A laboratory study of the transfer of 234U and 238U during water–rock interactions in the Carnmenellis granite (Cornwall, England) and implications for the interpretation of field data

Applied Radiation and Isotopes 54 (2001) 977–994

A laboratory study of the transfer of 234U and 238U during water–rock interactions in the Carnmenellis granite (Cornwall, England) and implications for the interpretation of field data D.M. Bonottoa,*, J.N. Andrewsb,1,X, D.P.F. Darbyshirec a

Departamento de Petrologia e Metalogenia, Universidade Estadual Paulista (UNESP), Caˆmpus de Rio Claro, Av. 24-A No.1515, C.P. 178, CEP 13506-900, Rio Claro, Sa˜o Paulo, Brazil b Postgraduate Research Institute for Sedimentology (PRIS), University of Reading, P.O. Box 225, Whiteknights, Reading RG6 2AB, UK c Isotope Geosciences Laboratory, Natural Environment Research Council, Kingsley Dunham Centre, Keyworth, Nottingham NG 12 5GG, UK Received 11 April 2000; received in revised form 11 August 2000; accepted 23 August 2000

Abstract Laboratory time-scale experiments were conducted on gravels from the Carnmenellis granite, Cornwall, England, with the purpose of evaluating the release of natural uranium isotopes to the water phase. The implications of these results for the production of enhanced 234U/238U activity ratios in Cornish groundwaters are discussed. It is suggested that the 234U/238U lab data can be used to interpret activity ratios from Cornwall, even when the observed inverse relationship between dissolved U and 234U/238U in leachates/etchates is taken into account. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Uranium; Isotopes; Granites

1. Introduction Uranium is among the main elements contributing to natural terrestrial radioactivity. It is a lithophile element, being concentrated preferentially in acid igneous rocks compared with intermediate, basic, and ultrabasic varities. Uranium occurs in crustal rock at an average concentration of about 2.5 ppm (Bowie and Plant, 1983), and has two primary isotopes, 238U and 235 U. 238U is the principal isotope of natural U (99.72% abundance). The isotope 234U is radiogenic and the decay chain from 238U to 234U proceeds as follows: 238U (4.46 Ga, a) ! 234Th (24.1 days, bÿ) ! 234 Pa (1.18 min, a) ! 234U (248 ka, a) !


. *Corresponding author. Tel.: +55-19-5262825; Fax: +5519-5249644. E-mail address: [email protected] (D.M. Bonotto) 1,X Deceased.

Since its discovery by Joly in 1908 (Mawson, 1969), radioactive disequilibrium in the U decay series has been extensively studied in almost all surficial environments. Because 234U is a heavy radiogenic isotope, it is essentially immune to mass-fractionation effects. Despite this, studies on secondary minerals and groundwaters have found radioactive disequilibrium between 238U and 234 U (Cherdyntsev, 1971). The isotopes 238U and 234U are in secular equilibrium only in minerals and rocks which are closed systems for U over a time-scale greater than 106 yr. Considering that the 234U/238U activity ratio is defined as the ratio of the 234U daughter activity to the 238U parent activity, an unit value for this parameter is achieved in the bulk of rock/mineral matrices under these conditions. However, rock–water interaction frequently results in 234U/238U activity ratios for dissolved uranium which are greater than unity (Osmond and Cowart, 1976; Ivanovich and Harmon, 1982).

0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 3 8 - 9

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Several alternative mechanisms have been suggested to explain the generation of the enhanced activity ratios in solution. For instance, Rosholt et al. (1963) proposed the occurrence of enhanced chemical solution of 234U due to radiation damage of crystal lattices or to autoxidation from U4+ to U6+ on decay of the parent 238 U. This model was supported by the results of Chalov and Merkulova (1966) who obtained activity ratios of up to 1.3 by leaching fresh, unaltered igneous rocks and showed that in 6 of 12 samples of uranium minerals 234 6+ U is enhanced relative to 238U6+. The experiments performed by Kigoshi (1971) and Fleischer and Raabe (1978a,b) showed that a-particle recoil ejection of the 234 U precursor, 234Th, into the solution may also generate enhanced activity ratios in the liquid phase. According to this model, if the interstices of a rock or assemblage of mineral grains are permeated with water, the water will absorb the recoiling, short-lived, 234Th nucleus from 238U decay, thereby enriching the solution and depleting the solid in 234U. Kigoshi (1971) also recognized the possibility that a-recoil tracks intersecting the surface of grains could provide paths of rapid diffusion that could allow 234U to diffuse outwards and be accessible to water that may later enter the pore spaces. Fleischer (1975) suggested a similar model involving the presence of a-recoil tracks, except that the damage track is removed chemically by the intergranular liquid: the recoil 234U nuclei are implanted when the interstitial space of the solid phase was dry during 238U decay, and chemical dissolution of the damage track after infiltration of water removes the 234U by etch solution. Some experimental evidence for this process was given by Fleischer (1980, 1982). In crystalline rocks, the most of the uranium is incorporated into accessory minerals such as monazite, allanite, sphene, and zircon so that uranium is not readily accessible for solution and available to secondary mineralization processes (Larsen and Phair, 1954; Brown and Silver, 1955; Gabelman, 1977; Speer et al., 1981; Tieh and Ledger, 1981). Uranium in the major, rock-forming minerals is more susceptible to leaching, particularly as the rock disintegrates during weathering. Brown and Silver (1955) studied the distribution of uranium and thorium in igneous rocks and concluded that less than one-third of the uranium is present as interstitial oxide or cryptocrystalline aggregate and available for leaching. Adams et al. (1959) estimated that 60–85% of the Th and U in igneous rocks is contained in resistates that are incorporated intact into sedimentary derivatives. Alteration of granitic rocks has been investigated since the beginning of the 20th century. For example, Goldich (1938) evaluated chemical weathering of granitic gneiss from southern Minnesota, USA, and observed that rapid loss of plagioclase and biotite, and slower removal of K-feldspar and quartz occurred,

allowing Goldich (1938) to show that the susceptibilities of the rock-forming silicate minerals to weathering are related to their position in Bowen’s reaction series (i.e. dependence on temperature of crystallization from a magma), in which olivine and Ca-plagioclase are considered as most susceptible followed by pyroxene, hornblende, biotite, and Na-plagioclase (Faure, 1991). Interest in these studies has increased in the last few years because of the importance of understanding chemical processes that affect the mobility of major, minor and trace-elements in the granitic matrices (Middelburg et al., 1988; Mongelli, 1993; Gouveia et al., 1993; Brown, 1993; Vanderweijden and Vanderweijden, 1995; Blum and Erel, 1997; Gavshin et al., 1997; Seimbille et al., 1998), with the purpose of determining the migration of radionuclides present from high-level wastes (from power reactors) buried in these rocks (Heath, 1984; Guthrie and Kleeman, 1986; delVillar et al., 1996; McAlister et al., 1997). Leaching experiments on igneous rocks have been performed under situations that approximate natural conditions, with the aim of determining the mobility of uranium (Szalay and Samsoni, 1969; Samsoni, 1969; Kovalev and Malyasova, 1971; Zielinski et al., 1981; Michel, 1984; Eyal and Olander, 1990). An acid (1– 6 N HCl) or carbonate [0.05–5% NaHCO3 or (NH4)2CO3] solution has been generally utilized for leaching powdered samples in order to characterize the mobile uranium. For example, Kovalev and Malyasova (1971) in multiple leaching experiments on various igneous rock types (granite, andesite, diorite, gabbro, basalt, etc.) using a solution of 5% (NH4)2CO3 found the greatest values (20–80%) for uranium leached from granites. Using 2% NaHCO3, a substantial amount of U in solution was readsorbed onto the rock particles, implying that adsorbed, mobile, leachable, and acidsoluble uranium cannot be distinguished, with acidsoluble uranium being considered to range between 20 and 70% (Szalay and Samsoni, 1969; Samsoni, 1969). Michel (1984) considered leachable uranium to be the fraction that is soluble under the experimental conditions, irrespective of how it is distributed in the rock or its behavior in nature, and thus the leachable uranium from parent rock may or may not represent uranium available for groundwater dissolution and transport. Leaching experiments involving 238U and 234U uranium isotopes were reported by Zielinski et al. (1981), who found that after 20 h, 2–6% of the total U was removed, with an average 234U/238U activity ratio of 1.72 in leach solutions. Eyal and Olander (1990) studied eight separate leaching series of monazite specimens in a bicarbonate–carbonate solution for durations up to 6.8 yr, and observed a fractionation of 234U relative to 238 U by a factor of 1.1–2 during the initial leaching period. Latham and Schwarcz (1987) proposed that a clear distinction must be made between etching and leaching

D.M. Bonotto et al. / Applied Radiation and Isotopes 54 (2001) 977–994

processes. Although both have been documented in the literature, they have not been distinguished during discussion of the data. If a rock/mineral is etched, each surface layer is removed successively, and the rate of removal of U is independent of the U concentration; thus, chemical etching (uniform dissolution) of a surface is a zero-order rate process. In contrast, if U is leached from a mineral or rock, all atoms are assumed to be equally removable, with the matrix little altered. The rate of U removal is therefore proportional to the concentration of U, and so chemical leaching (selective dissolution of one or more species) of a surface is a firstorder rate process, that depends on the surface concentration of the species being dissolved. In this paper, the effect of chemical etching and leaching on the 234U/238U activity ratio of dissolved U in the etch/leach solution at Carnmenellis granite gravels from Cornwall, England, is investigated on a laboratory time-scale. The implications of the results obtained in the laboratorial experiments for the production of enhanced activity ratios in natural saline groundwaters from Cornwall, England, is also discussed.

2. Geological and hydrological setting The rock types considered in this paper for the application of the laboratory/field experimental data consist of granites from Cornwall, extending from Land’s End to Dartmoor. The Carnmenellis granite is one of the five major plutons which form the Cornubian batholith in south-west England, being nearly circular in outcrop (Fig. 1). Its origin has been the subject of debate, but is generally considered to have intruded into Devonian argillaceous sediments during the Hercynian orogeny (Carboniferous/Permian) (Jackson et al., 1982) or during the post-Variscan cooling and uplift processes that took place 293 Ma ago (Chen et al., 1996). It is predominantly a biotite adamellite granite with primary quartz, plagioclase (usually oligoclase), orthoclase (often perthitic) and biotite. Muscovite is usually present, either as a late magmatic or hydrothermal mineral (Exley and Stone, 1964). The granite is enriched in volatile elements (B, Cl, F, Li) compared with many other granites in the UK and commonly carries accessory tourmaline, apatite and zircon. Tourmalinisation and greisen formation has occurred as the result of boron and fluorine metasomatism. Kaolinization, which is locally extensive, may be of hydrothermal or lowertemperature origin (Alderton and Rankin, 1983). The granite produced a low-grade thermal aureole with cordierite and andalusite. There is a strong hydrothermal mineralization with oxides and sulphides predominantly of Sn, Cu, Pb, and Zn that produced economic vein deposits of Variscan age, although there is an evidence for hydrothermal activity at stages later than

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Fig. 1. Location map showing the Carnmenellis granite (adapted from Edmunds et al., 1985).

this (Durrance et al., 1982). The principal mineral lodes strike ENE–WSW or E–W in the mineralized belt north of the Carnmenellis granite, but later-stage mineralization also occurs as NW(NNW)–SE(SSE) cross-courses (Edmunds et al., 1985). The Cornubian batholith is situated in an area with substantially higher heat flow than the average (60 mW mÿ2) for the UK. The Carnmenellis granite has an average heat flow of about 120 mW mÿ2 (Wheildon et al., 1977) while the South Crofty mine, situated on the northern edge of the Carnmenellis granite has na average heat flow of 129 mW mÿ2 (Tammemagi and Wheildon, 1974) (Fig. 1). The higher thermal conductivity of the granite in comparison with the surrounding metasediments results in the temperature gradients in the granite generally being lower than the temperature gradients across the metapelite. The average temperature gradient in the Carnmenellis granite, determined from several boreholes drilled across its outcrop, is 29.88C Kmÿ1, but is 388C Kmÿ1 in the mineralized satellite intrusion, at South Crofty, Carn Brea, which is observed to be in contact with Carnmenellis pluton at depth (Edmunds et al., 1985). The temperature gradient in the metapelites is rather more variable (20–508C kmÿ1), but in general the lower thermal conductivities in the aureole result in steeper gradients in the immediate vicinity of the granite. The local temperature gradient at Wheal Jane is 458C kmÿ1. An experimental facility was established in 1980 jointly by the European Economic Community and the Department of Energy (UK) for the investigation of the development of geothermal ‘‘hot dry rock’’ systems

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(Edmunds et al., 1985; Andrews et al., 1986). In order to realize the studies, a doublet system (wells RH 11/RH 12) was drilled to a depth greater than 2000 m in the Carnmenellis granite at Rosemanowes Quarry, Penryn, 12 km south of the area of active tin mining, where water from a surface reservoir was injected by well RH 12 and returned to the surface by well RH 11. Saline (up to 19,300 mg lÿ1 total mineralization) and thermal (up to 528C) groundwaters are observed in active and disused tin mines in the north of the Carnmenellis granite, and they were considered in detail as part of the research programme connected with exploration of the geothermal potential of the UK (Burgess et al., 1982; Edmunds et al., 1984). There are many inflows of saline warm water in workings in the tin mines in the granite and its aureole, with water inflows to the mines occurring along joints or fractures in the granite (Fig. 2). They generally issue from cross-courses with discharges between 1 and 10 l sÿ1 at depths between 200 and 700 m below surface. The discharge temperatures (up to 528C) are typically in excess of the average regional thermal gradients of 29.88C kmÿ1 in the granite and 508C kmÿ1 in the aureole (Wheildon et al., 1980). This implies that warmer saline fluids are upwelling by convective circulation (Fig. 2) and is borne out by the SiO2 and Na/K geothermometry, which suggests an equilibration temperature of around 548C in the granite, equivalent to a maximum circulation depth of 1200 m. The geochemistry of the saline groundwaters and the freshwater sources in the area has been discussed by Edmunds et al. (1984). Even the saline mine waters

contain tritium and it was shown that the observed saline chemistry must result from mixing between recent non-saline, shallow groundwaters (recent meteoric waters) and a much older brine (saline component) derived from the granite. The driving force for the current circulation system is the hydraulic sink created by the mining operations. Circulation of the groundwater must also have been taking place at a very slow rate over a geological time span in response to changes in hydraulic gradients (Durrance et al., 1982). The proportion of recent groundwater as indicated by tritium must be between 6 and 65%, implying that fluids with salinities higher than those measured could exist. The saline component has been shown from the radiogenic 4He contents and uranium series geochemistry to have had a likely residence time of at least 5  104 yr, and probably of the order 106 yr. The stable isotope compositions of all samples of recent and saline waters analysed by Edmunds et al. (1984) cluster near the meteoric line (d2H=ÿ29 to ÿ38%; d18O=ÿ5.2 to ÿ6.1%), with no systematic variation with salinity, indicating that all groundwaters in the granite are of meteoric origin. The most important features of the chemistry, in addition to the high chloride concentrations, are the depletion of Na+ relative to Clÿ, the enhanced Ca2+ levels and especially the significantly enriched Li+, with values as high as 125 mg lÿ1. The unusual chemistry combined with the oxygen and hydrogen stable-isotope compositions demonstrate that seawater can be ruled out as the source of any of the salinity.

Fig. 2. Schematic cross-section through the Carnmenellis granite showing the position of the tin mines and the pattern of circulation of groundwaters (adapted from Edmunds et al., 1985).

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3. Experimental techniques Selected specimens of the Carnmenellis granite were collected at Rosemanowes Quarry, Penryn, and South Crofty mine, Cornwall (Table 1). Determinations of 238U and 234U in the rock samples were made using standard alpha spectrometric techniques (Osmond and Cowart, 1976, 1981; Veselsky, 1974; Ivanovich and Harmon, 1982). About 1 g of material was crushed to 200 mesh, put in acid digestion bomb similar to Parr 4575 at temperature of 1508C and internal pressure of 1200 psig, and brought into complete solution with HF, HNO3 and HCl, which was then evaporated and the residue treated with 6 M HCl. About 500 mg of FeCl3 and a proper amount of the synthetic nuclide 232U was added to each sample, since it is a yield

tracer (spike) suitable for obtaining the uranium concentration and activity ratio data. The isotopes of uranium were co-precipitated on Fe(OH)3 by increasing the pH to 7–8 by the addition of concentrated NH4OH solution. The precipitate was recovered, dissolved in 9 M HCl and Fe3+ was extracted into an equal volume of methyl isobutyl ketone. The acid solution of uranium was further purified by anion exchange, first on a Clÿ and then on a NOÿ 3 column of Biorad AG1-X8 100–200 mesh resin. Uranium was finally eluted from the NOÿ 3 column with 0.1 M HCl, evaporated to dryness, dissolved in 10 cm3 2 M (NH4)2SO4 solution, and transferred to a Teflon electrolysis cell. Electrodeposition of U on a stainless-steel planchet was complete after 3 h at a current density of 1 Acmÿ2. The alpha activities were determined with 100 mm depletion depth, 450 mm2

Table 1 Carnmenellis granite gravels used for etch/leach experimentsa Physical parameters Sample no.

Location

1 2 3 4 5

Rosemanowes Quarry South Crofty Mine Rosemanowes Quarry Rosemanowes Quarry Rosemanowes Quarry

Density (g cm3) 2.5 2.5 2.5 2.5 2.5

Weight (kg)

Mean radius Specific surface Total surface (cm) area (cm2 gÿ1) area (m2)

2.80 1.75 } } }

0.41 0.25 0.16 0.32 0.50

Chemical characterization Parameter Unit

Sample 1

SiO2 Na2O K2O CaO MgO Al2O3 Fe2O3 MnO TiO2 P2O5 LOI U 234 U/238U activity ratio Si Na K Ca Mg Molar U/Si ratio Molar U/Na ratio Molar U/K ratio Molar U/Ca ratio Molar U/Mg ratio

69.23 69.44 3.48 3.16 5.04 5.89 1.22 0.93 0.60 0.45 16.18 15.81 2.54 2.29 0.05 0.04 0.32 0.26 0.30 0.27 1.02 1.44 71.90  13.53 83.25  21.20 0.99  0.05 0.99  0.04 323 993 325 003 25 863 23 418 41 870 48 904 8735 6623 3598 2709 2.62 3.02 2.68 3.43 2.82 2.80 1.39 2.12 2.04 3.14

a

wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% mg gÿ1 } mg gÿ1 mg gÿ1 mg gÿ1 mg gÿ1 mg gÿ1  10ÿ5  10ÿ4  10ÿ4  10ÿ3  10ÿ3

2.94 4.64 } } }

0.82 0.81 } } }

Sample 2

Sample descriptions: 1=granite taken from an outcrop; 2=granite taken from 335 m depth; 3–5=granite taken from an outcrop and crushed, respectively, to the size fractions 0.9–2.4 mm, 2.4–4 mm, and >4 mm, as reported by Andrews et al. (1987) LOI=Loss on ignition.

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area silicon surface barrier detectors, whose typical backgrounds in the 238U, 234U and 232U energy regions were 0.001  0.0002, 0.0009  0.0002 and 0.0028  0.0003 cpm, respectively. The spectra for natural U and 232U tracer extracted were recorded on a EG&G ORTEC 919 Spectrum Master Multichannel Buffer, where the concentration data were calculated from the counting rates of 238U and 232U peaks and the activity ratio data were calculated from the counting rates of 234 U and 238U peaks. A part of each rock sample was reserved for major element analysis by X-ray fluorescence spectrography. The experimental etch/leach on a laboratory timescale was performed using different size distributions of Carnmenellis granite gravels. Their specific surface area S (cm2 gÿ1) was determined by counting a large number of particles (at least 600 randomly selected), and weighing the total amount. After evaluating the average mass m of each particle distribution, the value of S was calculated from the equation S ¼ ð36p=r2 mÞ1=3 , where r is the rock density (g cmÿ3).The values of S were 2.94 and 4.64 cm2 gÿ1, and the other results of physical and chemical characterization of the analysed rocks are reported in Table 1. Freshly crushed and sized samples of the Carnmnellis granite were subjected to chemical dissolution in the laboratory under controlled conditions, where the gravels were initially washed with distilled water to remove any finely divided material. After drying, 1.8– 2.8 kg aliquots were weighed into 5 l glass bottles and subjected to chemical etch/leach at room temperature (208C) with distilled water equilibrated with the atmosphere (PCO2 10ÿ3.5 atm; pH=4.9; Eh=153 mV). The solutions were daily circulated through the rock aggregates, and they were periodically removed for analysis and replaced by a fresh one, where etching/ leaching was continued in this sequential manner for up to 200 days. The etch/leach solutions were filtered through a 0.45 mm Millipore membrane, an aliquot was reserved for Na+, K+, Ca2+, Mg2+, and Si measurements by atomic absorption spectrophotometry, and the remainder was acidified to pH less than 2 using HCl for U and acivity ratio determinations, following the analytical procedure described for the rocks, but using 236 U as a yield tracer (spike) and performing the record of spectra on a Canberra, 2048-channel, multichannel analyzer. The results of the measurements are reported in Table 2.

4. Experimental results 4.1. Chemical composition of the rock matrices The reported values in Table 1 show that the bulk chemical composition of both Carnmenellis granite

samples do not differ significantly. With the exception of oxygen, the major element abundances (higher than 1%) were found for Si, Al, K, Na, and Fe, concentrations that also decreased in the order Si> Al>K>Na>Fe for other samples referred by Andrews et al. (1987). Al-Turki and Stone (1978) estimated modal compositions of the main Carnmenellis granite rock type as quartz 31–36 vol%, K-feldspar 28–33 vol%, plagioclase 24 vol%, biotite 3–6 vol%, and muscovite 5–7 vol%; values that justify silica as the major constituent (69 wt%) of the samples (Table 1). Edmunds et al. (1985) reported values of SiO2 between 34.9 and 38.6 wt% in biotites of the Carnmenellis granite, which also were higher than those obtained for the other oxides. K2O constitutes the main component among the oxides of the major cations Na, K, Ca, and Mg, where the measured concentrations (5–6 wt%) reflect the chemical composition of K-feldspar, with some contribution from micas. Edmunds et al. (1985) reported values between 7.7 and 10.5 wt% for selected specimens of biotite. Plagioclase may be considered as the principal source of Na2O and CaO in the analysed samples, and the biotites are the potential source of MgO, with values between 1.9 and 5.7 wt% (Edmunds et al., 1985). However, despite the predominance of primary quartz, plagioclase, orthoclase, and micas in the Carnmenellis granite, it is not possible to ignore that accessory minerals and inclusions of olivine, smectite, corrensite, zircon, uraninite, monazite and tourmaline deposited as vein infills (Hussain, 1997) may also contribute to the whole rock chemistry reported in Table 1. Neuerburg (1956) considered that uranium may have six modes of occurrence in these types of rocks: (1) uranium minerals, (2) uranium substituting in minor amounts for cations in the structures of rock minerals, and uranium located in minor amounts in structural defects of rock minerals, (3) uranium held in cationexchange position, (4) uranium adsorbed on crystal surfaces, on surfaces of crystallographic discontinuities, and on surfaces of irregular cracks within crystals, (5) uranium dissolved in fluid inclusions contained within rock minerals, and (6) uranium dissolved in intergranular fluids. The measured U content of the Carnmenellis granite is higher than the average of 10.8 mg gÿ1 reported by Andrews et al. (1987) but given the experimental errors, the values obtained are similar. The large 1 s analytical uncertainty in U concentration reflects the low counting time used to evaluate the peak of the spike, whose activity corresponding to 3.39 dpm, was not high enough to yield a peak area equivalent to that of 238U and 234U. The measured 234U/238U activity ratios were equal to 1, within experimental error, indicating that secular radioactive equilibrium was established between 238U and 234 U in the bulk of both rock matrices (i.e. undisturbed

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D.M. Bonotto et al. / Applied Radiation and Isotopes 54 (2001) 977–994 Table 2 Dissolution of U and major cations from Carnmenellis granite gravels by chemical etch/leach Solution Type

Sample 1 Dist. water

Sample 2 Dist. water

Etch/leach Volume (l)

No.

234

U/238U

Dissolved

Activity

Cations dissolved (mg) +

(days)

(mg)

ratio

Na

K

+

Ca2+

Mg2+

Si

1.80 1.92 1.95 1.75 1.70

1 2 3 4 5 1+2 1+2+3 1+2+3+4 1+2+3+4+5

5.1 12.9 30.1 50.8 102.3 18.0 48.1 98.9 201.2

3.4 2.5 1.9 2.3 4.3 5.9 7.8 10.1 14.4

1.89  0.06 1.77  0.07 1.61  0.06 1.42  0.06 1.46  0.04 1.83 1.76 1.67 1.63

10.1 3.8 2.9 1.6 0.8 13.9 16.0 17.6 18.4

2.5 1.9 1.8 1.7 1.5 4.4 6.2 7.9 9.4

3.2 3.8 4.4 4.5 5.2 7.0 11.4 15.9 21.1

1.2 1.2 1.3 1.5 1.7 2.4 3.7 5.2 6.9

} 2.31 1.44 1.60 3.75 5.35 } 1.68 0.52

1.40 1.25 1.20 1.50 1.50

1 2 3 4 5 1+2 1+2+3 1+2+3+4 1+2+3+4+5

5.1 12.9 30.0 50.9 102.0 18.0 48.0 98.9 200.9

6.4 5.9 10.7 26.4 41.2 12.3 23.0 49.4 90.6

1.20  0.03 1.29  0.04 1.22  0.04 1.20  0.01 1.12  0.02 1.24 1.24 1.23 1.21

34.2 9.5 5.9 4.2 3.6 43.7 49.6 53.8 57.4

3.6 1.7 2.0 1.9 1.8 5.3 7.3 9.2 11.0

7.4 4.4 14.9 14.4 20.2 11.8 26.7 41.1 61.3

0.8 0.5 1.2 1.1 1.3 1.3 2.5 3.6 4.9

} 2.20 } 1.17 2.60 1.65 3.77 5.42

1 2 3 4 2+3 2+3+4

10.7 28.0 8.0 28.8 36 64.8

2.4 2.7 1.2 1.6 3.9 5.5

2.11  0.13 1.90  0.05 1.58  0.06 1.66  0.06 1.74 1.71

} 1.36 0.60 1.16 1.96 2.49

} } 1.02 2.18 } }

} 2.04 1.28 0.53 3.32 5.50

} 0.82 0.42

1 2 3 4 2+3

10.7 28.0 8.0 28.8 36.0

2.2 1.8 1.2 1.0 3.0

1.66  0.15 1.82  0.05 1.58  0.06 1.83  0.09 1.7

} 0.80 0.20 } 1.00

} 1.02 0.46 } 1.48

} 2.34 0.68 } 3.02

} 0.73 0.20 } 0.93

1 2 3 4 2+3 2+3+4

1.0 17.9 14.0 35.0 31.9 66.9

0.9 1.8 0.9 0.6 2.7 3.3

2.13  0.12 1.64  0.06 1.74  0.07 1.67  0.10 1.69 1.68

} 3.12 2.31 0.70 5.43 6.13

} 1.04 1.07 0.73 2.11 2.81

} 1.27 1.60 0.21 2.87 3.60

} 0.24 0.62

Sample 3a Dist. water

Sample 4a Dist. water

Sample 5a Dist. water

a

Time

U

1.24 1.77

0.86 1.07

Source: Andrews et al. (1987).

for at least 1 million years, and there has not been 234Uloss either due to a-recoil effects (Kigoshi, 1971) or by preferential leaching/etching of recoil-damaged sites (Fleischer, 1975)). However, it is necessary to take into account that 238U and 234U also may have been equally leached due to the effect of rock–water interactions (Latham and Schwarcz, 1987), yielding activity ratios in the rock matrices corresponding to secular equilibrium.

From Table 1, lower values were found for the molar U/ Si ratio than U/Ca and U/Mg ratios. 4.2. The progress of Si, Na, K, Ca, Mg, and U dissolution The results of the analyses of etch/leach solutions demonstrate that significant reaction took place in each set of experiments. The progress of Si, Na, K, Ca, Mg,

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and U dissolution is illustrated in Figs. 3 and 4, and it can be seen that the process is variable, i.e. it may be a single-stage process characterized by parabolic kinetics (Luce et al., 1972) as well a two-stage process characterized by parabolic/linear or parabolic/parabolic kinetics, in which the granite surfaces are dissolved by zero-order etch (constante etch rates with time) and firstorder chemical leach (decreasing or increasing leach rates with time). The results of lab experiments reported by Bonotto and Andrews (1993) for Carboniferous Limestone gravels were interpreted as a gradual change from a first-order reaction (rate diminishing with time) to a constant rate, zero-order process (i.e. a two-stage process characterized by parabolic and linear kinetics). Such behaviour was identified here in only three circumstances which involved the longest etch/leach times of up to about 200 days (i.e. for Na released from sample 1 of granite gravels and for U released from samples 1 and 2 of granite gravels). The variable kinetics

and order of the reactions for all of the elements may be related to the non-uniformity of exposed surfaces of the gravels associated with the different rock samples, which are irregularly shaped and have inhomogeneous Si, Na, K, Ca, Mg, and U distribution. When the cumulative curves obtained for the major cations dissolved from samples 1 and 2 of granite gravels are compared, it is evident to see that Na and Ca are dissolved to a much greater extent than K and Mg, and that dissolution of Na and Ca is much more pronounced in sample 2 than in sample 1, in spite of their similar exposed total surface area. Edmunds et al. (1985) concluded that hydrolysis of biotite could account for the build-up of K+, Mg2+, and other species in groundwaters of the Carnmenellis granite and that acid hydrolysis of plagioclase might be considered as the principal source of Na+ and Ca2+ in these groundwaters. Considering the possible dissolution of biotite, the experimental data obtained for the etchates/lea-

Fig. 3. Dissolution of (a) sodium, potassium, calcium and magnesium, (b) sodium, potassium, calcium and magnesium, (c) silicon, sodium, calcium and magnesium, and (d) silicon, sodium, potassium, calcium and magnesium during etch/leach of the Carnmenellis granite gravels by air-saturated distilled water at pH=4.9. Note that non-linear kinetics dominates the release of these elements to the liquid phase. The data for samples 3 and 5 are those reported by Andrews et al. (1987).

D.M. Bonotto et al. / Applied Radiation and Isotopes 54 (2001) 977–994

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Fig. 4. Dissolution of uranium from the Carnmenellis granite gravels corresponding to (a) samples 1 and 2, and (b) samples 3–5 during etching/leaching experiments using distilled water equilibrated with the atmosphere at pH=4.9. Note that non-linear kinetics characterizes the process, being a more pronounced dissolution rate achieved for experiments performed with sample 2. The data for samples 3–5 are those reported by Andrews et al. (1987).

chates indicate that the initial K+ dissolution rates of 0.49–0.7 mg dayÿ1, respectively, for samples 1 and 2, decreased 10–14 times to a value of 0.05 mg dayÿ1, whereas the initial Mg2+ dissolution rates of 0.24– 0.16 mg dayÿ1 decreased 8 times to values of 0.03– 0.02 mg dayÿ1, respectively, for samples 1 and 2. If the dissolution of plagioclase (oligoclase) is taken into account, then the initial Na+ dissolution rates of 1.98– 6.7 mg dayÿ1 decreased 22–24 times to values of 0.09– 0.28 mg dayÿ1, respectively, for samples 1 and 2, whereas the initial Ca2+ dissolution rates of 0.63–1.45 mg dayÿ1 decreased 6–5 times to values of 0.1–0.3 mg dayÿ1, respectively, for samples 1 and 2. Therefore, the experimental data obtained for the etchates/leachates in this investigation confirm those reported by Goldich (1938), since oligoclase usually contains 70–90% albite and 10–30% anorthite, a proportion that justify the observed faster decrease of the dissolution rate of Na-plagioclase relative to that of biotite, followed by that of Ca-plagioclase. The molar Na+/Ca2+ ratios observed in the etchates/leachates decreased of 5.5–8.0 to 1.5–1.6, respectively, for samples 1 and 2, in accordance with the faster decrease of the dissolution rate of Na from the rock gravels. Thus, the progress of the chemical dissolution of minerals in the granite samples suggest that several reactants were consumed and products were formed. It was observed that the rates of dissolved elements declined with time from some initial value until they

approached virtually zero as the reaction was trying to reach equilibrium conditions or to go to completion. However, in spite of this trend, it is not possible to evaluate the specific rate constant (timeÿ1 for first-order reaction) of the reaction, following the premises considered by Faure (1991), since the data do not fit an exponential curve characterizing the consumption of a reactant and the formation of a product with time. 4.3. Uranium relative to Si, Na, K, Ca and Mg dissolution The U content of the analysed solutions is plotted against their Si, Na+, K+ , Ca2+ and Mg2+ contents in Fig. 5. The amount of U dissolved compared to Si, Na, K, Ca and Mg dissolution is variable, and probably reflects the irregular distribution of U relative to other elements in the unequal exposed surfaces of the rock samples, with most of the U residing in microfractures and grain boundaries filled with secondary minerals and deposits (Hussain and Andrews, 1986). Photomicrographs of radiohaloes in biotites from the Carnmenellis granite also showed they have a width of less than 20 mm and are centred on radioactive inclusions (uraninite and zircon crystals), where, surrounding the inclusion, the biotite appears to be unaltered (Durrance, 1986; Jefferies, 1988). The preference for U relative to Si dissolution, PU=Si , may be calculated from the ratio PU=Si ¼ Rsolution =Rrock ,

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Fig. 5. The amount of uranium dissolved from Carnmenellis granite gravels corresponding to (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 5 in relation to the dissolution of silicon, sodium, potassium, calcium, and magnesium during etching/leaching experiments using air-saturated distilled water at pH=4.9. Note that variable curves are obtained, reflecting irregular distribution of U relative to other elements in the unequal exposed surfaces of the rock samples. The data for samples 3 and 5 are those reported by Andrews et al. (1987).

where Rrock is the molar U/Si ratio in the granites (reported in Table 1) and Rsolution represents the molar U/Si ratio in the etchates/leachates (reported in Table 2). The values obtained for this parameter are given in Table 3, which were not evaluated for samples 1 and 2 due to the lack of dissolved Si data, being calculated for samples 3–5 on assuming a molar U/Si ratio in rock

corresponding to the mean of the values obtained for samples 1 and 2 (Table 1). The reported values of PU=Si range from 2.6 to 6.4, indicating that uranium is preferentially etched/leached than silicon. Similarly, defined ratios PU=Na , PU=K , PU=Ca , and PU=Mg are reported in Table 3, where the same assumption relative to the molar U/Si ratio in rock was utilized, i.e. the

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Table 3 The preference for U relative to Si and major cation dissolution from Carnmenellis granite gravels by chemical etch/leach with distilled watera Sample

Etch/leach time (day)

PU/Si

PU/Na

1

5.1 18.0 48.1 98.9 201.2

} } } } }

0.12 0.15 0.18 0.21 0.28

2

5.1 18.0 48.0 98.9 200.9

} } } } }

0.05 0.08 0.13 0.26 0.44

3

28.0 36.0 64.8

4.89 4.35 4.30

0.63 0.63 0.70

4

28.0 36.0

4.48 5.71

0.71 0.95

5

17.9 31.9 66.9

6.44 3.00 2.55

0.18 0.16 0.17

PU/K

PU/Ca

PU/Mg

0.79 0.78 0.73 0.74 0.89

0.13 0.10 0.08 0.08 0.08

0.14 0.12 0.10 0.10 0.10

0.10 0.10 0.09 0.10 0.11

0.76 1.39 1.99 2.63 3.30

1.04 1.36 1.85 3.15 4.84

0.07 0.08 0.07 0.10 0.12

0.26 0.31 0.30 0.45 0.60

0.13 0.18 0.24 0.41 0.62

1.70 2.45 4.06 5.56 7.52

0.13 0.11 0.10

0.13 0.12 0.12

0.74 1.18

0.07 0.10

0.10 0.13

0.10 0.15

} }

1.01 0.75 0.69

0.14 0.09 0.09

0.30 0.12 0.12

0.13 0.10 0.09

} } }

} } }

PU/Kb

} } }

Depth of etch/leach (mm)

} } }

a PU/Si=molar U/Si ratio in solution divided by the molar U/Si ratio in the rock. PU/Na=molar U/Na ratio in solution divided by the molar U/Na ratio in the rock. PU/K=molar U/K ratio in solution divided by the molar U/K ratio in the rock. PU/Ca=molar U/Ca ratio in solution divided by the molar U/Ca ratio in the rock. PU/Mg=molar U/Mg ratio in solution divided by the molar U/Mg ratio in the rock. PU/Kb=molar U/K ratio in solution divided by the molar U/K ratio in the rock (without considering the contribution of K-feldspar). (U/Si)rock=2.82  10ÿ5; (U/Na)rock=3.06  10ÿ4; (U/K)rock=2.81  10ÿ4; (U/Ca)rock=1.75  10ÿ3; (U/Mg)rock=2.59  10ÿ3; (U/Kb)rock=2.18  10ÿ3.

molar U/Na, U/K, U/Ca and U/Mg ratios in rock samples 3–5 corresponded to the average of the values obtained for samples 1 and 2 (Table 1). The results show that the mineral surface being dissolved does not strongly affect the source of major cations in solution. It is evident that values higher, lower, and equal to 1are found, suggesting a similar dissolution of U and K in some cases for samples 2, 4 and 5 (values close to 1), the preferential dissolution of Na, Ca and Mg relative to U in all cases and of K relative to U in some cases for samples 1, 4 and 5 (values lower than 1) or that U is being preferentially dissolved relative to K in almost all cases in sample 2 (values higher than 1). Considering oligoclase as the main source of Na+ and Ca2+ in solution, it is clear from the PU=Ca and PU=Na ratios that Ca and Na are always etched/leached preferentially to U. This may be explained by the fact that plagioclase shows preferential alteration in the centres of most zoned crystals, favouring an enhancement of the aqueous Ca2+ (and certainly Na+) levels (Edmunds et al., 1985).

The low PU=Mg data (51) suggest that Mg is etched/ leached preferentially to U in all analysed circumstances. Olivine, like anorthite, is a mineral phase most susceptible to chemical weathering (Goldich, 1938), and its presence is often recognized as inclusions in the Carnmenellis granite. However, the contribution of olivine crystals to Mg2+ in solution is unlikely to be important, because the Mg2+ dissolution rates are very similar for samples 1 and 2, indicating that it is more reasonable to consider biotite as the main source of Mg2+ in solution. However, if biotite is also the main source of K+ in solution, then, it is necessary to recalculate the PU=K values reported in Table 3, since the molar U/K ratios referred in Table 1 include the contribution of K-feldspar to the amount of potassium evaluated for samples 1 and 2. The modal compositions for principal minerals of the Carnmenellis granite yield a biotite/K-feldpar+biotite ratio of 0.13, which allows us to calculate the other PU=K values (PU=Kb ) represented in Table 3. The values vary between 0.09 and 0.62 for samples 1, 2, 4 and 5, indicate that K is always etched/

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leached preferentially to U. This suggests that, with the exception of Si, all major cations Na+, K+, Ca2+ and Mg2+ were etched/leached preferentially to uranium during the experiments. The depth, d, of material which was removed by continuous surface etch/leach may be calculated from the mass, M, of each major element dissolved and the specific surface area S of the rock sample, using the equation d ¼ M=Sf r, where f is the fractional mass of each major element in the rock, and r is the rock density. The chemical data for the etchates/leachates relative to samples 3–5 (Table 2) allow us to estimate that the maximum contribution of Si to the parameter d corresponds to 1%, which, therefore, can be discarded on its evaluation. Thus, the depth of etch/leach ranged from 0.76 up to 3.3 mm for the set of experiments performed with sample 1, and from 1.7 up to 7.5 mm for the experiments realized with sample 2, where very close variations from 0.2–0.3 up to 0.8–0.9 mm were found for potassium (without considering the contribution of Kfeldspar), as well for magnesium (from 0.1–0.2 up to 0.9 mm), suggesting that biotite is the main source of these elements in solution. In contrast, the parameter d also allows us to demonstrate that oligoclase weathers more easily in sample 2 than in sample 1, since values ranging from 0.7 up to 1.2 mm and from 0.6 up to 4.6 mm were found, respectively, for Na and Ca in sample 2,

whereas values from 0.2 up to 0.3 mm and from 0.2 up to 1.2 mm were evaluated, respectively, for Na and Ca in sample 1. 4.4.

234

U excess in etch/leach solutions

Fig. 6 shows the activity ratios of dissolved U in cumulative leach/etch solutions corresponded to values of 1.2, 1.6, and 1.7. These values are higher than the value of 1.3 as reported by Bonotto and Andrews (1993) for etchates/leachates of carboniferous limestone samples. The observed cumulative activity ratios were not related to the total surface area of the gravels used for the etch/leach experiments. The limiting values approached for the cumulative dissolution and the different final values are probably dependent upon the inhomogeneous U distribution in the multi-grain aggregates, surface roughness of the rock pieces, unequal exposed areas to leaching/etching or irregular distribution of the minerals in the surface of the gravels, being difficult or impossible to properly evaluate most of these parameters. Bonotto and Andrews (1993, 1998) found that the activity ratio of dissolved U from samples of limestone and dolomite generally increased with each successive leach/etch. In contrast to this trend, our results show a decrease of the activity ratio of dissolved U with each

Fig. 6. The 234U/238U activity ratio for dissolved uranium from Carnmenellis granite gravels corresponding to (a) samples 1 and 2, and (b) samples 3–5 during etching/leaching experiments using distilled water equilibated with the atmosphere at pH=4.9. Note that the variation of the activity ratios over the duration of the experiments is between 1.2 and 2.1, always showing 234U excess in the leachates/ etchates. The data for samples 3, 4, and 5 are those reported by Andrews et al. (1987).

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successive leach/etch in the experiments with samples 1, 3, and 5 (Fig. 6). Comparing the U/Na, U/K, U/Ca, and U/Mg ratios for etchates/leachates relative to samples 1 and 2, which were subjected to more extensive etch/leach times, it is evident that the U/Ca and U/Mg ratios have opposite trends (i.e. they decrease with time for sample 1 but increase with time for sample 2). Considering the results obtained for the etchates/ leachates of sample 1, the initial U dissolution rate of 0.67 mg dayÿ1 for sample 1 decreased 9.6 times to a value of 0.07 mg dayÿ1, whereas the initial Ca2+ and Mg2+ dissolution rates decreased 6.3 and 8 times, respectively. The values obtained for sample 2 indicated that the initial U dissolution rate of 1.25 mg dayÿ1 decreased 2.8 times to a value of 0.45 mg dayÿ1, whereas the initial Ca2+ and Mg2+ dissolution rates decreased 4.8 and 8 times, respectively. Thus, the faster decrease of the U dissolution rate relative to that of Ca2+ and Mg2+ for sample 1 can justify the decrease of U/Ca and U/Mg ratios with etch/ leach time, while the slower decrease of the U dissolution rate relative to that of Ca2+ and Mg2+ for sample 2 can justify the increase of U/Ca and U/Mg ratios with etch/leach time, where the decrease of the cumulative 234 U/238U activity ratios of dissolved U with each successive leach/etch for sample 1 is accompanied by the decrease of U/Ca and U/Mg ratios. Such behaviour involving the activity ratios was also reported by Eyal and Olander (1990) who showed that the activity ratios in leachates/etchates of an untreated monazite specimen decreased from 2 towards the equilibrium value of 1 on a time-scale of up to 6.8 years. Eyal and Olander (1990) attributed this process to a preferential dissolution of 234 U from tracks produced by a-recoil atoms of 234Th. During the initial leaching period, these sites of severe crystal damage are more readily chemically attacked by the etchant/leachant than the undamaged or naturally annealed crystal sites, in which most of the 238U atoms reside. Because the 234U/Ca and 234U/Mg ratios have similar decreasing trends as U and U/Ca and U/Mg ratios, the mechanism proposed by Eyal and Olander (1990) suggests a possible interpretation of the decreasing activity ratios with etch/leach time. Preferential alteration of a-recoil tracks could occur in zoned Caplagioclase crystals, and within pleochroic haloes centred on radioactive inclusions, since these zones in biotite are caused by charge transfer affecting oxidation of Fe2+ to Fe3+, where uranium is more easily mobilized under oxidizing conditions. Fig. 7 shows the 234U/238U activity ratios for etchates/ leachates of sample 1 plotted against the U content and major cations concentrations. The shape of the curves plotted for Ca, K and Mg in Figs. 7(a) and (b) are similar, reinforcing the conclusion that Ca-plagioclase and biotite may be considered as important mineral

989

phases contributing to the release of 234U into solution. The dependency of the activity ratio on these three elements is confirmed again by the cumulative curve shown in Fig. 7(c) for total major cations, whose shape is practically the same. Curiously, it was found that the cumulative curve for uranium shown in Fig. 7(d) also has a similar shape, yielding an inverse but non-linear relationship between the amount of dissolved uranium and the 234U/238U activity ratio of dissolved U. Uranium is an element very sensitive to modifications on redox potentials, and so in solution it may be depleted or enriched, depending on oxidation–reduction conditions. If oxidizing conditions prevail, active solution of U occurs, and if the redox character changes towards reducing conditions (lower values of Eh), then, deposition of U takes place. Enhanced activity ratios for dissolved U at more reducing conditions are favoured due to 234U solution by the 234Th recoil process, and therefore lower U concentrations and higher activity ratios are generally expected at aquifer zones in natural systems. This has been reported, among other, for the waters of the Carrizo Sand Formation of South Texas and of the Lode`ve U deposit, SW of the French Massif Central (Cowart and Osmond, 1977; Toulhoat and Beaucaire, 1991). The results from laboratory time-scale experiments in this paper clearly demonstrate an inverse relationship between the activity ratio and U dissolved in a reducing solution (the Eh–pH data of the solution utilized for leach/etch experiments indicated that it is reducing in character despite its positive Eh value), without having to result to changing the 234U/238U activity ratio and U content values across a reducing barrier as is often referred to in the literature. 4.5. 234U/238U activity ratios generated in the leachates/ etchates and values in Cornish groundwaters Activity ratios measured in water samples from Cornwall (Table 4) range from 0.89 to 1.52 (average 1.05) for shallow groundwaters (Nos. 1–14), and from 1.26 to 2.79 (average 1.86) for mine waters (Nos. 15–24). The total variation of activity ratios for Cornish groundwaters is therefore 0.89–2.79 (average 1.39). The range of activity ratio values from our laboratory leach/etch experiments is between 1.21 and 1.72 (average 1.59). If we take into account the variation of the activity ratios over the duration of the experiments, the range is 1.12–2.13 (average 1.61). These data show a similar range in activity ratios between the field data and the lab data. A decrease in the amount of dissolved U and an increase in the activity ratio has commonly been observed for groundwater close to redox boundaries in groundwater systems (Cowart and Osmond, 1974; Andrews and Kay, 1982). Such changes have been attributed to an increased a-recoil rate caused by the

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Fig. 7. The 234U/238U activity ratio for dissolved uranium from sample 1 of Carnmenellis granite gravels etched/leached by airsaturated distilled water at pH=4.9 plotted against the (a) dissolved sodium and calcium contents, (b) dissolved potassium and magnesium contents, (c) total dissolved major ions content, and (d) dissolved uranium content. The filled symbols represent the values obtained during each sequential etch/leach, whereas the empty symbols are the cumulative values. Note the similar shape of the curves plotted for Ca, K, and Mg, as well the inverse non-linear relationship between the amount of dissolved uranium and 234U/238U activity ratio.

deposition of U at the commencement of the reducing zone. Andrews (1983) and Edmunds et al. (1984) showed that for the near-surface groundwaters from the Carnmenellis granite, the average U content is 1.4 mg kgÿ1, and the activity ratio is close to secular equilibrium, whereas with the exception of the sources in the Mount Wellington mine, all minewater samples have 238 U contents less than 0.5 mg kgÿ1 and activity ratios significantly enhanced (Table 4). Based on these data, Andrews (1983) and Edmunds et al. (1984) suggested that the South Crofty waters evolved through a shallow recoil-dominated zone of limited extent, where their activity ratios reached a value higher than or equal to 2 at a depth around 250 m, and that their present activity

ratios are the result of either ageing or mixing with groundwaters that are in secular equilibrium. The Eh–pH of the solution utilized for the leach/etch experiments indicates that it is reducing in character, despite its positive Eh value. The available Eh–pH data for Cornish groundwaters reported by Edmunds et al. (1984) show that they are transitional in character, tending to be oxidizing in the case of one analysed shallow groundwater (pH=4.8; Eh=370 mV), or reducing in the case of the samples of Pendarves (pH=6.5; Eh=30 mV), Wheal Jane (pH=4.9 and 5.0; Eh=148 and 216 mV) and South Crofty (pH=6.0 and 6.8; Eh=+188mV) mines. This suggests some similarity between the solution used in our experiments and

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Table 4 U content and 234U/238U activity ratio for shallow groundwaters and mine waters from the Cornish granites and their metamorphic aureole*. Data compiled from Andrews (1983) and Edmunds et al. (1984) No. location 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Average 15* 16* 17* 18 19 20 21* 22 23 24 Average

Praze Praze Praze Crowan Wendron Wendron Penryn Penryn St. Austell St. Ives St. Ives St. Ives St. Buryan St. Buryan Mt. Wellington Mine Mt. Wellington Mine Mt. Wellington Mine Pendarves Mine Pendarves Mine Pendarves Mine Wheal Jane Mine South Crofty Mine South Crofty Mine South Crofty Mine

Temp. (8C)

U(mg lÿ1)

234

Comments

30 36 } 0 18 36 6 41 23 57 } 0 31 32

10.8 5.9 12.4 11.5 10.1 10.7 } } 5.5 } } 8.8 11.0 11.2 16.8 16.4 21.6 18.2 21.4 } 24.8 43.0 41.5 41.5

1.52  0.11 0.89  0.03 0.99  0.10 1.08  0.07 1.09  0.02 1.04  0.02 0.93  0.03 0.97  0.04 1.07  0.05 1.14  0.06 0.99  0.02 1.09  0.06 1.01  0.04 0.92  0.08 1.05  0.05 2.04  0.19 1.26  0.06 1.90  0.08 2.06  0.39 2.79  0.50 1.96  0.16 1.59  0.26 1.61  0.26 1.77  0.31 1.67  0.17 1.86  0.24

Pumped sample Tank sample Tank sample Spring Pumped sample Pumped sample Pumped sample Pumped sample Pumped sample Pumped sample Tank sample Stream Depth sample Depth sample

210 240 240 230 260 260 300 690 690 690

0.80  0.05 0.90  0.04 0.34  0.03 0.68  0.07 1.85  0.03 1.37  0.03 4.76  0.23 1.18  0.06 1.96  0.11 1.41  0.07 0.46  0.01 0.30  0.01 1.51  0.06 1.82  0.07 1.38  0.06 0.53  0.04 2.90  0.08 1.47  0.03 0.14  0.02 0.25  0.03 0.45  0.01 0.11  0.03 0.33  0.04 0.11  0.003 0.25  0.004 0.65  0.03

Depth (m)

natural groundwater in terms of the redox conditions. Andrews (1983) and Edmunds et al. (1984) suggested that the minewater sources from the Carnmenellis granite are distinguished from the near-surface groundwaters by a change in uranium geochemistry. The minewaters have much lower U contents than the shallow groundwaters so that U deposition must have occurred during its migration to greater depths. However, considering the similar total surface area, etchant/ leachant and etch/leach time utilized during the etch/ leach experiments realized with samples 1 and 2, as well the very different results reported in Table 2 for dissolved U and activity ratios values, it is possible that the U release into the liquid phase is controlled by aspects related to the rock surfaces exposed to water, such as the inhomogeneous U distribution in the multigrain aggregates, surface roughness of the rock pieces, unequal exposed areas to leaching/etching or irregular distribution of the minerals at the rock–water interface, which, therefore, suggest other possibilities to explain the observed variations on the U contents and activity ratios values in the Cornish groundwaters. The a-recoil process requires either a residence time of some hundreds of thousands of years or a large U enrichment by deposition at a redox boundary to

U/238U activity ratio

Drill-hole, metapelite Lode, drive Cross-cut Fracture in granite Lode Work-face Lode, work-face Fracture in granite Cross-cut Cross-cut

significantly increase the activity ratio of dissolved U within relatively short time-scales (years to tens of years). Therefore, it is clear from the short duration of the laboratory experiments, from the inverse relationship between dissolved U and activity ratio in leachates/ etchates discussed in the last section, and from the higher U contents and lower activity ratios values found for etchates/leachates of sample 2 relative to sample 1 (Table 2) that a-recoil processes occurring at redox boundaries may not be important for the generation of enhanced activity ratios in the Carnmenellis granite groundwaters.

5. Conclusions The radioactivity due to uranium isotopes, 238U and U, observed in groundwaters from Carnmenellis granite, Cornwall, England, was evaluated from experiments using granite gravels that were etched/leached by air-saturated water for a duration of up to 200 days. The Si, Na, K, Ca, Mg, and U dissolution is variable, being a single-stage process characterized by parabolic kinetics, as well a two-stage process characterized by parabolic/ linear or parabolic/parabolic kinetics, in which the

234

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granite surfaces are dissolved by zero-order etch (constant etch rates with time) and first-order chemical leach (decreasing or increasing leach rates with time). The Cornish groundwaters are transitional in terms of redox conditions, showing both an oxidizing and a reducing character, whereas the solutions used in the laboratory experiments always were reducing. However, using a reducing solution it was possible to generate 234 U/238U activity ratios between 1.1 and 2.1 in the etchates/leachates. The observed cumulative activity ratios values and also the variation of the activity ratios with etch/leach time did not depend on the total surface area of the samples used in our experiments. Decreasing activity ratios probably are the result of inhomogeneous U distribution in the multi-grain aggregates, surface roughness of the rock pieces, unequal exposed areas to leaching/etching or irregular distribution of minerals in the surface of the gravels. Larger U dissolution rates relative to that of Ca2+ and Mg2+ as well the opposite trend also verified can justify the decrease/increase of the measured U/Ca and U/Mg ratios with etch/leach time, where the decrease of the cumulative activity ratios of dissolved U with each successive leach/etch is accompanied by the decrease of the U/Ca and U/Mg ratios. A possible mechanism to explain the decrease of the 234U/ Ca and 234U/Mg ratios with the etch/leach time is that, during the initial leaching period, the lines of severe crystal damage produced by a-recoil atoms of 234Th are more readily chemically attacked by the etchant/ leachant than the undamaged or naturally annealed crystal sites, in which most of the 238U atoms reside. The short duration of the laboratory experiments and the inverse relationship between dissolved U and activity ratio found in leachates/etchates suggest that a-recoil processes are apparently less important than chemical leaching/etching for the generation of enhanced activity ratios in the studied area.

Acknowledgements The experimental work was carried out whilst two authors (D.M.B. and J.N.A.) were at the School of Chemistry, University of Bath. D.M.B. thanks the CNPq, Brazil, for funding a visiting fellowship at the University of Bath, FUNDUNESP for financial support, and one anonymous referee for improving the readability of this paper.

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