[Show abstract][Hide abstract] ABSTRACT: Unmetamorphosed and undeformed marine siliciclastics rocks of the FB, FC and FD of the Francevillian series (Gabon) were deposited in an epicontinental basin. Clay minerals found in black shale, siltstone and sandstone are dominantly illite and chlorite except in two levels of the FB formation, which contain smectite-rich randomly ordered mixed layers. Their survival in a 2.1 Ga old sedimentary series is not related to the abundance of organic matter (total organic carbon or TOC), nor redox conditions at the time of deposition as indicated by the Fe speciation (FeHR/FeT and FePy/FeHR ratios). Rather it results from an incomplete illitization reaction that reflects potassium deficiency. The K2O/Al2O3 ratio of shale, siltstone and sandstone vary along the series, and appear to conserve the signature of the original chemical composition of the rocks. K-feldspars which are present in the FC and FD formations are missing in the FB formation. Consequently, the smectite layers do not appear to be inherited from a detrital input in the basin but must be considered as representative of an intermediate stage of the illitization reaction reached during diagenesis.
No preview · Article · Dec 2014 · Precambrian Research
[Show abstract][Hide abstract] ABSTRACT: The occurrence of the criticality with analogue modelled configurations of the fossil reactor zone n°9 (RZ9) from Oklo uranium deposit in Gabon is studied. The RZ9 and the other reaction zones 7 and 8, of the same deposit sector are characterized by a low thickness compared to other zones 1 to 6 and an important presence of organic matter with a lower U content. This makes the simulations performed in the past unable to explain the occurrence of that phe-nomenon in such zones. In the present work an extrapolation to 2 billion years ago as an initial state of the reactor is done using the MCNP Monte-Carlo based code. To make it more close to the geological reality experimental condi-tions and geometry, possible porosity, moderators and minimal Uranium content are considered. The minimal critical configuration is obtained by variation of a set of geometrical and physical parameters around the estimated composi-tion of the reactor area. A set of simulations computing the corresponding effective multiplication factor k eff , and reactivity are presented. The effect of organic matter as well as the influence on the criticality is discussed.
[Show abstract][Hide abstract] ABSTRACT: The Archean granitoids in the Kiéné area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1 Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals.
Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean – Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoïds is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite + phengite + hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U–Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0–2.9 Ga (magmatic), 2.6 Ga (propylitic alteration) and 1.9 Ga (diagenetic illitization). The calculation of geochemical mass balances suggests that the water–rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs.
Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.
[Show abstract][Hide abstract] ABSTRACT: The Paleoproterozoic Era witnessed crucial steps in the evolution of Earth's surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.
[Show abstract][Hide abstract] ABSTRACT: The Palaeoproterozoic Franceville basin, Gabon, is mainly known for its high-grade uranium deposits, which are the only ones known to act as natural nuclear fission reactors. Previous work in the Kiéné region investigated the nature of the fluids responsible for these natural nuclear reactors. The present work focuses on the top of the Archaean granitic basement, specifically, to identify and date the successive alteration events that affected this basement just below the unconformity separating it from the Palaeoproterozoic basin. Core from four drill holes crosscutting the basin–basement unconformity have been studied. Dating is based on U–Pb isotopic analyses performed on monazite. The origin of fluids is discussed from the study of fluid inclusion planes (FIP) in quartz from basement granitoids. From the deepest part of the drill holes to the unconformable boundary with the basin, propylitic alteration assemblages are progressively replaced by illite and locally by a phengite + Fe chlorite ± Fe oxide assemblage. Illitic alteration is particularly strong along the sediment–granitoid contact and is associated with quartz dissolution. It was followed by calcite and anhydrite precipitation as fracture fillings. U–Pb isotopic dating outlines three successive events: a 3.0–2.9-Ga primary magmatic event, a 2.6-Ga propylitic alteration and a late 1.9-Ga diagenetic event. Fluid inclusion microthermometry suggests the circulation of three types of fluids: (1) a Na–Ca-rich diagenetic brine, (2) a moderately saline (diagenetic + meteoric) fluid, and (3) a low-salinity fluid of probable meteoric origin. These fluids are similar to those previously identified within the overlying sedimentary rocks of the Franceville basin. Overall, the data collected in this study show that the Proterozoic–Archaean unconformity has operated as a major flow corridor for fluids circulation, around 1.9 Ga.
No preview · Article · Feb 2014 · Comptes Rendus Geosciences
[Show abstract][Hide abstract] ABSTRACT: The Archean granitoids of the South Gabon, dated from 3.0-to-2.5 Ga, were examined along the Archean-Proterozoic unconformity. The petrography was based on mineralogical identification using electron microscopy, X-ray diffraction, and the electron microprobe. The geochemistry was based on chemical analyses of major and trace elements, and on isotopic dating. Results of this work show that the granitoids have been affected by superimposed chloritization and illitization events that were dated by monazite U-Th-Pd isotopic data. The granitic basement shows three superimposed alteration events: 3.0-2.9 Ga primary magmatic event, 2.6 Ga propylitic, and a late oxic 1.9 Ga diagenetic event. (C) 2013 The Authors. Published by Elsevier B.V.
[Show abstract][Hide abstract] ABSTRACT: Clay minerals are exceptionally well preserved in marine black shale of the ca. 2.1 Ga Francevillian Group in southeastern Gabon. The FB Formation of the Francevillian Group is characterized by smectite-rich clay minerals including randomly ordered (R0-type) and ordered (R1-type) mixed layer illite/smectite (I/S). The preservation of R0-type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis, which is unique, considering the Paleoproterozoic age of the sedimentary rocks. R0- and R1-type, smectite-rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures, suggesting formation of clay–organic matter complexes. Our data suggests that clay minerals may have enhanced organic matter preservation, providing the oldest example where a link between clay minerals and organic matter sequestration can be established. Our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time.
[Show abstract][Hide abstract] ABSTRACT: RÉSUMÉ La présence d'une quinzaine de zones de réaction nucléaires (RZ) dans un milieu géologique reste un mystère quarante ans après leur découverte. Le présent travail donne pour la première fois une explication des évolutions ayant conduit au démarrage de ces coeurs au croisement de deux processus antagonistes, la concentration en uranium et l'appauvrissement progressif en son isotope 235. À partir de simulations Monte-Carlo, il a été possible de définir un espace de solutions réalistes – fonction des principaux paramètres tant physiques que géologiques – justifiant la criticité et expliquant l'expan-sion, le fonctionnement et les conditions d'arrêt des plus petits aux plus gros réacteurs d'Oklo.
[Show abstract][Hide abstract] ABSTRACT: The oxygen content of Earth's atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth's oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth's oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest-lived positive δ(13)C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.
Full-text · Article · Sep 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Explanation of the so-called Oklo phenomenon can only be found by associating neutron physics and geological observations. The framework of Reactor Zone #9 (RZ9) from the Oklo uranium deposit in Gabon is discussed. A realistic model of RZ9 is proposed and studied with present-day numerical tools. Anchor points for this model are set by means of a selection of relevant parameters acting on geological conditions and neutron-transport physics, including ore homogeneity, initial reactor thickness, reflector effect and initial neutron poison content. Based on Monte Carlo simulations, the present modeling enables discussion of start-up conditions of the RZ9 reactor at optimal overlap of two opposing long-term processes: geochemical enrichment of the uranium content of the ore and its progressive impoverishment in 235U. On the basis of the present work, the inception and operation of Oklo natural reactors can be discussed from the largest to smallest initial core size, whereas before, only the largest ones had found some explanation.
Full-text · Article · Feb 2013 · IEEE Transactions on Nuclear Science
[Show abstract][Hide abstract] ABSTRACT: Highly depleted C isotope composition of organic matters from the Onega (Fennoscandian shield) and Francevillian (Gabon) basins are differently interpreted. Kump et al. (2011) suggested the occurrence of a massive and global oxidation event during the period of 1980–2090 Ma, which follows the Great Oxidation Event (2450–2320 Ma) (Bekker et al., 2004). Inversely, Gauthier-Lafaye and Weber (2003) invoke the possible action of methanotrophic microorganisms to explain the δ13C values as low as –46‰ measured in the Franceville basin. Here we present the isotope data available in the Franceville basin in order to discuss these two interpretations. The lack of any δ13C correlation between organic matter and carbonate in the Franceville basin does not allow the consideration of a massive and global oxidation event.RésuméLes mesures de la composition isotopique du carbone des matières organiques des bassins d’Onéga (Bouclier fennoscandinave) et de Franceville (Gabon) ont révélé des valeurs très basses qui ont été différemment interprétées. Kump et al. (2011) suggèrent l’existence d’une oxydation globale et massive pendant la période 1980 à 2090 Ma, qui suit le Grand Évènement oxydant (GOE) (2450–2320 Ma) (Bekker et al., 2004). Inversement, Gauthier-Lafaye et Weber (2003) invoquent l’action possible de microorganismes méthanotrophes pour expliquer des valeurs de δ13C aussi basses que –46 ‰, mesurées dans le bassin de Franceville. Nous présentons ici les mesures isotopiques effectuées dans le bassin de Franceville pour discuter ces deux hypothèses. L’absence de corrélation entre les valeurs de δ13C des matières organiques et des carbonates ne permet pas d’envisager une période d’oxydation massive et globale dans le bassin de Franceville.
No preview · Article · Jan 2013 · Comptes Rendus Geosciences
[Show abstract][Hide abstract] ABSTRACT: The activity concentrations of radionuclides within the uranium and thorium series were determined in wheat and lettuce at five sites in France, and in their respective potential sources: crop soils of wheat and crop soils and irrigation waters of lettuce. These data were used to calculate concentration ratios and to enrich the database supported by the technical report series N°472 of the IAEA (2010). For wheat and lettuce, the activity concentrations were in the same range for all radionuclides studied, except for 210Pb, which had higher activity concentrations in wheat, ranging between 1.3 and 11 Bq kg−1 (fresh weight) as compared to 0.4 and 0.7 Bq kg−1 (fresh weight) for lettuce. For wheat, the range of activity concentrations (mBq kg−1; fresh weight) decreased as 210Pb > 226Ra (56–1511) ≈ 228Ra (86–769) > 228Th (19–176) ≈ 238U (11–169) ≈ 234U (12–150) ≈ 230Th (9.08–197.18) ≈ 232Th (8.61–121.45) > 235U (0.53–7.9). For lettuce, it decreased as 228Ra (<320–1221) > 210Pb (409–746) > 226Ra (30–599) ≈ 228Th (<29–347) > 238U (8–120) ≈ 234U (8–121) ≈ 230Th (5.21–134.63) ≈ 232Th (5.25–156.99) > 235U (0.35–5.63). The species differences may reflect different plant physiologies. Through the study of activity ratios of wheat and lettuce in relation with those of the various radionuclide sources it has been possible to highlight the contribution of the main sources of natural radionuclides. Indeed, irrigation water when the uranium concentration is enhanced (>30 mBq L−1) contributed significantly to the activity concentration of uranium in lettuces. Concerning the high activity concentrations of 210Pb, it could be explained by atmospheric particle deposition. The effect of soil particles resuspension and their adhesion to the plant surface seemed to be important in some cases. The soil-to-plant transfer factors were calculated for lettuce and wheat. The values were lower in wheat than in lettuce except for 210Pb which had similar values in the two species (0.11–0.13 respectively). For both species, 210Pb followed by 228Ra (0.015–0.10) and 226Ra (0.010–0.051) displayed the highest transfer factor, whereas 238U had intermediate values (0.0015–0.030) and 232Th exhibited the lowest (0.0014–0.013).
No preview · Article · Oct 2012 · Journal of Environmental Monitoring
[Show abstract][Hide abstract] ABSTRACT: Activity concentrations of the uranium and thorium series radionuclides were determined in chicken meat and eggs as well as in soil, water and other dietary intakes of poultry at five sites of the French territory. These data allow the calculation of transfer coefficients which enrich the database given by the technical report series no. 472 of the IAEA. In egg contents, the highest activity concentrations (in mBq kg(-1) fresh weight) are for (226)Ra, ranging between 136 and 190 and are much lower for uranium (between 0.51 and 1.30 for (238)U). In chicken meat, (238)U activity concentration is higher than in egg contents and ranges between 1.7 and 9.7. Concerning (232)Th, its activity concentration is lower than uranium and ranges between 0.5 and 4.9. Daily ingested activity concentration by the animals was assessed taking into account the activity concentrations measured in the grains, in the soil and in the drinking water. The activity concentration in grains and the daily intakes allow the calculation of concentration ratios and transfer coefficients for chicken meat and egg contents. In chicken meat the transfer coefficients (d kg(-1)) range between 0.0018 and 0.0073 for (238)U and between 0.0008 and 0.0028 for (232)Th. In egg contents they range from 0.00018 to 0.0018 for (238)U and are much higher for radium isotopes (0.10-0.23 for (226)Ra and 0.07-0.11 for (228)Ra).
No preview · Article · Jun 2012 · Journal of Environmental Monitoring