广西来宾栖霞组缺氧沉积环境的地球化学特征

地层的岩石地球化学特征是判断其形成环境氧化还原条件的重要手段之一，笔者对广西来宾铁桥剖面栖霞组中层纹状灰岩和含泥质灰岩进行岩石地球化学分析，结合栖霞组积学和古生态学特征，对栖霞组沉积环境进行了探讨，岩石微量元素含量、黄铁矿矿化程的DOP值、痕量元素V/Cr、Ni/Co、U/Th、V/（V Ni)等比值的研究结果显示，研究区具有缺氧沉积成因地球化学特征，是由温度引起的季节性盆氧。     

Rare Earth Elements and Yttrium (REY) Geochemistry of Reefal Limestones in the Ordovician,Tarim Basin,NW China and their Paleoenvironment Implications

This study examines the rare earth elements and yttrium (REY) concentrations of twenty-five samples from the reef outcrop exposed along the Lianglitage Mountain in the Ordovician, Tarim Basin in China. The concentration analysis provides constraints on the paleoenvironment during reef deposition. Based on the detailed sedimentology and petrographic work, we divide the reef facies into four sub-facies: the base facies, reef-core facies, reef-flank facies, and sealing facies. The geochemical data (such as major and trace elements, carbon and oxygen isotopes, and REYs) are further used to study the coeval seawater characteristics as well as potential diagenesis overprints. The result indicated that the diagenesis has little effect on the REY patterns of the reefal limestones. The REY concentrations of the reefal limestones are overall low (ranging from 3.69 to 19.60 ppm, arithmetic mean=10.22 ppm, SD=5.4). The PAAS-normalized REY patterns are consistently flat compared to the typical well-oxidized, shallow marine water patterns. However, the light REE (LREE) depletions, positive La anomalies, negative Ce anomalies and positive Y anomalies, suggest that these reefal limestones are likely an indicative of contemporaneous seawater REY signals. The seawater-like Y/Ho ratios (average at 37.51) further support that REY signals in these limestones are likely a reflection of seawater with little diagenetic modifications. The low Y/Ho ratios presented only in the reef-flank facies and sealing facies are likely a suggestion of detrital contamination. Hence, this study confirms that REY patterns of the limestones at the base facies and reef-core facies can record ancient seawater information, and reefs can be used as a potential geochemical proxy for paleoenvironment studies throughout the Earth’s history.     

Geochemistry of the Neoproterozoic Narji limestone,Cuddapah Basin,Andhra Pradesh,India: implication on palaeoenvironment

The Neoproterozoic Narji Formation of Cuddapah Basin, Southern India is mainly composed of limestones with minor amount of clastic rocks. Limestones are massive as well as laminated and occasionally chert bearing. Geochemistry (major, trace, and REE) of limestones is studied to strengthen the knowledge on depositional environment of Narji Formation in the direction to better figure out the development of Cuddapah Basin during Neoproterozoic era. Average SiO₂ (25.97), Al₂O₃/TiO₂ (16.67), and K₂O/Al₂O₃ (0.21) ratios suggest clastic contamination in the Narji limestones. PAAS (Post Archean Australian Shale) normalized REE?+?Y pattern of Narji limestones are showing seawater like REE?+?Y pattern. The Er/Nd and Y/Ho ratios (average 0.17 and 35.68, respectively) of Narji limestones indicate the retention of normal seawater character with the signatures of terrigenous input and diagenesis process. Positive Ce anomaly, high U/Th (>?1.25), and V/(V?+?Ni) (>?0.5) ratios of Narji limestones clearly indicate their deposition in dyoxic to anoxic condition.     

Competition between enzymatic and abiotic reduction of uranium(VI) under iron reducing conditions

Reduction of U(VI) under iron reducing conditions was studied in a model system containing the dissimilatory metal-reducing bacterium Shewanella putrefaciens and colloidal hematite. We focused on the competition between direct enzymatic uranium reduction and abiotic reduction of U(VI) by Fe(II), catalyzed by the hematite surface, at relatively low U(VI) concentrations (< 0.5 μM) compared to the concentrations of ferric iron (> 10 mM). Under these conditions surface catalyzed reduction by Fe(II), which was produced by dissimilatory iron reduction, was the dominant pathway for uranium reduction. Reduction kinetics of U(VI) were identical to those in abiotic controls to which soluble Fe(II) was added. Strong adsorption of U(VI) at the hematite surface apparently favored the abiotic pathway by reducing the availability of U(VI) to the bacteria. In control experiments, lacking either hematite or bacteria, the addition of 45 mM dissolved bicarbonate markedly slowed down U(VI) reduction. The inhibition of enzymatic U(VI) reduction and abiotic, surface catalyzed U(VI) reduction by the bicarbonate amendments is consistent with the formation of aqueous uranyl-carbonate complexes. Surprisingly, however, more U(VI) was reduced when dissolved bicarbonate was added to experimental systems containing both bacteria and hematite. The enhanced U(VI) reduction was attributed to the formation of magnetite, which was observed in experiments. Biogenic magnetite produced as a result of dissimilatory iron reduction may be an important agent of uranium immobilization in natural environments.     

确定砂岩型铀矿矿化地段的铀、钍同位素证据

根据铀、钍同位素地球化学基本原理,对新疆伊梨盆地５１２矿床含矿砂岩的Ｕ、Ｔｈ含量及其＾２３４Ｕ／＾２３８Ｕ、＾２３０Ｔｈ／＾２３２Ｔｈ进行了测试,并在此基础上对含矿砂岩的氧化带、还原带和氧化还原过渡带特征进行了研究。研究表明,与氧化带、还原带相比,氧化－还原过渡带具有较高Ｕ含量和＾２３４Ｕ／＾２３８Ｕ比值,较低Ｔｈ／Ｕ比值;矿石样品中的＾２３４Ｕ／＾２３８Ｕ、＾２３０Ｔｈ／＾２３８Ｕ比值位于过渡带     

诸广山中段“三九”地区铀矿放射性同位素特征及找矿潜力

“三九”地区位于诸广山复式岩体中段,隶属华南花岗岩型铀矿富集区。放射性同位素及其比值是反映铀成矿信息的重要参数,可直接反映浅部到深部的铀、镭富集的地球化学特征及其迁移规律。本文重点探讨“三九”地区不同地段、不同含量、不同埋深和不同矿体部位的²³⁸U、²³⁴U、²²⁶Ra、²³⁰Th和²³¹Pa同位素特征及其比值,分析铀、镭迁移富集规律。结果表明：“三九”地区铀矿体由近地表至深部铀镭平衡系数具有高-低-高变化特征;U元素的相对富集、Th元素相对减低及其U/Th比值的急剧升高等特征,与U、Th丰度区间具有明显分带特征关系密切;当地表有矿化或异常显示且浅、深部有工业矿时,其同位素组成具有铀、镭同时迁出,且铀大量迁出特征;当地表无异常显示时,其同位素组成具有铀大量迁出、镭明显迁入特征。大于1的²³⁴U/²³⁸U比值可有效指示找铀矿化。综上,利用放射性同位素及其比值法在“三九”地区预测深部铀成矿有利地段,取得较好应用效果,其中九龙江、石壁窝和木...     

Paleo-environment of iron-rich sedimentary rocks

It is possible to apply actualistic principles to the interpretation of the paleoenvironment of iron-rich sedimentary rocks, although few iron-rich sediments form today. The sedimentary textures and structures of cherty iron-formations, Minette-tpye ironstones and limestones are similar. These similarities prove that the hydrodynamic processes of the deposition of the three rock types are the same. Therefore, it is possible to define facies of cherty iron-formations and Minette-type ironstones on the basis of their sedimentary textures and structures, disregarding mineralogy, and to interprete the formative environment by comparison with Recent limestones. Shaly sulfide iron-formation and sideritic clay ironstones are ferriferous mudstones. Methods applied to the interpretation of iron-poor shales may also be applied to these two rock types. The mineralogy of iron-rich sedimentary rocks is determined by diagenetic processes. Depositional organic content of the sediment, sediment texture (which controls diffusivity and permeability) and the groundwater flow system during diagenesis are the primary controls of iron-mineral diagenesis. Paleogeography and basin bathymetry are indirect controls of iron mineralogy, because they affect organic content and sediment texture.     

Reduction of uranium(VI) under sulfate-reducing conditions in the presence of Fe(III)-(hydr)oxides

Hexavalent uranium [U(VI)] dissolved in a modified lactate-C medium was treated under anoxic conditions with a mixture of an Fe(III)-(hydr)oxide mineral (hematite, goethite, or ferrihydrite) and quartz. The mass of Fe(III)-(hydr)oxide mineral was varied to give equivalent Fe(III)-mineral surface areas. After equilibration, the U(VI)-mineral suspensions were inoculated with sulfate-reducing bacteria, Desulfovibrio desulfuricans G20. Inoculation of the suspensions containing sulfate-limited medium yielded significant G20 growth, along with concomitant reduction of sulfate and U(VI) from solution. With lactate-limited medium, however, some of the uranium that had been removed from solution was resolubilized in the hematite treatments and, to a lesser extent, in the goethite treatments, once the lactate was depleted. No resolubilization was observed in the lactate-limited ferrihydrite treatment even after a prolonged incubation of 4 months. Uranium resolubilization was attributed to reoxidation of the uraninite by Fe(III) present in the (hydr)oxide phases. Analysis by U L₃-edge XANES spectroscopy of mineral specimens sampled at the end of the experiments yielded spectra similar to that of uraninite, but having distinct features, notably a much more intense and slightly broader white line consistent with precipitation of nanometer-sized particles. The XANES spectra thus provided strong evidence for SRB-promoted removal of U(VI) from solution by reductive precipitation of uraninite. Consequently, our results suggest that SRB mediate reduction of soluble U(VI) to an insoluble U(IV) oxide, so long as a suitable electron donor is available. Depletion of the electron donor may result in partial reoxidation of the U(IV) to soluble U(VI) species when the surfaces of crystalline Fe(III)-(hydr)oxides are incompletely reduced.     

The effect of pH and natural microbial phosphatase activity on the speciation of uranium in subsurface soils

The biomineralization of U(VI) phosphate as a result of microbial phosphatase activity is a promising new bioremediation approach to immobilize uranium in both aerobic and anaerobic conditions. In contrast to reduced uranium minerals such as uraninite, uranium phosphate precipitates are not susceptible to changes in oxidation conditions and may represent a long-term sink for uranium in contaminated environments. So far, the biomineralization of U(VI) phosphate has been demonstrated with pure cultures only. In this study, two uranium contaminated soils from the Department of Energy Oak Ridge Field Research Center (ORFRC) were amended with glycerol phosphate as model organophosphate source in small flow-through columns under aerobic conditions to determine whether natural phosphatase activity of indigenous soil bacteria was able to promote the precipitation of uranium(VI) at pH 5.5 and 7.0. High concentrations of phosphate (1-3 mM) were detected in the effluent of these columns at both pH compared to control columns amended with U(VI) only, suggesting that phosphatase-liberating microorganisms were readily stimulated by the organophosphate substrate. Net phosphate production rates were higher in the low pH soil (0.73 ± 0.17 mM d⁻¹) compared to the circumneutral pH soil (0.43 ± 0.31 mM d⁻¹), suggesting that non-specific acid phosphatase activity was expressed constitutively in these soils. A sequential solid-phase extraction scheme and X-ray absorption spectroscopy measurements were combined to demonstrate that U(VI) was primarily precipitated as uranyl phosphate minerals at low pH, whereas it was mainly adsorbed to iron oxides and partially precipitated as uranyl phosphate at circumneutral pH. These findings suggest that, in the presence of organophosphates, microbial phosphatase activity can contribute to uranium immobilization in both low and circumneutral pH soils through the formation of stable uranyl phosphate minerals.     

Spatial and temporal variability of 234U/238U activity ratios in the Shu River,Central Asia

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.     

The interpretation of U-U-Th-Ra disequilibria produced by rock-water interactions

A mathematical model is set up to describe the evolution in time of the activity ratios of the long lived members of the ²³⁸U decay series following alteration of the initial state of secular equilibrium in the rock by a rock-water interaction. Both sudden (that is to say, of duration very much shorter than any of the half-lives involved) and continuously acting disturbances are considered. Continuous processes may involve either the removal or the deposition of uranium and these are represented in the formalism as first and zero order effects respectively. Difficulties of principle are identified, but in spite of these an attempt is made to show that the predictions of the model are useful in the interpretation of activity ratios obtained for suites of samples under circumstances in which complementary geochemical information is available.     

Speciation of naturally-accumulated uranium in an organic-rich soil of an alpine region (Switzerland)

Very high concentrations of uranium (up to 4000 ppm) were found in a natural soil in the Dischma valley, an alpine region in the Grisons canton in Switzerland. The goal of this study was to examine the redox state and the nature of uranium binding in the soil matrix in order to understand the accumulation mechanism. Pore water profiles collected from Dischma soil revealed the establishment of anoxic conditions with increasing soil depth. A combination of chemical extraction methods and spectroscopy was applied to characterize the redox state and binding environment of uranium in the soil. Bicarbonate extraction under anoxic conditions released most of the uranium indicating that uranium occurs predominantly in the hexavalent form. Surprisingly, the uranium redox state did not vary greatly as a function of depth. X-ray absorption near edge spectroscopy (XANES), confirmed that uranium was present as a mixture of U(VI) and U(IV) with U(VI) dominating. Sequential extractions of soil samples showed that the dissolution of solid organic matter resulted in the simultaneous release of the majority of the soil uranium content (>95%). Extended X-ray absorption fine structure (EXAFS) spectroscopy also revealed that soil-associated uranium in the soil matrix was mainly octahedrally coordinated, with an average of 1.7 axial (at about 1.76 Å) and 4.6 to 5.3 equatorial oxygen atoms (at about 2.36 Å) indicating the dominance of a uranyl-like (UO₂²⁺) structure presumably mixed with some U(IV). An additional EXAFS signal (at about 3.2 Å) identified in some spectra suggested that uranium was also bound (via an oxygen atom) to a light element such as carbon, phosphorus or silicon. Gamma spectrometric measurements of soil profiles failed to identify uranium long-life daughter products in the soil which is an indication that uranium originates elsewhere and was transported to its current location by water. Finally, it was found that the release of uranium from the soil was significantly promoted at very low pH values (pH 2) and increased with increasing pH values (between pH 5 and 9).     

砂岩型铀矿的微生物成矿作用研究述评

目前,砂岩型铀矿是全球应用最为广泛和最有前景的铀矿类型,也是我国最主要的工业铀矿类型。本文展示了当今世界最新的铀资源分布和组成,强调了砂岩型铀矿在世界和中国铀矿资源中的重要性,梳理了实验室条件下微生物对U(VI)的还原性富集和非还原性富集机理,归纳了地质条件下微生物参与砂岩型铀矿的成矿证据。微生物对铀富集作用的实验研究,主要体现在还原性和非还原性富集两个方面。微生物对U(VI)的还原性富集研究最为深入,包括微生物的细胞色素、菌毛和电子穿梭体在U(VI)的还原过程中的作用。微生物对U(VI)的非还原性富集表现在微生物表面吸附、表面络合沉淀和细胞内积聚作用。微生物参与砂岩型铀矿成矿作用的证据,可分为直接证据和间接证据。直接证据主要有铀矿物形态学特征、P元素含量和矿物纳米晶体尺寸;间接证据主要有黄铁矿硫同位素和方解石碳同位素组成以及相应烃类包裹体特征。在未来的研究工作中,应重视微区实验方法在砂岩型铀矿中的应用,以及含油气/煤盆地上覆地层的砂岩型铀矿找矿工作,应探索更加适当的指标和评价体系以量化微生物对砂岩型铀矿的成矿作用。     

热表面电离质谱法测定贫化铀样品痕量铀同位素比值及含量

选用磷酸为离子流发射剂,热表面电离质谱法分析纳克量级贫化铀样品的铀同位素比值,方法最大相对标准偏差2.9%。以233U为稀释剂,采用同位素稀释法对铀的含量进行测定,扩展不确定度为2.4%(K=2)。研究表明,在纳克量级的铀同位素比值测定中,来自铼带等铀本底的干扰影响不容忽视,需要进一步研究并扣除。     

粘土对地下水中U（VI）的吸附作用及其污染控制研究

进行了U（VI）在粘土上吸附的批实验，其中，粘土样采自我国南方某大型铀尾矿库库底。实验结果表明U（VI）在粘土上的吸附与浸泡液的pH值呈强烈的非线性关系，在pH值近中性时，U（VI）在粘土上的吸附达到了一个最大值，而在偏酸性或偏碱性条件下，U（VI）在粘土上的吸附迅速减少；运用表面络合理论建立了U（VI）在粘土上吸附的表面络合模型（DLM），该模型很好地拟合了实验数据。模型检验表明，它可以精确预测U（VI）在不同热力学条件下的吸附行为；此外，模拟结果表明，U（VI）的粘土上吸附在酸性条件下受固液比（M／V）影响明显，而在碱性条件下主要受浸泡液中HCO3^-和CO3^2-的控制。     

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.     

Interactions of uranium with bacteria and kaolinite clay

We assessed the accumulation of uranium (VI) by a bacterium, Bacillus subtilis, suspended in a slurry of kaolinite clay, to elucidate the role of microbes on the mobility of U(VI). Various mixtures of bacteria and the koalinite were exposed to solutions of 8 × 10^− 6 M- and 4 × 10^− 4 M-U(VI) in 0.01 M NaCl at pH 4.7. After 48 h, the mixtures were separated from the solutions by centrifugation, and treated with a 1 M CH₃COOK for 24 h to determine the associations of U within the mixture. The mixture exposed to 4 × 10^− 4 M U was analyzed by transmission electron microscope (TEM) equipped with EDS. The accumulation of U by the mixture increased with an increase in the amount of B. subtilis cells present at both U concentrations. Treatment of kaolinite with CH₃COOK, removed approximately 80% of the associated uranium. However, in the presence of B. subtilis the amount of U removed was much less. TEM–EDS analysis confirmed that most of the U removed from solution was associated with B. subtilis. XANES analysis of the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both revealed that it was present as U(VI). These results suggest that the bacteria have a higher affinity for U than the kaolinite clay mineral under the experimental conditions tested, and that they can immobilize significant amount of uranium.     

Impact of phosphate on U(VI) immobilization in the presence of goethite

Past mining, processing, and waste disposal activities have left a legacy of uranium-contaminated soil and groundwater. Phosphate addition to subsurface environments can potentially immobilize U(VI) in-situ through interactions with uranium at mineral-water interfaces. Phosphate can induce the precipitation of low solubility U(VI)-phosphates, and it may enhance or inhibit U(VI) adsorption to iron(III) (oxy)hydroxide surfaces. Such surfaces may also facilitate the heterogeneous nucleation of U(VI)-phosphate precipitates. The interactions among phosphate, U(VI), and goethite (α-FeOOH) were investigated in a year-long series of experiments at pH 4. Reaction time, total U(VI), total phosphate, and the presence and absence of goethite were systematically varied to determine their effects on the extent of U(VI) uptake and the dominant uranium immobilization mechanism. Dissolved U(VI) and phosphate concentrations were interpreted within a reaction-based modeling framework that included dissolution-precipitation reactions and a surface complexation model to account for adsorption. The best available thermodynamic data and past surface complexation models were integrated to form an internally consistent framework. Additional evidence for the uptake mechanisms was obtained using scanning electron microscopy and X-ray diffraction. The formation and crystal growth of a U(VI)-phosphate phase, most likely chernikovite, UO₂HPO₄·4H₂O_(s), occurred rapidly for initially supersaturated suspensions both with and without goethite. Nucleation appears to occur homogeneously for almost all conditions, even in the presence of goethite, but heterogeneous nucleation was likely at one condition. The U(VI)-phosphate solids exhibited metastability depending on the TOTU:TOTP ratio. At the highest phosphate concentration studied (130 μM), U(VI) uptake was enhanced due to the likely formation of a ternary surface complex for low (∼1 μM) to intermediate (∼10 μM) TOTU concentrations and to U(VI)-phosphate precipitation for high TOTU (∼100 μM) concentrations. For conditions favoring precipitation, the goethite surface acted as a sink for dissolved phosphate that resulted in higher dissolved U(VI) concentrations relative to goethite-free conditions. Based on the total uranium and available sorption sites, a critical phosphate concentration between 15 μM and 130 μM was required for preferential precipitation of uranium phosphate over U(VI) adsorption.     

Multiproxy approach to understanding the origin of Cretaceous pelagic limestone–marl alternations (DSDP site 391, Blake-Bahama Basin)

Limestone–marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals. However, uncertainty in such interpretations stems from the differential diagenesis that most limestone–marl alternations have undergone. Differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters. For an unequivocal interpretation of the origin of rhythmic alternations, diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required. The present study uses a multiproxy approach (independent biotic, sedimentary and geochemical parameters) in order to unravel the palaeoenvironmental signal recorded in Valanginian (Early Cretaceous) limestone–marl alternations from the Blake‐Bahama Basin (DSDP site 391). Using this approach, terrestrial and marine influences can be distinguished, changes in nutrient levels estimated and prediagenetic differences in the non‐carbonate fraction constrained. Surprisingly, no systematic variations in any of these parameters were observed between limestone and marl layers, implying that none of these was directly responsible for the formation of the rhythmic alternation. Hence, none of the current models of sedimentary formation of limestone–marl rhythmites is applicable here. Calcareous nannofossils are equally well preserved in limestone and marl layers, ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds. Sr values of 700–900 p.p.m. indicate that aragonite may have been present in the original, pelagic sediment. The assumption of fine‐grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites, including the source of the calcite cement observed in the limestones, the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers. The study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results. Clearly, careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions.