Geochimica et Cosmochimica Acta, Vol. 69, No. 6, pp. 1589–1606, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2004.07.013

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

LENA Z. EVINS,1,* KELD A. JENSEN,2,3 and RODNEY C. EWING2

1Swedish Museum of Natural History, Stockholm, Sweden2Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA

3The National Institute for Occupational Health, Copenhagen, Denmark

(Received September 16, 2003; accepted in revised form July 19, 2004)

Abstract—The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Francevillebasin, Gabon. The response of uraninite in the natural reactors to different geological conditions hasimplications 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 eventson the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklodeposit were completed to better constrain the age of the dyke, and thereby testing the link between the dykeand 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 radiogenicPb. Transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist ofmainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions haveelevated 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 (2�). Thus, the first major Pb loss from uraninite occurred at both Oklo and Bangombé during regionalextension and the intrusion of a dyke swarm in the Franceville basin, �860–890 Ma ago. Uraninite Pb isotopesfrom 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 nanocrystallitesin the reactor uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting inthe 6wt% PbO uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny wasimportant during this second Pb-loss event. Thus, there are indications that uraninite at both the Oklo andBangombé natural reactors has experienced at least two ancient episodes of Pb loss associated with internalrecrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission

0016-7037/05 $30.00 � .00

products compatible with the uraninite structure. Copyright © 2005 Elsevier Ltd

1. INTRODUCTION

The Oklo-Okélobondo and Bangombé U deposits are situ-ated in the ca. 2 Ga old Franceville basin, SE Gabon, andformed contemporaneously with the other U deposits found inthe Franceville Basin: Boyindzi, Mounana, Mikouloungou andKiene (Fig. 1). Unique to the Oklo-Okélobondo and Bangombédeposits is the occurrence of fossil natural fission reactorswithin high-grade volumes of the ore deposits (e.g., Gauthier-Lafaye et al., 1996; Jensen and Ewing, 2001). Fourteen naturalfission reactors have been found in the Oklo-Okélobondo de-posit. A last remaining natural fission reactor was found atBangombé, some 20 km to the south.

The natural fission reactors have been studied as naturalanalogues to a variety of processes relevant to nuclear wastedisposal. One of the major aims has been to improve ourunderstanding of the long-term behavior of uraninite (UO2�x),an analog to the UO2 matrix in spent nuclear fuel (Janeczek etal., 1996), in various geological environments. The naturalfission reactors in Gabon pose a unique opportunity to study anatural “spent nuclear fuel” that has been subjected to geolog-ical processes for almost 2 Ga. Here, we focus on the mobility

* Author to whom correspondence should be addressed, at School of

Earth Sciences, James Cook University, QLD 4811, Australia(Lena.Evins@jcu.edu.au).

1589

of radiogenic Pb, which in context of mineral chemistry anddetailed petrography, may be used to understand the geochemi-cal alteration of reactor zone uraninite. Mineral chemistry andtransmission electron microscopy (TEM) analyses show howthese alteration events have affected the uraninite grains andthe crystal structure on a nm scale. Isotopic analyses of Pbreveal the timing of the alteration, and thus can connect thealteration to geological events during the history of the naturalreactors.

Previous studies have shown that the Bangombé and Oklo-Okélobondo U deposits have been subjected to both chemicalalteration and supergene weathering (Janeczek and Ewing,1995; Jensen et al., 1997, 2002; Janeczek, 1999; Bros et al.,2000; Jensen and Ewing, 2001; Fayek et al., 2003). In general,the studies have focused on two periods of radionuclide migra-tion after the formation of the reactors. One is due to recentalteration and supergene weathering, the other is related toregional extension and the intrusion of a Neoproterozic dykeswarm in the Franceville basin (Gauthier-Lafaye et al., 1996).Efforts to date the Neoproterozoic dykes have been met withseveral difficulties and have yielded questionable or impreciseresults; however, they all indicate an age between 1000 and 700Ma (Weber and Bonhomme, 1975; Bonhomme et al., 1978;Sère, 1996). The dolerite dykes have been altered, evidenced as

widespread chloritization (Weber and Bonhomme, 1975; Sère,

1590 L. Z. Evins, K. A. Jensen, and R. C. Ewing

1996). This alteration may explain the difficulty in obtaining aprecise age of the dykes.

The U and Pb content of uraninite indicate typical lowerchemical ages of 450 to 600 Ma at both Oklo and Bangombé(Jensen and Ewing, 2000). However, even younger chemicalages of 200–300 Ma (i.e., lower Pb content) have been ob-served at both Bangombé and Oklo (Jensen and Ewing, 2000;Jensen and Ewing, 2001). Since the Pb loss related to thedolerite dyke intrusions occurred at 850–720 Ma (Gauthier-Lafaye et al., 1996), these young chemical ages indicate a laterPb mobility. The mechanism of Pb-loss is unclear—it may havebeen continuous diffusional Pb loss, episodic recrystallization,or by the formation of secondary uraninite.

In this contribution, we present a detailed study of uraniniteand galena from Reactor Zone 16, Oklo (RZ16) and ReactorZone Bangombé (RZB) to better define the timing and effect ofalteration events in the Oklo and Bangombé natural reactors.This study is prompted by observed variability in the chemicaland Pb isotopic compositions of uraninite and galena. A third ofthe galena model ages reported by Gauthier-Lafaye et al.(1996) are younger than 700 Ma, the lower limit of the ageestimate of the uraninite Pb loss related to the dolerite dykeintrusions (722 � 30 Ma; Gauthier Lafaye et al., 1996). Thereis also a time gap between the K-Ar 980 Ma age of the doleritedyke at Mikouloungou (Bonhomme et al., 1978) and the ob-served Pb loss in the reactor zones (850–720 Ma; Gauthier-Lafaye et al., 1996). Although there is a clear relation betweenthe dolerite dyke intrusions and uraninite Pb loss, these resultsalso suggest the possibility of another, later Pb-loss event in theOklo and Bangombé reactor zones. For example, the event thatcaused alteration of the dolerite dykes may also have affectedthe uraninite in the reactor zones. This possibility is explored,

Fig. 1. Geologic map of the Franceville basin, showing the lithologicdistribution of the Francevillian Series (formations FA to FE) andlocation of the most important uranium deposits in the basin (Jensenand Ewing, 2001).

and the temporal connection between the dolerite dykes and

uraninite Pb loss is tested by dating the main dyke at Oklo byion microprobe U-Pb analysis on zircon.

2. GEOLOGICAL SETTING

The Franceville basin formed ca. 2 Ga ago on 2.7 Ga oldbasement (Bonhomme et al., 1982; Caen-Vachette et al., 1988;Bros et al., 1992). The Francevillian Series is divided into fiveformations: FA to FE, and reach a thickness of 1000–4000 m(Bonhomme et al., 1982; Gauthier-Lafaye and Weber, 1989).The lowermost formation, FA, is 100–1000 m thick, consists ofsandstone and conglomerate, and contains all the uraniumdeposits of the Franceville basin. The FB formation consistsmainly of black shales. The formations FC-FE are more heter-ogeneous, containing chert, dolomite, and volcanic rocks. Broset al. (1992) determined the age of early diagenesis of theFrancevillian series to be 2036 � 79 Ma using the Sm-Ndsystematics of the authigenic clays. The Francevillian U depos-its are all found in the southern part of the basin, in an areabetween Mounana (near Oklo) and Franceville (Fig. 1). Ura-nium ore formation occurred during the early stages of burialand diagenesis of the Franceville basin (Gauthier-Lafaye andWeber, 1989). Reducing fluids rich in organic matter causeduraninite precipitation after interaction with oxidizing uranif-erous basin fluids (Gauthier-Lafaye and Weber, 1989, 2003).

The majority of the numerous dolerite dykes in the Francevillebasin trend approximately E-W (Fig. 1). There are also somemajor N-S structures, on aeromagnetic maps, which have beeninterpreted as dolerite dykes (Weber and Bonhomme, 1975).These E-W trending dykes also intruded the Oklo-Okélobondodeposit—one 20–25 m thick dyke cuts through the Oklo deposit(the “main” dyke, ca. 150 m south of RZ16; Fig. 2), and one thin,40–50 cm wide dyke parallel to the thick Oklo dyke was found ingallery Accés D70 (N175°75°S) and OP50 (N100°80°S), ca. 80 mS of the main dyke (Peycelon, 1993).

The Oklo-Okélobondo U deposit is located ca. 50 km NW ofFranceville (Fig. 1). The ore body has an area of ca. 0.9 km2

and occurs within the upper 10 m of the FA formation. Thenormal ore grade is 0.1%–1% UO2, while high grade volumescontain up to 15% UO2 (excluding the reactor zones). Thenatural fission reactors formed in high-grade areas of the orewhere intense hydrofracturing enabled the presence of moder-ator fluids which led to sustained nuclear fission chain reactions(Gauthier-Lafaye et al., 1989).

Fourteen reactor zones have been found in the Oklo-Okélo-bondo U deposit. A typical reactor zone is 10–30 m wide and0.5–1 m thick. The reactor zone has a uraninite-rich core (� ca.80 wt% U) with an illitic matrix surrounded by a hydrothermalalteration halo consisting mainly of chlorite and minor illite,that formed during criticality �1950 Ma ago (Gauthier-Lafayeet al., 1989). Photographs of these lithological units can be seenin Jensen and Ewing (2001). RZ16 was located at �325 m’sbelow the original surface level before mining and 150 m N ofthe main dolerite dyke at Oklo (Fig. 2). The size of RZ16 hasnot been determined, but field studies suggest that it is up to10 m wide and more than 20 m in length (Jensen and Gauthier-Lafaye, 1998).

The Bangombé U deposit is located ca. 20 km SE of Oklo(Fig. 1). The Bangombé ore body has an area of ca. 1.5 km2 and

also occurs within the upper 10 m of the FA formation. RZB is

1591Uraninite Pb loss in the Oklo and Bangombé natural reactors

located between 10.4 and 12 m below the surface, and has beensubjected to supergene weathering during the last 350,000 yr(Bros et al., 2000; Salah, 2000; Jensen et al., 2002).

3. ANALYTICAL METHODS

3.1. Transmission Electron Microscopy

Analytical transmission electron microscopy (TEM) and high-reso-lution TEM (HRTEM) were performed on TEM samples from thecores of RZB (BA145(4) with internal sample number UNM705) andRZ16 (PIX3-1.0 and PIX3-1.42). The TEM analyses were completedon a JEOL 2000FX and a JEOL JEM 2010 electron microscope with apoint-to-point resolution of 2 nm and 1.5 nm, respectively. All micro-scopes were operated at 200 kV and were equipped with energydispersive spectrometers (EDS) for in situ chemical analysis.

TEM samples were prepared from �30-�m-thick waxed-on doublypolished thin sections. Areas of interest were mounted on 800 �m Curings or oval Mo rings and cut out of the thin sections. Thin films wereproduced by mechanical polishing on 0.1-�m sheets followed by 6 to27 hr of ion milling using 6 kV Ar� ions in a Gatan Dual Ion Mill

�

Fig. 2. Map of the Oklo-Okélobondo mine at Mounana, showing thelocation of the natural fission reactors. The contours are on the surfaceof the FA sandstone formation. Reactor zones (RZ) 10, 13 and 16 andthe Okélobondo reactor zone (RZOKE) are located underground. Theother reactor zones are in the Oklo open pit. The drill core OK84bis andRZOKE are located at the southern part of the ore deposit (Jensen andEwing, 2001).

Model 600 or 4–5 kV Ar ions in a Gatan Dual high-precision ion millat a gun current below 5 mA.

3.2. Electron Microprobe Analysis

Quantitative electron microprobe analyses (EMPA) of the uraninitegrains analyzed by ion microprobe were performed with a 30 nA, 2–10�m beam on carbon coated mounts using a Cameca SX50 WDSelectron microprobe operated at 20 kV. The counting time per elementwas 10–60 s. The PAP program, including ZAF corrections, was usedfor data reduction. Nine elements were analyzed simultaneously (Si, P,S, Ca, Pb, Th, U, Fe, La). Analytical standards were uraninite (U),vanadinite (Pb), wollastonite (Si, Ca), apatite (P), sphalerite (S), mon-azite (Th, La) and hematite (Fe). Oxygen was determined by stoichi-ometry. Detection limits were � 0.15%. Low totals may be due toundetected elements, as well as the presence of hexavalent uranium asU was calculated as UO2.

3.3. Ion Microprobe Analyses

3.3.1. Uraninite and Galena

Selected crystals on gold-coated thin sections and polished rockchips were analyzed in situ for Pb and U isotopes with a Cameca IMS1270 at the NORDSIM facility in Stockholm. To correct for instru-mental mass fractionation, a galena standard (NRM-990118) and auraninite standard (P88) were analyzed during the same sessions as theunknowns. The standards and the analytical setup of the NORDSIMinstrument is described elsewhere (Evins et al., 2001). For small(�30–10 �m) crystals, the primary beam current was reduced andfocused to a spot size down to ca. 5 �m in diameter.

The Pb isotopic ratios in galena were corrected for a fractionation of0.5 � 0.1% (2�)/amu in favor of the lighter isotope. This is inagreement with earlier studies (Hart et al., 1981; Meddaugh et al.,1982; Evins et al., 2001). U isotopes fractionate strongly during ionmicroprobe analysis of uraninite (Holliger, 1992; Evins et al., 2001).The U fractionation factor was 0.95 (1.7 � 0.3%/amu) for RZ16 and0.97 (1.1 � 0.3%/amu) for RZB U isotopic measurements.

Uraninite Pb isotopic ratios often suffer from hydride interferences,causing an estimated elevation in the measured 207Pb/206Pb of ca. 1%(e.g., Reed et al., 1988; Evins et al., 2001). In comparison, Pb isotopicfractionation is insignificant (Holliger, 1988; Cathelineau et al., 1990)and no instrumental fractionation correction has been performed for Pbin uraninite. Hydride interference on the order of 1% corresponds to ca.20 Ma in model age calculations. Owing to the observed oxidativealteration at Bangombé, extra care was taken to reduce hydride inter-ferences. The procedures involved are described by Evins et al. (2001).The maximum effect of hydride interferences, as observed on theuraninite standard, was on the order of 0.8%. This is in the range ofanalytical precision and no correction for hydride interferences wasmade. During uraninite analyses, a small amount of Pb from submi-croscopic Pb-rich inclusions, which are too small to detect and avoid,may accidentally have been mixed with uraninite Pb.

Reactor zone uranium contains less 235U than other natural uranium,and the 207Pb/206Pb is corrected for this 235U depletion by multiplying(207Pb/206Pb)measured by the ratio (235U/238U)normal / (235U/238U)measured.Since the 235U depletion may vary from grain to grain, galena Pbisotopic compositions were corrected using the average U isotopiccomposition of the uraninite in the sample or, in the case of a galenainclusion in uraninite, the U isotopic composition of the surroundinguraninite grain was used. Model age calculations for large galena grainsalso required a correction for a small component of 2 Ga old averageterrestrial Pb (Stacey and Kramers, 1975).

Model ages for galena are calculated using the function AgePb76 inIsoplot/Ex (Ludwig, 2000), and each model age has, on average, aprecision of � 100 Ma (2�). For reactor core samples, T2 is set to theage of the reactor, 1950 Ma, since the primary uraninite crystallized atthat time (Gauthier-Lafaye, 1996). This model is justified for the oldestgalena generation, but not for younger grains, since it only applies to asystem which has suffered one episode of Pb loss and galena crystal-lization.

3.3.2. Zircon

The heaviest mineral fraction was separated from a sample of themain dolerite dyke at Oklo using standard gravity and magnetic meth-

1592 L. Z. Evins, K. A. Jensen, and R. C. Ewing

1593Uraninite Pb loss in the Oklo and Bangombé natural reactors

ods. �100 zircons were handpicked from the resulting heavy fraction.All zircons were mounted in epoxy together with chips of the standardzircon Geostandards 91500 (Wiedenbeck et al., 1995). The sample waspolished and coated with gold.

Cathodoluminescence (CL) was performed at Stockholm Universitybefore isotopic analyses of zircon by ion microprobe. U-Pb isotopeanalyses were conducted at the NORDSIM facility Cameca IMS 1270ion microprobe using the methods described by Whitehouse et al.(1997, 1999). Measured isotopic ratios were corrected for common Pb,assuming a composition of recent average terrestrial Pb (Stacey andKramers, 1975).

4. RESULTS

4.1. Petrography

4.1.1. The Reactor Zones

Nine thin sections from RZ16 were studied, of which threeare reactor core samples (PIX3-0.9, -1.0 and -1.42), the rest arefrom within 3 m of the core. The uranium mineralogy of RZBhas been described previously in great detail (e.g., Janeczek andEwing., 1996; Jensen et al., 1997, 2000, 2002; Jensen andEwing, 2000). In this work only one sample (UNM705; orig-inal sample nr BA145(4)) was characterized in detail for ionmicroprobe analysis. UNM705 is from the reactor core inter-cepted by drill core BA145 at a depth of 11.00–11.80 m.

The reactor core samples from both RZ16 and RZB have ahigh concentration of �0.5-mm-sized uraninite in an illiticmatrix with minor chlorite and kaolinite (Fig. 3). Most ura-ninite grains are intimately associated with galena and havecorroded rims enriched in silica; probably owing to partialcoffinitization of the outer few �m’s. Outside of the reactorzones, uraninite predominantly occurs in �5-�m-sized inclu-sions in organic nodules and as well as in veinlets and fracturefillings. Organic nodules with uraninite inclusions also occur asfragments in calcite veinlets �3 m from the core of RZ16(Fig. 3C).

Numerous �10 �m size anhedral galena inclusions occur inuraninite from RZ16. These small galena inclusions are partic-ularly abundant near grain boundaries (Fig. 3B). Interestingly,galena inclusions are rarely observed in the uraninite in thesample from RZB. However, larger (0.5–4 mm) corroded an-hedral galena grains occur in both RZ16 and RZB (Figs. 3D,G).Smaller grains (5–100 �m) also occur in the hydrothermalphyllosilicates. Additionally, galena and phyllosilicates occa-sionally fill microcracks in the uraninite. Outside of the reactorzones, galena (up to �10 �m) also occurs in quartz, calcite andzircon.

The textures of UNM705 show that RZB has been subjectedto a more advanced degree of oxidative alteration than RZ16.The phyllosilicate matrix in RZB is almost entirely impreg-

Fig. 3. Reflected light micrographs of reactor zone s. Umatter; Cc � calcite; Qz � quartz; C � coffinite; B � bascore (PIX-1.0). (B) Grain boundaries between uraninite auraninite “network” (PIX-1.42). (C) Fragmented organic nof RZ16 (PIX-4.0). (D) Large, millimeter-sized galena(PIX-0.90). (E) Overview of the “porphyritic” texture inrounded shape (UNM705). (G) Large galena crystal (wh

Uraninite vein in sandstone beneath the reactor core (UNM801)grains are altered to coffinite, while thin veins of bassetite cross

nated with red Fe-oxyhydroxides in accord with the ferraliticweathering model (Salah, 2000). Uraninite appears to be morecorroded and several different secondary uranium minerals,dominated by phosphatian coffinite, uranyl phosphates andsulfates, have been observed in and around the core of RZB(Fig. 3H). Phosphatian coffinitization, however, has mainlyoccurred in the FA-sandstone immediately below the core ofRZB (Janeczek and Ewing, 1996; Jensen et al., 2000). Jensen etal. (2002) gives a detailed description of the oxidative alterationproducts in the Bangombé U deposit.

Transmission electron microscopy was used to assess thepurity and to obtain micro- to nanoscale information on theuraninite from the reactor zones. Analysis showed that cores ofthe larger-sized uraninite grains in both reactor cores normallyconsist of large-sized crystallites of several hundred nm size oreven larger (Figs. 4AB). Structural heterogeneities are mainlyobserved at the crystallite grain boundaries and at the border ofthe grains (Fig. 4C). Locally, however, small islands of nano-crystalline domains in a poorly crystalline matrix occur in thecenter of grains (Fig. 4D). This is also observed by the weakring patterns in the insert of Figure 4A. Locally, mineralimpurities were observed in the uraninite. Chlorite, galena andan unidentified Si-rich phase were observed in this study (Figs.4E-F).

At the uraninite grain boundaries, the crystallites decrease insize to less than 20 nm (Figs. 4EF). Semiquantitative EDSanalysis showed that these nanocrystallite regions have lowerPb contents (�4–5 wt% Pb) as compared to the typical con-centrations (�8 wt% Pb) in the uraninite interior. The regionswith lower Pb contents had higher Si contents (6–7 wt% Si)than the coarser crystallite uraninite interior (�4 wt% Si),suggesting a connection between the fine nanoscale crystallin-ity and initial coffinitization, leaching, or accumulation ofincompatible impurities. Elemental high-resolution scanningtransmission electron microscopy of uraninite from other Okloreactor zones have shown similar results with uraninite crys-tallite islands in a Si-enriched matrix (Fayek et al., 2003). Animportant observation is that these nanocrystalline domains arenot restricted to uraninite in a near-surface environment, butalso occur in uraninite at greater depth (� 325 m, RZ16).

4.1.2. The Dolerite Dykes

Samples from the main dolerite dyke at Oklo have an ophitictexture and a grain size of 0.5–1 mm (Fig. 5A). Alteration ofthe main minerals—plagioclase, clinopyroxene and an opaquemixed ilmenite-titanite phase—is common. The alteration ischaracterized by abundant chlorite, sericite, highly altered or-thopyroxene or olivine, and veins of phyllosilicates (mainly

inite; G � galena; Ph � phyllosilicates; OM � Organic) Overview of uraninite and phyllosilicates in the reactor

losilicates in the reactor core. Galena grains occur in theith uraninite inclusions in calcite vein �3 m from the corerroded grain boundaries, surrounded by phyllosilicatesof RZB (UNM705). F) Small, anhedral uraninite with a

a phyllosilicate vein in the reactor core (UNM705). (H)

� uransetite. (And phylodule wwith co

the coreite) in

, showing signs of hydrous alteration. The center of the-cut the uraninite.

1594 L. Z. Evins, K. A. Jensen, and R. C. Ewing

lmenite

1595Uraninite Pb loss in the Oklo and Bangombé natural reactors

chlorite; Fig. 5B) with associated thin, pale green amphiboleneedles. The ilmenite-titanite grains are gray with whitish-graysmall-scale micron-size laminations resembling trellis texture(Fig. 5C; Haggerty, 1976; Force et al., 1996). The majority ofthe zircons, recovered after crushing, are 60–100 �m. Most ofthe zircon crystals have dark and very diffuse CL patterns. Twocrystals have brighter CL than the others, and have clear

Fig. 4. Transmission electron microscopy images of urrystalline uraninite from RZB with nanometer-sized inclushow that the uraninite contains nanocrystallites as indicshowing well-crystalline uraninite from RZ16. The inserFourier Transformed (FFT) imaging (PIX3-1.0). (C) Highthe boundary between two large-size crystallites in theLow-angle subgrains have formed owing to accumulatiouraninite from RZ16 showing low-angle nanocrystallites iFFT electron diffraction pattern of the poorly (left box) a(E) Example of the fine nanocrystalline rims of uraninite a

Fig. 5. Micrographs of dolerite samples from Oklo. PO � probably altered olivine and/or orthopyroxene; Chl �light). (B) Veins of chlorite and altered plagioclase andphase. The titanite phase is filled with submicroscopic iabundant calcite. (OP50/3, transmitted light).

in the chlorite (UNM 705). (F) HRTEM showing an example of nafrom RZ16 (PIX3-1.0).

zonation patterns commonly interpreted as magmatic (Hancharand Miller, 1993). Both CL bright crystals are small, clear andrelatively euhedral, while many of the CL dark zircons arecloudy and fractured. Two thin sections (OP50/1 and OP50/3)originate from the thin, ca. 40 cm wide dyke 80 m south of themain dyke (Peycelon, 1993). This thin dyke is fine grained andabundant alteration features, related to a network of calcite

from RZB and RZ16. (A) TEM image showing microc-phyllosilicates. The inserted electron diffraction patternsthe weak ring patterns (UNM 705). (B) HRTEM image

s a perfect electron diffraction pattern obtained by Fastion TEM image showing a structurally disordered zone atr of a several-micron-sized uraninite grain from RZB.efects (UNM705). (D) HRTEM image of the center ofrly crystalline to amorphous matrix. The inserts show thecrystalline (right box) regions as indicated (PIX3-1.42).

rder to chlorite in RZB. Note the �2-nm uraninite islands

ioclase; C � clinopyroxene; T � titanite; I � ilmenite;te; G � galena. (A) Overview of the texture (transmittedroxene (transmitted light). (C) Titanite-ilmenite opaque(reflected light). (D) Galena in altered plagioclase with

aninitesions ofated byt show

-resolutinterion of d

n a poond wellt the bo

� plagchlori

clinopy

nocrystalline uraninite at the grain boundaries of uraninite

1596 L. Z. Evins, K. A. Jensen, and R. C. Ewing

veinlets, are visible under the microscope. Galena is common,both in the calcite veinlets and in the altered parts of the rock(Fig. 5D).

4.2. Uraninite Composition and Chemical Alteration

4.2.1. Uraninite Chemistry

EMPA results show only minor differences between thechemical composition of uraninite from RZ16 and RZB(Tables 1 and 2). The major elements are U, Pb, Si, Ca andFe. The average PbO content was slightly lower in RZ16(5.83 � 1.12 wt%) than in RZB (6.37 � 0.21 wt%); how-ever, the lower average Pb content in RZ16 was mainlycaused by four low-lead uraninite analyses (2.62– 4.92 wt%PbO). The observed PbO contents are within the typicalrange in uraninite from the natural fission reactors, butsignificantly lower than that of original 2 Ga old uraninite(�19 –24 wt%), found in RZ10 and RZ2 (Jensen and Ewing,2000).

Among other notable differences, the CaO content in RZBuraninite was 1.71 � 0.28 wt% compared to 1.43 � 0.34 wt%in RZ16; whereas, the SiO2 content was slightly higher in RZ16uraninite (0.81 � 0.23 wt%) than in RZB (0.45 � 0.12 wt%).Elevated Si contents in uraninite may arise from chemicalalteration and partial coffinitization (see below). FeO occurswith an average of ca. 0.48 wt% in both reactor cores. Sulfur(�1.39 wt% SO3) was found above the detection limit ineleven and three uraninite analyses from RZ16 and RZB,respectively. Sulfur in uraninite is most likely caused by con-tamination from submicroscopic galena inclusions in theuraninite.

4.2.2. Pb Loss Mechanism

Scatter plots (not shown) using the chemical composition ofuraninite from this study showed that the PbO contents de-crease; whereas, CaO and FeO increase as function of increas-ing SiO2. The enrichment in Ca, Fe and Si may be the result ofuraninite recrystallization and partial coffinitization during ep-isodic Pb loss 1000–750 Ma ago (Janeczek and Ewing, 1995;Jensen and Ewing, 2000, 2001). However, diffusion or leachingmay also result in Pb loss from uraninite.

The extent and types of Pb loss was evaluated by plotting theU-normalized impurity content, (Si�Ca�Fe)/U vs. Pb/U (Fig.6A). Compared to the trend for all uraninite compositions atOklo-Okélobondo and Bangombé previously presented inJensen and Ewing (2000), Figure 6A suggests that the overallmain mechanism forming the 6 wt% PbO population was Pbloss by diffusion, leaching or recrystallization. However, fo-cusing on the data range of the current uraninite analysis alone(insert in Fig. 6A), it is clear that uraninite, especially fromRZB, plot on a straight vertical line. This suggests that U andPb leaching during replacement by Ca, Si and Fe also played asignificant role in the RZB and RZ16 uraninite. The uraninitefrom RZ16 may have suffered from secondary coffinitization aswell or mixed leaching and diffusion owing to the slightly

inclined data trend.

4.2.3. Chemical Ages

Chemical ages have previously been used to infer formationages of uraninite (e.g., Bowles, 1990). Using the atomic per-centage of Pb, U and Th, chemical ages were calculated usingCameron-Schiman’s equation given by Bowles (1990):

t � Pb � 1010 ⁄ (1.612U � 4.95Th), (1)

which yields the age in years (Fig. 6B). The average chemicalages of the RZB and RZ16 uraninite analyzed by ion micro-probe are the same within error: 555 � 38 Ma and 534 � 82Ma (2�), respectively. Excluded from these calculations aredata with S over the detection limit and the four data pointswith PbO contents below 5 wt% in RZ16. The four low-leaddata points yield chemical ages down to 222 Ma. Previouslyobtained data for all uraninite and partially coffinitized ura-ninite (Jensen and Ewing, 2000) are plotted for comparisonshowing that coffinitization results in much younger chemicalages with a steep increase in impurity contents; whereas, chem-ical ages of �1700 to 2400 Ma are obtained for high Pb-uraninite in RZ2 and RZ10.

4.3. Geochronology

Uraninite U and Pb isotopic compositions are given in Table 3.The 235U/238U in the reactor core uraninite varies between0.00659 and 0.00627. Corrected for 235U depletion, uraninite datafrom RZ16 and RZB form two linear arrays in an inverse (207Pb/206Pb vs. 204Pb/206Pb) Pb-Pb plot (Fig. 7A). These arrays are theresult of mixing various amounts of two Pb end members. Thepurely radiogenic end member has a 207Pb/206Pb corresponding tothe y-axis intercept in Figure 7A, which is the same for RZB andRZ16 (0.05795 � 0.0019 and 0.0575 � 0.0029, respectively). Theless radiogenic end members differ for RZB and RZ16, but thecomposition of both are clearly different from average terrestrialPb (see insert in Fig. 7A).

Galena Pb isotopes (Tables 4 and 5) give clues to the compo-sition of the less radiogenic end members. The Pb isotopic com-position of large, millimeter-sized galena in both RZB and RZ16plot on or very close to the mixing lines (Fig. 7B), stronglysuggesting that they correspond to the less radiogenic end mem-bers. Large galena contains Pb from two different sources: one isradiogenic (207Pb/206Pb �0.155), and one is average terrestrial Pb.The radiogenic Pb in large galena evolved from �1950 Ma to�850 Ma and was subsequently isolated from its parent U. Small-sized galena in RZ16 has lower 207Pb/206Pb than the large galena,and plot on or scatter around the RZ16 mixing line (Fig. 7B). Thisindicates that the small galena contains mixed Pb from the radio-genic (uraninite) and less radiogenic (large galena) end members.The observed x-axis scatter is probably due to variable amounts ofaverage terrestrial Pb in these crystals. Galena in sample OP50/1contains Pb with a very radiogenic composition, indicating anorigin from the surrounding U ore. The amount of average terres-trial Pb in these crystals is only 2%, and the radiogenic 207Pb/206Pb is similar to that of the large galena.

The large galena grains form a separate group with a ho-mogenous radiogenic composition. Since they also contain Pbwith the highest 207Pb/206Pb, they represent the oldest galena inthe samples and single-stage model ages have been calculated

for the large galenas. In RZ16, the model ages range between

Table 1. RZ16 uraninite WDS analyses. Concentration is given in wt% oxide. The average includes data below detection limit. The error on the average is one standard deviation of the dataa

Crystal SiO2 �� P2O5 �� SO3 �� CaOb �� PbO �� ThO2 �� UO2 �� FeO �� La2O3 �� SUM ��

1RCa 0.50 0.01 � 1.39 0.08 1.20 0.11 6.97 0.13 � 87.68 0.45 0.30 0.03 � 98.0 0.81RCa1 1.05 0.01 0.17 0.08 � 1.42 0.11 4.92 0.12 � 87.68 0.45 0.65 0.03 � 95.9 0.81RCb 0.95 0.01 0.23 0.07 � 1.38 0.11 5.44 0.12 � 88.34 0.45 0.65 0.03 � 97.0 0.81RCc 1.15 0.02 0.24 0.07 0.43 0.06 1.69 0.11 5.56 0.12 � 86.53 0.44 0.69 0.03 � 96.3 0.91RCd 1.06 0.02 0.19 0.08 � 1.54 0.10 4.41 0.11 � 88.98 0.45 0.76 0.03 � 97.0 0.82RCa 0.36 0.01 � � 1.08 0.09 6.46 0.12 � 88.0 0.43 0.26 0.02 � 96.2 0.73RCa1 0.86 0.01 � 0.16 0.05 1.03 0.09 6.68 0.12 0.15 0.04 86.11 0.42 0.39 0.02 � 95.4 0.83RCb 0.99 0.01 0.17 0.07 0.16 0.05 1.25 0.10 5.77 0.12 � 87.53 0.45 0.57 0.03 � 96.4 0.83RCc 1.15 0.02 0.17 0.07 0.16 0.05 1.36 0.11 5.91 0.12 � 87.52 0.45 0.53 0.03 � 96.8 0.94RCa 0.46 0.01 � 0.11 0.05 1.26 0.10 6.96 0.13 � 87.75 0.45 0.24 0.03 � 96.8 0.86RCa 0.74 0.01 � � 1.77 0.11 6.46 0.13 � 86.59 0.44 0.45 0.03 � 96.0 0.76RCb 0.76 0.01 0.22 0.06 � 1.78 0.11 5.77 0.13 � 86.78 0.44 0.48 0.03 � 95.8 0.86RCc 0.77 0.01 � � 1.65 0.11 5.95 0.13 � 86.91 0.44 0.50 0.03 � 95.8 0.77RCa 0.75 0.01 � 0.14 0.05 1.62 0.11 6.04 0.13 0.15 0.05 87.25 0.45 0.50 0.03 � 96.5 0.87RCb 0.75 0.01 � � 1.49 0.11 6.22 0.13 � 86.23 0.44 0.40 0.03 � 95.1 0.77RCc 0.75 0.01 0.18 0.07 0.19 0.05 1.86 0.11 5.28 0.12 � 87.59 0.45 0.49 0.03 � 96.3 0.88RCa 0.60 0.01 � 0.10 0.05 1.65 0.11 6.34 0.13 � 86.49 0.44 0.40 0.03 � 95.6 0.88RCb 0.57 0.01 � � 1.72 0.11 6.75 0.13 � 86.82 0.44 0.34 0.03 � 96.2 0.78RCc 0.58 0.01 0.15 0.07 � 1.54 0.11 6.14 0.13 � 87.99 0.45 0.44 0.03 � 96.8 0.89RCa 0.78 0.01 � � 1.64 0.11 5.86 0.12 � 86.83 0.44 0.48 0.03 � 95.6 0.79RCb 0.77 0.01 � � 1.72 0.11 5.68 0.12 � 87.15 0.45 0.48 0.03 � 95.8 0.710OM

a1.03 0.02 � 0.11 0.05 1.75 0.11 7.32 0.13 � 87.65 0.45 0.59 0.03 � 98.4 0.8

11RCa 1.09 0.02 0.40 � 0.46 0.10 2.62 0.10 � 88.58 0.45 0.43 0.03 � 93.6 0.811RCb 1.11 0.02 0.55 0.09 � 0.95 0.10 3.13 0.10 � 87.90 0.45 0.51 0.03 � 94.1 0.8Average 0.81 0.23 0.14 0.12 0.14 0.28 1.43 0.34 5.83 1.12 0.07 0.04 87.51 1.00 0.48 0.13 � 96.1 1.0

a � � below detection limit.b Ca analyzed on the Ca�� line.

1597U

raninitePb

lossin

theO

kloand

Bangom

bénatural

reactors

Table 2. RZB uraninite WDS analyses. Concentration is given in wt% oxide. The average includes data below detection limit. The error on the average is one standard deviation of the data.a

Crystal SiO2 �� P2O5 �1� SO3 �1� CaOb �1� PbO �1� ThO2 �1� UO2 �1� FeO �1� La2O3 �1� SUM �1�

rb1 0.45 0.03 � � 1.79 0.15 6.60 0.10 0.14 0.02 85.37 0.34 0.51 0.02 � 94.8 0.7rb2 0.61 0.03 0.15 0.04 � 2.02 0.16 6.29 0.10 0.12 0.02 85.96 0.34 0.58 0.02 � 95.7 0.7rb3 0.48 0.03 0.19 0.04 0.12 0.04 1.78 0.15 6.68 0.10 0.14 0.02 83.59 0.34 0.50 0.02 � 93.5 0.8rc1 0.51 0.03 � 0.32 0.05 1.77 0.15 6.84 0.11 0.11 0.02 84.80 0.34 0.45 0.02 � 94.8 0.7rc2 0.50 0.03 0.11 0.04 � 2.03 0.15 6.56 0.10 � 85.16 0.34 0.50 0.02 � 94.9 0.7rc3 0.69 0.03 0.17 0.04 � 2.06 0.16 6.23 0.10 0.14 0.02 84.42 0.34 0.55 0.02 � 94.3 0.7re1 0.56 0.03 0.08 0.04 � 2.17 0.16 6.43 0.10 0.14 0.02 85.31 0.34 0.48 0.02 � 95.2 0.7re2 0.63 0.03 0.15 0.04 � 2.12 0.15 6.37 0.10 0.14 0.02 84.91 0.34 0.59 0.02 � 94.9 0.7rf1 0.55 0.03 0.18 0.05 � 1.82 0.15 6.59 0.10 0.13 0.02 86.39 0.34 0.62 0.02 � 96.3 0.7G 0.35 0.03 0.10 0.04 � 1.48 0.15 6.23 0.10 � 84.92 0.34 0.37 0.02 � 93.4 0.7J1 0.35 0.03 0.09 0.05 � 1.68 0.15 6.14 0.10 0.11 0.02 85.97 0.34 0.48 0.02 � 94.8 0.7J2:1 0.44 0.03 0.24 0.04 0.08 0.04 1.56 0.15 6.43 0.10 0.12 0.02 84.22 0.34 0.49 0.02 � 93.6 0.8J2:2 0.38 0.03 0.15 0.05 � 1.32 0.15 6.12 0.10 0.10 0.02 85.94 0.34 0.50 0.02 � 94.5 0.7K 0.35 0.03 0.15 0.04 � 1.27 0.15 6.30 0.10 0.11 0.02 85.60 0.34 0.46 0.02 � 94.2 0.7M 0.39 0.03 0.13 0.05 � 1.35 0.15 6.27 0.10 0.15 0.02 87.16 0.34 0.43 0.02 � 95.9 0.7N 0.38 0.03 0.12 0.04 � 1.75 0.15 6.40 0.10 0.12 0.02 86.09 0.34 0.40 0.02 � 95.3 0.701b 0.27 0.03 0.15 0.04 � 1.53 0.15 6.04 0.10 0.11 0.02 85.06 0.34 0.43 0.02 � 93.6 0.701f 0.33 0.03 � � 1.60 0.15 6.26 0.10 0.11 0.02 85.40 0.34 0.42 0.02 � 94.1 0.701k 0.35 0.03 0.16 0.04 � 1.42 0.15 6.34 0.10 0.11 0.02 87.06 0.34 0.40 0.02 � 95.8 0.7Average 0.45 0.12 0.13 0.06 0.04 0.07 1.71 0.28 6.37 0.21 0.12 0.02 85.44 0.91 0.48 0.07 � 94.7 0.7

a � � below detection limit.b Ca analyzed on the Ca�� line.

1598L

.Z

.E

vins,K

.A

.Jensen,

andR

.C

.E

wing

1599Uraninite Pb loss in the Oklo and Bangombé natural reactors

960 Ma and 826 Ma, with an average of 856 � 97 Ma (2�). InRZB, two galena grains were analyzed, yielding a model age of892 � 15 (2�)Ma.

The reason uraninite plots on a mixing line between purelyradiogenic uraninite Pb and large galena Pb is probably due tosubmicroscopic galena inclusions or other exsolved Pb phasescontaminating the ion microprobe analysis. This kind of con-tamination is difficult to avoid, as shown by the evidence ofgalena contamination (� 1.1 wt% with one exception) in someelectron microprobe analyses, and the �100 nm heterogeneitiesand galena inclusions shown by TEM. Therefore, the purelyradiogenic end member is the most reliable estimate of theuraninite 207Pb/206Pb. Using the radiogenic 207Pb/206Pb at they-axis intercepts (Fig. 7A), ages were calculated using Iso-plot/Ex (Ludwig, 2000). The ages are the same within error:512 � 110 Ma (2�) for RZ16 and 528 � 71 Ma (2�) for RZB.The large errors for RZB may be due to the relatively highanalytical errors related to the use of a smaller primary beam(�0.2 nA). Data from RZ16 show some nonanalytical, i.e., realor “geological,” scatter (MSWD 2.2) which is reflected by thelarge error. Possible reasons for scatter are the involvement ofsmall and variable amounts of average terrestrial Pb, and slightvariations in the amount of Pb loss.

Data from the ion microprobe analyses of zircon are given in

Fig. 6. Chemical plots for showing the composition of uraninitegrains analyzed by SIMS (squares, this study) compared with data fromprevious analyses (Jensen and Ewing, 2000). (A) Scatter plot forevaluating the mechanism of Pb loss from uraninite by U-normalizedimpurity contents. (B) Chemical age plotted vs. U-normalized impuritycontent in uraninite.

Table 6 and plotted on a concordia diagram in Figure 8. The

majority of zircon crystals yield a concordant age of ca. 1880Ma. Two crystals, 4a and 7a, with distinctly different, brightCL images (Fig. 8, upper left), yield a concordant age of 860 �39 Ma (2�). The measured Th/U is higher for these two young,CL-bright crystals than the rest (Table 6), except one discordantcrystal (5a). The most discordant data point (9b) is from thevery CL-dark rim of grain 9. The data point from the center ofthe same grain, 9a, is concordant.

Inherited zircon is abundant in the main dyke at Oklo (Sère,1996) and is a common feature of continental tholeiites (Camp-bell, 1985). The 1880 Ma old zircons found in the present studywere probably inherited and not further discussed here. The agegiven by the two CL-bright zircons is considered to represent theintrusion age of the dolerite dyke.

5. DISCUSSION

5.1. Timing and Mechanisms of Uraninite Alteration andPb Loss

The 5–6 wt% PbO content of the uraninite analyzed in thisstudy is similar to a majority of uraninite in Bangombé andOklo-Okélobondo (Janeczek and Ewing, 1995; Jensen et al.,1997; Jensen and Ewing, 2000, 2001). The PbO content trans-lates to a chemical age of ca. 550 Ma. Since earlier studies haveshown that uraninite formed ca. 2 Ga ago, and that fissionproducts have been retained in the uraninite since that time(e.g., Gauthier-Lafaye et al., 1996), this chemical age indicatesthat substantial Pb loss has occurred. The chemical age alsoindicates that the process that caused this Pb loss was active ator after ca. 550 Ma.

Large galena grains, in RZ16 and RZB, yield model ages ofca. 890–860 Ma, indicating the time of the first major Pb lossepisode. The majority of galena model ages presented in pre-vious studies are slightly younger, between 800 and 700 Ma, asis the age given for Neoproterozoic Pb loss from uraninite,850–720 (Gauthier-Lafaye et al., 1996), but nevertheless thegalena model ages presented here agrees with a major Pb lossat 900–700 Ma.

In this study, the intrusion age of the dolerite dyke is con-strained to 860 � 39 Ma (2�), which firmly consolidates theproposed link between the dolerite dyke intrusions and the firsturaninite Pb loss. Previously, the temporal link between thedolerite dykes and the uraninite Pb loss was relatively weak,due to lack of a reliable age of the dykes. K-Ar on whole rockand feldspar fractions suggested ages of 980–850 Ma, but theK-Ar system had clearly been disturbed by a later event (Weberand Bonhomme, 1975; Bonhomme et al., 1978). Conventionalzircon U-Pb dating and Sm-Nd dating of the main dolerite dykeat Oklo resulted in the age of 746 � 16 Ma and 783 � 55 Ma,respectively, (2�; Sère, 1996); however, both of these ages arebased on two-point regressions. This, and the existence ofinherited zircon, render these ages questionable.

Since galena model ages at both RZ16 and RZB (20 kmapart) indicate a Pb loss at 890–860 Ma, the Pb loss at this timewas not a local effect of the main dolerite dyke at Oklo, but aregional event. The Pb released during this regional extensionalevent crystallized as millimeter-sized galena in the clays andhydrothermal alteration halo of the reactor zones. Galena crys-

tals in the 40-cm-wide and altered Oklo dyke appear to be

1600 L. Z. Evins, K. A. Jensen, and R. C. Ewing

coeval with this event. The timing of the regional extension inthe Franceville basin suggests that it may be related to a1000–900 Ma major tectono-thermal event in the Congo basin,and the breakup of Rodinia (Cahen, 1982; Tack et al., 2001).

Uraninite Pb-Pb isotope systematics yield ages around 500 Ma(RZ16: 512 � 110 Ma and RZB: 528 � 71 Ma). These ages arethe same, within error, to the chemical ages obtained in this studyand similar to chemical ages presented in previous studies (e.g.,Janeczek and Ewing, 1995). As with the chemical ages, theseisotopic ages indicate that Pb loss has occurred at or after 600–500Ma. From previous U-Pb isotope studies of uraninite, it has beenestablished that recent Pb loss occurred in the Oklo reactors (e.g.,Gauthier-Lafaye et al., 1996). Recent Pb loss does not change thePb isotopic composition of uraninite due to the long half-lives ofthe U isotopes. There is no time for accumulation of radiogenic Pbafter the recent Pb loss, and thus, no time to change the isotopiccomposition. Therefore, the observed Pb loss must have been

Table 3. Uraninite U and Pb isotopic io

Crystal 207Pb/206Pb � (%) 208Pb/206Pb � (%)

Bangombérb1 0.0578 1.4 0.00131 9.6rb2 0.0583 1.1 0.00199 5.1rb3 0.0552 1.4 0.00169 7.9rc1 0.0606 1.0 0.00237 2.7rc2 0.0564 0.7 0.00191 2.3rc3 0.0561 1.0 0.00158 4.5re1 0.0573 0.8 0.00190 4.6re2 0.0557 0.8 0.00174 2.5rf 0.0620 1.3 0.00304 2.2G 0.0686 0.8 0.00457 2.0J1 0.0662 0.7 0.00419 2.1J2 0.0653 0.8 0.00429 2.6K 0.0682 0.9 0.00448 2.1M 0.0651 1.3 0.00368 2.7N 0.0679 1.1 0.00423 2.101k 0.0696 1.1 0.00467 1.7

RZI61-RC 0.0604 2.7 0.00236 4.61-RC 0.0631 0.7 0.00264 2.31-RC 0.0612 0.9 0.00228 1.71-RC 0.0634 2.2 0.00284 3.92-RC 0.0561 0.4 0.00118 1.73-RC 0.0589 0.2 0.00174 1.23-RC 0.0582 0.3 0.00163 1.53-RC 0.0597 0.2 0.00202 0.54-RC 0.0586 0.3 0.00157 0.75-RC 0.0504 0.3 0.00176 0.76-RC 0.0592 0.2 0.00137 0.96-RC 0.0581 0.3 0.00123 1.36-RC 0.0589 0.2 0.00132 0.97-RC 0.0588 0.3 0.00142 0.77-RC 0.059 0.2 0.00146 0.87-RC 0.0592 0.3 0.00138 0.98-RC 0.0579 0.4 0.00126 1.48-RC 0.0588 0.4 0.00147 1.28-RC 0.0587 0.3 0.00139 1.09-RC 0.0585 0.3 0.00140 0.89-RC 0.0591 0.5 0.00144 1.2

10-OM 0.066 1.4 0.00096 7.011-RC 0.0567 0.7 0.00111 2.0

a 235U/238U is corrected for fractionation: fractionation factor 0.97 (b Corrected for 235U depletion.

ancient, but still later than the dolerite dyke intrusions. This later

Pb loss may have been caused by an episodic event or by acontinuous process, like volume diffusion.

To evaluate the possibility of continuous Pb loss via volumediffusion, we need information regarding the closure tempera-ture (Tc) and diffusional parameters of Pb in uraninite, of whichvery little is available. However, since the single stage modelworks well for large galena in this study, the uraninite U-Pbsystem must have been closed from 1950 Ma to ca. 860 Ma.Thus, the Tc of Pb in uraninite must be higher than the tem-perature of the basin during that time. Probably the temperaturewas at or below 180°C, the maximum burial temperature of theFA formation (Gauthier-Lafaye and Weber, 1989).

Tc of a mineral may be estimated using the concept of ionicporosity, i.e., the percentage of unit cell volume not occupiedby ions (Dahl, 1997). A high ionic porosity corresponds to alow Tc, and vice versa. Ideal uraninite (UO2) has a ionicporosity of 34.9%, while natural uraninite tends to have a

oprobe data. Errors are 1� given in %.

6Pb � (%) 235U/238Ua � (%) 207Pb/206Pbb � (%)

3 27.9 0.00635 1.0 0.0660 1.75 13.7 0.00640 1.0 0.0660 1.54 26.7 0.00635 1.2 0.0630 1.84 10.2 0.00641 0.6 0.0685 1.23 15.1 0.00640 0.5 0.0639 0.93 21.1 0.00638 0.5 0.0638 1.19 6.5 0.00631 0.5 0.0658 0.93 15.4 0.00631 0.7 0.0640 1.16 17.1 0.00639 0.4 0.0703 1.40 13.7 0.00633 0.7 0.0786 1.10 6.0 0.00627 0.7 0.0765 1.00 11.2 0.00637 0.5 0.0743 0.99 10.5 0.00632 0.6 0.0782 1.17 9.9 0.00636 0.6 0.0742 1.48 11.7 0.00644 0.7 0.0764 1.30 11.1 0.00638 0.7 0.0791 1.3

4 18.5 0.00651 0.8 0.0673 2.85 5.9 0.00645 0.9 0.0709 1.14 10.7 0.00652 0.8 0.0681 1.24 17.4 0.00640 2.0 0.0718 3.02 18.7 0.00646 0.6 0.0630 0.73 8.9 0.00636 0.6 0.0671 0.62 12.5 0.00647 1.0 0.0652 1.03 7.8 0.00642 0.8 0.0674 0.83 8.1 0.00659 0.8 0.0645 0.92 8.5 0.00636 0.8 0.0575 0.92 3.8 0.00647 0.6 0.0663 0.62 3.8 0.00643 0.5 0.0655 0.62 3.4 0.00643 0.5 0.0664 0.53 4.0 0.00645 0.5 0.0661 0.62 2.5 0.00650 0.6 0.0658 0.62 2.0 0.00646 0.6 0.0664 0.72 4.4 0.00653 0.6 0.0643 0.73 1.7 0.00649 0.6 0.0657 0.73 2.1 0.00651 0.6 0.0654 0.72 2.5 0.00651 0.6 0.0651 0.73 2.8 0.00651 0.6 0.0658 0.82 18.5 0.00711 0.9 0.0673 1.73 6.1 0.00654 1.7 0.0629 1.8

bé) and 0.95 (RZ16).

n micr

204Pb/20

0.00000.00000.00000.00000.00000.00000.00000.00000.00000.00010.00010.00010.00000.00000.00000.0001

0.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.0000

Bangom

somewhat lower ionic porosity due to the presence of large ions

1601Uraninite Pb loss in the Oklo and Bangombé natural reactors

like Pb2� and Ca2� and the smaller unit cell volume caused bythe presence of U6� (Janeczek and Ewing, 1992). According toDahl (1997), a 34.9% ionic porosity corresponds to a Tc of 630to 590°C. This can be compared to the diffusion coefficient (D)of Pb in uraninite given by Yershov (1974): logD � �17 at700°C. This D value is similar to that of titanite at 750°C, andtitanite has a Tc of ca. 620 to 680°C for a 1 cm grain andcooling rate of 1°C/Ma (Dahl, 1997).

Thus, it appears as if a ca. 400°C (590–180 � 410) temper-ature rise was needed to keep the temperature close to or abovethe Tc of the uraninite U-Pb system. This would require achange in geothermal gradient from �45°C/km to �140°C/km, which is improbable. Moreover, the significance of ther-mal-only volume diffusion, for several isotopic systems, isdisputed in the literature (e.g., Villa, 1998). Instead, the impor-

Fig. 7. Pb isotopic compositions of uraninite and galena. 207Pb/206Pb^ is corrected for 235U depletion. Error bars are 2�. (A) Uraninitein RZ16 (diamonds; no error bars) and RZB (circles). Linear regres-sions (mixing lines) and ages are calculated with Isoplot/Ex (Ludwig,2000). Data excluded from the regressions are plotted as white sym-bols. B) Uraninite (squares) and galena (circles) mixing lines. Crosses� galena in sample OP50/1; white symbols � RZ16; black symbols �RZB; large circles � large galena grains. Uraninite and galena form anarray, coinciding with the mixing lines (dashed lines) in (A). Largegalena grains from RZ16 and RZB, and galena from OP50/1, plot on aline (dotted) between radiogenic Pb and average terrestrial Pb (Staceyand Kramers, 1975).

tance of fluid availability and fluid-assisted recrystallization is

stressed in a number of studies (Dahl, 1997; Villa, 1998; Vanceet al., 2003 and references therein). Nanocrystallites in ura-ninite, observed by TEM in this study (also observed by Fayeket al., 2003) formed through internal recrystallization of theuraninite lattice, caused either by fluid interaction or self-annealing. Self-annealing, i.e., spontaneous repair of radiationdamage, occurs continuously and rapidly in uraninite (half-lifeof damage is �15,000 yr; Eyal, 1985). It is possible thatself-annealing accelerates Pb loss, causing the observed low Pbcontent. However, as with Pb diffusion, if this was a significantPb loss mechanism, we would expect Pb loss between 1950 and860 Ma. The lack of significant Pb loss during that timeindicates that continuous Pb loss due to self-annealing was notan important Pb-loss mechanism. This prompts us to considerthe possibility that an episodic, fluid-assisted recrystallizationevent caused the observed �500 Ma Pb loss from uraninite.The arguments in favor of this hypothesis are:

1. Leaching of U and Pb occurred during the formation of the�6wt% PbO-uraninite in RZ16 and RZB.

2. The dolerites in the Franceville basin are altered. Twodifferent studies indicate that the alteration, mainly chloritiza-tion, may be of Pan-African age (Weber and Bonhomme, 1975;Sère, 1996).

3. Granodiorites in the basement underneath the Francevillebasin have been altered, resulting in the crystallization ofsecondary chlorite, epidote (including allanite), actinolite andwhite mica. It is suggested that this alteration is related toPan-African green schist metamorphism (Sère, 1996).

4. U-Pb dating of zircon from the FD volcanics of theFrancevillian Series yield discordant data indicating a Pb lossfrom zircon at ca. 500 Ma (Horie et al., in press).

5. Two late Proterozoic fluids (type 9 and 10; Michaud andMathieu, 1998) and two late Proterozoic stages of sulfideprecipitation (Raimbault et al., 1996) have been recorded in theOklo region. The separate fluids have been connected to theseparate sulfide groups. One of these fluids is probably relatedto the extension/intrusion of the dyke swarm; however, theother could be related to the Pan-African orogeny.

6. The Pan-African orogeny in the region occurred ca. 600–540 Ma (Maurin et al., 1991; Hanson, 2003), and subjectedneighboring basins (200 km S and 300 km N of Oklo) to greenschist facies metamorphism (Vicat et al., 1991; Vicat andPouclet, 1995).

7. Orogenies may cause fluid flow in areas up to 700 kmaway (Romer and Wright, 1993) and Oklo is ca. 150 km awayfrom present limit of the Pan-African West Congolian belt.Thus, it is possible that the Pan-African orogeny triggered fluidflow in the Franceville basin, as well as in the basement.

Arguments against this hypothesis are:1. The majority of previous isotopic studies have not indi-

cated any uraninite Pb loss during Pan-African times (e.g.,Gauthier-Lafaye et al., 1996).

2. The late Proterozoic fluids (Michaud and Mathieu, 1998,and references therein) are both believed to be related to theregional extension and dolerite dyke intrusions. This is basedon single stage model ages of galena (830–790 and 725–695Ma).

3. The green schist paragenesis in the granodioritic base-

ment, interpreted by Sère (1996) to be related to the Pan-

1602 L. Z. Evins, K. A. Jensen, and R. C. Ewing

African orogeny, could have formed during cooling of thegranodioritic intrusions (Michaud and Mathieu, 1998).

We find, that there are several arguments supporting aninterpretation for a Pan-African episodic Pb-loss, probablyrelated to fluid circulation and alteration of uraninite, whilethere is no firm evidence against this interpretation. Thus, weinterpret the isotopic Pb-Pb ages found in this study as signif-icant. They have resulted from Pb loss related to internal,within-grain recrystallization of uraninite, which probably oc-curred episodically at 600–500 Ma. This coincides with the ageof the Pan-African orogeny in Equatorial and Southwest Africa(Hanson, 2003), and fits well with the typical chemical ages ofuraninite in this and previous studies (e.g., Janeczek and Ew-ing, 1995).

The difference between the age of uraninite Pb loss (�500Ma) and that of large galena crystals (860–890 Ma) observedin this study implies that there have been at least two ancientepisodes of radiogenic Pb loss in RZ16 and RZB. Fayek et al.(2002) report resetting of the U-Pb system in uraninite at 1180� 47 Ma and 898 � 46 Ma (1�). In the same study, three lowerintercepts of U-Pb discordias indicate Pb loss at 570 � 48 Ma,375 � 53 Ma and 17 � 141 Ma (1�). The present studyprovides support for a possible Pb loss at 500–600 Ma, but nosupport for the 1180 and 375 Ma ages. These two ages arederived from one U-Pb discordia, thus implying that there is aseparate group of uraninite grains that recrystallized at 1180and subsequently suffered Pb loss at 375 Ma (fig. 3c in Fayeket al., 2002). Fayek et al. (2002) propose that the carbonatediagenesis, related to fluids circulating in the Franceville basinduring uplift, may have caused the recrystallization at ca. 1180Ma. There are two main arguments against this interpretation:1) the carbonate diagenesis was suggested as the cause forresetting of the Rb-Sr and K-Ar isotopic systems in clays fromthe Franceville basin at 1870–1700 Ma (Bonhomme et al.,1982; Michaud and Mathieu, 1998), not ca. 1180 Ma. 2) Basin

Table 4. Pb isotopic composition of large galena grains measured byin %. Measured isotopic ratios are corrected for instrumental fractionat

Crystal 207Pb/206Pb � (%) 208Pb/206Pb � (%) 204Pb/206P

Bangombé1-L 0.144 0.1 0.0208 0.2 0.000471-L 0.144 0.2 0.0218 0.3 0.000491-L 0.143 0.1 0.0207 0.2 0.000481-L 0.143 0.1 0.0204 0.2 0.000461-L 0.143 0.1 0.0199 0.2 0.000451-L 0.143 0.1 0.0203 0.2 0.000461-L 0.143 0.1 0.0203 0.2 0.000472-L 0.144 0.1 0.0214 0.2 0.00050

RZI61-L 0.141 2.3 0.0166 1.2 0.000341-L 0.143 2.1 0.0167 0.7 0.000311-L 0.141 0.5 0.0153 0.8 0.000311-L 0.143 0.3 0.0166 0.7 0.000351-L 0.143 0.5 0.0171 0.7 0.000358-L 0.142 0.3 0.0169 0.5 0.00035

19-L 0.139 0.8 0.0165 1.5 0.0003319-L 0.141 0.8 0.0161 1.6 0.0003420-L 0.147 1.2 0.0158 1.9 0.00034

a Corrected for 235U depletion.b Corrected for common Pb and 235U depletion.

uplift occurred at the time of uranium mineralization, ca. 2 Ga

(Gauthier-Lafaye and Weber, 1989; Gauthier-Lafaye et al.,1996), and probably from Eocene and onwards (Gauthier-Lafaye and Weber, 2003).

The Pb isotopic compositions of small galena crystals inRZ16 appear to reflect a mixture between large, ca. 860 Ma oldgalena Pb and uraninite Pb. Galena is a U- and Th-free mineraland the Pb isotopic composition does not evolve in galena aftercrystallization. Instead, the Pb isotopic composition of galenarepresents a sample of the U-Pb system from which the Pb wasextracted at the time of galena crystallization. Since the mixtureobserved in small galena crystals involve Pb that accumulatedin uraninite from ca. 500 Ma until recently, this mixture musthave occurred recently. There are two possibilities: either Pbwas mixed, accidentally, during the ion microprobe analysis, orPb was mixed during a recent fluid event, combining the twotypes of Pb into new galena grains. Due to lack of correlationbetween Pb isotopic composition of small galena with neithergrain size nor proximity to uraninite, we consider the secondpossibility to be most likely. In addition, recent Pb loss in RZ16is reflected by the lower PbO content in some uraninite grains(1-RC, 11-RC) that still have the same isotopic composition asthe rest of the analyzed uraninite.

5.2. Potential Implications for Nuclear Waste Storage

The mechanisms behind migration of actinides and fissionproducts are vital for our understanding of the behavior of spentnuclear fuel in a natural environment. With the help of TEManalysis, we have been able to demonstrate the formation ofnanocrystallites in uraninite, which indicates that internal re-crystallization of uraninite occurred in association with radio-genic Pb loss. Although this study does not provide informationregarding fission product retention, previous studies show thatfission products have been well retained in RZB (Hidaka andGauthier-Lafaye, 2000). Uraninite in RZB appears to have

roprobe, and derived single-stage model ages. The errors are 1� givenanium isotopic ratio is the average isotopic composition of the sample.

(%) 235U/238U 207Pb/206Pba � (%) 207Pb/206Pbb Model age

.5 0.00636 0.164 0.7 0.157 903

.4 0.00636 0.164 0.7 0.157 901

.5 0.00636 0.163 0.7 0.156 881

.6 0.00636 0.163 0.7 0.156 885

.5 0.00636 0.163 0.7 0.156 888

.6 0.00636 0.163 0.7 0.157 895

.0 0.00636 0.163 0.7 0.156 888

.9 0.00636 0.164 0.7 0.156 891

.9 0.00647 0.158 2.5 0.153 826

.3 0.00647 0.160 2.3 0.155 865

.6 0.00647 0.158 1.0 0.154 845

.2 0.00647 0.160 0.9 0.156 884

.4 0.00647 0.160 1.0 0.155 865

.5 0.00644 0.160 1.4 0.154 845

.8 0.00648 0.156 1.1 0.151 785

.2 0.00648 0.158 1.1 0.153 826

.1 0.00648 0.164 1.4 0.160 960

ion micion. Ur

b �

00000010

172222565

undergone internal recrystallization and Pb loss without signif-

1603Uraninite Pb loss in the Oklo and Bangombé natural reactors

icantly remobilizing compatible fission products like REE. Thisobservation is important, since it illustrates that fission productsmay be retained within the uraninite structure, even wheninternal recrystallization of the lattice takes place. However,since fissiogenic metals (i.e., Ru, Rh, Pd, Mo), like Pb, areincompatible with the uraninite structure, these metals mayhave been mobilized during the Pb-loss events, as discussed byJaneczek and Ewing (1995) and Gauthier-Lafaye et al. (1996).Ru-S-arsenides have been observed at the grain boundaries ofuraninite from the natural fission reactors, suggesting a behav-ior similar to Pb of these elements (Jensen and Ewing, 2001).

6. CONCLUSIONS

Uraninite in both RZ16 and RZB contains ca. 6 wt% PbOgiving a chemical age of ca. 550 Ma. Submicron-sized galenainclusions in RZ16 are numerous, and the uraninite grainsanalyzed in this study are relatively rich in SiO2, indicating ahigher degree of alteration than in previously studied uraninitefrom RZ16. TEM analysis of both RZB and RZ16 showed that

Table 5. Pb isotopic composition of small galena grains measured binstrumental fractionation. The U isotopic ratio is the average isotopic

Crystala 207Pb/206Pb � (%) 208Pb/206Pb � (%)

RZI62-U 0.107 0.2 0.0108 0.33-Q 0.132 0.3 0.0155 0.44-S 0.197 3.0 0.0106 3.35-U 0.105 0.4 0.0101 0.66-U 0.132 0.3 0.0153 0.57-S 0.133 0.2 0.0159 0.49-S 0.116 1.2 0.0115 1.2

10-S 0.141 0.6 0.0150 1.411-S 0.134 1.2 0.0153 1.312-S 0.114 1.2 0.0117 1.713-S 0.113 0.9 0.0110 2.014-S 0.120 0.6 0.0132 1.115-S 0.110 0.7 0.0106 1.016-S 0.093 1.0 0.0069 2.417-S 0.115 0.9 0.0117 1.118-S 0.107 1.3 0.0103 2.021-S 0.101 1.5 0.0091 2.022-S 0.128 0.9 0.0148 1.223-S 0.128 1.1 0.0147 1.524-Q 0.153 1.0 0.0206 1.925-S 0.124 0.7 0.0142 0.526-Q 0.265 0.9 0.1118 0.927-Q 0.118 0.7 0.0111 1.028-C 0.120 1.3 0.0115 3.029-Z 0.100 1.6 0.0095 3.130-S 0.123 0.7 0.0134 0.831-S 0.118 0.4 0.0123 0.832-S 0.119 0.2 0.0128 0.333-S 0.095 0.3 0.0088 0.634-Z 0.072 0.3 0.0062 0.635-S 0.077 0.5 0.0068 0.6

OP50/1d-1a 0.164 0.3 0.0612 0.2d-1b 0.164 0.4 0.0642 0.1d-2 0.169 1.1 0.0682 0.1d-3 0.169 0.7 0.0676 0.1

a Letter in the crystal name indicates surrounding mineral: U � urab Corrected for 235U depletion.

the microscopic uraninite grains consist of nanocrystalline to

microcrystalline aggregates. At the grain boundaries, where theSi content is slightly higher and Pb content lower, the nano-crystallites are smaller than in the interior of the uraninite. Thismay be due to incipient coffinitization or leaching.

Large, millimeter-sized galena in the core of RZ16 is 856� 97 Ma (2�) old. The same age, 892 � 15 Ma, is found fortwo large galena grains in RZB. These ages are in agreementwith the intrusion age of the main dolerite dyke at Oklo, 860 �39 Ma (2�), as determined on two magmatic zircon crystals.This concurrence of ages substantiates the connection betweenthe intrusion of dolerite dykes in the Franceville basin and thefirst important Pb loss in the Oklo and Bangombé naturalreactors.

Uraninite Pb isotopes from both RZ16 and RZB indicate asecond Pb loss at �500 Ma. From isotopic data alone it is notclear if this age is a result of an episodic event, or if the Pb lossoccurred continuously; however, Pb loss occurred during inter-nal recrystallization and Fe, Ca and Si enrichment, and wasprobably associated with leaching. This, together with indica-

. Errors are 1� given in %. Measured isotopic ratios are corrected forsition of U in the sample.

Pb/206Pb � (%) 235U/238U 207Pb/206Pbb � (%)

.00021 1.9 0.00646 0.120 0.6

.00033 2.3 0.00644 0.149 1.4

.00017 9.4 0.00644 0.222 3.3

.00019 4.9 0.00642 0.119 1.1

.00033 3.4 0.00659 0.145 0.9

.00033 1.4 0.00644 0.150 1.4

.00023 5.3 0.00648 0.130 1.4

.00031 6.8 0.00648 0.158 0.9

.00034 4.1 0.00648 0.150 1.4

.00026 3.9 0.00648 0.128 1.4

.00024 2.3 0.00648 0.126 1.1

.00027 4.0 0.00648 0.134 0.9

.00021 4.1 0.00648 0.123 1.0

.00014 3.1 0.00648 0.104 1.2

.00028 3.4 0.00648 0.129 1.1

.00020 4.3 0.00648 0.120 1.5

.00018 6.3 0.00725 0.101 1.5

.00031 3.5 0.00725 0.128 0.9

.00040 3.6 0.00725 0.128 1.1

.00029 4.8 0.00725 0.153 1.0

.00028 2.8 0.00725 0.124 0.7

.00296 1.4 0.00725 0.265 0.9

.00021 4.2 0.00725 0.118 0.7

.00025 6.4 0.00725 0.120 1.3

.00018 7.3 0.00654 0.111 2.3

.00026 3.5 0.00654 0.136 1.8

.00027 1.3 0.00654 0.131 1.7

.00024 1.2 0.00654 0.132 1.7

.00017 1.4 0.00654 0.105 1.7

.00004 8.7 0.00654 0.080 1.7

.00013 1.5 0.00654 0.085 1.8

.00119 0.7 0.00725 0.164 0.3

.00129 1.5 0.00725 0.164 0.4

.00120 6.5 0.00725 0.169 1.1

.00135 2.0 0.00725 0.169 0.7

Q � quartz; S � sheet silicates; C � calcite; Z � zircon.

y SIMScompo

204

0000000000000000000000000000000

0000

ninite;

tions of Pan-African (600–540 Ma) fluid circulation in the

1604 L. Z. Evins, K. A. Jensen, and R. C. Ewing

Oklo region, leads us to interpret the �500 Ma age as signif-icant. Recent Pb loss is also indicated at RZ16, where fine-grained galena (5–100 �m) contains Pb both from 860 Ma oldgalena crystals and Pb recently lost from uraninite.

The results of this study indicate that reactor zone uraninitein nature may restabilize not only during one, but several eventsinvolving fluid interaction and internal recrystallization, andstill retain most of the fission products within its structure.However, incompatible elements, such as Ru, Tc, Rh, Pb andMo, may be segregated from the matrix to form their ownphases. It has important implications for the storage of spent

Table 6. Ion microprobe U-Th-Pb data of zircon crystals and der204Pb.a

Grain/spot no.

[U]ppm

[Th]ppm

[Pb]ppm

Th/Umeas. f206 (%)

207Pb/206Pb

E(

1a 177 33 71 0.19 0.03* 0.1151 02a 148 33 61 0.22 0.00* 0.1147 03a 467 66 182 0.14 0.01* 0.1149 04a 265 168 48 0.63 0.04* 0.0670 05a 2246 942 569 0.42 8.5 0.0981 36a 243 39 98 0.16 0.01* 0.1153 07a 177 140 34 0.79 0.00* 0.0694 18a 125 27 51 0.22 0.45 0.1100 19a 144 22 58 0.15 0.03* 0.1147 09b 5124 952 760 0.19 6.82 0.0912 3

10a 132 24 54 0.18 1.19 0.1210 111a 361 68 158 0.19 0.02* 0.1122 012b 406 81 164 0.20 1.26 0.1144 013a 160 35 66 0.22 0.02* 0.1147 014a 223 37 92 0.16 0.01* 0.1147 015a 207 42 83 0.20 0.03* 0.1151 016a 1069 248 441 0.23 0.01* 0.1154 017a 211 34 85 0.16 0.02* 0.1140 018a 1973 741 718 0.38 0.94 0.1139 0

a An asterisk indicates possibly insignificant common Pb. Errors onas 1� absolute errors. D � discordance at the 2� error limit.

Fig. 8. Concordia plot and cathodoluminescence (CL) images ofanalyzed zircon crystals from the main dyke at Oklo. Numbers refer tozircon crystals (Table 6). The concordia plot and age calculation isperformed with Isoplot/Ex (Ludwig, 2000). Circles on the CL images

indicate the location of ion microprobe analyses. 4a and 7a are the onlycrystals with bright CL.

nuclear fuel, in the case of a broken canister and interactionbetween fuel and fluids.

Acknowledgments—We thank F. Gauthier-Lafaye and L. Pourcelot forproviding the well-documented samples. We thank P. Allart for helpwith sample preparations. H. Harryson, D. Holtstam and U. Håleniuskindly helped us with the electron microprobe analysis. We thank C. S.Palenik for conducting the HRTEM analysis of RZ16 samples atUniversity of Michigan, USA. We are greatly indebted to the staff atthe NORDSIM facility who provided technical assistance and invalu-able help during completion of the ion microprobe analyses. We areespecially grateful to K. Högdahl for the help during final preparation,CL imaging and ion microprobe analyses on the zircons. S. Claesson,V. Oversby, U. Hålenius and I. Williams are thanked for their carefulreviews of earlier versions of the manuscript as well as for providingimportant suggestions. This work was supported by the Swedish Nu-clear Fuel and Waste Co. AB (SKB), Stockholm, Sweden under EUOklo Natural Analog Project—Phase II (LZE and KAJ) and the Sci-ence and Technology Program of the Office of Civilian RadioactiveWaste Management (RCE). The NORDSIM ion microprobe facility inStockholm is funded by the natural science funding agencies of Swe-den, Denmark, Finland and Norway and the Swedish Museum ofNatural History. This is NORDSIM contribution nr. 105. This manu-script improved greatly from the reviewers’ comments.

Associate editor: E. Ripley

REFERENCES

Bonhomme M. G., Leclerc J., and Weber F. (1978) Etude radiochro-nologique complémentaire de la série du Francevillien et de sonenvironnement. In Natural Fission Reactors (ed. IAEA), pp. 19–23. IAEA

Bonhomme M. G., Gauthier-Lafaye F., and Weber F. (1982) Anexample of Lower Proterozoic sediments: The Francevillian inGabon. Precambr. Res. 18, 87–102.

Bowles J. F. W. (1990) Age dating of individual grains of uraninite inrocks from electron microprobe analyses. Chem. Geol. 83, 47–53.

es. f206 (%) � amount of common 206Pb, estimated from measured

06Pb/238U

Err(%)

207Pb/235U

Err(%)

207Pb/206Pbage (Ma)

206Pb/238Uage (Ma) D %

.342 1.9 5.43 1.9 1881 � 8 1897 � 31

.348 1.9 5.50 1.9 1875 � 8 1925 � 31

.338 1.9 5.36 1.9 1879 � 5 1878 � 30

.143 1.9 1.32 1.9 837 � 13 864 � 15

.182 1.9 2.46 3.9 1588 � 63 1077 � 19 �18

.349 1.9 5.55 1.9 1885 � 7 1929 � 31

.146 1.9 1.40 2.5 911 � 33 879 � 15

.354 1.9 5.37 2.1 1799 � 18 1955 � 31 4

.351 3.3 5.56 3.3 1875 � 8 1941 � 56

.125 2.3 1.57 4.3 1451 � 67 757 � 16 �31

.342 2.3 5.71 2.7 1970 � 24 1898 � 38

.377 3.0 5.84 3.1 1835 � 13 2064 � 53 7

.343 2.2 5.41 2.3 1871 � 9 1900 � 36

.351 3.9 5.55 3.9 1876 � 9 1938 � 65

.358 3.9 5.67 4.0 1875 � 14 1975 � 67

.342 3.0 5.42 3.1 1881 � 1 1895 � 49

.349 3.0 5.56 3.0 1887 � 4 1932 � 50

.349 3.0 5.48 3.0 1864 � 7 1928 � 50

.300 3.1 4.71 3.2 1863 � 11 1690 � 47 �4

c ratios are given as 1� relative errors in %. Errors on ages are given

ived ag

rr%)

2

.5 0

.5 0

.3 0

.6 0

.5 0

.4 0

.6 0

.0 0

.4 0

.6 0

.4 0

.7 0

.5 0

.5 0

.8 0

.6 0

.2 0

.4 0

.6 0

isotopi

Bros R., Stille P., Gauthier-Lafaye F., Weber F., and Clauer N. (1992)Sm-Nd isotopic dating of Proterozoic clay material: An example

1605Uraninite Pb loss in the Oklo and Bangombé natural reactors

from the Francevillian sedimentary series, Gabon. Earth Planet.Sci. Lett. 113, 207–218.

Bros R., Andersson P., Roos P., Claesson S., Holm E. and Smellie J.(1998) Swedish investigations on the Bangombé reactor zone.U-series disequilibrium and time scale of radionuclide mobilizationprocesses. In OKLO Working Group Proceedings of the First JointEC-CEA Workshop on the OKLO—Natural Analogue Phase IIProject Held in Sitjes, Spain, from 18 to 20 June 1997 (eds. D.Louvat and C. Davies) pp. 187–196. Nuclear Science and Technol-ogy, EUR 18314 EN.

Bros R., Roos P., Andersson P., Holm E., Griffault L., and Smellie J.(2000) Mobilization of fission products and 238U-series radionu-clides in the Bangombé reactor zone—Migration trends and time-scale evaluation. In OKLO Working Group Proceedings of theThird and Final EC-CEA workshop on OKLO—Phase II, Held inCadarache, France, on 20 and 21 May 1999 (eds. V. Louvat H.Michaud and von Maravic) pp. 227–241. Nuclear Science andTechnology, EUR 19137 EN.

Caen-Vachette M., Vialette Y., Bassot J.-P., and Vidal P. (1988)Apport de la geochronologie isotopique à la connaissance de lageologie gabonaise. Chron. Rech. Min. 491, 35–53.

Cahen L. (1982) Geochronological correlation of the Late Precambriansequences on and around the stable zones of Equatorial Africa.Precambr. Res. 18, 87–102.

Campbell J. H. (1985) The difference between oceanic and continentaltholeiites: A fluid dynamic explanation. Contrib. Min. Petrol. 91,37–43.

Cathelineau M., Boiron M. C., Holliger P., and Poty B. (1990) Metal-logenesis of the French part of the variscan orogen. Part II: Time-space relationships between U, Au and Sn-W ore deposition andgeodynamic events—Mineralogical and U-Pb data. Tectonophysics177, 59–79.

Dahl P. S. (1997) A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implica-tions for geochronology. Earth. Planet. Sci. Lett. 150, 277–290.

Evins L. Z., Sunde T., Fayek M., and Schöberg H. (2001) U and Pbisotope analysis of uraninite and galena by ion microprobe. Tech-nical Report TR-01-35. SKB.

Eyal Y. (1985) Preferential leaching and the age of radiation damagefrom alpha decay in minerals. Geochim. Cosmochim. Acta 49,1155–1164.

Fayek M., Jensen K. A., Ewing R. C., Riciputi L. R. (2002) In situisotope analysis of uraninite microstructures from the Oklo-Okélo-bondo natural fission reactors, Gabon. MRS Symp. Proc. 713,849–856.

Fayek M., Utsunomiya S., Ewing R. C., Riciputi L. R., and JensenK. A. (2003) Oxygen isotopic composition of nano-scale uraniniteat the Oklo-Okélobondo natural fission reactors, Gabon. Am. Min.88, 1583–1590.

Force E. R., Richards R. P., Scott K. M., Valentine P. C., and FishmanN. S. (1996) Mineral intergrowths replaced by “elbow-twinned”rutile in altered rocks. Can. Mineral. 34, 605–614.

Gauthier-Lafaye F. and Weber F. (1989) The Francevillian (LowerProterozoic) uranium ore deposits of Gabon. Econ. Geol. 84, 2267–2285.

Gauthier-Lafaye F., Weber F., and Ohmoto H. (1989) Natural fissionreactors of Oklo. Econ. Geol. 84, 2286–2295.

Gauthier-Lafaye F., Holliger P., and Blanc P.-L. (1996) Natural fissionreactors in the Franceville basin, Gabon: A review of the conditionsand results of a “critical event” in a geologic system. Geochim.Cosmochim. Acta 60, 4831–4852.

Gauthier-Lafaye F. and Weber F. (2003) Natural nuclear fission reac-tors: Time constraints for occurrence and their relation to uraniumand manganese deposits and to the evolution of atmosphere. Pre-cambr. Res. 120, 81–100.

Haggerty S. E. (1976) Oxidation of opaque mineral oxides in basalts.In Oxide Minerals: M SA Short Course Notes (ed. D. Rumble III,pp. )Hg-1–100. Mineralogical Society of America.

Hanchar J. M. and Miller C. F. (1993) Zircon zonation patterns asrevealed by cathodoluminescence and backscattered electron im-ages: Implications for interpretation of complex crustal histories. In

Geochemistry of Accessory Minerals: Papers Presented at the

Third V. M. Goldschmidt Conference (eds. E. B. Watson, T. M.Harrison, C. F. Miller and F. J. Ryerson), pp. 1–13. Elsevier.

Hanson R. E. (2003) Proterozoic geochronology and tectonic evolutionof southern Africa. In Proterozoic East Gondwana: SupercontinentAssembly and Breakup (eds. M. Yoshida B. F. Windley and S.Dasgupta), pp. 427–463. Special Publication 206. Geological So-ciety, London.

Hart S. R., Shimizu N., and Sverjensky D. A. (1981) Lead isotopezoning in galena: An ion microprobe study of a galena crystal fromthe Buick Mine, Southeast Missouri. Econ. Geol. 76, 1873–1878.

Hidaka H. and Gauthier-Lafaye F. (2000) Redistribution of fissiogenicand non-fissiogenic REE, Th and U in and around natural fissionreactors at Oklo and Bangombe, Gabon. Geochim. Cosmochim.Acta 64, 2093–2108.

Holliger P. (1988) Ages U-Pb définis in situ sur oxydes d’uranium àl’analyseur ionique: Méthodologie et conséquences géochimiques.C. R. Acad. Sci. Paris, ser. 2, 307, 367–372.

Holliger P. (1992) SIMS isotope analysis of U and Pb in uraniumoxides: Geological and nuclear applications. In Secondary Ion MassSpectrometry (SIMS VIII) (eds. A. Benninghoven, K. T. F. Jansen,J. Tümpner, and H. W. Werner), pp. 719–722. Wiley.

Horie K., Hidaka H., and Gauthier-Lafaye F. (in press) U-Pb geochro-nology and geochemistry of zircon from the Franceville series atBidoudouma, Gabon. Goldschmidt 2005, abstract.

Janeczek J. (1999) Mineralogy and geochemistry of natural fissionreactors in Gabon. Rev. Mineral. 38, 321–392.

Janeczek J. and Ewing R. C. (1992) Structural formula of uraninite.J Nucl. Mat. 190, 128–132.

Janeczek J. and Ewing R. C. (1995) Mechanism of lead release fromuraninite in the natural fission reactors in Gabon. Geochim.Cosmo-chim. Acta 59, 1917–1931.

Janeczek J. and Ewing R. C. (1996) Phosphatian coffinite with rareearth elements and Ce-rich françoisite-(Nd) from sandstone beneatha natural fission reactor at Bangombé, Gabon. Mineral. Mag. 60,665–669.

Janeczek J., Ewing R. C., Oversby V. M., and Werme L. O. (1996)Uraninite and UO2 in spent nuclear fuel: A comparison. J. Nucl.Mat. 238, 121–130.

Jensen K. A., Ewing R. C., and Gauthier-Lafaye F. (1997) Uraninite: A2 Ga spent nuclear fuel from the natural fission reactor at Bangoméin Gabon, West Africa. MRS Symp. Proc. 465, 1209–1218.

Jensen K. A. and Ewing R. C. (2000) Microtexture and chemistry of“unaltered” uraninite in the Oklo, Okélobondo and Bangombénatural fission reactors. In OKLO Working Group: Proceedings ofthe Second EC-CEA Workshop on OKLO—Phase II, Helsinki, 16 to18 June (eds. H. von Maravic and D. Louvat), pp. 61–91. NuclearScience and Technology, EUR 19116 EN.

Jensen K. A. and Ewing R. C. (2001) The Okélobondo natural fissionreactor: Geology, mineralogy and retardation of nuclear reactionproducts. GSA Bull. 113, 32–62.

Jensen K. A., Janeczek J., Ewing R. C., Stille P., Gauthier-Lafaye F.,and Salah S. (2000) Crandallites and coffinite: Retardation ofnuclear reaction products at the Bangombé natural fission reactor.MRS Symp. Proc. 608, 525–532.

Jensen K. A., Palenik C. S., and Ewing R. C. (2002) U6� phases in theweathering zone of the Bangombé U-deposit: Observed and pre-dicted mineralogy. Radiochim. Acta 90, 761–769.

Ludwig K. R. (2000) User’s Manual for Isoplot/Ex version 2.4. Specialpublication No. 1a. Berkeley Geochronology Center.

Maurin J.-C., Boudzoumou F., Djama L.-M., Gioan P., Michard A.,Mpemba-Boni J., Peucat J.-J., Pin C., and Vicat J.-P. (1991) Lachaîne protérozoïque ouest-congolienne et son avant-pays au Con-go: Nouvelles données géochronologiques et structurales, implica-tions en Afrique centrale. C. R. Acad. Sci. Paris, ser. 2, 312,1327–1334.

Meddaugh W. S., Holland H. D. and Shimizu N. (1982) The isotopiccomposition of lead in galenas in the uranium ores at Elliot Lake,Ontario, Canada. In Ore Genesis: The State of the Art (eds. G. C.Amstutz et al.), pp. 25–37. Springer-Verlag.

Michaud V. and Mathieu R. (1998) Synthesis of existing informationon the geological history of the Franceville basin and the Oklo-

Okélobondo and Bangombé nuclear reactor zones (Gabon). Com-missariat l’Energie Atomique/DCC/DESD/SESD, Centre de Ca-

1606 L. Z. Evins, K. A. Jensen, and R. C. Ewing

darache 13108 Saint-Paul-les-Durance, France. Rapport technique,v. DESD no. 98/176, 65 p.

Peycelon H. (1993) Suivi des travaux miniers et échantillonnage.Mission de septembre 1990 à décembre 1991. In Oklo, AnalogueNaturels de Stockage de Déchets Radioactifs, Rapport avancementau 30 Juin 1992—Contrat FI22W-CT91-OO71 (eds. P. L. BlancA. M. Chapuis), pp. 1-63 IPSN.

Raimbault L., Peycelon H., and Blanc P.-L. (1996) Characterization ofnear- to far-field ancient migrations around Oklo Reaction Zones(Gabon) using minerals as geochemical tracers. Radiochim. Acta74, 283–287.

Reed S. J. B., Trueman N. A., and Long J. V. P. (1988) Ion microprobeU-Pb dating of pitchblende. In Secondary Ion Mass Spectrometry(SIMS VI) (eds. A. Benninghoven A. M. Huber H. W. Werner), pp.945–948. Wiley.

Romer R. L. and Wright J. E. (1993) Lead mobilization during tectonicreactivation of the western Baltic Shield. Geochim. Cosmochim.Acta 57, 2555–2570.

Salah S. (2000) Weathering processes at the natural fission reactor ofBangombé (Gabon). Identification and geochemical modeling ofthe retention and migration mechanisms of uranium and rare earthelements. Ph.D. thesis. Université Louis Pasteur de Strasbourg.

Sère V. (1996) Géochimie des minéraux néoformés à Oklo (Gabon),histoire géologique du bassin d’Oklo: Une contribution pour lesétudes de stockages géologiques de déchets radioactifs. Ph.D. the-sis. Université Paris VII.

Stacey J. S. and Kramers J. D. (1975) Approximation of terrestial leadisotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26,207–221.

Tack L., Wingate M. T. D., Liegeois J. P., Fernandez A. M., andDeblond A. (2001) Early Neoproterozoic magmatism (1000–910

Ma) of the Zadinian and Mayumbian groups (Bas-Congo): Onset of

Rodinia rifting at the western edge of the Congo Craton. Precambr.Res. 110, 277–306.

Vance D., Müller W., and Villa I. M. (2003) Geochronology: Linkingthe isotopic record with petrology and textures—An introduction.In Geochronology: Linking the Isotopic Record with Petrology andTextures, pp. 1–24. Special Publication 220. Geological Society,London.

Vicat J.-P. and Pouclet A. (1995) Nature du magmatisme lié à uneextension pré-panafricaine: Les dolérites des bassins de Comba etde Sembé-Ouesso (Congo). Bull. Soc. Géol. France 166, 355–364.

Vicat J. P., Gioan P., and Caby R. (1991) Mise en évidence d’unmétamorphisme de faciès schiste vert dans la série de la Bouenzasur le flanc sud oriental du massif du Chaillu (Congo). C. R. Acad.Sci. Paris, ser. 2, 312, 517–522.

Villa I. M. (1998) Isotopic closure. Terra Nova 10, 42–47.Weber F. and Bonhomme M. (1975) Données radiochronologiques

nouvelles sur le Francevillien et son environnement. In The OkloPhenomenon (ed. IAEA), pp. 17–33. IAEA.

Whitehouse M. J., Claesson S., Sunde T., and Vestin J. (1997) Ionmicroprobe U-Pb zircon geochronology and correlation of Archaeangneisses from the Lewisian Complex of Gruinard Bay, northwesternScotland. Geochim. Cosmochim. Acta 61, 4429–4438.

Whitehouse M. J., Kamber B. S., and Moorbath S. (1999) Age signif-icance of U-Th-Pb zircon data from early Archaean rocks of WestGreenland; a reassessment based on combined ion-microprobe andimaging studies. Chem. Geol. 160, 201–224.

Wiedenbeck M., Allé P., Corfu F., Griffin W. L., Meier M., Oberli F.,vonq Uadt A., Roddick J. C., and Spiegel W. (1995) Three naturalzircon standards for U-Th-Pb, Lu-Hf trace element and REE anal-ysis. Geostand. Newsl. 19, 1–23.

Yershov V. M. (1974) A method of examining the diffusion parametersof lead in uranium minerals (translated in Geochemistry Interna-

tional 11, 1099–1101). Geokhimyia 10, 1565–1568.