Dissolved Uranium and 234U/238U in the Yamuna and the Chambal Rivers, India

Dissolved uranium concentration and ²³⁴U/²³⁸U activity ratio have been measured in two distinctly different Indian drainage systems: the Yamuna headwaters in the Himalaya and the Chambal river system in the plains to study the weathering and mobility of uranium in these watersheds. The dissolved uranium in the Chambal river system ranges from 0.2 to 1.74 μg L⁻¹ during September (tail end of monsoon), whereas in the Yamuna river system, its concentration varies from 0.1 to 3.18 μg L⁻¹ during October (post-monsoon) and from 0.09 to 3.61 μg L⁻¹ in June (summer). In the Yamuna main stream, uranium is highest at its source and decreases steadily along its course, from 3.18 μg L⁻¹ at Hanuman Chatti to 0.67 μg L⁻¹ at Batamandi, at the base of the Himalaya. This decrease results mainly from mixing of the Yamuna mainstream with its tributaries, which are lower in uranium. The high concentration of uranium at Hanuman Chatti is derived from weathering of the Higher Himalayan Crystalline series (HHC) and associated accessary minerals, which may include uranium-mineralised zones. The ²³⁴U/²³⁸U activity ratios in the samples from the Chambal watershed are in the range of 1.15±0.05 to 1.67±0.04; whereas in the Yamuna the ratios vary from 0.95±0.03 to 1.56±0.07, during post-monsoon and from 0.98±0.01 to 1.30±0.03, during summer. The relatively high ²³⁴U/²³⁸U activity ratios in the Yamuna system are in its tributaries from the lower reaches viz., the Amlawa, Aglar, Bata, Tons and the Giri. It is estimated that ~9×10³ and ~12 × 10³ kg of dissolved uranium are transported annually from the Yamuna at Batamandi and the Chambal at Udi, respectively. This corresponds to uranium weathering rates of 0.9 and 0.09 kg U km⁻² y⁻¹ in the basins of the Yamuna and the Chambal headwaters. This study confirms that uranium weathering rate in the Himalaya is far in excess (by about an order of magnitude) of the global average value of ~0.08 kg U km⁻² y⁻¹.     

A study of234U/238U ratios in groundwaters of Taiyuan area,Shanxi Province

Uranium contents and²³⁴U/²³⁸U ratios have been determined on 29 water samples from the Taiyuan area, Shanxi Province. The results show that the same artesian aquifer has similar uranium contents and²³⁴U/²³⁸U activity ratios, and the deeper aquifers have higher A. R. values but lower uranium contents. The A. R. values increase slightly towards groundwater flow in the artesian aquifers dominated by oxidizing ground waters. The Lancun Spring and the famous Jinci Spring belong to two different karst groundwater systems, i.e., the east and west karst groundwater systems. The recharge area of the Lancun Spring should cover the wide limestone outcrops of middle Ordovician in the northeast. The Ordovician fissure-karst ground water to the Jinci Spring is extensively mixed with fissure water in Carboniferous-Jurassic formations and seepage water from the Fenhe River.     

Rates of silicate dissolution in deep-sea sediment: In situ measurement using U/U of pore fluids

Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the ²³⁴U/²³⁸U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).The pore water ²³⁴U/²³⁸U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment ²³⁴U/²³⁸U activity ratios are close to 1.0. The ²³⁴U/²³⁸U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess ²³⁴U to the pore fluid by α-recoil injection of ²³⁴Th. The fraction of ²³⁸U decays that result in α-recoil injection of ²³⁴U to pore fluid is estimated to be 0.10 to 0.20 based on the ²³⁴U/²³⁸U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10⁻⁷ to 2 × 10⁻⁶ yr⁻¹. There is significant down-hole variability in pore water ²³⁴U/²³⁸U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 10⁴ times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials.The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (R_d) of soils and deep-sea sediments can be approximately described by the expression R_d ≈ 0.1 Age⁻¹ for ages spanning 1000 to 5 × 10⁸ yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.     

Uranium isotopes in rivers,estuaries and adjacent coastal sediments of western India: their weathering,transport and oceanic budget

The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and ${}^{238}\text{U}{}^{238}\text{U}$ activity ratios.The ²³⁸U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, σ Na + K + Mg + Ca, and with the HCO₃^? ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that ²³⁸U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between ²³⁸U and total dissolved salts for selected rivers of the world yield an annual dissolved ²³⁸U flux of 0.88 × 10¹⁰g/yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 10¹⁰g/yr, estimated based on its correlation with HCO₃^? contents of rivers.In the estuaries, both ²³⁸U and its great-grand daughter ²³⁴U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02–0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation.A review of the uranium isotope measurements in river waters yield a discharge weighted-average ²³⁸U concentration of 0.22 μg/l with a ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio of 1.20 ± 0.06ismissing. The residence time of uranium isotopes in the oceans estimated from the ²³⁸U concentration and the ${}^{234}\text{U}{}^{238}\text{U}$ A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional ²³⁴U flux of about 0.25 dpm/cm₂·10³ yr into the oceans (about 20% of its river supply) is necessitated.     

Weathering rates at Alto Sorocaba basin,Brazil, using U-isotopes and major cations

The aim of this paper was to evaluate the chemical weathering rate at Alto Sorocaba basin, São Paulo State, Brazil, as it is an important parameter on geomorphological characterization of continents. Several tools may be used for such evaluation, and this work compares the results obtained from analytical data involving the uranium concentration, the ²³⁴U/²³⁸U activity ratio and the major cation concentration for samples of waters, soils, and rocks from that basin. The use of the natural uranium as a tracer in weathering rate rocks studies is advantageous since this element is virtually absent in rainwater and also allows obtaining the dissolution rocks coefficient by the ²³⁴U/²³⁸U activity ratio. The Alto Sorocaba basin has serious environmental problems in terms of the quality of the rainwater and river waters, and, consequently, the geochemical balance using the main cations presented some difficulties. However, measuring of the weathering rate utilizing the U-isotopes method corresponded to 14 μm/year that allows estimating a time of 72,000 years to weather 1 m of rock at Sorocaba River upstream from Itupararanga Reservoir, under the present climatic conditions.     

Uranium series disequilibria in the sedimentary uranium deposit at Yeelirrie,Western Australia

The weighted mean values of the ²³⁴U/²³⁸U and the ²³⁰Th/²³⁴U ratios in ore samples collected above the modern water table of the Yeelirrie uranium deposit in Western Australia are 1.38 ± 0.10 and 0.83 ± 0.28, respectively. The relatively larger variability in the latter ratio is evidence for the translocation of uranium subsequent to deposition. Daughter product separation is due to differences in the mobilities of the thorium and uranium species. Factors involved in interpreting the observed ratios, in terms of the dynamics of the accumulation of the deposit, are assessed in terms of an open system model. The observed disequilibria provide a quantitative basis for the evaluation of hypotheses developed from geological and other considerations.     

U-Pb systems and U isotopic composition of the sandstone-hosted paleovalley Dybryn uranium deposit, Vitim uranium district, Russia

The isotopic (U-Pb, ²³⁸U-²³⁵U, ²³⁴U-²³⁸U) and chemical study of whole-rock samples and finegrained fractions of rocks in a vertical section of the terrigenous sequence at the Dybryn uranium deposit in the Khiagda ore field shows that a wide U-Pb isotopic age range (26.9-6.5 Ma) is caused by oxidation and disturbance of the U-Pb isotopic system in combination with protracted uranium ore deposition. The oxidation of rocks resulted in the loss of uranium relative to lead and eventually to an overestimated ²⁰⁶Pb/²³⁸U age at sites with a low U content. The ²³⁸U/²³⁵U ratios in the studied samples are within the range of 137.74–137.88. Samples with a high uranium content are characterized by a decreasing ²³⁸U/²³⁵U ratio with a decrease in ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U ages. A nonequilibrium ²³⁴U/²³⁸U ratio in most studied samples furnishes evidence for young (<1.5 Ma) transformation of the Miocene uranium ore, which is responsible for uranium migration and its redeposition.     

Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands,Quebec: The potential role of natural rock fracturing

The goal of this study is to explain the origin of ²³⁴U–²³⁸U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with ³He/⁴He ratios, to gain insight regarding the evolution of groundwater in the region. (²³⁴U/²³⁸U) activity ratios, or (²³⁴U/²³⁸U)_act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (²³⁴U/²³⁸U)_act, 1.14 ± 0.01, was measured in Ca–HCO₃-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little ²³⁴U–²³⁸U isotopic fractionation. The (²³⁴U/²³⁸U)_act increases to 6.07 ± 0.14 in Na–HCO₃–Cl-type groundwater. Preferential migration of ²³⁴U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (²³⁴U/²³⁸U)_act and ³He/⁴He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (²³⁴U/²³⁸U)_act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (¹⁴C ages of 6.6 kyrs), with (²³⁴U/²³⁸U)_act of ≥6.07 and large amounts of radiogenic ⁴He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (²³⁴U/²³⁸U)_act and the addition of excess ⁴He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced ⁴He and ²³⁴U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic ⁴He excesses measured in groundwater.     

234U/238U evidence for local recharge and patterns of ground-water flow in the vicinity of Yucca Mountain,Nevada, USA

Uranium concentrations and ²³⁴U/²³⁸U ratios in saturated-zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of Yucca Mountain, a potential high-level radioactive waste disposal site. The U data were obtained by thermal ionization mass spectrometry on more than 280 samples from the Death Valley regional flow system. Large variations in both U concentrations (commonly 0.6–10 μg l⁻¹) and ²³⁴U/²³⁸U activity ratios (commonly 1.5–6) are present on both local and regional scales; however, ground water with ²³⁴U/²³⁸U activity ratios from 7 up to 8.06 is restricted largely to samples from Yucca Mountain. Data from ground water in the Tertiary volcanic and Quaternary alluvial aquifers at and adjacent to Yucca Mountain plot in 3 distinct fields of reciprocal U concentration versus ²³⁴U/²³⁸U activity ratio correlated to different geographic areas. Ground water to the west of Yucca Mountain has large U concentrations and moderate ²³⁴U/²³⁸U whereas ground water to the east in the Fortymile flow system has similar ²³⁴U/²³⁸U, but distinctly smaller U concentrations. Ground water beneath the central part of Yucca Mountain has intermediate U concentrations but distinctive ²³⁴U/²³⁸U activity ratios of about 7–8. Perched water from the lower part of the unsaturated zone at Yucca Mountain has similarly large values of ²³⁴U/²³⁸U. These U data imply that the Tertiary volcanic aquifer beneath the central part of Yucca Mountain is isolated from north-south regional flow. The similarity of ²³⁴U/²³⁸U in both saturated- and unsaturated-zone ground water at Yucca Mountain further indicates that saturated-zone ground water beneath Yucca Mountain is dominated by local recharge rather than regional flow. The distinctive ²³⁴U/²³⁸U signatures also provide a natural tracer of downgradient flow. Elevated ²³⁴U/²³⁸U in ground water from two water-supply wells east of Yucca Mountain are interpreted as the result of induced flow from 40 a of ground-water withdrawal. Elevated ²³⁴U/²³⁸U in a borehole south of Yucca Mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic Tertiary volcanic aquifer where it becomes diluted by regional flow in the Fortymile system.     

Use of 234U/238U disequilibrium in measuring chemical weathering rate of rocks

The rate of chemical weathering of rocks has been determined by using uranium as a natural isotopic tracer. The concentration of uranium and ²³⁴U/²³⁸U ratio in natural waters, rocks, and soils of the Preto river basin (Bahia State, Brazil) was measured by alpha-ray spectroscopy.The activity ratio U234/U238 measured in the various samples indicates the uranium fraction which is dissolved from rocks during the weathering process. The results obtained show that 1 m of rock needs 25,000 yr to be weathered in this region under present climatic conditions.     

Role of Suspended Particulate Matter in Regulating the Behavior of Dissolved Uranium in the Yellow River Estuary

In order to examine the mixing behavior of dissolved uranium (U) in estuaries under different suspended particulate matter (SPM) regimes, three laboratory-based experiments were conducted by mixing seawater with river water containing different concentrations of SPM. Comparing this study with other field and laboratory-based experiments, dissolved U behaved differently depending upon the concentration of SPM. When SPM concentrations are >?0.8 g/L in the Yellow River, desorption/dissolution of U from SPM becomes predominant and dissolved U is enriched relative to the theoretical mixing line. However, when SPM concentrations are <?0.8 g/L, dissolved U behaves conservatively with some extent of removal during estuarine mixing. ²³⁴U/²³⁸U activity ratios were somewhat constant showing no measurable isotopic fractionation during physical mixing and U sorption/desorption to/from particles. Addition of dissolved ²³⁸U desorbed/dissolved from SPM during the annual Yellow River water-sediment regulation scheme (Jun 30th–Jul 14th, 2014) was estimated at 6.4?×?10¹¹ dpm, about 9% of the total riverine flux of dissolved ²³⁸U during that same period. This study represents a contribution to studies of dissolved U in muddy rivers and estuaries throughout the world. Results reported here provide not only a perspective to better estimate U flux from rivers to the ocean but also new insights into better understanding its estuarine mixing behavior and controlling factors.     

Geo-chemical Behavior of Uranium in the Sambhar Salt Lake, Rajasthan (India): Implications to "Source" of Salt and Uranium "Sink"

Among several salt lakes in the Thar Desert of western India, the Sambhar is the largest lake producing about 2 × 10⁵ tons of salt (NaCl) annually. The “lake system” (lake waters, inflowing river waters, and sub-surface brines) provides a unique setting to study the geo-chemical behavior of uranium isotopes (²³⁸U, ²³⁴U) in conjunction with the evolution of brines over the annual wetting and evaporation cycles. The concentration of ²³⁸U and the total dissolved solids (TDS) in lake water increase from ~8 μg L⁻¹ and ~8 g L⁻¹ in monsoon to ~1,400 μg L⁻¹ and 370 g L⁻¹, respectively, during summer time. The U/TDS ratio (~1 μg g⁻¹ salt) and the ²³⁴U/²³⁸U activity ratio (1.65 ± 0.05), however, remain almost unchanged throughout the year, except when U/TDS ratio approaches to 3.8 at/or beyond halite crystallization. These observations suggest that uranium behaves conservatively in the lake waters during the annual cycle of evaporation. Also, uranium and salt content (TDS) are intimately coupled, which has been used to infer the origin and source of salt in the lake basin. Furthermore, near uniform ratios in evaporating lake waters, when compared to the ratio in seawater (~0.1 μg g⁻¹ salt and 1.14 ± 0.02, respectively), imply that aeolian transport of marine salts is unlikely to be significant source of salt to the lake in the present-day hydrologic conditions. This inference is further consistent with the chemical composition of wet-precipitation occurring in and around the Sambhar lake. The seasonal streams feeding the lake and groundwaters (within the lake’s periphery) have distinctly different ratios of U/TDS (2–69 μg g⁻¹ salt) and ²³⁴U/²³⁸U (1.15–2.26) compared to those in the lake. The average U/TDS ratio of ~1 μg g⁻¹ salt in lake waters and ~19 μg g⁻¹ salt in river waters suggest dilution of the uranium content by the recycled salt and/or removal processes presently operating in the lake during the extraction of salt for commercial use. Based on mass-balance calculations, a conservative estimate of "uranium sink" (in the form of bittern crust) accounts for ~5 tons year⁻¹ from the lake basin, an estimate similar to its input flux from rivers, i.e., 4.4 tons year⁻¹.     

Uranium isotopes as a tracer of sources of dissolved solutes in the Han River,South Korea

The uranium (U) content and ²³⁴U/²³⁸U activity ratio were determined for water samples collected from Korea's Han River in spring, summer, and winter 2006 to provide data that might constrain the origin of U isotope fractionation in river water and the link between U isotope systematics in river waters and the lithological nature of the corresponding bedrock. The large difference in the major dissolved loads between the two major branches of the Han River, the North Han River (NHR) and South Han River (SHR), is reflected in the contrasting U content and ²³⁴U/²³⁸U activity ratio between the tributaries: low U content (0.08–0.75 nM; average, 0.34 nM) and small ²³⁴U/²³⁸U activity ratio (1.03–1.22; average, 1.09) in the NHR; and high U content (0.65–1.98 nM; average, 1.44 nM) and large ²³⁴U/²³⁸U activity ratio (1.05–1.45; average, 1.24) in the SHR. The large spatial differences in U content and ²³⁴U/²³⁸U activity ratio are closely related to both lithological differences between the two tributaries and groundwater input. The low U content and small ²³⁴U/²³⁸U activity ratio in the NHR arise mainly from a combination of surface and meteoric weathering of the dominant silicate rocks in this branch and congruent dissolution of already weathered (secular equilibrium) materials. In contrast, the high U content and large ²³⁴U/²³⁸U activity ratio in the SHR are ascribed to the dissolution of carbonates and black shales along with significant inputs of deep groundwater.     

Uranium geochemistry and dating of Pacific island apatite

Uranium-series disequilibrium dating of island phosphate deposits is evaluated in terms of known associated coral ages, uranium geochemistry, and stratigraphic sequences as well as the concordance between the geochronometers ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁸U. U(VI) is the predominant oxidation state of uranium in island phosphorites and by analogy to the youngest surficial deposits, most of the uranium initially bound is in the form of U(VI) sorbed by surfaces from seawater. Insular deposits contain more organic matter than even very young ocean floor samples and this leads to a greater probability of reduction of available recoil uranium than occurs in marine deposits. As a consequence, R(VI) ? R(T) ? R(VI), where R represents the ²³⁴U/²³⁸U activity ratio. This situation is completely opposite from that observed for marine-origin phosphorites. We determined that a fraction of U(VI) in ancient insular phosphorites is very labile and lost to alkaline carbonate solutions with a uranium activity ratio even more depleted in ²³⁴U than the bulk R(VI).Most younger samples appear to have no more discordance between ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁸U than marine phosphorites of similar apparent age. Young, surficial atoll-rim apatite cements and unconsolidated phosphorites date in the range of 1500–8000 years B.P., consistent with the concept of partial submergence of low-lying coral islands prior to 2000 years B.P. Sub-surficial samples in the same environment date older at about 20,000 years B.P. Violation of the closed system assumption occurs in at least 6 out of 13 ancient (> 800,000 year) samples. Uranium-series disequilibrium dating of insular apatite shows some promise as a recorder of climatic/sea level events, but the assumptions necessary for valid ages must be carefully evaluated for each occurrence.     

Fixation of radionuclides in the 238U decay series in the vicinity of mineralized zones: 1. The Austatom Uranium Prospect,Northern Territory,Australia

The minimum age of a zone of secondary uranium mineralization, located at the Austatom Prospect in the Alligator Rivers region of Australia, is estimated to be 3.6 × 10⁵y. This is derived from a geochronological model based on retarded leaching of ²³⁴U with respect to ²³⁸U and on ratios within the ore of these members of the ²³⁸U decay series. Although kaolinite is a dominant mineral in the weathered schist-host-rocks, retarded dissolution of ²³⁴U occurs only in the presence of the clay minerals illite and montmorillonite. In their absence the reverse occurs. A model is proposed to explain the results. Ratios of ²³⁰Th to ²³⁸U indicate that the mineralization has probably remained stationary within the weathered schist for at least 1 to 2 × 10⁵y. Future use of clay minerals as buffers in radioactive waste repositories is supported by the excellent long-term retention obtained for oxidized uranium, probably due in part to isomorphic substitution into the clay crystal lattice.     

Experimental evidence for U-U fractionation during granite weathering with implications for U/U in natural waters

The daughter to parent (²³⁴U/²³⁸U) activity ratio in natural waters is often out of secular radioactive equilibrium. The major reason for this disequilibrium is related to the energetic α-decay of ²³⁸U and differential release of ²³⁴U relative to ²³⁸U. This disequilibrium originates from (1) preferential release of more loosely bound ²³⁴U from damaged mineral lattice sites or; (2) direct recoil of ²³⁴Th into surrounding media from near mineral surface boundaries, however, it is unclear which of the two mechanisms is most important in nature. To better quantify the effects of preferential release of ²³⁴U, two continuous laboratory granite leaching experiments conducted over 1100 h were performed. The leachates were characterized by declining U concentrations with time and (²³⁴U/²³⁸U) initially greater than unity (up to 1.15), which changed to below unity during leaching (∼0.95). The early elevated (²³⁴U/²³⁸U) suggests that additional ²³⁴U is released into solution by preferential release of ²³⁴U from mineral phases. However, the excess ²³⁴U constitutes a finite pool of easy leachable ²³⁴U and the (²³⁴U/²³⁸U) values become lower than unity when this pool is used up. A model based on first-order kinetics, dissolution rates and preferential release of ²³⁴U from damaged lattice sites was developed and is able to quantitatively predict the observed pattern of (²³⁴U/²³⁸U) values and U concentrations for the two granite leaching experiments. Extending the modeling to longer time scales more comparable to natural systems shows that the production of waters with high (²³⁴U/²³⁸U) ratios can be achieved in two distinct regimes (1) slow weathering where the rate of directly recoiled ²³⁴U near mineral surfaces into waters is high; (2) fast weathering where the role of incipient chemical weathering and preferential release of loosely bound ²³⁴U are important. The model is able to explain apparent opposite correlations between physical erosion rates and (²³⁴U/²³⁸U) in waters and it provides a new framework that will be useful for examining weathering regimes, their timescales and their coupling with physical erosion.     

Preferential solution of234U from recoil tracks and234U/238U radioactive disequilibrium in natural waters

A mathematical model to calculate the²³⁴U/²³⁸U activity ratio (AR) in an aqueous phase in contact with rock/soil is presented. The model relies on the supply of²³⁸U by dissolution and that of²³⁴U by dissolution and preferential release from radiation damaged regions (recoil tracks). The model predicts that values of²³⁴U/²³⁸U AR>1 in the aqueous phase can be obtained only from weathering “virgin” surfaces. Thus, to account for the observed steady-state supply of²³⁴U excess to the oceans by the preferential leaching model, ‘virgin’ rock/soil surfaces would have to be continually exposed and weathered. The²³⁸U concentration and²³⁴U/²³⁸U AR in continental waters allow us to estimate the exposure rates of “virgin” rock/soil surfaces.     

Uranium series dating of stalagmites from Blanchard Springs Caverns,U.S.A.

Because stalagmites are enriched in ²³⁴U beyond the equilibrium value at the time of deposition, they are potentially datable by measuring their progress toward equilibrium. Procedures for uranium-thorium and uranium disequilibrium dating of speleothems were developed, making use of alpha spectrometry for thorium determinations and mass spectrometry for uranium determinations. The stalagmites tested were generally beyond the age range of the U-Th age dating method and, as a result, ²³⁴U/²³⁸U ratios provided the most useful dating technique. The estimated age range for a stalagmite from Blanchard Springs is 135,000 to 785,000 years BP with an interval of slow or suspended growth from about 320,000 to 570,000 years BP.     

砂岩铀矿成矿过程与氧化还原分带: 铀系不平衡证据

铀系不平衡技术被用来研究新疆伊犁盆地库捷尔太砂岩型铀矿床的氧化还原分带和成矿过程.取自该矿床的26个样品的铀、钍含量以及234 U/ 238U, 230 Th/ 234 U和230 Th/ 238U活度比值分别用ICP -MS和α能谱仪进行了测量.不同氧化还原带岩石的铀、钍含量和钍/铀比明显不同: 强氧化带岩石的U、Th含量和Th/U比分别为12.4 μg/g, 4.5 μg/g和0.48; 弱氧化带分别为20.4 μg/g, 5.0 μg/g和0.38; 过渡带(矿化带)分别为169.7μg/g, 4.7μg/g和0.07; 还原带(未蚀变带)分别为6.8μg/g, 3.7μg/g和0.87.其同位素特征亦有明显差异: 氧化带岩石234 U/ 238U大多大于1, 过渡带(矿化带)岩石部分大于或等于1, 部分小于1, 还原带(未蚀变带)岩石大多大于1; 氧化带岩石230 Th/ 234 U和230 Th/ 238U大多大于1, 过渡带(矿化带)岩石大多小于或等于1, 还原带(未蚀变带)岩石大多大于1.这可作为砂岩型铀矿床矿体定位的指示剂.铀系不平衡特征还示踪了该矿床的成矿作用过程.      

Uranium isotopes (234U/238U) in rivers of the Yukon Basin (Alaska and Canada) as an aid in identifying water sources, with implications for monitoring hydrologic change in arctic regions

The ability to detect hydrologic variation in large arctic river systems is of major importance in understanding and predicting effects of climate change in high-latitude environments. Monitoring uranium isotopes (²³⁴U and ²³⁸U) in river water of the Yukon River Basin of Alaska and northwestern Canada (2001?C2005) has enhanced the ability to identify water sources to rivers, as well as detect flow changes that have occurred over the 5-year study. Uranium isotopic data for the Yukon River and major tributaries (the Porcupine and Tanana rivers) identify several sources that contribute to river flow, including: deep groundwater, seasonally frozen river-valley alluvium groundwater, and high-elevation glacial melt water. The main-stem Yukon River exhibits patterns of uranium isotopic variation at several locations that reflect input from ice melt and shallow groundwater in the spring, as well as a multi-year pattern of increased variability in timing and relative amount of water supplied from higher elevations within the basin. Results of this study demonstrate both the utility of uranium isotopes in revealing sources of water in large river systems and of incorporating uranium isotope analysis in long-term monitoring of arctic river systems that attempt to assess the effects of climate change.