Oklo Natural Fission Reactors Research Articles

Oklo natural fission reactors and dynamical models of dark energy.

Bounds on the cosmological variation of the fine structure constant α inferred from Oklo neutron capture dataare sometimes taken cum grano salis. It is possible to quantify uncertainties related to the treatment of excitation, deformation and the Coulomb interaction. On the basis of this analysis, it is concluded that Oklo data imply the relative change in α over the last 1.9 billion years is < 0.01ppm (95% CL). Accommodation of this constraint represents a challenge to dark energy models that predict that fundamental constants do change.

Oil-bearing fluid inclusions from the Palaeoproterozoic: A review of biogeochemical results from time-capsules &gt;2.0 Ga old

The observation of oil inclusions trapped prior to 2.0 Ga in Palaeoproterozoic rocks and the ability to obtain detailed molecular geochemical information from them provide a robust way for understanding the early biogeochemical evolution of the Earth. Oil-bearing fluid inclusions (FI) in ca. 2.45 Ga fluvial metaconglomerate of the Matinenda Formation at Elliot Lake, Canada were trapped in quartz and feld-spar during diagenesis and early metamorphism of the host rock, probably before ca. 2.2 Ga. The 2.1 Ga FA Formation sandstone of the Franceville Basin in Gabon that hosts the Oklo natural fission reactors has also been discovered to contain abundant Palaeoproterozoic oil-bearing FIs. This oil occurs within H2O and CO2-dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz, and was most likely trapped 2.1–1.98 Ga. Molecular geochemical analyses of both FI oils reveal a wide range of compounds, including n-alkanes, isoprenoids, monomethylalkanes, aromatic hydrocarbons, and trace amounts of complex multi-ring biomarkers including terpanes, hopanes, methylhopanes, steranes and diasteranes. To ensure a reliable interpretation of oil inclusions, a comprehensive series of outside-rinse blanks and procedural system blanks was analysed by gas chromatography-mass spectrometry; quantitative amounts of the hydrocarbons in these blanks were compared to the FI extracts, so as to provide confidence limits on the experimental integrity of each compound class. Maturity ratios based on reliably detected compound classes show that the FI oils were generated in the oil window, with no evidence of extensive thermal cracking. The presence of biomarkers for cyanobacteria and eukaryotes derived from and trapped in rocks deposited prior to 2.0 Ga is consistent with early evolution of oxygenic photosynthesis and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time. The extraction of biomarker molecules from Palaeoproterozoic oil-bearing FIs thus establishes a new method, using low detection limits and system blank levels, to trace evolution through Earth’s early history that avoids the potential contamination problems affecting shale-hosted hydrocarbons.

Oil and its biomarkers associated with the Palaeoproterozoic Oklo natural fission reactors, Gabon

The 2.1 Ga FA Formation sandstone of the Franceville Basin in Gabon that hosts the Oklo natural fission reactors has been discovered to contain abundant Palaeoproterozoic oil-bearing fluid inclusions inside which syngenetic biomarkers are preserved. The oil occurs within H 2O and CO 2-dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz. Homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and CO 2 with liquid hydrocarbons prior to trapping. Textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca. 115 to 200 °C soon after deposition of the sequence at ca. 2.1 Ga and secondly during a hydrothermal event at 200 to > 285 °C linked to the operation of the reactors ca. 1.98 Ga. A series of sequential outside-rinse blanks of framework grains analysed by gas chromatography–mass spectrometry (GC–MS) prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co-occurring solid bitumens, which have a different geochemical distribution compared to the fluid inclusion oil. The crush–leach extract containing oil from fluid inclusions is markedly richer than the final outside-rinse blank, displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen. The fluid inclusion oil has a high maturity, is non-biodegraded and lacks clear effects of radiolysis. It is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor-heated fluids rather than by the criticality event itself. Biomarkers include hopanes, 2α-methylhopanes, terpanes and steranes, indicative of bacteria, including cyanobacteria and of eukaryotes. The relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution. Trace amounts of C 30 n-propylcholestanes indicate a contribution from marine algae. The oil was likely derived from conformably overlying marine black shales that were deposited during the Great Oxidation Event during which the Earth's atmosphere became oxygenated. The biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2.7 and 1.4 Ga. It is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period, despite apparent fluctuations of atmospheric O 2.

97/04684 Chemical characterization of solid graphitic carbonaceous matter associated with the Oklo natural fission reactors and uranium ore deposits, Gabon (West Africa) (radioactive waste storage): Rigali, M. J. 1997, 188 pp. Avail. UMI, Order No. DA9729437. From Diss. Abstr. Int., B., 1998, 58, (58), 1756

The boundary between the Arabian and African plates, is marked in the Middle East by one of the most prominent deep-seated lineamentary structures, called the Dead-Sea-Transform Fault System (DSTFS). Structural and mineralogical processes related to the DSTFS were correlated with equivalent processes leading to the alteration of pyroclastic deposits of alkali-olivine basaltic to nepheline basaltic composition which formed during a time span of less than 0.5 Ma. The large deposit of Tell Rimah, Jordan, is operated for the exploitation of zeolites, tuffs, and as pozzolana raw material. Four discrete stages of mineralizations have been distinguished from each other within these volcanic-hosted mineral deposits.(1) Hypogene syneruptive alteration of pyroclastic rocks produced siliceous gels (“allophane”), smectite, analcime, and phillipsite in vesicles when the groundwater level was low in the rift basin of the DSTFS. The lake-level lowstand caused the fluid system in the pyroclastic cone to become self-sufficient and it has been considered as a closed hydrothermal system. (2) Periods of tectonic and magmatic quiescence grinded the detrital sedimentation in the rift basin to a halt, while triggering a supergene alteration in the eruptive cones on the adjacent Arabian Plate. (3) Epigenetic alteration affected the pyroclastic rocks in the distal part of the DSTFS as a result of a rising water level. The water gradually filled the pore space of the permeable pyroclastic deposits almost to completeness and caused meniscus and blocky cements of calcite, phillipsite and chabazite to develop. In the rift basin, contemporaneously with the alteration of the pyroclastic rocks, freshwater limestones formed on calcareous bedrocks. Ba and Mn minerals in these freshwater limestones were supplied by subaquatic brines. Subsequently, a drastic lowering of the lake water level in the DSTFS converted the system of subaquatic freshwater limestones into subaerial tufa and travertine. As long as the basal parts of the pyroclastic units at Tell Rimah were in the reaches of the saline groundwaters, calcite and faujasite developed in the pyroclastic host rocks. (4) Another lake level lowstand within the rift basin caused the pyroclastic host rocks to get emerged and forced zeolite–carbonate mineralization in the tuffs to a complete stillstand. Hypogene and supergene alteration in these phreatomagmatic–strombolian pyroclastic cones of the Pleistocene x were correlated with lake high- and lowstands in the adjacent rift basin along the DSTFS.The results obtained by current tectono-morphological studies of the rift-related alteration of pyroclastic rocks along the DSTFS may also be applied to basin-and-swell-topographies elsewhere in the world. The current studies involved microscopy supplemented by SEM-EDX, X-ray diffraction analysis, mid (MIR) and far (FIR) infrared spectroscopy. Major and trace elements were analyzed by X-ray fluorescence spectrometry (XRF). C- and O isotope analyses were conducted on carbonate minerals, which were also targeted on by radiocarbon dating.

Organic free radicals and micropores in solid graphitic carbonaceous matter at the Oklo natural fission reactors, Gabon

The presence, concentration, and distribution of organic free radicals as well as their association with specific surface areas and microporosities help characterize the evolution and behavior of the Oklo carbonaceous matter. Such information is necessary in order to evaluate uranium mineralization, liquid bitumen solidification, and radionuclide containment at Oklo. In the Oklo ore deposits and natural fission reactors carbonaceous matter is often referred to as solid graphitic bitumen. The carbonaceous parts of the natural reactors may contain as much as 65.9% organic C by weight in heterogeneous distribution within the clay-rich matrix. The solid carbonaceous matter immobilized small uraninite crystals and some fission products enclosed in this uraninite and thereby facilitated radionuclide containment in the reactors. Hence, the Oklo natural fission reactors are currently the subjects of detailed studies because they may be useful analogues to support performance assessment of radionuclide containment at anthropogenic radioactive waste repository sites. Seven carbonaceous matter rich samples from the 1968 ± 50 Ma old natural fission reactors and the associated Oklo uranium ore deposit were studied by electron spin resonance (ESR) spectroscopy and by measurements of specific surface areas (BET method). Humic acid, fulvic acid, and fully crystalline graphite standards were also examined by ESR spectroscopy for comparison with the Oklo solid graphitic bitumens. With one exception, the ancient Oklo bitumens have higher organic free radical concentrations than the modern humic and fulvic acid samples. The presence of carbon free radicals in the graphite standard could not be determined due to the conductivity of this material. Oklo solid bitumen samples were subjected to various pressures of O 2, a paramagnetic gas. O 2-organic free radical interactions, as revealed by ESR spectral line broadening, indicate that the organic free radicals of the Oklo solid bitumens are, with one exception, accessible to O 2. This indicates that the organic free radicals are located at or very near the surfaces of these organic solids, including the internal surfaces of pores. The presence of pores was deduced from the BET analyses. The presence of porosity and related permeability has important implications for fluid flow through the uraniferous graphitic bitumen affecting radionuclide containment at Oklo. However, the very small average pore sizes (18.5–38.1Å) together with the non-wettability by aqueous solutions of the uraniferous graphitic bitumen indicate that the pores did not serve as effective conduits for the transport of radionuclides by aqueous solutions through and out of the solid bitumen matrix. The ESR data obtained from the O 2 experiments is comparable to the BET N 2 adsorption data, indicating that the ESR O 2 experiment reflects physical adsorption, and not chemisorption, of O 2 on the Oklo bitumen surfaces.

Isotopic evidence for the retention of 90Sr inferred from excess 90Zr in the Oklo natural fission reactors: Implication for geochemical behaviour of fissiogenic Rb, Sr, Cs and Ba

In order to investigate the mobility of fissiogenic 90Sr in the geological environment, the Zr isotopic compositions of seven samples from one of the newly formed Oklo natural reactor zones (i.e., reactor core and adjacent rocks (10, SF84)) in the Republic of Gabon were determined with an inductively coupled plasma mass spectrometer (ICP-MS). Zr isotopes in uraninite grains from different reactor zones were also measured by secondary ion mass spectrometry (SIMS). Fissiogenic Zr isotopic abundances of three samples from the reactor core have excess 90Zr, which has never before been formed in previous Oklo samples. In this paper, the geochemical behaviour of 90Zr is discussed by making use of the relative retentivity inferred from the isotopic abundance of Sr. The excess in 90Zr suggests dependence on the degree of retention/migration of 90Sr, the precursor of 90Zr in the fission chain. In the aqueous phase, chemical fractionation between Sr and Zr could have occurred before radioactive 90Sr decayed. Considering the halflife of 90Sr ( t 1/2 = 29.1y ), considerable amounts of the latter have been produced during criticality. Sr and Zr (including 90Zr) could have been redistributed between the reactor core and its vicinity. The retentivity of fissiogenic 90Zr in reactor core 10 is not homogeneous. In addition, the distributions of Rb, Cs and Ba is also heterogeneous.

Illite in the Oklo Natural Fission Reactors in Gabon: Considerations for Cs Containment

AbstractThe ∼2 Ga old Oklo, Okelobondo and Bangombé natural reactors in the Republic of Gabon contain solid graphitic bitumens and clay minerals, both of which have effected the containment, or partial containment, of 235U and several fission products. In laboratory experiments, sorption of 134Cs by illite, and illite coated with petroleum was measured in aqueous NaCl solutions to simulate subsurface (connate) waters in sedimentary rocks. Elevated temperatures and increasing salinity of the NaCl solutions facilitated the removal of sorbed cesium from illite.

Role of organic matter in the Proterozoic Oklo natural fission reactors, Gabon, Africa

Of the sixteen known Oklo and the Bangombe natural fission reactors (hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals), reactors 1 to 6 at Oklo contain only traces of organic matter, but the others are rich in organic substances. Reactors 7 to 9 are the subjects of this study. These organic-rich reactors may serve as time-tested analogues for anthropogenic nuclear-waste containment strategies. Organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions. Liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality. The bitumen soon became a solid, consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite, which enclosed and immobilized uraninite and the fission-generated isotopes entrapped in uraninite. This mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1.2 b.y. 24 refs., 4 figs.

Fission-product retentivity in peripheral rocks at the Oklo natural fission reactors, Gabon

Thermal ionisation mass spectrometry has been used to measure the isotopic and elemental abundances of Mo, Ru, Pd, Ag, Cd, Sn and Te in a number of sandstones and shales in the immediate vicinity of Reactor Zone 9 at the Oklo mine site. The mass spectrometric isotope dilution technique was used to measure the elemental abundances of these elements. The data show that considerable amounts of the fissiogenic Mo, Ru, Pd, Ag and Te which escaped from the reactor zone, have been retained in the peripheral rocks, although Cd and Sn were not contained to any significant extent in these samples. Some information on the retention of Sb and Tc was also obtained. Evidence of element fractionation has also been demonstrated in that 99Tc and Sn were fractionated from Ru and Te, respectively within 1 Ma of the end of reactor criticality. The presence of excess 125Te in some of the peripheral rock samples indicates that Sb was mobilised from Reactor Zone 9 within tens of years of the completion of reactor criticality. The implications of these results to the storage of radioactive wastes in natural geological repositories is discussed.

Fission product retention in the Oklo natural fission reactors

Eight samples from a planar cross section of one of the natural fission reactor zones at the Oklo U mine were analyzed to determine the abundances and isotopic composition of U, Mo, Ru, Pd, Ag, Cd, Sn, Te and Nd. Fission product concentrations were calculated from the isotopic compositions. The relative abundances of these fission products are different from those produced by fission i.e. portions have been lost or gained from all the samples. The proportions of Te, Ru, 99Ru, Pd and Mo in the eight samples are invariant. We attribute this regularity to retention at the site of production—primary retention. Based upon this interpretation, we suggest that fixed proportions of Ru, Tc, Pd and Mo have been removed from the reactor zone. In contrast, Nd and Sn have been depleted in some regions of the zone and enriched in others. Portions of Ag and virtually all the fissiogenic Cd have been removed from the reactor zone. By analogy with anthropogenic spent fuel, we suggest that the degree of primary retention was controlled by phase assemblages formed in the Oklo natural reactor fuel in response to microscale conditions of pressure, temperature and composition produced by the nuclear reactions. The ability of these putative minerals to retain nuclear products was a function of their stability under physiochemical conditions established by the geological environment over the last 2 Ga. An integrated study of the natural reactors would test our hypothesis and provide valuable information for evaluating the long-term effectiveness of anthropogenic spent fuel as a container of nuclear waste in the geochemical environment.