Radionuclide migration over recent geologic time in a granitic pluton

doi:10.1016/0168-9622(86)90058-8

Chemical Geology: Isotope Geoscience section

Volume 59, 1986, Pages 75-85

Chemical Geology: Is...

https://doi.org/10.1016/0168-9622(86)90058-8 Get rights and content

Abstract

Measurements of isotope activity ratios in the ²³⁸U decay series have been made for core samples obtained from boreholes up to 1-km depth in the Eye-Dashwa Lakes granite pluton, Atikokan, northwest Ontario, Canada. This work was done to determine the significance of recent radionuclide migration in plutonic rocks for the Canadian Nuclear Fuel Waste Management Programme. Most samples displayed secular equilibrium between ²³⁸U, ²³⁴U, ²³⁰Th and ²²⁶Ra, indicating that there has been no significant radionuclide migration in these samples over periods as long as the last 1 Ma. Disequilibrium was observed in some altered near-surface and deeper fracture-filling mineral samples with ²³⁴U deficiencies of up to 20%. A similar deficiency of ²²⁶Ra was found at ∼ 1-km depth in a gypsum-infilled core sample. These results indicate that U and Ra leaching by groundwater has occurred within periods 1 Ma and 8 ka ago, respectively. One instance of ²²⁶Ra excess was found at 94-m depth, indicating deposition of ²²⁶Ra in the last 8 ka. All cases of disequilibrium involved fracture-infilling minerals or altered granite cores from fracture zones, indicating that radionuclide migration occurred via fractures. Examples were found, however, of altered samples or mineral infillings that showed no detectable disequilibrium. These locations have been closed to migration in the last 1 Ma. Relative age estimates are made for some of the infilling minerals, based on their U-series isotopic characteristics.

References (31)

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²³⁴U/²³⁸U activity ratios of dissolved uranium in ground-waters from a Jurassic limestone aquifer in England
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²³⁰Th-²³⁸U disequilibria in historical lavas from Iceland
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²³⁵U fission tracks and ²³⁸U-series disequilibria as a means to study recent mobilization of uranium in Archaean pyritic conglomerates
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Uranium-series disequilibrium in the sedimentary uranium deposit at Yelirrie, Western Australia
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Drilling and core logging programs at the Atikokan Research Area

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The use of uranium-series disequilibrium to determine radionuclide migration on geologic time scales

M. Gascoyne

The determination of U-Th-Ra isotopic ratios in granitic rocks

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Uranium-series disequilibrium studies of Precambrian Shield rocks and ground-waters

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High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba

(1986)

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Saline groundwaters and brines in plutons in the Canadian Shield

Geol. Assoc. Can., Spec. Publ.

(1986)

Cited by (28)

226Ra/238U and 228Th/228Ra disequilibrium as weathering indices in beach sand sediments associated with granitoids from Cyclades. Greece
2019, Applied Geochemistry
Citation Excerpt :
All the above can cause the mobilisation of the U isotopes (238U, 234U) and 226Ra which under oxidising conditions are significantly more mobile than 230Th. The latter is geochemically immobile and cannot be removed easily under the pH conditions expected during weathering (circumneutral) (Gascoyne and Schwarcz, 1986). There is also a possibility for its isotopic system to be out of secular equilibrium due to the presence of a-particle recoil.
Radioactive secular disequilibrium measured as the absolute difference from 1 of the ²²⁶Ra/²³⁸U and ²²⁸Ra/²²⁸Th ratios have been used to determine weathering extent. Twenty-eight samples from beach sands of Cyclades islands, all of them being the weathering products of the local granitic plutons, are studied for their secular disequilibrium in both ²³⁸U and ²³²Th series (²²⁶Ra/²³⁸U and ²²⁸Th/²²⁸Ra respectively). The results are compared to weathering indices already in use worldwide and suggest a significant linear correlation of U-series disequilibrium with WIP (alkaline and alkaline-earth element leaching) and PWI (silica leaching) for the beach sands studied and significant non-linear correlations with CIA (chemical index of alteration) and PIA (plagioclase index of alteration). Considering only the heavy mineral enriched samples, significant correlations exist between U-series disequilibrium and CIA, CIW (chemical index of weathering) and PIA. Therefore, with an appropriate choice of the weathering index and samples both U-series and weathering index can recover the extent of chemical weathering. On the other hand, the use of Th-series disequilibrium as a weathering index is limited, due to ²²⁸Th and ²²⁸Ra geochemistry and especially due to their short radionuclide half-lives. U-series disequilibrium can therefore be used as a relatively sensitive weathering index in sediments consistent with CIA, CIW and PIA indices. However, the mineralogical composition must be considered. More specifically the mineral content of the sediments should involve considerable amounts of sensitive to weathering minerals (e.g. Fe-Mg minerals) as well as feldspars.
U-Pb, Rb-Sr, and U-series isotope geochemistry of rocks and fracture minerals from the Chalk River Laboratories site, Grenville Province, Ontario, Canada
2013, Applied Geochemistry
Citation Excerpt :
The process of selection of radioactive waste disposal sites must include characterization not only of the current hydrogeology, but also of the long-term geologic stability of the host rocks. Investigations of rock and fracture mineral geochemistry are an important part of site characterization for radioactive waste disposal programs in crystalline rock (Smellie and Stuckless, 1985; Gascoyne and Schwarcz, 1986; Milton and Brown, 1987; Krauskopf, 1988; Guthrie, 1991; Rasilainen et al., 2003; Hu and Möri, 2008; Tullborg et al., 2008; Blyth et al., 2009; Drake and Tullborg, 2009). Establishing geochronological framework at the CRL site is needed to understand the history of rock forming and rock alteration processes in this part of the Canadian Shield.
As part of the Geologic Waste Management Facility feasibility study, Atomic Energy of Canada Ltd. (AECL) is evaluating the suitability of the Chalk River Laboratories (CRL) site in Ontario, situated in crystalline rock of the southwestern Grenville Province, for the possible development of an underground repository for low- and intermediate-level nuclear waste. This paper presents petrographic and trace element analyses, U–Pb zircon dating results, and Rb–Sr, U–Pb and U-series isotopic analyses of gneissic drill core samples from the deep CRG-series characterization boreholes at the CRL site. The main rock types intersected in the boreholes include hornblende–biotite (±pyroxene) gneisses of granitic to granodioritic composition, leucocratic granitic gneisses with sparse mafic minerals, and garnet-bearing gneisses with variable amounts of biotite and/or hornblende. The trace element data for whole-rock samples plot in the fields of within-plate, syn-collision, and volcanic arc-type granites in discrimination diagrams used for the tectonic interpretation of granitic rocks.
Zircons separated from biotite gneiss and metagranite samples yielded SHRIMP-RG U–Pb ages of 1472 ± 14 (2σ) and 1045 ± 6 Ma, respectively, in very good agreement with widespread Early Mesoproterozoic plutonic ages and Ottawan orogeny ages in the Central Gneiss Belt. The Rb–Sr, U–Pb, and Pb–Pb whole-rock errorchron apparent ages of most of the CRL gneiss samples are consistent with zircon U–Pb age and do not indicate substantial large-scale preferential element mobility during superimposed metamorphic and water/rock interaction processes. This may confirm the integrity of the rock mass, which is a positive attribute for a potential nuclear waste repository. Most ²³⁴U/²³⁸U activity ratios (AR) in whole rock samples are within errors of the secular equilibrium value of one, indicating that the rocks have not experienced any appreciable U loss or gain within the past 1 Ma. However, ²³⁴U/²³⁸U AR in fracture mineral samples collected down to borehole lengths of about 740 m deviate from the secular equilibrium value and ²³⁴U/²³⁸U model ages calculated for fracture mineral samples showing excess ²³⁴U range from 593 to 1415 ka, thus providing evidence of fracture flow in the associated bedrock during the past 1.5 Ma. Rare earth element patterns are variable in fracture-filling calcites and Fe oxides/hydroxides but are similar to those observed in associated whole-rock samples. The observed Ce anomalies are very small ( ${Ce}_{N} / {Ce}_{N}^{*} \approx 1$ ), do not vary with depth, and, therefore, do not contain evidence that the studied fracture minerals precipitated from oxidizing waters at the conceptual depth of a repository.
Radioactive secular equilibrium in 238U and 232Th series in granitoids from Greece
2013, Applied Radiation and Isotopes
Citation Excerpt :
The use of uranium series disequilibrium in the geological sciences also includes applications such as determining the age of deposits younger than 1 Ma (Schwarcz and Gascoyne, 1984), mantle evolution (Condomines et al., 1988), hydrological tracing (Guttman and Kronfeld, 1982) and nuclear waste disposal (Schwarcz et al., 1982, Smellie, 1982). In the latter application, the 238U series technique examines the state of radioactive equilibrium, existing between the radionuclides of the 238U-decay series (Schwarcz et al., 1982; Smellie et al., 1986; Gascoyne and Schwarcz, 1986; Chapman and Smellie, 1986). When an isotopic system is undisturbed, it can be considered as closed.
Granitoid rocks belonging to plutons of Greece, covering a wide range of compositions and rock-types, have been studied for their specific activity (Bq/kg) of ²³⁸U and ²²⁶Ra from ²³⁸U radioactive series and ²²⁸Ra and ²²⁸Th from ²³²Th radioactive series by using gamma-ray spectroscopy. Results on the radioactive secular equilibrium of both ²³⁸U and ²³²Th radioactive series are presented by studying the ²²⁶Ra/²³⁸U and ²²⁸Ra/²²⁸Th ratios. The majority of the samples are in radioactive secular equilibrium for ²²⁶Ra/²³⁸U±1σ. However, several samples exhibit ²²⁶Ra/²³⁸U ratios significantly different from 1. The distortion of the secular equilibrium in the ²²⁶Ra–²³⁸U isotopic system of those samples has occurred over the last 1 Ma, and can be associated with post-magmatic processes, mainly rock–water interactions. All studied samples have ²²⁸Ra/²²⁸Th ratios equal to unity ±1σ. Consequently, they can be considered to be in secular equilibrium for the last 40 a.
Transport of 234U in the Opalinus Clay on centimetre to decimetre scales
2009, Applied Geochemistry
Citation Excerpt :
Hence, studies of natural radionuclides, mostly those of the long-lived U-series are useful. They may provide relevant information concerning (sub-) recent mobility of these radionuclides in rock–water systems being considered for a radioactive waste repository (Schwarcz et al., 1982; Gascoyne and Schwarcz, 1986; Smellie et al., 1986; Luo et al., 2000; Neymark and Paces, 2000; De Craen et al., 2000; Paces et al., 2002; Gascoyne et al., 2002; Deschamps et al., 2004). The 238U-series radionuclides are of interest as they represent elements that will constitute a large body of radioactive waste (Langmuir, 1997) and may also serve as analogues of other radionuclides such as Pu (Choppin, 2006).
The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of ²³⁸U, ²³⁴U and ²³⁰Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of ²³⁴U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both ²³⁸U and ²³⁰Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of ²³⁴U from the rock matrix to the pore water by the decay of ²³⁸U will be established. Diffusive redistribution separates ²³⁴U from its immobile parent ²³⁸U resulting in bulk rock ²³⁴U/²³⁸U activity disequilibria. These may provide a means of estimating the mobility of ²³⁴U in the rock if the diffusion rate of ²³⁴U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements.
Samples were digested using aqua regia followed by total HF dissolution, yielding two fractions. In all studied samples U was found to be concentrated in the HF digestion fraction. It has a high U/Th ratio and a study by SEM-EDS points to sub-μm up to several μm in size zircon grains as the main U-rich phase. This fraction consistently has ²³⁴U/²³⁸U activity ratios below unity. The minute zircon grains constitute the major reservoir of U in the rock and act as constant rate suppliers of ²³⁴U into the rock matrix and the pore water. The aqua regia leach fraction was found to be enriched in Th, and complementary to the HF fraction, having ²³⁴U/²³⁸U activity ratios above unity. It is believed that these U activity ratios reflect the surplus of ²³⁴U delivered from the zircon grains. Some cm-spaced samples show bulk rock ²³⁴U/²³⁸U activity ratios that are markedly out of equilibrium. In most of them a striking negative correlation between the total U content and the bulk rock ²³⁴U/²³⁸U activity ratios is observed. This is interpreted to indicate net ²³⁴U transfer from regions of higher supply of ²³⁴U towards those of lower supply which is, in most cases, equivalent to transfer from clayey towards carbonate/sandy portions of the rock. In contrast, the 25 cm averaged samples all have uniform bulk rock ²³⁴U/²³⁸U activity ratios in equilibrium, indicating U immobility in the last 1–1.5 Ma on this spatial scale. It is concluded that the small-scale lithological variations which govern U spatial distribution in the Opalinus Clay are the major factor determining ²³⁴U in situ supply rates, regulating its diffusive fluxes and controlling the observed bulk rock ²³⁴U/²³⁸U activity ratios. A simple box-model is presented to simulate the measured bulk rock ²³⁴U/²³⁸U activity ratios and to give an additional insight into the studied system.
Further investigations on optimized tail correction and high-precision measurement of uranium isotopic ratios using multi-collector ICP-MS
2003, Chemical Geology
Citation Excerpt :
Absolute dating of marine carbonates such as corals, used for paleo-sea level studies (Stirling et al., 2001; Gallup et al., 2002) and paleo-reconstitution of (234U/238U) seawater (Henderson, 2002), for instance. Studies of radionuclide migration in deep geological formations conducted to assess the safety of radioactive waste disposal in such environments (see, for example, Schwarcz et al., 1982; Smellie and Stuckless, 1985; Gascoyne and Schwarcz, 1986; Smellie et al., 1986; Gascoyne and Cramer, 1987; Ivanovich et al., 1992; Griffault et al., 1993). The second application is primarily concerned with determining whether a geological system is at secular equilibrium or not.
In the present paper, we further examine the optimum conditions for rapid, precise and accurate determination of ²³⁴U/²³⁸U ratios in geological materials by multiple collector (magnetic-sector) inductively coupled plasma mass spectrometry (MC-ICP-MS). In our experiments, isotopic measurements were performed on a Micromass IsoProbe™ instrument, using Faraday collectors in static mode only. Unlike the ion counting system coupled with an energy filter, this technique eliminates the difficulty of proper calibration of the Daly/Faraday gain ratio. On the other hand, since our Micromass instrument has a poor abundance sensitivity (the proportion of the ²³⁸U ion beam measured at mass 237 is ∼25 ppm), the major issue to be addressed is the tail correction. For this purpose, we have developed a tail correction method slightly modified from Thirlwall [J. Anal. At. Spectrom. 16 (2001) 1121]. This method is based on correction of the actual tail contribution under each peak, as assessed by the tail shape measurements on mono-isotopic ion beams, instead of the usual half-mass zeroes baseline estimation. Our approach enabled us to correct for the large offset that can be observed on isotopic data when tail correction is done by means of linear interpolation between half-mass zeroes, and showed that this latter tail correction method results in nearly 3% underestimation of ²³⁴U/²³⁸U ratios on the GEOTOP IsoProbe™ for material at secular equilibrium.
A ²³⁶U–²³³U double spike was employed to correct for mass discrimination bias. Using an Aridus™ micro-concentric, desolvating nebulizer sample introducing system, a minimum of 200 ng of sample-U was consumed to carry out a precise ²³⁴U/²³⁸U analysis, thereby allowing a ²³⁴U signal of ∼4–5 mV to be monitored for 50 measurement cycles of 5 s each. This time-consuming, 10-min procedure allowed us to obtain an external reproducibility of 0.8‰ (2σ) for isotopic measurements of the NBL-112a standard solution. Replicate measurements of this reference material yielded a mean δ²³⁴U value of −36.42±0.80‰ (2σ, n=19), which is highly consistent with values reported by other laboratories. The total reproducibility, including both chemical separation and spectrometric measurement, was assessed using geological samples (a coral and a carbonate rock); the long-term reproducibility obtained was about 1.3‰ (2σ).
Uranium isotopic data in uraninite spent fuel from the Bangombe natural nuclear reactor (Gabon) and its surroundings
2000, Applied Radiation and Isotopes
In the framework of the “Oklo-Natural Analogue Phase II” Project, uraninite from the Bangombé natural reactor and samples from its host rock were analyzed to determine their uranium isotopic composition by thermal ionisation mass spectrometry, inductively coupled plasma mass spectrometry and alpha spectrometry. There were several objectives for this work: (i) to validate the $^{235} U /^{238} U$ isotopic ratios obtained by these techniques; (ii) to test the use of the $^{235} U /^{238} U$ ratio of uraninite as a tracer of migration/retention processes of uranium from the source term to the far field; (iii) to evaluate the most recent migration/retention processes of uranium in the system by U-series disequilibrium.

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Chemical Geology: Isotope Geoscience section

Abstract

References (31)

²³⁴U/²³⁸U activity ratios of dissolved uranium in ground-waters from a Jurassic limestone aquifer in England

Earth Planet. Sci. Lett.

²³⁰Th-²³⁸U disequilibria in historical lavas from Iceland

Earth Planet. Sci. Lett.

Tracing interaquifer connections in the Kefar Uriyya-Agur region (Israel), using natural uranium isotopes

J. Hydrol.

The mobility of thorium in natural waters at low temperatures

Geochim. Cosmochim. Acta

Uranium-series dating of Quaternary deposits

Uranium-series disequilibrium studies of granitic rocks

²³⁵U fission tracks and ²³⁸U-series disequilibria as a means to study recent mobilization of uranium in Archaean pyritic conglomerates

Earth Planet. Sci. Lett.

Uranium-series disequilibrium in the sedimentary uranium deposit at Yelirrie, Western Australia

J. Geol. Soc. Aust.

Drilling and core logging programs at the Atikokan Research Area

The use of uranium-series disequilibrium to determine radionuclide migration on geologic time scales

The determination of U-Th-Ra isotopic ratios in granitic rocks

Uranium-series disequilibrium studies of Precambrian Shield rocks and ground-waters

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba

Saline groundwaters and brines in plutons in the Canadian Shield

Geol. Assoc. Can., Spec. Publ.

Cited by (28)

<sup>226</sup>Ra/<sup>238</sup>U and <sup>228</sup>Th/<sup>228</sup>Ra disequilibrium as weathering indices in beach sand sediments associated with granitoids from Cyclades. Greece

U-Pb, Rb-Sr, and U-series isotope geochemistry of rocks and fracture minerals from the Chalk River Laboratories site, Grenville Province, Ontario, Canada

Radioactive secular equilibrium in <sup>238</sup>U and <sup>232</sup>Th series in granitoids from Greece

Transport of <sup>234</sup>U in the Opalinus Clay on centimetre to decimetre scales

Further investigations on optimized tail correction and high-precision measurement of uranium isotopic ratios using multi-collector ICP-MS

Uranium isotopic data in uraninite spent fuel from the Bangombe natural nuclear reactor (Gabon) and its surroundings

Research paperRadionuclide migration over recent geologic time in a granitic pluton

Abstract

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

J. Hydrol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Uranium-series disequilibrium in the sedimentary uranium deposit at Yelirrie, Western Australia

J. Geol. Soc. Aust.

Drilling and core logging programs at the Atikokan Research Area

The use of uranium-series disequilibrium to determine radionuclide migration on geologic time scales

The determination of U-Th-Ra isotopic ratios in granitic rocks

Uranium-series disequilibrium studies of Precambrian Shield rocks and ground-waters

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba

Saline groundwaters and brines in plutons in the Canadian Shield

Geol. Assoc. Can., Spec. Publ.

Research paper
Radionuclide migration over recent geologic time in a granitic pluton