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1.
R.J.N. Brits 《Chemical Geology》1979,25(4):347-353
A simple method to determine the ratio in water by α-spectrometry is presented. The thiocyanate complex of uranium is extracted from the water by anion exchange. After elution of the U from the resin it is further purified by extraction from nitrate medium into ether. The source for α-spectrometry is prepared by oxalate electrodeposition. The overall yield is 70%. 相似文献
2.
The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U-233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41 ± 0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old. 相似文献
3.
内蒙古二连盆地、鄂尔多斯盆地典型含铀碳酸型盐湖水、岩两相234U/238U综合分析表明,碳酸型盐湖铀来源于大气降水和潜水对盐湖盆地周围中生代到现代富铀沉积物的溶滤、浸出,具有快速、近源物质来源特点。盐湖卤水和对应沉积物234U/238U值一般为0.8～1.2,湖卤水和潜卤水（晶间卤水）-岩两相中的铀处于沉积平衡状态。早白垩世～上新世含膏盐地层对比研究证实了富铀岩层234U/238U值随铀含量增大而减小,并趋近于1。室内盐湖水蒸发模拟实验发现残余卤水、沉积物234U/238U具有随蒸发程度增大逐渐减小的变化特征。盐湖现代沉积物物相研究发现铀主要以碳酸铀酰和吸附形式赋存在富含有机物和碎屑成分的含盐粘土沉积中,铀在盐类晶体中含量极少,仅存在于封闭水和结晶水中。卤水和沉积物ARu值是盐湖铀源及铀含量水平的指示标志之一。 相似文献
4.
5.
《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
6.
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. 相似文献
7.
Julien Mercadier Michel Cuney Michel Cathelineau Mathieu Lacorde 《Mineralium Deposita》2011,46(2):105-135
Proterozoic basement-hosted unconformity-related uranium deposits of the Athabasca Basin (Saskatchewan, Canada) were affected
by significant uranium redistribution along oxidation–reduction redox fronts related to cold and late meteoric fluid infiltration.
These redox fronts exhibit the same mineralogical and geochemical features as the well-studied uranium roll-front deposits
in siliclastic rocks. The primary hydrothermal uranium mineralisation (1.6–1.3 Ga) of basement-hosted deposits is strongly
reworked to new disseminated ores comprising three distinctly coloured zones: a white-green zone corresponding to the previous
clay-rich alteration halo contemporaneous with hydrothermal ores, a uranium front corresponding to the uranium deposition
zone of the redox front (brownish zone, rich in goethite) and a hematite-rich red zone marking the front progression. The
three zones directly reflect the mineralogical zonation related to uranium oxides (pitchblende), sulphides, iron minerals
(hematite and goethite) and alumino-phosphate-sulphate (APS) minerals. The zoning can be explained by processes of dissolution–precipitation
along a redox interface and was produced by the infiltration of cold (<50°C) meteoric fluids to the hydrothermally altered
areas. U, Fe, Ca, Pb, S, REE, V, Y, W, Mo and Se were the main mobile elements in this process, and their distribution within
the three zones was, for most of them, directly dependent on their redox potential. The elements concentrated in the redox
fronts were sourced by the alteration of previously crystallised hydrothermal minerals, such as uranium oxides and light rare
earth element (LREE)-rich APS. The uranium oxides from the redox front are characterised by LREE-enriched patterns, which
differ from those of unconformity-related ores and clearly demonstrate their distinct conditions of formation. Uranium redox
front formation is thought to be linked to fluid circulation episodes initiated during the 400–300 Ma period during uplift
and erosion of the Athabasca Basin when it was near the Equator and to have been still active during the last million years.
A major kaolinisation event was caused by changes in the fluid circulation regime, reworking the primary uranium redox fronts
and causing the redistribution of elements originally concentrated in the uranium-enriched meteoric-related redox fronts. 相似文献
8.
拉布拉多(Labrador)地处加拿大地盾东部边缘,其U矿化类型多样,但最重要的是赋存于变质火山岩和沉积岩内的矿化,主要分布于Makkovik省中心矿带区(CMB)的古元古代表壳岩和侵入岩中。该区矿化有岩浆型(伟晶岩和酸性火山岩)、后生--热液型(有些类似于Fe氧化物--Cu--Au(IOCG)型矿床)、剪切带型(可能是变质--交代成因)和砂质沉积岩中的层状矿化,但未发现典型的不整合型。根据其矿化类型,结合已发现矿床的成矿地质特征和矿化地质资料分析,得出该区的一些成矿远景区有较大的勘探潜力,圈定White Bear湖U矿勘查战略选区和Nash湖U矿勘查战略选区。 相似文献
9.
Constancy of Nb/U in the mantle revisited 总被引：5，自引：0，他引：5
Weidong Sun Yanhua Hu Vadim S. Kamenetsky Ming Chen 《Geochimica et cosmochimica acta》2008,72(14):3542-3549
It has long been proposed that MORB and OIB have constant supra-primitive mantle (PM) Nb/U values identical to each other. This fact together with complementary sub-PM values for the continental crust (CC), are taken as fundamental evidence, linking the mantle sources of MORB and OIB to the formation of the CC. Given that plate subduction at convergent margins is the major known process that dramatically fractionates Nb from U, and consequently that subducted oceanic slabs are the main primary carriers of supra-PM Nb/U, a constant supra-PM Nb/U in MORB mantle implies that the mixing of subducted oceanic crust is essentially finished or the newly recycled oceanic crust has Nb/U close to that of the mantle. The similarity between Nb and U as well as the constancy of Nb/U in MORB are revisited here based on MORB glass data obtained using laser ablation ICP-MS. The result shows that Nb/U is not correlated with Nb/Hf, supporting that Nb and U are similarly incompatible. Further investigation shows that Nb is not perfectly identical to, but is faintly more incompatible than U as indicated by the good correlation between log(U) and log(Nb) with a slope of 0.954, very close to 1. Nonetheless, the similarity between Nb and U is high enough, such that the average Nb/U value of MORB glasses should be very close to that of the MORB mantle. By contrast, the difference between Ce and Pb is more obvious. Ce is more incompatible than Pb with a slope of 1.13 in a log(Pb) versus log(Ce) diagram. Therefore, the Ce/Pb of MORB should be a little bit higher than that of the mantle source. The Nb/U value is not as uniform as expected for the similar incompatibility in studied MORB glasses, but varies by a factor of ∼2, suggesting that MORB mantle source is not yet homogenized in term of Nb/U. This indicates that the mixing back of subducted oceanic crust is still an ongoing process, i.e., subducted oceanic crust is recycling back after staying in the lower mantle for billions of years. 相似文献
10.
11.
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. 相似文献
12.
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
13.
B. Uralbekov M. Burkitbayev B. Satybaldiyev I. Matveyeva T. Tuzova D. Snow 《Environmental Earth Sciences》2014,72(9):3635-3642
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
14.
Owing to the rapid increase in available data on the natural variations of the 238U/235U ratio, new isotopic geochemical mark of redox processes are beginning to emerge. In this connection, numerical estimates of the 238U and 235U fractionation factor (α(UIV?UVI)) accompanying the reduction UVI → UIV are needed. Such an estimate has been obtained for hydrothermal pitchblende formation based on results of high-precision (±0.06‰) measurements of the 238U/235U ratio in local microsamples of coarse spherulitic pitchblende from carbonate-pitchblende veins at the Oktyabr’sky deposit (Strel’tsovsky uranium ore field, eastern Transbaikal region). For this purpose, we used the formation temperature of hydrothermal pitchblende and a maximum estimate of the fractionation factor for 238U and 235U isotopes in the solution-solid phase system under normal (25°C) conditions (Murphy et al., 2014). The most probable isotopic fractionation factor accompanying pitchblende crystallization from hydrothermal solution at T = 320?250°C falls into the interval α(UIV?UVI) = 1.00020?1.00023. 相似文献
15.
通过对降扎铀矿床地质条件的分析,总结了矿床围岩蚀变、控矿构造形式、矿石矿物特征、铀矿体主要特征、矿物组合、成矿流体、盲矿体地表放射性物化探异常等矿床特征和成矿规律,指出向阳沟矿段等已知矿段深部、向阳沟以西地段、热乎东地段为以后的找矿靶区. 相似文献
16.
To increase the understanding of uranium transport in the environment and in the presence of steel corrosion products, the interaction of U(VI) with natural magnetite has been studied. Sorption studies have been carried out using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The XPS results clearly indicate the reduction of U(VI) to U(IV) on the surface of magnetite facilitated by electron transfer between the Fe and U, leading to a coupled oxidation of Fe(II) to Fe(III). 相似文献
17.
The possibility that the atomic ratio 235u/238U may be slightly variable due to natural nuclear chain reactions such as have been identified in Gabon, West Africa, has prompted a re-examination of very precise gas mass spectrometric isotopic ratio data from a number of laboratories. Two modal values of the isotopic ratio exist in the data. Their relative difference, 0.03%, is statistically significant. The lower mode is due principally to ores from the Colorado Plateau. This difference was recognized in 1963 by the U.S. Atomic Energy Commission but the substantiating data have not been openly published. Insufficient data presently exist to attribute this difference to chemical differentiation of the uranium isotopes, presumably in the ‘sandstone’ type deposits of the Colorado Plateau, or to dilution with Precambrian ore deposits, depleted in 235U by nuclear reactions. 相似文献
18.
岩石样品U0和ΔU的计算 总被引：1，自引：0，他引：1
本文给岩石原始铀含量和铀变化系数以确切定义 ,并系统介绍了U0 和ΔU的计算方法。计算岩石原始铀含量 (U0 )有Th/U比值法和U Pb同位素法 ,后者又包括两阶段法和三阶段法 ;铀的变化系数 (ΔU)包括近代 (现代 )和成矿时铀的变化。这一手段或方法对研究地浸砂岩型铀矿成矿机理和成矿预测有重要意义。 相似文献
19.
Barney J Szabo 《Geochimica et cosmochimica acta》1982,46(9):1675-1679
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 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 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
20.
南京直立人的U/Th和U/Pa年代 总被引：2，自引：0，他引：2
中国直立人化石的准确定年对于研究人类演化有着极为重要的意义。1993～1994年在南京汤山葫芦洞发现的两个直立人头盖骨化石和一枚牙化石被称为“南京直立人”。其中1号头盖骨化石之上的方解石钙板的U/Th年龄为53.3_(-1.2)~(+1.5)万年;但考虑到定年的准确性,则为53.3_(-3.0)~(+3.5)万年。其~(231)Pa/~(235)U活度比值为0.998±0.006。这表明“南京直立人”的年代应该大于50万年。与”南京直立人”伴生的动物牙化石U/Th年代为18.5～29.0万年;U/Pa年代为13.7～17.2万年。此外,对于同一颗牙化石,牙釉的年龄小于牙本质的年龄。同一样品的U/Pa年龄也显著小于其U/Th年龄。因此,牙化石的U摄取过程并不符合U早期摄取模式。多数牙化石分析点在~(234)U/~(238)U-~(230)Th/~(238)U图上落在位于U早期摄取和线性摄取模式曲线之间,指示牙化石的U摄取过程很可能介于上述两种模式之间。如果这一假设成立,那么牙化石的U/Th和U/Pa线性摄取模式年龄则为其年代的上限。因为不受U摄取过程~(234)U/~(238)U变化的影响,U/Pa线性摄取模式年龄比U/Th较为可靠。最小的U/Pa线性摄取模式年龄为1Ma,这是”南京直立人”上限年龄的估计。从定年结果看,”南京直立人”可能生活在海洋同位素(MIS)16阶段,但这不是最终结论。 相似文献