Alteration of a basaltic glass in an argillaceous medium:

Geochimica et Cosmochimica Acta, Vol. 65, No. 7, pp. 1071–1086, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 ⫹ .00

Pergamon

PII S0016-7037(00)00583-4

Alteration of a basaltic glass in an argillaceous medium: The Salagou dike of the Lode`ve Permian Basin (France). Analogy with an underground nuclear waste repository ISABELLE TECHER,1,2,* JOE¨ L LANCELOT,2 NORBERT CLAUER,3 JEAN MICHEL LIOTARD,2 and THIERRY ADVOCAT1 Commissariat a` l’E´nergie Atomique, Centre de Recherches de la Valle´e du Rhoˆne DCC/DRRV/SCD, BP 171, 30207 Bagnols-sur-Ce`ze Cedex, France 2 Laboratoire de Ge´ochimie Isotopique, ISTEEM, Universite´ de Montpellier II, CC 066, place E. Bataillon, 34095 Montpellier Cedex 5, France 3 Centre de Ge´ochimie de la Surface, EOST, Universite´ de Strasbourg, 1 rue Blessig, 67084 Strasbourg Cedex, France 1

(Received January 26, 2000; accepted in revised form September 1, 2000)

Abstract—Volcanic basaltic glasses are commonly considered to be very suitable natural analogs of vitrified waste packages intended for geological disposal. The studied basaltic glass of the Salagou dike (Lode`ve Basin, France) intruded into a Permian argillaceous formation about 1.4 Ma ago, provides a means of assessing the long-term behavior of nuclear waste glass in an argillaceous repository concept. This study is based on combining chemical, mineralogical and isotopic investigations. The occurrence of a palagonite film no more than 1.2 mm thick characterizes the basaltic glass alteration in contact with the argillaceous host rock. The chemical and structural properties of the palagonite suggest constant volume alteration. The alteration rates estimated from palagonite thickness and age of the glass are comparable to those measured on natural glasses altered in nonargillaceous media. The occurrence of the studied argillaceous material in contact with the basaltic glass did not result in long-term alteration kinetics significantly different from those measured in simple glass/water systems. Mineralogical, chemical, and isotopic data obtained on the basaltic and argillaceous components suggest that an aqueous fluid flowed primarily at the glass/argillite interface and did not propagate in the argillaceous host rock beyond 5 cm from the basaltic dike. The elements released by alteration of the basaltic glass—notably strontium— did not diffuse into the surrounding clay. This conclusion is important from the perspective of a natural analog for a deep geological repository for nuclear waste, and highlights the major role of the structural properties of the clayey barrier. Copyright © 2001 Elsevier Science Ltd behavior of nuclear glass in a clay medium. We have therefore considered in the present study a new approach to gain insights into the problems of nuclear waste containment in investigating the alteration of an old volcanic basaltic glass in a natural argillaceous environment. Basaltic glasses are known to be very good natural analogs of nuclear containment glasses, and thus to provide information on the long-term and very longterm behavior of nuclear glasses in a given disposal scenario. In southern France, basalt dikes—some of them with vitreous chilled margins—intruded Thuringian argillaceous formation of the Lode`ve Permian Basin about 1.4 Ma ago (Fig. 1) (Gastaud et al., 1983). The investigation of natural glass alteration in this environment addresses two main objectives: 1) characterize the alteration products formed in the presence of argillaceous material; 2) characterize the interactions between the glass and the argillaceous material using a mineralogical chemical and isotopic approach. To what degrees do elements released during glass alteration remain confined, and do the released elements diffuse in the clay unit and to what extent? Natural strontium isotopes were selected as tracers of the exchange processes; they are chemical analogs of the Sr isotopes produced in reactors by nuclear fission (90Sr, with a half-life of 29.1 yr). Strontium is also a highly mobile element during glass alteration, and the contrasting Sr concentrations and isotopic compositions of the basaltic glass and the argillaceous host material serve well to detect exchanges between these two rock types.

1. INTRODUCTION

High-level nuclear waste produced by reprocessing spent nuclear fuel has been confined in a glass matrix for over two decades in France (Bonniaud et al., 1980; Jouan et al., 1986). The waste packages could be placed in deep geological formations following concepts based on the multiple containment barrier principle. Three barriers are considered: the waste package itself (the glass block, a metal canister, and a possible overpack), a nearfield engineered barrier, and the geological barrier or host rock. These barriers are required to protect the package from external fluids and to ensure containment of any radionuclide released. The primary function of the engineered barrier will be to minimize exchanges between the geological barrier and the waste package. One of the scenaris under consideration calls for the use of a clay nearfield barrier. Moreover, an argillaceous formation could itself constitute the geological barrier. An assessment of storage in such a scenario requires characterization of the interaction mechanisms between the nuclear glass and the argillaceous material. In this respect, laboratory experiments have been carried out on nuclear glass alteration in clay media (Godon et al., 1989; Marples et al., 1990), as well as simulations of glass/clay interactions (Curti et al., 1993). But these studies do not provide information on the long-term

* Author to whom correspondence should be addressed (TECHER. [email protected]). 1071

1072

I. Techer et al.

Fig. 1. Geological map of the Lode`ve Permian Basin with location of the Salagou lake and of the studied sector SA4. Argillite specimens A and B were sampled at right angles to the basalt dike over a distance of 1 m in the same sedimentary layer (inset).

2. WHAT ABOUT NATURAL ANALOGS?

The studied geological context of the area of concern is highly relevant for the stated objectives with a focus on the long-term behavior of nuclear waste stored in a geological environment. The site offers basaltic glass, which is a very good natural analog of borosilicate nuclear glass used for waste containments. This analogy has been demonstrated from standpoint of their aqueous alteration products (Crovisier et al., 1983; Malow and Lutze, 1984; Crovisier et al., 1989; Murukami et al., 1989; Jercinovic et al., 1990a; Morgenstein and Shettel, 1994), as well as for their alteration mechanisms and kinetics (Techer et al., 2000). Thus, secondary products formed at the surface of basaltic glass during laboratory experiments are comparable with secondary products formed on the surface of nuclear glass in the same experimental conditions: precipitation of an amorphous hydrated silicate (called palagonite in case of basaltic glasses, gel in case of nuclear glasses), formations of oxyhydroxides, phyllosilicates, and zeolites for the most encountered. From a standpoint of the alteration mechanisms, dissolution of basaltic glass is controlled by three successive mechanisms: 1) selective dissolution depending on temperature and pH, which is controlled by exchange between the modifying elements of the glass and hydrogen of the solution (Daux et al., 1997); 2) congruent dissolution controlled by the hydrolysis of the vitreous matrix; 3) while the reaction progresses, secondary mineral phases form at the glass surface and dissolution becomes incongruent. The same alteration

mechanisms are observed during alteration of borosilicate nuclear glasses (Advocat et al., 1991). For identical experimental conditions, further alteration rates of the basaltic and nuclear glasses are very close and have the same evolution in time (Techer et al., 2000). The processes controlling the alteration kinetics for basaltic glasses and for nuclear glasses are the same: a control by chemical affinity and by the formation and the development of the alteration rim (Techer et al., 2000). The studied site presents additional interest for its environmental analogy towards geological storage concepts: the “basaltic glass/argillaceous rocks” system is comparable to the “nuclear glass/argillaceous barrier” system. Careful evaluation of this analogy is made farther from the standpoints of the argillaceous rock composition, the geological formation structure and the “temperature/time” systems. 3. SON 68 NUCLEAR WASTE GLASS ALTERATION IN CLAY MEDIA

The short-term behavior of SON 68 nuclear glass in clay media has been investigated for over a decade through laboratory experiments (Godon et al., 1989; Marples et al., 1990, Curti et al., 1993) as part of an assessment of geological disposal of nuclear waste. Some of these experiments have shown that the initial maximum glass alteration rate is maintained for several hundred days in the presence of some clays, e.g., smectite 4a or Boom clay (Godon et al., 1989), compared with only a few days in pure water under the same experimental

Alteration of the Salagou basaltic glass in an argillaceous medium

1073

Fig. 2. Sketch geological representation of the studied SA4 area of the Lode`ve Basin 1.4 million years ago, and the present geological situation after recent erosion. The studied basaltic dike outcrops 160 m below the basaltic plateau in sector SA4.

conditions. The presence of these clays appears to delay the onset of the steady-state SON 68 glass dissolution rate. Conversely, other clays (e.g., bentonite) are relatively neutral; the nuclear glass alteration rates are low in the presence of such clays, comparable to those measured in pure water at high glass surface-area-to-solution-volume (S/V) ratios (Godon et al., 1989). Silica sorption and/or precipitation processes have been suggested to account for the higher glass alteration rate in clay media (Godon et al., 1989; Curti et al., 1993). Elements released into solution during glass alteration are captured by the clay. As a result, the alteration rim that forms on the glass surface at the interface with the clay (which is less retentive for elements such as Si, Al, or Ca) does not constitute an alteration barrier. Only a few studies have been published to date on the behavior of volcanic basaltic glass in the presence of an argillaceous material (Kamei et al., 2000) simulating the behavior of nuclear glass in a deep geological repository scenario. Previous studies were performed particularly on basaltic glasses altered by seawater or meteoric water without clay in contact with the glass (Peacock, 1926; Noack, 1981; Byers et al., 1987, Crovisier et al., 1985, Crovisier et al., 1989; Jercinovic et al., 1990b; Thorseth et al., 1991). Characterization of basaltic glass alteration in the Salagou dikes in contact with an argillaceous host rock of the Lode`ve Basin is a first step toward this objective.

4. GEOLOGICAL SETTING

The studied geological formations (Fig. 1) are exposed along Salagou lake in the Permian basin of Lode`ve (France). Located to the south of the Massif Central, the Lode`ve Basin corresponds to a half-graben extending over 150 km2, limited to the south and east by late Variscan faults. The Lode`ve Permian sedimentary layers are inclined 10° to 12° to the south. They form a pile of some 3000 m consisting of pelites, sandstones, siltites, and argillites. The lower portion of the sedimentary series corresponds to reduced (gray Permian) facies containing cinerite layers and uraniferous mineralizations that were mined until 1996 (Lancelot et al., 1995). The upper portion of the series corresponds to oxidized (red Permian) facies, consisting of a continental fluviatile detrital assemblage of fine to coarse sandstones and argillites deposited under semi-arid conditions, and dated by pollen analysis to be of Thuringian age (Odin et al., 1986). The red argillites comprise a “quartz, albite, illite, clinochlorite, montmorillonite, calcite, analcime, dolomite and hematite” assemblage (Fabre et al., 1989). Discordant Plio-Quaternary basalt flows overlie the red Permian sediments (Fig. 2). The basalt emissions in the Salagou region have been found to have K-Ar dates ranging from 1.6 to 1.4 Ma (Gastaud et al., 1983). In addition to the basalt effusions, which constitute today tabular table-lands 150 to 200 m above the valleys formed by recent erosion, the Salagou volcanism also produced an extensive network of necks and

1074

I. Techer et al.

dikes. Basalt dikes crop out mainly in the valleys in a succession nearly 3 km long, crosscutting Permian red argillites. The N30 direction corresponds to a fracture parallel to the Ce´vennes fault forming the eastern border of the Lode`ve Basin (Fig. 1). In the studied sector (SA4), a basaltic dike outcrops along the Salagou lake (Fig. 1) and shows a vitreous rim less than 1 cm thick formed as the basalt vein cooled in contact with the surrounding red argillaceous rocks (Fabre et al., 1989). The Permian argillite in contact with this vitreous basaltic dike was not “baked.” In the same area, many basaltic dikes or necks intruded Permian sedimentary formations but without vitreous rims. In these cases “baked” shales outline the contact with volcanic intrusions (Fabre et al., 1989). 5. METHODS 5.1. Sampling Methods The study concerned Sector SA4 located along the Salagou lake (Fig. 1), 160 m below the base of the surrounding tabular basalt flows (Alabouvette et al., 1982). The dike is 60 cm thick at this point. Samples were taken from the center and edge of the dike, including palagonite and glass specimens. Five Permian argillaceous samples (B1, B2, B3, B4, and B5) were taken with a micro-core drilling device at increasing distance (5, 10, 25, 50, and 100 cm) from the dike perpendicularly to the dike wall. All these samples were collected from the same Permian layer (Fig. 1). Two other argillaceous samples (A1 and A2) immediately adjacent to the dike were taken by hammer from two very small furrows located between 0 and 2.5 cm (A1) and between 2.5 and 4.5 cm (A2) from the dike. Two straight joints filled with fibrous calcite (FFR according to Lopez, 1992) were also sampled, one perpendicular and the other parallel to the dike. 5.2. Analytical Methods The fringes of the basalt veins were observed by polarizing optical microscopy (POM) and by scanning electron microscopy (SEM). The scanning electron microscope (Cambridge F360, Cambridge, MA, USA) was equipped with an energy-dispersive spectrometry (EDS) system for semi-quantitative overall or localized analyses. The contents of the major elements (Si, Al, Na, K, Mg, Ca, Fe, Ti, and Mn) were obtained for selected points in the alteration film on the same samples using an electron microprobe (Camebax, Paris, France). The analytical points were situated at intervals of 2 to 10 ␮m along a profile at right angles to the specimen surface, i.e., through the alteration film. These analyses were performed with special care as the alteration products are rich in water, implying a possible release of alkali metals under electron bombardment. The parameters used for this study were those recommended by Jercinovic et al. (1990b) for this type of sample: 2 ␮m diameter beam, 15 kV potential acceleration, 2 nA current. The analytical error was ⫾10% for the alkali metals and alkaline earths, and better than ⫾5% for the other major elements. The glassy film was separated from the basalt by using a microsaw and the alteration film was manually recovered under a binocular magnifier; the overall chemical compositions (major and trace elements) of these three basaltic components, the argillite samples and the calcite filling joints were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) and by inductively coupled plasma mass spectrometry (ICP-MS) (Samuel et al., 1985) having analytical uncertainties of 2% on the major elements and 5% on the trace elements. The mineralogical compositions of the argillaceous whole rocks, the fine fractions (⬍0.2 ␮m) recovered by sedimentation in distilled water, and the alteration film, were determined by X-ray diffraction analysis on rock powder specimens and on fine-fraction smear slides. The fine fractions were treated with ethylene glycol and heated for 2 h at 490°C. Mineralogical percentages were determined semi-quantitatively by comparing specific 001 peaks of each clay component with standard values. The values are given with an uncertainty of 5%. The Sr concentrations and isotopic compositions (87Sr/86Sr) were

determined using two solid-source thermal ionization mass spectrometers (VG-Sector, Manchester, UK and Cameca TSN 206S, Paris, France). The samples were dissolved in hot acid. Disseminated calcite in the Permian argillites and the FFR calcite were dissolved in acetic acid. Separation of Sr for isotopic determination was performed according to Birck’s method (1979) using Teflon FEP columns containing 1 mL of AG 50 W-X12 resin (200 – 400 mesh). The separated Sr was deposited on a W filament with a Ta activator. An average of 100 ratios were measured by mass spectrometry to ensure an analytical error of about 1 to 5 ⫻ 10⫺5 on the 87Sr/86Sr ratios. The external repeatability of the isotopic measurements on the VG-Sector was verified by regular analysis of the NBS987 standard, providing a mean 87 Sr/86Sr isotopic ratio of 0.7102225 ⫾ 4 (10 runs during the analysis period). The ⬍0.2 ␮m fractions were K-Ar dated following a method close to that of Bonhomme et al. (1975). The K concentrations were determined by atomic absorption and the Ar concentrations and isotopic compositions by mass spectrometry (VG 1200). The decay constants used were those recommended by Steiger and Ja¨ger (1977). The overall error on the calculated ages is estimated to be better than ⫾2%. 6. RESULTS

A five-component system was identified, comprising the basalt, the basaltic glass, the glass alteration film, the argillaceous host rocks, and the FFR calcites. The data of the basaltic material and the host sediment are presented successively. 6.1. Basalt, Basaltic Glass and Glass Alteration Film The SA4 dike corresponds to an olivine alkali basalt (Table 1) characterized by high Sr concentrations (on the order of 1200 ppm) and a 87Sr/86Sr isotopic ratio of 0.70328 ⫾ 2. The 1 cm thick basaltic glass was distinguished optically on the basis of its brown to black color. Primary magmatic crystals (olivine, clinopyroxene) represented 11% to 12% of the glass composition (Fabre et al., 1989). The mean chemical composition of the basaltic glass determined from 100 electron microprobe analyses, the Sr concentration, and the 87Sr/86Sr isotopic ratio were comparable to those of the basalt at the center of the dike (Table 1). At the glass surface in contact with the Thuringian argillite, a palagonite film developed to thicknesses ranging from 20 ␮m to 1.2 mm. The thickest films appeared macroscopically by a brownish color. Optical microscopic observation revealed a sharp interface between the slightly anisotropic orange-yellow palagonite and the isotropic brown or black basaltic glass (Fig. 3a). The palagonite was found not only on the glass surface, but also as concentric yellow bands around the gas vesicles and along fissures or fractures through the unaltered glass (Fig. 3b,d). Numerous microfractures were observed throughout the film thickness cross-cutting the concentric palagonite bands (Fig. 3c). This type of fracturation is attributable to sample preparation in the laboratory. The primary magmatic crystals (forsterite and augite) were found unaltered in the palagonite (Fig. 3a,c). The occurrence of these crystals in the rims confirms that palagonite replaces the basaltic glass. Very discrete secondary mineralization was associated with the palagonite; it was not detectable by scanning microscopy, but appeared on the XRD spectra with peaks, especially the (060), that are characteristic of nontronite, a variety of smectite. A 20-␮m-thick film of silica was observed in contact with the palagonite along the open fractures or in the gas vesicles (Fig. 3e,f).

43.2 14.5 6.18 8.74 12.5 0.03 3.11 3.82 2.89 1.10 1183 n.d. 0.70330 1 —

44.7 14.8 6.17 8.01 13.3 0.21 3.26 4.36 2.91 1.10 1162 1121 0.70325 3 —

Basalt adjacent 45.9 17.4 3.4 9.04 11.1 0.21 2.58 4.87 4.04 n.d. 1141 1252 0.70322 2 —

Basaltic glass* 41.9 15.4 1.14 3.54 9.65 0.12 2.40 1.06 1.40 n.d. n.d. 331 0.70659 2 —

Palagonite* 51.5 22.4 3.15 0.99 9.20 0.05 0.79 2.01 3.93 0.21 96 103 0.72755 3 0.70978 2

A1

A2 46.7 20.8 2.64 4.6 8.10 0.11 0.69 3.23 3.91 0.19 161 186 0.71853 0.8 0.71003 1.6

B1 46.7 19.8 2.69 5.16 8.00 0.09 0.68 3.45 3.85 0.18 178 n.d. 0.71811 0.9 0.71002 0.6

B2

Argillite

48.5 19.8 2.67 4.87 8.10 0.11 0.7 3.5 3.98 0.19 164 n.d. 0.71778 0.4 0.70996 1.4

B3 45.1 19.3 2.53 6.2 7.30 0.13 0.62 3.66 3.7 0.18 173 n.d. 0.71748 1 0.71019 0.5

B4

48.2 19.3 2.73 4.14 7.60 0.10 0.69 3.56 3.81 0.20 172 206 0.71650 1 0.71023 1.4

B5

47.0 19.8 2.65 4.99 7.82 0.11 0.68 3.48 3.85 0.19 170 196 0.71768 1 0.71009 1.4

Samples B average

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 539 n.d. 0.71053 0.2

0.71054 0.2

2 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 553 n.d.

1

Calcite (FFR)

Sr/86Sr isotopic ratios in studied components (basalt, basaltic glass, palagonite, argillite, and calcite veins).

50 21.8 2.80 1.44 9.00 0.10 0.74 2.57 4.03 0.2 96 109 0.72590 5 0.70968 1.8

87

* The oxide content was determined by ICP-AES (⫾2%) for the basalt and argillite specimens, and by electron microprobe analysis for the glass and palagonite. The Sr concentrations were determined by ICP-MS (⫾5%) and by mass spectrometry. The 87Sr/86Sr isotopic ratios were determined by mass spectrometry (n.d., not determined; TR, total rock, CB, carbonate phase).

SiO2 Al2O3 MgO CaO Fe2O3 MnO TiO2 Na2O K2O P2O5 SrICP-MS TR Srspectrometry (87Sr/86Sr)TR ⫾ (87Sr/86Sr)CB ⫾

Basalt central

Table 1. Major element concentrations (wt.%), Sr concentration (ppm), and

Alteration of the Salagou basaltic glass in an argillaceous medium 1075

1076

I. Techer et al.

Fig. 3. POM (a and b) and SEM (c to f) microphotographs of palagonite film on Salagou basaltic glass. Palagonite forms on the glass surface (a) around the vesicles and along cracks and microcracks (b, d, e). The interface with the pristine glass is sharp (a) and the primary magmatic crystals (a, c) are unaffected by alteration. A siliceous deposit is visible along the edges of a fracture and around the rim of a vesicle (e, f). (u.g., unaltered glass; p, palagonite; c, primary crystal; v, vesicle; f, fracture; Si, silica).

The palagonite that developed on the glass surface in contact with the argillite or along the fractures was characterized by lower alkali, alkaline earth and silica concentrations than the basaltic glass (Table 2). The alkali metals and alkaline earths accounted for more than 20 oxide wt.% of the total glass composition (4.9% Na2O, 4.0% K2O, 9.0% CaO, 3.4% MgO),

compared with less than 5% in the palagonite. The Al2O3, FeO, and TiO2 concentrations varied along the analysis profiles, ranging above and below the corresponding concentrations in the glass. The FeO, TiO2, and CaO variations were intercorrelated, and inversely correlated with the Al2O3 and SiO2 variations (Fig. 4). All these variations reflect the heterogeneity

Alteration of the Salagou basaltic glass in an argillaceous medium

1077

Table 2. Mean chemical composition of glass (100 analyses) and palagonite obtained by electron microprobe analysis.

Glassy E.C.

P1 observed on the surface of the glassy edges

P2 observed along the fracture. Presence of a silica rim

P3 observed along the fracture without silica rim

Distance/xurface (␮m)

SiO2 45.9 0.80

Al2O3 17.4 0.35

Na2O 4.87 0.36

K2O

CaO

MgO

FeO

TiO2

4.04 0.16

9.0 0.43

3.40 0.17

10.0 0.27

2.58 0.10

MnO 0.21 0.06

Total 97.4

1 4 10 14 20 24 30 34 38 44 50 60 70 80 100 122 200 600 980

34.6 42.2 39.0 37.4 30.2 29.6 29.4 34.5 34.9 29.9 28.3 36.0 21.4 23.4 25.4 32.8 26.7 31.5 36.6

22.4 23.5 22.9 22.1 14.9 13.8 14.8 19.1 18.5 11.6 12.1 17.2 7.2 7.6 9.8 15.4 19.4 22.3 20.2

0.08 0.02 0.11 0.14 0.00 0.03 0.18 0.09 0.13 0.18 0.15 0.07 0.09 0.12 0.09 0.00 0.15 0.12 0.14

0.62 0.81 0.85 0.48 0.47 0.38 0.60 0.98 0.67 0.47 0.72 1.04 0.40 0.28 0.28 0.46 0.35 0.36 0.59

0.65 1.33 2.09 1.93 2.94 2.96 2.53 1.87 2.33 3.04 2.18 2.19 2.86 4.56 3.18 2.74 2.90 2.51 3.87

0.59 0.83 0.73 0.58 0.55 0.53 0.45 0.70 0.65 0.62 0.45 0.72 0.73 1.26 0.27 0.41 0.12 0.27 0.68

6.63 13.5 14.8 16.7 24.6 24.5 23.2 18.4 19.5 27.5 21.7 18.7 26.6 29.3 24.8 19.8 13.9 10.5 14.5

0.60 4.60 4.21 5.36 10.83 9.90 8.40 5.31 6.63 10.0 7.32 4.57 9.58 10.8 9.36 5.90 4.81 3.57 4.22

0.37 0.00 0.00 0.00 0.00 0.02 0.55 0.00 0.00 0.03 0.00 0.06 0.04 0.25 0.06 0.11 0.01 0.05 0.00

66.6 87.1 85.0 88.1 84.7 81.6 80.3 80.9 83.4 83.5 73.0 80.9 69.0 77.6 73.5 77.9 68.4 71.2 80.8

3 5 7 20 22 26 36 40 50 70 80

84.4 88.1 92.1 92.5 57.7 55.7 55.0 48.4 44.4 49.7 55.7

2.5 2.0 0.63 0.68 13.7 14.3 15.9 15.1 7.7 9.5 10.4

0.07 0.02 0.04 0.00 0.08 0.13 0.04 0.02 0.23 0.03 0.77

0.20 0.32 0.19 0.05 1.13 0.90 0.30 0.82 1.46 0.78 1.97

0.87 0.39 0.58 0.57 1.83 2.09 1.50 2.01 2.45 2.46 2.88

0.18 0.13 0.04 0.12 1.07 1.31 1.39 1.22 1.09 1.13 0.67

0.71 0.32 0.00 0.45 7.57 9.85 10.8 8.69 12.7 13.9 7.09

0.07 0.04 0.00 0.00 1.35 1.95 2.14 1.60 2.94 4.31 2.31

0.92 0.00 0.32 0.00 1.00 0.02 0.00 0.23 0.00 0.03 0.13

89.8 91.3 93.9 94.3 85.5 86.2 87.0 78.0 72.9 81.8 81.9

2 8 10 14 18 22 24 26 28 30 34 38 54

58.6 53.0 50.8 51.0 52.5 60.2 51.0 48.1 48.0 47.2 46.8 47.0 48.5

14.7 13.8 13.9 13.6 13.6 10.0 17.8 16.5 17.1 17.1 16.8 16.6 16.7

0.12 0.11 0.30 0.24 0.09 0.51 5.0 5.5 5.7 5.6 5.2 4.9 4.8

0.84 1.05 2.03 2.15 0.52 1.79 3.8 3.8 3.9 4.0 4.2 3.9 4.3

1.89 2.24 2.13 2.18 2.85 2.84 8.1 8.6 8.3 8.8 9.0 8.6 8.8

0.85 0.48 0.13 0.47 0.44 0.35 3.3 3.4 3.4 3.3 2.7 3.4 3.3

6.93 8.94 10.3 10.6 10.7 4.71 9.23 9.91 9.37 10.2 10.3 10.3 10.2

1.84 2.20 3.32 2.89 3.79 3.18 2.93 2.82 2.68 2.82 2.16 2.70 2.47

0.02 0.13 0.00 0.00 0.04 0.05 0.18 0.17 0.34 0.11 0.29 0.13 0.30

85.8 81.9 91.6 83.2 84.4 83.5 101 98.8 98.6 99.1 97.4 97.6 99.4

Distribution of oxide percentage concentrations in the alteration film vs. distance from surface.

of the alteration film. The total oxide concentration in the palagonite was always less than 100% (ranging from 70 – 90%). If the remainder is attributed to the H2O content, the results suggest significant hydration of the alteration product. The analyses performed in the 20-␮m film on the palagonite surface in the fractures showed that it consisted of more than 88% SiO2, with less than 1% Al2O3 and negligible quantities of other oxides. The Sr concentration of the palagonite (331 ppm) at the glass surface was lower than the Sr concentration in the glass (about 1200 ppm) (Table 1). Conversely, the 87Sr/86Sr ratio of the palagonite (0.70659 ⫾ 2) was higher than in the basaltic glass (0.70322 ⫾ 2) (Table 1).

6.2. Argillaceous Host Rocks The host rock specimens sampled between 5 and 50 cm away from basaltic dikes exhibited similar mineralogical compositions: ⬍5% quartz, 7% albite, 10% calcite, 12% dolomite, ⬍5% hematite, 18% analcime, and 47% clay minerals (Table 3). In this “reference” Thuringian argillite, the fraction ⬍0.2 ␮m consisted of a mixture of 64% illite, 8% chlorite, and 28% analcime; the latter was related to Permian volcanism, as suggested by the occurrence of cinerites in the Thuringian series (Laversanne, 1976; Nmila et al., 1989). The proportion of chlorite, and to some extent of illite, increased as the distance to the dike decreased from 100 to 5 cm (Fig. 5). Mineralogical

1078

I. Techer et al.

from the dike (Table 1). Next to the dike, the SiO2, Al2O3, and Fe2O3 concentrations rose by about 15% from the mean reference composition, whereas the Na2O and CaO concentrations decreased by more than 30%. The 87Sr/86Sr ratio of the argillaceous specimen farthest from the dike was 0.71650 ⫾ 1. The ratio increased in the specimens nearer the dike, only slightly at first (from 0.71650 at 100 cm to 0.71853 at 5 cm), then more significantly in the 2 cm nearest the dike (from 0.72590 ⫾ 5 to 0.72755 ⫾ 3). This increase contrasts with the Sr concentrations, which decreased relative to increasing distance to the dike: slowly at first (from 200 ppm at 100 cm to 170 ppm at 5 cm), then more rapidly (to 103 ppm in contact with the dike) (Table 1, Fig. 6). The carbonate phase disseminated in the argillite provides relatively homogeneous 87Sr/86Sr ratio, although it may be assumed that the ratio decreased slightly near the dike (0.71023 ⫾ 2 in specimen B5 relative to 0.71003 ⫾ 2 in specimen B1) and to a larger degree in contact with the dike (0.70978 ⫾ 2 in the specimens A1 and A2). The calcites sampled from the two FFR straight joints yielded 87Sr/86Sr ratios slightly higher than in the disseminated carbonate phase (0.71053 ⫾ 2) with Sr contents ranging from 540 to 550 ppm (Table 1). 7. DISCUSSION

7.1. Origin of the Basaltic Glass and Impact on the Surrounding Argillaceous Host Rock

Fig. 4. Oxide percentage versus depth (first 500 ␮m) in a palagonite film 1 mm thick (P1).

variations were found in specimens A1 (3.5 cm from the dike) and A2 (1.25 cm): depletion of analcime, calcite and dolomite, and enrichment of hematite (Table 3). Specimens A1 and A2 contained no chlorite, which was replaced by smectite (Fig. 5). The illite in these two specimens was better crystallized (Fig. 5). Higher crystallinity indices of illite were also observed, although to a lesser extent, beyond 5 cm from the dike. The K-Ar ages were determined on the ⬍0.2 ␮m fraction of specimens B5 to A1 (Perry, 1974; Hoffman et al., 1974; Aronson and Hower, 1976). The ages were identical within analytical uncertainty, with a mean value of 257.7 ⫾ 6.5 Ma (Table 3, Fig. 6). Alternatively, if considering only the analytical accuracy, a slight decreasing trend is noticeable in the K-Ar data from 260.8 to 253.7 Ma as the distance from the dike decreases, suggesting a slight loss (about 3%) of radiogenic 40 Ar near the dike. Like the mineralogical composition, the chemical composition of the argillite remained unchanged at distances 5 cm away

In the Lode`ve Basin, as well as in the neighboring Causses region, basalt intrusion emplacements have been favored by structural discontinuities along a major N30 direction, but vitreous chilled margins are not systematically present at the dike borders. Maury (1976) and Fabre et al. (1989) showed that this random occurrence is related neither to the thickness nor to the depth of the veins: for two dikes of the same thickness located in the same area, one developed a vitreous rim and the other did not. Vitrification of the magma along the dike borders thus does not appear to be attributable to heat dissipation alone within the host rocks. In the studied case, the occurrence of fluids in structural discontinuities within the surrounding sedimentary rocks could account for the peripheral vitrification of some basaltic dikes. In the environment of the Cigar Lake U deposit, Philippe et al. (1993) demonstrated that surface water can penetrate along fracture lines to depths exceeding 400 m. This suggests that meteoric water could have flowed in discontinuities controlling the intrusion emplacement within the argillaceous material of the Lode`ve Basin to depths of 150 or 200 m. Such meteoric waters would have interacted locally with the intruding basaltic magma. In addition, during the intrusion of this magma, volcanic gases freeing may have produced acidification of the water present in the structural discontinuities. To study the chemical interactions between the basaltic glass and the argillite, distinction is needed between the thermal effect of the basaltic dike intrusion on the surrounding rock and the effect of migrating meteoritic water in the structural discontinuity. The K-Ar ages near and far from the dike confirm that the intrusion of basaltic magma did not generate a major thermal gradient in the surrounding argillite, as no significant change in the dates was noticed. The values obtained on the ⬍0.2 ␮m fraction are identical within analytical uncertainty, irrespective of the distance from the dike. The mean value of

Alteration of the Salagou basaltic glass in an argillaceous medium

1079

Table 3. Mineralogical composition (XRD ⫾5%) of Thuringian argillite, total rock, and fraction ⬍0.2 ␮m vs. distance from dike. Argillite Distance from dike (cm) Mineralogical data

K-Ar data on ⬍0.2-␮m fraction

Quartz Albite Calcite Dolomite Hematite Analcime ⬍0,2 ␮m fraction illite chlorite smectite analcime %K2O 40 Ar (10⫺6 cc/g) age (Ma) ⫾

A1 1.25

A2 3.5

B1 5

B2 10

B3 25

B4 50

⬍5 8 3 6 6 6 65 62.5 — 38.5 — 4.37 38.3 253.7 6.0

⬍5 8 3 7 7 9 62 70 — 30 — 4.60 41.0 257.2 7.2

⬍5 7 11 11 ⬍5 14 48 65 10 — 25 4.72 41.8 256.0 6.0

⬍5 7 10 11 ⬍5 17 48 65 10 — 25 4.86 43.2 256.8 6.0

⬍5 7 8 12 ⬍5 19 46 62.5 7.5 — 30 5.00 43.0 259.3 6.0

⬍5 7 11 12 ⬍5 17 46 62.5 5 — 32.5 4.85 43.8 260.3 6.1

B5 100 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 60 5 — 35 4.93 44.6 260.8 8.0

Samples B average ⬍5 7 10 11.5 ⬍5 16.8 63.8 8.1 — 28.1

Total K2O composition (%K2O tot) in fraction ⬍0.2 ␮m measured by ICP-AES (⫾0.5%); radiogenic 40Ar concentration (10⫺6 cc/g) measured by ICP-MS; mineral age (Ma).

257.7 ⫾ 6.5 Ma agrees with the age determined from pollen analysis (Odin et al., 1986). It must be emphasized, however, that this value can also account for a late-Hercynian diagenetic event. Considering the possibility that the K-Ar age decreases from 260.8 to 253.7 Ma towards the dike, this decrease may be attributed to a slight opening of the K-Ar isotopic chronometer when the basaltic magma intruded into the argillite, which can be quantified by a 3% drop in the radiogenic 40Ar content of the illite. Illite loses all its radiogenic argon at a mean temperature of 260 ⫾ 30°C (Hunziker et al., 1986), suggesting that the intrusion of the basaltic magma did not generate such a high temperature in the surrounding argillaceous formation. However, the better crystallinity of the illite and the increasing of the chlorite and illite content towards the dike, between 100 and 5 cm, suggest a very slight recrystallization effect due to the dike intrusion. These variations are noticeable up to 100 cm from the dike and differ from the mineralogical variations restricted to the first 4 cm in contact with the dike: depletion in carbonate minerals, enrichment in hematite (i.e., oxidation of the material), and appearance of smectite. From a crystallochemical viewpoint, simultaneous crystallizations of smectite and well-formed illite appears incompatible. These results imply the occurrence of two processes: 1) One accounting for the mineralogical and chemical evolution of the illite and chlorite (noticeable up to 1 m) related to the intrusion of the basaltic dike. 2) One accounting for the mineralogical and chemical variations limited to a few centimeters in contact with the dike; these variations indicate an alteration process with dissolution of carbonate and analcime, oxidation of the material, and neoformation of smectite. 7.2. Basaltic Glass Alteration in Contact with the Argillaceous Rock 7.2.1. Palagonite The palagonite formed by alteration of the basaltic glass in contact with the Thuringian argillite is morphologically iden-

tical to palagonite developed during glass alteration by seawater or fresh water in the absence of surrounding clay minerals (Honnorez, 1978; Noack, 1981; Crovisier et al., 1983; Byers et al., 1987; Jercinovic et al., 1990b). The occurrence of the silica film at the surface of the palagonite rim along some microcracks (Fig. 3e) indicates undersaturation of the fluid phase with respect to silica released during alteration of the glass in the confined medium. Silica also precipitated inside gas vesicles (Fig. 3f). Similar siliceous precipitates have been observed during alteration of basaltic glasses by marine or continental waters at low temperatures (Hay and Iijima, 1968a; Hay and Iijima, 1968b; Hekinian and Hoffert, 1975; Byers et al., 1985); in these cases as well, the precipitates were restricted to confined volumes (cavities and microcracks). Laboratory experiments have shown that silica can form at low temperatures (45–70°C) in the presence of pure or meteoric water in basaltic glass cavities during alteration in static conditions (Gislason and Eugster, 1987; Gislason et al., 1993). For the Salagou palagonite sample, very slightly crystallized nontronite is the only secondary mineral detected. Nontronites are routinely observed in low-temperature alteration products of basaltic glasses (Grambow et al., 1985; Crovisier et al., 1987; 1989; Murukami et al., 1989; Jercinovic et al., 1990b; Thorseth et al., 1991). Simulations have shown that this mineral forms readily in the presence of seawater or fresh water at temperatures ranging from 0°C to 90°C (Crovisier et al., 1985; Grambow et al., 1985). Occurrence of nontronite and silica precipitates suggests low-temperature alteration of the basaltic glass in the Salagou region (at temperatures not exceeding 100°C). Contacts between the palagonite and the glass along the fissures and between the palagonite and the primary crystals indicate an isovolumic palagonitization process (Fig. 3c,d); in fact, microscopic observations have shown that when the basaltic glass is palagonitised along a fissure (Fig. 3d), no cracks or micro-cracks are visible in the surrounding glass, and that no empty voids developed. Alternatively, if the process had increased the rock volume, cracks, and micro-cracks should be

1080

I. Techer et al.

Fig. 5. Clay composition (⫾5%) of the argillite versus distance from basalt dike. XRD spectra of samples in contact with the dike (A1) and far from the dike (B4): (a) untreated specimens; (b) specimens treated with ethylene glycol; (c) specimens heated: analcime (3.32 Å), illite (10 Å), chlorite (14 Å), smectite (14 Å).

observed, and if it had reduced the rock volume, empty voids should be observed (in the fissure zone and around the primary crystals) which is not the case (Fig 3a,d). Constant-volume palagonitization was also reported by Hay and Iijima (1968a), Furnes (1974), Jakobsson and Moore (1986), and Jercinovic et al. (1990b). A few protruding phenocrysts (Fig. 3a) can be attributed to removal of the first few micrometers of the film, either during sampling or during the specimen preparation. Oxide gains or losses during palagonitization were calculated assuming a constant-volume alteration process. The ratio (i) of oxide mass per unit volume in the palagonite to oxide mass per unit volume in the glass was calculated using the relation: 共%i xO y兲 palagonite ⫻ ␳ palagonite i⫽ (1) 共%i x O y 兲 glass ⫻ ␳ glass where ␳ is the material density. A value of i greater than 1 indicates an oxide gain during palagonitization. The densities of the basaltic glass and of the palagonite considered were 2.75 g/cm3 (Jercinovic et al., 1990b) and 2.2 g/cm3, respectively.

Note the implicit approximation in this approach, based on the mean value calculated for several palagonite samples (Hay and Iijima, 1968a; Furnes, 1978; Zhou and Fyfe, 1989). The calculations show that the palagonite formed at the surface of the solidified edges is highly depleted in alkali metals and alkaline earths (⫺98% for Na2O, ⫺89% for K2O, ⫺77% for CaO, ⫺86% for MgO), in SiO2 (⫺45%) and in Al2O3 (⫺24%). Conversely, it is enriched in TiO2 (⫹107%) and Fe2O3 (⫹55%) (Fig. 7a). This relative mobility of the elements during palagonitization of Salagou basaltic glass is similar to the relative mobility of the same elements observed during chemical weathering of glassy and crystalline basalts in Iceland, namely along the following decreasing sequence: Na ⬎ K ⬎ Ca ⬵ Mg ⬎ Sr ⬎ Si ⬎ Al ⬎ Ti ⬵ Fe (Gislason et al., 1996). The Ti-Fe enrichment is not observed in the palagonite formed along glass fractures (Fig. 7b), in which the Fe and Ti concentrations are slightly lower than in the glass. The same calculation applied to Sr which yielded an i value of 0.2, indicating a loss of 80% during palagonitization.

Alteration of the Salagou basaltic glass in an argillaceous medium

1081

Fig. 6. Alkali and alkaline-earth concentrations, 87Sr/86Sr isotopic ratio and K-Ar age of the various components of the studied SA4 system (basalt, basaltic glass, Thuringian argillite, disseminated calcite, calcite veins, clay minerals).

7.2.2. Alteration kinetics The alteration rate of the natural basaltic glass can be evaluated on the basis of the palagonite thickness and of the age of the glass (Byers et al., 1987; Jercinovic and Ewing, 1987; Jakobsson and Moore, 1986). In this approach, the glass is assumed to have been subjected to alteration processes since its formation and the palagonite thickness is assumed equal to the altered glass thickness. The alteration rate is then determined and expressed in micrometers of palagonite per unit time (␮m/ 1000 yr). In case of the Salagou basaltic glass, the mineralogy of the palagonite (evidence of nontronite and silica precipitates) has point towards low-temperature alteration. Moreover, after the dike emplacement, the contact dike/argillite has provided a new discontinuity which will favour the advent of new meteoric waters in the system with time. Thus we envisage that

alteration in such a context occurred probably during 1.4 Ma. We so calculated an alteration rate for the Salagou basaltic glass considering a 1.4-Ma duration (with an uncertainty of 50% considering that the glass/water contact could have lasted less than the glass history). The thickness of the palagonite film developed during this time is of 20 ␮m to 1.2 mm. Considering an uncertainty of 50% on the thickness (possibility of removal a few micrometers of palagonite during sampling) the alteration rate of the Salagou basaltic glass ranges from 0.014 to 0.71 ␮m/1000 yr. Many authors have investigated basaltic glass alteration in continental or oceanic settings (including Hekinian and Hoffert, 1975; Grambow et al., 1985; Jakobsson and Moore; 1986; Byers et al., 1987; Jercinovic and Ewing, 1987; Jercinovic et al., 1990b; Crovisier et al., 1989). They reported that the

1082

I. Techer et al.

Fig. 7. Palagonite/glass composition ratios per unit volume; values below 1 indicate loss of an element during palagonitization; values obtained for few points into a palagonite rim developed at the surface of the Salagou basaltic glass (a) and along a fracture in the Salagou basaltic glass (b).

glasses were weathered by rainwater or surface percolating waters or seawater at low temperatures (0 –15°C). The palagonite thicknesses measured at the surface of these glasses are reported in function of the age of the vitrified materials (Fig. 8). As previously, with the same hypothesis, the basaltic glasses alteration rate is expressed in terms of thickness of altered glass (palagonite) by unit time (1000 yr). The literature reported values (Hekinian and Hoffert, 1975; Grambow et al., 1985; Jakobsson and Moore, 1986; Byers et al., 1987; Jercinovic and Ewing, 1987; Jercinovic et al., 1990b, Crovisier et al., 1989) range from 70 to 10⫺4 ␮m/1000 yr. The alteration kinetics of the Salagou basaltic glass in contact with the Thuringian argillaceous rocks appear comparable to those measured in aqueous media in the absence of clay minerals in contact with the glass. No acceleration in the glass corrosion was observed in the case of studied glass samples over a period of several thousand years. 7.2.3. Glass/argillite interactions The alteration of basaltic glass produced a loss of more than 75% of the alkali and alkaline earth elements. Prior alteration studies of basaltic glass and aluminosilicate mineral (Berger et al., 1987; Gislason and Eugster, 1987; Atassi, 1989; Crovisier et al., 1989; Guy and Schott, 1989) indicated preferential re-

lease of the alkali metals and alkaline earths from the material into the aqueous leachate. Strontium depletion was also observed in weathering profiles of natural rocks (Bottino and Fullagar, 1968; Dash, 1969; review in Clauer and Chaudhuri, 1995). The argillite near the basaltic dike is not enriched in these released elements (Table 1), suggesting that no elemental migration occurred in the host material. On the contrary, a decrease in the Na, Ca, and Sr and an increase in the Mg concentrations were observed in the 4 cm of argillite from the contact with the basaltic dike. The whole-rock K2O content does not change significantly, but a slight decrease in the K2O content occurred for the ⬍0.2-␮m fraction (Table 3). The chemical variations closely relate to mineralogical variations in the argillite which are unrelated to thermal phenomena. For instance, the depletion of Ca minerals and the reduction in the analcime content in the 5 cm near the dike would account for Ca, Sr, and Na losses. Hematite enrichment accounts for the higher Fe concentrations in these specimens (Table 1). The Mg concentrations increased, contrary to the expected decrease due to dolomite depletion and disappearance of chlorite. The increase can be explained by the appearance of a Mg-enriched smectite in contact with the dike. All these variations record the alteration of the argillaceous rock, affecting the sedimentary material to a distance of 4 cm from dike/argillite interface.

Alteration of the Salagou basaltic glass in an argillaceous medium

1083

Fig. 8. Palagonite thickness measured at the surface of volcanic basaltic glass (in continental and oceanic media— literature data) versus age of glasses. The alteration rates (in ␮m/1000 yr) are defined by the fulled and dashed lines crosscutting the diagram (from 0.001 to 70 ␮m/1000 yr).

The combination of chemical, isotopic, and mineralogical analyses provides a better assessment of the interaction processes between the basaltic and argillaceous materials. The elemental concentrations, Sr isotopic compositions, and mineralogical proportions of the basaltic and argillaceous components converge toward the dike/argillite interface, revealing a preferential alteration zone related to fluid flow, which extends no more than 5 cm into the argillaceous rock. The alteration fluid was probably meteoric water percolating through the argillite discontinuities that constituted zones favorable for the intrusion of the basaltic magma at the origin of Salagou dike swarm. However, the depletion of calcium minerals— calcite,

dolomite—is probably due to the circulation of acid fluids resulting from the magma degassing in the first step of the basalt geological story. Systematic Sr isotopic composition analyses performed on the various SA4 components (basalt, basaltic glass, palagonite, argillite, calcite) provide information about the fluid-rock interactions. The 87Sr/86Sr ratio increased for the palagonite and the altered argillite near the dike (Fig. 6). Two processes can account for this enrichment: ●

Leaching of the carbonate phases (87Sr/86Sr ⫽ 0.7105) disseminated in the Thuringian argillite during the alteration

1084

●

I. Techer et al.

process; the dissolution of these Sr-enriched phases lead to a relative enrichment in radiogenic Sr rich argillaceous minerals and caused the 87Sr/86Sr ratio of the total-rock specimens to increase in contact with the dike. Flowing of a radiogenic Sr-enriched fluid: investigations of alteration of mid-ocean-ridge basalt (MORB) have shown that mixing during solid/liquid interactions leads to enrichment of the altered phases in radiogenic 87Sr from the present-day seawater, in which the 87Sr/86Sr ratio (0.7091) is higher than in the MORB (0.7025– 0.7031) (McCulloch et al., 1981). The composition of the aqueous alteration fluid in the system studied here is unknown. Negrel and Roy (1998) measured the Sr concentrations and isotopic compositions of rainwater in the Massif Central in 1994 to 1995, and reported 87 Sr/86Sr ratios ranging from 0.70920 to 0.71314. The 87Sr/ 86 Sr ratio of the meteoric water responsible for weathering in the studied system was probably higher than in the initial rainwater, considering that the water flowed to depths of 150 to 200 m in the argillaceous sediments and could thus have become enriched in Rb-rich radiogenic 87Sr. Moreover, the solubilization of radiogenic carbonates (87Sr/86Sr ⫽ 0.71009) during argillite alteration—notably occurred during the intrusion of the magma and the gas freeing—also contributed to the 87Sr enrichment of the alteration fluid. Finally, in southern France, rainwater may be enriched in 87Sr through contamination by wind-borne dust particles from North Africa and the Sahara (Molinaroli et al., 1993; Wagenbach et al., 1996); this additional source of radiogenic 87Sr could also account for the high Sr isotopic ratios found for the palagonite rim and the Thuringian argillite and assumed for the waters which have flown though the discontinuity. These various explanations for the enrichment in radiogenic 87 Sr of the surface infiltration water are not indissociable.

The intrusion of basaltic magma and the water/rock interactions had little effect on the surrounding Permian argillite. The geochemical and mineralogical results presented here suggest that the Salagou basaltic glass was altered by rainwater percolating along the dike/argillite interface. This water did not affect the argillaceous rock beyond 5 cm from the interface. Moreover, the elements released during the glass alteration— notably Sr— did not diffuse in the argillaceous rock. This argillaceous material, in the present-day context, constitutes a suitable barrier against diffusion of the elements released by glass alteration. Confinement of the water to the dike interface alone could be a consequence of the low porosity of the argillite, established since the late Permian period (Jalabert, 1998). This is an important conclusion from the standpoint of an analogy with a deep geological repository. 7.3. Implications for a Nuclear Waste Storage in an Argillaceous Environment The presence of basaltic glasses, analogs of nuclear glasses, in an argillaceous environment in which aqueous fluids migrated, leads to consider the studied geological setting as a good analog for a deep geological storage with a concept of an argillaceous nearfield engineered barrier. Moreover, the Thuringian argillite from Salagou site is indurated and very slightly permeable. In Europe, for several years now, interest

for studies concerning potential nuclear waste storage in indurated argillaceous media has developed. These media were in fact considered as potential host rocks for nuclear waste storages (de Windt et al., 1999). In addition, the Thuringian argillite of the Lode`ve Basin has a mineralogical composition that is close to other indurated argillites selected as references in western Europe (de Windt et al., 1999; Thury and Bossart, 1999). The main differences being the oxidizing state of the Lodeve argillite and, of course, its lack of organic matter. Our study of a natural basaltic-argillaceous analog underlines the importance of discontinuities—like the dike/argillite interface—for the fluids to access to the glass and for the chemical elements to be moved. In addition, for a geological storage, we have to consider the risk of natural or induced discontinuities (for example by gallery excavations) creating unfavorable fluid drains. We also demonstrate that if these structures may represent preferential zones for fluids flows, they do not necessarily constitute preferential zones for element transfers among glass and argillaceous formation. Thus, although unfavourable context, an argillaceous formation such as the Thuringian argillite may represent an efficient barrier against elementary diffusion. This role is probably directly linked with the physical properties of the studied argillaceous formation. It is necessary to keep in mind that the primary function of an argillaceous barrier will be to minimize exchanges between the geological barrier and the waste package, to ensure containment of any radionuclides released, but also to avoid the radionuclides to reach the biosphere. The analogy between the studied geological environment and a nuclear waste storage could also be seen in the standpoint of a “temperature/time” system. The thermal behavior of different storage concepts has been modelised in PAGIS program (Performance Assessment of Deep Geological Isolation System for radioactive wastes). It has been outlined that a temporal temperature gradient consisting in a quick heating of the surrounding rocks— during about 10 yr—will exist with a maximum value of about 100°C to 200°C according to the storage concept. During the basaltic magma intrusion at the Salagou site, the temperature did not reach the value of 260°C in the surrounding argillite and the present-day temperature is close to 15°C. These values suggest that the temperature system in the natural geological site is close to that of a geological storage one. On the other hand, the time system in the two environments is not comparable. In the case of a waste disposal in deep geological environment, the fall of temperature in the vicinity of the radioactive package will be very low and the geothermal value will be reach after 1000 yr (Poinssot, 1997), whereas in the geological case studied it is probable that the temperature has decreased very quickly and has reached the geothermal value in a few days (Fabre et al., 1989; Smith et al., 1991). 8. CONCLUSION

Alteration of the basaltic glass of the Salagou dike of the Lode`ve Basin in contact with argillaceous rocks, and the resulting transfer of elements between the glass and clay via water/rock interactions, were investigated using chemical, isotopic, and mineralogical analyses. Glassy margins were formed by interaction between the basaltic magma and meteoric water initially present in fractures of the Thuringian argillite that

Alteration of the Salagou basaltic glass in an argillaceous medium

favored the intrusion of the basaltic magma. Aqueous fluids with a similar origin altered the basaltic glass in contact with the argillite, forming a palagonite film no more than 1 mm thick. The structural, mineralogical, and chemical properties of this alteration film are similar to those of palagonite formed in aqueous media in the absence of argillaceous material. The alteration kinetics estimated from thickness of the measured film are also similar to those measured in continental or oceanic environments in the absence of a clay environment. Thus, the occurrence of the argillaceous material near the basaltic glass had no effect on the long-term reaction process. The argillite in contact with the glass was also altered, but the fluids were never able to propagate more than 4 cm into the argillite. Compared with the alkali basalt, these fluids with an meteoric origin were probably enriched in 87Sr through: ● ●

●

radiogenic components initially present in rain waters (i.e., dust particles from north-western Africa), dissolution of disseminated carbonates of the argillite host rocks on contact with the dike; enhanced by acidification of percolating waters by gases released from the dike 1.4 Ma, contamination by high radiogenic 87Sr from clay minerals during percolation of waters along the dike/argillite interface.

From the standpoint of the analogy with a geological repository for nuclear waste in a clay medium, the results of this study highlight the absence of any catalyzing effect of the argillaceous materials on the glass alteration kinetics. Recent laboratory investigations of glass alteration in argillaceous media have shown that, in the presence of such material, the glass alteration rates were higher than in simple glass/water media: under such conditions, the maximum initial rate (noted r 0 ) was maintained for several months. The results presented here suggest that the initial rate r 0 is not maintained over the long term, but is merely a transient state. Moreover, this investigation demonstrates the absence of migration of elements released from the glass into the surrounding argillaceous rocks in the absence of structural or sedimentary discontinuities, on a time scale of 1.4 Ma. The barrier properties appear to be governed by the physical and structural characteristics of the argillaceous material (porosity, permeability, discontinuities, etc.), which could be the goal of a further study. Acknowledgments—The authors are grateful to Dr. M. Pre´vost and his geophysical research group for lending the micro-core drilling device. This study was carried out in the frame of a contract between the University of Montpellier II, the CNRS, and the French Atomic Energy Commission (CEA VALRHO). Associate editor: E. Merino REFERENCES Advocat T., Crovisier J. L., Vernaz E., Ehret G., and Charpentier H. (1991) Hydrolysis of R7T7 nuclear waste glass in dilute media: Mechanisms and rate as a function of pH. Mat. Res. Soc. Symp. Proc. 212, 57– 64. Alabouvette B., Aubagne M., Bambier A., Feist R., and Paloc H. (1982) Carte ge´ologique de Lode`ve au 1/50 000, Ed. BRGM. Aronson J. L. and Hower J. (1976) Mechanism of burial metamorphism of argillaceous sediment: Radiogenic argon evidence. Geol. Soc. Am. Bull. 87, 738 –744. Atassi H. (1989) Evaluation de la re´sistance a` la corrosion en solution aqueuse de quelques verres silicate´s. Ph.D. Thesis, Univ. Strasbourg.

1085

Berger G., Schott J., and Loubet M. (1987) Fundamental processes controlling the first stage of alteration of a basalt glass by seawater: An experimental study between 200 and 320°C. Earth Planet. Sci. Lett. 84, 431– 445. Birck J. L. (1979) Chronologie primitive des objets plane´taires diffe´rencie´s. Ph.D. Thesis, Univ. of Paris VII. Bonhomme M. R., Thuizat R., Pinault Y., Clauer N., Wendling R., and Winkler R. (1975) Me´thode de datation potassium-argon. Appareillage et technique. NT. Inst. Geol. Univ. Strasbourg 3. Bonniaud R., Jouan A., and Sombret C. (1980) Large-scale production of glass for high level radioactive waste. Nuclear Chem. Waste Management 1, 3–19. Bottino M. L. and Fullagar P. D. (1968) The effects of weathering on whole-rocks Rb-Sr ages of granitic rocks. Am. J. Sci. 266, 661– 670. Byers C. D., Jercinovic M. J., Ewing R. C., and Keil K. (1985) Basalt glass: an analogue for the evaluation of the long term stability of the nuclear waste form borosilicate glasses. Mat. Res. Soc. Symp. Proc. 44, 583–590. Byers C. D., Jercinovic M. J., and Ewing R. C. (1987) A study of natural glass analogues as applied to alteration of nuclear waste glass. NUREG/CR-4842 ANL-86-46, 150. Clauer N. and Chaudhuri S. (1995) Clays in crustal environments. Isotope dating and tracing. Springer-Verlag. Crovisier J. L., Thomassin J. H., Juteau T., and Eberhart J. P. (1983) Experimental seawater-basaltic glass interaction at 50°C: Study of early developed phases by electron microscopy and X-ray photoelectron spectrometry. Geochim. Cosmochim. Acta 47, 377–387. Crovisier J. L., Fritz B., Grambow B., and Eberhart J. P. (1985) Dissolution of basaltic glass in seawater: Experiments and thermodynamic modeling. Mat. Res. Soc. Symp. Proc. 50, 237–280. Crovisier J. L., Honnorez J., and Eberhart J. P. (1987) Dissolution of basaltic glass in seawater: Mechanism and rate. Geochim. Cosmochim. Acta 51, 2977–2990. Crovisier J. L., Atassi H., Daux V., Honnorez J., Petit J. C., and Eberhart J. P. (1989) A new insight into the nature of the leached layers formed on basaltic glasses in relation to the choice of the constraints for long term modeling. Mat. Res. Soc. Symp. Proc. 127, 41– 48. Crovisier J. L. (1989) Dissolution des verres basaltiques dans l’eau de mer et dans l’eau douce. Essai de mode´lisation. Ph.D. Thesis. Univ. Strasbourg. Curti E., Godon N., and Vernaz E. (1993) Enhancement of the glass corrosion in the presence of clay minerals: testing experimental results with integrated glass dissolution model. Mat. Res. Soc. Symp. Proc. 294, 163–170. Dash E. J. (1969) Strontium isotopes in weathering profiles, deep-sea sediments and sedimentary rocks. Geochim. Cosmochim. Acta 33, 1521–1552. Daux V., Guy C., Advocat T., Crovisier J. L., and Stille P. (1997) Kinetic aspects of basaltic glass dissolution at 90°C: Role of aqueous silicon and aluminum. Chem. Geol. 142, 109 –126. De Windt L., Cabrera J., and Boisson J. Y. (1999) Etude ge´ochimique des proprie´te´s de confinement de la formation argileuse de Tournemire. Rapport de l’Institut de Protection et de Suˆrete´ Nucle´aire DPRE/SERGD n°99/17. Fabre P., Kast Y., and Girod M. (1989) Estimation of flow duration of basaltic magma in fissures. J. Volc. Geoth. Res. 37, 167–186. Furnes H. (1974) Volume relations between palagonite and authigenic minerals in hyaloclastites and its bearing on the rate of palagonitization. Bull. Volcanol. 38, 173–186. Furnes H. (1978) Element mobility during palagonitization of a subglacial hyaloclastite in Iceland. Chem. Geol. 22, 249 –264. Gastaud J., Campredon R., and Fe´raud G. (1983) Les syste`mes filoniens des Causses et du Bas Languedoc (Sud de la France): Ge´ochronologie, relations avec les pale´ocontraintes. Bull. Soc. Geol. France 5 (t. XXV), 737–746. Gislason S. R., and Eugster H. P. (1987) Meteoric water-basalt interactions. I: A laboratory study. Geochim. Cosmochim. Acta 51, 2827– 2840. Gislason S. R., Veblen D. R., and Livi K. J. T. (1993) Experimental meteoric water-basalt interactions: Characterization and interpretation of alteration products. Geochim. Cosmochim. Acta 57, 1459 – 1471.

1086

I. Techer et al.

´ rmannsson H. (1996) Chemical Gislason S. R., Arno´rsson S., and A weathering of basalt in southwest Iceland: effects of Runoff, age of rocks and vegetative/glacial cover. Am. J. Sci. 296, 837–907. Godon N., Vernaz E., Thomassin J. H., and Touray J. C. (1989) Effect of environmental materials on aqueous corrosion of R7T7 glass. Mat. Res. Soc. Symp. Proc. 127, 97–104. Grambow B., Jercinovic M. J., Ewing R. C., and Byers C. D. (1985) Weathered basalt glass: A natural analogue for the effects of reaction progress on nuclear waste glass alteration. Mat. Res. Soc. Symp. Proc. 50, 263–272. Guy C. and Schott J. (1989) Multisite surface reaction versus transport control during the hydrolysis of a complex oxide. Chem. Geol. 78, 181–204. Hay R. L. and Iijima A. (1968a) Petrology of palagonite tuffs of Koko Craters, Oahu, Hawaii. Contrib. Mineral. Petrol. 17, 141–154. Hay R. L. and Iijima A. (1968b) Nature and origin of palagonite tuffs of the Honolulu Group on Oahu, Hawaii. Geol. Soc. Am. Mem. 116, 331–376. Hekinian R. and Hoffert M. (1975) Rate of palagonitization and manganese coating on basaltic rocks from the rift Valley in the Atlantic Ocean near 36°50N. Mar. Geol. 19, 91–109. Hoffmann A. W., Mahoney J. W., and Giletti B. J. (1974) K-Ar and Rb-Sr data on the detrital and post-depositional history of Pennsylvania clay from Ohio and Pennsylvania. Geol. Soc. Am. Bull. 85, 639 – 644. Honnorez J. (1978) Generation of phillipsites by palagonitization of basaltic glass in seawater and the origin of K-rich deep-sea sediments. In Natural Zeolites, Occurrence Properties, Use. (eds., S. B. Sand and F. A. Mumpton), pp. 245–258, Pergamon Press. Hunziker J. C., Frey M., and Clauer N. (1986) The evolution from illite to muscovite: Mineralogical and isotopic data from Glarus Alps, Switzerland. Contr. Min. Petrol. 92, 157–180. Jakobsson S. P. and Moore J. G. (1986) Hydrothermal minerals and alteration rates at Surtsey volcano, Iceland. Geol. Soc. Am. Bull. 97, 648 – 659. Jalabert X. (1998) Caracte´risation structurale et ge´ochimique a` l’aide du rapport isotopique (87Sr/86Sr) des colmatages carbonate´s des fentes de tension dans la se´rie saxono-thuringienne du bassin permien de Lode`ve, DEA University of Montpellier II. Jercinovic M. J. and Ewing R. C. (1987) Basaltic glasses from Iceland and deep sea: Natural analogues to borosilicate nuclear waste-form glass. In JSS PROJECT 88-01. Jercinovic M. J., Kaser S. A., Ewing R. C., and Lutze W. (1990a) Comparison of surface layers formed on the synthetic basaltic glass, French R7T7 and HMI borosilicate nuclear waste form glasses: Materials interface interactions test, waste isolation pilot plant. Mat. Res. Soc. Symp. Proc. 176, 355–362. Jercinovic M. J., Keil K., Smith M. R., and Schmitt R. A. (1990b) Alteration of basaltic glasses from north-central British Columbia, Canada. Geochim. Cosmochim. Acta 54, 2679 –2696. Jouan A., Ladirat C., and Moncouyoux J. P. (1986) Present status of the French continuous fission product vitrification process. Advances in Ceramics, Nuclear Waste Management 20, pp. 95–106. Kamei G., Yusa Y., and Arai T. (2000) A natural analogue of nuclear waste glass in compacted bentonite. App. Geoch. 15, 141–155. Lancelot J. R., Briqueu L., Respaut J. P., and Clauer N. (1995) Ge´ochimie isotopique des syste`mes U-Pb/Pb-Pb et e´volution polyphase´e des gıˆtes d’uranium du Lodevois et du sud du Massif Central. Chron. Rech. Min. 521, 3–18. Laversanne J. (1976) Se´dimentation et mine´ralisation du Permien de Lode`ve, He´rault. Ph.D. Thesis, Univ. Paris-Sud (Orsay). Lopez M. (1992) Dynamique du passage d’un appareil terrige`ne a` une plate-forme carbonate´e en domaine semi-aride: le Trias de Lode`ve, sud de la France. Ph.D. Thesis, Univ. Montpellier II. Malow G. and Lutze W. (1984) Alteration effects and leach rates of basaltic glasses: Implications for the long term stability of nuclear waste form borosilicate glasses. J. Non Cryst. Sol. 67, 305–321. Marples J. A. C., Lutze W., Kawanishi M., and Iseghem P. V. (1990) A comparison of the behavior of vitrified HLW in repositories in salt and granite. Part II: Results. Mat. Res. Soc. Symp. Proc. 176, 275– 282.

Maury R. C. (1976) Contamination (par l’encaissant et les enclaves) et cristallisation fractionne´e de se´ries volcaniques alcalines, continentales (Massif Central franc¸ais) et oce´aniques (Pacifique central): l’origine des laves acides. Ph.D. Thesis, Univ. Orsay. McCulloch M. T., Gregory R. T., Wasserburg G. T., and Taylor H. P. (1981) Sm-Nd, Rb-Sr and 18O-16O isotopic systematics in an oceanic crustal section. Evidence from the Semail ophiolite near Ibra, Oman. J. Geoph. Res. 86, 27–35. Molinaroli E., Guerzani S., and Rampazzo G. (1993) Contribution of Saharan dust to the Central Mediterranean Basin. Geol. Soc. Am. 284, 303–312. Morgenstein M. E. and Shettel D. L. (1994) Volcanic glass as a natural analogue for borosilicate waste glass. Mat. Res. Soc. Symp. Proc. 333, 15–27. Murukami T., Banba T., Jercinovic M. J., and Ewing R. C. (1989) Formation and evolution of alteration layers on borosilicate and basalt glasses: initial stage. Mat. Res. Soc. Symp. Proc. 127, 65–72. Negrel P. and Roy S. (1998) Chemistry of rainwater in the Massif Central (France): a strontium isotope and major element study. App. Geoch. 13, 941–952. Nmila P., Cabanis B., Dardel J., Martin J. S., and Treuil M. (1989) Reliques volcaniques dans le remplissage permien du bassin de Lode`ve: Incidence me´talloge´nique. C.R. Acad. Sci. Paris 309, 1931– 1938. Noack Y. (1981) La palagonite: caracte´ristiques, facteurs d’e´volution et mode de formation. Bull. Mine´ral. 104, 36 – 46. Odin B., Doubinger J., and Conrad G. (1986) Attribution des formations de´tritiques rouges du Permien du sud de la France au Thuringien, d’apre`s l’e´tude du Bassin de Lode`ve: implications pale´ontologiques et pale´oclimatiques. C.R. Acad. Sci. Paris 16, 302, 1015– 1020. Peacock M. A. (1926) The volcano-glacial palagonite formation of Iceland. Geol. Mag. 63, 385–399. Perry E. A. (1974) Diagenesis and the K-Ar dating of shales and clay minerals. Geol. Soc. Am. Bull. 85, 827– 830. Philippe S., Lancelot J. R., and Clauer N. (1993) Formation and evolution of the Cigar Lake uranium deposit based on U-Pb and K-Ar isotope systematics. Can. J. Earth Sci. 30, 720 –730. Poinssot C. (1997) Interactions solide/solution et transferts de matie`re dans un gradient de tempe´rature: application au confinement des de´chets nucle´aires de haute activite´. Ph.D. Thesis, Univ. Pierre et Marie Curie, Paris VI. Samuel J., Rouault R., and Besnus Y. (1985) Analyse multi-e´le´mentaire standardise´e des mate´riaux ge´ologiques en spectrome´trie d’e´mission par plasma a` couplage inductif. Analysis 13, 312–317. Smith B., Bonneville A., and Hamzaoui R. (1991) Flow duration of a dike constrained by palaeomagnetic data. Geophys. J. Int. 106, 621– 634. Steiger R. M. and Ja¨ger E. (1977) Subcommission on geochronology, convention on the use of decay constants in geochronology and cosmochronology. Earth Planetary Sci. Lett. 36, 359 –362. Techer I., Advocat T., Lancelot J., Liotard J. M., and Vernaz E. (in press) Dissolution kinetics of basaltic glasses: Control by solution chemistry and protective effect of the alteration film, submitted to Chem. Geol. Thorseth I. H., Furnes H., and Tumyr O. (1991) A textural and chemical study of Icelandic palagonite of varied composition and its bearing on the mechanism of glass-palagonite transformation. Geochim. Cosmochim. Acta 55, 731–749. Thury M. and Bossart P. (1999) Mont Terri rock laboratory: results of the hydrogeological, geochemical and geotechnical experiments performed in 1996 and 1997. Geol. Reports 23. Wagenbach D., Preunkert S., Schafer J., Jang N., and Tomadin L. (1996) Northward transport of Saharan dust recorded on deep alpine ice cores. In The Impact of Desert Dust Across the Mediterranean. (eds., Guerzani S. and Chester R.), pp. 291–300. Kluwer Academic Publishers. Zhou Z. and Fyfe W. S. (1989) Palagonitization of basaltic glass from DSDP Site 335, Leg 37: Textures, chemical composition and mechanism of formation. Am. Miner. 74, 1045–1053.