Dissolution kinetics of basaltic glasses: control by solution chemistry and protective effect of the alteration film

Chemical Geology 176 Ž2001. 235–263 www.elsevier.comrlocaterchemgeo

Dissolution kinetics of basaltic glasses: control by solution chemistry and protective effect of the alteration film Isabelle Techer a,b,) , Thierry Advocat a , Joel ¨ Lancelot b, Jean-Michel Liotard b a

b

´ Commissariat a` l’Energie Atomique (CEA), Rhone ˆ Valley Research Center, DCCr DRRV r SCD, BP 171, 30207 Bagnols-sur-Ceze ` Cedex, France Laboratoire de Geochimie Isotopique, ISTEEM UniÕersite´ de Montpellier II, CC 066, 1 place E. Bataillon, ´ 34095 Montpellier Cedex 5, France Received 26 May 2000; accepted 15 September 2000

Abstract Basaltic glasses are considered as natural analogs for industrial nuclear aluminoborosilicate glasses. Alteration experiments were conducted in closed and open systems at 908C with a synthetic basalt glass doped with 1% lithium Ždissolution tracer.. The evolution of the alteration kinetics over time was assessed by comparison of reaction progress at different degrees in closed system experiments. The maximum dissolution rate Žinitial rate, r 0 . was comparable to the value observed for an SON68-type nuclear glass; the basaltic glass alteration rate subsequently dropped by four orders of magnitude. The kinetic models currently proposed in the literature to account for the alteration kinetics of basaltic glasses, nuclear glasses or aluminosilicate minerals are based on the concept of chemical affinity: the chemical affinity alone is assumed to control the dissolution kinetics. When applied to the experimental data for the closed system tests with basaltic glass, these models failed to account for the low measured rates. An inhibiting effect of dissolved silica was then investigated through open system basaltic glass alteration experiments with silicon-enriched solutions. The basaltic glass dissolution rate dropped by a factor not exceeding 200 Žor about two orders of magnitude. compared with r 0 at the high imposed silicon concentrations Ž130 ppm.. A protective effect of the alteration film was advanced to account for the four-orders-of-magnitude rate drop observed in closed system experiments, based notably on an examination of natural basaltic glasses ranging in age from several thousand to a few million years. The mean alteration rates can be estimated from the measured palagonite thicknesses, taking into account the age of the natural glasses; the rates were comparable to those measured in the laboratory for high reaction progress. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Basaltic glasses; Natural analogs; Dissolution; Kinetics; Palagonite

1. Introduction Basaltic glasses have been proposed as natural analogs for the aluminoborosilicate nuclear glasses ) Corresponding author. Laboratoire de Geochimie Isotopique, ´ ISTEEM Universite´ de Montpellier II, CC 066, 1 place E. Bataillon, 34095 Montpellier Cedex 5, France.

ŽEwing and Haaker, 1979; Allen, 1982; Malow et al., 1984; Byers et al., 1985; Lutze et al., 1985; Ewing and Jercinovic, 1987; Petit, 1992. that currently constitute the reference material for conditioning fission product solutions produced by reprocessing spent fuel in France ŽBonniaud et al., 1980; Jouan et al., 1986.. The long-term behavior of these nuclear glasses is assessed by models based on the knowl-

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 0 . 0 0 4 0 0 - 9

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edge of the reaction mechanisms and dissolution kinetics of the material on laboratory time-scale. To validate these models, one approach consists to model the long-term behavior of a basaltic glass and then, to compare the results with field data extracted from basaltic glass altered over geological time scale. Investigating the chemical durability of natural glass thus helps validate long-term behavior models for nuclear glasses. The proposed approach, therefore, determines the reaction mechanisms and dissolution kinetics of the basaltic glass. Considerable work has already been done on the initial dissolution of basaltic glass, and the initial alteration rates are now known for this material over a wide range of temperatures and pH values ŽCrovisier et al., 1985, 1987, 1989a; Berger et al., 1987, 1994; Gislason and Eugster, 1987; Guy and Schott, 1989; Atassi, 1989.. The nature of the products formed by aqueous alteration of basaltic glass has also been extensively studied, and they have been demonstrated to be analogous to the alteration products formed by the French reference nuclear glass ŽCrovisier et al., 1983, 1989a; Malow et al., 1984; Murukami et al., 1989; Jercinovic et al., 1990a; Morgenstein and Shettel, 1994; Abdelouas et al., 1994.. However, no published studies have yet characterized the evolution of the alteration kinetics of basaltic glass as the reaction progresses, when the solution becomes saturated and alteration products develop on the glass surface. This characterization was the primary objective of the present study, and was performed by means of closed system alteration experiments on a synthetic basaltic glass. The second objective was to investigate the mechanisms controlling the alteration kinetics. The models currently proposed in the literature based on chemical affinity concepts ŽAagaard and Helgeson, 1982; Grambow, 1985; Berger et al., 1994; Daux et al., 1997. were applied to the dissolution of basaltic glass and tested with experimental data. A specific effect of dissolved silicon was also investigated through open system basaltic glass alteration experiments with silicon-enriched solutions. Finally, the hypothetical control of the kinetics by the alteration gel was examined by studying natural basaltic glasses altered over time periods representative of those envisaged for a vitrified nuclear waste repository, i.e. from about 10 000–100 000 years.

2. Review of kinetic concepts The kinetics of aqueous dissolution of aluminoborosilicate glasses was first described by Grambow Ž1985. as an application of transition state theory ŽTST., considering the limiting step of the glass hydrolysis reaction to be the desorption of a purely siliceous surface complex. The kinetic model derived from this approach expresses the dissolution of aluminoborosilicate glass as a function of the initial maximum dissolution rate r 0 and of the orthosilicic acid activity in solution and at saturation; these are inherent parameters of the test material. This first-order law is expressed as follows: r s r0 1 y

H 4 SiO48 H 4 SiO4 sat8

,

Ž 1.

where r is the aluminoborosilicate glass dissolution rate, r 0 the initial dissolution rate, H 4 SiO48 the orthosilicic acid activity in solution, and H 4 SiO4 sat8 the orthosilicic acid activity at saturation with the pristine glass. This last parameter is an empirical parameter, determined during static leach tests conducted at high SrV ratio Žsurface area of the glass powders divided by the volume of leachate.. The initial dissolution rate r 0 can also be expressed as a function of the solution pH ŽAdvocat et al., 1991.: n

r 0 s kq w Hq x ,

Ž 2.

where kq is a rate constant and wHqx is the proton activity in solution, raised to the nth power; kq and n are temperature- and pH-dependent parameters. In the case of the SON68 glass, r 0 was determined at 908C in both acidic and basic media; the numerical values for kq and n in basic media are 10y1 0 mol my2 sy1 and y0.41, respectively ŽAdvocat et al., 1991. ŽTable 4.. Although further work has been carried out in recent years with basaltic glass to understand describe the aqueous dissolution kinetics ŽBerger et al., 1994; Daux et al., 1997., no previously published work on basaltic materials has adopted the same approach as Grambow Ž1985. proposed for aluminoborosilicate glass. Berger et al. Ž1994. described the dissolution kinetics of basaltic glass under hydrothermal conditions by assuming that glass hydrolysis is controlled

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by hydrolysis of the glass network-forming oxides. They modeled the dissolution in accordance with TST by using the silica hydrolysis reaction, considering that 5SiOH is the most reactive surface species at neutral pH. The basaltic glass dissolution rate was then determined from the following relation:

r s kq CŽi. 1 y

Q

ž / K

0.33

,

Ž 3.

where r is the dissolution rate, AiB the precursor speciess SiOH, kq and CŽi. the rate constant and the concentration related to this precursor species, Q the activity product and K the solubility constant for the amorphous silica hydrolysis reaction. The A0.33B value was determined from experimental findings obtained during the study. These results indicate that when basaltic glass is altered at high temperatures in the presence of silica-enriched solutions, the residual gel comprises two-thirds of the available silica. Numerous studies of basaltic glass alteration in aqueous media and observations of natural alteration products ŽFurnes, 1978; Noack, 1981; Gislason et al., 1993; Crovisier et al., 1983, 1985; Daux et al., 1994. clearly demonstrate, however, that the silicon retention fraction in the alteration gel varies according to temperature, the glass-surface-area-to-solutionvolume-ratio, the alteration time and other factors. The model proposed by Berger et al. in Eq. Ž3. thus cannot be generalized to describe the dissolution of basaltic glass. The first work addressing the dissolution kinetics of a synthetic basaltic glass at lower temperatures was carried out by Daux et al. Ž1997.. A synthetic basaltic glass doped with 4% Li 2 O Žas a tracer. was altered at 908C in initially pure water and in siliconenriched solutions Ž12–73 mg ly1 .. In this approach, based on work by Bourcier et al. Ž1992., the overall basaltic glass dissolution reaction was assumed to be controlled by the hydrolysis reaction of the hydrated basaltic glass ŽHBG. formed on the surface. This HBG was formed in the initial stage by exchange reactions between the modifying elements of the glass and ions of the solution. These dissolution mechanisms are now well known ŽGislason and Eugster, 1987; Berger et al., 1987; Guy and Schott, 1989; Crovisier et al., 1989a.. The diffusion rate,

237

during exchange reactions, decreases with time because of an increase of the hydrated glass thickness. At the same time, hydrolysis phenomena occur at the HBGrsolution interface. Initially, the hydrolysis rate is lower than the diffusion rate; with time, the layer thickness and the rate get equal. The dissolution becomes congruent. At 908C in basic media, the congruent state is instantaneous. Daux et al. Ž1997. consider that the basaltic glass dissolution at 908C corresponds to the HBG hydrolysis formed in the first stage of the interaction. The stoichiometric composition of this HBG is the same as unaltered basaltic glass, without alkali and earth-alkali. Daux et al. considered the HBG to be composed of Si, Al, Fe and H 2 O. The dissolution rate is given by the equation: rmol cm y2 sy 1 s r 0 1 y

ž

Q 8.2 = 10y5

/

,

Ž 4.

where r 0 s Ž 3.10 = 10y10 . w OHy x

0.39

,

Ž 5.

and y 0.36

Q s w H 4 SiO48 x Al Ž OH . 4 = w OHy x

y0 .36

Fe Ž OH . 38

0.18

Ž 6.

The stoichiometric coefficients assigned to the Ž . activities of the H 4 SiO48, AlŽOH.y 4 and Fe OH 38 species in solution are directly based on the surface gel composition. This kinetic model, which is based on the postulated composition of the altered material, was never applied to experimental data other than those on which it was initially established, and particularly to basaltic glass dissolution data obtained in closed system experiments. We therefore proposed to determine the response to Eq. Ž4. in the case of basaltic glass dissolution at 908C in a closed medium at different degrees of reaction progress; this response was also compared with experimental data obtained under the same conditions. We also tested a third kinetic model, which is routinely used to describe dissolution–precipitation reactions involving aluminosilicate materials, by applying it to basaltic glass dissolution. The silicate

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mineral dissolution–precipitation reactions have been widely investigated in recent years ŽNagy et al., 1991; Casey and Sposito, 1992; Nagy and Lasaga, 1992; Oelkers et al., 1994; Gautier et al., 1994; Welch and Ullman, 1996; Xiao and Lasaga, 1996., and were considered here from the standpoint of TST. The model proposed to account for the dissolution kinetics ŽNagy et al., 1991; Nagy and Lasaga, 1992; Lasaga et al., 1994. involves the notion of reaction affinity Ž A. in the A DGr B term corresponding to the free energy of the reaction Ž A s yDGr .: r s kq Ł a mj j f Ž DGr . ,

Ž 7.

j

where r is the mineral dissolution rate and kq the rate constant. The P a mj j term represents the activity product of the species in solution, and describes the catalytic and inhibiting effects of the reaction; the pH, a known reaction catalyst, is thus expressed in this term; f Ž DGr . is the free Gibbs energy function, and may be written in the following form, for example ŽAagaard and Helgeson, 1982; Nagy et al., 1991; Lasaga et al., 1994.:

ž

f Ž DGr . s 1 y exp n

DGr RT

/

Ke

s exp

DGr

ž / RT

,

Ž 9.

in which Q is the ion activity product and K e the solubility product. Eq. Ž7. can then be simplified:

ž

r s kq 1 y

Q Ke

/

log K e s Ý x i log K i q Ý x i log x i i

Ž 11 .

i

This method of estimating the solubility products was described for glass compositions with nuclear applications by Bourcier et al. Ž1992., Advocat et al. Ž1998., and Leturcq et al. Ž1999.. The kinetic models in Eqs. Ž1., Ž4. and Ž10. were applied to basaltic glass dissolution in order to account for the kinetic control of this reaction. The evolution of the material dissolution kinetics was therefore characterized by means of aqueous leaching experiments in initially pure water in closed systems at 908C at different glass-surface-area-tosolution-volume Ž SrV . ratios to simulate different degrees of reaction progress. Open system basaltic glass dissolution experiments with silicon-enriched solutions were also carried out to assess the specific effect of dissolved silica on the glass alteration kinetics.

Ž 8.

where R is the ideal gas constant and T is the temperature ŽK.. For an elementary reaction, n is often assumed to be equal to 1. As the material dissolves, the saturation state between the aqueous fluid and the solid can then be expressed by the following relation: Q

products K i of each oxide are known; the intrinsic solubility of the material is thus determined from the ideal solid solution relation:

Ž 10 .

In order to apply a kinetic model such as Eq. Ž10. to basaltic glass, the intrinsic solubility K e of the material must first be defined. The theoretical approach proposed by Paul Ž1977. consists in defining the glass solubility by considering the material as an oxide mixture. The molar fractions x i and solubility

3. Experimental procedures 3.1. Basaltic glass synthesis and specimen preparation A tholeiitic basaltic glass was synthesized by melting a powder mixture of oxides, carbonates, nitrates and phosphates ŽTable 1. in alumina crucibles for 3 h at 15008C. The resulting melt was then poured into graphite crucibles preheated to 7008C. The glass was heat treated at 6708C for 1 h, then cooled to room temperature in 10 h. X-ray diffraction analysis confirmed the vitreous nature of the material ŽFig. 1.. Rectangular prismatic glass coupons measuring 23 = 23 = 6 mm were cut and wet polished with silicon carbide to a final grade 4000 finish. The coupons were then cleaned ultrasonically in ultrapure water Ž18.2 M V resistivity using a Millipore MiliQplus device. and then in acetone before weighing to within 0.1 mg on a Mettler precision balance. The

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Table 1 Components used for basaltic glass synthesis and chemical composition of the glass Component

Supplier

Oxide form

Oxide wt.%

Oxiderelement weight conversion ratio Ž R c .

SiO 2 Na 2 CO 3 Al 2 O 3 MgO CaO LiNO 3 FeC 2 O4 P 2H 2 O AlPO4 P 4H 2 O SrŽNO 3 . 2 MnO 2 KNO 3 TiO 2

SIFRACO PROLABO PROLABO PROLABO PROLABO PROLABO PROLABO PROLABO PROLABO PROLABO PROLABO PROLABO

SiO 2 Na 2 O Al 2 O 3 MgO CaO Li 2 O Fe 2 O 3 P2 O5 SrO MnO 2 K 2O TiO 2

49.34 2.67 14.79 7.89 10.85 1.0 10.85 0.1 0.36 0.19 0.19 1.77

2.139 1.348 1.889 1.667 1.399 2.153 1.286 2.291 1.183 1.291 1.205 1.668

Concentration ratio ŽirSi.

0.086 0.339 0.206 0.336 0.020 0.366 0.002 0.013 0.005 0.007 0.046

MW a s 65.3 g moly1 . a Molecular weight of glass Žg moly1 . calculated from Eq. Ž12..

mean specimen surface area, measured with an electronic gauge with 0.01 mm accuracy, was 1.6 = 10y3 m2 . A fraction of the melt was crushed with a hammer, then ground to a powder in a Fritsch tungsten ball mill. The 40–50 and 100–125 mm size fractions were recovered after screening and cleaned ultrasonically in pure water, then in acetone. The specific surface area of the powder samples, as measured by krypton adsorption using the BET method, was 8.42 = 10y2 m2 gy1 for the 40–50 mm fraction and 4.21 = 10y2 m2 gy1 for the 100–125 mm fraction. The glass composition determined by ICP-AES analysis ŽJobin-Yvon JY66. after dissolving the powder in nitric acid is indicated in Table 1.

The synthetic material was doped with 1 wt.% lithium as an alteration tracer compared with a natural tholeiitic glass. The formula of the glass is:

Ž SiO 2 . 0.537 Ž Al 2 O 3 . 0.095 Ž Na 2 O . 0.028 Ž CaO. 0.126 Ž K 2 O . 0.0013 Ž MgO. 0.128 Ž Li 2 O . 0.022 Ž Fe 2 O 3 . 0.044 Ž SrO. 0.0023 Ž MnO 2 . 0.0014 Ž P2 O5 . 0.0005 Ž TiO 2 . 0.0145 According to this formula and as calculated from Eq. Ž12. below, the mass of 1 mol of glass is 65.3 g. MWglass s Ý Ž moles of oxide . MWoxide

Ž 12.

3.2. Closed system experiments

Fig. 1. X-ray diffraction image of synthetic material.

Three alteration experiments were carried out in closed system at 908C in initially ultrapure water, at various SrV ratios for several durations Ž t .. The reaction progress was estimated from the products Ž SrV . = t. Alteration experiments at 10 my1 were performed with glass coupons. Three specimens were placed in a Savillex w reactor containing 500 ml of pure water inside an oven at 908C ŽFig. 2.. At each specified time interval Ž0.9, 1.2, 2, 2.1, 3, 3.2, 3.9, 4.2, 7 and 9 days., 2 ml of solution were sampled from the

240

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Fig. 2. Schematic representation of basaltic glass leaching experiments at 10 and 50 my1 .

reactor using a syringe and filtered to 0.45 mm cellulose acetate filters. When each sample was taken, the solution pH was measured within 0.05 unit at 908C by inserting an Orion 8103 RS combined electrode directly into the reactor. After sampling, the reactor was kept at temperature without any adjustment of solution. The slight decrease of the volume is taken into account in the calculations of the normalized mass losses and dissolution rates. This experiment was carried out in duplicate. A second series of experiments was carried out at 50 my1 . Two glass coupons prepared as mentioned above were placed in a Savillex w reactor containing 60 ml of ultrapure water ŽFig. 2.. Seven reactors were thus prepared; each one was devoted to the study of a specified duration Ž3, 7, 14, 28, 56, 91 and 112 days.. At a given time, 3 ml of solution were sampled from the reactor using a syringe and filtered to 0.45 mm. The solution pH was then measured directly in the reactor at 908C. Each experiment was carried out in duplicate. Experiments were also conducted at a high SrV ratio Ž33 700 my1 . by adding 2 g of glass powder from the 40–50 mm size fraction to 5 ml of ultrapure water in a Savillex w reactor. The water volume was measured with precision using a Metrohm Dosimat 665 solution distributor. Each experiment was carried out in triplicate and corresponds to a specified duration ŽFig. 3.. The three reactors for each time period were inserted into a Pyrex tube containing a few milliliters of water and sealed to prevent evaporation. The total duration was 281 days. At the sampling intervals Ž8, 51, 58, 88, 186 and 281 days. the reactors were weighed and 3 ml of solution were sampled, then ultrafiltered in Sartorius cells with a cutoff threshold of 10 000 Da. The pH was measured

directly in the reactor at 908C after each sample was taken. 3.3. Open system experiments Basaltic glass was also altered in open system at 908C in pure water and in silicon-enriched solutions. The flowing solution of known composition was maintained in contact with the glass at a known, constant flow rate ŽF , ml miny1 . throughout the duration of each experiment. A 1.3 g powder sample from the 100–125 mm size fraction was placed in a 55 ml Savillex w reactor, two fluid lines were connected to the cover, and the unit was placed in an oven at 908C ŽFig. 4.. Four tests Ždesignated Vh1– Vh4. were carried out with initially ultrapure water

Fig. 3. Schematic representation of basaltic glass leaching experiments at 33 700 my1 .

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241

Fig. 4. Schematic representation of experimental device used for open system alteration experiments with pure water and silicon-enriched solutions.

at flow rates ranging from 0.02 to 0.63 ml miny1 . The parent solution, maintained at room temperature above the oven, was fed to the reactor through Pharmede flexible ŽPTFE. tubing; the leachate was also recovered from the reactor through flexible tubing. The flow rate was controlled by a peristaltic roller pump. Seven tests ŽVSi1–VSi7. were conducted with silicon-enriched solutions, prepared with anhydrous amorphous silica gel Ž15–40 mm size fraction.. The required quantity of silica gel was weighed and dissolved in 11 of ultrapure water adjusted to pH 8 by adding 0.1 M KOH. The stirred solutions were maintained at a temperature of 908C for 1 month; the pH was then measured and the Si concentration determined by ICP–AES after ultrafiltration to 10 000 Da in Sartorius cells. The concentrations of solutions VSi1 to VSi7 ranged from 15 to 129 ppm ŽTable 8.. The silicon-rich solution contained was placed directly in the oven at 908C, above the leaching reactor. As in the pure water tests, the solution was fed to the reactor via Pharmexe tubing and the flow rate was controlled by a roller pump ŽFig. 4.. The time Žt . required to reach steady-state conditions in all the tests was calculated by the following relation, considering that at the calculated value of t ,

the concentration in solution Ž C . was equal to 99% of the steady-state concentration Ž Csat .:

t s ln Ž 0.01 .

ž

Csat Csat y C

V

V

/ž / ž / f

s 4.6

f

Ž 13 .

V represents the solution volume Žin l. in the reactor, and f the rate in l dayy1 . The t value is indicated for each experiment in Tables 7 and 8. After t , 2-ml solution samples were taken with a syringe directly from the reactor at regular intervals. The sample pH was measured at 908C, and the sample was then filtered to 0.45 mm. Only the final sample was ultrafiltered to 10 000 Da to check for possible colloids. 3.4. Analysis After filtration or ultrafiltration, the solutions were acidified with 14 N HNO 3 . The Si, Al, Na, Li, K, Ca, Mg, Sr, Fe and Ti concentrations in solutions were measured by ICP–AES for the closed system tests Žthe detection limits were: 4 mg ly1 for Sr, 10 mg ly1 for Si, Al, Mg, Fe; 20 mg ly1 for Na, Li, Mg; and 40 mg ly1 for K.. The Li, Al, Ti and Sr concentrations were determined by ICP–MS for the

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242

open system tests Žwith detection limits of 1–10 mg ly1 .. The measurement precision was on the order of 6% for Si, and 3% for the other elements. The potassium concentration was measured only in the

basaltic glass alteration leachates recovered from the tests at 33 700 my1 , the only experiment in which the pH was measured after sampling the analysis fraction; in the other experiments, the direct pH

Table 2 Experimental results of synthetic basaltic glass alteration at 10 and 50 my1 Ž908C, initially pure water, closed systems. Days

Concentrations Žppm. Si

Al

pH Na

Li

Ca

Mg

Normalized mass loss Žg glass my2 .

Alteration rate Ž10y7 mol glass my2 sy1 . ŽEq. Ž16..

NL ŽNa.

NL ŽLi.

Dt

2.52 2.98 2.97 3.21 3.66 4.12 3.90 4.12 4.34 4.78 4.58 5.22 4.78 5.25 5.03 5.28 6.19 6.42 6.86 7.05

rŽNa.

rŽLi.

S r V s 10 m y 1 0.9 4.4 1.3 0.55 0.12 1.2 0.63 0.9 3.7 1.1 0.59 0.14 0.9 0.44 1.2 4.9 1.3 0.44 0.14 1.7 0.80 1.2 4.6 1.3 0.42 0.15 1.6 0.71 2.0 6.4 1.7 0.57 0.17 2.1 1.01 2.0 5.7 1.6 0.55 0.19 2.1 0.93 2.1 7.4 1.9 0.59 0.18 2.3 1.08 2.1 6.0 1.7 0.57 0.19 2.2 0.95 3.0 7.6 2.2 0.69 0.20 2.6 1.21 3.0 7.3 2.1 0.69 0.22 2.5 1.10 3.2 8.0 2.3 0.69 0.21 2.8 1.25 3.2 7.8 2.3 0.75 0.24 2.7 1.13 3.9 8.8 2.3 0.74 0.22 2.7 1.19 3.9 8.0 2.3 0.75 0.25 2.7 1.11 4.2 8.5 2.4 0.78 0.24 2.8 1.15 4.2 8.0 2.3 0.77 0.25 2.7 1.07 7.0 9.6 2.4 0.98 0.29 3.3 0.79 7.0 9.3 2.3 0.99 0.30 3.3 0.77 8.9 10.3 2.4 1.10 0.32 3.5 0.62 8.9 10.1 2.4 1.08 0.33 3.6 0.58 For all durations: Sr - 0.04; Fe - 0.1; Ti - 0.1; Mn - 0.1; P - 0.2

8.38 8.42 8.45 8.44 8.59 8.63 8.64 8.64 8.59 8.60 8.59 8.88 8.94 8.60 8.61 8.58 8.61 8.60 8.64 8.55

2.71 2.92 2.18 2.07 2.81 2.71 2.92 2.81 3.45 3.43 3.45 3.73 3.69 3.71 3.87 3.83 4.84 4.89 5.46 5.38

Days

pH

Normalized mass loss Žg glass my2 .

Alteration rate Ž10y7 mol glass my2 sy1 . ŽEq. Ž16..

NL ŽNa.

NL ŽLi.

Dt

rŽNa.

rŽLi.

1.11 1.25 2.05 1.53 1.56 1.76 2.08 2.04 1.93 1.80 2.64 3.16 3.87 4.01

2.14 2.65 3.03 2.68 2.90 3.19 3.20 3.09 3.93 3.72 3.49 3.77 3.51 3.68

1.5 1.5 5 5 10.5 10.5 21 21 42 42 73.5 73.5 101.5 101.5

0.66 0.74 0.43 0.12 y0.12 0.05 0.07 0.04 y0.02 y0.02 0.04 0.07 0.11 0.07

1.26 1.56 0.39 0.02 y0.04 0.12 0.04 y0.02 0.05 0.04 y0.02 0.004 0.002 y0.01

Concentrations Žppm. Si

Al

Na

Li

Ca

Mg

0.5 0.5 1.1 1.1 1.6 1.6 2.0 2.0 2.6 2.6 3.1 3.1 3.6 3.6 4.1 4.1 5.6 5.6 8.0 8.0

5.07 5.46 y3.51 y5.55 1.52 1.51 1.26 1.26 1.05 1.24 0.02 2.43 0.60 y0.01 1.26 0.90 0.60 0.67 0.58 0.44

4.71 5.57 2.96 1.52 1.63 2.16 2.69 0.04 0.89 1.33 1.88 3.51 0.50 0.09 1.81 0.21 0.73 0.73 0.62 0.58

y1

S r V s 50 m 3 12.0 3.0 1.1 0.50 5.1 3 12.7 2.9 1.3 0.62 5.7 7 16.5 3.4 2.1 0.70 6.5 7 16.5 3.4 1.5 0.62 5.8 14 14.0 3.1 1.6 0.67 6.2 14 17.8 3.6 1.8 0.74 6.3 28 16.4 3.5 2.1 0.74 6.6 28 14.9 3.3 2.0 0.72 7.2 56 16.8 3.2 1.9 0.91 6.9 56 16.0 3.2 1.8 0.86 6.9 91 16.1 2.7 2.6 0.81 6.7 91 16.9 3.2 3.2 0.88 6.7 112 17.6 3.3 3.9 0.81 6.7 112 20.0 3.3 4.0 0.85 6.3 For all durations: Mg - 0.1; Ti - 0.1; Mn - 0.1; P - 0.2

0.14 0.12 0.15 0.13 0.14 0.14 0.14 0.14 0.17 0.17 0.20 0.19 0.20 0.18

8.90 8.85 8.80 8.75 8.87 8.79 8.75 8.75 8.77 8.78 8.90 8.92 8.70 8.55

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measurements in the reactors resulted in potassium contamination. 3.5. Calculation of alteration parameters 3.5.1. Calculating the altered glass mass In the closed system experiments, the quantity of altered glass per unit area was calculated from the elemental analysis data and expressed in terms of normalized elemental mass loss, NLŽi.: Ci R c NL Ž i . s Ž 14 . S i V where Ci is the concentration Žppm. of element i in solution, R c is the oxiderelement mass conversion ratio in the glass, i is the mass fraction of oxide i in the glass, and SrV is the glass-surface-area-to-solution-volume ratio Žmy1 .. The normalized elemental mass loss is expressed in grams of altered glass per square meter Žg my2 .. For the open system tests, the quantity of altered glass was determined from the following relation: NL Ž i . t s NL Ž i . tyl q

Ci t q Ci ty 1 y 2Cb

q Ž V Ž Ci t y Ci ty 1 . .

2 Rc %Ž i . S

f Ž t y Ž t y 1. .

243

Ž1. When the evolution of the normalized Li or Na mass loss is linear, NLŽi. s aŽ t . q b, the alteration rate can be determined by linear regression applied to the data points. This method was used for the initial time intervals for the tests at 10 my1 and for the open system tests. Ž2. When the normalized mass losses follow a logarithmic curve, NLŽi. s alnŽ t . q b, the rate is calculated at each time interval from the art ratio. This method was applied for the tests at 33 700 my1 . Ž3. The alteration rate between two sampling points can also be calculated by the relation: r Ž i . tq Ž tq1. s 2

NL Ž i . tq1 y NL Ž i . t

Ž t q 1. y t

Ž 16 .

The rate can be followed in this way over time, but the method tends to overestimate or underestimate the rate when the evolution of the normalized mass loss is irregular. This method was applied when the altered glass mass followed neither a linear regression nor a logarithmic regression Ži.e. for the tests at 50 my1 and the final samples at 10 my1 .. 3.5.3. Calculating the actiÕity of species in solution The objective here was to determine the degree of leachate saturation with respect to the glass and the

Ž 15 .

where C b is the concentration Žppm. of element i in the initial solution, t the time Ždays., f the flow rate Žl dayy1 ., V the vector volume Žl. and S the glass surface area Žm2 .. The altered glass quantity NLŽi. in g my2 corresponds in this case to a cumulative mass loss. The normalized elemental mass loss NLŽi. accounts for the degree of retention of each element during alteration. The alkali metals Li and Na are considered as alteration tracers: the altered glass mass is therefore expressed in terms of the calculated normalized mass losses for lithium and sodium. 3.5.2. Calculating the alteration rate Three methods were used to calculate the alteration rates Žexpressed in mol my2 sy2 using the value of the molecular weight of glass of 65.3 g moly1 ..

Fig. 5. Concentration ratios vs. time of element i to silicon in solution of basaltic glass alteration experiments at 908C and 10 my1 .

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HBG in order to test the kinetic models proposed by Grambow Ž1985; Eq. Ž1.. and by Daux et al. Ž1997; Eq. Ž4.., as well as the general dissolution model ŽEq. Ž10... The activities of the species in solution were calculated using the Equil subroutine of the

KINDIS geochemical code ŽFritz, 1975; Made´ et al., 1994.. This program determines the element distribution between simple and complex ions to the point of molality convergence, based on the initial concentrations and data concerning gasrsolution equilibrium

Fig. 6. Altered basaltic glass mass vs. time, calculated from Na and Li mass losses during closed system alteration at 908C at 10, 50 and 33 700 my1 ; comparison with altered SON68 glass mass vs. time measured from B mass loss under similar experimental leaching conditions Ž908C at 10, 50 and 20 000 my1 ..

Table 3 Experimental results of synthetic basaltic glass alteration at 33 700 my1 Ž908C, initially pure water, closed systems. Days

Concentrations Žppm.

pH

Al

Na

Li

K

Ca

Sr

Fe

24.2 24.7 24.3 32.3 33.2 31.5 29.7 30.5 29.6 33.7 33.8 33.9 31.3 31.2 30.9 33.3 31.8 32.0

5.5 5.6 5.4 6.9 7.2 6.7 5.8 5.9 5.8 8.2 8.0 8.1 10.5 9.2 8.9 11.0 10.7 12.0

14.1 14.1 14.3 19.4 20.5 19.2 20.1 20.6 20.7 29.0 28.1 28.6 24.8 32.2 31.4 39.4 39.9 40.8

4.06 4.07 4.09 5.86 6.21 5.81 6.12 6.34 6.33 7.99 8.03 7.98 9.12 8.82 8.69 10.2 9.5 11.7

0.87 0.63 0.70 1.30 1.36 1.16 1.24 1.18 1.23 1.92 1.68 2.15 1.88 1.87 1.96 1.96 2.06 2.08

2.9 2.8 2.6 4.6 5.8 4.6 5.0 5.3 5.3 4.6 4.8 4.9 4.0 4.3 4.1 4.4 4.3 4.2

0.05 0.05 0.05 - 0.06 - 0.06 - 0.06 - 0.05 - 0.05 - 0.05 - 0.05 - 0.05 - 0.05 0.075 0.096 0.094 0.097 0.102 0.098

0.22 0.23 0.24 - 0.14 - 0.14 - 0.14 - 0.12 - 0.12 - 0.12 - 0.13 - 0.14 - 0.13 0.30 0.26 0.28 0.17 0.18 0.20

Normalized mass loss Žg my2 .

Alteration rate Ž10y10 mol my2 sy1 . Žlog reg..

NLaŽNa.

NLaŽLi.

rŽNa.

rŽLi.

0.021 0.021 0.021 0.029 0.031 0.029 0.030 0.031 0.031 0.044 0.042 0.043 0.052 0.048 0.047 0.059 0.060 0.061

0.026 0.026 0.026 0.038 0.039 0.037 0.039 0.041 0.041 0.051 0.051 0.051 0.058 0.056 0.056 0.066 0.061 0.075

2.30

2.48

0.37

0.39

0.32

0.34

0.21

0.23

0.11

0.11

0.07

0.07

y1

Sr V s 33 700 m 8 8 8 51 51 51 58 58 58 88 88 88 186 186 186 281 281 281

9.02 9.05 9.10 9.39 9.41 9.42 9.48 9.48 9.50 9.58 – 9.58 9.81 9.78 9.72 9.94 9.95 9.98

3.8ey04 3.7ey04 3.4ey04 2.8ey04 2.8ey04 2.6ey04 2.2ey04 2.2ey04 2.1ey04 2.8ey04 2.4ey04 2.6ey04 2.8ey04 2.5ey04 2.5ey04 2.6ey04 2.5ey04 3.2ey04

I. Techer et al.r Chemical Geology 176 (2001) 235–263

Si

H 4 SiO48

For all durations: Mg - 0.2; Mn- 0.02; Ti - 0.02; P - 0.3. a NL ŽNa. s1.066=lnŽ t .y0.6104 Ž r 2 s 0.85.; NL ŽLi. s1.123=lnŽ t .y0.083 Ž r 2 s 0.89..

245

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I. Techer et al.r Chemical Geology 176 (2001) 235–263

Ž pCO 2 , solution pH, etc.. The H 4 SiO48, AlŽOH.y 4 , Naq, Ca2q, Kq, Mg 2q, Liq, Sr 2q, Mn2q, and PO43q activities were thus determined from the total measured elemental concentrations in solution. The Fe 3q and TiŽOH.y 5 activities were determined by considering them equal to the total Fe and Ti concentration.

4. Closed system experiments: results and discussion 4.1. Results at S r V s 10 m y 1 The measured elemental concentrations in the basaltic glass alteration leachates at 10 my1 were the same within the experimental error margin Ž3–6%. for both series of experiments ŽTable 2.. The Si, Al, Na, Li and Ca concentrations in solutions increased in time, while the solution pH rapidly stabilized at 8.4 and remained constant at that value throughout the 9-day tests. The irSi concentration ratios were compared with the same ratios in the glass ŽFig. 5.. The AlrSi and MgrSi ratios were always lower than the glass reference values, indicating that these elements contributed to the formation of secondary phases. The nature of the potential mineral phases which might have precipitated on the glass surface will be evaluated later using EquilŽT . subroutine of KINDIS program. The NarSi and the LirSi ratios were higher than in the glass in the first days. This could indicate preferential extraction of the alkali metals in agreement with the observations of Crovisier et al. Ž1989a. who has previously demonstrated that the basaltic glass alteration at 908C is initially selective. The altered glass quantity, based on Li and Na, increased in a linear manner over the first few time intervals. The maximum rate r 0 was determined by linear regression over the data points between 1 and 3.2 days. The value was 1.4 = 10y7 mol my2 sy1 . The rate decreased after 4 days of alteration ŽFig. 6a. and by the end of the experiment was only a third of r 0 ŽTable 2.. 4.2. Results at S r V s 50 m y 1 The elemental concentrations measured in the glass alteration leachates at 50 my1 ŽTable 2. reflect

the experimental repeatability. The Si, Al, Ca and Li concentrations in solution increased rapidly over the first 28 days, and at a much slower pace thereafter; the Na concentration continued to increase for 112 days. The alkali metal concentrations in solution indicate an initially rapid and continuous increase in the quantity of altered glass, which became much lower after 14 days ŽFig. 6b.. The alteration rates determined at each time interval using Eq. Ž16. decreased in time. After 3 days of alteration the rate was less than half of r 0 ; by 112 days it had decreased by a factor of 700 at the maximum, or almost three orders of magnitude below r 0 ŽTable 2.. 4.3. Results at S r V s 33 700 m y 1 The Si concentration in solution reached a mean steady-state value of 32 ppm after 50 days of basaltic glass alteration at 33 700 my1 . This phenomenon was not observed for the other alkali metals ŽNa, Li, K. or for the alkaline earth elements ŽCa, Sr., for which the concentrations in solution continued to rise. The solution pH became sharply basic after the first 14 days of alteration ŽTable 3.. The altered glass quantity based on the Na and Li release increased logarithmically over time ŽFig. 6c.:

Ž r 2 s 0.85 . NL Ž Li . s 1.123ln Ž t . y 0.0803 Ž r 2 s 0.89 . NL Ž Na . s 1.066ln Ž t . y 0.6104

The alteration rates determined by logarithmic regression at each time step were low and decreased as the reaction progressed ŽTable 3.. The numerical values were the same for Na or Li. After 8 days of alteration, the rate r was 600 times lower than r 0 ; after 281 days the drop was by a factor of 20 000, or four orders of magnitude below r 0 . 5. Discussion The alteration rates are comparable between the calculations based on Li or Na. These two alkali elements can therefore be considered as good tracers of the basaltic glass alteration at 908C. Two sections of altered glass—obtained at the end of 10 and 50 my1 experiments—were analyzed by ionic beam ŽSIMS. which confirm that these elements were not incorporated in secondary phases.

I. Techer et al.r Chemical Geology 176 (2001) 235–263

247

Fig. 7. Initial alteration rate Ž r 0 . vs. temperature for basaltic glass and for SON68 reference nuclear glass; dissolution reaction activation energy for both glasses Ž Ea : kJ moly1 ..

Basaltic glass dissolution is selective at the outset of the alteration in initially pure water at 908C. For the experimental conditions studied ŽT equal to 908C, mean pH equal to 8.6. in the first few days, the alteration rate is at its maximum value of 1.4 = 10y7 mol my2 sy1. This Ainitial rateB Ž r 0 . is near the rate measured by Delage and Dussossoy Ž1991. under the same experimental conditions at 908C for the ASON68B reference nuclear glass: r 0ŽS ON68. s 1.8 = 10y7 mol my2 sy1 . Since the 1980s, several authors have investigated the initial alteration of basaltic glass at temperatures ranging from 38C to 3008C, including Crovisier et al. Ž1985, 1989a., Berger et al. Ž1994., Daux et al. Ž1997., Gislason and Eugster Ž1987., Guy and Schott Ž1989., Atassi Ž1989.. The reported values of r 0 are plotted logarithmically vs. the reciprocal of the temperature in Fig. 7, forming a linear relation throughout the considered temperature range:

ln Ž r 0 . s ln Ž A . q

yEa RT

,

Ž 17 .

where r 0 is the initial alteration rate Žmol my2 sy1 ., A the preexponential factor Žmol my2 sy1 ., R the ideal gas constant and T the temperature ŽK.. The activation energy Ea ŽJ P moly1 . is the product of s = R, where s is the slope of the regression line. The literature data are corrected to a same value of pH of 8.4 Žvalue obtained in our experiments., using Guy and Schott Ž1989. data on the evolution of r 0 with pH for temperatures ranging from 508C to 2008C. The activation energy of the initial basaltic glass alteration reaction is constant between 38C and 3008C at a value of 72.4 kJ moly1 . This is comparable to the value of 71 kJ moly1 obtained by Delage and Dussossoy Ž1991. for the SON68 nuclear glass dissolution reaction ŽFig. 7.. The initial alteration of basaltic glass could thus be characterized by mechanisms similar to those involved during SON68 glass alteration. The initial rate r 0 did not persist as the reaction progressed: the basaltic glass alteration rate quickly dropped by several orders of magnitude. After 9 days at 10 my1 the alteration rate was r 0r3; at the highest experimental reaction progress Ž281 days at

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I. Techer et al.r Chemical Geology 176 (2001) 235–263

33 700 my1 . the rate was 20 000 times lower than r 0 , i.e. four orders of magnitude. At high degrees of reaction progress the leaching solution pH continued to rise. The pH quickly became basic as a result of inter-diffusion of species in the first instants of the reaction ŽVernaz and Dussossoy, 1992.. The continuing rise during the experiments at 33 700 my1 caused the distribution of species in solution to vary over time. It is important to note the significant dissociation of orthosilicic acid H 4 SiO48, the species considered by Grambow Ž1985. as the major parameter controlling the glass dissolution kinetics. The H 4 SiO48 activity diminished during the reaction ŽTable 3.; thus, while experiments at high SrV ratios are a means of reaching steady-state Si concentrations Ž32 ppm: Table 3., they do not result, here, in a constant steady-state H 4 SiO48 activity, because of the constant rise in the pH. The kinetics of basaltic glass alteration in pure water and in closed system experiments at 908C are thus characterized by an initial maximum rate that drops by four orders of magnitude over time, i.e. the same evolution as observed for SON68 nuclear glass ŽFig. 6.. The analogy between basaltic glass and SON68 nuclear glass has been based to date only on characterization studies of the alteration products from both materials ŽLutze et al., 1985; Byers et al., 1985; Ewing and Jercinovic, 1987; Jercinovic et al., 1990a.. The experimental findings presented here further support this assertion by highlighting the similarity of the pure-water alteration mechanisms and kinetics of basaltic glass and SON68 nuclear glass. Previous studies of the dissolution of glass ŽGrambow, 1985; Berger et al., 1994; Daux et al., 1997. and aluminosilicate minerals ŽNagy et al., 1991; Casey and Sposito, 1992; Nagy and Lasaga, 1992; Oelkers et al., 1994; Gautier et al., 1994; Welch and Ullman, 1996; Xiao and Lasaga, 1996. provided the basis for understanding the alteration kinetics of these materials. All these studies showed that the kinetics were probably controlled by the solution chemistry, either by inhibition or by reaching a solidrsolution saturation state. We propose to apply these kinetic models to basaltic glass alteration in closed system. Comparing the experimental data with the results predicted by these models provides conclusive evidence of the precise role of the solu-

tion chemistry with regard to the glass alteration kinetics. 5.1. Application of Grambow’s model In order to apply Grambow’s model ŽEq. Ž1.. to the basaltic glass dissolution at 908C, the initial rate expression and the activity of orthosilicic acid obtained in saturation state, H 4 SiO4 sat8, must be determined at that temperature. The kinetic parameters of the initial alteration of basaltic glass were determined by Guy and Schott Ž1989. over a wide pH range at temperatures of 508C, 1008C, 1508C and 2008C ŽTable 4.. As with SON68 glass, the initial basaltic glass dissolution rate, r 0 , can be expressed as a function of the Hq concentration in solution together with a coefficient n and an initial rate constant kq ŽEq. Ž2... The following expression for r 0 at 908C in alkaline media was derived by interpolation of the data reported by Guy and Schott Ž1989.: n

r 0 mol m y2 sy 1 s kq w Hq x s 3.3 = 10y11 w Hq x

y0.44

Ž 18 . In the case of SON68 glass, the activity of orthosilicic acid in saturation state, H 4 SiO4 sat8, was determined by experiments simulating conditions near Si saturation at 20 000 my1 ŽAdvocat, 1991.; the H 4 SiO48 activity was constant at a mean steadystate value that was considered to be the saturation value: wH 4 SiO48xsat s 10y3 .00 ŽVernaz and Dussossoy, 1992.. For basaltic glass, because of the constant rise in the pH Ž) 9. during the experiments at 33 700 my1 , the H 4 SiO48 activity in the solution

Table 4 Kinetic parameters from Eq. Ž2. for initial alteration of basaltic glass ŽGuy and Schott, 1989. and for SON68 reference nuclear glass at 908C ŽAdvocat et al., 1991. in basic medium Sample

T Ž8C.

kq Žmol my2 sy1 .

n

Basaltic glass

50 100 150 200 90

1.7=10y1 5 2.0=10y9 1.7=10y8 6.2=10y6 1=10y1 0

y0.72 y0.35 y0.27 y0.06 y0.41

SON68 nuclear glass

I. Techer et al.r Chemical Geology 176 (2001) 235–263

decreased over time ŽTable 3.: no steady-state value was observed for orthosilicic acid during basaltic glass alteration at a high SrV ratio at 908C. So, if we want to apply the first-order law to basaltic glasses we have to fix an approximate value for the H 4 SiO48 activity. We can chose the maximum value measured experimentally: H 4 SiO48 s 10y3 ,30 In this condition, as the orthosilicic acid activity decreases with time, the dissolution rate increases, and at 281 days it is not far from the value of r 0 ŽTable 5.. This is not the case experimentally. This suggests that the first-order rate law may not be applicable to basaltic glass, and that the alteration of this material is not governed by desorption of a siliceous surface complex. Once again, this is analogous to the alteration kinetics of the SON68 reference nuclear glass, which—particularly in highly concentrated media—do not correspond to a siliceous complex desorption process covered by Grambow’s model ŽJegou et al., 1998.. ´ 5.2. Application of Daux’ model The kinetic model ŽEq. Ž4.. proposed by Daux et al. Ž1997. has been established for a temperature of 908C and a mean pH of 8.3. However, Daux et al. have never mentioned that pH was a limit condition to the application of their model, and this let us applied it to our experimental data, with reservations that species activities in solution are well determined. The kinetic model takes into account the stoichiometry of the HBG. For the basaltic glass investigated here, the kinetic model takes the following form:

249

equation was calculated for different stages of reaction progress based on the results of the closed system experiments. In some of the tests, where iron was not detected, the maximum value for calculation purposes was assumed equal to the analytical detection limit. The rates predicted by Eq. Ž19. are compared with the experimentally measured dissolution rates at various stages of reaction progress as estimated by the Ž SrV . t parameter ŽFig. 8.. During the first instants of the reaction, when Ž SrV . t - 20 my1 day, Eq. Ž19. underestimates the measured rates. However, at a slightly more advanced stage of reaction progress Ž20 - Ž SrV . t - 50 my1 day., the predicted and measured values are comparable ŽFig. 8a.. The deviation between the predicted and measured rates then becomes more and more pronounced when the reaction progresses still further, as Eq. Ž19. increasingly overestimates the actual rate ŽFig. 8b and c.. The model proposed by Daux et al. Ž1997. thus does not account for the alteration kinetics of basaltic glass at 908C. It fails to account for the observed drop in the alteration rate, but instead predicts a virtually constant initial rate that increases as the reaction progresses Ž8 = 10y8 mol my2 sy1 for Ž SrV . t s 10 my1 day vs. 3.4 = 10y7 mol my2 sy1 for Ž SrV . t s 9.47 = 10 6 my1 day., overestimating the rate by a factor of 40 000 Žor four orders of magnitude. at the highest Ž SrV . t value. We must specify that, if the Fe concentration in solution were 10 or 100 times lower than the considered value Žas we haven’t detected it in solution most of the time., we would find a higher dissolution rate. The assumption made for the Fe concentrations doesn’t change drastically the conclusion on the validity of the model.

rmo l cm y2 sy t s 4.3=10y15 w Hq x

ž

= 1y

5.3. Application of a general dissolution model

y0 .39

w H 4 SiO48 x w Al Ž OH. 4y x

0 .35

w Fe Ž OH. 38 x 0 .18 w OHy x y0 .35

8.2=10y5

/

,

Ž 19 . where it is assumed that wHqxw OHyx s 10y1 2.42 at 908C ŽStumm and Morgan, 1981.. The basaltic glass alteration rate in pure water 908C predicted by this

In order to express the alteration kinetics of basaltic glass as for minerals, by a model taking into account an overall chemical affinity effect as saturation conditions are approached ŽEq. Ž10.., the solubility product, K e , of basaltic glass ŽEq. Ž11.. must first be determined, and therefore the solubility products of each constituent oxide, K i . The latter are

I. Techer et al.r Chemical Geology 176 (2001) 235–263

250

Table 5 Alteration rates predicted by Eqs. Ž1. and Ž19. as applied to basaltic glass alteration in closed system experiments at 10, 50 and 33 700 my1 Experiments at 10 my1 Days

Experiments at 50 my1

Predicted rate

a

Grambow Ž1985. ŽEq. Ž1.. 0.9 1.22e y 07 0.9 1.34e y 07 1.2 1.28e y 07 1.2 1.29e y 07 2.0 1.38e y 07 2.0 1.52e y 07 2.1 1.38e y 07 2.1 1.51e y 07 3.0 1.26e y 07 3.0 1.31e y 07 3.2 1.24e y 07 3.2 1.91e y 07 3.9 1.98e y 07 3.9 1.24e y 07 4.2 1.21e y 07 4.2 1.20e y 07 7.0 1.10e y 07 7.0 1.11e y 07 8.9 1.10e y 07 8.9 9.47e y 08 Daux et al. Ž1997. ŽEq. Ž19.. 0.9 7.96e y 08 0.9 8.25e y 08 1.2 8.47e y 08 1.2 8.40e y 08 2.0 9.61e y 08 2.0 9.96e y 08 2.1 1.00e y 07 2.1 1.00e y 07 3.0 9.61e y 08 3.0 9.69e y 08 3.2 9.61e y 08 3.2 1.25e y 07 3.9 1.32e y 07 3.9 9.69e y 08 4.2 9.78e y 08 4.2 9.52e y 08 7.0 9.78e y 08 7.0 9.69e y 08 8.9 1.00e y 07 8.9 9.27e y 08 a

Experiments at 33 700 my1 Days

Predicted rate a

1.49e y 07 1.27e y 07 7.49e y 08 6.39e y 08 1.19e y 07 5.93e y 08 6.52e y 08 8.06e y 08 6.47e y 08 7.61e y 08 1.06e y 07 1.03e y 07 4.25e y 08

8 8 8 51 51 51 58 58 58 88 88 88 186 186 186 281 281 281

6.14e y 08 6.78e y 08 9.23e y 08 1.75e y 07 1.81e y 07 2.02e y 07 2.58e y 07 2.52e y 07 2.73e y 07 3.03e y 07 y4.65e y 11 3.01e y 07 5.14e y 07 4.88e y 07 4.37e y 07 6.29e y 07 6.47e y 07 6.76e y 07

1.27e y 07 1.21e y 07 1.16e y 07 1.11e y 07 1.24e y 07 1.15e y 07 1.11e y 07 1.11e y 07 1.13e y 07 1.14e y 07 1.27e y 07 1.29e y 07 1.06e y 07 9.27e y 08

8 8 8 51 51 51 58 58 58 88 88 88 186 186 186 281 281 281

1.41e y 07 1.45e y 07 1.52e y 07 1.97e y 07 2.01e y 07 2.02e y 07 2.14e y 07 2.14e y 07 2.17e y 07 2.34e y 07 4.29e y 11 2.34e y 07 2.87e y 07 2.80e y 07 2.65e y 07 3.23e y 07 3.26e y 07 3.35e y 07

Days

Predicted rate

3 3 7 7 14 14 28 28 56 56 91 91 112 112

3 3 7 7 14 14 28 28 56 56 91 91 112 112

a

Rates are expressed in mol glass my1 sy1 .

calculated from thermodynamic data using the Van’t Hoff relation: log K i ŽT . s log K i ŽTr . y

DHrŽTr .8 2.303 R

ž

1

1 y

T

Tr

/

,

Ž 20 .

where T is the reaction temperature Ž363.15 K., Tr the reference temperature Ž298.15 K., DHr the reaction enthalpy ŽkJ moly1 ., and R the ideal gas constant. The K e value for basaltic glass is 10 3.42 ŽTable 6..

I. Techer et al.r Chemical Geology 176 (2001) 235–263

251

Fig. 8. Comparison between experimental alteration rates for basaltic glass in closed system Ž10, 50 and 33 700 my1 . and rates predicted from Eq. Ž19. Žmodel of Daux et al.. based on element concentrations in solution Žerror 10% on the rate..

The proposed approach also requires consideration of a glass dissolution reaction that may be written as follows in basic media:

q 0.126Ca2qq 0.0026Kqq 0.128Mg 2q

Ž Si 0.537 Al 0.190 Na 0.056 Ca 0.126 K 0.0026 Mg 0.128 Li 0.044 =Fe 0.089 Sr0.0023 Mn 0.0014 P0.0009Ti 0.0145 O 1.83 .

q 0.0014Mn2qq 0.0009PO43y

q1.1479H 2 O q 0.6811Hqq 0.0028ey ° 0.537H 4 SiO48 q 0.190Al Ž OH .

y q 4 q 0.056Na

q 0.044Liqq 0.089Fe 3qq 0.0023Sr 2q

y

q 0.0145Ti Ž OH . 5

The ion activity product Q associated with this dissolution reaction was calculated for each sampling interval of each alteration experiment. The activities

252

I. Techer et al.r Chemical Geology 176 (2001) 235–263

Table 6 Oxide dissolution equations and logŽ K i 908C . at 908C; Basaltic glass solubility product K e Oxide dissolution equation

Log K i 908C

Refs.

Oxide x i mol%

SiO 2 glass q 2H 2 0 s H 4 SiO48 q Al 2 O 3 q q 5H 2 O s 2AlŽOH.y 4 q 2H q q Na 2 O q 2H s 2Na H 2 O Li 2 O q 2Hqs 2Liqq H 2 O CaO q 2Hqs Ca2qq H 2 O MgO q 2Hqs Mg 2qq H 2 O Fe 2 O 3 q 6Hqs 2Fe 2qq 3H 2 O P2 O5 q 3H 2 O s 2PO43yq 6Hq SrO q 2Hqs Sr 2qq H 2 O MnO 2 q 4Hqq 2eys Mn2qq 2H 2 O K 2 O q 2Hqs 2Kqq H 2 O q TiO 2 q 3H 2 O s TiŽOH.y 5 qH

y2.22 y24.29 56.3 38.33 26.63 16.85 y8.29 y8.48 31.78 33.24 70.76 y10.93

Phillips et al. Ž1988., Babushkin et al. Ž1985. Phillips et al. Ž1988. Tachikawa Ž1985., Cox et al. Ž1989. Cox et al. Ž1989. Cox et al. Ž1989. Cox et al. Ž1989. Phillips et al. Ž1988. Paul Ž1982., Grenthe et al. Ž1992. Barin et al. Ž1977., Grenthe et al. Ž1992. Phillips et al. Ž1988. Tachikawa Ž1985., Cox et al. Ž1989. Phillips et al. Ž1988., James and Johnson Ž1985.

0.5339 0.0943 0.0280 0.0218 0.1258 0.1273 0.0490 0.0005 0.0023 0.0014 0.0013 0.0144

Log K e s 3.42 Žcalculated from Eq. Ž11...

taken into account for the species in solution were determined from the measured elemental concentrations in solution using the KINDIS code ŽFritz, 1975; Made´ et al., 1994., or by equating the element with . the species ŽTiŽOH.y 5 . A maximum concentration equal to the analysis detection limit was assigned to the elements undetected in solution. Applying Eq. Ž9. yielded a DGr value for each experimental time interval. The experimentally measured alteration rate is plotted vs. DGr in Fig. 9. The higher the free energy of the reaction, the closer the system is to saturation conditions, and the lower the alteration rate. However, Eq. Ž10. implies that the alteration rate far from saturation state is independent of the reaction affinity; on the contrary, the data compiled

here show that even near the initial rate conditions Ž r 0 ., the alteration rate diminishes as DGr increases. Similarly, at a more advanced stage of reaction progress, the experimental results diverge significantly from the theoretical curve corresponding to kinetic control by the chemical reaction affinity described by Eq. Ž10.. Using available data, Linard et al. Ž1998. have calculated the Gibbs-free energies of formation of a set of silicate glasses for which a calorimetric determination was possible. With these results, the predictions of the model of Paul Ž1977. for calculating Gibbs-free energies of dissolution were assessed. The differences between the two types of data were between a factor 0.7 to 4. Such important variations would nevertheless not conduct to a

Fig. 9. Basaltic glass alteration rate Ždetermined experimentally in closed system tests. vs. free energy of reaction DGr Žcalculated from Eq. Ž9. for each increment of reaction progress based on a basaltic glass solubility product K e of 10 3.42 ..

I. Techer et al.r Chemical Geology 176 (2001) 235–263

better fitting of our experimental data with the theoretical curve reported in Fig. 9. The diminishing alteration kinetics of basaltic glass are therefore not simply related to the chemical reaction affinity calculated from a deviation from saturation state. Nevertheless, the DGr calculations are based on an empirical determination of the basaltic glass solubility product, K e , which assumes that the reaction at saturation state is a reaction between the glass and the solution. The proposed approach requires further improvement. A thermodynamic study is now in progress to determine DGr experimentally, as previously done for borosilicate nuclear glasses ŽLinard et al., 1998.. Taking into account the different kinetic models systematically overestimates the dissolution rate. The discrepancy between the theoretical rates obtained by applying these models and the experimental ones is enough great to avoid an artefact due to inconsistent experimental conditions in the laboratory which have permitted to established the models and ours. Other inhibiting factors must be considered in order to account for the low measured alteration rates. Silica is one of the major components of basaltic glass and, as a dissolved species in solution, can be expected to play a role in inhibiting the dissolution reaction—as previously observed for the SON68 reference nuclear glass by Advocat et al. Ž1998.. Open system experiments with silicon-enriched solutions were therefore carried out to examine this hypothesis with basaltic glass. Such kind of experiments have already been carried by Daux et al. Ž1997.. The synthetic basaltic glass studied was doped with 4% Li 2 O, and the silicon-enriched solution reached to the maximum 73 mg ly1 of Si. The same approach is proposed for the 1% Li 2 O synthetic basaltic glass, with data which will supplement the Si-concentration field studied by Daux et al. Ž1997..

6. Open system experiments: results and discussion 6.1. Results In all the open system experiments, steady-state concentrations and solution pH were observed after t

253

ŽEq. Ž13... During basaltic glass alteration with ultrapure water, the pH increased as the flow rate decreased and the concentrations of the species in solution logically increased as well ŽTable 7.. The highest mean silicon concentration reached was 9.4 ppm during the test Vh4. The mean Na concentrations ranged from 0.4 to 1.2 ppm. The measured Li concentrations in solution were low, and were comparable regardless of the experimental flow rate; considering the limits detection of the analytical method for this element Ž- 0.01 mg ly1 ., no significant evolution could be identified. The Na concentrations in solution are higher, so we chose to use Na as an alteration tracer Žthe closed system experiments demonstrated the tracer behavior of this element.. The altered glass quantity calculated from the Na release data increased in a linear manner throughout the experiment, and the alteration rate was therefore determined by linear regression. The basaltic glass dissolution rate decreased as the flow rate decreased, and thus with the increasing silicon concentration in solution ŽFig. 10.. During the open system tests with silicon-enriched solutions, the solution pH decreased as the silicon concentration increased. The Na, Al, Mg and Ca concentrations in solution also decreased as the Si concentration increased. Li, Sr, Fe, Ti, Mn and P were not detect in solution ŽTable 8.. The alteration rates determined from the linear sodium mass losses decreased as the silicon concentration increased in solution ŽFig. 10.. The minimum value Ž9 = 10y1 0 " 10y1 0 mol my2 sy1 . was observed during the experiment with a very high imposed silicon concentration of 130 ppm. 6.2. Discussion The alteration rates of basaltic glass at 908C decreases with the Si or orthosilicic acid concentration in solution. The greatest drop—by a factor of 200 compared with the initial rate measured in closed system tests at 908C Ž r 0 s 1.4 = 10y7 mol my2 sy1 . —was observed during the tests at a high imposed silicon concentration ŽVSi6, VSi7.. The alteration rate was never observed to drop to zero, confirming that the glass alteration kinetics are not controlled by a simple function of the orthosilicic acid activity in solution as assumed by the first-order law ŽEq. Ž1...

I. Techer et al.r Chemical Geology 176 (2001) 235–263

254

Table 7 Conditions and results of basaltic glass alteration experiments in open system in initially ultrapure flowing water Ž908C. Experiment

Days

Concentrations Žppm. a Si

Vh1 F s 0.63 ml miny1 , t s 0.28 day

1 1.2 4 1.1 8 1.0 14 1.2 Mean 1.1 I.C. 0.10 b rŽNa. s 5.8 = 10y8 " 4 = 10y9 mol my2 sy1 Vh2 F s 0.12 ml miny1 , 2 5.7 t s 1.5 days 5 6.3 9 6.3 19 6.0 Mean 6.1 I.C. 0.34 b rŽNa. s 2.3 = 10y8 " 2 = 10y9 mol my2 sy1 Vh3 F s 0.05 ml miny1 , 2 7.8 t s 3.8 days 5 8.4 8 8.2 19 7.9 Mean 8.1 I.C. 0.37 b rŽNa. s 1.1 = 10y8 " 2 = 10y9 mol my2 sy1 Vh4 F s 0.02 ml miny1 , 20 9.7 t s 9.9 days 26 10.1 30 8.9 40 8.8 Mean 9.4 I.C. 0.80 b rŽNa. s 5.3 = 10y10 " 4 = 10y10 mol my2 sy1

pH Ž908C.

Normalized mass loss Žg my2 .

Al

Na

Li

Ca

Mg

NL ŽNa.

NL ŽLi.

0.43 0.40 0.39 0.33 0.38 0.05

0.36 0.34 0.30 0.40 0.39 0.06

- 0.10 - 0.10 - 0.11 - 0.13

0.38 0.49 0.61 0.48 0.49 0.13

- 0.21 - 0.20 - 0.21 - 0.16

7.68 7.73 7.66 7.41 7.62 0.20

0.35 1.33 2.54 4.52

= = = =

2.4 2.6 2.5 2.2 2.4 0.21

0.70 0.72 0.75 1.30 0.87 0.37

0.16 0.17 0.17 0.19 0.17 0.01

1.8 2.1 2.1 2.0 2.00 0.20

1.02 0.97 0.87 0.74 0.90 0.15

8.28 8.29 8.36 7.88 8.20 0.26

0.27 0.6 1.06 2.75

0.25 0.58 1.03 2.22

3.3 3.3 3.2 2.9 3.2 0.27

0.88 0.92 0.86 1.20 1.00 0.20

0.15 0.17 0.19 0.13 0.16 0.03

2.5 2.7 2.8 2.8 2.70 0.18

1.4 1.1 1 0.9 1.10 0.03

8.3 8.5 8.59 7.74 8.30 0.53

0.15 0.32 0.48 1.18

0.11 0.23 0.37 0.82

3.5 3.6 3.2 3.1 3.4 0.30

1.10 1.20 1.20 1.20 1.20 0.04

0.25 0.12 0.14 0.14 0.16 0.07

3.5 3.6 3.2 3.1 3.40 0.30

0.78 0.86 0.79 0.77 0.80 0.05

8.22 7.77 7.82 8.53 8.09 0.43

0.65 0.82 0.94 1.22

0.56 0.64 0.7 0.85

a

Fe, Sr, Mn, Ti and P concentrations were always below the detection limit. Alteration rates are calculated by linear regression from NL ŽNa. Ž rŽNa. : mol glass my2 sy1 ..

b

A power relation can account for the drop in the rate with the wH 4 SiO48x activity in solution: rmol m y2 sy 1 s 10y11 w H 4 SiO48 x

y0 .87

Ž r 2 s 0.90 . Ž 21 .

However, this equation is not applicable when wH 4 SiO48x is equal to 0. This case referred to initial dissolution stage where the equation r 0 Ž r 0 function of pH and temperature. is available. The pH is known to have an catalytic effect on the initial alteration kinetics of basaltic glass, as indicated in Eq. Ž18. at 908C. During the open

system experiments, the pH was never the same, but varied with the solution flow rate and composition: the greater the silicon enrichment, the lower the pH. This behavior could be due to metal hydrolysis reactions, which are known to occur, or to silicon condensation on the glass surface, in quantities increasing with the initial imposed concentration. The difference between the silicon quantity in solution at the end of the experiment and the silicon quantity in initial solution Žin percent., decreases with the imposed concentration. Hydrolysis phenomena affecting the M`OH bonds of the glass network would then be less significant because of the supposed siliceous film that developed at the glassrsolution

I. Techer et al.r Chemical Geology 176 (2001) 235–263

255

Fig. 10. Basaltic glass alteration rate in open system tests at 908C vs. total Si concentration or orthosilicic acid ŽH 4 SiO48. activity determined using KINDIS code ŽEquil subroutine.. Comparison with SON68 alteration rates obtained with the same experimental conditions ŽAdvocat et al., 1998..

interface. This hypothesis implies that glass alteration should be more limited in a solution with a high silicon concentration, as was effectively observed. However, the assumption of the silicon condensation must be verified by surface investigations. Disregarding this pH effect in order to characterize an absolute rate decrease, the following empirical relation can then be derived between the ratio of the measured rate to the initial rate and the dissolved silica concentration ŽFig. 10.. The alteration rate r at 908C in basic media can be written as follows: rrr 0 s 8 = 10y3 w H 4 SiO48 x

y0 .62

Ž 22 .

or: r s 3.3 = 10y11 w Hq x y3

= 10

y0 .44

w H 4 SiO48 x

comes clear that at an advanced stage of reaction progress the kinetic expression in Eq. Ž23. alone cannot account for the measured results. The role of the solution chemistry in controlling the alteration kinetics of basaltic glass is thus insufficient in a kinetic model to account for the considerable drop in the alteration rate over time Žsome four orders of magnitude with respect to r 0 .. However, one fundamental aspect has not been considered. As with nuclear glass, during alteration, secondary products form on the surface of the basaltic glass. The alteration film so formed could constitute a diffusion barrier that would contribute to the control of the reaction kinetics.

=8

y0 .62

Ž 23 . y2

7. Basaltic glasses alteration products

y1

where r and r 0 are expressed in mol m s , and wHqx and wH 4 SiO48x in mol ly1 . However, this equation cannot account for a drop in the rate by more than factor of 200. As in the case of SON68 nuclear glass, the inhibiting effect of dissolved silica in solution is low, and does not account for the drop of four orders of magnitude in the rate observed experimentally at high SrV ratios in closed system experiments Ž20 000–30 000 my1 for durations exceeding 1 year: Advocat et al., 1998.. In attempting to determine the rate values from closed system experimental alteration data for basaltic glass, it quickly be-

7.1. Nature of potential alteration products in closed system experiments During closed system experiments, secondary phases may have precipitated on the basaltic glass surface, as deduced from solutions analysis. Using EquilŽT . subroutine of KINDIS computer program, we have evaluated the nature of these potential phases. KINDIS program is based on the equilibrium constant approach and permits determination the saturation state of a solution with respect to different

I. Techer et al.r Chemical Geology 176 (2001) 235–263

256

Table 8 Conditions and results of basaltic glass alteration experiments in open system in silica-enriched solutions Ž908C. Experiment

Days

Concentrations Žppm. Si

VSi 1 Im-posed wSix 15 ppm, f s 0.016 ml min -1 , t s 12 days

Al

pH Na

20 24.6 2.7 1.2 26 23.2 2.4 1.1 29 23.0 2.2 1.1 32 24.1 2.5 1.4 Mean 23.7 2.4 1.2 I.C. 0.91 0.20 0.18 a rŽNa. s 5.1 = 10y9 " 9 = 10y10 , rŽLi. s 3.7 = 10y9 " 9 = 10y10 VSi 2 Imposed wSix 28 ppm, 20 35.0 1.1 0.81 f s 0.010 ml miny1 , 26 36.1 1.1 0.83 t s 19 days 29 34.5 1.0 0.90 40 32.4 1.1 0.66 Mean 34.5 1.1 0.8 I.C. 1.84 0.04 0.12 a rŽNa. s 2.1 = 10y9 " 4 = 10y10 VSi 3 Imposed wSix 38 ppm, 20 46.1 0.95 0.54 f s 0.011 ml miny1 , 26 46.7 0.89 0.54 t s 18 days 29 45.3 0.83 0.50 40 46.6 1.00 0.48 Mean 46.2 0.92 0.52 I.C. 0.7 0.09 0.04 a rŽNa. s 1.2 = 10y9 " 2 = 10y10 VSi 4 Imposed wSix 70 ppm, 20 71.3 0.36 0.34 f s 0.03 ml miny1 , 26 75.9 0.38 0.41 t s 6 days 29 72.1 0.39 0.33 40 73.0 0.37 0.45 Mean 73.1 0.37 0.38 I.C. 2.4 0.02 0.07 a rŽNa. s 3.4 = 10y9 " 7 = 10y10 VSi 5 Imposed wSix 90 ppm, 21 102 0.12 0.56 f s 0.012 ml miny1 , 25 104 0.18 0.48 t s 16 days 30 104 0.22 0.48 39 106 0.33 0.51 Mean 104 0.21 0.51 I.C. 2.20 0.11 0.04 a rŽNa. s 1.6 = 10y9 " 3 = 10y10 VSi 6 Imposed wSix 109 ppm, 25 131 - 0.10 0.33 f s 0.007 ml miny1 , 30 132 - 0.10 0.49 t s 28 days 35 132 - 0.10 0.30 45 139 - 0.11 0.28 Mean 133 0.37 I.C. 4.4 0.13 a rŽNa. s 7.1 = 10y10 " 3 = 10y10 VSi 7 Imposed wSix 129 ppm, 25 151 0.11 0.38 f s 0.009 ml miny1 , 30 149 0.12 0.39 t s 21 days 39 152 0.10 0.42 Mean 151 0.11 0.40 I.C. 2.3 0.02 0.03 a rŽNa. s 8.9 = 10y1 0 " 1 = 10y1 0 a

Li

Ca 0.20 0.19 0.19 0.26 0.21 0.041

Mg

Sr

Normalized mass loss Žg my2 . NL ŽNa.

NL ŽLi.

3.0 2.8 2.7 3.0 2.9 0.20

0.80 0.66 0.61 0.57 0.66 0.12

0.10 0.09 0.09 0.09 0.09 0.001

7.87 8.10 8.40 8.42 8.2 0.32

0.61 0.76 0.84 0.94

0.43 0.54 0.59 0.67

- 0.11 - 0.10 - 0.10 - 0.11

1.8 1.9 1.7 1.6 1.8 0.19

0.80 0.82 0.72 0.59 0.73 0.13

0.05 0.06 0.05 0.04 0.05 0.010

7.70 7.70 7.82 7.70 7.7 0.07

0.26 0.33 0.37 0.47

- 0.14 - 0.18 - 0.20 - 0.27

- 0.10 - 0.11 - 0.10 - 0.11

1.5 1.7 1.5 1.4 1.50 0.15

0.58 0.68 0.59 0.56 0.60 0.06

- 0.04 - 0.04 - 0.04 - 0.04

7.30 7.50 7.60 7.60 7.5 0.17

0.18 0.23 0.25 0.32

- 0.15 - 0.19 - 0.20 - 0.28

- 0.11 - 0.11 - 0.10 - 0.11

0.8 0.7 0.8 0.9 0.83 0.10

- 0.21 - 0.21 - 0.21 - 0.22

- 0.04 - 0.04 - 0.04 - 0.04

n.d. 7.20 7.13 6.90 7.1 0.19

0.11 0.14 0.24 0.26

- 0.14 - 0.18 - 0.30 - 0.32

- 0.10 0.03 0.01 - 0.02

0.90 0.75 0.82 0.93 0.85 0.10

0.24 0.23 0.26 0.24 0.24 0.016

- 0.04 0.01 0.01 - 0.04

0.22 0.25 0.29 0.37

- 0.18 0.18 0.18 - 0.20

- 0.10 - 0.10 - 0.10 - 0.11

0.62 0.77 0.56 0.70 0.66 0.11

0.14 0.12 0.12 0.15 0.13 0.014

- 0.04 - 0.04 - 0.04 - 0.04

0.09 0.12 0.12 0.15

- 0.12 - 0.14 - 0.16 - 0.20

- 0.01 - 0.01 - 0.02

0.73 0.71 0.88 0.77 0.14

0.13 0.16 0.17 0.15 0.02

- 0.01 - 0.01 - 0.01

0.14 0.17 0.22

- 0.01 - 0.01 - 0.02

Alternation rates calculated by linear regression from NL ŽNa. or NL ŽLi. Žmol glass my2 sy1 ..

6.66 7.17 7.23 7.0 0.38

7.00 6.82 6.62 6.8 0.23 6.54 6.85 6.87 6.75 0.26

I. Techer et al.r Chemical Geology 176 (2001) 235–263

mineral phases. The mathematical formulation of the program and example of its application on the determination of the saturation state of solutions with respect to mineral phases have been published, respectively by Fritz Ž1975. and Made´ et al. Ž1994. and by Crovisier Ž1989.. For each experimental duration studied in 10, 50, and 33 700 my1 tests, EquilŽT . has calculated the distribution and activity of the various chemical species in aqueous solution and have tested the saturation state with respect to different mineral phases. The minerals used for the calculations were chosen considering natural phases currently observed in the nature as a result of basaltic glasses dissolution ŽHay and Iijima, 1968; Noack, 1981; Grambow et al., 1985; Byers et al., 1985; Crovisier et al., 1989b; Jercinovic et al., 1990b.. The clay minerals were represented for a part by a T.O.T. ideal solid solution model Ž15 end members.. In the code, equilibrium with respect to the solid solution is reached when the sum of the molar fraction of each member is equal to 1: Qi

Ý Xi s Ý K

s1

Ž 24 .

i

where X i is the molar fraction of end member i, Qi and K i are, respectively the ionic product and the equilibrium constant of the dissolution reaction of the end member i. Equilibrium constants of the tested mineral phases and of the 15 end members of the solid solution used for the calculations were reported for the temperature studied Ž908C. in Table 9. Fig. 11 presents the results of calculations in terms of degree of saturation of the solutions, defined as the ionic productrsolubility product ratio. At 908C, in the experimental conditions studied, the solutions were saturated with respect to goethite, nontronite and Fe-saponite, and in the highest reaction progresses Žover 1 day my1 . to calcite and illite. In terms of the T.O.T. clay, the ferrimuscovite and phlogopite end members were supersaturated. However, the composition of the result clay could not be obtained as the program is given this only for saturation state. These results are in agreement with literature data: the mineral phases which might have precipitated are currently observed in the nature as

257

secondary products of basaltic glasses alteration at low temperature, by seawater or meteoric waters ŽHay and Iijima, 1968; Furnes, 1978; Noack, 1981; Byers et al., 1985; Grambow, 1985; Crovisier, 1989; Jercinovic et al., 1990b.. Numerous studies of experimental basaltic glass alteration and modelizations at low temperature Ž- 908C. in presence of seawater andror meteoric waters have shown that such minerals phases are likely to form ŽGrambow, 1985; Crovisier et al., 1985; Gislason and Eugster, 1987.. Considering the results of these geochemical calculations, we notice the presence of alkali elements Žnotably Na. in the basaltic glass alteration products which are supposed to precipitate, and thus even in the first stage of the reaction. Nevertheless, ionic beam ŽSIMS. analyses performed on two sections of the glass altered in 10 and 50 my1 experiments Žsee discussion above. have not allowed to detect Na in the secondary products formed at the glass surface. It is so essential to keep in mind that such calculations are only qualitative and give a potential indicator of what could be the geochemistry of secondary products. It is not a clear demostration of the existence of such phases during the leach tests. However, in order to confirm the hypothesis that Na is a good tracer of the basaltic glass alteration— no incorporation of Na in secondary products—the presence of this alkali element as well as the quantity mobilized in secondary products must be verified and determined. At this stage of the study, as the secondary mineral phases mass is unknown a quantitative estimation of the Na incorporated in these secondary products is impossible. Only microscopic identifications ŽTEM combined with an EDS analytical system . could emphasize the real compositions of the thin alteration zones Ž- 1 mm in thickness.. 7.2. ProtectiÕe role of the alteration film The potential protective role of the alteration film was investigated by examining natural basaltic glass specimens. Basaltic glass alteration in natural environments has been widely investigated in the past ŽPeacock, 1926; Hay and Iijima, 1968; Noack, 1981; Jakobsson and Moore, 1986; Byers et al., 1987; Jercinovic and Ewing, 1987; Crovisier et al., 1989a,b; Jercinovic et al., 1990b; Thorseth et al., 1991.. The

I. Techer et al.r Chemical Geology 176 (2001) 235–263

258

Table 9 Mineral phases used for calculations with KINDIS and respective equilibrium constant at 908C Name

Dissolution equation

log K Ž908C.

Oxides–hydroxides goethite FeŽO.OH q 3Hqs Fe 3q 2H 2 O gibbsite AlŽOH. 3 q 3Hqs Al 3qq 3H 2 O amorphous silica SiO 2 q 2H 2 O s H 4 SiO4 Carbonates calcite hydrotalcite Phyllisilicates montmorillonite nontronite

9.19 y12.49 y2.25

CaCO 3 s Ca2qq CO 32y Mg 6 Al 2 CO 3 ŽOH.16 q 6Hqs 6Mg 2qq 2Al 3qq CO 32yq 6H 2 O q 10OHy

y9.22 75.14

kaolinite illite

i 3.83 Al 1.84 Mg 0.38 O10 ŽOH. 2 K 0.4 q 3.32H 2 O q 6.68Hqs 3.83H 4 SiO4 q 1.84 Alq3 q 0.38Mg 2qq 0.4 Si 3.84 Al 0.94 Fe 0.9 Mg 0.33 O10 ŽOH. 2 Ca 0.105 Na 0.05 K 0.2 q 3.36H 2 O q 6.64Hqs 3.84H 4 SiO4 q 0.94Al 3q q0.9Fe 3qq 0.33Mg 2qq 0.105Ca2qq 0.05Naqq 0.2Kq Si 3.67 Al 0.8 Fe 0.46 Mg 1.52 O10 ŽOH. 2 Ca 0.11 Na 0.11 K 0.17 q 2.68H 2 O q 7.32Hqs 3.67H 4 SiO4 q 0.8Al 3q q0.46Fe 3qq 1.52Mg 2qq 0.11Ca2qq 0.11Naqq 0.17Kq Si 2 Al 2 O5 ŽOH.4 q 6Hqs 2H 4 SiO4 q 2Al 3qq H 2 O Si 3.5 Al 2.3 Mg 0.25 O10 ŽOH. 2 K 0.6 q 8Hqq 2H 2 O s 3.5H 4 SiO4 q 2.3Al 3qq 0.25Mg 2qq 0.6Kq

y4.17 y32.70 y36.13

Clay T.O.T. pyrophyllite talc Fe 3-pyrophyllite K-muscovite Ca-muscovite Mg-muscovite Na-muscovite K-ferrimuscovite Ca-ferrimuscovite Mg-ferrimuscovite Na-ferrimuscovite K-phlogopite Ca-phlogopite Mg-phlogopite Na-phlogopite

Si 4 Al 2 O10 ŽOH. 2 q 6Hqq 4H 2 O s 4H 4 SiO4 q 2Al 3q Si 4 Mg 3 O10 ŽOH. 2 q 6Hqq 4H 2 O s 4H 4 SiO4 q 3Mg 2q Si 4 Fe 2 O10 ŽOH. 2 q 6Hqq 4H 2 O s 4H 4 SiO4 q 2Fe 3q Si 3 Al 3 O10 ŽOH. 2 K q 10Hqs 3H 4 SiO4 q 3Al 3qq Kq Si 3 Al 3 O10 ŽOH. 2 Ca 0.5 q 10Hqs 3H 4 SiO4 q 3Al 3qq 0.5Ca2q Si 3 Al 3 O10 ŽOH. 2 Mg 0.5 q 10Hqs 3H 4 SiO4 q 3Al 3qq 0.5Mg 2q Si 3 Al 3 O10 ŽOH. 2 Na q 10Hqs 3H 4 SiO4 q 3Al 3qq Naq Si 3 AlFe 2 O10 ŽOH. 2 K q 10Hqs 3H 4 SiO4 q Al 3qq 2Fe 3qq Kq Si 3 AlFe 2 O10 ŽOH. 2 Ca 0.5 q 10Hqs 3H 4 SiO4 q Al 3qq 2Fe 3qq 0.5Ca2q Si 3 AlFe 2 O10 ŽOH. 2 Mg 0.5 q 10Hqs 3H 4 SiO4 q Al 3qq 2Fe 3qq 0.5Mg 2q Si 3 AlFe 2 O10 ŽOH. 2 Na q 10Hqs 3H 4 SiO4 q Al 3qq 2Fe 3qq Naq Si 3 AlMg 3 O10 ŽOH. 2 K q 10Hqs 3H 4 SiO4 q Al 3qq 3Mg 2qq Kq Si 3 AlMg 3 O10 ŽOH. 2 Ca 0.5 q 10Hqs 3H 4 SiO4 q Al 3qq 3Mg 2qq 0.5Ca2q Si 3 AlMg 3 O10 ŽOH. 2 Mg 0.5 q 10Hqs 3H 4 SiO4 q Al 3qq 3Mg 2qq 0.5Mg 2q Si 3 AlMg 3 O10 ŽOH. 2 Na q 10Hqs 3H 4 SiO4 q Al 3qq 3Mg 2qq Naq

y37.72 19.73 9.87 y43.68 y41.58 y41.97 y41.97 1.85 3.95 3.56 3.61 14.28 16.38 15.99 16.04

saponite Fe

present study is not concerned with the alteration conditions or with characterization of the alteration products, but rather with assessing the alteration kinetics of various glasses already studied in the literature. The only parameter relevant to the alteration kinetics of natural glass specimens is the thickness of the alteration film. A simple relation between the palagonite thickness and the age of the glass is used to determined the bulk alteration rate. The glass is assumed to have been subjected to alteration processes since its formation; the alteration time is thus assumed equal to the age of the volcanic formation.

y29.70 y20.52

The alteration rate is then determine and expressed in micrometers of palogonite per unit time Žmm per 1000 years.. This approach is based on many hypotheses and uncertainties, including the glassrsolution contact time Žclosed or open system., the actual palagonite thickness Žis it representative of the alteration thickness?., the composition of the alteration fluid etc., but is the only one available for estimating alteration rates in the natural environment. The glass sample studied in the literature and taken into account in this approach came from a great variety of environments. Volcanic eruptions

I. Techer et al.r Chemical Geology 176 (2001) 235–263

259

Fig. 11. Results of KINDIS calculations: saturation state of experimental solutions obtained in 10, 50 and 33 700 my1 tests, with respect to many mineral phases.

beneath Iceland glaciers have occured since the Miocene Žy23.5 to y5.3 Ma1 ., resulting in vitreous or hyaloclastic formations Žy0.008 and y3.1 Ma.. These glass formations and their alteration by meteoric water have been extensively studied ŽByers et al., 1987; Crovisier et al., 1989a; Jercinovic et al., 1990b.. The same type of volcanic activity created vitreous formations in British Colombia that have been altered by fresh water for 10–15 000 years ŽGrambow et al., 1985; Byers et al., 1987; Jercinovic et al., 1990b.. Byers et al. Ž1987. also refer to the vitreous formations along the Columbia River, which have been subjected to alteration by surface water or percolation for 12–14 Ma. Glass formed on the seabed Ždredged or cored basaltic glass. has also frequently been studied ŽHekinian and Hoofer, 1975;

1

Ma: mega annum Žmillions years..

Grambow et al., 1985; Jakobsson and Moore, 1986; Jercinovic and Ewing, 1987.. The bulk alteration rates determined from the Apalagonite thicknessrglass ageB relation for the above-mentioned continental and oceanic formations range from 10y4 to 70 mm per 1000 years ŽFig. 12.. The data collected for a given volcanic formation are subject to wide dispersion because of the acknowledged Žand sometimes significant. uncertainty on the age of the glass: e.g. 0.01–0.002 Ma for the British Colombian glasses, and 0.008–0.8 Ma for the Icelandic glasses studied by Byers et al. Ž1987. and Jercinovic and Ewing Ž1987.. Variations in the palagonite thickness measured on the surface of a single volcanic series also account for scattered rate values. Nevertheless, the general tendency is to accept a bulk rate of less than 10 mm per 10 3 years Žthe rates exceeding this value were all measured on glass samples less than 0.1 Ma old.. Moreover, the

I. Techer et al.r Chemical Geology 176 (2001) 235–263

260

Fig. 12. Palagonite thickness vs. glass age Žconsidered to represent the alteration time. for basaltic glasses altered in a continental Ža. or oceanic Žb. environment. The alteration rates determined from this relation are represented by broken lines and expressed in micrometer per 1000 years. The initial alteration rates r 0 Žin micrometer per 1000 years. are calculated from Eq. Ž17. for 08C and 158C.

alteration rate tends to diminish with the age of the sample. In order to compare these bulk rates with the initial rate r 0 characterizing these systems, a representative temperature must be assumed; the initial rate is then determined from Eq. Ž17., given the activation energy of the basaltic glass dissolution reaction, i.e. 72.4 kJ moly1 ŽFig. 7. ln r 0 s ln A y

Ea

s 8.3 y

72.4 = 10 3

Ž 25 . RT RT Temperature of the continental Žsubglacial. and seabed environments are estimated to be below 158C.

Calculating r 0 from Eq. Ž25. for temperatures of 08C Žminimum representative value. and 158C yields values of 40 and 206 mm per 1000 years, respectively, assuming a basaltic glass density of 2.75 g cmy3 ŽJercinovic et al., 1990b.. The bulk rates determined by measuring the palagonite thickness of natural glass samples are systematically one to five orders of magnitude lower than these calculated initial rates. A study of natural basaltic glass samples shows that the long-term alteration of these materials does not occur at the initial rate, but at a rate several orders of magnitude lower, confirming the results of

I. Techer et al.r Chemical Geology 176 (2001) 235–263

laboratory studies at a high degree of reaction progress, i.e. at high Ž SrV . t ratios. Several parameters can account for the low bulk alteration rates. Ž1. The first is directly related to the uncertainties on the alteration time. In the proposed approach, the alteration time is assumed equal to the age of the glass, which itself is often uncertain. Overestimating the alteration time systematically leads to underestimating the alteration rate. Ž2. The palagonite thickness is poorly representative of the actual altered glass thickness.Assuming, for example, that a basaltic glass sample is altered at a rate of 100 mm per 1000 years Žnear the initial rate., the palagonite layer observed after 1 Ma should be 0.1 m thick. Few vitreous formations were initially thick enough to exhibit an altered glass thickness of this magnitude today: basaltic glass is found as millimetric or centimetric fragments Žhyaloclastic particles. or centimetric thick chilled margin of basaltic dykes. This implies that more recent formations Žless than 1000 years old. should be examined intead: moreover, the older the sample, the greater the uncertainty on the alteration time and conditions. Ž3. The third parameter capable of explaining the low measured alteration rates is related to the alteration conditions: sedimentary or detrital deposits are liable to seal off the volcanic formation, limiting fluid access. Ž4. Finally, the alteration film Žpalagoniteq secondary minerals. that forms on the basaltic glass surface could constitute a barrier against diffusion of reactive species or reaction products. The role of this barrier is probably nil during the initial stages of the reaction, hence the high measured rates for the recent glass sample. As the reaction progresses, however, the barrier could likely become the controlling factor leading to the low measured alteration rates on the older glass samples. Laboratory experiments are now in progress to demonstrate the protective effect of the alteration film suggested by the natural basaltic glass samples ŽTecher, 1999..

8. Conclusion The objective of this study was to examine the phenomena controlling the aqueous alteration of basaltic glass. When the dissolved element concentrations in solution increase and alteration products

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form a surface film, the glass alteration rate decreases by four orders of magnitude from the initial maximum rate. This drop in the reaction kinetics is not attributable to the solution chemical composition alone Ži.e. to an inhibiting effect or to a glassrsolution saturation state condition.. A study of natural basaltic glass sample altered over thousands or millions of years suggests that the palagonite film that forms on the glass surface constitutes a diffusion barrier for reactive species ŽH 2 O, H 3 Oq. and reac. tion products ŽH 4 SiO48, AlŽOH.y 4 , etc. . Allowance for the protective role of the alteration film on basaltic glass could account for the low measured alteration rates that are quickly reached in closed system alteration experiments. The magnitude of this protective effect could be demonstrated by measuring the H 2 O, Naq and H 4 SiO48 diffusion coefficients in a palagonite film, and by specific experiments.

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