Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo–Okélobondo and Bangombé natural nuclear reaction zones (Franceville basin, Gabon (original) (raw)
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Geochimica et Cosmochimica Acta, 2005
In order to study geochemical processes for migration and fixation of fissiogenic rare earth elements (REE) in association with uranium dissolution, in-situ isotopic analyses using an ion microprobe were performed on U-and REE-bearing secondary minerals, such as coffinite, françoisite, uraniferous goethite, and uraninite found in a sandstone layer 30~110 cm beneath a natural fission reactor at Bangombé, Gabon. Phosphate minerals such as phosphatian coffinite and françoisite with depleted 235 U ( 235 U/ 238 U=0.00609~0.00638) contained large amount of fissiogenic light REE, while micro-sized uraninite grains in a solid bitumen aggregate have normal U isotopic values ( 235 U/ 238 U=0.00725) and small amount of fissiogenic REE components.
Geochemical Characteristics of an Ancient Nuclear Reactor ^|^ldquo;Oklo^|^rdquo
Journal of Nuclear and Radiochemical Sciences, 2007
The Oklo uranium deposit at the Republic of Gabon, central Africa, had partly functioned as natural fission reactors. Many elements of the Oklo reactor zones and the related samples show the variations in the isotopic composition caused by a combination of nuclear fission, neutron capture, and radioactive decay. Isotopic studies provide useful information to estimate reactor conditions and to understand behavior of radionuclides in geological media. In my recent work, in-situ REE, Pb, and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a Sensitive High Resolution Ion Micro-Probe (SHRIMP). The data suggest the adsorption property of apatite in trapping fissiogenic LREE and Pu migrated from the reactor zone, and distribution of fissiogenic REE under oxidizing atmosphere.
Geological Society of America Bulletin, 2001
Oklo-Okélobondo uranium deposit in southeast Gabon. The Okélobondo reactor zone (RZOKE, ϳ310 m depth) was the last reactor zone to be excavated before mining was terminated in December 1997. RZOKE is situated at the base of a 2.5 m deep and 2.7 m wide reactor synform located between the FA sandstone and hydrothermally altered black shales-argile de pile-of the FB Formation. The reactor synform developed by hydrothermal dissolution of the FA sandstone during criticality at ca. 2 Ga. The core of RZOKE is up to ϳ55 cm thick and contains Յ90 vol% uraninite (Յ91.24 ؎ 0.91 wt% UO 2 and Յ7.22 ؎ 0.53 wt% PbO) embedded in a matrix of Si-rich illite (6.08 to 7.91 Si per formula unit [p.f.u.]). The argile de pile (typically Յ60 cm) consists mainly of donbassitic chlorite (7.84 to 8.07 octahedrally coordinated Al p.f.u.) cut by fine illite veinlets. Fe-rich chlorite (0.708 to 1.427 Fe p.f.u.) is abundant at the edges of the reactor zone. Organic matter in both the reactor zone and near-field surroundings played an important role in the enhancement of porosity and stabilization of U during formation and operation of RZO-KE. Fissiogenic Zr, Ce, Nd, and Th (daughter of 240 Pu and 236 U generated by neutroncapture reactions) are well retained in uraninite and (U,Zr)-silicate. The (U,Zr)silicate formed during local migration of Zr, Sr, U, Pu, and lanthanide fission prod-*Present address:
Applied Geochemistry, 2003
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 UO 2 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. #
A semi-empirical model is presented, which describes the processes of formation, migration, and accumulation of radiogenic lead (PbRn) in opals deposited in open cavities. In contrast to lead that forms in situ through radioactive decay of uranium trapped by opal, PbRn is produced from uranium disseminated in the rock enclosing the cavities. Its incorporation into the opal is described by the following chain of processes: decay of parental uranium to yield radon -emanation and diffusion migration of radon from the rock into the cavity -decay of radon to yield lead -diffusion migration of lead in the cavityadsorption of lead on colloidal silica particles -coagulation and settling of the colloidal particles -formation of opal. Besides the colloidal adsorption, PbRn can also be incorporated into the growing opal through direct diffusion flow onto its surface. The latter mechanism is also relevant to minerals crystallizing from ionic solutions; it is less efficient than the mechanism of colloidal adsorption. Distribution of PbRn isotopes throughout a cavity depends on the cavity geometry and the half-life of the parental Rn isotope. In cavity filled with stagnant water, the concentration profiles of PbRn show maxima at some distance from the cavity wall. The movement of water through a cavity leads to a more complex distribution of PbRn isotopes. The model describes accumulation of PbRn on silica micelles during their growth until the critical size of coagulation (5-10 nm) is reached and during the subsequent coagulation. For micelles 5-50 nm in size, the calculated concentrations of PbRn agree with those in young (Miocene and younger) natural opals.
Geochimica et Cosmochimica Acta, 2007
The isotopic analyses of rare earth elements (REE), Pb and U in several kinds of minerals from the clay and black shale layers above the Bangombé natural reactor, Gabon, were performed using a sensitive high-resolution microprobe (SHRIMP) to investigate the migration and retardation processes of fission products released from the reactor. REE isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays, which reveals that the clays play an important role in preventing fission products from spreading.
Radiogenic Isotope Geochemistry
2016
What is a model age, and why do we need to know it? How can we test whether ore-forming fluids carry a mantle signature? What is the difference between a SHRIMP and a LA-ICPMS U-Pb age and is it significant? Isotopes provide important information on many geological processes, with key relevance to the mining and petroleum industries, yet the techniques to obtain, process and interpret the data can be complex to master. This accessible book provides broad coverage of radiogenic isotopes in geochronology and geochemistry, explaining the basic principles and state-of-the-art techniques used to study them, with an emphasis on industry applications. The major isotopic systems are fully summarised with relation to real-world applications, enabling readers to decide which technique is most relevant for the problem they want to solve, and then to rigorously evaluate existing data, or recalculate and reassess data sets to avoid duplication of effort. Written at a level appropriate for advanced undergraduate students, the book also includes detail which allows more experienced practitioners to maximise the potential value of isotopic data sets. bruce f. schaefer is a Senior Lecturer in isotope geochemistry at Macquarie University, Australia. His work uses isotopes to solve a broad range of geological problems, by identifying and subsequently applying, or on occasion developing, an appropriate isotopic tracer to solve the issue. His research interests include planetary differentiation, mantle plumes, Precambrian geodynamics and magmatic processes. Dr. Schaefer was awarded the Tate Memorial Medal for original research from Adelaide University, where he graduated. Subsequently he has held positions at the Open University, UK, and Monash University, Australia, before moving to Macquarie. His publications include work on magmatism, Precambrian tectonics, mineralised systems, groundwater, weathering rates, geophysics and meteorites.
Acta Geologica Hungarica, 2000
In southern Hungary a thick Upper Permian claystone formation has been selected as a possible repository of high-level nuclear waste. Detailed petrographic and geochemical characterizations of the formation are given, using the results of microscopic observations, X-ray diffractometric modal and phyllosilicate crystallinity, X-ray fluorescence major element bulk chemical, K-Ar isotope geochronologic and stable C, O and H isotope ratio analyses. On the basis of peculiar bulk chemistries and modal compositions characterized by extremely high, authigenic albite, low quartz, high hematite and moderate carbonate contents, the formation is considered to have been deposited in a shallow-water lacustrine environment, under semi-arid to arid climatic conditions and highly alkaline, strongly oxidative hydrological ones. Illite and chlorite crystallinity as well as vitrinite reflectance data point to late or deep diagenesis (max. 200-250 °C) that - according to the K-Ar ages of the <2 fim grain-size fraction K-white micas - culminated in the Lower Jurassic. Post-diagenetic fracturing caused by repeated brittle deformations of the rigid rock mass is a common phenomenon. In the most frequent fault gouges phyllosilicate retrogression (i.e. chlorite alteration, smectite and mixed-layered clay mineral formation, etc.) occurs. K-Ar ages show that these processes proceeded under disequilibrium conditions. C and O isotope compositions of whole rock carbonates indicate sedimentary origin, while whole rock carbonates with high d13C and variable d180 values suggest pervasive diagenesis. Ubiquitous fract.ure fillings are grouped into calcite-, baryte +quartz-and anhydrite-dominated veins, the barytic one with traces of sulfidic mineralizations. The combined H-C-O isotopic study of fissure-filling carbonates and fluid inclusions suggests three fluid generations acting in fractures, namely: magmatic fluid and meteoric waters related to warm and cold climates, respectively. A significant effect of recent meteoric water in fracture fillings is unlikely.
Applied Geochemistry, 2002
Hectometer wide cryptokarsts in Paleozoic limestone from Southern Belgium have been studied, to determine to what extent U, Th, Pb and rare earth elements (REE) have been mobilized in the karst sedimentary filling, during a Miocene weathering event. The weathering process resulted in the massive halloysite/kaolinite formation at the karst wall. As with most fossil systems, data on weathering fluid chemistry are lacking, hence it is difficult to quantify relevant parameters such as pH, Eh, and to address solution chemistry. However, on the basis of both field studies of more recent systems, and of geochemical modeling, it is proposed that moderately acid fluids percolated through a multi-layer sedimentary filling, in near-surface conditions and in a temperate/warm climate. Special attention is paid to the trace element immobilization/trapping processes, in newly crystallized REE phosphates, at the karst wall. Analytical methods used include major/trace element geochemistry (emission ICP, ICP-MS) and mineralogy (XRD, SEM, TEM, microprobe). The results suggest that both the sandy sediments that are in contact with the karst carbonate wall, and the carbonate wall itself acted as a kind of geochemical ''barrier''. Mineralization cells settled there, at the decimeter to meter scale. This results in sequential trace element (Pb, Th, REE, U) trapping, according to the affinity of these elements for the aqueous solution. At the end of the sequence, minute U-rich automorphic (Ce, Nd) monazite crystals (from 3 nm upwards) formed on kaolinite flakes. Though the analogy between the studied cryptokarst and planned surface-based repositories for low-level radioactive waste (LLW) in argillaceous context is far from complete, the results outlined here are relevant because they show that even in natural-i.e. intrinsically uncontrolled and unmonitored-systems, ''pollutant'' radionuclide (U, Th, REE, Pb) migration paths are often limited in space. Various processes converge towards trapping of these elements, that are present in the radioactive waste. #
Isotopic study of natural fission reactors at Oklo and Bangombé, Gabon
Journal of Radioanalytical and Nuclear Chemistry, 1999
The Oklo and Bangomb6 uranium ores in the Republic of Gabon are fossils of natural fission reactors. Many elements in these natural fission reactors show isotopic anomalies derived from fission and neutron capture reactions. Isotopic analyses of uraninites and some other minerals provide useful information on the geochemical behavior of fission products and nuclear chemical characterization of the reactors. Integrated isotopic measurements by whole rock analysis with inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS) and by in-situ analysis with secondary ion mass spectrometry (SIMS) make it possible to clarify the migration processes of fissiogenic nuclides over a range of scales from micro meters to meters.