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1.
Aya Sakaguchi Masayoshi Yamamoto Junpei Tomita Kenta Mino Keiichi Sasaki Kenji Kashiwaya Takayoshi Kawai 《Quaternary International》2009,205(1-2):65
Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in 238U–234U–230Th–232Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The 238U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high 238U peak at 23.8 m in depth, while the 232Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk 238U into authigenic and terrigenous 238U fractions was attempted, based on the measured 232Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic 238U was higher in interglacials and lower in glacials. This depth profile of authigenic 238U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic 238U contents were associated with the environmental change around Lake Hovsgol. 相似文献
2.
V. N. Golubev I. V. Chernyshev A. V. Chugaev A. V. Eremina A. N. Baranova V. V. Krupskaya 《Geology of Ore Deposits》2013,55(6):399-410
The isotopic (U-Pb, 238U-235U, 234U-238U) and chemical study of whole-rock samples and finegrained fractions of rocks in a vertical section of the terrigenous sequence at the Dybryn uranium deposit in the Khiagda ore field shows that a wide U-Pb isotopic age range (26.9-6.5 Ma) is caused by oxidation and disturbance of the U-Pb isotopic system in combination with protracted uranium ore deposition. The oxidation of rocks resulted in the loss of uranium relative to lead and eventually to an overestimated 206Pb/238U age at sites with a low U content. The 238U/235U ratios in the studied samples are within the range of 137.74–137.88. Samples with a high uranium content are characterized by a decreasing 238U/235U ratio with a decrease in 207Pb/235U and 206Pb/238U ages. A nonequilibrium 234U/238U ratio in most studied samples furnishes evidence for young (<1.5 Ma) transformation of the Miocene uranium ore, which is responsible for uranium migration and its redeposition. 相似文献
3.
Natural radionuclides in the uranium and thorium series were measured in solid tidal phases (suspended particles, bottom sediment, surface microlayer colloids) of a salt marsh in lower Delaware. The purpose was to identify potential processes responsible for trace element cycling (sources, redistribution and exchange) in salt water marshes and with their coastal waters. Generally, concentrations of U, Th,210Pb, and210Po on the tidal solid phases suggest a general mechanism by which tidal marshes appear to be trapping the nuclides into their interiors. The processes may include transport of enriched fine particles into the marsh, capture by salt marsh grass and chemical fixation by redox processes at the sediment surface. Specifically, the uranium contents of most of the samples are similar with activity ratios234U238U≧1, indicating a mixture of detrital and nondetrital (authigenic) uranium inputs such as seawater or ground water. Since the230Th daughter is generally deficient by about 50%, the authigenic enrichment process appears to favor uranium and is potentially linked to the extensive diagenetic sulfur redox cycle of salt marsh sediments. The210Po/210Pb activity ratio is less than one on Spartina adsorbed solids, and could suggest a general process in salt marshes which favors210Pb enrichment by atmospheric fallout over enrichment of210Po on time scales of weeks which correspond to complete tide marsh exchange. A228Th/232Th activity ratio of less than unity on the solids adsorbed onto marsh grass suggests a net process whereby diffusive loss of the intermediate daughter228Ra from the adsorbed solids to tidal waters dominates over potential228Th scavenging by suspended sediment. 相似文献
4.
V. N. Golubev E. O. Dubinina I. V. Chernyshev T. A. Ikonnikova 《Doklady Earth Sciences》2016,466(1):28-31
The uneven character of the distribution of 18O/16O and 234U/238U values was established in the vertical cross section of the productive sequence of the Dybryn uranium deposit (Vitim uranium-ore region, Buryatia). Both a deficiency and an excess of 234U in relation to the equilibrium 234U/238U ratio in the vertical sequence may provide evidence for the extremely low rate of the infiltration water flow. The behavior of oxygen isotope characteristics for different size fractions of terrigenous rocks provides evidence for active uranium redistribution and openness of the isotope system of this element during interaction of terrigenous–sedimentary rocks with infiltration waters. 相似文献
5.
Yehia H. Dawood Hamdy H. Abd El-Naby Bassam Ghaleb 《Arabian Journal of Geosciences》2014,7(7):2881-2892
Uranium and thorium isotopic composition of kasolite [Pb(UO2)SiO4-(H2O)] from Jabal Sayid area was determined by thermal ionization mass spectrometry. Secondary electron imaging, back-scattered electron imaging, and energy dispersive spectral scans were used to investigate the mineralogical characteristics of this uranyl mineral phase. Distinct crystal faces and crystal growth of kasolite from the study area confirm mineral precipitation near the surface from the circulating groundwater. The obtained data were used to interpret the mechanism of uranium mobility in Jabal Sayid weathering profile and to construct a tentative model to explain the isotopic evolution of uranium and thorium. This model indicates that (1) uranium was leached at depth, (2) uranyl mineralization was precipitated along fractures and cavities in the host rocks during humid conditions and pluvial periods, (3) preferential leaching of 234U from uranyl mineralization by recoil processes was continuous indicative of a weakly circulating groundwater, and (4) 234U-deficiency resulted in isotopic signatures characterized by low 234U/238U and high 230Th/234U ratios. The modification pattern of these activity ratios suggests that uranyl mineralization of Jabal Sayid, most probably, has been precipitated during the same Late Quaternary pluvial periods responsible for the formation of the corresponding mineralizations in the Eastern Desert of Egypt. 相似文献
6.
7.
G. P. Kiselev E. Yu. Yakovlev S. V. Druzhinin A. S. Galkin 《Geology of Ore Deposits》2017,59(5):391-406
The contents of radioactive elements and the uranium isotopic composition of kimberlite in the Arkhangelskaya pipe at the M.V. Lomonosov deposit and of nearby country rocks have been studied. A surplus of 234U isotope has been established in rocks from the near-pipe space. The high γ = 234U/238U ratio is controlled by the geological structure of the near-pipe space. A nonequilibrium uranium halo reaches two pipe diameters in size and can be regarded as a local ore guide for kimberlite discovery. The rocks in the nearpipe space are also characterized by elevated or anomalous U, Th, and K contents with respect to the background. 相似文献
8.
确定砂岩型铀矿矿化地段的铀、钍同位素证据 总被引：6，自引：1，他引：5
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 相似文献
9.
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
10.
Richard A. Mortlock Richard G. Fairbanks Tzu-chien Chiu 《Geochimica et cosmochimica acta》2005,69(3):649-657
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
11.
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
12.
Lycia Maria Moreira-Nordemann 《Geochimica et cosmochimica acta》1980,44(1):103-108
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
13.
《Geochimica et cosmochimica acta》1987,51(6):1743-1754
Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210Pb-derived sedimentation rate decreases from ~80 mg/cm2-yr at the slope to ~20 mg/cm2-yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments.The 232Th-228Th disequilibrium in sediments indicates that 228Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228Th and 234Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm2/yr and a sedimentation rate close to that based on 210Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210Pb, 230Th and 231Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause. 相似文献
14.
高分辨电感耦合等离子体质谱法测定铀矿石样品中234U/238U、230Th/232Th和228Ra/226Ra同位素比值 总被引：1，自引：0，他引：1
采用氢氟酸-硝酸-盐酸混合酸密闭消解含铀矿石样品,用阴离子交换树脂、阳离子交换树脂和锶特效树脂逐级分离富集铀、钍和镭。使用高分辨电感耦合等离子体质谱(HR-ICPMS)测定分离纯化液中234U/238U2、30Th/232Th和228Ra/226Ra同位素。比值的测量精密度取决于比值的大小和对应核素浓度的大小。对质量浓度为10 ng/mL天然铀测量液,234U/238U的测量精密度优于1.2%;对230Th质量浓度为0.6ng/mL且230Th和232Th质量浓度接近的测量液,230Th/232Th的测量精密度为1.2%;对228Ra质量浓度为0.48 pg/mL且228Ra和226Ra质量浓度接近的测量液,228Ra/226Ra的测量精密度为4.0%。 相似文献
15.
R. Carpenter M.L. Peterson J.T. Bennett B.L.K. Somayajulu 《Geochimica et cosmochimica acta》1984,48(10):1949-1963
Activity profiles of excess 234Th, excess 210Pb, 232Th, 230Th, 234U and 238U, and 228/232Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of 228Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and 210Pb in this coastal zone.Comparison of excess 234Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column.Surface sediment horizons with excess 234Th have no excess 228Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of 228Th due to loss of soluble 228Ra from pore water to the overlying water persist to 20–30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound.Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. 232Th and 230Th activities and 230/232Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic 230Th. Steady state 210Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved 210Pb in sea waters adverting into Puget Sound.Excess 234Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess 210Pb profiles. At least six of eight 234Th profiles show that mixing within the 210Pb-defined surface mixed layer is depth dependent. In three profiles, 234Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the 234Th data are consistent with 210Pb profiles determined for the same sediments, strongly suggesting that 234Th and 210Pb are mixed equivalently and in a multilayered manner. 相似文献
16.
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. 相似文献
17.
Uranium-series mass spectrometric analyses of corals from the uplifted last glacial terraces at Huon Peninsula, Papua New Guinea, that grew from 50,000 years ago to 30,000 years ago show systematically low values of 234U/238U, at the time of coral growth, compared with modern corals. When combined with coral data from other studies a systematic trend emerges indicating shifts in the 234U/238U ratio at times of major glacial-interglacial transitions that involve large variations in sea-levels. From last glacial to Holocene, the rate of change in δ234U is approximately 1‰ per thousand years. The variations in the U budget of the oceans appear to be due to accumulation of excess 234U in near shore areas in anoxic and suboxic sediments, in salt marshes and mangroves, in estuaries, and in continental margins during periods of warm climate and high sea-levels. These near-shore areas are exposed during periods of low sea level resulting in rapid oxidation of U into highly soluble phases. The subsequent release of 234U-enriched uranium into the oceans occurs over a sustained period, in step with rising sea-levels. 相似文献
18.
TIMS测定铀矿石样品中^234U/^238U、^230Th/^232Th、^228Ra/^226Ra的方法研究 总被引：1，自引：0，他引：1
研究了TIMS测定铀矿石样品中234U/238U、230Th/232Th、228Ra/226Ra的方法。建立了铀矿石密闭混酸一次溶样的方法和采用阴离子、阳离子和Sr特效树脂逐级离子交换分离纯化U、Th和Ra的流程,满足了TIMS测量要求。测定结果表明：100～1000 ng的天然铀中234U/238U,其测量精密度从静态多接收的2.34%提高到动态多接收的0.47%;对230Th与232Th丰度接近、质量为1μg左右的钍,采用三带点样技术和法拉第多接收技术测定230Th/232Th,其内精度平均值为0.0048%,外精度为0.028%;采用单带加钽发射剂,ETP跳峰测定50～100 fg镭-228稀释剂中的228Ra/226Ra,其内精度小于0.10%,外精度小于0.20%。比较TIMS和HR-ICP-MS、α能谱法测定234U/238U、230Th/232Th、228Ra/226Ra结果,三者结果相吻合。TIMS测量法样品用量少、快速、准确、精密度高,是U、Th、Ra同位素比值测定方法的又一补充。 相似文献
19.
The concentrations of 238U, 234Th, 226Ra, 222Rn and 210Pb and 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
20.
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