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1.
Extensive hydrogeological investigations followed by three-dimensional groundwater flow and contaminant transport modelling were carried out around a proposed uranium tailings pond at Seripalli in Andhra Pradesh, India, to estimate its radiological impact. The hydrogeological parameters and measured groundwater level were used to model the groundwater flow and contaminant transport from the uranium tailings pond using a finite-element-based model. The simulated groundwater level compares reasonably with the observed groundwater level. Subsequently, the transport of long-lived radionuclides such as 238U, 234U, 230Th and 226Ra from the proposed tailings pond was modelled. The ingrowths of progenies were also considered in the modelling. It was observed that these radionuclides move very little from the tailings pond, even at the end of 10,000 y, due to their high distribution coefficients and low groundwater velocities. These concentrations were translated into committed effective dose rates at different distances in the vicinity of the uranium tailings pond. The results indicated that the highest effective dose rate to members of the public along the groundwater flow pathway is 2.5 times lower than the drinking water guideline of 0.1?mSv/y, even after a long time period of 10,000 y.  相似文献   

2.
《Applied Geochemistry》2003,18(7):1095-1110
The exchange of 226Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O2, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ∼3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; 226Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of 226Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of 226Ra, resulting in a relatively large flux of 226Ra from the tailings to the water column. The strong correlation between the porewater profiles of 226Ra and Ba (r2=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase [(Ra,Ba)SO4]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO4 reduction. Flux calculations suggest that the increase in 226Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l−1) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of 226Ra.  相似文献   

3.
《Applied Geochemistry》1999,14(3):365-385
The abundance and chemical/mineralogical form of 226Ra, 238U and 232Th were determined on samples of soil and associated vegetation at 12 sites in the eastern United States. Progressive, selective chemical extraction plus size fractionation determined the abundance and radiometric equilibrium condition of these nuclides in 6 operationally defined soil fractions: exchangeable cations, organic matter, “free” Fe-oxides, sand, silt, and clay.In soils, profile-averaged 226Ra/238U activity ratios (AR) are within 10% of unity for most sites, implying little fractionation of U and Ra when the entire soil profile is considered. However, 226Ra greatly exceeds 238U activity in most surface soil (AR up to 1.8, av 1.22), in vegetation (AR up to 65, av. 2.8), in the exchangeable+organic fraction (AR up to 30, av. 13), in some soil Fe oxides (AR up to 3.5, av. 0.83) and in the C horizons of deeply weathered soils (AR up to 1.5).A major factor in Ra behavior is uptake by vegetation, which concentrates Ra>U and moves Ra from deeper soil to surface soil. Vegetation is capable of creating the observed Ra excess in typical surface soil horizons (AR up to 1.8, av. 1.22) in about 1000 a. Of the total Ra in an average A horizon, 42% occurs as exchangeable ions and in organic matter, but only 6–8% of the parent U and Th occur in these soil forms. In contrast, U is slightly enriched relative to Ra in Fe-oxides of A horizons, implying rapid chemical partition of vegetation-cycled U and Ra.In deeper horizons, transfer by vegetation and/or direct chemical partitioning of Ra into organic and exchangeable forms provides a source for unsupported 226Ra in Ra-rich organic matter, and leaves all soil minerals Ra-poor (AR=0.73). Organic matter evidently has a strong affinity for Ra.The phenomena discussed above are relevant to evaluation of indoor Rn hazard, and behavior of Ra at sites affected by radioactive waste disposal, phosphate tailings, Ra-rich brine, and uraniferous fertilizer.  相似文献   

4.
Factors controlling the groundwater transport of U, Th, Ra, and Rn   总被引：1，自引：0，他引：1  
A model for the groundwater transport of naturally occurring U, Th, Ra, and Rn nuclides in the238U and232Th decay series is discussed. The model developed here takes into account transport by advection and the physico-chemical processes of weathering, decay, α-recoil, and sorption at the water-rock interface. It describes the evolution along a flowline of the activities of the238U and232Th decay series nuclides in groundwater. Simple sets of relationships governing the activities of the various species in solution are derived, and these can be used both to calculate effective retardation factors and to interpret groundwater data. For the activities of each nuclide, a general solution to the transport equation has been obtained, which shows that the activities reach a constant value after a distance ϰi, characteristic of each nuclide. Where ϰi is much longer than the aquifer length, (for238U,234U, and232Th), the activities grow linearly with distance. Where gKi is short compared to the aquifer length, (for234Th,230Th,228Th,228Ra, and224Ra), the activities rapidly reach a constant or quasi-constant activity value. For226Ra and222Rn, the limiting activity is reached after 1 km. High δ234U values (proportional to the ratioɛ234Th/W238U) can be obtained through high recoil fraction and/or low weathering rates. The activity ratios230Th/232Th,228Ra/226Ra and224Ra/226Ra have been considered in the cases where either weathering or recoil is the predominant process of input from the mineral grain. Typical values for weathering rates and recoil fractions for a sandy aquifer indicate that recoil is the dominant process for Th isotopic ratios in the water. Measured data for Ra isotope activity ratios indicate that recoil is the process generally controlling the Ra isotopic composition in water. Higher isotopic ratios can be explained by different desorption kinetics of Ra. However, the model does not provide an explanation for228Ra/226Ra and224Ra/226Ra activity ratios less than unity. From the model, the highest222Rn emanation equals 2ɛ. This is in agreement with the hypothesis that222Rn activity can be used as a first approximation for input by recoil (Krishnaswamiet al 1982). However, high222Rn emanation cannot be explained by production from the surface layer as formulated in the model. Other possibilities involve models including surface precipitation, where the surface layer is not in steady-state.  相似文献   

5.
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   

6.
Analyses have been made of many groundwater samples, some of which were collected from the vicinity of uranium deposits and others from unmineralized areas, for dissolved uranium and for the four naturally occurring isotopes of radium: 226Ra (238U decay series, y), 228Ra and 224Ra (232Th decay series, y and 3.8 d) and 223Ra (235U decay series, d). The radium isotopes 226Ra, 224Ra and 223Ra, are measured by alpha-spectrometry after extraction from a water sample soon after collection and 228Ra at a later time by determining the amount of ingrown 212Po.  相似文献   

7.
《Applied Geochemistry》1987,2(4):385-398
The source of Ra has been determined in water samples from four areas in Australia where anomalously high surface concentrations of226Ra have accumulated from groundwaters. All four anomalies were located adjacent to sandstone formations, and the groundwaters, which were generally all acidic and low in dissolved salts, appeared to be meteoric water with short ground-residence times. Uranium,226Ra and228Ra concentrations of waters feeding the anomalous areas were comparable to those found in standing waters within the sandstones. The226Ra/228Ra isotopic ratios were distributed about a median of 1.1 which suggests that the waters are in contact with rocks with near-normal U/Th ratios and, hence, that the Ra in the anomalies was derived from within the sandstones.The presence of the short-lived Ra isotopes,223Ra and224Ra, in high concentrations in most spring waters feeding these anomalies suggests that Ra enters groundwaters by recoil following alpha decay of a precursor parent radionuclide within mineral grains. Thus, although three of the areas were considered prospective for U, the radioactive anomalies studied appear to be due to natural transfer of Ra from the sandstones to the surface environment. In no case were the anomalies related to nearby known or undiscovered U deposits. Accordingly, a geochemical procedure, which includes Ra isotopic measurements, is recommended for evaluating radioactive anomalies for U exploration. This procedure should enable selection of only those anomalies with the highest potential for further exploration by more expensive techniques.  相似文献   

8.
Simultaneous in situ immobilisation of uranium (U) and radium (226Ra) by injectible amounts of grey cast iron (gcFe), nano-scale iron (naFe) and a gcFe/MnO2 mixture (1:1) was studied in batch and column tests. Both 0.5 g/L naFe and gcFe are effective in 226Ra and U removal from mine water, whereas MnO2 addition clearly increased the efficiency of gcFe for 226Ra and U immobilisation. In a column test with 0.6 wt% gcFe/MnO2 mixture (1:1), neither 226Ra nor U was detected in the effluent after replacement of 45 pore volumes. A sequential extraction under flow condition revealed 226Ra to be mostly occluded in manganese oxides. Uranium was mostly sorbed onto poorly crystalline iron hydroxides, but a significant part was found to be occluded in manganese oxides also. The results of this study suggest that MnO2 promotes iron hydroxide formation under slightly reducing environmental conditions resulting in an increased pollutant retention capacity.  相似文献   

9.
This study was conducted to define the geochemical controls on 226Ra during raffinate (pH = 1.2) neutralization to pH 10 at the Key Lake U mill in northern Saskatchewan, Canada. High activities (120–150 Bq/L) of aqueous phase 226Ra are present in raffinate produced during milling of U ore. The solubility control of 226Ra in the SO4-rich, hydrometallurgical raffinate solutions often involves the addition of BaCl2 to form a radium-barite co-precipitate (Ba(Ra)SO4). As such, neutralization experiments were conducted with samples of mill raffinate using Ca(OH)2 or NaOH with and without the addition of BaCl2. Radium-226 activity decreased from 150 to <4 Bq/L for all combinations of neutralizing agents with Ca(OH)2 + BaCl2 being the most effective combination (final activity ∼1.0 Bq/L; ∼99.3% removal). In the absence of BaCl2, Ca(OH)2 more efficiently removed 226Ra than NaOH between pH 4 and 8, due to the co-precipitation of 226Ra with gypsum. Overall, neutralization with the addition of BaCl2 reduced 226Ra activities at lower pH values (by pH 4.5), due to co-precipitation of 226Ra with BaSO4. At varying concentrations of BaCl2, aqueous phase activities of 226Ra converged, but did not attain steady-state values during neutralization and would continue to decrease with time. Sequential extractions indicated that 226Ra in precipitates formed during neutralization of the mill raffinate is dominated by amorphous and crystalline Fe hydroxide phases, consistent with raffinate neutralization experiments that showed that adsorption onto ferrihydrite can remove most 226Ra in the raffinate. Data generated in this study are being used to define the long-term geochemical controls on 226Ra in U mill processes and tailings.  相似文献   

10.
In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e.226Raex/226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/234U chronometer. Analytical solutions of the coupled 234U/230Th/226Ra radionuclide system predicted that the 226Raex/226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Raex/226Ra(0) ages agreed well with those derived from 230Th/234U, (ii) all data plot within uncertainty on the 226Raex/226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Raex/226Ra(0) should be applicable to materials which are suitable for 230Th/234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/234U for samples that have been formed a few thousand years ago.  相似文献   

11.
Precise measurements of 238U-230Th-226Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr-Nd-Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess 238U-230Th-226Ra disequilibria with 230Th (18-24%) and 226Ra (9-21%) excesses, and there exists a positive correlation in a (226Ra/230Th)-(230Th/238U) diagram. The extent of 238U-230Th-226Ra disequilibria is markedly different in lavas of individual eruption ages, although the (230Th/232Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (230Th/238U)-(206Pb/204Pb) and (226Ra/230Th)-(206Pb/204Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high DU and DU/DTh, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios (206Pb/204Pb > 20.47) due to late Mesozoic metasomatism.  相似文献   

12.
We present data for U and its decay series nuclides 230Th, 226Ra, 231Pa, and 210Po for 14 lavas from Kick’em Jenny (KEJ) submarine volcano to constrain the time-scales and processes of magmatism in the Southern Lesser Antilles, the arc having the globally lowest plate convergence rate. Although these samples are thought to have been erupted in the last century, most have (226Ra)/(210Po) within ±15% of unity. Ten out of 14 samples have significant 226Ra excesses over 230Th, with (226Ra)/(230Th) up to 2.97, while four samples are in 226Ra-230Th equilibrium within error. All KEJ samples have high (231Pa)/(235U), ranging from 1.56 to 2.64 and high 238U excesses (up to 43%), providing a global end-member of high 238U and high 231Pa excesses. Negative correlations between Sr, sensitive to plagioclase fractionation, and Ho/Sm, sensitive to amphibole fractionation, or K/Rb, sensitive to open system behavior, indicate that differentiation at KEJ lavas was dominated by amphibole fractionation and open-system assimilation. While (231Pa)/(235U) does not correlate with differentiation indices such as Ho/Sm, (230Th)/(238U) shows a slight negative correlation, likely due to assimilation of materials with slightly higher (230Th)/(238U). Samples with 226Ra excess have higher Sr/Th and Ba/Th than those in 226Ra-230Th equilibrium, forming rough positive correlations of (226Ra)/(230Th) with Sr/Th and Ba/Th similar to those observed in many arc settings. We interpret these correlations to reflect a time-dependent magma differentiation process at shallow crustal levels and not the process of recent fluid addition at the slab-wedge interface.The high 231Pa excesses require an in-growth melting process operating at low melting rates and small residual porosity; such a model will also produce significant 238U-230Th and 226Ra-230Th disequilibrium in erupted lavas, meaning that signatures of recent fluid addition from the slab are unlikely to be preserved in KEJ lavas. We instead propose that most of the 238U-230Th, 226Ra-230Th, and 235U-231Pa disequilibria in erupted KEJ lavas reflect the in-growth melting process in the mantle wedge (reflecting variations in U/Th, daughter-parent ratios, fO2, and thermal structure), followed by modification by magma differentiation at crustal depths. Such a conclusion reconciles the different temporal implications from different U-series parent-daughter pairs and relaxes the time constraint on mass transfer from slab to eruption occurring in less than a few thousand years imposed by models whereby 226Ra excess is derived from the slab.  相似文献   

13.
This study of environmental radioactivity was carried out in the soils of an urban area. Naturally occurring gamma-emitting radionuclides and man-made 137Cs were found in the soil profiles collected from four parks in the central Belgrade city area and the soil layer was examined every 10 cm and to a depth of 50 cm. Radioisotope activity concentrations (Bq kg?1) in the samples of urban soil using the gamma-ray spectrometry method were in the range of 14–46 for 238U, 33–50 for 226Ra, 29–63 for 210Pb, 1.2–3.4 for 235U, 28–50 for 232Th, 424–576 for 40K and 0.7–35.8 for 137Cs. Some of the basic physicochemical soil properties (pH, organic matter content, calcium-carbonate content, particle size distribution) were determined to investigate the impact on the vertical distribution of radionuclides. The results of this investigation showed that variations of activity concentration ratios of radionuclides that belong to the same (238U/226Ra) or different radioactive series (232Th/226Ra; 235U/238U), including 210Pb/137Cs ratios could well be explained by the properties of the soil. Alkaline pH reaction, the accumulation of organic matter in the uppermost and of carbonates in the deepest layers of urban soil had an effect on 238U/226Ra, and 210Pb/137Cs activity concentration ratio values, while 232Th/226Ra and partially 235U/238U ratios were associated with the particle sizes vertical distribution. A study of radionuclides in the samples of leaves of two deciduous tree species common for these parks was also conducted and 210Pb and 40K were found concentrated in leaves rather than other investigated radionuclides.  相似文献   

14.
The geochemistry of Ba, Ra, Th, and U and the potential of using 226Ra/Ba ratios as an alternative dating method are explored in modern and Holocene marine mollusc shells. Five modern shells of the Antarctic scallop Adamussium colbecki collected from the present day beach and six radiocarbon dated specimens from Holocene beach terraces of the Ross Sea region (Antarctic) between 700 and 6100 calibrated yr BP old have been analysed by mass spectrometry. In clean shells 226Ra concentrations and 226Ra/Ba ratios show a clear decrease with increasing age, suggesting the possibility of 226Ra dating. Limiting factors for such dating are Ba and 226Ra present in surface contaminants, and ingrowth of 226Ra from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using 232Th. Sub-samples from the same shell with different proportions of contamination form a mixing line in a 226Ra/Ba-232Th/Ba graph, and the 226Ra/Ba of the pure shell can be derived from the intercept on the 226Ra/Ba axis. Contaminant corrected 226Ra/Ba ratios of late-Holocene 14C-dated samples fall close to that expected from simple 226Ra excess decay from seawater 226Ra/Ba values. 226Ra ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of post mortem U uptake into the shell makes a correction for 226Ra ingrowth from secondary U difficult to achieve. In the A. colbecki shells, 226Ra ingrowth from such secondary U becomes significant only when ages exceed ∼2500 yr. In younger shells, 226Ra/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between 226Ra and Ba in the ocean can be confirmed for that time scale, the 226Ra/Ba chronometer may enable the reconstruction of variability in sea surface 14C reservoir ages from mollusc shells and allow its use as a paleoceanographic tracer.  相似文献   

15.
The increasing interest in radioactivity has brought about the need for an assessment of human exposure to radiation. It is, therefore, necessary to examine naturally occurring radioactivity in the environment, especially its occurrence in groundwater. The aim of this work was then to study the levels and behavior of the most significant natural radionuclides, also in order to improve the knowledge of the hydrochemical processes involved in the selected groundwater systems. Natural radioactivity in fifteen Calabrian groundwaters for human use was investigated through high-resolution gamma spectrometry (with a negative-biased Ortec HPGe detector) and liquid scintillation measurements. Particular attention was given on those radionuclides (3H, 238U, 226Ra, and 228Ra), which contribute in a significant way to the overall effective dose received by members of the public due to the intake of drinking water originating from groundwater systems. The activity concentration of 238U varied from 1 to 51 mBq/L, as a result of the geology of the investigated area and of the oxidizing conditions that favored U dissolution. 226Ra presented a broad range of activity concentrations (0.011–0.14 mBq/L), lower than uranium ones because radium occurs in groundwater under reducing conditions. Some heavy metals (Cd, Pb, Be, Hg, Ag, As, Tl, Sb, Se, and Ni) were also investigated through ICP-MS measurements and compared with the limits set by the Italian Legislation. Metals are released into the environment by both natural and anthropogenic sources; they leach into underground waters, moving along water pathways and eventually depositing in the aquifer.  相似文献   

16.
228Ra, 226Ra, and 222Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log 228Ra and log 226Ra (n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median 228Ra226Ra activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal 232Th238U activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log 222Rn and log 226Ra for Piedmont (n = 68, r = 0.62) and Coastal Plain (n = 89, r = 0.64) ground water. However, the median 222Rn226Ra activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess 222Rn activities may be due to greater retention of 226Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log 228Ra and log 226Ra was used to predict total Ra (228Ra + 226Ra) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of 226Ra for 228Ra analysis.  相似文献   

17.
厦门火烧屿裸露岩石的铀放射系不平衡   总被引：2，自引：1，他引：2  
用HPGeγ谱方法测定了厦门火烧屿裸露岩石天然放射性核素^40K、^228Ra、^228Th、^238U、^226Ra和^210Pb含量，对其铀系不平衡关系进行了讨论，发现钍系核素^228Ra和^228Th基本上是平衡的，而大部分样品^226Ra相对于^228U、^210Pb相对于^226Ra亏损。由此推论，水体作用下岸边岩石中^226Ra直接进入水体，可以是海水中^226Ra的一个来源；岸边岩石中^222Rn逸出后，衰变^210Pb再进入水体，可以是海水中^210Pb的一个来源。  相似文献   

18.
In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469-2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show 238U-230Th-226Ra disequilibria with excess 238U and 226Ra, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest (230Th/232Th) ratio compared to older Miyakejima eruptives, yielding the youngest 238U-230Th model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a (230Th/232Th)-(238U/232Th) diagram and (2) a positive linear correlation in a (226Ra/230Th)0 − 1/230Th diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of 230Th and 226Ra, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in (230Th/232Th) and (226Ra/230Th)0 ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr-Nd-Pb isotopic systems. In such cases, Th isotopes and (226Ra/230Th) ratio are excellent geochemical tracers of magmatic evolution.  相似文献   

19.
The Sarcheshmeh is one of the largest Oligo-Miocene porphyry Cu deposits in the world. Comparative hydrochemical, mineralogical and chemical fractionation associated with mining efflorescence salts and processing wastes of this mine are discussed. Hydrochemical results showed that rock waste dumps, reject wastes and old impoundments of tailings are the main sources of acid mine drainage waters (AMD) that contain potentially toxic metals such as Cd, Co, Cu, Mn, Ni and Zn as well as Al. Episodic fluxes of highly contaminated acidic waters were produced in a tailings dam over a short period of time. Secondary soluble minerals provide important controls on the quality of AMD produced, especially in old, dry tailings impoundments. Secondary sulfate minerals such as gypsum, magnesiocopiapite, hydronium jarosite, kornelite and coquimbite were found in rock waste drainages and in old weathered reject wastes. Highly soluble secondary minerals such as gypsum, eriochalcite, and bonattite are also observed in an evaporative layer on old tailings impoundments. Chemical fractionation patterns of potentially toxic elements showed that the geochemical behavior of metals is primarily controlled by the mineralogical composition of waste samples. Elements such as Co, Cr, Cu, Mn, Ni and Zn are readily released into the water soluble fraction from efflorescence salts associated with rock waste drainages, as well as from the evaporative layer of old tailings. Potentially toxic elements, such as As, Mo and Pb, are principally adsorbed or co-precipitated with amorphous and crystalline Fe oxides, but they may also be associated with oxidizing, primary sulfides and residual fractions. Following the development of the dammed tailings pond, the secondary minerals were dissolved, producing acidic waters contaminated by Al (154 mg L−1), Cu (150 mg L−1), Cd (0.31 m gL−1), Co (2.13 mg L−1), Mn (73.7 mg L−1), Ni (1.74 mg L−1), Zn (20.3 mg L−1) and Cl (1690 mg L−1). Therefore, the potential use of recycled water from the Sarcheshmenh dammed tailings pond is diminished by the presence of corrosive ions like Cl− in highly acidic fluids that promote corrosion of pipes and pumps in the water recycling system.  相似文献   

20.
U-Pb systems were examined in samples (ranging from 4 to 10 cm3 in volume) of ore material taken from along a 3.5-m profile across a zone of U mineralization exposed in an underground mine at the Strel’tsovskoe U deposit in eastern Transbaikalia. The behaviors of two isotopic U-Pb systems (238U-206Pb and 235U-207Pb) are principally different in all samples from our profile. While the individual samples are characterized by a vast scatter of their T(206Pb/238U) age values (from 112 to 717 Ma), the corresponding T(207Pb/235U) values vary much less significantly (from 127 to 142 Ma) and are generally close to the true age of the U mineralization. The main reason for the distortion of the U-Pb system is the long-lasting (for tens of million years) migration of intermediate decay products in the 238U-206Pb(RD238U) in the samples. This process resulted in the loss of RD238U from domains with high U concentrations and the subsequent accommodation of RD238U at sites with low U concentrations. The long-term effect of these opposite processes resulted in a deficit or excess of 206Pb as the final product of 238U decay. The loss or migration of RD238U are explained by the occurrence of pitchblende in association with U oxides that have higher Si and OH concentrations than those in the pitchblende and a higher +6U/+4U ratio. The finely dispersed character of the mineralization and the loose or metamict texture of the material are the principal prerequisites for RD238U loss and an excess of 206Pb in adjacent domains with low U concentrations. Domains with low U contents in the zone with U mineralization serve as geochemical barriers (because of sulfides contained in them) at which long-lived RD238U(226Ra, 210Po, 210Bi, and 210Pb) were accommodated and subsequently caused an excess of 206Pb. The 235U-207Pb system remained closed because of the much briefer lifetime of the 235U decay products. This may account for the significant discrepancies between the T(206Pb/238U) and T(207Pb/235U) age values. RD238U was most probably lost via the migration of radioisotopes at the middle part and end of the 238U family (starting with 226Ra). The heavy Th, Pa, and U radioisotopes (234Th, 234Pa, 234U, and 230Th) that occur closer to the beginning of 238U decay, before 226Ra, only relatively insignificantly participated in the process. Our results show that the loss and migration of RD238U are, under certain conditions, the main (or even the only) process responsible for the distortion of the U-Pb system.  相似文献   

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