cenozoic topographic and climatic response to changing tectonic boundary conditions in western north america abstract this study presents an oxygen isotopic record from the paleocene to the pliocene based on the analysis of predominantly paleosol carbonate from intermontane basins in southwestern montana and eastern idaho 18 o values of calcite decrease by 7 to 10 between 50 and 47 ma in southwestern montana and idaho most likely as a result of an increase in elevation of 2 5 to 3 5 km this rise in elevation is roughly contemporaneous with the emplacement of the nearby challis volcanics and the formation of metamorphic core complexes in the hinterland of the sevier thrust belt moreover when compared to previous oxygen isotopic studies that show oxygen isotopic shifts of similar magnitude occurring later in the late eocene to early oligocene in northeastern nevada and late oligocene to miocene in southern nevada the results of this study add to a growing body of evidence for a spatial and temporal migration of high surface elevations from north to south in the great basin of western united states this surface uplift history supports tectonic models calling for north to south removal of the farallon slab or delamination of the mantle lithosphere

transport and selective uptake of radium into natural clay minerals abstract understanding of the environmental behavior of ra is important from the viewpoint of the long termed repository safety of radioactive waste but investigation of ra behavior in natural environment is difficult to detect we found isotopic evidence of ra transportation and its selective uptake into clay minerals from pb isotopic analyses illite grains found in calcite veins included in sandstone near the oklo uranium deposit republic of gabon show extremely low 207 pb 206 pb 0 0158 isotopic ratios although the pb isotopic ratios of calcite and quartz coexisting with illite indicate the formation age of each component those of illite do not in addition illite grains having low 207 pb 206 pb isotopic ratios contain a strongly large amount of ba 1230 to 6010 ppm in contrast with low contents of ba in calcite and quartz 207 pb 206 pb isotopic data suggest an excess of 206 pb due to selective adsorption of 226 ra and also ba into illite grains this is a very rare example to show evidence of the selective adsorption behavior of ra from the isotopic excesses of 206 pb although the adsorption ability of ra itself in nature was largely reported

ree mobility in groundwater proximate to the natural fission reactor at bangombe gabon the natural nuclear fission reactor at bangombe is located at a depth of approximately 12 m and has undergone supergene weathering and chemical exchange with groundwater under moderately oxidizing conditions this reactor has been studied as an analogue for the migration of radionuclides in a geologic repository five water samples were taken from a drill hole in the reactor zone and from drill holes that are situated along the direction of the groundwater flow dissolution of phosphates in the weathering profile provided an important mechanism for the mobilization of phosphorous and ree this phosphorous allowed the formation of secondary minerals e g phosphatian coffinites and fe uranyl phosphate hydroxide hydrates the filtered groundwaters dissolved phase obtained directly from the reactor zone had anomalous 143nd 146nd 145nd 146nd and 149sm 147sm isotope ratios of 0 7235 0 4933 and 0 843 respectively confirming mixing between a fissiogenic and normal ree component in this dissoved phase a small fraction of the nd 2 3 at is of fissiogenic origin the suspended loads filtered particulates of the same groundwater from within the reactor show similar isotopic anomalies with a fissiogenic nd contribution of 3 2 at similar fissiogenic nd contributions are observable for groundwater samples from outside the reactor zone 3 m distance both dissolved and suspended load samples from the more distant well 25 m have normal isotopic ratios thus the prevailing physico chemical conditions within this groundwater system allow the migration of the fissiogenic ree over at least 3 m but less than 25 m although ree phases are abundant in the reactor zone the ree concentrations of the water in contact with the sediments of this zone are very low 17 ppt the most important lanthanide and actinide bearing phases are uraninite coffinites and fe uranyl phosphate hydroxide hydrate the presence of all of these phases retards the mobility of the fissiogenic lanthanides and actinides

late pleistocene holocene ruptures of the lima reservoir fault sw montana abstract active tectonics within the northern basin and range province provide a natural laboratory for the study of normal fault growth linkage and interaction here we present new geologic mapping and morphologic fault scarp modeling within the centennial valley montana to characterize pleistocene holocene ruptures of the young and active lima reservoir fault geologic relationships and rupture ages indicate middle pleistocene activity on the henry gulch 50 ka and 23 10 ka trail creek 43 ka and 13 ka and reservoir 23 ka segments offset quaternary deposits also record holocene rupture of the reservoir segment 8 ka but unfaulted modern streams show that no segments of the lima reservoir fault have experienced a large earthquake in at least several millennia the clustered pattern of rupture ages on the lima reservoir fault segments suggests a seismogenic linkage though segment length and spacing make a physical connection at depth unlikely trail creek and reservoir segment slip rates were non steady and appear to be increasing the fault helps accommodate differential horizontal surface velocity measured by gps geodesy across the boundary between the northern basin and range province and the snake river plain

evidence for synchronous thin skinned and basement deformation in the cordilleran fold thrust belt the tendoy mountains southwestern montana the tendoy mountains contain the easternmost thin skinned thrust sheets in the cordilleran fold thrust belt of southwestern montana and are in the zone of tectonic overlap between the rocky mountain foreland and the cordilleran fold thrust belt the three frontal thrust sheets of the tendoy mountains are from north to south the armstead mckenzie and tendoy sheets near the southeastern terminus of the tendoy thrust sheet is a lateral ramp in which the tendoy thrust climbs along strike from the upper mississippian lombard limestone to lower cretaceous rocks this ramp coincides with the southeastern side of the paleozoic snowcrest trough and projection of the range flanking basement thrust of the blacktail snowcrest uplift suggesting either basement or stratigraphic control on location of the lateral ramp axes of major folds on the southern part of the tendoy thrust sheet are parallel to the direction of thrust transport and to the trend of the snowcrest range they are a result of 1 pre thrust folding above basement faults 2 passive transportation of the folds from a down plunge position 3 minor reactivation of basement faults and 4 emplacement of blind sub tendoy thin skinned thrust faults the tendoy sheet also contains a major out of sequence thrust fault that formed in thick upper mississippian shales and created large overturned foreland verging folds in upper mississippian to triassic rocks the out of sequence fault can be identified where stratigraphic section is omitted and by a stratigraphic separation diagram that shows it cutting down section in the direction of transport the prominent lateral ramp at the southern terminus of the tendoy thrust sheet is a result of fault propagation through strata folded over the edge of the blacktail snowcrest uplift

provenance of synorogenic foreland basin strata in southwestern montana requires revision of existing models for laramide tectonism north american cordillera

isotopic evidence for trapped fissiogenic ree and nucleogenic pu in apatite and pb evolution at the oklo natural reactor abstract a part of the boundary layer of reactor zone 10 at the oklo natural reactor shows a unique petrologic texture which contains high grade uraninite and massive apatite concretions in order to study distribution behavior of fission products around the boundary between the reactor zone and the wall rock and to clarify the relation of migration mechanisms of fission products with geochemical factors in situ isotopic analyses of nd sm gd pb and u in uraninite and apatite from the sample were performed by sensitive high resolution ion microprobe shrimp sm and gd isotopic ratios of uraninite and apatite show evidence of neutron irradiation with fluence between 4 4 6 8 10 19 n cm 2 judging from the isotopic anomalies of nd and u the apatite coexisting with the uraninite plays an important role in trapping fissiogenic lree and nucleogenic 239 pu into the structure systematic pb isotopic data from apatite uraninite galena and minium suggest the following chronological interpretations 1 the apatite formed 1 92 0 01 ga ago and trapped fissiogenic light ree and nucleogenic 239 pu that migrated from the reactor during the criticality 2 the uraninite around the boundary between reactor and sandstone dissolved once 1 1 1 2 ga ago 3 galena grains were formed by u pb mobilization in association with the intrusion of dolerite dyke 0 45 0 83 ga ago 4 minium was derived from recent dissolution of galena under locally oxidizing conditions

post glacial arid episodes in ethiopia have implications for climate prediction several lines of geomorphological and stratigraphical evidence suggest that the widespread early post glacial wet phase in tropical africa between 12 500 and 5 000 yr bp refs 1 2 was interrupted by severe dry episodes during the intervals 11 000 10 000 8 500 6 500 and 6 200 5 800 yr bp refs 3 9 the magnitude of the second event has recently been questioned10 we present new stratigraphical and dating evidence from the ziway shala lake basin ethiopia indicating the severity and timing of dry phases during the interval 12 000 5 000 yr bp our findings have important implications for predictions of the climatic impact of increasing atmospheric levels of co2

sedimentation and recent history of a freshwater wetland in a semi arid environment loboi swamp kenya east africa loboi swamp is a 1ae 5k m 2 freshwater wetland situated near the equator in the kenya rift valley the climate is semi arid precipitation is 700 mm year 1 and evapotranspiration is 2500 mm year 1 some of the wetland water is currently used for irrigation an interdisciplinary study was conducted on the geology hydrology pedology and biology of the wetland to determine its origin and history and to assess its longevity under present hydrological conditions sedimentary records from two piston cores 1ae8 and 4 m long indicate that the present wetland developed during the late holocene on a lowrelief alluvial plain floodplain deposits sandy silts are capped with wetland sediments organic rich clay and peat which began to form at 700 bp the swamp is dominated by typha domingensis pers 80 and floating cyperus papyrus l 20 it is fed by warm springs t 35 c ph 6ae4 6ae9 emanating from grid faults of the rift floor water compositions suggest that sources are dominated by shallow meteoric water with little contribution from deeper geothermal fluids siderite concretions in the floodplain silts reflect the fe reducing conditions that developed as the surface became submerged beneath the water table the pollen record captured both local and more regional vegetation showing the prevailing dry rift valley climate despite development of the wetter conditions on the valley floor the diatom record also suggests a dramatic change in local hydrology the combined biological records of this semi arid wetland indicate an abrupt change to wetter conditions most probably as a result of a regional change in climate rift tectonics provided accommodation space maintained the wetland at or below the water table and enabled spring recharge the size of the modern wetland has been reduced by about 60 since 1969 which suggests that the system may now be under hydrological stress due to anthropogenic impacts from land use change

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

zeolitic diagenesis of late quaternary fluviolacustrine sediments and associated calcrete formation in the lake bogoria basin kenya rift valley late quaternary fluviolacustrine siltstones mudstones and claystones loboi silts on the northern margins of the saline alkaline lake bogoria in the kenya rift valley contain up to c 40 authigenic analcime and minor natrolite the zeolitic sediments are reddish brown and up to 1 m thick the amount of analcime increases upward in the profile but decreases with distance from the lake the altered sediments show many pedogenic features including zeolitic root mats rootmarks concretions and carbonate rhizoliths residual patches of calcrete locally cap the zeolitic rocks the profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake the analcime occurs as submicroscopic 0 5 2 5 m subhedral and euhedral crystals which have an average si a 1 ratio of 2 33 as determined by x ray microanalysis or 2 18 d value of 639 analcime peak the analcime formed in lake marginal sediments soils by reaction of silicate detritus with na2co3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration poorly ordered clay minerals were probably the main reactants authigenic illite may have been a by product of the reactions chemical analyses suggest that pore waters supplied some of na and possibly k and sio2 the associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration the ca2 may have originated from infiltrating dilute runoff and groundwater authigenic smectite was precipitated in open porosity following analcime formation the zeolitic alteration at lake bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline alkaline lake sediments

late quaternary vegetation and climate of the sahel abstract pollen and phytogeographic evidence provides a vegetational history of the sahel for the period 0 18 000 yr b p the zonal vegetation fluctuated latitudinally and its most extreme positions occurred at 18 000 and 8500 yr b p the first involved a southward shift of the sahelian wooded grassland to 10 n under the arid conditions of the last glacial maximum the second change shows a rapid northward migration of humid vegetation guinean elements reach 16 n and sahelo sudanian elements extend to the southern margin of the modern sahara 21 n when the atlantic monsoon flux increased in the middle holocene the extensive spread of sudanian elements into the modern sahelian zone suggests the appearance of a markedly dry season the modern sahelian semiarid conditions appeared abruptly at 2000 yr b p

plio pleistocene lacustrine stromatolites from lake turkana kenya morphology stratigraphy and stable isotopes abell p i awramik s m osborne r h and tomellini s 1982 plio pleistocene lacustrine stromatolites from lake turkana kenya morphology stratigraphy and stable isotopes sediment geol 32 1 26 a sequence of fossil stromatolites from lake turkana in kenya was examined for 51so and 513c content these stromatolites ranging in age from holocene 10 000 yrs b p to middle pliocene 3 m y showed a variety of growth forms from oncolitic columnar layered to bulbous heads the stromatolites used in our study contain filamentous blue green algae of one morphological type and rare coccoids thus the stromatolites are considered biogenic the stable isotope ratios for oxygen and carbon indicate changing climatic conditions ranging from a cool wet climate prior to ca 1 9 m y to much drier warmer conditions around 1 4 m y followed in turn by a somewhat cooler and wetter climate at the end of the pleistocene

late holocene and recent hydroclimatic variability in the central kenya rift valley the sediment record of hypersaline lakes bogoria nakuru and elementeita abstract the sedimentology salt mineralogy and stable oxygen and carbon isotope signatures of a mid lake sediment sequence from hypersaline lake bogoria in kenya reveal marked climate driven changes in water column mixing regime and salinity over the past 1700 years combined with sedimentological data on short sediment sequences from nearby lakes nakuru and elementeita this results in a preliminary reconstruction of hydroclimatic variability in the central kenya rift valley since ca 300 ad stratigraphic analyses of bulk sediment composition texture and magnetic susceptibility of all three cores and smear slide xrd and stable isotope analyses on carbonate minerals in the bogoria sequence allowed to define a succession of sedimentary units corresponding to distinct phases in lake history in the lowermost unit four characteristic trona layers na3 hco3 co3 2h2o are attributed to predominantly dry conditions during the second half of the first millennium ad and the period equivalent to the medieval climate anomaly until the first half of the 12th century ad lake bogoria was probably polymictic at that time its surface level standing significantly lower than today the second unit displays uniform deposition of nahcolite nahco3 indicative of a strongly stratified lake where a layer of less saline surface water near permanently covers a hypersaline water mass with high pco2 we propose that this depositional phase reflects the rising lake level related to the transition from dry to more humid climate conditions at the onset of the little ice age as previously documented for other parts of equatorial east africa the third uppermost bogoria unit contains only sporadic depositions of magadiite na2si14o29 oh 3 11h2o and various sodium carbonate minerals this together with low magnetic susceptibility is interpreted to indicate humid little ice age conditions with highest lake level and reduced salinity around the late 15th to early 16th century ad from the late 18th to early 19th century and again in the 1870 s the region experienced two episodes of drought more severe than any recorded in historical time tentative evidence for these events in the bogoria record is not well constrained in time but dated desiccation surfaces in the nakuru and elementeita records confirm their widespread nature across eastern equatorial africa

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

mobilization and mechanisms of retardation in the oklo natural reactor zone 2 gabon inferences from u ree zr mo and se isotopes abstract mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides to address these issues in situ isotopic analyses by shrimp and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products exchangeable and non exchangeable and their association with mineral phases the distribution patterns of u ree zr and mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality the results emphasize the mobility of zr at oklo this element being often considered as immobile during water rock interactions the main output is the demonstration of the net effects of sorption and coprecipitation processes chlorite and to a lesser extent illite were found to have adsorbed significant amounts of u ree zr and probably th and less sorbing elements such as mo coprecipitation of secondary uo2 and p rich coffinite within the alteration halo is also an important means of retardation the concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour no retention effect was found for se in the immediate vicinity of the reactor and this element may have moved farther from its source of production these results have interesting implications for the evaluation of long term containment of radionuclides they provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary u minerals this study also demonstrates the robustness of these retention processes over extremely long periods of time

redistribution of ree pb and u by supergene weathering studied from in situ isotopic analyses of the bangombe natural reactor gabon abstract the isotopic analyses of rare earth elements ree pb and u in several kinds of minerals from the clay and black shale layers above the bangombe natural reactor gabon were performed using a sensitive high resolution microprobe shrimp to investigate the migration and retardation processes of fission products released from the reactor ree isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays which reveals that the clays play an important role in preventing fission products from spreading zircon crystals in the clays heterogeneously contain high u regions up to 28 3 wt with normal 235 u 238 u ratios 0 00725 and significant amounts of fissiogenic ree which suggest the occurrence of significant chemical fractionation between ree and u during the dissolution of reactor uraninite and the recrystallization of secondary u minerals the pb data suggest that galena grains in the clays were also formed by the mixing of the two components during a recent alteration event and that a significant amount of pb was derived from 2 05 ga old original uraninite rather than reactor uraninite the u pb systematics of zircon provide chronological information on the old igneous activity associated with the basement rock formation at 2 8 ga and geochemical evidence of the incomplete mixing of independent pb and u sources this result is consistent with previous chronological results in this area

the cenozoic climatic and topographic evolution of the western north american cordillera herein we present oxygen isotope records from cretaceous to recent terrestrial sediments in the western north american cordillera the purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the cenozoic to do this we constructed 18o maps of surface waters for time intervals that trace the development of topography of western north america these maps are based on 4861 oxygen isotope analyses from both published 4478 and new 383 data we determined the 18o values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors these data suggest that in the late cretaceous to early eocene the sevier hinterland formed a plateau of unknown height around 50 ma a topographic wave developed in british columbia and eastern washington that swept southward reaching northeastern nevada at 40 to 38 ma and southern nevada 23 ma this southward encroachment of an eocene plateau sweep caused reorganization of drainage patterns such that the intraforeland basins of wyoming and utah drainages extended deep within the sevier hinterland as the wave swept southward the landscape within the sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of 4 km and relief of 1 5 km this eocene highland was bordered on the west by a high sierra nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands the spatial and temporal evolution of this highland correlates with the timing of volcanism and extension these observations support tectonic models that call for north to south removal of the farallon slab or piecemeal removal of mantle lithosphere the isotopic data show that prior to growth of this highland the north american monsoon nam penetrated much farther north in the paleocene eocene than today the combined effects of global cooling increasing latitudinal temperature gradients and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the nam front by the oligocene the hydrologic regime that we observe today was in place it has been modified since then as a result of basin and range extension and collapse of the highlands in the mid miocene this collapse allowed the nam to penetrate farther north into the great basin of nevada and utah

isotopic evidence for the retention of90sr inferred from excess90zr in the oklo natural fission reactors implication for geochemical behaviour of fissiogenic rb sr cs and ba in order to investigate the mobility of fissiogenic90sr in the geological environment the zr isotopic compositions of seven samples from one of the newly formed oklo natural reactor zones i e reactor core and adjacent rocks 10 sf84 in the republic of gabon were determined with an inductively coupled plasma mass spectrometer icp ms zr isotopes in uraninite grains from different reactor zones were also measured by secondary ion mass spectrometry sims fissiogenic zr isotopic abundances of three samples from the reactor core have excess90zr which has never before been formed in previous oklo samples in this paper the geochemical behaviour of90zr is discussed by making use of the relative retentivity inferred from the isotopic abundance of sr the excess in90zr suggests dependence on the degree of retention migration of90sr the precursor of90zr in the fission chain in the aqueous phase chemical fractionation between sr and zr could have occurred before radioactive90sr decayed considering the halflife of90sr t1 2 29 1y considerable amounts of the latter have been produced during criticality sr and zr including90zr could have been redistributed between the reactor core and its vicinity the retentivity of fissiogenic90zr in reactor core 10 is not homogeneous in addition the distributions of rb cs and ba is also heterogeneous

methodology for bounding calculations of nuclear criticality of fissile material accumulations external to a waste container at yucca mountain nevada abstract the possibility of nuclear criticality however remote in the vicinity of the proposed repository at yucca mountain nevada generates justified concerns and may impact the performance of the repository a heuristic approach is presented here for determining the amount spatial distribution and other characteristics of fissile material accumulation in the rock beneath a waste package that could contribute to such an event this study is concerned primarily with waste packages containing special spent fuel from the department of energy and high level nuclear waste glass mixing with less alkaline waters and the subsequent drop in ph is the mechanism that is most efficient for precipitating fissile material from the waste package internal leachate in contrast to natural deposits in which redox changes are the main precipitation driver external accumulation size is determined by 1 computing the chemical composition of the leachate leaving a package as its internal materials degrade with the batch geochemical code eq3 6 2 determining precipitation of fissile material into mineral phases using the 1d geochemical code phreeqc as the effluent mixes with percolation water and 3 heuristically scaling results to a 3d volume and computing the criticality coefficient using the code mcnp loci for accumulation are the multiple lithophysal cavities and the fracture system a bounding conservative approach is used by necessity in step 3 nuclear criticality is sensitive to small variations in the distribution of fissile material and parameters of natural systems vary by orders of magnitude because the most likely combinations of parameters are not conducive to nuclear criticality this study focuses on extreme values of parameter probabilistic distributions such as limited flow into the package associated with a large percolation rate combinations of material degradation rates favoring actinide release and very high host rock porosity values by considering these combinations most favorable to criticality but unlikely it was concluded that external nuclear criticality is not a concern at the proposed repository

evidence of fissiogenic cs estimated from ba isotopic deviations in an oklo natural reactor zone abstract isotopic studies of many elements from the uranium ore natural nuclear reactors at oklo provide useful information on the migration of radioactive nuclides the fissiogenic isotopic composition of ba is particularly interesting as it is an important indication in the search for fissiogenic cs in this report we detail the detection of remarkable isotopic deviations of ba in the oklo samples and estimate the geochemical behaviour of fissiogenic cs from excess ba isotopes six samples systematically collected from borehole sf84 zone 10 at the oklo uranium mine have been analyzed isotopic deviations of ba indicate the existence of fissiogenic cs and ba a good correlation between the elemental abundance of cs and isotopic abundances of excess 135 ba and 137 ba suggests that fissiogenic 135 ba and 137 ba behaved as cs rather than ba

role of organic matter in the proterozoic oklo natural fission reactors gabon africa of the sixteen known oklo and the bangombe natural fission reactors hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals reactors 1 to 6 at oklo contain only traces of organic matter but the others are rich in organic substances reactors 7 to 9 are the subjects of this study these organic rich reactors may serve as time tested analogues for anthropogenic nuclear waste containment strategies organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality the bitumen soon became a solid consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite which enclosed and immobilized uraninite and the fission generated isotopes entrapped in uraninite this mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1 2 b y 24 refs 4 figs

textural and paleo fluid flow control on diagenesis in the paleoproterozoic franceville basin south eastern gabon abstract the paleoproterozoic 2 15 ga franceville basin south eastern gabon is a continental sedimentary basin that host unmetamorphosed sediments this study involve detailed mineralogy sedimentology and petrography of the basal sedimentary units of fa and lower fb fb1 formations from the basin margin to centre in relation to mineral paragenesis and fluid flow the fa formation conglomerate sandstone and mudstone consists lithofacies of mixed fluvial and fluvio deltaic transitional origin while the overlying fb1 formation includes alternating organic rich black shale and sandstone of marine deposit the medium to coarse grained fluvio deltaic quartz arenite in upper part of fa formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history this provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis in contrast the clay and unstable detrital grains rich fluvial arkosic to sub arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts these arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities from the proximal to distal basin and within sample suites there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore fluid the predominance of 1mt illite polytype in most lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid rock ratio dolomite anhydrite barite and fe oxides are the main crystallized authigenic minerals aside illite and fe rich chlorite clay minerals the mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the fa formation in the franceville basin are mainly controlled by depositional facies

natural fission reactors of oklo fourteen natural fission reactors are known to date in three different ore deposits located in the franceville basin the most famous and the first discovered are those in the oklo uranium ore deposit petrographic geochemical and oxygen isotope studies have been performed in an attempt to answer these questions 1 what are the main characteristics of the uranium ores in which fission reactions took place 2 what are the processes that created such exceptionally high grade ores and 3 what are the effects of such fission reactions on the host rocks the ore which hosts the fission reactions at oklo is characterized by a gangue of clay minerals whereas the normal ore of oklo consists of sandstones these clay minerals are the result of hydrothermal alteration of sandstone hydrothermal circulation was induced by the heat produced by fission reactions the reactor ores are further characterized by very high uranium contents 20 60 and high uranium accumulation by means of silica depletion and by uranium enrichment of the normal ore without uranium depletion hydrothermal convective circulation induced by the fission reactions created very high grade ore next to the reactors hydrothermal processes are thus critical to the propagation of the fission reactions

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

the oxygen isotopic composition of uranium minerals a review abstract uranium ore is an essential material in the preparation of nuclear fuel for civilian as well as military uses uranium is first extracted from uranium bearing minerals using a variety of reagents and is precipitated from solutions as yellow cake prior to isotope enrichment processes the disintegration of the former eastern bloc in the 1990s and frequent unrest in the middle east have underscored the need for better characterizing source uranium ores for forensic and attribution purposes the world s major deposits of u occur in several distinctly different geological environments fourteen principal types of u deposits and rocks with elevated uranium contents are recognized with more than 40 subtypes combining our own analysis and literature data we have amassed over 250 oxygen isotope data from 13 major u producing countries which vary widely from 32 to 11 however interpreting the oxygen isotopic composition of uraninite in terms of the composition of the fluid from which it precipitated or interacted with requires knowledge of the fractionation factor and temperature of interactions which are not always available since each deposit type occurs within a unique geologic setting and is generally formed from chemically distinct fluids the chemical compositions of the uranium ores are also distinct uranium deposits that form in igneous rocks have higher trace element compositions relative to sandstone hosted deposits our data shows that one of the most useful techniques for distinguishing between uranium ore is to use a combination of 18 o values and rare earth elements e g la yb ratios these methods may allow investigators to trace uranium ore back to the source

organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in brazilian marginal basins abstract an investigation of bulk isotopic and molecular features of a wide selection of source rocks and oils from brazilian marginal basins ranging from lower cretaceous to tertiary in age provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo environments the methods employed include evaluation of organic carbon contents rock eval pyrolysis vitrinite reflectance measurements determination of carbon isotope ratios elemental and visual kerogen analysis assessment of oils and solvent extracts by liquid and gas chromatography and quantitative biological marker investigations using gc ms for aliphatic hydrocarbons and uv vis spectrophotometry for metalloporphyrins selected data were evaluated using principal component analysis the results enable the recognition and differentiation of seven depositional regimes lacustrine freshwater lacustrine saline water marine evaporitic marine carbonate marine deltaic marine highly anoxic with a predominance of calcareous mudstone lithology and marine anoxic with predominance of siliciclastic lithology diagnostic features in the biological markers from these depositional environments include the carbon number distributions of n alkanes pristane phytane ratios absolute concentrations of carotane higher acyclic isoprenoids c 30 hopane and steranes and the abundance of the latter relative to 4 methylsteranes the occurrence and abundance of particular biological markers such as gammacerane tricyclic and tetracyclic terpanes 18 h oleanane 28 30 bisnorhopane and 25 28 30 trisnorhopane and the relative abundances of nickel and vanadyl porphyrins

uraninite recrystallization and pb loss in the oklo and bangombe natural fission reactors gabon abstract the oklo and bangombe natural fossil fission reactors formed ca 2 ga ago in the franceville basin gabon the response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the uo 2 in spent nuclear fuel uraninite and galena from two reactor zones rz16 at oklo and rzb at bangombe were studied to clarify the chronology and effect of alteration events on the reactor zones in addition ion microprobe u pb analysis of zircons from a dolerite dyke in the oklo deposit were completed to better constrain the age of the dyke and thereby testing the link between the dyke and an important alteration event in the reactor zones the analyzed uraninite from rz16 and rzb contains ca 6 wt pbo indicating a substantial loss of radiogenic pb transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect free nanocrystalline to microcrystalline aggregates however the nanocrystalline regions have elevated si contents and lower pb contents than coarser uraninite crystallites single stage model ages of large millimeter sized galena grains at both rz16 and rzb correlate well with the age of the oklo dolerite dyke 860 39 ma 2 thus the first major pb loss from uraninite occurred at both oklo and bangombe during regional extension and the intrusion of a dyke swarm in the franceville basin 860 890 ma ago uraninite pb isotopes from rz16 and rzb give lower ages of ca 500 ma these ages agree with the chemical ages of the uraninite and show that an ancient pb loss occurred after the intrusion of the dolerite dykes the presence of nanocrystallites in the reactor uraninite indicates internal recrystallization which may have occurred around 500 ma resulting in the 6wt pbo uraninite it is suggested that leaching by fluid interaction triggered by the pan african orogeny was important during this second pb loss event thus there are indications that uraninite at both the oklo and bangombe natural reactors has experienced at least two ancient episodes of pb loss associated with internal recrystallization these recrystallization events have occurred without significantly depleting the 2 ga fission products compatible with the uraninite structure

oil and its biomarkers associated with the palaeoproterozoic oklo natural fission reactors gabon abstract the 2 1 ga fa formation sandstone of the franceville basin in gabon that hosts the oklo natural fission reactors has been discovered to contain abundant palaeoproterozoic oil bearing fluid inclusions inside which syngenetic biomarkers are preserved the oil occurs within h2o and co2 dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and co2 with liquid hydrocarbons prior to trapping textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca 115 to 200 c soon after deposition of the sequence at ca 2 1 ga and secondly during a hydrothermal event at 200 to 285 c linked to the operation of the reactors ca 1 98 ga a series of sequential outside rinse blanks of framework grains analysed by gas chromatography mass spectrometry gc ms prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co occurring solid bitumens which have a different geochemical distribution compared to the fluid inclusion oil the crush leach extract containing oil from fluid inclusions is markedly richer than the final outside rinse blank displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen the fluid inclusion oil has a high maturity is non biodegraded and lacks clear effects of radiolysis it is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor heated fluids rather than by the criticality event itself biomarkers include hopanes 2 methylhopanes terpanes and steranes indicative of bacteria including cyanobacteria and of eukaryotes the relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution trace amounts of c30n propylcholestanes indicate a contribution from marine algae the oil was likely derived from conformably overlying marine black shales that were deposited during the great oxidation event during which the earth s atmosphere became oxygenated the biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2 7 and 1 4 ga it is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period despite apparent fluctuations of atmospheric o2

shrimp in situ isotopic analyses of ree pb and u in micro minerals bearing fission products in the oklo and bangombe natural reactors a review of a natural analogue study for the migration of fission products abstract in the precambrian parts of the oklo okelobondo and bangombe uranium deposits of the republic of gabon central africa functioned as natural fission reactors many elements in the oklo and bangombe uranium deposits show variations in isotopic composition caused by a combination of nuclear fission neutron capture and radioactive decay isotopic studies provide useful information to understand the behavior of radionuclides in geological media in our recent work in situ ree pb and u isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a shrimp sensitive high resolution ion microprobe the isotopic results of the shrimp analyses on micro minerals found in and around the oklo and bangombe natural reactors are reviewed in this paper the data suggest the selective uptake behavior of 1 ra into illite and 2 pu into apatite 3 the formation process of secondary minerals bearing fissiogenic ree and depleted u 4 evidence of nuggets particles bearing fissiogenic platinum group elements pge and 5 from the u pb systematics of highly altered zircons the redistribution of u and pb

constraining provenance for the uraniferous paleoproterozoic francevillian group sediments gabon with detrital zircon geochronology and geochemistry abstract the paleoproterozoic sandstones of the ca 2 1 ga francevillian group southeast gabon host the oldest known redox controlled high grade uranium u deposits uranium bearing detrital minerals e g monazite in the francevillian group sediments were derived by erosion of archean basement rocks from the east gabonian block of the northwestern congo craton surrounding the francevillian basin they are viewed as primary sources for u but the nature of these sources remains poorly constrained here u pb ages and hf isotope data for detrital zircon grains are combined with literature geochemical data for igneous rocks from the archean basement to characterize the nature age and origin of the provenance for the francevillian basin and the ultimate source of u for the high grade uranium deposits detrital zircon ages range from ca 3 1 to 2 6 ga with a dominant population between ca 2 90 and 2 80 ga suggesting a major contribution from ca 2 88 to 2 81 ga high k granitoids with a limited contribution from ca 3 1 to 2 84 ga ttgs and 2 75 to 2 7 ga granitoids of the east gabonian block most detrital zircon grains share similar depleted mantle hf model ages between ca 3 08 and 3 30 ga indicating significant recycling of a paleoarchean to mesoarchean crust we suggest that ca 2 9 to 2 8 ga high k u and th granitoids which reflect hybrid melts of both mantle and crustal parentage were the major detrital sources of u to the francevillian basin and ultimately for the u deposits we propose that ca 2 9 to 2 8 ga detrital zircons with hf tdm ages between ca 3 1 and 3 3 ga may thus allow for vectoring u deposits in sedimentary basins across the congo craton

geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at bangombe gabon to study geochemical processes for migration and fixation of fissiogenic rare earth elements ree in association with uranium dissolution in situ isotopic analyses using an ion microprobe were performed on u and ree bearing secondary minerals such as coffinite francoisite uraniferous goethite and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at bangombe gabon phosphate minerals such as phosphatian coffinite and francoisite with depleted 235u 235u 238u 0 00609 to 0 00638 contained large amount of fissiogenic light ree while micro sized uraninite grains in a solid bitumen aggregate have normal u isotopic values 235u 238u 0 00725 and small amount of fissiogenic ree components the proportions of fissiogenic and non fissiogenic ree components in four samples from the core of bax03 vary in depth ranging from 30 cm to 130 cm beneath the reactor which suggests mixing between fissiogenic isotopes from the reactor and non fissiogenic isotopes from original minerals in the sandstone significant chemical fractionation was observed between ce and the other ree in the secondary minerals which shows evidence of an oxidizing atmosphere during their formation pb isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of u pb decay system due to recent geologic alteration however systematic pb isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes pb and u from the reactor in association with dolerite dyke intrusion 0 798 ga ago and the formation of the secondary minerals by mixing event between 2 05 ga old original minerals and reactor materials due to recent alteration

carbonaceous substances associated with the paleoproterozoic natural nuclear fission reactors of oklo gabon paragenesis thermal maturation and carbon isotopic and trace element compositions field relationships textural and reflectance characteristics and carbon isotopic compositions indicate that at least eight different categories of carbonaceous substances cs occur in the black shales uranium ores and fossil nuclear fission reactors of oklo the categories include kerogens of the mikouloungou coal in the fa formation the c1 pelites of the fa formation the black shale of the fb formation and bitumens present in veinlets in the basal black shale of the fb formation in the fa formation in normal ore and in hydraulic fractures in and near the fossil reactors and in late dolerite dikes bitumen migration occurred at oklo during at least three intervals 1 ca 2 0 ga ago when petroleum liquids derivatives of which are preserved as solid bitumen were generated by thermal maturation of black shale kerogen 2 ca 1 968 50 ma ago when 15 or more pockets of uranium ore achieved criticality and 3 977 to 981 27 ma ago when dolerite dikes intruded radionuclide migration occurred during intervals 2 and 3 bitumenization trends established for organic matter which has undergone thermal maturation fail to discriminate between heat affected and normally matured cs at oklo in and near oklo reactors where several types of solid bitumen commonly occur in close association carbon of the youngest low reflectance bitumen is isotopically heavier than that of older high reflectance bitumen due to thermal cracking this process together with enhanced radiolysis of bitumen in and near the reactors have combined to increase its 13c avg 25 8 compared to that of bitumen samples in normal ore avg 35 5 distant from the reactors graphitized bitumen inhibited migration of 235u and some fission products in uraninite of the natural reactors however alkaline earth elements such as ba sr and mo and alkaline elements rb and cs not compatible in the uo2 crystalline structure have migrated in hydrothermal solutions and in the once fluid oklo bitumens in situ analysis of solid bitumens by laser ablation icp ms detects differences in trace element composition developed as a result of their varying geological histories and evolutionary trends

basement and cover rock deformation during laramide contraction in the northern madison range montana and its influence on cenozoic basin formation two major laramide fault systems converge in the northwestern madison range the northwest striking southwest vergent spanish peaks reverse fault and the north striking east vergent hilgard thrust system analysis of foliation attitudes in basement gneiss north and south of the spanish peaks fault indicates that the basement in thrusted blocks of the hilgard thrust system has been rotated by an amount similar to that of the basement cover contact steeply dipping north striking breccia zones enclosing domains of relatively undeformed basement may have permitted domino style rotation of basement blocks during simple shear between pairs of thrusts in most places along the hilgard thrust system a large basement overhang produced by thrusting of archean blocks above rocks as young as late cretaceous overlies a tight footwall syncline this tight folding is largely concentric and was accommodated by flexural slip resulting in severe crowding in synclinal hinges that resulted in observed or inferred features such as bedding plane slip imbricate and out of syncline thrusting and hinge collapse the north striking madison normal fault system a zone of tertiary and quaternary valley forming normal faults is approximately parallel to the hilgard thrust system in some places normal faults are reactivated thrusts on which large basement overhangs of the hilgard thrust system were dropped back into the madison valley and covered by tertiary basin fill deposits leaving only the rocks of the footwall synclines exposed in other places both the thrusted archean blocks and the near isoclinal footwall synclines are well preserved this paired fault system the madison normal fault system and the hilgard thrust system of the northern madison range is strikingly similar to other paired systems in southwestern montana along and adjacent to the western margins of the ruby range snowcrest range greenhorn range tobacco root mountains and bridger range such systems may be the result of collapse of the crestal zones of large laramide basement uplifts arches during tertiary extension no hydrocarbon discoveries have been made in this unique structural province however petroleum exploration here has focused on basement cored anticlines both surface and subthrust related to the two major laramide fault systems and on the fault bounded blocks of tertiary rocks within the post laramide extensional basins the interplay of the two laramide fault systems during both laramide shortening and tertiary extension has produced a variety of possible structural traps in the madison range that have not yet been thoroughly investigated

extra basinal fluid infiltration mass transfer and volume strain during folding insights from the idaho montana thrust belt finite strain and geochemical variations along strain gradients were used to study cleavage development in carbonates in the lost river range idaho deformation accommodating layer parallel shear was partitioned into thin deforma tion zones during folding of willow creek anticline cleavage intensity is strong to very strong in deformation zones and weak in surrounding rocks strain magnitudes range from e s0 32 1 15 0 93 0 73 x y z principal axes of strain outside to e s0 64 1 29 0 80 0 52 inside deformation zones we found a positive linear correlation between strain cleavage development and negative dilation volume loss at a cubic centimeter scale ranges from 12 percent to 49 percent cleavage selvages are depleted in ca and 18 o and enriched in other elements relative to microlithons and less deformed protolith carbonates mass balance considerations indicate that cleavage was formed by incongruent pressure solution leading to a passive concentration of less soluble components during ca loss and metasomatic additions of si al and k to produce authigenic clay minerals in selvages data for the willow creek locality and elsewhere in the lost river range davidson and others 1998 pioneer mountains and the tendoy range bebout and others 2001 show that across the sevier orogen fluid infiltration was heterogeneous at centimeter kilometer scales and resulted from positive feedback between deforma tion and far traveled surficial fluids metasomatic strain softening enhanced deforma tion zone development which generated increased permeability as the thrust belt evolved from a porosity based closed system to a discontinuity based open system increased fluid infiltration and isotopic exchange was associated with volume loss increasingly prolate strains and crystallization of clays to produce the cleavage selvages mass transfer was accommodated by diffusion early and advection later in the deformation history mesoscopic structures deformation zones faults veins focused fluid flow and were kinematically related to larger scale structures faults fault related folds the inferred addition of surficial fluids to depths of 7 kilometers in the thrust belt implies a fluid regime involving significant topographically driven recharge deformed whole rocks and microsamples are lower in 18 o than unde formed samples which have o and c isotope compositions similar to those of marine carbonates veins are even lower in 18 ov smow with minimum values of 5 permil reflecting penetration of the crust by ocean derived precipitation with 18 o near to somewhat lower than 0 permil range of 7 5 to 2 5 calculated for h2 oi n equilibrium with these veins the inferred penetration into the thrust belt of nearshore meteoric waters is consistent with proximity to the reconstructed western interior seaway later fluid infiltration locally lowered the 18 o of carbonates even further based on the o isotope compositions of veins related to younger compres sional deformation and to the onset of crustal scale extension in the eocene this progression toward lowered 18 o of the surficial waters is compatible with the retreat of the seaway during emergence of the thrust wedge and paleogene extension uplift and subaerial volcanism

14 c and th u dating of pleistocene and holocene stromatolites from east african paleolakes during recent humid episodes stromatolites were built along paleolake margins some 60 m above the modern water level of lakes natron and magadi southern gregory rift valley three generations of stromatolites are observed the more recent ones frequently covering pebbles and boulders eroded from the older ones the youngest one yielded 14c ages ranging from approximately 12 000 to 10 000 yr b p their 13c values 2 6 suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric co2 thereby lending credence to the reliability of the 14c an initial 230th232th ratio in the detrital component was determined by thu measurements on the 14c dated stromatolites using this value a 230th234u chronology for the older stromatolites was calculated ages of 240 000 and 135 000 10 000 yr were obtained for the first and second generations respectively a humid episode apparently characterized eastern africa during each glacial interglacial transition 18o and 13c measurements on stromatolites when compared to values on modern waters and carbonates provide paleohydrological information long residence time of the paleolake waters and less seasonally contrasted regimes are inferred

hydrothermal and supergene clays of the oklo natural reactors conditions of radionuclide release migration and retention abstract mineralogical chemical and o isotopic analyses were performed on the clay matrices surrounding two natural fission reactors of the oklo deposit in the deeply preserved reactor 10 450 m clays are composed of hydrothermal mg chlorites and sudoites al chlorites the wide range of their o isotopic compositions from 0 0 to 15 1 smow is consistent with crystallisation during conditions of criticality when the reactors created a thermal gradient of 100 c m the temperature in the core of reactor is calculated to have been approximately 400 c in contrast in the shallow reactor 9 120 m deep vermiculitized al chlorites are the products of the weathering of the hydrothermal chlorites these vermiculitized al chlorites are 18 o enriched 18 o 18 smow and underwent interaction with surface fluids 2 to 5 149 sm 147 sm isotopic ratios of the clays of reactor 9 show a wide dispersion 2 3 m of the fissiogenic sm around the reactor the migration of the fissiogenic sm which is usually retained in the deep reactors is here related to the circulation of meteoric water under oxidising conditions

redistribution of fissiogenic and non fissiogenic ree th and u in and around natural fission reactors at oklo and bangombe gabon abstract elemental abundances and isotopic compositions of rare earth elements ree th and u of twenty eight samples from four kinds of natural fission reactors at the oklo and bangombe uranium ores were determined by mass spectrometry fissiogenic and non fissiogenic ree components of each sample were calculated from the isotopic compositions the geochemical distribution of both fissiogenic and non fissiogenic ree differs among the four reactors each reactor shows the following features possibly associated with the local geochemical conditions 1 reactor 9 at the shallow depth of the oklo deposit contains higher amounts of non fissiogenic ree than the other reactors the distribution of non fissiogenic ree does not reflect the primordial ree abundance patterns and might have been disturbed in and around the reactor due to supergene alteration our results suggest that significant amounts of fissiogenic lighter ree lree such as la ce and pr were mobile in the reactor 2 reactors 10 and 13 located deep underground have much lower contents of non fissiogenic ree than reactor 9 all fissiogenic ree show high retention in the reactors non fissiogenic ree of reactor 10 inform of the primordial ore formation without any geochemical disturbance little redistribution of both fissiogenic and non fissiogenic ree in the reactor 10 was observed and 3 the bangombe reactor located at shallow depth has apparently well preserved fissiogenic ree non fissiogenic ree in the bangombe reactor might have been disturbed possibly by hydrothermal processes the distribution behavior of fissiogenic ree is considered to be related with physico chemical reactor conditions such as nuclear reaction temperature and chemical compositions of each reactor but clear relationship between nuclear parameters and the ree distribution could not be obtained in this study

three dimensional strain produced by 50 my of episodic extension horse prairie basin area sw montana u s a abstract the horse prairie basin of southwestern montana is a complex east dipping half graben that contains three angular unconformity bounded sequences of tertiary sedimentary rocks overlying middle eocene volcanic rocks new mapping of the basin and its hanging wall indicate that five temporally and geometrically distinct phases of normal faulting and at least three generations of fault related extensional folding affected the area during the late mesozoic to cenozoic all of these phases of extension are evident over regional or cordilleran scale domains the extension direction has rotated 90 four times in the horse prairie area resulting in a complex three dimensional strain field with 60 east west and 25 north south bulk extension extensional folds with axes at high angles to the associated normal fault record most of the three dimensional strain during individual phases of extension phases 3a 3b and 4 cross cutting relationships between normal faults and tertiary volcanic and sedimentary rocks constrain the ages of each distinct phase of deformation and show that extension continued episodically for more than 50 my gravitational collapse of the sevier fold and thrust belt was the ultimate cause of most of the extension

hydrothermal clay mineral formation in the uraniferous paleoproterozoic fa formation francevillian basin gabon abstract the spatial distribution of neoformed clay minerals was investigated in the paleoproterozoic fa formation of the francevillian basin south east gabon which hosts high grade u ore deposits associated with the only known occurrence of natural nuclear reactors illite appears as the main clay phase in the lower fluvial unit in the mineralized upper fluvio deltaic tidal unit the clay assemblage is more diversified and commonly characterized by illite chamosite berthierine and chamosite berthierine mixed layers usually in association with alteration products of u bearing minerals the clay mineral assemblage thus potentially constitutes a mineralogical marker for the regional distribution of u ore deposits polytype species of the clay phases indicate mineral transformations in an environment characterized by a high fluid rock ratio according to crystalline structure mineral chemistry thermodynamic modeling and geochronology clay phases seem to be mainly hygrometer rather than exclusively a thermometer and their formation as well as associated dissolution precipitation of u bearing minerals took place between ca 2040 and 2010 ma ago at temperatures of about 240 30 c using previous burial estimates this suggests the operation of hydrothermal processes controlled by an external heat source likely associated with volcanism during deposition of the fd and fe formations hydrothermal activity involving oxidized fluids would have driven u remobilization and ultimately formation of high grade u ore deposits the thermal history proposed here allows for a better understanding of the conditions during burial of the sedimentary succession and the origin of its rich u endowment

uranium and sm isotope studies of the supergiant olympic dam cu au u ag deposit south australia the olympic dam cu u au ag deposit in the archean proterozoic gawler craton south australia is a type example of the iron oxide copper gold iocg spectrum of deposits and one of the largest cu u au resources known mineralization is hosted in a lithologically and texturally diverse hematite rich breccia complex developed within a granite of the 1 59 ga gawler silicic province emerging evidence indicates that both the breccia complex and its metal content developed over 1000 ma responding to major tectonic events e g at 1300 1100 825 and 500 ma however metal sources and exact mechanism s of ore formation remain poorly known new high precision 238u 235u data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range 238ucrm112a 0 56 to 0 04 at the scale of sampling there is no correlation of 238u with lithology degree of alteration or u mineralogy although ores with u 5 wt have subtly higher 238u values 0 20 to 0 00 than the majority of samples 0 7 wt u 0 56 to 0 23 the new u isotope data are consistent with published data for uraninites from olympic dam and with published results from high temperature u deposits they overlap completely with the range of 238u values in granitoids including the host granite 0 18 to 0 32 and with estimates of the upper continental crust in general this similarity suggests that olympic dam 238u values reflects the crustal sources of u which probably include felsic volcanic rocks and granitoids the isotopic homogeneity suggests depositional mechanisms that involve minimal isotopic fractionation of u alternatively primary fractionation signatures may have been erased during the long history of the u mineralization high grade u ores may record isotopic neutron capture effects related to fissionogenic neutrons high precision sm isotope data for five high u 5 wt u u sm 500 olympic dam ores define a neutron capture line with correlated depletions in 149sm up to 2e units and excesses in 150sm up to 4e units but fission fragment contributions to sm are below detection these observations provide evidence for small scale neutron capture effects with calculated neutron fluences of 1015 to 1016 n cm 2 similar to those observed in several proterozoic and phanerozoic u deposits the apparent lack of fission fragment contributions in olympic dam high grade ores can be explained with an age of u deposition or re deposition that is substantially younger than the initial 1 59 ga age of the oldest iocg style mineralization the results presented here thus i suggest uranium sources in common likely igneous upper crustal lithologies ii support geochronological evidence for gradual addition of u in several stages over 1000 ma at elevated temperatures of mineralization and iii do not show the high 238u signatures expected from low temperature reworking of older low 238u ores

natural fission reactors in the franceville basin gabon a review of the conditions and results of a critical event in a geologic system abstract natural nuclear fission reactors are only known in two uranium deposits in the world the oklo and bangombedeposits of the franceville basin gabon since 1982 five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative european program new geological mineralogical and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2 0 ga years the franceville basin and the uranium deposits remained geologically stable over a long period of time therefore the sites of oklo and bangombeare well preserved for the reactors two main periods of actinide and radionuclides migration have been observed during the criticality under p t conditions of 300 bars and 400 500 c respectively and during a distention event which affected the franceville basin 800 to 900 ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors new isotopic analyses on uranium dioxides clays and phosphates allow us to determine their respective importance for the retention of fission products the uo 2 matrix appears to be efficient at retaining most actinides and fission products such as rees y and zr but not the volatile fission products cd cs xe and kr nor rb sr and ba some fissiogenic elements such as mo tc ru rh pd and te could have formed metallic and oxide inclusion in the uo 2 matrix which are similar to those observed in artificial spent fuel clays and phosphate minerals also appear to have played a role in the retention of fissiogenic rees and also of pu

fate of trace elements during alteration of uraninite in a hydrothermal vein type u deposit from marshall pass colorado usa alteration of uraninite from a hydrothermal vein type u deposit in marshall pass colorado has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements w as mo zr pb ba ce y ca ti p th fe si al during alteration under both reducing and oxidizing conditions the release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the uo2 in spent nuclear fuels uraninite occurs with two different textures 1 colloform uraninite and 2 fine grained uraninite the colloform uraninite contains 1 04 1 75 wt of wo3 0 16 1 70 wt of as2o3 0 06 0 88 wt of moo3 whereas the fine grained uraninite retains 2 25 4 93 wt of wo3 up to 5 76 wt of moo3 and 0 26 0 60 wt of as2o3 the near constant concentration of incompatible w in the colloform uraninite suggests w incorporation into the uraninite structure or homogeneous distribution of w rich nano domains incorporation of w and mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of cs during the corrosion of spent nuclear fuel as there is a strong affinity of cs with w and mo zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite for unaltered zones the calculated amount of oxygen ranges from 2 08 to 2 32 apfu atom per formula unit and defines the stoichiometry as uo2 x and u4o9 whereas for the altered zones of the colloform texture the oxygen content is 2 37 2 48 apfu which is probably due to the inclusion of secondary uranyl phases mainly schoepite the supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite four stages of colloform uraninite alteration are proposed i formation of an oxidized layer at the rim ii corrosion of the oxidized layer iii precipitation of u6 phases with well defined cleavage and iv fracture of the uraninite surface along the cleavage planes of the u6 phases

florencite la with fissiogenic rees from a natural fission reactor at bangombe gabon florecite la la ce 1 09 with fissiogenic rees and florecite ce la ce 0 62 have been identified in illite from the clay mantle surrounding a natural 2 ga fission reactor at bangombre and in sandstone beneath the reactor zone respectively florencite ce is apparently unrelated to nuclear processes and occurs with monazite ce apatite tio sub 2 probably anatase zircon and illite grains of florencite ce contain inclusions of thorite chalcopyrite and galena florencite la was found 5 cm from the open quotes core close quotes of the reactor and contains inclusions of galena and u ti bearing phases secondary uraninite and coffinite have precipitated on some of the florencite grains the chemical composition of florencite la as determined by electron microprobe analysis is la sub 0 38 ce sub 0 35 nd sub 0 06 sm sub 0 01 ca sub 0 03 sr sub 0 17 al sub 2 98 fe sub 0 02 sup 3 po sub 4 po sub 3 80 oh sub 0 20 oh sub 6 secondary ion mass spectrometry revealed that between 27 and 30 of nd and 67 and 71 of sm in florencite la is fissiogenic the presence of fissiogenic rees in open quotes florencite close quotes from the reactor zone in bangombe and their preferential concentration in florencite relative to the bulk sample of clay demonstrate that aluminous phosphates may have played a more significant role in the fixation of fissiogenic rees released from uraninite after the sustained fission reactions than sorption ontomore clays 30 refs 3 figs 2 tabs less

biogenic structures in exhumed surfaces around saline lakes an example from lake bogoria kenya rift valley abstract five overprinted suites of animal and plant traces around saline alkaline lake bogoria kenya are described from a set of interfingering late pleistocene and holocene exhumed surfaces that reveal a complex story of changes in lake level and environmental controls on trace assemblages through time these stratigraphic surfaces are amalgamated and form a co planar surface the sediments were first deposited during intermediate lake levels then reworked and cemented during low lake levels and were subsequently overlain by transgressive lacustrine sediments during higher lake levels ongoing exhumation of the surface near the present shoreline has reactivated the sediments as a substrate for the activities of epifaunal and infaunal animals as well as plant growth the modern environmental setting at lake bogoria is very similar to that of the preserved exhumed surfaces which allows comparisons between observed sets of environmental factors affecting animal behaviour and their representation in the fossilized examples pronounced lateral environmental heterogeneity around saline and hypersaline lakes may lead to concentrations of life around sites with vital resources such as freshwater at lake bogoria groups of fault controlled hot springs represent these oases and have been a relatively consistent feature in and around the lake during the development of the exhumed surfaces together the diverse assemblage of animal trace suites from lake bogoria represent the mermia and scoyenia ichnofacies but can be divided into five suites that reveal details of laterally heterogeneous sets of environmental factors and are useful indicators of lake level substrate moisture content substrate cohesion etc suite 1 comprises the traces of chironomid larvae diptera chironomidae formed in subaqueous lacustrine settings suite 2 includes the traces of flamingos phoeniconaias minor and phoenicopterus ruber formed at the shoreline suite 3 compares well with the mermia ichnofacies and comprises trails and burrow systems associated with relatively fresh saturated to extremely shallow subaqueous substrates suite 4 is comparable to both the pre and post desiccation suites of the scoyenia ichnofacies is the most diverse suite at lake bogoria and mainly comprises the burrows of beetles earwigs and their larvae as well as a diverse set of rhizoliths and vertebrate tracks suite 5 contains a moderately diverse set of traces attributed to termites and ants

characterization of organic matter associated with uranium deposits in the francevillian formation of gabon lower proterozoic abstract elemental analysis organic petrography and high resolution transmission electron microscopy were used to study organic matter in lower proterozoic rocks of the francevillian series in africa results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil and deposition of uraninite ores evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones moreover the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization

the neodymium isotope system as a tool for petroleum exploration abstract mineralogical petrological geochemical and neodymium isotopic investigations strongly suggest isotopic and chemical exchange between authigenic silicate phases and petroleum alteration of organic matter sulfur reduction generation of aqueous acidic solutions with the resultant dissolution of k feldspar leads to in situ crystallization of illite and chlorite as well as neodymium isotopic equilibration between the silicate phases and petroleum these findings are of significance not only to understand the mechanisms leading to neodymium isotopic equilibration between authigenic clay minerals in detrital sediments but also to source rock petroleum in reservoir correlations in particularly when biomarker molecules i e isoprenoid hydrocarbons have been obliterated by prolonged severe diagenesis

oxygen isotopic composition of nano scale uraninite at the oklo okelobondo natural fission reactors gabon high spatial resolution 10 30 m in situ oxygen isotopic analyses by secondary ion mass spectrometry sims coupled with high resolution transmission electron microscopy hrtem were used to show that uraninite from the oklo okelobondo natural fission reactors that occur in near surface environments have low 1 8 o values and nanotextures that are consistent with interaction with ground water these low 1 8 o values 14 4 to 8 5 suggest that the minerals exchanged with meteoric groundwater in contrast reactor zones that occur at depth have largely retained their original o isotopic composition 10 2 to 5 6 and uraninites are well crystallized and essentially defect free these observations clearly demonstrate that by combining both hrtem and in situ o isotopic analyses by sims it is possible to characterize the nano scale porosity and post depositional alteration of u bearing phases

the corrosion of basaltic dykes in evaporites ar sr nd isotope and rare earth elements evidence abstract the corrosion of basaltic dykes from central germany which intruded during the tertiary into rock and potash salts of upper permian age zechstein has been studied the corrosion behavior of these glassy basalts can serve as a natural analogue for the long term corrosion expected for nuclear waste glasses in a salt repository our data demonstrate mobility and fractionation of the rare earth elements ree during a postintrusive circulation of salt brines the processes controlling this behavior of the ree were dissolution and reprecipitation of phosphates the k ar data document the synintrusive assimilation of a salt phase followed by a postintrusive fluid circulation in the peripheral parts this circulation removed the assimilated salt and imported highly radiogenic salt ar which was integrated into newly formed sheet silicates the central part of the dyke was not affected by this circulation and could retain a mantle gas phase the sr isotope data document a synintrusive assimilation of salt sr and a strong exchange with salt fluids during the postintrusive corrosion which was strongest in a zone next to the chilled dyke margins this locally stronger alteration is probably due to different cooling and shrinking rates of the basalt at the margins and in the core of the dyke which led to the formation of a highly permeable fractured zone