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Mostafa Fayek - One of the best experts on this subject based on the ideXlab platform.
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mass bias corrections for u pb isotopic analysis by secondary ion mass spectrometry implications for u pb dating of Uraninite
Rapid Communications in Mass Spectrometry, 2016Co-Authors: Ryan Sharpe, Mostafa FayekAbstract:RATIONALE: Uranium (U)-lead (Pb) geochronology of Uraninite is critical to the study of the genesis of U deposits throughout the world. Previous attempts at developing a technique to date Uraninite using secondary ion mass spectrometry (SIMS) have, however, had limited success. Improper correction of mass bias results in incorrect reported U-Pb ratios. When these results are plotted on Concordia diagrams they produce erroneous ages, complicating the study of U deposits. METHODS: Uranium and Pb isotope ratios were measured in three Uraninite reference materials (RMs) with varying Pb content and three samples with unknown U-Pb isotope compositions using a CAMECA 7f SIMS instrument. Measurements were made using a primary beam of O(-) accelerated at 12.5 kV. A mass resolving power of 1300 and a 50-V offset were used to minimize interferences. RESULTS: The study demonstrates that the mass bias for U-Pb isotope ratio measurements in Uraninite by SIMS varies as a function of Pb content. A three-point calibration curve was developed using Uraninite RMs with low-, intermediate- and high-Pb contents. Corrected ratios for both concordant and discordant Uraninite were plotted on a Concordia diagram to demonstrate the effect that different correction techniques have on the resulting age. CONCLUSIONS: Accurate determination of U-Pb ratios in Uraninite using a SIMS instrument requires a suite of RMs with varying Pb content and the construction of a calibration curve, or a Uraninite RM with a Pb content similar to that of the unknowns. Failure to standardize correctly will result in erroneous ages being calculated using Concordia diagrams. Copyright © 2016 John Wiley & Sons, Ltd.
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in situ sims Uraninite u pb dating and genesis of the xianshi granite hosted uranium deposit south china
Ore Geology Reviews, 2015Co-Authors: Ruizhong Hu, Mostafa Fayek, Chusi Li, Xianwu Bi, Yassir A Abdu, Youwei ChenAbstract:Abstract The southeastern part of the Nanling metallogenic province, China is host to numerous granite-hosted vein-type hydrothermal uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because the uranium minerals in these deposits are usually fine grained, and may have formed in several stages. Therefore, the ages previously obtained by the bulk dating techniques are possibly a mixed age. The Xianshi uranium deposit, located in the southeastern part of the Guidong granite complex, is a major uranium deposit in South China. The uranium mineralization from this deposit is mainly fine grained Uraninite in quartz or calcite veins which are spatially associated with the Cretaceous mantle-derived mafic dykes. Micro-Raman spectroscopy and X-ray diffraction analyses indicate that the dominant uranium mineral occurs as a rare form of Uraninite (U 3 O 7 ). Three distinct generations of uranium minerals have been identified based on petrographic and field relations. Stage 1 Uraninite has the lowest UO 2 and highest PbO contents whereas Stage 3 Uraninite has the highest UO 2 and lowest PbO contents. Uraninite from the Xianshi deposit was dated using an in-situ SIMS U–Pb dating technique. The results show three distinct age groups: 135 ± 4 Ma, 113 ± 2 Ma and 104 ± 2 Ma, which are in excellent agreement with the ages of three episodes of mantle-derived mafic dykes. Therefore, the Xianshi uranium deposit has experienced at least three hydrothermal events that are responsible for the deposition of uranium ores, which are genetically related to the emplacement of three sets of mafic dykes.
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THE WORLD’S OLDEST OBSERVED PRIMARY Uraninite
Canadian Mineralogist, 2011Co-Authors: Ryan Sharpe, Mostafa FayekAbstract:Secondary ion mass spectrometry (SIMS), an in situ micro-analytical technique, was used to date Uraninite from the Huron Claim pegmatite in southeastern Manitoba. Detailed petrography shows that there are zones in the Uraninite grains that have undergone varying degrees of alteration. Altered and relatively unaltered zones were analyzed on the scale of a few micrometers. In situ U–Pb isotopic analysis shows that the Uraninite from the Huron Claim pegmatite is highly discordant (>5%) with an upper intercept of 2575 ± 38 Ma. Lead (207Pb/206Pb) isotope ages range from 2514 ± 3 to 2573 ± 18 Ma, with an average age of 2550 ± 17 Ma. Therefore, the crystallization age of the Uraninite is interpreted to be 2575 ± 38 Ma. A comparison of this age with Uraninite from other deposits suggests that the Huron Claim pegmatite Uraninite is currently the oldest known example of primary Uraninite. Evidence for the presence of very old primary Uraninite is an important part of the global evolution of uranium minerals. Detrital accumulations such as the Witwatersrand and Elliot Lake deposits have inferred ages much greater than 2.2 Ga. However, no primary Uraninite approaches the age of these deposits except for the Huron Claim Uraninite.
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the world s oldest observed primary Uraninite
Canadian Mineralogist, 2011Co-Authors: Ryan Sharpe, Mostafa FayekAbstract:Secondary ion mass spectrometry (SIMS), an in situ micro-analytical technique, was used to date Uraninite from the Huron Claim pegmatite in southeastern Manitoba. Detailed petrography shows that there are zones in the Uraninite grains that have undergone varying degrees of alteration. Altered and relatively unaltered zones were analyzed on the scale of a few micrometers. In situ U–Pb isotopic analysis shows that the Uraninite from the Huron Claim pegmatite is highly discordant (>5%) with an upper intercept of 2575 ± 38 Ma. Lead (207Pb/206Pb) isotope ages range from 2514 ± 3 to 2573 ± 18 Ma, with an average age of 2550 ± 17 Ma. Therefore, the crystallization age of the Uraninite is interpreted to be 2575 ± 38 Ma. A comparison of this age with Uraninite from other deposits suggests that the Huron Claim pegmatite Uraninite is currently the oldest known example of primary Uraninite. Evidence for the presence of very old primary Uraninite is an important part of the global evolution of uranium minerals. Detrital accumulations such as the Witwatersrand and Elliot Lake deposits have inferred ages much greater than 2.2 Ga. However, no primary Uraninite approaches the age of these deposits except for the Huron Claim Uraninite.
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o and h diffusion in Uraninite implications for fluid Uraninite interactions nuclear waste disposal and nuclear forensics
Geochimica et Cosmochimica Acta, 2011Co-Authors: Mostafa Fayek, Lawrence M Anovitz, David R Cole, Debra A BostickAbstract:Abstract Diffusion coefficients for oxygen and hydrogen were determined from a series of natural Uraninite–H 2 O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400–600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e −5 exp (−123,382 J/RT) cm 2 /s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e −10 exp (−62484 J/RT) cm 2 /s, where the activation energies for Uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in Uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e −6 exp (−156,528 J/RT) cm 2 /s for temperatures between 450 and 700 °C and D = 1.39e −14 exp (−34518 J/RT) cm 2 /s for temperatures between 50 and 400 °C, where the activation energies for Uraninite are 156.5 and 34.5 kJ/mol, respectively. Results from these new experiments have implications for isotopic exchange during natural UO 2 –water interactions. The exceptionally low δ 18 O values of natural Uraninites (i.e. − 32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental Uraninite–water and UO 3 –water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ 18 O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ 18 O values of ca. −18‰, despite petrographic and U–Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the Uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for Uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material.
Rodney C. Ewing - One of the best experts on this subject based on the ideXlab platform.
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fate of trace elements during alteration of Uraninite in a hydrothermal vein type u deposit from marshall pass colorado usa
Geochimica et Cosmochimica Acta, 2007Co-Authors: Artur P Deditius, Satoshi Utsunomiya, Rodney C. EwingAbstract:Alteration of Uraninite from a hydrothermal vein-type U-deposit in Marshall Pass, Colorado, has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements W, As, Mo, Zr, Pb, Ba, Ce, Y, Ca, Ti, P, Th, Fe, Si, Al, during alteration, under both reducing and oxidizing conditions. The release of trace elements from Uraninite is used to establish constraints on the release of fission product elements from the UO2 in spent nuclear fuels. Uraninite occurs with two different textures: (1) colloform Uraninite and (2) fine-grained Uraninite. The colloform Uraninite contains 1.04-1.75 wt% of WO3, 0.16-1.70 wt% of As2O3, 0.06-0.88 wt% of MoO3; whereas, the fine-grained Uraninite retains 2.25-4.93 wt% of WO3, up to 5.76 wt% of MoO3, and 0.26-0.60 wt% of As2O3. The near constant concentration of incompatible W in the colloform Uraninite suggests W-incorporation into the Uraninite structure or homogeneous distribution of W-rich nano-domains. Incorporation of W and Mo into the Uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of Cs during the corrosion of spent nuclear fuel, as there is a strong affinity of Cs with W and Mo. Zoning in the colloform texture is attributed to variation in the amount of impurities in Uraninite. For unaltered zones, the calculated amount of oxygen ranges from 2.08 to 2.32 [apfu, (atom per formula unit)] and defines the stoichiometry as UO2+x and U4O9; whereas, for the altered zones of the colloform texture, the oxygen content is 2.37-2.48 [apfu], which is probably due to the inclusion of secondary uranyl phases, mainly schoepite. The supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of Uraninite. Four stages of colloform Uraninite alteration are proposed: (i) formation of an oxidized layer at the rim, (ii) corrosion of the oxidized layer, (iii) precipitation of U6+-phases with well-defined cleavage, and (iv) fracture of the Uraninite surface along the cleavage planes of the U6+-phases.
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Uraninite recrystallization and pb loss in the oklo and bangombe natural fission reactors gabon
Geochimica et Cosmochimica Acta, 2005Co-Authors: Lena Z Evins, Keld Alstrup Jensen, Rodney C. EwingAbstract:Abstract The Oklo and Bangombe natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of Uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO 2 in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombe, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones. The analyzed Uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic Uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser Uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2σ). Thus, the first major Pb loss from Uraninite occurred at both Oklo and Bangombe during regional extension and the intrusion of a dyke swarm in the Franceville basin, ∼860–890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the “chemical” ages of the Uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor Uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO Uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that Uraninite at both the Oklo and Bangombe natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the Uraninite structure.
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oxygen isotopic composition of nano scale Uraninite at the oklo okelobondo natural fission reactors gabon
American Mineralogist, 2003Co-Authors: Mostafa Fayek, Rodney C. Ewing, Keld Alstrup Jensen, Satoshi Utsunomiya, Lee R RiciputiAbstract:High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that Uraninite from the Oklo-Okelobondo natural fission reactors that occur in near surface environments, have low δ 1 8 O values and nanotextures that are consistent with interaction with ground water. These low δ 1 8 O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and Uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and post-depositional alteration of U-bearing phases.
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O and Pb isotopic analyses of uranium minerals by ion microprobe and U-Pb ages from the Cigar Lake deposit
Chemical Geology, 2002Co-Authors: Mostafa Fayek, T. Mark Harrison, Rodney C. Ewing, Marty Grove, Christopher D. CoathAbstract:Abstract We apply a rapid and accurate in situ technique to make U–Pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of Uraninite from the unconformity-type Cigar Lake uranium deposit. Secondary uranium minerals intergrown with Uraninite, such as coffinite, USiO4·nH2O and calciouranoite, CaU2O7·5H2O, were identified by high-resolution transmission electron microscopy (HRTEM). In situ U–Pb results from three stages of Uraninite and coffinite define well-correlated arrays on concordia with upper intercepts of 1461±47, 1176±9, and 876±14 Ma (±1σ). These ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration (∼1477 Ma) associated with these unconformity-type uranium deposits, the ages of magnetization events at 1600–1450 and 900 Ma from the Athabasca Basin, and the Grenvillian Orogeny at ∼1100 Ma. In situ U–Pb isotopic analyses of Uraninite and coffinite can document the Pb*/U heterogeneities that occur on the scale of 15–30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits. The high spatial resolution and precision of the ion microprobe allow us to measure δ18O values of 20–100 μm unaltered portions of Uraninites from Cigar Lake. The range of δ18O values (−33.9 to −20.4‰) are among the lowest reported for unconformity-type deposits, confirming that conventional fluorination analyses of material sampled at the mm-scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite.
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in situ isotopic analysis of Uraninite microstructures from the oklo okelobondo natural fission reactors gabon
MRS Proceedings, 2002Co-Authors: Mostafa Fayek, Rodney C. Ewing, Kathleen Jensen, Lee R RiciputiAbstract:Uranium deposits can provide important information on the long-term performance of radioactive waste forms because Uraninite (UO 2+X ) is similar to the UO 2 in spent nuclear fuel. The Oklo-Okelobondo U-deposits, Gabon, serve as natural laboratory where the long-term (hundreds to billions of years) migration of uranium and other radionuclides can be studied over large spatial scales (nm to km). The natural fission reactors associated with the Oklo- Okelobondo U-deposits occur over a range of depths (100 to 400 m) and provide a unique opportunity to study the behavior of Uraninite in near surface oxidizing environments versus more reducing conditions at depth. Previously, it has been difficult to constrain the timing of interaction between U-rich minerals and post-depositional fluids. These problems are magnified because Uraninite is susceptible to alteration, it continuously self-anneals radiation damage, and because these processes are manifested at the nm to μm scale. Uranium, lead and oxygen isotopes can be used to study fluid-Uraninite interaction, provided that the analyses are obtained on the micro-scale. Secondary ionization mass spectrometry (SIMS) permits in situ measurement of isotopic ratios with a spatial resolution on the scale of a few μm. Preliminary U-Pb results show that Uraninite from all reactor zones are highly discordant with ages aaproaching the timing of fission chain reactions (1945±50 Ma) and resetting events at 1180±47 Ma and 898±46 Ma. Oxygen isotopic analyses show that Uraninite from reactors that occur in near surface environments (δ 18 O= −14.4‰ to −8.5‰) have reacted more extensively with groundwater of meteoric origin relative to reactors located at greater depths (μ 18 O= −10.2‰ to −7.3‰). This study emphasizes the importance of using in situ high spatial resolution analysis techniques for natural analogue studies.
Keld Alstrup Jensen - One of the best experts on this subject based on the ideXlab platform.
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Uraninite recrystallization and pb loss in the oklo and bangombe natural fission reactors gabon
Geochimica et Cosmochimica Acta, 2005Co-Authors: Lena Z Evins, Keld Alstrup Jensen, Rodney C. EwingAbstract:Abstract The Oklo and Bangombe natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of Uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO 2 in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombe, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones. The analyzed Uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic Uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser Uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2σ). Thus, the first major Pb loss from Uraninite occurred at both Oklo and Bangombe during regional extension and the intrusion of a dyke swarm in the Franceville basin, ∼860–890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the “chemical” ages of the Uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor Uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO Uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that Uraninite at both the Oklo and Bangombe natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the Uraninite structure.
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oxygen isotopic composition of nano scale Uraninite at the oklo okelobondo natural fission reactors gabon
American Mineralogist, 2003Co-Authors: Mostafa Fayek, Rodney C. Ewing, Keld Alstrup Jensen, Satoshi Utsunomiya, Lee R RiciputiAbstract:High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that Uraninite from the Oklo-Okelobondo natural fission reactors that occur in near surface environments, have low δ 1 8 O values and nanotextures that are consistent with interaction with ground water. These low δ 1 8 O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and Uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and post-depositional alteration of U-bearing phases.
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Uraninite a 2 ga spent nuclear fuel from the natural fission reactor at bangombe in gabon west africa
MRS Proceedings, 1996Co-Authors: Keld Alstrup Jensen, Rodney C. Ewing, Francois GauthierlafayeAbstract:Uraninites from the Bangombe natural fission reactor (RZB) and normal uranium-ore occur as fine veins in the sandstone host-rock as well as altered, broken, and slightly displaced grains in an illitic matrix, and in nodules and veins of solid bitumen. Inclusions of galena, (Y,Gd)-rich phosphates, a Pb-oxide and a Ti-oxide? were observed. Uraninites just below RZB were partially altered to a uranyl-sulfate. Three generations of Uraninite were identified based on their PbO-contents of 8--11.06 wt%, 6 wt% (the largest population), and a younger generation with 3 wt%. Diffusional loss of Pb is indicated by the presence of a Pb-oxide at the interface to the Uraninites. The behavior of the metallic fission products, incompatible with the Uraninite structure, may mimic the behavior of Pb in these Uraninites. The averaged impurity-content ranges from 4.29 to 6.89 wt%, and consists mainly of SiO{sub 2}, TiO{sub 2}, ZrO{sub 2}, FeO, CaO, Al{sub 2}O{sub 3} and P{sub 2}O{sub 5}. The averaged content of Y{sub 2}O{sub 3} and the Ln`s is less than 0.78 wt% and there is a scattered positive correlation with P{sub 2}O{sub 5}. The content of Y + Ln`s is generally highest in the Uraninites from RZB. Uraninite hydration and the formation ofmore » uranopelite/zippeite have caused complete loss of Y and the Ln`s. The analytical results indicate that Y and the Ln`s, which are high yield fission products, may be released from Uraninite during alteration in the presence of P.« less
Lee R Riciputi - One of the best experts on this subject based on the ideXlab platform.
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oxygen isotopic composition of nano scale Uraninite at the oklo okelobondo natural fission reactors gabon
American Mineralogist, 2003Co-Authors: Mostafa Fayek, Rodney C. Ewing, Keld Alstrup Jensen, Satoshi Utsunomiya, Lee R RiciputiAbstract:High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that Uraninite from the Oklo-Okelobondo natural fission reactors that occur in near surface environments, have low δ 1 8 O values and nanotextures that are consistent with interaction with ground water. These low δ 1 8 O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and Uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and post-depositional alteration of U-bearing phases.
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in situ isotopic analysis of Uraninite microstructures from the oklo okelobondo natural fission reactors gabon
MRS Proceedings, 2002Co-Authors: Mostafa Fayek, Rodney C. Ewing, Kathleen Jensen, Lee R RiciputiAbstract:Uranium deposits can provide important information on the long-term performance of radioactive waste forms because Uraninite (UO 2+X ) is similar to the UO 2 in spent nuclear fuel. The Oklo-Okelobondo U-deposits, Gabon, serve as natural laboratory where the long-term (hundreds to billions of years) migration of uranium and other radionuclides can be studied over large spatial scales (nm to km). The natural fission reactors associated with the Oklo- Okelobondo U-deposits occur over a range of depths (100 to 400 m) and provide a unique opportunity to study the behavior of Uraninite in near surface oxidizing environments versus more reducing conditions at depth. Previously, it has been difficult to constrain the timing of interaction between U-rich minerals and post-depositional fluids. These problems are magnified because Uraninite is susceptible to alteration, it continuously self-anneals radiation damage, and because these processes are manifested at the nm to μm scale. Uranium, lead and oxygen isotopes can be used to study fluid-Uraninite interaction, provided that the analyses are obtained on the micro-scale. Secondary ionization mass spectrometry (SIMS) permits in situ measurement of isotopic ratios with a spatial resolution on the scale of a few μm. Preliminary U-Pb results show that Uraninite from all reactor zones are highly discordant with ages aaproaching the timing of fission chain reactions (1945±50 Ma) and resetting events at 1180±47 Ma and 898±46 Ma. Oxygen isotopic analyses show that Uraninite from reactors that occur in near surface environments (δ 18 O= −14.4‰ to −8.5‰) have reacted more extensively with groundwater of meteoric origin relative to reactors located at greater depths (μ 18 O= −10.2‰ to −7.3‰). This study emphasizes the importance of using in situ high spatial resolution analysis techniques for natural analogue studies.
Kurtis T Kyser - One of the best experts on this subject based on the ideXlab platform.
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measurement of u pb ages of Uraninite and davidite by laser ablation hr icp ms
American Mineralogist, 2007Co-Authors: Don Chipley, Paul A Polito, Kurtis T KyserAbstract:Laser-ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS) is a rapid, accurate, and inexpensive technique for making in situ U-Pb isotopic measurements of Uraninite and davidite. The advantages of this method include: (1) mineral separation and chemical digestion are not required; (2) measurements on complex samples are feasible because significant isobaric interferences can be resolved; and (3) accurate U-Pb and 207Pb/206Pb dates on 10–25 μm spots can be obtained rapidly. The LA-HR-ICP-MS method is applied to U oxide minerals from four deposits and prospects from northern Australia, and the new dates are compared to previously published conventional thermal ionization mass spectroscopy (TIMS) dates, and to known ages of geologically important events. These comparisons permit us to assess elemental fractionation of U and Pb for Uraninite and davidite of the new method, relative to zircon, as well as its geochronological accuracy and precision. U-Pb apparent ages measured previously agree well with our measurements for El Sherena, Palette, and Mt. Isa. Additionally, the upper-intercept 207Pb/206Pb dates for Adelaide River (701 ± 190 Ma, 1σ) and Palette (841 ± 94 Ma, 1σ) Uraninite, measured here, are similar to those previously obtained for Palette (730 Ma), Nabarlek (920 Ma), and Koongarra (870 Ma), and the upper-intercept date for El Sherena Uraninite (1573 ± 160 Ma) is within error of that previously determined for Ranger (1550 ± 15 and 1472 ± 40 Ma). Such apparent-age agreement for Uraninite (and similarly for davidite) indicates that U and Pb fractionations are within error of that for zircon, whereas the inherent imprecision of our dates and their associated MSWD values greater than 2.5 probably indicate that multiple resetting events affected our samples. Analytically, these results demonstrate that the LA-HR-ICP-MS technique provides excellent spatial resolution while also removing argide, phosphide, sulfide, and halide interferences that can otherwise lead to erroneous data when using quadrupole-ICP-MS. Geologically, the individual 207Pb/206Pb and upper-intercept U-Pb dates of Uraninite from Adelaide River and Palette are ca. 800 Ma, possibly reflecting recrystallization of Uraninite during the break-up of Rodinia.
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low temperature oxygen isotopic fractionation in the Uraninite uo3 co2 h2o system
Geochimica et Cosmochimica Acta, 2000Co-Authors: Mostafa Fayek, Kurtis T KyserAbstract:Oxygen-isotope fractionation factors for Uraninite–water and UO3–water were determined from a series of Uraninite–CO2 and UO3–CO2 exchange experiments between 100 and 300°C. The measured fractionation factors are similar to the theoretical fractionation factors of Hattori and Halas (1982) at high temperatures and Zheng (1991) at low temperatures and are nonlinear over the temperature range examined. Regression through these curve gives 1000 ln αUO2–H2O = 16.58 (106/T2) − 77.52 (103/T) + 77.48, 1000 ln αUO3–H2O = −2.21 (106/T2) + 25.06 (103/T) − 45.90 with 2σ errors of ±2.5‰. Diffusion coefficients of oxygen with temperature for Uraninite and UO3 can be represented by D(Uraninite) = 3.1 × 10−10 exp (−63981/RT) and D(UO3) = 1.3 × 10−15 exp (−21,197/RT), where the activation energies for Uraninite and UO3 are 64.0 ± 0.2 kcal/mole and 21.2 ± 0.2 kcal/mol, respectively. Exceptionally low δ18O values of natural Uraninites (i.e., −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental Uraninite–water and UO3–water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ18O values have been interpreted as having resulted from the low temperature recrystallization of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ18O values of ca. −18‰, despite petrographic and U–Pb isotope data that indicate limited alteration. Therefore, it appears that uranium minerals can exchange oxygen isotopes with later fluids, with only limited actinide remobilization and minor disturbances to their original chemical compositions and textures.
