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

controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic granitic catchment basin massif central france abstract we present rare earth element ree patterns of small streams from a catchment basin in the massif central france in order to characterize the individual fractionation stages for the dissolved ree from the source to the catchment outlet the upper part of the catchment is located on a basalt plateau followed downstream by deep and narrow valleys within granitic and orthogneissic bedrock basalt normalized 0 45 m filtered stream water has ree patterns slightly depleted in the light ree la sm lree on the basalt plateau close to the source followed by a continuous amplification of this lree depletion downstream at the same time also a negative ce anomaly develops in the adsorbed fraction of 0 45 m particles which has been isolated by leaching with 1 m hcl strontium and neodymium isotope ratios of stream water demonstrate that the dissolved ree are essentially of basaltic origin even in the lower granitic and gneissic part of the catchment mixing with gneiss or granite derived ree thus cannot explain the observed evolution of the ree patterns our data suggest that the ree of the 0 45 m particles in contrast to earlier studies we found no link between ree behavior and organic colloids or organic complexes this scenario is confirmed by a detailed analysis of the ree patterns at 3 stream confluences the results show that the ree do not behave conservatively during mixing at stream confluences but that the lree and ce are preferentially removed similarly to what has been observed at catchment scale our study thus underlines the importance of fe colloids for the transport and fractionation of the ree in stream water

weathering process at the natural fission reactor of bangombe the uranium deposits in the basin of franceville gabon host the only natural fission reactors known in the world unique geological conditions favoured a natural fission reaction 2 ga ago this was detected by anomalous isotopic compositions of uranium and rare earth elements ree which are produced by the fission reaction in total 16 reactor zones were found most of them are mined out the reactor zone of bangombe is only 10 11 m below the surface this site has been influenced by surface weathering processes six drill cores have been sampled at the site of the reactor zone of bangombe during the course of the study and only one drill core bax 08 hit the core of the reactor from these data and previous drilling campaigns the reactor size is estimated to be 10 cm thick 2 3 m wide and 4 6 m long the migration of fission products can be traced by the anomalous isotope ratios of ree because of the fission process the 149sm 147sm ratio close to the reactor zone is only 0 28 normal 0 92 because of the intense neutron capture of 149sm and subsequent transmutation whereas 147sm is enriched by the fission reaction similar changes in isotopic patterns are detectable on other ree the isotope ratios of sm and nd of whole rock and fracture samples surrounding the reactor indicate that fission genic ree migrated only a few decimetres above and mainly below the reactor zone organic matter bitumen seems to act as a trap for fission genic ree additional ree patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model

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

magma dynamics at the base of an evolving mafic magma chamber incompatible element evidence from the partridge river intrusion duluth complex minnesota usa abstract a characteristic feature of the partridge river intrusion of the keweenawan duluth complex is the approximately fivefold to ninefold increase in the concentrations of incompatible elements in the lower zone compared with cumulates stratigraphically higher the concentrations of incompatible elements decrease from the lower zone upward to steady state values which is ascribed to variations in the proportions of trapped liquid rather than variable degrees of fractional crystallization of a single parental magma the calculated average composition of trapped liquid using our algorithm is similar to typical keweenawan low alumina high ti p basalts associated with the duluth complex but is different from the leading edge ferrodioritic liquid quenched in the chilled margin of the intrusion this difference suggests that the chilled margin does not represent the original parental magma composition from which the whole intrusion solidified and that the enrichment of incompatible elements may be related to the local flotation of magmatic suspensions to test the latter hypothesis numerically we have used heat mass transfer models assuming a sheet like magma chamber to calculate the parameters of the model that best reproduce the observed distribution of incompatible elements in a mush zone at the base of the partridge river intrusion the results indicate that a mush zone enriched in the incompatible elements is produced if the velocity of movement of the lower solidification front into the magma body was less than the floating velocity of the bulk crystal mush the dynamic parameters that best reproduce the observed distribution of incompatible elements include a magma emplacement pressure of 2 kbar critical crystallinities of 50 68 in the mush zone from which the liquid is being expelled and an emplacement temperature of 1160 c for the initial magma

correlation of plate motions with continental tectonics laramide to basin range some of the major tectonic and magmatic events of the last 150 ma in the western cordillera can be correlated with a new model for the displacement histories between western north america and adjacent oceanic plates sierra nevada plutonism ended and laramide compression began during increasingly rapid convergence of the farallon plate with north america and during a moderate increase in the westward motion of north america in the hotspot reference frame the end of the laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions the spectacular slowing of faralion north america convergence is attributed to the decreasing age of the farallon plate entering the trench and thus to a change from negative to positive buoyancy the transition from widespread arc related magmatism and rapid extension to basaltic volcanism and moderate extension in the basin and range province finds no ready explanation in the plate motions of the pacific basin a change to oblique spreading in the basin and range province accompanied growth in length of the san andreas fault as the mendocino triple junction progressed northward

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

strontium dissolved and particulate loads in fresh and brackish waters the baltic sea and mississippi delta a study was conducted of the isotopic composition and concentration of sr and of major elements in dissolved and suspended loads of fresh and brackish waters the purpose was to establish the contributions of different parent rocks and minerals to sr during weathering and transport and to identify the role of fe mn oxyhydroxides in the redistribution of sr in the water column during the sedimentary cycle studies were conducted on a profile across an oxic anoxic boundary in the baltic and on rivers covering behavior over an annual cycle in general the 87sr 86sr ratios differ between particulate and dissolved loads with more radiogenic sr in the particulate loads these differences are attributed to differential weathering of minerals where high rb sr minerals dominate the particulate load and low rb sr the dissolved load there is broad correlation of 87 sr 86 sr with k al in the suspended load the differences in 87 sr 86 sr between suspended and dissolved load are highly variable and are related to the fe or mn concentration on the particulates in samples with high fe al the difference becomes small a good correlation was found between sr al and fe al or mn al in the particulates both in brackish and fresh waters sr is removed from solution both in rivers and in the baltic sea whenever there is formation of fe mn oxyhydroxide particulates this precipitation greatly diminishes the difference in isotopic composition of the dissolved and suspended loads as the particles containing fe mn oxyhydroxides settle they dissolve in anoxic zones and release sr this provides a mechanism for sr redistribution in the water column sr is thus only quasi conservative in environments where fe mn oxyhydroxides form or dissolve from consideration of the isotopic differences in sr between dissolved and suspended loads it follows that the net sr input depends upon weathering characteristics of the contributing mineral phases changes in weathering mechanisms due to climate change may cause sr isotopic shifts in the marine environment

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

clay mineralogy and isotope geochemistry of the alteration halo at the cigar lake uranium deposit the clay size fraction of the alteration halo associated with the cigar lake uranium deposit consists of illite with subordinate kaolinite sudoite al chlorite fe chlorite hematite and traces of an illite chlorite mixed layer mineral the kubler index of illite crystallinity ranges from 2 0 to 6 7 the most altered samples average 4 0 these data indicate that high grade diagenetic to anchizonal conditions have been reached within the alteration halo intensity ratios of the 002 and 001 illite reflections i 002 i 001 are generally greater than 0 4 the phengite muscovite boundary reflecting aluminum rich illites illite composition approaches hydromuscovite whereas 1m and 2m polytypes prevail regionally the presence of 3t and 2m polytypes in the alteration halo suggests that locally higher temperatures have been attained than the regional estimates of 180 220 c k ar dates for illite rich samples from 1255 to 1148 ma could reflect variable resetting of the k ar system during hydrothermal f

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

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

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

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

rare earth elements in river waters we measured rare earth element ree concentrations in river waters to characterize the suspended and dissolved river flux of the ree to the oceans the ree pattern of river suspended materials is sensitive to drainage basin geology a positive correlation is observed between la yb ratios and nd model ages for the rivers studied major rivers have light ree enriched patterns relative to the north american shale composite nasc with la yb n 1 6 2 7 river water dissolved material 0 2 m is heavy ree enriched relative to suspended material and the most pronounced negative ce anomalies occur in rivers of high ph light ree concentrations vary by approximately 3 orders of magnitude and are inversely related to ph and major cation concentrations from these data we estimate that typical major river runoff has heavy ree depleted suspended material with la yb n 1 9 we conclude that the terrigenous input to the oceans from major rivers is heavy ree depleted relative to shales from the available data average river water dissolved material appears to be heavy ree enriched with la yb n 0 4 estuarine removal processes lower the dissolved ree river flux by approximately 60 and result in a flux that is more heavy ree enriched with la yb n 0 2 calculated oceanic residence times with respect to river input range from 2300 to 21 000 years are shortest for ce and greatest for the heavy ree and la such long residence times may suggest the presence of additional sources of ree in seawater

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

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

scully field marion county kansas abstract the scully field is a multipay new field discovery located in the southern end of salina basin marion county kansas since discovery in november of 1981 r j walker oil co inc has drilled and completed successfully 15 wells within the field productive depths are 3 300 ft 1 000 m and production capabilities of individual wells can exceed 300 bbl of oil day 24 hr gauges oil is trapped structurally within the viola and simpson units ordovician and trapped by a combination of erosional truncation and structure in the mississippian units the scully field was discovered using a combination of satellite imagery and subsurface control structural lineaments recognized from satellite imagery in conjunction with an understanding of the structural timing and framework of the salina basin enabled the definition of the structural unit which contains the scully field subsurface control prior to discovery although relatively sparse was sufficient in light of these structural elements to define the prospective area of the scully structure subsequent drilling has not altered significantly the potentially productive area the overall trapping mechanism at the scully field is anticlinal closure infield drilling has demonstrated however that significant stratigraphic variations do exist within the productive area the stratigraphic variations within the ordovician reservoirs are controlled strongly by paleostructure the simpson sands have been subdivided in five separate units which range from 4 to 12 ft 1 to 4 m in thickness three of these are of economic importance in the field in general the sands with the most economic potential are distributed within relative ordovician paleolows core analysis demonstrates that pay intervals have porosities of 14 to 18 and permeabilities of 200 to 500 md air whole core dean stark analysis the viola has four main lithologic divisions the uppermost of these is a relatively thin dolomite cap which ranges from 2 to 15 ft 1 to 5 m this upper dolomite is the primary viola pay zone core analysis indicates that this interval has porosities of 6 to 12 and permeabilities of 30 to 140 md air whole core dean stark analysis in general this dolomite is best developed on the ordovician paleohighs the mississippian section is eroded deeply over the scully structure and demonstrates about 70 ft 20 m of thinning the potential pay interval is chert which has 25 to 30 porosity based on log analysis the pay interval is absent over the portion of the field that is highest on present day structure at the ordovician level the trapping mechanism is a combination of erosional truncation and structural closure in addition to the structural information obtained from satellite imagery r j walker oil co inc evaluated the hydrocarbon potential of t18s r1e marion county kansas which contains the scully field using remote sensing technology developed by earth reference systems of long beach california the technology involves direct detection of hydrocarbons in place using satellite data nonlinear mathematics and the fundamental principles of molecular structure and electromagnetic wave propagation this analysis was made several months after the discovery well had been drilled in the scully field the conformance of these data within the structural and geologic confines of the scully field after 14 development wells drilled by r j walker oil co inc and two recent dry holes rilled by outside operators provides an interesting glimpse at technology that may revolutionize the way the oil and gas industry searches for new reserves end of article last page 1327

variscan potassic dyke magmatism of durbachitic affinity at the southern end of the bohemian massif lower austria dykes in the strudengau area sw moldanubian zone austria can be mineralogically divided into lamprophyres spessartites and kersantites and felsic dykes granite porphyries granitic dykes and pegmatoid dykes geochemical analyses of 11 lamprophyres and 7 felsic dykes show evidence of fractional crystallization the lamprophyres are characterized by metaluminous compositions intermediate sio2 contents and high amounts of mgo and k2o these rocks have high ba 800 3000 ppm and sr 250 1000 ppm contents as well as an enrichment of large ion lithophile elements over high field strength elements typical for enriched mantle sources with variable modifications due to fractionation and crustal contamination this geochemical signature has been reported from durbachites biotite and k feldspar rich mela syenites particularly characteristic of the variscan orogen in central europe for most major elements calculated fractionation trends from crystallization experiments of durbachites give an excellent match with the data from the strudengau dykes this suggests that the lamprophyres and felsic dykes were both products of fractional crystallization and subsequent magma mixing of durbachitic and leucogranitic melts rb sr geochronological data on biotite from five undeformed kersantites and a locally deformed granite porphyry gave cooling ages of c 334 318 ma indicating synchronous intrusion of the dykes with the nearby outcropping weinsberger granite part of the south bohemian batholith c 330 310 ma oriented matrix biotite separated from the locally deformed granite porphyry gave an rb sr age of c 318 ma interpreted as a deformation age during extensional tectonics we propose a large scale extensional regime at c 320 ma in the strudengau area accompanied by plutonism of fractionated magmas of syncollisional mantle derived sources mixed with crustal components this geodynamic setting is comparable to other areas in the variscan belt documenting an orogenic wide extension by the end of the carboniferous

reply to comment on steady state 226ra 230th disequilibrium in mantle minerals implications for melt transport rates in island arcs by r george m reagan s turner j gill b bourdon abstract we agree with george et al that high 226 ra 230 th and correlated ba th 238 u 230 th and other elemental and isotopic ratios are the result of fluid addition to the wedge the feineman and depaolo earth planet sci lett 215 2003 339 355 manuscript addresses more specifically the pathway followed by the fluid en route to the surface in particular we address whether the 226 ra 230 th data directly yield the total time available for the fluid to migrate from the point of origin in the slab to the melting region in the wedge and ultimately to the surface with the melt all of the processes involved in generating arc volcanism would of necessity have to take place very rapidly within the constraints proposed by george et al the alternative is that the upward fluid movement is hindered by the fluid flow regime and solid mantle flow allowing more time for reaction with the mantle wedge there is evidence in the u series data for more than one time scale and our model helps to explain how this could come about the extent to which grainscale 226 ra 230 th disequilibrium affects the inferred melt transport time is yet another issue and this could be achieved either with the small degree melt mechanism we discussed or by additional effects associated with melt and fluid migration at later stages in the melt generation process we agree that the final melt transport event is likely to happen quickly one of the most puzzling features of volcanic arcs is that the volcanic front emerges considerably behind the presumed location of primary water release from the subducting slab depth to slab is 120 km beneath the volcanic front as opposed to the predicted 80 km this offset between the expected site of fluid release and the location of the volcanic front can be attributed to coupled transport of the fluid in the form of hydrous minerals such as amphibole and or phlogopite in the wedge j geophys res 97 1992 2037 2070 or redirection of the melt due to flow and stresses in the mantle matrix as the magma slowly percolates upward j geol 95 1987 285 307 in either case the fluid and or melt is required to ascend slowly at least for some part of its journey to explain the location of the volcanic front furthermore 238 u 230 th disequilibria in arc lavas seem to suggest a relatively long interval for fluid addition 30 60 ka science 292 2001 1363 1366 earth planet sci lett 179 2000 581 593 geochim cosmochim acta 61 1997 4855 4884 which is difficult to reconcile with the 226 ra 230 th disequilibrium observed in the lavas for these reasons we sought to find a means whereby apparently short lived isotopic disequilibria could be maintained during complex fluid and melt migration in the mantle wedge below we address a few of the main issues presented by george et al earth planet sci lett this issue in their comment

rare earth elements in the sediments of lake baikal lake baikal is the deepest and probably oldest lake on earth its water column is pervasively oxic and sedimentation rates are very low which leads to the formation of a dynamic iron fe and manganese mn enrichment below the mn ii o2 boundary these often massive accumulations can be buried within the reducing part of the sediments and give rise to complex and cryptic redox cycles the mobility of rare earth elements rees is influenced by the dissolution and reduction dynamics of the ferromanganese oxides the present study offers an overview of the ree chemistry in lake baikal and its catchment area and more specifically ree distribution in five 11 to 26 cm long sediment cores situated across the lake at different water depths we analysed and discussed normalised ree patterns and their consequential cerium ce anomalies while particulate ree concentrations are mainly influenced by processes above or near the surface of lake baikal such as the development of a widespread negative ce anomaly processes occurring during early diagenesis in the sediment are most reflected in pore water rees the dissolution of ferromanganese oxides at the mn ii o2 boundary remobilizes significant amounts of ree into the pore water whereby some are likely adsorbed onto colloidal fe oxides however besides the tendency of ce being associated with mn oxides pore water rees fractionate predominantly around the buried fe mn accumulation where light rees preferentially adsorb onto fe oxides

pbpe systematics on diagenetic clays an example from proterozoic black shales of the franceville basin gabon this study presents the first dating attempt of diagenetic clay material by the pb cpb isotope method pb pb isotopic measurements were performed on authigenic 0 4 m clay fractions illite 1m fe mg chlorite extracted from black shales highly enriched in organic matter of the francevillian lower proterozoic sedimentary series in gabon the clay residues and acid leachates 1 n hcl yield a pb pb isochron age of 2328 130 ma mswd 1 24 even if this age is within error limits identical with two sm nd isochron ages previously reported for the same material it is not considered as a true isochron age the pb isotopic data of the residues indicate that the clay minerals are not in isotopic equilibrium however the isotope data of the leachates separately define an isochron age of 2265 150 ma mswd 0 36 which is closer to the sm nd age it is suggested that the leachates are the most appropriate phases for dating diagenetic processes with pb isotopes the u th pb fractionation in the clay fractions is related to the occurrence of organic matter in the sediments the upb and uth ratios of the clays correlate with their organic carbon contents this fractionation is probably due to differential complexation of u th and pb by organic ligands during the dissolution of detrital mineral phases and clay mineral authigenesis

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

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

rare earth elements ree of dissolved and suspended loads in the xijiang river south china abstract the xijiang river is the main channel of the zhujiang pearl river the second largest river in china in terms of water discharge and flows through one of the largest carbonate provinces in the world the rare earth element ree concentrations of the dissolved load and the suspended particulate matter spm load were measured in the xijiang river system during the high flow season the low dissolved ree concentration in the xijiang river is attributed to the interaction of high ph and low doc concentration the paas normalized ree patterns for the dissolved load show some common features negative ce anomaly progressively heavy ree hree enrichment relative to light ree lree similar to the world s major rivers the absolute concentration of the dissolved ree in the xijiang river are mainly ph controlled the degree of ree partitioning between the dissolved load and spm load is also strongly ph dependent the negative ce anomaly is progressively developed with increasing ph being consistent with the oxidation of ce iii to ce iv in the alkaline river waters and the lack of ce anomalies in several doc rich waters is presumably due to both ce iii and ce iv being strongly bound by organic matter the paas normalized ree patterns for the dissolved load and the spm load in rivers draining the carbonate rock area exhibit middle ree mree enrichment and a distinct maximum at eu indicating the preferential dissolution of phosphatic minerals during weathering of host lithologies compared to the xijiang river waters the mree enrichment with a maximum at eu disappeared and light ree were more depleted in the south china sea scs waters suggesting that the ree sourced from the xijiang river must be further fractionated and modified on entering the scs the river fluxes of individual dissolved ree introduced by the xijiang river into the scs vary from 0 04 to 4 36 10 4 mol a 1

origin of minette by mixing of lamproite and dacite magmas in veliki majdan serbia composite dykes consisting of leucominette and dacite as well as discrete dykes and flows of minette and lamproite composition occur in the veliki majdan area western serbia this area is part of the serbian tertiary magmatic province which consists of numerous small occurrences of ultrapotassic igneous rocks the composite dykes have leucominette margins up to 150cm thick enclosing a central part of dacite up to 100m in width between these two lithologies a decimetre sized transition zone may occur petrography mineral chemistry and bulk rock geochemistry including sr nd and pb isotopes provide evidence that the minettes and leucominettes formed by hybridization between a felsic magma similar in composition to dacite and a mantle derived lamproitic magma the leucominettes and minettes contain all phenocryst types biotite plagioclase quartz present in the dacites but in partly resorbed and reacted form the mica displays a great diversity of resorption textures as a result of partial dissolution incipient melting and phlogopitization suggesting superheating of the felsic melt during hybridization the mineral modes and mineral compositions of the leucominettes and minettes resemble those in the lamproites a model for the modification of lamproite melt towards minette is presented in which minette is formed by mixing of lamproite and530 felsic magma the lack of any significant correlation between pb isotopic ratios and some of the mixing indices sio2 zr zr nb 143 nd 144 ndi recognized in the hybridization model for the veliki majdan dykes may be a result of similarity of the pb isotopic signature in the two end members highly phlogopitized biotite xenocrysts in the minettes are ascribed to the retention of volatile components after magma mixing and crystallization of a new generation of phlogopite from the hybridized magma the magma mixing model explains the reverse zoning and resorption features of phlogopite macrocrysts commonly recognized in calcalkaline lamprophyres elsewhere therefore this mixing mechanism may be globally applicable for the origin of minettes associated with calcalkaline granitic plutonism in postorogenic settings

geological environment of the cigar lake uranium deposit the cigar lake uranium deposit occurs within the athabasca basin of northern saskatchewan canada like other major uranium deposits of the basin it is located at the unconformity separating helikian sandstones of the athabasca group from aphebian metasediments and plutonic rocks of the wollaston group the athabasca group was deposited in an intra continental sedimentary basin that was filled by fluviatile terrestrial quartz sandstones and conglomerates the group appears undeformed and its actual maximum thickness is about 1500 m on the eastern side of the basin the detrital units correspond to the manitou falls formations where most of the uranium deposits are located the lower pelitic unit of the wollaston group which lies directly on the archean basement is considered to be the most favourable horizon for uranium mineralization during the hudsonian orogeny 1800 1900 ma the group underwent polyphase deformation and upper amphibolite facies metamorphism the hudsonian orogeny was followed by

comment on steady state 226ra 230th disequilibrium in mantle minerals implications for melt transport rates in island arcs by m d feineman and d j depaolo earth planet sci lett 215 2003 339 355

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

surface expressions of the ghawar structure saudi arabia abstract the ghawar anticline is approximately 225 km long and 25 km wide in the subsurface but the surface structural expression is not obvious identifying subtle structural imprints in the young mio pliocene sedimentary cover is therefore of great importance in developing a structural growth model for the ghawar field the ghawar area between the jafurah sand desert to the east and the rubayda dahna sand deserts to the west is characterized by a rougher topography when compared to the surrounding rather smooth flat areas this rough geomorphology of the structure can also be noticed on the satellite images a geomorphologic elevation map of the area and a subsurface structural contour map of the top arab d upper jurassic reservoir reveal very similar geometric shapes calcareous sandstone deposits of the youngest hofuf formation mio pliocene cap the structural highs along the axis of the anticline and spectacular fractures caves mesas and monumental geomorphologic features have developed in the escarpments fractures in the dam formation middle miocene are not as conspicuous as in the hofuf formation but indigenous fractures are clearly recognized in this formation a match between the directions of some topographic lineaments and projected surface traces of subsurface faults from seismic cross sections can be observed however at the field locations faults have yet to be defined on the surface surface indications suggest that the structure has been active until the present day elevations of a bedding plane on the shedgum plateau reveal a 0 25 dip angle which cannot be distinguished on the surface by the human eye the movements that occurred during the 4 million years since the pliocene period reveal an average tilting of 0 06 per one million years

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

geochemical behaviour of rare earth elements ree along a river reach receiving inputs of acid mine drainage abstract total and dissolved rare earth elements ree were studied in a river reach affected by several inputs of acid mine drainage the first four acidic discharges were of lesser importance compared to the last agrio river coming from the rio tinto mines which transported high loads of fe and al 2 1 and 4 2 ton day respectively together with ree 16 4 kg day and other trace elements in the acid mine drainage amd sources practically all the ree were dissolved and the north american shale composite nasc normalized patterns showed an enrichment in medium ree although some differences exist between the patterns of each source the ph values in the river reach upstream of the confluence with the agrio river ranged between 8 01 and 7 03 and most of the fe and al from the amd sources precipitated downstream of this acidic discharge the ph decreased to 2 98 upstream of the first amd input the dissolved and total concentrations of ree were very low 7 concentrations of ree increased up to 25 g l mainly transported by the particulate phase in the reach downstream of the agrio river with acidic conditions ree behaved conservatively the ree nasc normalized patterns of the river samples resemble that of the amd sources although an enrichment of heavy ree in the dissolved phase is observed linked to complexation by carbonates cerium and particularly la also show higher dissolved percentages similar to hree which must be due to the lower affinity of these elements to be sorbed onto fe oxyhydroxides variations in ce and eu anomalies are observed along the river reach as a consequence of the different amd inputs however the values of the ce anomaly are higher in total samples than in dissolved samples speciation results indicate that these differences are not caused by differences in oxidation states but by slight differences in the hydrogeochemical behaviour of ce with respect to la and nd

solid liquid ree fractionation in the lateritic system of goyoum east cameroon the implication for the present dynamics of the soil covers of the humid tropical regions the ree th weathering geochemistry and mineralogy has been investigated in a lateritic soil cover in relation to the close hydrographical system at goyoum east cameroon the stutied area composed of gneissic hills covered by humid tropical forest belongs to the sanaga river basin and corresponds to a transition vegetation zone between rainforest and savannah a representative soil catena was taken as a reference zone close to the sanaga river the gneissic parent rock soil and groundwater samples have been cut off in pits in the lower part of the catena waters were also sampled both in brooks rich in organic matter and in the sanaga river all the waters were sampled seasonally during the rainy and dry periods the water rock interactions and the exportation of ree and th have been discussed as a function of the evolution of specific ratios considering th as the less mobile element during laterization variation of parent rock pr normalized ratios ln th pr vs depth show that the upper ferruginous horizons are depleted in ree while the basal saprolite presents accumulation zones which are differently located for ce and the other lrees the hrees are also depleted the la accumulations are located near the saprolite gneiss weathering front associated with negative ce anomalies ce ce 0 4 the ce accumulations inducing strong positive ce anomalies ce ce 4 are located in the upper saprolite beneath the ferruginous horizons all along the soil catena the secondary lree bearing minerals which reflect lree fractionation during incipient weathering of the gneiss are hydrous phosphates rhabdophane lreepo4 nh2o and cerianite ceo2 whose precipitation depends on the eh ph changes in the soil profiles on the other hand hree depletion is controlled by the dissolution of xenotime the major hree bearer in the parent gneiss in the 0 45 m filtered acidic groundwater ph 5 5 which have flooded the saprolite for a long residence time th contents are extremely low 0 th 10 ppt while lree contents are high 90 la 450 ppt in these filtered waters th seems to be strongly controlled by minerals i e thorianite which lead to its immobilization in the soil while lree remain in the soluble and or colloidal pools moreover the groundwaters show significant negative ce anomalies throughout the year 0 1 ce ce 0 4 the mobilization of lree except redox active ce results from the dissolution and or the mobilization in colloidal form of the secondary phosphates which may present major negative ce anomalies as indicated by the bulk soil analyses at the saprolite gneiss weathering front therefore the groundwaters have the signature of saprolitic materials we propose that the strong positive ce anomalies of the upper saprolite result from ancient weathering processes these processes would have led to much stronger ce iii ce iv dissolution reprecipitation conditions than those occurring at the present time in the saprolite in the 0 45 m filtered brook acidic blackwaters lree and th contents are high 100 th 400 ppt 400 la 1700 ppt in this organic matter rich environment lree and th are highly mobilized the colloial pool controls half of the whole lree th load the geochemical signature of these waters is independent of the gneissic parent rock and soil lree contents there is no ce anomaly this signature is similar to those of other organic rich rivers draining humid tropical ecosystems in the world in the 0 45 m filtered sanaga waters lree and th contents are lower than in the brooks 15 th 40 ppt 40 la 300 ppt these concentrations are dependent on the time and on the colloidal composition however 90 of these elements are mainly controlled by the suspended pool

remobilization of u and ree and the formation of secondary minerals in oxidized u deposits the hydrothermal uranium vein type deposits of menzenschwand and wittichen in the schwarzwald in southwestern germany have been investigated with regards to their primary and secondary mineralization primary magmatic uraninite i from the host granite of menzenschwand primary hydrothermal uraninite ii and secondary supergene uranyl silicates uranophane and cuprosklodowskite uranyl phosphates torbernite and uranocircite and uranyl arsenates zeunerite heinrichite novacekite walpurgite and uranospinite were analyzed for their ree contents by la icp ms together with uraninite ii samples from other known schwarzwald uraninite ii occurrences for comparison water samples were taken from drillings and abandoned mines and were analyzed for their major and trace element composition including u and ree the ree patterns of uraninite show significant eu anomaly variations negative eu anomalies are related to granitic host rocks whereas positive eu anomalies imply a gneissic ree source this is in agreement with the observation of eu anomalies in granite and gneiss derived waters which display negative and positive eu anomalies respectively rare earth element distributions in the secondary uranyl minerals provide information about the sequence of mineral precipitation and the degree of remobilization of u cerium anomalies or their absence imply that uranyl silicates formed during an earlier stage of weathering under more reduced conditions than the uranyl arsenates and phosphates the ree patterns of the uranyl silicates in wittichen are similar to those of uraninite ii suggesting a very local redistribution on the millimeter to centimeter scale in contrast the ree patterns of uranyl arsenates and phosphates are different from uraninite ii patterns and resemble those of the waters this shows that the uranyl phosphates and arsenates are formed not only by redistribution of u in the hydrothermal veins but are influenced by waters from the host rock implying that u was probably transported over a greater distance these conclusions are supported by field observations where uranyl silicates are often found in the vicinity of uraninite ii whereas uranyl phosphates and arsenates are commonly found on fissures in the host rock to explain specific features in the sulfide rich pyrite chalcopyrite menzenschwand deposit we conducted thermodynamic reaction modeling using the phreeqc computer code sulfide oxidation was modeled by reaction of a u rich water in equilibrium with atmospheric oxygen with fes2 and cufes2 a water that initially precipitates uranophane gradually evolves to more acidic ph values thereby reaching torbernite saturation this is in agreement with observed paragenetic sequences ongoing reaction results in a further decrease of ph and in the destabilization of uranophane and torbernite since goethite precipitates even at low ph values these calculations can explain observed pseudomorphs of goethite after both uranophane and torbernite which are frequently observed at menzenschwand if the reaction proceeds and f o2 in the system is buffered by the surrounding minerals u6 can be reduced to u4 and uraninite can form and replace pyrite and earlier formed secondary uranium minerals such as ianthinite also these textures have been observed we show that the combination of ree geochemistry careful paragenetic observations and thermodynamic modeling allows to reconstruct the formation and weathering of uranium deposits in great detail

rare earth elements and sr and nd isotopic compositions of dissolved and suspended loads from small river systems in the vosges mountains france the river rhine and groundwater abstract the aim of this study is to characterize the evolution of the rare earth elements ree in non mature streams from small catchment areas in the vosges mountains downstream to more mature plain rivers including the river rhine the dissolved load ree distribution patterns of the low ph vosges streams are very different from those of high ph plain rivers indicating that different physico chemical parameters control ree transport in these different water systems the plain rivers and groundwater show similar ree distribution patterns with a strong negative ce anomaly and heavy rare earth element hree enrichment in addition the river rhine has a positive gd anomaly which is of anthropogenic origin similar to the world s major rivers the light rare earth element lree enrichment in the plain rivers is mainly ph controlled their sm nd and 143 nd 144 nd isotope ratios are close to average continental crust values this is not the case for the less evolved non mature and low ph vosges streams their high sm nd and 143 nd 144 nd ratios but low 87 sr 86 sr ratios suggest that chemical alteration of accessory middle rare earth element mree enriched minerals such as apatites from rocks in the catchment area control the ree abundances of these waters a comparison of the dissolved load ree distribution patterns with those of the principal lithologies in the corresponding drainage basins illustrates that especially the eu anomalies of the vosges streamlets are strongly lithology dependent leaching experiments indicate that the suspended load is isotopically very inhomogeneous the ree distribution patterns of the suspended load leachates have no similarities with those of the corresponding dissolved load indicating that the leachable reservoir not only contains adsorbed ree but also ree from leachable mineral phases their 143 nd 144 nd sm nd and 87 sr 86 sr ratios support this suggestion being always higher and lower respectively than those of their corresponding dissolved loads they rather point to the presence of relic primary apatite in the leachable portion of the suspended load the flat paas normalized ree distribution patterns the high 87 sr 86 sr and low 143 nd 144 nd isotopic ratios of the corresponding residues suggest secondary silicate phases such as clay minerals in the residual phase of the suspended load leachates and corresponding residual phases define alignments in the sm nd isochron diagram whose slopes define ages ranging between 200 and 390 ma indicating that the mineral phases in the suspended load retain some memory of their primary precursor minerals in the hercynian granitic source rocks

ree fractionation during granite weathering and removal by waters and suspended loads sr and nd isotopic evidence very few studies deal with ree rare earth element mobility within the system soil soil solution streamwater in this article we try to characterize the fractionation and the migration of the ree in a granite derived soil system located in a small catchment of the vosges mountains icp ms and tims measurements were performed on both solid samples fresh granite soil and suspended load of the stream and waters soil solutions springwater and streamwater to determine their respective ree concentrations and sr and nd isotopic compositions the paas normalized ree pattern of the bedrock is characterized by a strong depletion in hree heavy ree and a negative eu anomaly 0 46 similarly the granite normalized ree distribution patterns of the soil samples show hree depletions that become more important with decreasing depth the correlative behavior between p2o5 th and ree with depth indicates that besides apatite other phosphate minerals such as monazite are the most important phases controlling the th and ree budget in the soil profile on the other hand at greater depth zircon seems to be another important mineral phase controlling especially the hree enrichment as shown by the positive relationship between zr content and the yb ho ratio different grain size fractions show similar ree distribution patterns and are only weakly fractionated compared with bulk soil sample however the finest fraction 0 20 m is more enriched in sr and ree suggesting a stronger concentration of ree carrying minerals in this fraction the suspended and dissolved load of the stream show as a whole an enrichment in hree if compared with the granite or with the different soil samples however compared with the uppermost soil samples the suspended load is significantly more enriched in hree its ree distribution pattern is more similar to that of the finest fraction of the deeper soil sample and to the fresh granite thus most probably the ree of the suspended load originated from a source with ree characteristics found in the deep soil horizons this source might have been situated in the uppermost soil profile which is actually ree depleted the weathering process can be compared with a leaching experiment where the waters correspond to the leachate and the soil to the residual phase of the granite the sr isotope data indicate that the suspended load originates from the finest soil fraction the sr and nd isotopic data of the suspended load suggest that it contains up to 3 sr and nd from apatite and up to 97 from feldspar most of the sr and nd in the waters originate from apatite leaching or dissolution

the aurora volcanic field california nevada oxygen fugacity constraints on the development of andesitic magma the aurora volcanic field located along the northeastern margin of mono lake in the western great basin has erupted a diverse suite of high k and shoshonitic lava types with 48 to 76 wt sio2 o ver the last 3 6 million years there is no correlation between the age and composition of the lavas three quarters of the volcanic field consists of evolved 4 wt mgo basaltic andesite and andesite lava cones and flows the majority of which contain sparse euhedral phenocrysts that are normally zoned there is no evidence of mixed hybrid magmas the average eruption rate over this time period was 200 m 3 ykm 2 yyear which is typical of con tinental arcs and an order of magnitude lower than that for the slow spreading mid atlantic ridge all of the au rora lavas display a trace element signature common to subduction related magmas as exemplified by baynb ratios between 52 and 151 pre eruptive water contents ranged from 1 5 wt in plagioclase rich two pyroxene andesites to 6 wt in a single hornblende lamprophyre and several biotite hornblende andesites calculated oxygen fugacities fall within 0 4 and 12 4 log units of the ni nio buffer the aurora potassic suite follows a classic calc alkaline trend in a plot of feo t ymgo vs sio 2 and displays linear decreasing trends in feo t and tio2 with sio2 content suggesting a prominent role for fe ti oxides during differentiation however develop ment of the calc alkaline trend through fractional crys tallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the ni nio buffer nor can fractionation of hornblende be invoked since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron together with a thermal stability limit that is lower than the eruption temperatures of several andesites 1040 10808c derived from two pyroxene ther mometry unless some progressive oxidation process occurs fractionation of titanomagnetite or hornblende cannot explain a calc alkaline trend in which all erupted lavas have oxygen fugacites the ni nio buffer in con trast to fractional crystallization closed system equi librium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt however the eruption of nearly aphryic lavas ar gues against tapping from a magma chamber during equilibrium crystallization a process that requires crys tals to remain in contact with the liquid a preferred model involves the accumulation of basaltic magmas at the mantle crust interface which solidify and are later remelted during repeated intrusion of basalt as an end member case closed system equilibrium crystallization of a basalt followed by equilibrium partial melting of the gabbro will produce a calc alkaline evolved liquid namely high sio 2 and low feo t ymgo with a relative fo2 corrected for the effect of changing temperature that is similar to that of the initial basalt differentiation of the aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large stable magma chambers is consistent with the low eruption rate at the aurora vol canic field

petrological characterization of the mantle source of mediterranean lamproites indications from major and trace elements of phlogopite lamproites are among the most enigmatic mantle generated melts and are typically abundant in phlogopite we investigated the major and trace element chemistry of phlogopite crystals from mediterranean lamproite occurrences in spain serbia and turkey the most primitive lamproite samples were selected in order to evaluate the generation of the different phlogopite populations in their distinct mantle setting s phlogopite hosted in mediterranean lamproites can be grouped into different compositional arrays on the basis of their major element chemistry using al2o3 feot and tio2 concentrations contents of cr f and presumably ti allow discrimination of the macrocrysts into phlogopite phenocrysts which are liquidus and near liquidus phases and mantle xenocrysts further distinction among different populations of mantle derived phlogopite can be achieved considering trace element and isotopic ratios the latter also distinguish between crustal and asthenospheric imprints on the source of the lamproitic parental melt

estimating abundances of volatile and other mobile components in evolved silicic melts through mineral melt equilibria to variable degrees of accuracy from source partitioning between and earths residual minerals and anatectic melts through melt fractionation partitioning between igneous minerals and residual melts to the end stages of magma solidification where higher concentrations of normally trace elements may promote saturation in their crystalline phases the magmatic abundance of rare alkalis and alkaline earths

controls on the geochemistry of rare earth elements in sediments and groundwaters of the aquia aquifer maryland usa abstract groundwater samples were collected along two flow paths from the aquia aquifer in maryland usa to investigate the rare earth element ree geochemistry of the flow system as a function of evolving groundwater composition neodymium nd concentrations of aquia aquifer groundwater range from 56 6 to 71 4 pmol kg 1 along the eastern shore flow path i e eastern shore of the chesapeake bay and from 41 5 to 77 9 pmol kg 1 on the western shore flow paths and consequently fall within the range reported for other circumneutral to alkaline ph groundwaters shale normalized ree patterns of eastern and western shore aquia groundwaters are similar i e relatively flat and do not vary substantially along either flow path for example yb nd nasc for aquia aquifer groundwaters exhibit a limited range between 0 7 and 1 1 aquia aquifer sediments were analyzed for rees following a sequential extraction procedure neodymium concentrations for the bulk aquifer sediment range from 26 to 70 mol kg 1 3 7 to 10 1 ppm or a factor of 2 5 to 7 lower than the nd content of the upper continental crust the sequential extractions reveal that the majority of the rees in the aquifer sediments occur associated with the abundant carbonate shell fragments that are ubiquitous within the aquia aquifer sediments furthermore the sequential extraction data indicate that the light rees lree are more readily exchangeable i e outer sphere surface complexed on aquifer mineral surfaces than the heavy rees hree and the hrees and middle rees mree exhibit a greater association with fe mn oxides oxyhydroxides within the sediments analysis of the abundant glauconite pellets within aquia aquifer sediments indicates an overall enrichment in the hrees relative to the lrees for this mineral compared to shale our data suggest that the chief source of rees to aquia aquifer groundwater is dissolution of carbonate biominerals shell fragments that occurs in the recharge zone and before groundwaters become saturated with respect to calcite and aragonite additionally weakly sorbed rees outer sphere surface complexed contribute lrees and mrees to aquia aquifer groundwater as ph increases above 8 leading to the relatively flat shale normalized ree patterns that characterize aquia aquifer groundwaters once released to the solution ree form strong aqueous complexes with carbonate ions lnco 3 and ln co 3 2 with the dicarbonato complexes i e ln co 3 2 predominating the ree are subsequently transported with flowing groundwater in the aquia aquifer as the negatively charged dicarbonato complexes exhibit a low affinity to sorb to negatively charged mineral surfaces in the aquifer sediments

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

petrogenesis of the ultrapotassic lincoln syenite maine late silurian early devonian melting of a source region modified by subduction driven metasomatism the late silurian early devonian 418 1 ma lincoln syenite exposed discontinuously over a distance of 75 kilometers in south central and mid coastal maine represents one of the most unusual igneous rocks in the appalachian orogen detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post magmatic recrystallization however a small proportion of the syenites in south central maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis the syenites are characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar clinopyroxene orthopyroxene biotite oxides the rocks are intermediate in terms of sio2 content 55 59 wt yet are ultrapotassic 6 7 wt k2o and relatively primitive in terms of their mgo mg 67 cr 400 ppm and ni 150 ppm concentrations additionally the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements 200 300 chondritic abundances the syenites show limited radiogenic isotopic variability with relatively high initial 87sr 86sr values 0 707 to 0 710 relatively low initial end values 4 8 to 6 7 and initial pb isotopic compositions characteristic of igneous rocks in central maine for example 207pb 204pb 15 577 to 15 639 the geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization facilitated the crystallization of mg rich biotite and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts correlation with the geochemically similar parks pond pluton in eastern maine extends the distribution of late silurian early devonian ultrapotassic magmatism in maine to an orogen parallel distance of over 150 km comparisons with rocks of similar composition in known tectonic environments suggest the lincoln syenite and by association the parks pond pluton formed as a result of lithospheric extension superimposed on a sub continental lithospheric mantle source region previously modified by subduction driven metasomatism the timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab break off or the convective removal of lithospheric mantle delamination in association with acadian convergence both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes

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

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

phase equilibria of phlogopite lamprophyres from western mexico biotite liquid equilibria and p t estimates for biotite bearing igneous rocks olivine and augite minette powders have been equilibrated from one bar to nearly 2 0 kbar water saturated and from 900 to 1300 c and then quenched rapidly at oxygen fugacities controlled between the nickel nickel oxide nno and hematite magnetite hm oxygen buffers the liquidus of both samples is suppressed 100 c at water saturated conditions and 1500 bar both lavas contained between 3 and 4 wt water at the stage of phenocryst precipitation the partitioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f o2 units when these determinations are combined with previous studies fe2o3 feo total of mg rich biotite can be calculated knowing log fo2 t and xfe thermodynamic modelling of biotite liquid equilibria results in two expressions for calculating activity coefficients for annite and phlogopite in natural biotites based on the partitioning of bao and tio2 between biotite and liquid we have formulated a thermometer and barometer over the range of 400 c tio2 partitioning between phlogopite and liquid is a function of temperature 50 c and is insensitive to pressure and h2o and o2 activities bao partitioning between phlogopite and liquid is a function of both temperature and pressure 4 kbar the latter being most important applying the tio2 and bao partitioning expressions to lamprophyre and lamproite suites shows that mexican minettes equilibrated at low pressures 5 to 15 kbar 4 kbar and temperatures 1090 to 1160 c 50 c while australian lamproites equilibrated at higher p up to 30 kbar 4 kbar and t 1125 to 1400 c 50 c experimental glass compositions and phenocryst fractionation calculations together with the bao and tio2 based pressure calculations indicate that felsic minettes from the mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures

a mossbauer study of pseudotachylytes redox conditions during seismogenic faulting the redox conditions during frictional melting provide information on the physical and chemical conditions during seismic slip in the crust here we examine frictional melts from five localities by analyzing host rocks and corresponding pseudotachylytes using mossbauer spectroscopy the faults examined are located at south mountain arizona fort foster maine two localities long ridge fault north carolina and the homestake shear zone colorado the main iron bearing phases in the pseudotachylytes are phyllosilicates biotite muscovite and clays and iron oxides magnetite and hematite and minor pyrite the ferrous ferric ratios of the phyllosilicates in the host rocks are the same as those in the pseudotachylytes except for the hematite bearing pseudotachylyte from the long ridge fault which is more oxidized the magnetites in the host rocks and the corresponding pseudotachylytes have different ferric and ferrous iron distributions which is attributed to different cation chemistry rather than redox conditions with the exception of the south mountain locality the ferric ferrous ratios of the micas are interpreted to record the primary redox state of the pseudotachylyte melt as the calculated oxygen fugacities are consistent with magnetite and hematite equilibria pyrite bearing pseudotachylytes plot 0 1 log10 units above the fayalite magnetite quartz fmq buffer magnetite bearing pseudotachylytes plot 2 4 log10 units above the fmq buffer and hematite bearing pseudotachylytes plot 3 5 log10 units above the hematite magnetite hm buffer hematite bearing pseudotachylytes together with previous oxygen isotope data are inferred to represent melting in the presence of externally derived pressurized water other localities are inferred to represent melting under rock buffered closed system conditions if the localities studied are representative of seismogenic faulting the calculated oxygen fugacities indicate that in the system c o h s h2o and co2 should be the dominant fluid species this is the first detailed study of the redox state of pseudotachylytes

the age of unconformity related uranium mineralization in the athabasca basin northern saskatchewan age data are presented for major athabasca basin uranium deposits at cigar lake cluff lake collins bay dawn lake eagle point mcarthur river midwest and rabbit lake as well as for several minor or undeveloped deposits including hughes lake and nisto the best constrained data indicate that almost all the deposits formed in a restricted time interval between about 1330 and 1380 ma this range of ages is believed to be real and not the result of uncertainties in the calculation of ages based on discordant data the one major exception is the recently discovered nias free deposit at mcarthur river for which a well determined age of 1514 18 ma 2 has been obtained even this deposit yields an age in the1330 1380 ma range for some material periods of reworking redeposition occurred at 1280 1000 575 and 225 ma these may be basin wide affecting to some degree all the deposits that we have studied other times of redeposition are less well determined but may be present as well no ages tha

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

the impact of vegetation on ree fractionation in stream waters of a small forested catchment the strengbach case previous studies on waters of a streamlet in the vosges mountains eastern france have shown that strontium and rare earth elements ree mainly originate from preferential dissolution of apatite during weathering however stream water ree patterns normalized to apatite are still depleted in the light ree lree la sm pointing to the presence of an additional lree depleting process vegetation samples are strongly enriched in lree compared to stream water and their sr and nd isotopic compositions are comparable with those of apatite and stream water thus the preferential lree uptake by vegetation might lead to an additional lree depletion of surface runoff in the forested catchment mass balance calculations indicate that the yearly lree uptake by vegetation is comparable with the lree export by the streamlet and therefore might be an important factor controlling lree depletion in river water this is underlined by the observation that rivers from arctic and boreal regions with sparse vegetation appear to be less depleted in lree than rivers from tropical environments or boreal environments with a dense vegetation cover

high ti amphibole as a petrogenetic indicator of magma chemistry evidence for mildly alkalic hybrid melts during evolution of variscan basic ultrabasic magmatism of central iberia central iberian variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous high k calc alkaline ultrabasic to intermediate rock series despite their overall calc alkaline affinity the mafic ultramafic members contain high ti amphibole oikocrysts rimmed by lower ti amphibole cummingtonite and high ti amphibole replacing early phlogopite to understand the factors controlling the saturation of high ti amphibole in the parental magmas clinopyroxene melt phlogopite melt and amphibole melt relationships are reviewed this analysis reveals that for melts with intermediate compositions the affinity of tio2 for amphibole rises in alkalic magmas accordingly mildly alkalic trachytoid to subalkaline medium to high k andesite and dacite compositions are estimated for interstitial high ti amphibole saturated melts amphibole ce pb ratios reveal a mantle crust hybrid nature for interstitial melts with subalkaline trachytoid compositions the hydrous character of the variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of na2o with respect to silica during amphibole crystallization

the eruptive history of the mascota volcanic field western mexico age and volume constraints on the origin of andesite among a diverse suite of lamprophyric and calc alkaline lavas the mascota volcanic field is located in the jalisco block of western mexico where the rivera plate subducts beneath the north american plate it spans an area of 2000 km2 and contains 87 small cones and lava flows of minette absarokite basic hornblende lamprophyre basaltic andesite and andesite there are no contemporary dacite or rhyolite lavas new 40ar 39ar ages are presented for 35 samples which are combined with nine dates from the literature to document the eruptive history of this volcanic field the oldest lavas 2 4 to 0 5 ma are found in the southern part of the field area whereas the youngest lavas predominantly 0 5 ma are found in the northern portion on the basis of these ages field mapping and the use of ortho aerial photographs and digital elevation models it is estimated that a combined volume of 6 8 3 1 km3 erupted in the last 2 4 myr which leads to an average eruption rate of 0 003 km3 kyr and an average volume per eruptive unit of 0 1 km3 the dominant lava type is andesite 2 1 0 9 km3 followed by absarokite 1 6 0 8 km3 basaltic andesite 1 2 0 5 km3 basic hornblende lamprophyre 1 0 0 4 km3 and minette 0 9 0 5 km3 thus the medium k andesite and basaltic andesite comprise approximately half 49 of the erupted magma with twice as much andesite as basaltic andesite and they occur in close spatial and temporal association with the highly potassic lamprophyric lavas there is no time progression to the type of magma erupted a wide variety of evidence indicate that the high mgo 8 9 wt basaltic andesites 52 53 wt sio2 were formed by h2o flux melting of the asthenopheric arc mantle wedge whereas the mafic minettes and absarokites were formed by partial melting induced by thermal erosion of depleted lithospheric mantle containing phlogopite bearing veins there is only limited differentiation of the potassic magmas with none more evolved than 55 4 wt sio2 and 4 4 wt mgo this may be attributable to rapid crystallization of the mantle derived melts in the deep crust owing to their low volumes thus the andesites 58 63 wt sio2 are notable for being both the most voluminous and the most evolved of all lava types in the mascota volcanic field which is not consistent with their extraction from extensively crystallized 60 70 low volume intrusions instead the evidence supports the origin of the andesites by partial melting of amphibolitized mafic lower crust driven by the emplacement of the minettes absarokites and the high mg basaltic andesites

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

oligocene shoshonitic rocks of the rogozna mts central balkan peninsula evidence of petrogenetic links to the formation of pb zn ag ore deposits article this study focuses on age and evolution of the oligocene quartz latite of the rogozna mts central balkan peninsula in order to better understand the link between magmatism and formation of pb zn ag min eralization new 40 ar 39 ar biotite and amphibole plateau ages sugg est that the rogozna mts quartz latite originated through a continuous volcanic episode from 27 3 0 1 to 29 5 0 1 ma which was immediate ly followed by a hydrothermal phase the quartz latites are hypocrystalline porphyritic with phenocrysts and microphenocrysts 60 vol of plagioclase an37 49 biotite mg 100 mg mg fetot b 50 cal cic amphibole quartz sanidine clinopyroxene and phlogopite mg 79 to 84 the rocks display nu merous disequilibrium textures such as sieved plagioclase phenocrysts dissolution effects on quartz phlogopitized biotite and amphibole crystals and phlogopite microphenocrysts showing effects of incomplete growth or dissolution and biotitization the rogozna mts quartz latites are shoshonitic in character with na2o k2ob 1 high lile hfse ratios strong depletions at nb and ti and k pb and u peaks on primitive mantle normalized diagrams they are similar to other potassic ultrapotassic rocks in this region in particular to those of veliki majdan and rudnik west serbia which are also related to pb zn deposits the evolution of the rogozna mts quartz latite is modeled using a trace element bina ry mixing model adopting a lamproite magma and a dacite like calc alkaline melt as end members the model implies that a fractionating magma chamber 4 5 9 5 km undergoes cooling in the range of 850 c 720 c and injection of lamproite like melts the injection causes an increase of temperature and a decrease of viscosity of the resulting hybrid magma facilitating its upwelling and triggering pyro clastic eruptions the addition of new volatiles by lamproitic melts most probably established the condi tions for a hydrothermal phase above the magma chamber that was previously degassed explosively this implies that magma mixing processes can be of great importance for the formation of pb zn deposits similar processes are likely to have occurred in other areas with economically significant pb zn ag other metal mineralization in the region of the central balkan peninsula veliki majdan rudnik golija kopaonik avala etc

origin and fluxes of atmospheric ree entering an ombrotrophic peat bog in black forest sw germany evidence from snow lichens and mosses the fate of the rare earth elements ree were investigated in different types of archives of atmospheric deposition in the black forest southern germany 1 a 70 cm snow pack collected on the domed part of a raised bog and representing 2 months of snow accumulation 2 a snow sample collected close to the road about 500 m from the peat bog 3 two species of lichens and 4 a peat profile representing 400 years of peat accumulation as well as a preanthropogenic sample and the living moss layer from the top of the core ree concentrations in peat are significantly correlated to ti which is a lithogenic conservative element suggesting that ree are immobile in peat bog environments snow lichens and peat samples show similar paas post archean australian shale normalized ree distributions suggesting that the complete atmospheric ree signal is preserved in the peat profile however the annual flux of ree accumulated by the peat is ca 10 times greater than that of the bulk winter flux of ree this difference probably indicates that the ree concentrations in the snowpack are not representative of the average ree flux over the whole year despite the pronounced geological differences between this site granite host rock and a previously studied peat bog in switzerland limestone host rock similar ree distribution patterns and accumulation rates were found at both sites given that both sites confirm an upper continental crust signature the data suggests both sites are influenced by regional and not local soil derived lithogenic aerosols

application of landsat imagery to oil exploration in niobrara formation denver basin wyoming the niobrara formation produces oil from fractures in several places in the denver basin the niobrara is an oil prone mature source rock that entered the oil generating window during the laramide orogeny the laramide orogeny began with maximum compressive stress oriented east northeast during the late cretaceous to paleocene and ended with maximum stress oriented to the northeast in the eocene we believe the eocene phase activated northeast trending extension fractures that may have acted as pathways for migration and loci for storage of oil locally generated in the niobrara theoretically the fracture pressures related to oil generation in the niobrara would preferentially open and fill this northeast trending fracture system using landsat imagery to map fracture in the northern part of the denver basin we have identified areas prospective for niobrara oil production within an exploration fairway that is based on subsurface isopach and resistivity mapping support for this concept is the location of wells reported to produce oil from the niobrara along a zone of northeast trending lineaments

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

impact of atmospheric deposition biogeochemical cycling and water mineral interaction on ree fractionation in acidic surface soils and soil water the strengbach case leaf litter contains not only important quantities of elements derived from decomposition of organic matter but also atmospheric compounds this is illustrated by sr and nd isotope data from soils of the strengbach catchment which show that soil litter contains more than 50 of atmospheric sr and nd downward migrating soil water becomes strongly acidic during flow through litter and humus layers ph 3 8 4 at 5 10 cm depth causing not only intense mobilization of elements in the underlying acid brown and podzolic soils at depths between 10 and 50 cm but also the downward transfer of some litter derived compounds the sr and nd isotope data allow to quantify that soil water from 10 to 70 cm depth contains 45 to 65 of litter derived nd and sr sequential leaching experiments have been performed on soil samples in order to recover adsorbed trace elements or those fixed in acid soluble mineral phases such as fe mn oxyhydroxides and phosphate minerals the isotope data of these leachates of the uppermost soil horizons point similarly to the presence of important quantities of litter derived sr and nd in the uppermost 30 cm of the soils with the exception of the litter layer the soil profiles are substantially depleted in ca and p down to 50 cm depth which can be related to low apatite and plagioclase contents these minerals have probably been preferentially weathered and ca and p have been removed by percolating soil water and plant uptake leaching of primary granite derived apatite controls rare earth element ree fractionation only below 50 cm depth rhabdophane ree po4 has been observed as secondary replacement mineral of apatite this replacement is reflected by ca p ratios that are low at the surface and which increase regularly with depth implying that some of the apatite derived p has been integrated into newly formed rhabdophane and some of the p and ca have been exported either by soil solutions or plant uptake during downward transport litter derived elements are adsorbed on soil particles and or coprecipitate with authigenic minerals such as fe mn oxyhydroxide or rhabdophane alternatively litter derived elements may remain in solution or like ca be absorbed by vegetation the soil solutions from the uppermost 30 cm of the soil profiles and the corresponding soil leachates show very similar ree distribution patterns and are compared to the waters of the strengbach stream which drains the forested catchment strongly enriched in light rare earth elements lree however soil solutions from 60 to 70 cm depth are compared to litter strongly lree depleted and show stream water like ree distribution patterns it is suggested that this evolution of the ree in the soil solutions is due to the following processes taking place above 50 cm depth 1 precipitation of lree rich phosphate minerals like rhabdophane 2 diminution of the formation of organic dissolved or colloidal phases in association with fe mn and al oxyhydroxides and 3 preferential lree uptake by vegetation below 50 cm the ree distribution patterns are further modified by dissolution precipitation reactions and adsorption the combination of all these fractionation processes finally leads to the ree patterns of the strengbach stream at the catchment outlet