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1.
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.  相似文献   

2.
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.  相似文献   

3.
This study presents the temporal and spatial variability of 234U/238U activity ratios in the Shu River and provides interpretation to explain the downstream changes of uranium and the 234U/238U activity ratios in the study area. The positive linear correlation (R 2 = 0.98, p < 0.001) between uranium concentration and specific electrical conductance is consistent with rock weathering and leaching as the major contributor of dissolved uranium in the studied area of the river. The 234U/238U activity ratio ranged between ~1.6 in the upper reaches of the river to ~1.15 furthest downstream. Activity ratios at specific sampling points do not show significant seasonal variability.  相似文献   

4.
《Applied Geochemistry》2000,15(3):369-383
Surface water and peat in the northern Everglades have very low natural concentrations of U and are therefore sensitive to the addition of small amounts of U from anthropogenic sources such as fertilizer. Peat samples collected along a nutrient gradient in the northern Everglades have unusually high concentrations of U (>1 μg/g, dry basis) and also have a distinctive 234U/238U activity ratio (AR). AR values for U-enriched peat fall in the narrow range of AR values for commercial phosphate fertilizer (1.00±0.05). In contrast, AR values for low-U peat from background sites exceed 1.05. The spatial distribution of anomalous U concentration, and of fertilizer-like AR values in peat, parallel a previously documented pattern of P enrichment. These results strongly suggest that some of the U in nutrient-impacted peatlands is fertilizer-derived. Agricultural drainage water sampled in the northern Everglades has high concentrations of dissolved U (0.3–2.4 μg/l) compared to surface water from background sites (<0.1 μg/l). Measured AR values in drainage water (0.949–0.990) are also permissive of a fertilizer origin for the U and are different from AR values in surface water or peat at background sites (AR>1.05). Synoptic sampling of surface water along drainage canals indicate that Lake Okeechobee, and some drainage from agricultural fields, are sources of dissolved U, whereas wetlands farther downstream act as sinks for U. Historically cultivated agricultural soil has only a marginally elevated (+0.2 μg/g) average concentration of U compared to nearby uncultivated soil and incorporates only 20% of the U from an aqueous solution that was slurried with the soil. In contrast, a similar experiment with fresh Everglades peat indicated uptake of 90% of the added U. These experiments support the proposed removal of U from agricultural fields and concentration of U in downstream peatlands. The methodology of this study can be used to describe the behavior of fertilizer-derived U in other low-U environments.  相似文献   

5.
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 (Rd) of soils and deep-sea sediments can be approximately described by the expression Rd ≈ 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.  相似文献   

6.
Isotopic fractionation as great as 1600% exists between 234U and 238U in spring waters, sediments, and fossils in the Pomme de Terre Valley, southwestern Missouri. The activity ratios of 234U238U in five springs range from 7.2 to 16 in water which has been discharged for at least the past 30,000 years. The anomalies in 234U238U ratio in deep water have potential usefulness in hydrologic investigations in southern Missouri. Clayey units overlying the spring bog sediments of Trolinger Spring are enriched in 230Th relative to their parent 234U by as much as 720%. The results indicate that both preferential displacement via alpha recoil ejection and the preferential emplacement via recoiling and physical entrapment are significant processes that are occurring in the geologic environment.  相似文献   

7.
The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements.  相似文献   

8.
《Applied Geochemistry》2002,17(6):751-779
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.  相似文献   

9.
The concentrations of 238U, 234Th, 226Ra, 222Rn and 210Pb and 234U238U activity ratios have been measured in several groundwater samples from Gujarat, India. In the aqueous phase the abundances of 234Th and 210Pb are grossly deficient relative to their parents, 238U and 222Rn respectively. The deficiency is ascribable to the impact adsorption of 234Th and 210Pb atoms onto particle surfaces which are very abundant in the groundwater regimes. The scavenging residence times for both these nuclides is about a day, suggesting that irreversible removal of ‘reactive’ metals and pollutants in groundwaters can occur on very short time scales. The fast removal of 234Th onto particles necessitates that in these groundwaters 234U ‘excess’ has to originate through leaching of soil grains rather than through in situ decay of dissolved 234Th in the water.  相似文献   

10.
《Applied Geochemistry》1998,13(3):339-347
Activities of the naturally occurring radionuclide 234Th were determined in water samples of Mecklenburg Bay (SW Baltic Sea) using a new Th-specific diatomite adsorption technique followed by liquid scintillation spectrometry. Activities of “dissolved” (operationally defined as Th in the centrifugate) and particulate 234Th varied in the range of 1.4–6.9 and 0.9–9.3 mBq l−1, respectively. A significant correlation between Kd and SPM concentration was found. From this particle-concentration effect, the “colloidal pumping” model predicts that 98% of the “dissolved” Th is associated with colloids rather than being truly dissolved. Relative to calculated activities of the parent nuclide 238U in the Bay, the 234Th data yielded mean 234Th scavenging residence times in the range of 1.2–9.7 days. Particulate 234Th activities are inversely correlated to SPM concentrations. Particle residence times ranged from a few days in winter up to 20 days in spring characterized by less intense bottom currents. The hydrodynamic regime is the master variable controlling scavenging of Th and other similarly particle-reactive elements in Mecklenburg Bay.  相似文献   

11.
This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large 230Th excess relative to 238U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (234U/238U) ratios in interstitial water are consistent with solid samples showing 234U depletion near the surface and an excess 234U at depth. A leaching experiment performed on chalk shows that the excess 234U in natural waters percolating through carbonate rocks results both from preferential 234U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates.  相似文献   

12.
Pore water chemistry and 234U/238U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent reactive transport code Crunch. The reactive transport model includes a solid-solution formulation to enable the use of the 234U/238U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still exists when a mechanistic reactive transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m2/g based on both grain geometry and on the depletion of 234U/238U in the sediment via α-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water 234U/238U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10-20% of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 102 to 105 times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The reactive transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (105), less substantial with the Al-inhibition formulation (103), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (102). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of reactive sites on silicate surfaces with time. The results imply that methods of mineral surface characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.  相似文献   

13.
The radioactivity due to 238U and 234U in three aquifer systems occurring within the Paraná sedimentary basin, South America, has been investigated. Uranium is much less dissolved from fractured igneous rocks than from the porous sedimentary rocks as indicated by the U-mobility coefficients between 7.6 × 10?6 and 1.2 × 10?3 g cm?3. These values are also compatible with the U preference ratios relative to Na, K, Ca, Mg and SiO2, which showed that U is never preferentially mobilized in the liquid phase during the flow occurring in cracks, fissures, fractures and faults of the igneous basaltic rocks. Experimental dissolution of diabase grains on a time-scale laboratory has demonstrated that the U dissolution appeared to be a two-stage process characterized by linear and second-order kinetics. The U dissolution rate was 8 × 10?16 mol m?2 s?1 that is within the range of 4 × 10?16–3 × 10?14 mol m?2 s?1 estimated for other rock types. The 234U/238U activity ratio of dissolved U in solutions was higher than unity, a typical result expected during the water–rock interactions when preferential 234U-leach from the rock surfaces takes place. Some U-isotopes data allowed estimating 320 ka for the groundwater residence time in a sector of a transect in São Paulo State. A modeling has been also realized considering all U-isotopes data obtained in Bauru (35 samples), Serra Geral (16 samples) and Guarani (29 samples) aquifers. The results indicated that the Bauru aquifer waters may result from the admixture of waters from Guarani (1.5 %) and Serra Geral (98.5 %) aquifers.  相似文献   

14.
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.  相似文献   

15.
Stilbite from Malmberget and Svappavara is part of hydrothermal mineral assemblages occupying regionally occurring open Palaeoproterozoic fractures in northern Sweden. At these locations, stilbite is characterized by Pbrad excess relative to U and by activity ratios of [234U]/[238U] > 1 and [230Th]/[238U] > 1. The activity disequilibrium requires a disturbance of the U-Th systematics within the last one million years. Leaching and infiltration experiments on Malmberget stilbite demonstrate: (i) preferential leaching in the order Pb >U >Th and uptake in the order Pb > U, and (ii) isotopic fractionation of U by preferential mobilization of 238U and 235U relative to 234U. Stepwise-leaching further indicates that the bulk of U is hosted in the channel sites of stilbite. The Th-U disequilibrium systematics observed in untreated Malmberget and Svappavara stilbite can be explained by: (1) addition of U with [234U]/[238U] > 1 from a fluid, or alternatively (2) loss of U from a two-component system, consisting of a component that is “open” or accessible and a component that is “closed” or inaccessible to mobilization. U addition requires a multistage history involving multiple gain or loss of U and/or Pb. In contrast, U loss does not necessarily require multistage processes but can also be explained by preferential removal of 238U (and 235U) relative to recoiled daughter isotopes such as 234U, 230Th, and 206Pb (and 207Pb) during a single event. Such a behavior could be obtained if the recoiled daughter isotopes of channel-sited uranium are implanted into the crystal lattice and, in such a way, become less mobile than their parent isotopes. This case implies an open-system behavior for ions in the channel sites and a closed-system behavior for ions in the silicate framework of stilbite. Each α-recoil directly or indirectly, i.e., through its recoil cascade, damages the silicate framework. Subsequent (continuous) low-temperature annealing of the damaged stilbite lattice could trap the recoiled daughter isotopes in the repaired crystal lattice or sealed-off channels. Such immobile recoiled material can, in part, represent the “closed” component of the system. This model can account for all observations regarding the Th-U-Pb systematics, including the Th-U disequilibrium systematics, the similarity in Th/U as deduced from Th-U disequilibrium and Pb isotope data, and the excess of radiogenic Pb (208Pb-parents also had been multiply recoiled). These two contrasting explanations involve either multistage or multicomponent systems. They do not permit the derivation of an accurate age.  相似文献   

16.
234Th produced from 238U within sea water was found to be in radioactive disequilibrium with respect to its progenitor nuclide 238U in the surface layer of the ocean. The median value for 234Th238U activity ratio is 0.80 in the upper 200 m layer. A box-model calculation gives a removal residence time of thorium of about 0.38 yr. This suggests that 234Th is scavenged from the surface layer by the uptake of thorium by biota.  相似文献   

17.
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.  相似文献   

18.
U-modeled groundwater residence times were estimated by changes in 234U/238U activity ratio (AR) and U content of groundwaters from Poços de Caldas city situated at the Poços de Caldas alkaline massif, Brazil. The estimated ages are more realistic than others generated by the use of hydraulic conductivity data and available information about the weathering rate of rocks in the plateau. The 234U/238U AR and reciprocal of the dissolved U-content data also defined a ternary plot that allowed the calculation of the relative volume of three source waters for the mixed water. Such U-isotopes modeling for mixing calculations indicated that the dominant phase (68%) in the mixture is thermal.  相似文献   

19.
The 230Th/234U/238U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system 230Th/234U/238U ages is elevated 234U/238Uinitial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have 234U/238Uinitial values close to seawater, there is a need for age validation. Redundant 230Th/234U/238U and 231Pa/235U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both 230Th/234U/238U and 231Pa/235U ages in fragments as small as 500 mg. We have obtained excellent agreement between 230Th/234U/238U and 231Pa/235U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the 230Th/234U/238U age accuracy.Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby 238U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of 230Th/234U/238U and 231Pa/235U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for 231Pa and from undetermined sources currently limit the 231Pa/235U age uncertainty to about ±2.5%. U isotope data and 230Th/234U/238U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.  相似文献   

20.
Both chemical weathering rates and fluid flow are difficult to measure in natural systems. However, these parameters are critical for understanding the hydrochemical evolution of aquifers, predicting the fate and transport of contaminants, and for water resources/water quality considerations. 87Sr/86Sr and (234U/238U) activity ratios are sensitive indicators of water-rock interaction, and thus provide a means of quantifying both flow and reactivity. The 87Sr/86Sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate. Similarly, the (234U/238U) ratio of natural ground waters is a balance between the flow rate and the dissolution of solids, and α-recoil loss of 234U from the solids. By coupling these two isotope systems it is possible to constrain both the long-term (ca. 100’s to 1000’s of years) flow rate and bulk dissolution rate along the flow path. Previous estimates of the ratio of the dissolution rate to the infiltration flux from Sr isotopes (87Sr/86Sr) are combined with a model for (234U/238U) to constrain the infiltration flux and dissolution rate for a 70-m deep vadose zone core from Hanford, Washington. The coupled model for both (234U/238U) ratios and the 87Sr/86Sr data suggests an infiltration flux of 5 ± 2 mm/yr, and bulk silicate dissolution rates between 10−15.7 and 10−16.5 mol/m2/s. The process of α-recoil enrichment, while primarily responsible for the observed variation in (234U/238U) of natural systems, is difficult to quantify. However, the rate of this process in natural systems affects the interpretation of most U-series data. Models for quantifying the α-recoil loss fraction based on geometric predictions, surface area constraints, and chemical methods are also presented. The agreement between the chemical and theoretical methods, such as direct measurement of (234U/238U) of the small grain size fraction and geometric calculations for that size fraction, is quite good.  相似文献   

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