Behavior of the rare earth elements in the Oklo natural reactor

A systematic analysis of the abundance and isotopic composition of ten REE (and Ba) in a sample from the reactor zone at the Oklo (Gabon) ore deposit is presented. From this analysis some constraints on the behavior of the REE group during and after the reaction time are determined. Taking into account the complex state of this sample, the main characteristics of the reactor are deduced.From the abundances of isotopes with large neutron capture cross sections the amount of REE (supposed to be natural) added after the reaction time is calculated first. The comparison of the fluence values obtained respectively from the analysis of the Nd and Dy isotopic spectrum indicates a rather good stability of the REE group during the reaction time. The total number (²³⁵U)_f of ²³⁵U nuclei that fissioned during the reaction calculated from different elements of the REE group provides an indication on the relative migration of these elements since the reaction time. Results of these computations suggest a migration of the heavy REE relative to the light REE to outside the reaction zone. As a consequence of these deductions on the behavior of the REE in the Oklo ore, the light REE appear the most appropriate to study the characteristics of the reactor. The reactor characteristics we deduced from this sample (fluence, epithermal index, proportion of nuclides that fissioned, duration of the reactor) are within the range of the ones determined in other studies dealing with REE analysis.     

Formation and geochemical significance of micrometallic aggregates including fissiogenic platinum group elements in the Oklo natural reactor, Gabon

Metallic aggregates with a size of a few tens μm and consisting mainly of Ru, Rh, Pd, Te, Pb, As, Sb, S and Bi were found in the acid residue of SD37-S2/CD uraninite taken from Oklo natural reactor zone (RZ) 13. Quantitative analyses of major elements using an electron probe microanalyzer and in situ isotopic analyses of Zr, Mo, Ru, Pb and U using a sensitive high-resolution ion microprobe were performed on the metallic aggregates to determine the geochemical behaviors of fission products and actinides and to ascertain the processes of formation of the aggregates in the RZs. The chemical compositions of the aggregates investigated in this study are significantly different from those reported previously, showing lower Pb content and no correlation between the contents of Pb and S in the individual grains. The ²³⁵U/²³⁸U ratios in metallic aggregates vary significantly from 0.00478 to 0.01466, indicating chemical fractionation between U and Pu during the formation of the aggregates. The Pb isotopic data indicate that most of the Pb in the aggregates decayed from 2.05 Ga-old uraninite that existed in the RZ originally and that there was chemical fractionation between U and Pb in some aggregates. The Zr and Mo isotopic ratios, ⁹⁰Zr/⁹¹Zr and ⁹⁵Mo/⁹⁷Mo, for most of the aggregates had small variations, which can be simply explained by constant separate mixing of fissiogenic and nonfissiogenic components. On the other hand, a large variation in the ⁹⁹Ru/¹⁰¹Ru ratio (0.324-1.73) cannot be explained only by a two component mixing theory; thus, chemical fractionation between Tc and Ru during the reactor criticality is suggested. The large variations in the ²³⁵U/²³⁸U and ⁹⁹Ru/¹⁰¹Ru isotopic ratios suggest that the aggregates formed under various redox conditions owing to the radiolysis of water.     

Isotopic evidence for trapped fissiogenic REE and nucleogenic Pu in apatite and Pb evolution at the Oklo natural reactor

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 4.4-6.8×10¹⁹ n/cm². 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 ²³⁹Pu 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 ²³⁹Pu 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.

     

Migration behavior of platinum group elements in natural and technogeneous systems

Data were summarized on the speciation of the main platinum group elements (PGE) platinum, palladium, and rhodium in aqueous media, forms of their input into the environment, and mechanisms of accumulation by natural sorptive phases. In some cases, the results obtained for PGE were compared with those for gold. Data on PGE speciation in a number of natural environments were analyzed. It was found that the main factor controlling the migration ability of PGE (Pd > Pt»Rh) is the formation of stable compounds with dissolved organic matter, hydroxyl, and thiosulfate ions and nanometer-sized particles. The transport of dissolved PGE species by marine, riverine, and technogeneous suspended materials was evaluated as an alternative way of an increase in PGE mobility. Significant differences were revealed in the migration behavior of platinum, palladium, and rhodium indicating the dominance of palladium dissipation in a dissolved state. The possibility of the prediction of PGE accumulation and dissipation in technogeneous systems was demonstrated.     

Weathering process at the natural fission reactor of Bangombé

 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 Bangombé, 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 Bangombé 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. Received: 26 November 1999 · Accepted: 2 May 2000     

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

The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO₂ in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombé, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones.The analyzed uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2σ). Thus, the first major Pb loss from uraninite occurred at both Oklo and Bangombé during regional extension and the intrusion of a dyke swarm in the Franceville basin, ∼860-890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the “chemical” ages of the uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that uraninite at both the Oklo and Bangombé natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the uraninite structure.     

Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombé, 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, françoisite, uraniferous goethite, and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at Bangombé, Gabon. Phosphate minerals such as phosphatian coffinite and françoisite 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.     

Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon)—inferences from U, REE, Zr, Mo and Se isotopes

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.     

稀土元素在热液中的迁移与沉淀

近年来,高新技术产业的不断发展使得全球稀土资源需求不断上涨,此外稀土元素作为研究地球起源与演化等有关地球科学问题的重要工具也深受地球科学界的重视。而热水溶液体系中稀土元素迁移、富集、沉淀及分馏机制等相关理论知识的不断完善对于理解稀土成矿作用、稀土地球化学特征以及微量元素示踪等方面均具有重要意义。本文总结了近年来有关稀土元素在热液体系中迁移、沉淀以及轻、重稀土分馏的最新研究进展,对不同稀土络合物的稳定性、溶解度以及搬运稀土元素的能干性进行了阐述,最后展望了"稀土元素在热液中迁移与沉淀"研究中仍需完善及拓展的方向。     

Migration of transuranic elements in groundwater from the near-surface radioactive waste site

Field experiments and laboratory studies were performed to investigate migration processes of plutonium isotopes from a near-surface radioactive waste trench to the underlying sandy aquifer at the Red Forest waste dump in the Chernobyl zone. The objectives of these experiments were to characterize the spatial distribution and possible migration mechanisms of plutonium in the aquifer. During 2002–2007 experimental investigations were carried out and spatial distributions of plutonium isotopes (^239,240Pu, ²³⁸Pu), ⁹⁰Sr and major ions in the aquifer in the direction of the groundwater flow were obtained. Specific activities of radionuclides in groundwater depended on the location of the piezometer and varied in the range of 1–360 mBq kg⁻¹ for ^239,240Pu, 0.5–180 mBq kg⁻¹ for ²³⁸Pu and n–n·10⁴ Bq kg⁻¹ for ⁹⁰Sr. It was found that the spatial features of the distributions of plutonium and strontium specific activities in the upper eolian aquifer were similar, i.e. there was a correlation between the positions of the activity maxima of the radionuclides. The Pu isotopes plume in the aquifer spreads about 15 m downstream of the radionuclides source. Characterization of the initial radionuclide composition of the waste showed that all plutonium in the aquifer originated from the trench. The ratio of plutonium isotopes (^239,240Pu/²³⁸Pu) at the sampling time was the same in waste material and in groundwater samples. In situ ultrafiltration of several groundwater samples was carried out. The size fractionation data obtained suggest that a significant part of plutonium (50–98%) in the groundwater sampled close to the source from the upper part of the aquifer is associated with a very low molecular weight fraction (<1 kDa).     

Migration and contamination of major and trace elements in groundwater of Madras city,India

Groundwater samples collected from both open and bore wells in an area of about 270 km² from Madras City, India, have been analyzed for major ions (HCO₃, Cl, Si, Na, Ca, and Mg) and trace elements (As, Se, B, V, Cr, Fe, Co, Pb, Cu, Zn, Cd, Mn, Ni, Mo, and Ba). The study reveals that the quality of potable water has deteriorated to a large extent. Seawater intrusion into the aquifer has been observed in nearly 50 percent of the study area. The toxic elements (As and Se) have already exceeded the maximum permissible limits of drinking water in almost the entire city. A positive correlation of As and Se with other toxic metals such as V, Cr, Fe, B, etc., indicates that all these elements are anthropogenic in origin. Applying multivariate analysis, the source for trace elements in groundwater has been grouped into two major factors: pollution and mobilization factors. The groundwater in the study area is largely contaminated by organic effluents and reflects the intensity of pollution caused by the overlying soil sediment and rapid infiltration of the pollutants.     

青藏高原自然资源要素综合观测实施进展与展望

青藏高原不仅是中国重要的生态安全屏障,在全球自然地理环境中也扮演着重要的角色.然而,受限于严酷的自然环境,青藏高原的自然资源要素观测数据较为缺乏.在青藏高原组建自然资源要素综合观测网络,建立一支长期稳定的综合观测专业队伍,开展全要素、多尺度、持续性的系统观测,是一项十分必要且紧迫的工作.从站网布设思路、观测指标体系、重...     

Stabilisation of divalent rare earth elements in natural fluorite

Summary ?The occurrence of divalent rare earth elements (Sm²⁺, Yb²⁺, Tm²⁺, and Ho²⁺) in natural fluorite is evaluated using a suite of 37 samples deriving mainly from Sn–W deposits in the Erzgebirge (Germany), Central Kazakhstan, and the Mongolian Altai. Trace element composition was determined by ICP-AES and ICP-MS. The defect structure of the samples was studied by cathodoluminescence (CL), electron paramagnetic resonance (EPR), and optical absorption spectroscopy. Reduction of cubic Sm³⁺, Yb³⁺, Tm³⁺, and Ho³⁺ under radioactive irradiation produces the corresponding divalent centres. Our data suggest a preferable formation of Sm²⁺ and Yb²⁺ under thorium and of Tm²⁺ and Ho²⁺ under uranium irradiation. Irradiation (indicated by intense brownish (thorium) and deep purple (uranium) coloration of fluorite) gives rise to a population of divalent centres in equilibrium with their decay. However, sporadic radioactive irradiation and stabilisation of the divalent state of the REE by other electron defects were found in most cases. Three models of stabilisation of Sm²⁺, Yb²⁺, Tm²⁺, and Ho²⁺ are discussed. The most effective mechanism for Sm, Yb, Tm, and Ho is coupling with Fe³⁺ centres (REE³⁺+Fe²⁺ → REE²⁺+Fe³⁺). Accordingly, the occurrence of Fe³⁺ centres in natural fluorite is regarded to indicate not an oxidising, but rather a reducing environment during fluorite precipitation. Originally incorporated in the divalent form, Fe²⁺ was converted to Fe³⁺ by radioactive irradiation. Such a conclusion is in agreement with the finding of high contents of interstitial fluorine providing tetragonal local compensation of trivalent REE centres in crystals with high Fe³⁺. If Fe is not present, compensation of divalent Sm, Yb, and Tm is achieved by radiogenic oxidation of Ce(Pr, Tb)³⁺ accompanied by charge transfer (REE³⁺+Ce(Pr, Tb)³⁺ → REE²⁺+ Ce(Pr, Tb)⁴⁺). Ho²⁺ is sometimes stabilised by a hole trapped by an electron localised on a F vacancy (Ho³⁺+e⁻ on □_F → REE²⁺+ self-trapped exciton). Because Sm²⁺ is optically active, the stabilisation by Fe³⁺ (stable up to temperatures above 350 °C) or Ce(Pr, Tb)⁴⁺ (unstable even under visible light) in samples may be determined by careful observations in the field. Institut für Geotechnik, ETH Zürich, ETH-H?nggerberg, Zürich, Switzerland Stanford Linear Accelerator Center, Menlo Park, CA, USA Received January 8, 2002; revised version accepted June 10, 2002     

SHRIMP in situ isotopic analyses of REE,Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé 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 Bangombé 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.     

河北开滦矿区煤洗选过程中 15种主要有害微量元素的迁移和分配特征

以河北开滦矿区晚古生代煤及其洗选产品为例,运用电感耦合等离子体质谱 (ICP-MS)和冷原子吸收光谱 (CV-AAS)方法对煤中主要微量有害元素 (包括 Be、 Cr、 Co、 Ni、 Cu、 Zn、 Ga、 As、 Se、 Mo、 Tl、 Pb、 Th、 U和 Hg)的含量及其在洗选过程中的迁移和分配特征进行了研究.通过对开滦矿区 10个矿井 47个煤层刻槽样品主要有害微量元素含量的统计,发现开滦矿区晚古生代煤中 Cr、 Ni、 Cu、 Zn、 Pb和 As富集.通过对原煤精煤中煤尾煤煤泥两套系列样品的分析,发现主要有害微量元素在精煤中都有不同程度的脱除,中煤中相对富集的元素有 Se和 Th,明显被脱除的元素有 Co和 Tl.除 Tl以外,尾煤中主要微量有害元素均有不同程度的富集,以 As的富集率最高,可达 34.4%, Hg的富集率最低,为 1.1%.煤泥中所有微量有害元素均相对富集,以 Hg的相对富集率最高,达 78.4%.因此,如果对尾煤和煤泥加以利用,需要特别关注 As和 Hg等微量有害元素对环境的影响.主要微量有害元素在洗选过程中的分配行为主要受控于它在煤中的赋存状态.     

Mobility and retention of trace elements in hardpan-cemented cassiterite tailings, north Queensland, Australia

This study reports on the mobility and retention of trace elements in cassiterite tailings at the inactive Jumna mill, tropical north Queensland. Since the 1980s, the uncapped tailings have developed laterally discontinuous Fe-rich hardpans, which are located in the higher parts of gently sloping tailings masses and at the top (<50 cm) of the tailings piles. Hardpan-cemented tailings comprise thin layers (typically ∼0.2–2 mm thick) of HFO (hydrous ferric oxides) and sulfate efflorescences cementing tailings grains. In comparison to the tailings, the hardpan-cemented tailings contain significantly higher median As, Ca, Cd, Co, Cu, Fe, In, Mn, Mo, S_total, Th, U, Y and Zn values. Partial leaching studies of tailings and pond water analyses indicate that wetting and acidification of Fe-cemented tailings removes significant proportions of trace elements into pore and surface waters. Tin shows no mobility due to the presence of weathering-resistant cassiterite (SnO₂) and, As and Pb display limited mobility possibly due to their coprecipitation with jarosite-type phases and HFO materials at the top of the tailings profile. By contrast, the trace elements Cd, Ce, Cu, La, Ni, Pb, U and Zn display the greatest mobility, possibly due to their incorporation in soluble sulfate efflorescences and sorption onto mineral and HFO surfaces. Hence, the Fe-rich hardpans do not protect the sulfidic tailings from further oxidation nor do they cause permanent sequestration of trace elements.     

自然资源要素综合观测一体化平台建设探索与实践

自然资源要素综合观测一体化平台是集站网运维管理和观测数据采集、传输、保存、挖掘、共享及成果产品于一体的平台,是观测站网的核心.从总体框架、管理系统、特点及优势3方面介绍了自然资源要素综合观测一体化平台的建设情况;结合开展项目,通过数据集成、多源数据融合和模型加载应用阐述了自然资源要素综合观测一体化平台的建设实践;最后,对一体化平台今后工作的重点、方向和技术应用进行了展望.     

全国自然资源要素综合观测体系建设需求及发展动态

自然资源要素综合观测工程围绕国家重大战略需求和自然资源部"两统一"职责展开,是一项具有战略性、基础性、紧迫性的系统工程,合理分析自然资源要素综合观测体系构建的各项需求,进一步明确建设思路,厘清目前发展动态,有利于自然资源要素综合观测工程下一步工作的开展.通过需求分析,明确了全国自然资源要素综合观测体系建设的各项需求,并...