厦门火烧屿裸露岩石的铀放射系不平衡

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。     

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 

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%。     

Uranium, Thorium and Potassium In Reference Samples

Coherent values of U, Th and K concentrations obtained by γ-spectrometry are recommended for 43 international rock reference samples.     

TIMS测定铀矿石样品中^234U/^238U、^230Th/^232Th、^228Ra/^226Ra的方法研究

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。     

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.     

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     

参加BRIUGIC-2012铀矿地质样品成分分析实验室间比对的结果

核工业地质分析测试研究中心于2012年8月组织有关实验室进行了"BRIUG IC-2012铀矿地质样品成分分析实验室间比对"活动。本实验室在低本底铅室内用HPGeγ谱仪对5个样品进行了测量,通过相对比较法给出了各样品放射性核素238 U、232 Th、226 Ra、40 K的含量。比对结果仅Albriug-4样品的40 K测量值不满意,其他均为满意。对该次比对测量中存在的问题进行了初步分析和讨论,认为Albriug-4样品的测量值偏高是核素228 Ac的1459.2keV的特征峰干扰所致,测量值经干扰校正后,结果令人满意。而样品Albriug-1的232 Th测试项目给出的结果虽然为满意,但本实验室结果24.0μg/g与中位值49.3μg/g的偏差很大,认为可能是样品的钍系不平衡所致。     

Uranium anomalies in paleo-aquifers near sandstone-type uranium deposits in the devonian catskill formation of Pennsylvania

Background values of U and Th in 371 clastic sedimentary rocks from the Catskill Formation correlate negatively with Si and positively with Al and most other major elements because of the low content of U and Th in quartz and the relative enrichment in clays, Fe oxides, and other fine-grained components. Background U also correlates closely with Th. Similar results are obtained from 100 samples from western Colorado near the Uravan mineral belt. Weak anomalies in U are much more easily recognized by study of residuals or deviations from regressions of U against Al or Th than in the raw data.At Penn Haven Junction near Jim Thorpe, Pa., roll-type uranium deposits similar to those in Wyoming are localized around an iron-stained paleo-aquifer of conglomeratic sandstone. Twenty-four rock samples from this paleo-aquifer contain 1–12 ppm U; eight of the samples contain less than 4 ppm U, the background level for shales of the area. Uranium residuals from regressions against Al or Th are clearly anomalous for all samples. Experiments on another group of samples suggest that readily extractable U (H₂O₂-acetic acid leach) would also show the paleo-aquifer to be anomalous.Anomalies of this type in paleo-aquifers should be useful in evaluation of drill holes, outcrops, and radiometric data in prospective districts, especially when paleo-aquifers are difficult to recognize visually. The anomalies also indicate that U is added to sandstones of the paleo-aquifer rather than leached out to form the ore bodies in reduced rocks at the margins of a geochemical cell.     

中国东部全新世火山的镭-钍同位素年代学

活火山是指1万年来有过喷发历史的全新世火山。火山的高分辨年代学对火山灾害评估和火山分类具有重要意义。对于缺乏历史记载的全新世火山,直接对火山岩进行同位素定年很困难。本文利用具有高时间分辨率的镭-钍-铀非平衡确定中国东部年轻火山的年龄。根据镭-钍-铀同位素,海南岛的马鞍岭和雷虎岭是全新世火山(马鞍岭:4.3ka;雷虎岭:4.7ka);镜泊湖火山(4.9ka)也是全新世火山;龙岗火山存在晚更新世和全新世活动(7.0ka,15.0ka);大兴安岭阿尔山和诺敏河Ra/Th非平衡消失但~(230)Th/~(238)U非平衡显著,属于晚更新世喷发(阿尔山:63ka;诺敏河:71ka)。海南岛的马鞍岭火山、雷虎岭火山和东北地区的龙岗火山、镜泊湖火山,是4座活火山。至于东北地区的阿尔山和诺敏河火山是否是活火山,有待测试更多样品的Ra/Th同位素。五大连池老黑山和火烧山有历史喷发记录,这与它们都存在显著Ra/Th非平衡一致。五大连池老黑山和火烧山的岩浆滞留年龄分别小于4.2ka和3.2ka,岩浆上升速率 18～23m/y。     

Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of ²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, ²³⁰Th excesses and ²²⁶Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of ²²⁶Ra, this inverse correlation between ²³⁰Th and ²²⁶Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, ²²⁶Ra and ²³⁰Th excesses also vary with lava composition: ²²⁶Ra excesses are negatively correlated with Na₈ and La/Yb and positively correlated with Mg#. Conversely, ²³⁰Th excesses are positively correlated with Na₈ and La/Yb and negatively correlated with Mg#. Th/U, ²³⁰Th/²³²Th, and ²³⁰Th excesses are also variable and correlated to one another. ²³¹Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na₈. The isotope ratios ¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁸Pb/²⁰⁶Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, ²³⁰Th, and ²²⁶Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.     

U-series disequilibria in Kick’em Jenny submarine volcano lavas: A new view of time-scales of magmatism in convergent margins

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.     

Geochemistry of radium in soils of the Eastern United States

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.     

Uranium series disequilibrium in a young surficial uranium deposit,northeastern Washington,U.S.A.

A recently discovered ore-grade accumulation of U in organic-rich sediments of late Quaternary age provides an opportunity for studying the early association of U, U-daughters, and organic matter in a natural setting. The U occurs in valley-fill sediments of peat, peaty clay, silt, and sand along the north fork of Flodelle Creek, Stevens County, Washington. Radiometric techniques (delayed neutron, high-resolution gamma-ray spectrometry, thin-source alpha spectrometry) were employed to determine the abundance and distribution of U-series nuclides, the extent of secular equilibrium within the U decay series, and the apparent U-series ages of U incorporation.Sixteen lithologically distinct intervals were sampled from a 292 cm core. Uranium contents range from 140 to 2790 ppm and are positively correlated with organic contents. Measured alpha activity ratios of ²³⁴U/²³⁸U (1.31–1.38) are very similar to those reported in coexisting waters, suggesting a rather constant isotopic composition of introduced U. Much lower Th contents of <10–40 ppm are controlled by the type and abundance of silicate detritus. The youth of the host sediments (<15 000 a) and the paucity of associated radioactivity suggested large excesses of U relative to radioactive daughters and such excesses were observed, particularly in the shallowest intervals. Apparent ages of U emplacement determined by the (alpha) activity ratio of ²³⁰Th daughter to ²³⁴U parent show a general increase with depth and fair agreement with estimated depositional ages. This observation suggests dominantly syndepositional or early post depositional emplacement of U followed by decay-generated buildup of ²³⁰Th daughter with time. However, interval by interval comparisons of the relative abundances of other daughters, particularly ²²⁶Ra and ²¹⁰Pb, indicate variability caused by processes other than closed-system growth and decay, probably because chemically diverse daughters that are decay-generated in situ have differing mobilities and because upwelling ground water continuously adds more U and minor amounts of daughters. If ²³⁰Th is considered the least susceptible to these modifications, the data suggest some addition of ²³⁴U in the deepest intervals and some loss of ²²⁶Ra and/or gain of ²²²Rn throughout the studied core.     

平面型高纯锗低能γ谱仪测定铀矿样中^235U,^238U,^226Ra及^232Th含量

本文对选用低能γ射线测定铀矿中的~(235)U、~(238)U、~(226)Ra、~(232)Th含量进行了讨论,提出了一种对样品进行自吸收校正的简单准确方法,对铀矿样中的~(235)U、~(238)U、或~(235)U/~(238)U比值测定获得了较高的分析精度和准确度。     

Uranium and thorium in achondrites

The abundances of U and Th in 19 achondrites and two pallasite olivines have been measured by radiochemical neutron activation analysis. Brecciated eucrites are enriched relative to chondrites in both elements by factors between 10 and 20, perhaps as a result of a magmatic differentiation process. Two unbrecciated eucrites are far less enriched, possibly due to their origin as igneous cumulates. The diogenites Johnstown and Shalka contain approximately chondritic levels of U and Th, but Ellemeet is 10 times lower. The abundances in three howardites are in good agreement with those expected from major element data for a mixing model with eucrite and diogenite end members. The high O¹⁸ basaltic achondrites Nakhla, Shergotty and Angra dos Reis have a range of U and Th abundances similar to the brecciated eucrites and howardites, but have systematically higher Th/U ratios. The Bishopville aubrite has U and Th abundances and Th/U ratios similar to those of several enstatite chondrites, suggesting a genetic relationship. The Norton County aubrite has a low Th/U, similar to that observed in recrystallized and metamorphosed terrestrial ultrabasic rocks, indicating a more complex history. Pallasite olivines have low U and Th contents (0.5.4 ppb and 1.4.3 ppb, respectively) similar to those in terrestrial dunites. The Goalpara ureilite has very low U (<0–6 ppb) and Th (2.7 ppb) abundance consistent with an origin from carbonaceous chondrites by partial melting.     

Uranium-series disequilibrium in tuffs from Yucca Mountain,Nevada, as evidence of pore-fluid flow over the last million years

Samples of tuff from boreholes drilled into fault zones in the Exploratory Studies Facility (ESF) and relatively unfractured rock of the Cross Drift tunnels, at Yucca Mountain, Nevada, have been analysed by U-series methods. This work is part of a project to verify the finding of fast flow-paths through the tuff to ESF level, indicated by the presence of ‘bomb’ ³⁶Cl in pore fluids. Secular radioactive equilibrium in the U decay series, (i.e. when the radioactivity ratios ²³⁴U/²³⁸U, ²³⁰Th /²³⁴U and ²²⁶Ra/²³⁰Th all equal 1.00) might be expected if the tuff samples have not experienced radionuclide loss due to rock-water interaction occurring within the last million years. However, most fractured and unfractured samples were found to have a small deficiency of ²³⁴U (weighted mean ²³⁴U/²³⁸U=0.95±0.01) and a small excess of ²³⁰Th (weighted mean ²³⁰Th/²³⁴U 1.10±0.02). The ²²⁶Ra/²³⁰Th ratios are close to secular equilibrium (weighted mean=0.94±0.07). These data indicate that ²³⁴U has been removed from the rock samples in the last ∼350 ka, probably by pore fluids. Within the precision of the measurement, it would appear that ²²⁶Ra has not been mobilized and removed from the tuff, although there may be some localised ²²⁶Ra redistribution as suggested by a few ratio values that are significantly different from 1.0. Because both fractured and unfractured tuffs show approximately the same deficiency of ²³⁴U, this indicates that pore fluids are moving equally through fractured and unfractured rock. More importantly, fractured rock appears not to be a dominant pathway for groundwater flow (otherwise the ratio would be more strongly affected and the Th and Ra isotopic ratios would likely also show disequilibrium). Application of a simple mass-balance model suggests that surface infiltration rate is over an order of magnitude greater than the rate indicated by other infiltration models and that residence time of pore fluids at ESF level is about 400 a. Processes of U sorption, precipitation and re-solution are believed to be occurring and would account for these anomalous results but have not been included in the model. Despite the difficulties, the U-series data suggest that fractured rock, specifically the Sundance and Drill Hole Wash faults, are not preferred flow paths for groundwater flowing through the Topopah Spring tuff and, by implication, rapid-flow, within 50 a, from the surface to the level of the ESF is improbable.     

Ba, Ra, Th, and U in marine mollusc shells and the potential of Ra/Ba dating of Holocene marine carbonate shells

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.     

Precise two chronometer dating of Pleistocene travertine: The Th/U and Raex/Ra(0) approach

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.²²⁶Ra_ex/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 ²²⁶Ra_ex/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 ²²⁶Ra_ex/²²⁶Ra(0) ages agreed well with those derived from ²³⁰Th/²³⁴U, (ii) all data plot within uncertainty on the ²²⁶Ra_ex/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, ²²⁶Ra_ex/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.     

Distribution of radon sources and effects on radon emanation in granitic soil at Ben Lomond,California

The abundance and distribution of radioelements on bulk and microscopic scales were investigated in residual granitic-derived soil at a facility for investigating the movement of radon into structures. In bulk soil samples, Ra concentrations range from 0.6 to 1.3 pCi/g, and variations in Ra, Th, and K appear to be controlled mainly by heterogeneities inherited from the parent granitic rock, which contains abundant dikes and inclusions. U in soil and parent rock is concentrated in primary minerals (mainly zircon and sphene), and in secondary sites that are of greater importance for Rn emanation. The main U-bearing secondary sites are weathered sphene, grain boundary coatings, weathered biotite and plagioclase, as well as dense Fe-rich coatings and a REE-phosphate mineral present in near-vertical fracture zones in saprolite underlying shallow loam. Elevated U in these sites generally correlates with high Ti, Al, Fe, and/or P. Preferential distribution of U and Ra on grain boundaries and porous weathered minerals is reflected in relatively high Rn emanation rates in the soil. Highest emanation occurs between 1.3 and 2.3 m depth, where fine pedogenic phasesgibbsite and amorphous silica and Fe-OOH—are most abundant; it is related to fixation of Ra by these phases, which precipitate close to the surface and accumulate at these depths by illuviation. Separation of Ra from U may occur locally, given remobilization of U-series elements from secondary sites, and large differences between Ra and U sorption capabilities of several phases present in the soil. Concentration of U along permeable fracture zones in saprolite suggests that contribution of soil-gas Rn from depth (> 2 m) could be significant to Rn availability near the surface.     

The source of radium in anomalous accumulations near sandstone escarpments,Australia

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.