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

lead mobilization during tectonic reactivation of the western baltic shield abstract lead isotope data from sulfide deposits of the western part of the baltic shield define mixing lines in the 206 pb 204 pb 207 pb 204 pb diagram lead from two types of sulfide deposits have been investigated 1 1 exhalative and volcanogenic deposits that are syngenetic with their host rocks 2 2 vein deposits the syngenetic deposits locally show a very wide range of lead isotopic compositions that reflect a variable addition of highly radiogenic lead while the vein deposits although they have radiogenic lead isotopic compositions exhibit only limited isotopic variations in different provinces of the shield both types of deposits fall on the same lead mixing array the slope of the lead mixing lines varies as a function of the age of basement rocks and the age of the tectonic event which produced the lead mobilization and therefore relates the source rock age with the age of lead mobilization calculated mixing ages fall into several short time periods that correspond either to orogenic events or to major phases of continental rifting the orogenic events are the ca 360 430 ma caledonian ca 900 1100 ma sveconorwegian and the ca 1800 1900 ma svecofennian orogenic cycles the rifting events correspond to the formation of the ca 280 ma oslo rift and the ordovician ca 450 ma graben system in the area of the present gulf of bothnia each mixing age indicates that lead was mobilized probably as a consequence of mild thermal disturbances and that the crust was permeable to lead migration the data show that the geographic distribution of sulfide deposits with highly radiogenic lead isotopic compositions coincides with old graben systems orogenic belts and orogenic forelands on the baltic shield the ages of vein deposits and their geographic distribution demonstrate multiple tectonic reactivation of the interior of the baltic shield in response to orogenic events at its margin

the susceptibility of uk groundwaters to acidic deposition the weathering of minerals at shallow depths provides much of the sink for acidic atmospheric deposition the geology of the uk has been divided into four classes according to the extent to which the bedrock is likely to provide buffering and possibly give rise to acidic surface or groundwaters other modifying factors including soils superficial deposits and land use are also considered the likely sources and sinks of h in shallow groundwater systems are also reviewed most available groundwater data are derived from major aquifers often in carbonate bearing lithologies which afford excellent buffering and which are unlikely ever to be adversely affected by acidic deposition relatively few data exist for shallow groundwaters widely used for domestic supplies in rural areas especially in western and northern britain where groundwaters might be most affected acidic groundwaters and or surface waters are defined on the basis of their alkalinity and calcite saturation index si c acidic groundwaters are most likely to occur where 1 acid soils are found and glacial drift is thin or absent 2 the source of acidic inputs is high 3 carbonate minerals are absent 4 the residence time of water is short low alkalinity groundwater is found in many sandstone lithologies including the sherwood sandstone a major uk aquifer many lower palaeozoic strata and granites however existing data for susceptible areas are insufficient to show whether or not there have been significant long term changes in groundwater acidity or alkalinity

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

satellite observations of circulation in the eastern bering sea satellite infrared observations of the eastern bering sea shelf and basin were analyzed from december 1979 to august 1981 a region of warm surface water which extends from the gulf of alaska through the aleutian passes into the bering sea was observed coincident with a band of warm surface water and with eddies along the shelf break the spatial and temporal relationships of these patterns imply a surface inflow through unimak and other western aleutian passes flow along the shelf break including eddies along the bering slope current and cyclonic flow in the basin the proposed generating mechanisms for eddies along the shelf break include topographic interactions baroclinic instabilities and rossby waves

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

low frequency current regimes over the bering sea shelf abstract using direct current measurements made during the period 1975 81 we describe the general circulation over the southeastern bering sea and differentiate it by regimes related to depth and forcing mechanisms three regimes are present delineated by water depth z the coastal z 50 m the middle shelf 50 z 100 m and the outer shelf z 100 m these are nearly coincident with previously described hydrographic domains statistically significant mean flow 1 to 10 cm s 1 exists over the outer shelf generally directed toward the northwest but with a cross isobath component flow of similar magnitude 1 6 cm s 1 occurs in the coastal regime paralleling the 50 m isobath in a counterclockwise sense around the shelf mean flow in the middle shelf is insignificant kinetic energy at frequencies 0 5 cycle per day cpd is greater over the outer shelf than in the other two regimes suggesting that oceanic forcing is important there but does not affect the remainder of the shelf kinetic

on the bering sea ice edge front an oceanographic field program was carried out along the bering sea ice edge in winter 1982 83 to investigate the temperature salinity front associated with and paralleling the winter ice edge currents were measured using taut wire moorings that were in place over winter and an intensive ctd survey was carried out of the frontal structure in midwinter february march the midwinter front separated cold 0 c more saline 32 5 outer shelf water the front had northwestward baroclinic currents paralleling the front ice edge with near surface speeds of 5 6 cm s and an associated transport of about 0 3 106 m3 s these baroclinic currents were superimposed on a regional barotropic along isobath flow resulting in north westward midwinter upper layer flows with monthly mean speeds exceeding 10 cm s an upper layer flow component was directed beneath the ice and provided heat adequate to melt the ice and limit advance of the edge the resulting low salinity meltwater was mixed vertically to maintain the front by wind acting across a highly mobile hydrodynamically rough ice cover subtidal fluctuations were not significantly affected by the front or the ice cover and local winds accounted for less than one quarter of the observed current variability the remainder being probably due to regional variations in the barotropic flow

dynamical analysis of a satellite observed anticyclonic eddy in the northern bering sea the characteristics and evolution of a satellite observed anticyclonic eddy in the northern bering sea during march and april 1999 are investigated using a three dimensional princeton ocean model pom the anticyclonic like current pattern and asymmetric feature of the eddy were clearly seen in the synthetic aperture radar sar sea surface temperature and ocean color images in april 1999 the results from model simulation reveal the three dimensional structure of the anticyclonic eddy its movement and dissipation energy analysis indicates that the barotropic instability bti is the main energy source for the growth of the anticyclonic eddy the momentum analysis further reveals that the larger magnitude of the barotropic pressure gradient in the meridional direction causes the asymmetry of the anticyclonic eddy in the zonal and meridional directions while the different magnitudes of the meridional baroclinic pressure gradient are responsible for the different intensity of currents between the northern and southern parts of the anticyclonic eddy

the bering slope current system revisited mean circulation and water properties within the aleutian basin of the bering sea are investigated using hydrographic and subsurface park pressure displacement data from a regional array of 14 profiling ctd floats after 10 days drifting at 1000 dbar each float measures temperature and salinity profiles as it rises to the surface and then transmits these data via satellites which also make several fixes of the float surface positions before it sinks again every fourth cycle the floats sink from 1000 dbar to a 2000 dbar target just prior to ascent to measure deeper profiles the 1000 dbar displacements estimated from the float surface position fixes reveal a coherent few centimeters per second northwestward flow along the northeastern boundary the deep signature of the bering slope current middepth water property distributions are consistent with cyclonic advection of warm water from the south around the basin eastward in the aleutian north slope current and then northwestward in the bering slope current geostrophic transport estimates relative to 1000 dbar also show cyclonic motion although with significant noise likely owing to the influence of mesoscale eddies the mean alongslope geostrophic transport of the bering slope current is determined between 0 and 1900 dbar relative to 1000 dbar and then combined with mean along slope velocities at 1000 dbar the result is an absolute geostrophic transport estimate with 95 confidence intervals for the along slope current offshore of the 1000 m isobath and between 0 and 1900 dbar of 5 8 61 7 3 106 m3 s 21

surface current patterns suggested by suspended sediment distribution over the outer continental margin bering sea abstract samples of total suspended matter tsm were collected at the surface over the northern outer continental margin of the bering sea during the summers of 1980 and 1981 volume concentrations of surface tsm averaged 0 6 and 1 1 mg l 1 for 1980 and 1981 respectively organic matter largely plankton made up about 65 of the near surface tsm for both years distributions of tsm suggested that shelf circulation patterns were characterized either by meso and large scale eddies or by cross shelf components of flow superimposed on a general northwesterly net drift these patterns may be caused by large submarine canyons which dominate the physiography of this part of the bering sea continental margin

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

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

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

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

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

preferential leaching and the age of radiation damage from alpha decay in minerals preferential release into solution of radionuclides that result from alpha decay is a contributor to natural isotopic disequilibrium such release is critical to geochemical understanding of alpha decay products and would be an undesirable characteristic of solidified nuclear waste in deep geological formations spontaneous annealing of the radiation damage that is responsible for the preferential release is shown to diminish the effect in thorianite and uraninite which we have found to be 550 to 1000 m y old respectively leaching experiments and measurements of the thorium and uranium isotopes show that there is strongly enhanced release of short lived 228th relative to 232th but only slight enhancement of long lived 234u and 230th relative to 238u and 232th these results are interpreted in terms of natural annealing of damage occurring on a time scale between the 10 and 105 years of the alpha decay products with a simple assumption actual times for the survival of the enhanced leaching in these minerals are 15 000 years

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

uraninite from the olympic dam iocg u ag deposit linking textural and compositional variation to temporal evolution the olympic dam iocg u ag deposit south australia the world s largest known uranium u resource contains three main u minerals uraninite coffinite and brannerite four main classes of uraninite have been identified uraninite occurring as single grains is characterized by high pb and ree y rey but based on textures can be classified into three of these classes typically present in the same sample primary uraninite class 1 is smallest 10 50 m displays a cubic euhedral habit and both oscillatory and sectorial zoning zoned uraninite class 2 is coarser sub euhedral and combines different styles of zonation in the same grain cobweb uraninite class 3 is coarser still up to several hundred micrometers has variable hexagonal octagonal morphologies varying degrees of rounding and features rhythmic intergrowths with sulfide minerals in contrast the highest grade u in the deposit is found as micrometer sized grains to aphanitic varieties of uraninite that form larger aggregates up to millimeter and vein fillings massive class 4 and have lower pb and rey but higher ca nanoscale characterization of primary and cobweb uraninite shows these have defect free fluorite structure both contain lattice bound pb rey which for primary uraninite is concentrated within zones and for cobweb uraninite is within high pb rey domains micro fractured low pb rey domains sometimes with different crystal orientation to the high pb rey domains in the same cobweb grain contain nanoscale inclusions of galena cu fe sulfides and rey minerals the observed pb zonation and presence of inclusions indicates solid state trace element mobility during the healing of radiogenic damage and subsequent inclusion nucleation recrystallization during formula 1 driven percolation of cu bearing fluid tetravalent lattice bound radiogenic pb is proposed based on analogous evidence for u bearing zircon calculating the crystal chemical formula to uo2 stoichiometry the sum of cations m is 1 for most classes but the presence of mono di and trivalent elements rey ca etc drive stoichiometry toward hypostoichiometric m o2 x in the absence of measured o and constraints of hypostoichiometric fluorite structure charge balance calculations showing o deficit in the range 0 15 0 36 apfu is compensated by assumption of mixed u oxidation states crystal structural formulas show up to 0 20 apfu pb and 0 25 apfu rey in classes 1 3 while for class 4 these are an order of magnitude less low pb and rey subcategories of classes 2 and 3 are similar to massive uraninite with 0 2 apfu ca other elements si na mn as nb etc show two distinct geochemical trends 1 across classes 1 3 and 2 class 4 whereby low pb rey sub populations of classes 2 and 3 are part of trend 2 for certain elements plots of alteration factor cao sio2 fe2o3 vs pb u suggest two uraninite generations early high pb rey classes 1 3 and late massive class 4 there is evidence of pb loss from diffusion leaching and or recrystallization for classes 2 3 low pb rey domains micro analytical data and petrographic observations reported here including nanoscale characterization of individual uraninite grains support the hypothesis for at least two main uraninite mineralizing events at olympic dam and multiple stages of u dissolution and reprecipitation early crystalline uraninite is only sparsely preserved with the majority of uraninite represented by the massive aphanitic products of post 1590 ma dissolution reprecipitation and possibly addition of uranium into the system coupled dissolution reprecipitation reactions are suggested for early uraninite evolution across classes 1 to 3 the calculated oxidation state u6 u4 u6 of the early and late populations point to different conditions at the time of formation charge compensation for rey rich early fluids rather than auto oxidation of uraninite late uraninites may have formed hydrothermally at lower temperatures t 250 c 1 embed mml math 1 gif

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

lead isotope zoning in galena an ion microprobe study of a galena crystal from the buick mine southeast missouri ion microprobe techniques have been used to determine the variations of pb isotope ratios within a single octahedral crystal of galena from the buick mine southeast missouri the crystal shows uniform concentric zonation of pb isotope ratios with a total variation of approximately 3 percent in 208 pb 206 pb and approximately 4 percent in 207 pb 206 pb and with the crystal margin being less radiogenic lower 206 204 than the core the range in the observed 208 pb 206 pb ratio is larger than that observed previously for bulk galena analyses from throughout the mine and is almost as large as the total variation reported for the whole southeast missouri ore district the ion probe results are demonstrated to be accurate to 0 1 0 2 percent by comparison with conventional mass spectrometric analyses performed on microsamples of the same crystal the ion probe technique holds promise for establishing a pb isotope chronostratigraphy for the ore bearing solutions which will be useful in delineating temporal and spatial aspects of the fluid circulation patterns

isotope geochronology geochemistry and mineral chemistry of the u bearing and barren granites from the zhuguangshan complex south china implications for petrogenesis and uranium mineralization abstract the zhuguangshan complex carries some of the most important granite hosted uranium deposits in south china here we investigate the changjiang and jiufeng granites which represent typical u bearing and barren granites in the complex using zircon u pb ages whole rock geochemistry sr nd isotopic and zircon hf isotopic data and mineral chemistry to constrain the petrogenesis and uranium mineralization la icp ms zircon u pb dating shows that both the changjiang and jiufeng granites were emplaced ca 160 ma these rocks show high silica weakly to strongly peraluminous compositions enrichment in rb th and u and depletion in ba nb sr p and ti these features coupled with the high initial 87 sr 86 sr ratios negative end t values and ehf t values and the paleoproterozoic two stage model ages of these two granites suggest that the two granites belong to s type granites and the parental magmas of the two granites were derived from the paleoproterozoic metasedimentary rocks however the granitoids show different mineralogical characteristics the biotite in the changjiang granite belongs to siderophyllite marking higher degree of chloritization whereas the biotite in the jiufeng granite is ferribiotite characterized by only slight chloritization compared with the jiufeng granite the biotite in the changjiang granite has lower crystallization temperature and oxygen fugacity but higher f content and the uraninite has higher uo 2 content but lower tho 2 content and stronger corrosion the chemical ages of uraninites from both granites are within error consistent with the zircon u pb ages and are considered to represent the emplacement ages of granites chemical ages of pitchblende in the changjiang granite yield 118 8 ma 87 4 ma and 68 6 ma representing multiple episodes of hydrothermal events that are responsible for the precipitation of u ores in the changjiang uranium ore field our study suggests that the degree of magma differentiation and physicochemical conditions of the magmatic hydrothermal system are the key factors that control the different u contents of these two granites the mineralogical characteristics of uraninite and biotite can be used to distinguish between u bearing and barren granites and serve as a potential tool for prospecting granite hosted uranium deposits

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

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

on exchange of water between the gulf of alaska and the bering sea through unimak pass we present the first long term current and bottom pressure observations from unimak pass alaska and adjacent locations on the gulf of alaska and bering sea shelves vector mean current recorded between march and august 1980 was 12 cm s toward 284 t or onto the bering sea shelf however magnitude decreased from 20 cm s to 6 cm s between the first and second halves of the record at shorter periods 3 10 days current fluctuations in the pass were strongly coherent k2 0 7 with the pressure difference measured along the axis of the pass and this was coherent with geostrophic wind estimates the results indicated that wind induced sea level perturbation was the dominant forcing mechanism for fluctuations relations among current bottom pressure pressure difference and geostrophic wind time series also showed that dynamic variaton on the gulf of alaska shelf was primarily responsible for current fluctuations in the pass hydrographic data indicated that a baroclinic current existed along the gulf of alaska coast and this flow became the long term mean flow through the pass we suggest that this feature was the extension of the kenai current the historic supposition that water is transported from the gulf of alaska into the bering sea was verified however the waters are of coastal origin rather than from the alaskan stream

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

mooring observations of the thermal structure salinity and currents in the se bering sea basin utilizing recently developed deep sea mooring technology we present the first interannual time series measurements of the ocean temperature salinity and velocity fields in the bering sea basin these were made during spring and summer of 1992 and 1993 and late winter to summer of 1994 in 2195 m of water results show a weak background flow of 5 cm s with semidiurnal tides superimposed anticyclonic clockwise eddies pass by sporadically inducing currents up to 80 cm s and depressing the isopycnals and isotherms up to 200 m the thermal structure in the upper 400 m shows a deep temperature maximum from warm alaskan stream inflows and a minimum above due to winter cooling a sea surface cooling event was observed in the late winter of 1994 with cold water penetrating to 120 m depth spring warming begins in late april to early may of each year marked by the arrival of the 3 75 c isotherm the local wind plays little role in forcing ocean currents at the site except at the inertial frequency

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

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

altimeter observations of the bering slope current eddy field topex altimeter data is analyzed to describe the mesoscale eddy field associated with the bering slope current in the central bering sea the observed eddy field is well defined seasonally modulated and interannually variable the most energetic mesoscale eddy activity typically occurs during the spring and summer months and is characterized by a peak along isobath wavelength of 212 km and a representative wave period of 4 months spatial variability of annual period phase indicates that the axis of bering slope current exhibits seasonal offshore migration

observations of topographic planetary waves in the bering slope current using the geosat altimeter topographic planetary waves associated with the downstream leg of the bering slope current are observed in measurements of sea surface height anomalies acquired by the geosat altimeter during 1987 and 1988 the waves are most prominent from may to august 1987 during which time the estimated wave period and wavelength are about 72 days and 103 km respectively the direction of propagation is roughly north northeast

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

geochronological correlation of the late precambrian sequences on and around the stable zones of equatorial africa abstract very few radiometric age determinations have a bearing on the late precambrian sequences of the stable zones of equatorial africa a geochronological correlation of these sequences must therefore rest upon a compilation of ages from these stable zones and the corresponding mobile zones about 15 ages of varying precision more or less well tied up with the stratigraphical sequences together with results of the study of stromatolite assemblages allow a provisional correlation to be made one conclusion of this correlation is that most mixtites surrounding the congo basin are in a 950 50 ma age bracket and are presumably to be correlated

on the processes controlling shelf basin exchange and outer shelf dynamics in the bering sea abstract we use a 9 km pan arctic ice ocean model to better understand the circulation and exchanges in the bering sea particularly near the shelf break this region has historically been undersampled for physical chemical and biological properties very little is known about how water from the deep basin reaches the large shallow bering sea shelf to address this we examine here the relationship between the bering slope current and exchange across the shelf break and resulting mass and property fluxes onto the shelf this understanding is critical to gain insight into the effects that the bering sea has on the arctic ocean especially in regard to recent indications of a warming climate in this region the bering sea shelf break region is characterized by the northwestward flowing bering slope current previously it was thought that once this current neared the siberian coast a portion of it made a sharp turn northward and encircled the gulf of anadyr in an anticyclonic fashion our model results indicate a significantly different circulation scheme whereby water from the deep basin is periodically moved northward onto the shelf by mesoscale processes along the shelf break canyons along the shelf break appear to be more prone to eddy activity and therefore are associated with higher rates of on shelf transport the horizontal resolution configured in this model now allows for the representation of eddies with diameters greater than 36 km however we are unable to resolve the smaller eddies

processes controlling acid attenuation in the unsaturated zone of a triassic sandstone aquifer u k in the absence of carbonate minerals abstract continuous core samples were taken through the unsaturated zone at three sites on the outcrop of permo triassic sandstone in the british west midlands sample sites were chosen for lack of recent direct anthropogenic disturbance and for differing vegetation heathland birch woodland and conifer forest interstitial water was extracted and analyzed for 32 major and trace elements solid phases were analyzed for exchangeable cations and mineralogy the rate of recharge calculated using a cl mass balance method was three times greater below heathland than below afforested sites owing to higher evapotranspiration rates in the woodlands carbonate minerals were absent from the unsaturated zone at each site soil solutions were acidic and soils at the woodland sites were more acidic ph 4 0 than those at the heathland site ph 4 5 acidic interstitial water solutions were found to up to 5 0 m depth in the unsaturated zone and are partially neutralized by two aluminosilicate mineral reactions in the unsaturated zone cation exchange and k feldspar dissolution the rate at which these acid neutralizing reactions act to neutralize acidity is revealed by the rate of depletion of base cations from the unsaturated zone in recharge solutions k dissolution ca 2 mg 2 cation exchange the total base cation depletion rate was greatest below heathland this can be attributed mainly to the greater rate of so 4 assimilation by the woodland biome

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

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

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

the haida current we present evidence for a narrow poleward flowing surface current over the continental slope of northwestern british columbia and southwestern alaska named for a prominent native indian tribe of the region the haida current occurs predominantly between october and april maximal flow takes place in midwinter from november through february characteristic features of the current include a minimal extent of 200 300 km a width of 20 30 km a depth scale of 500 m and near surface temperature signatures of up to 1 c within the core of the current as well as large amplitude o 10 km mesoscale waves and eddies along its flank the wind stress and alongshore sea surface slope are shown to be the primary driving mechanisms for the current

towards more detailed methods for quantifying the acid susceptibility of rocks and soils the sensitivity of rocks and soils to acidification is best described by their acid base titration curves these curves tend to be relatively featureless but can be usefully summarized in terms of their initial ph the buffer capacity at this ph and the total acidity to ph 8 3 unfortunately the shapes of soil titration curves depend on the type and concentration of background electrolyte and the rate of titration therefore while a set of standard titration conditions can be defined the resulting titration curves will not simulate those applicable to in situ field conditions as required for modelling purposes to progress it is necessary to gain a better understanding of the sources of and sinks for acidity in rocks and soils for soils partitioning of the acidity into soil solution neutral salt extractable and solid phase but not neutral salt extractable components provides a promising approach this is demonstrated using a series of soil samples derived from an extremely acid humus iron podzol overlying eocene sand near wokingham berks

age dating of individual grains of uraninite in rocks from electron microprobe analyses it is shown that an electron microprobe analysis of the u th and pb content of a single uraninite uo2 grain may be used to calculate the age of formation of that grain this is known as a chemical age examples are shown for a range of natural samples with a wide range of ages comparison with ages derived from isotopic measurements shows the results to be valid approximate formulae which have been proposed for the calculation of the chemical age are not sufficiently reliable especially where the th content is high and for older ages a method for calculating the age by iteration is presented this uses the full and exact formula and is shown to produce a reliable age for the uraninite compared to isotopic measurements the method is particularly useful for samples where the minerals and mineral textures record information on more than one event two examples of samples recording information on dual ages are discussed to show how these ages can be related to events in the formation of the host rocks in one example an alluvial heavy mineral concentrate is shown to contain uraninites from two distinct sources with significantly different ages in the other example two phases of u mineralisation are distinguished in the same rock sample one phase relates to the genesis of the sediments from which the rock was formed the second phase records remobilisation of the u during later tectonic movement

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

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

seasonal and interannual variability of the bering slope current abstract time series of sea surface height anomalies derived from satellite altimeters and absolute dynamic topography are used to examine variability in the geostrophic surface currents in the eastern bering sea the data suggest that the primary source of water to the eastern boundary currents of the bering sea is flow from the north pacific through amukta pass an aleutian pass located at 172 w the aleutian north slope current ansc is strongest and most variable in the winter months upon turning the corner to feed the bering slope current bsc the ansc bifurcates resulting in a westward flow at 54 n in addition to the northwestward flowing bsc this westward countercurrent has not been previously observed in the winter the bsc flows strongly to the northwest and is located near the shelf break during the rest of the year the mean flow is broadly northwestward but weaker and farther from the shelf break a continuous northwestward flowing current is rarely observed in the altimetry data except during winter months as it is overwhelmed by mesoscale variability the strength of the bsc in winter is correlated with the multivariate enso index and the north pacific index on interannual timescales

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