, suggesting that distribution of uranium and thorium was at least controlled by magmatic differentiation, while, the positive correlation between eU and eU/eTh ratios indicates enrichment of uranium through post magmatic processes. Some precautions and recommendations are proposed to avoid any possible environmental impacts from shear zone areas with high intensity of natural radiation sources.     

The use of neutrons and monochromatic X-rays for non-destructive testing in geological materials

In the paper we present our results for element sensitive high-energy X-ray tomography and high-speed thermal neutron tomography applied to geological samples. The tuneable monochromatic X-ray beam was used for element sensitive tomography of geological samples containing heavy elements using the K-edge dichromatic scanning technique. This type of NDT (non-destructive testing) allowed us to reveal the quantity and location of uranium, lead and mercury in Oklo samples (natural nuclear reactor) and Cinnabar stones. Fast 3-D thermal neutron imaging of dynamical processes was successfully tested at the high flux neutron tomography station of the ILL. The qualitative investigation of water repellents and consolidants applied on sandstones of Bray and limestones of Maastricht were the first samples to be studied with the new imaging technique.     

A determination of the decay constant for spontaneous fission of natural uranium using fission track accumulation

The decay constant for spontaneous fission of natural uranium has been determined by accumulating fission fragment tracks in Lexan plastic held adjacent to uranium metal for one year. The value, 6.8 ± 0.6. 10⁻¹⁷ yr⁻¹, is consistent with values most commonly used in fission track dating.     

Weathering process at the natural fission reactor of Bangombé

 The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them are mined out. The reactor zone of Bangombé, is only 10–11 m below the surface. This site has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombé during the course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE because of the fission process. The ¹⁴⁹Sm/¹⁴⁷Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of ¹⁴⁹Sm and subsequent transmutation, whereas ¹⁴⁷Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE. Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model. Received: 26 November 1999 · Accepted: 2 May 2000     

Uranium redistribution in weathered conglomerates of the early precambrian pongola supergroup, South Africa - inferences from a study by alpha spectrometry and fission track micromapping

Samples of potentially uranium-bearing conglomerates from deeply weathered outcrops of the 2900 to 3200 m.y. old Pongola Supergroup were investigated from three different localities in the southeastern Transvaal and northern Zululand. Uranium isotope and uranium fission track analyses were carried out to study and to unravel the complex uranium redistribution processes which took place and which are still in progress in the weathering zone of the conglomerates. It is proposed that the combination of the two radiochemical methods can provide valuable information assisting the exploration of uranium mineralization in early Precambrian quartz-pebble conglomerates.     

Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Ba isotopic studies of the Oklo and Bangombé natural fission reactors in east Gabon provide information on the geochemical behavior of radioactive Cs (¹³⁵Cs and ¹³⁷Cs) in a geological medium. Large isotopic deviations derived from fissiogenic Ba were found in chemical leachates of the reactor uraninites. The fissiogenic Ba isotopic patterns calculated by subtracting the non-fissiogenic component are classified into three types that show different magnifications of chemical fractionation between Cs and Ba. In addition, the isotopic signatures of fissiogenic ¹³⁵Ba, ¹³⁷Ba and ¹³⁸Ba suggest an early differentiation between Cs and Ba of less than 20 years after the production of fissiogenic Cs and Ba. On the other hand, only small excesses of ¹³⁵Ba (ε < +1.8) and/or ¹³⁷Ba (ε < +1.3) were identified in some clay samples, which might have resulted from selective adsorption of ¹³⁵Cs and ¹³⁷Cs that migrated from the reactors by differentiation.     

The variability of the natural abundance of U

The possibility that the atomic ratio ²³⁵u/²³⁸U may be slightly variable due to natural nuclear chain reactions such as have been identified in Gabon, West Africa, has prompted a re-examination of very precise gas mass spectrometric isotopic ratio data from a number of laboratories. Two modal values of the isotopic ratio exist in the data. Their relative difference, 0.03%, is statistically significant. The lower mode is due principally to ores from the Colorado Plateau. This difference was recognized in 1963 by the U.S. Atomic Energy Commission but the substantiating data have not been openly published. Insufficient data presently exist to attribute this difference to chemical differentiation of the uranium isotopes, presumably in the ‘sandstone’ type deposits of the Colorado Plateau, or to dilution with Precambrian ore deposits, depleted in ²³⁵U by nuclear reactions.     

核子在地学中的应用与发展──缅怀导师侯德封先生

核子是组成物质的基本粒子。自然界物质千差万别，就其本质来说是由核子决定的。对核子的研究不仅是物理学，也是地学及其它自然科学的一个主题。核子理论在地学中得到广泛应用。侯德封先生提出的核子地质学，开拓了地学研究的新领域。核转变能是地质作用的主要动力，放射性衰变规律为地球演化建立了准确时钟，同位素分馏为探讨地球演化、环境变迁提供了重要依据，同位素示踪是地质探测、环境监测的重要手段，核子地质学把地球科学与基础物理学、天文学、古气候学及其它科学和现代技术紧密地结合在一起。     

核废物处置安全评价的地质类比研究

本文简要论述了利用天然地质类似物（或类比体），类比评价核废物处置系统的安全性能。地质类比研究是安全评价核废物处置系统的重要方法之一，也是地质学科的新研究领域。目前已有的地质类比研究对象主要有火山玻璃，陨石玻璃，粘土矿物，铁陨石，铜矿床，铀矿床，含含Ｔｈ和ＲＥＥ的铁矿床，天然核反应堆（一种特殊的铀矿床）和火成岩接触带等。最后对我国今后从事同类研究提出了若干建议。     

The Equilibrium Coefficient Kp and Petroleum Exploration:Design of the New Generation of Petroleum Exploration Instrument

The main reason why the application of nuclear technology in petroleum exploration has not yet been accepted by most exploration workers is that they are not clear about the homologous distribution features of oil and gas fields and radioactive radiation. The authors hold that the disequilibrium of uranium, radium and radon as a natural radioactive series is the basic feature in the use of this technology in petroleum exploration. The invention Gamma-ray Spectral Measurement of the Equilibium Coefficient Kp and Its Embodiment of the senior author now can readily solve that problem and replace the impedient measure of normalization of uranium and potassium to thorium that had to be proposed before. Application of this impedient measure has some limitations. In areas where the surface is covered by beach or river sands, thorium minerals such as monazite may be concentrated by placering. This could result in local thorium highs that would yield local uranium and potassium lows after normalization to thoriu     

Thermoluminescence as a tool in uranium exploration

Fundamental principles of thermoluminescence (TL) and the special role of TL in uranium exploration techniques are given in this paper with examples from Australia illustrating its application.TL can be used in two ways in uranium exploration. The first usage stems from the property of minerals capable of TL, to act as dosimeters accounting for charging of available traps by ionizing radiation. The second usage relates to ionizing radiation interfering with the crystal structure of the host mineral and thereby changing the capability for TL. Studies at the Beverley and Westmoreland uranium deposits utilize this second property.At Beverley a continuous drop in TL intensity is observed upon approaching the orebody with TL intensity at a minimum within the orebody. Recognition of ore type TL glow curves and sensitized (or marginal) TL glow curves within the area surrounding the orebody have enlarged the exploration target beyond that of the orebody and its associated radiometric halo.Traverses examining the variations in glow peak intensities, ratios and temperatures across the Junnagunna orebody at Westmoreland show that the glow peak intensity of the high-temperature glow peak increases, that of the low-temperature peak fluctuates though generally decreases, the high temperature/low-temperature glow peak ratio increases upon approaching the orebody. The high-temperature glow peak temperature is at a minimum within the orebody. Similar variations detected nearby may indicate further potential exploration sites.An example from an unmineralized area at Bremer River has indicated a lack of widespread radiation sensitization indicating the unsuitability of the area for economic uranium accumulation.