Geochemical evolution of basaltic rocks subjected to weathering: Fate of the major elements, rare...

Geochimica et Cosmcchimica Acta, Vol. 58, No. 22, pp. 4941-4954, 1994

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Geochemical evolution of basaltic rocks subjected to weathering: Fate of the major elements, rare earth elements, and thorium

V. DAUX, I* J. L. CROVISIER, ’ C. HEMOND,~‘+ and J. C. PETITE

‘Centre de Geochimie de la Surface, 1 rue Blessig 67084 Strashourg Cedex, France 2Abteilung Geochemie, Postfach 3060, D-6500 Mainz, Germany

3Commissariat 2 I’Energie Atomique, Centre de Fontenay, DCC/SCS, Section de geochimie BP 6, 92265 Fontenay-aux-Roses Cedex, France

(Received November 20, 1993; accepted in revised,form June 2, 1994)

Abstract-Eleven Icelandic hyaloclastites altered in freshwater have been studied. The weathering of ba&ic gJa.r,s, which is their primary constituent, leads to pr~ipi~tion of clayey and possibly ZeOiitiC

phases. The dissolution reaction progress (mass of dissolved glass per liter of solution) governs the chemistry of the secondary phases, which control the residence time of the solution through their influence on rock pet-me&ility. They contribute to the regulation of reaction advancement. The reaction progress, 4, can be calculated according to:

[ = [Srw*(Zw - Isp)*(Io + 9.375)]/[Sro+(Zsp - Zo)*(lw + 9.375)1,

where Zw, Io, and I& are the isotopic s~ontium ratios of initial water, of pristine glass, and secondary products, respectively, and Srw and Sro the strontium contents of initiai water and glass. The amount of dissolved glass per liter of solution is estimated to be in the order of 0.0 1-O. 1 g for samples 90000- 100000 years old. A global agreement is found between this result and estimations that can be made on the basis of thermodynamic modelling.

Expressing the geochemical budget as a function of the reaction progress allows a look at the evolving aspect of the rock/ water interaction: ( 1) According to the budget calculated for the major elements, the rock undergoes a global loss of matter which decreases with increasing reaction progress (from -45 to about 0% Josses). For the most evolved samples, the transformation of the pristine basaltic glass to the alteration products is a nearly conservative process. (2) There is increasing evidence that rare earth elements, particularly LREEs, can be mobilized during alteration processes. Our study corroborates this view as we demonstrate that REEs can be significantly mobilized during basaltic glass weathering. Nev- ertheless, no fractionation among the suite of REEs was observed.

We show that Th and REE behaviours are similar. Mass balance calculations account for Th and REE losses up to 40%. These losses are not related to the reaction progress but could be due to a colloidal or particulate transport.

The REE contents of the secondary clayey phases is showed to be linked to their degree of crystallinity. The need to take into account the latter parameter to modelize the long-term behaviour of the elements adsorbed onto clay minerals is outlined.

THE DISSOLUTION MECHANISM of basaltic glass in distilled water at low temperature ( < 150°C) is well documented (see for instance DECARREAU, 1977; THOMASSIN and TOURAY, 1979; LUTZE et al., 1985; CROVISIER et al., 1986, 1990, 1992; THORSETH et al., 199la and references mentioned in the following section). The info~ation comes from both laboratory experiments and extensive field investigations. In particular, experiments (conducted at temperatures lower than 150°C; CROV~SIER et al., 1989a,b, 1990; GUY and SCHOTT, 1989) showed that the initial process consists in a selective extraction of the alkaline and alkaline-earths from the glass with a pH increase of the solution up to 7 leading to a first stage of incongruent dissolution. Dissolution then

* Present address: Commissariat B 1’Energie atomique, DAM LDG/GEG, BP 12,91580 BruyZres-le-ChStel.

+ Present address: Laboratoire de G&ochimie, DRO-GM, I~EMER-BREST, BP 70,292SO Plouzane, France.

becomes cong~ent, and a subsequent incon~ency arises from precipitation of secondary phases.

The nonstoichiometric character of the initial reaction should lead to the formation of a residual surface layer depleted in the network modifying constituents such as Na, Mg, and Ca. GUY and Scnon ( 1989) reported that the cation exchange is quite ins~ntaneous. For instance, compared to the silicium release, the preferential release of Na does not last more than 150 hours (at 50°C pH 5.6). CROVISIER et al. ( 1990) outlined that the preferential release of alkaline and alkaline earth in a closed-system cannot be observed after 5 day experiments (60°C pH increasing from 5.5-7).

To our knowledge, a depleted surface layer of which calculated thickness could not exceed a few pms ( THORSETH et al., 199 1 a) has never been observed at low temperature. The first incongruent stage can thus be neglected. At low temperature a basalt glass dissolves nearly stoichiomettically (pH > 7,25”C, GfsLAsoN and EUGSTER, 1987a). Microbial activity through the alteration of Eh and pH conditions may influence locally the glass corrosion mechanism ( THORSETH

4941

V. Daux et al

et al., 1991 b. 1994). Although this process could be an im- paleostratigraphy inferred from successive glacial events) portant one, this aspect is not addressed in the present paper. (Table 1 ).

Field investigations indicate that the products of the glass corrosion vary widely according to the geochemical environment (NAYUDU, 1964; HAY and IJIMA, 1968a; HAY and JONES, 1972; HONNOREZ, 1972, 1978; SINGER, 1974; FURNES, 1978, 1980; KRISTMANNSDOTTIR, 1982: STAUDIGEL and HART, 1983; WIELEMAKER and WAKATSUKI, 1984; KONTA, 1986; VAN DER GAAS~ et al., 1986; G~SLASON and EUGSTER, 1987a,b; CROVISIER et al., 1987. 1989a; CROVISIER and DAUX, 1990; THORSETH et al.. 199 la). The meteoric alteration of basaltic glass leads to the precipitation of a series of amorphous and/or crystalline phases: Fe- and Al-hydroxide, clay minerals, and eventually zeolites. Indeed, CROVISIER et al. ( 1992) observed such a mineralogical sequence in Ice- landic hyaloclastites and could simulate it using the kinetic and thermodynamic computer code DISSOL (FRITZ, 1975; TARDY and FRITZ, 1981; MADE et al., 1990).

In addition, two alteration products (palagonite and clay minerals) with similar compositions in their major elements and different crystallinities coexist in the samples. This as-

sociation could allow us to examine the influence of the crystallinity parameter on the retention capacity of secondary phases regarding trace elements.

SAMPLES

Settling and Alteration Conditions

The fate of trace elements during the alteration of basaltic glass is investigated in few studies (NESBITT and WILSON, 1992 ) . Some aspects of this question are evidently relevant to the long-term disposal of nuclear waste glasses. The main point about the effectiveness of such a disposal in a geological environment relates to the possibility that water may reach the repository and leach the waste. Natural glasses having reacted with water during periods of time comparable to the half-life period of the elements of concern (a thousand to a million years) have been proposed as analogues for nuclear glasses to evaluate the long-term release of radio toxic elements into the geosphere. However, transuranic elements, which do not have any nonradioactive isotopes are, if any, very scarce in natural materials. To decipher their long-term geochemical behaviour one must thus relate on chemical analogues such as rare earth elements (REEs) and thorium (BROOKINS, 1984; KRAUSKOPF, 1986): REEs and the acti- nides Am and Cm have many common features (same ox- idation state, close ionic radii, similar hydrolysis constants); Th is a fairly satisfactory analogue of Pu and Np in the tet- ravalent state (comparable stability of the dioxides, similarity of the formation constants for their complex ions).

The eleven samples studied are: ( 1) 86/61 from KaldB, Htisafell, SW Iceland, 2.4 m.y. old, (2) 86/75 from StrGtur, Hisafell, SW Iceland, 0.5 m.y. old, (3 and 4) 86/ 10 and 86/ 15 from the Nesjask6gar formation, in Hengill, SW Iceland, 100000 y old, (5-10) 86/21-86/26 from the Hdhryggur for-

mation, in Hengill, 90000 y old, ( 1 I ) 86/83 from the Mjba- nesoddi, Thingvellir, SW Iceland, 2000 y old.

The geological history of the sampling areas can be found in SAEMUNDSSON (1967), SAEMUNDSSON and NOLL ( 1974), and ARNASSON et al., (1986) and the exact location of the samples in CROVISIER et al. ( 1992 ). These rocks are volcanic rocks erupted under subglacial conditions (except sample 86 / 83 which is of phreatomagmatic origin) 2000 years to 2,4 My ago. They are generally covered by massive basaltic lavas. Just after settling, the hyaloclastites were porous aggregates of basaltic glass fragments. These rocks were affected by subglacial weathering at first. Secondly, after the glacier with- drawal and until they were sampled, they have been weathered by meteoric waters. The temperature of both glacier melt waters and Icelandic meteoric waters is close to 0°C. The initial solutions (rains and snow) are at equilibrium with the atmosphere. Their typical pH is 5-6 (G~SLASON and Euc- STER, 1987b). After initial saturation with the atmospheric CO*, dissolution and precipitation reactions take place. De- pending on the access to the atmosphere and water rock con- tact area, the pH of the solution increases to values ranging from 7-10 (GfsLAsoN and EUGSTER, 1987b; G~SLASON, 1989).

The aim of the present work is to describe the mineralogical (nature and crystallinity of the secondary phases) and geochemical (major elements, REEs, Th, and 87Sr/86Sr) aspects of the evolution of a basaltic rock submitted to weathering. Beside its fundamental relevance, this study is expected to guide our thoughts to the likely long-term fate of the transuranic counterparts of REEs and thorium. We have chosen to study basaltic hyaloclastites from Iceland because they constitute a relatively simple and well characterised water/ rock system: ( 1) These glassy rocks were submitted to alteration by low concentrated solutions (subglacial and meteoric waters). CROVISIER et al. ( 1992) has shown in particular the absence of significant seawater influence. (2) The rocks were altered at low temperature (CROVISIER et al., 1992) which makes the system simpler than in the case of hydrothermalism (convection transfer). (3) The ages of the samples are known from the studies of SAEMUNDSSON ( 1967)) SAEM~JNDSSON and NOLL (1974), and ARNASSON et al. ( 1986) (K/Ar dating of the associated basal& 14C for the most recent sample and

The Sr/Cl ratio in Icelandic precipitation is the same as the ratio in seawater as reported by RILEY and CHESTER ( 197 1) and S. R. Gislason ( pers. commun.) . The average, present, chlorine concentrations of precipitation in the Hi- safe11 is 4-5 ppm and in Hengill it is 7-10 ppm according to SIGURDSSON ( 1990). Thus, one can calculate the average present Sr concentration of precipitation. Today, it is 2 ppb for Hhsafell and it is 4 ppb for the Hengill area. In reference to compositions of various rains and snows, the 07Sr/86Sr can reasonably set to 0.709 (ABERG et al., 1989; JACKS et al., 1989; PROBST et al., 1992 ), which is the seawater isotopic signature.

As they percolate in the rock, the solutions dissolve the basaltic glass and, as a result, chemical species are liberated in solution. When they reach their saturation limit secondary phases can precipitate. The altered hyaloclastites consist in fragments of basaltic glass, surrounded by an alteration rim, and cemented by submicroscopic clayey intergranular products (Fig. 1).

Geochemistry of basalt weathering 4943

Table I : Ages of the samples (My) and major elements contents of the glasses, palagonites and

products (in wt%). < : below detection limit; n : not meawed

Sample 1 Age 1 SiO, 1 AhO, 1 TiO, 1 Fe74 1 MgO 1 Na?O I CaO 1 KTO 1 EGO I Tot

EEEI

1 PALAGONITES 1

intergranular

INTRRGRANULAR PRODUCTS

148.30 1 II.10 1 2.81 1 14.30 1 4.15 1 0.14 1 5.10 1 0.37 1 II.73 1 98.00 1

The alteration rim formed on basaltic glass at low temperature is called palagonite ( VON WALTERSHAUSEN, 1845).

It is a compact product, though flimsy, adherent to the glass. As this product is relatively coherent and macroscopically distinct from the intergranular clayey products, it was possible to separate it from the other constituents. The palagonitization process has been a matter of debate for a long time. Several mechanisms relevant to various glass compositions, temperature, and pH conditions, have been proposed (PEA- COK, 1930; MORGENSTEIN and RILEY, 1974; MARSHALL,

196 1; HONNOREZ, 1972; MOORE, 1965; MELSON and

THOMPSON, 1973; G~SLASON and EUGSTER, 1987a,b; THOR- SETH et al., 199 1 a). The model of congruent dissolution followed by precipitation (HAY and IJIMA, 1968b CROVISIER et al., 1987, 1992) suited for meteoric alteration of basaltic glass at low temperature, accounts for the palagonitization of the studied hyaloclastites.

ANALYTICAL METHODS

The submicroscopic clayey intergranular products were assumed to correspond to a <2 pm fraction. They were siphoned from a sus- pension in distilled water and then centrifuged.

In order to separate the residual glass fragments and the surrounding palagonite, the following, soft method was applied: ( 1) the samples were desegregated in distilled water by repeated freezing-thawing cycles and sieved into > 150 prni SO- 150 pm, 150 pm fractions; (2) separation of the components making up the 50-150 pm fraction was obtained using bromoform/ethyl alcohol mixtures whose density was made to vary to 2.85, 2.60, 2.40, 2.20, 2.10, and 2.05. The 2.60- 2.85 fractions were mainly composed of brown glass, the 2.20-2.60 of glass-palagonite, the 2.05-2.20 of white zeolites and orange palagonite; (3) purification of the obtained fractions was performed by handpicking under binocular microscope.

The respective cross contaminations between the glass and the palagonite fractions do not exceed 5%, and the <2 pm fractions appear purer (from optical, SEM, TEM, and XRD investigations). The re- ciprocal contamination of palagonite and glass does not alter greatly their respective chemical compositions. To exploit the accuracy of the method used to determine the isotopic strontium compositions, the glass samples were subjected to further cleaning by acid attack.

The mineralogical compositions of the glasses, palagonites, and intergranular products were determined by X-ray diffraction and electron micro-diffraction (Transmission Electron Microscope). The zeolites have been identified by XRD and SEM. The major elements and rare earth elements in the glasses, palagonites, and I.P. were analysed by inductively coupled plasma atomic emission spectrometry (RIP-AES), except for the major elements of glasses 86/ 10, / 15, /21, /22, /75, and /83 which were analysed by electron microprobe under the following conditions: I5 kV, 12 nA, IO s counting time

4944 V. Daux et al.

Residual MinepI

FIG. I. Drawing from a thin section of an altered hyaloclastite. Sample 86/23.

for each element. The precision of the ICP-AES analyses is estimated to be better than 1% for all the major elements and better than 4- 5% for all the suite of lanthanides (GOVINDARAJU and MEVELLE, 1987). The water content, reported in Table I, was established by measuring the loss on ignition at 1000°C.

The major element compositions of the bulk rocks were performed by ICP-AES. The magmatic minerals were analysed by electron microprobe. The chemical compositions of the bulk rocks and the magmatic minerals are used for the computation ofthe modal composition of the hyaloclastites. The data relevant to sample 86/ 15 are reported in Table 2 as an example. Detailed presentation of these data is found in DAUX (1992).

The Th concentrations of the glasses, palagonites, and intergranular products of samples 86/15, 86/2l, 86/23, 86/25, and 86/26 were determined by using isotope dilution in mass spectrometry on a MAT 261 mass spectrometer. The strontium isotopic compositions of the glasses and I.P. of these samples were measured with the same in- strument using multicollection mode. The glass phases were leached before the Sr chemistry for an hour in hot HCI 6 N as described by SHIMIZU and HART ( 1973). s’Sr/%r values were normalised to *%r/ 8sSr = 0.1194. The NBS987 standard gave 0.710219 f 0,000012 (2~) from 8 measurements performed during the period of time of the analysis of the samples.

MINERALOGY OF THE ROCK FORMING COMPONENTS

Thin sections of the hyaloclastite samples were studied with a transmitted light polarizing microscope. They are composed of vesicular grains of yellow glass with size ranging from 10 pm to a few millimeters. The glass is optically iso- tropic and contains primary minerals such as olivine, plagioclase, and clinopyroxene which do not seem to be altered. The glass grains are surrounded by a palagonitic rim (Pal.) and cemented by clayey intergranular products (I.P.). The

palagonite color changes from sample to sample, from a bright golden yellow to a very pale yellow. In samples 86/24 and 86/83, palagonite and intergranular products are respectively missing.

In the oldest samples (samples 86/ 10, 86/75, and 86/ 6 1 ), zeolites also fill the intergranular medium.

XRD, TEM, and SEM

The palagonites are X-ray amorphous, except 86/ 10 and 86/6 I, whose patterns are typical of ttioctahedral smectites. Investigations by TEM revealed that palagonites are made up of small particles morphologically identical to smectites. Some of these particles are indeed crystallised ( CROVISIER et al., 1992).

A number of studies have been devoted to the mineralogical characterization of palagonite. They have shown that it consists of an amorphous to poorly crystalline product with properties of clay minerals (MATTHEWS, 1962; NAYUDU,

1964; HAY and IJIMA, 1968a,b; HONNOREZ, 1972; FURNES and EL-ANBAAWY, 1980; CROVISIER et al., 1987; THORSTEH et al., 1991a,b). Our observations are in agreement with these studies.

Some of the intergranular products exhibit a X-ray diffraction pattern characteristic of trioctahedral smectites; this is corroborated by the TEM observation that shows, in the I.P. fractions, the presence of numerous particles with a smectitic morphology and electronic diffraction spectra compatible with this type of clay mineral (see CROVISIER et al., 1992).

The intensity of the X-ray diffraction peaks (here we infer intensity from the height of the peaks) is proportional to the degree of crystallinity of a product. Insofar as no spike was added to the powders, we cannot pretend to quantify the crystallinity. Nevertheless, the diffraction patterns of an alteration crust and corresponding intergranular products can be compared as the diffraction patterns were determined in identical conditions. We observe that Pal. and I.P. of samples 86/15, 121, 122, 123, 125, and 86175 are amorphous or poorly crystalline (Fig. 2A); the Pal. of sample 86/26 is poorly crystalline, whilst the 1.P. are very crystalline; the Pal. and the I.P. of samples 86/ 10 and 86/61 are crystalline. In the former case the crystallinity of the I.P. is fairly superior to the crystallinity of the Pal. (Fig. 2B); in the latter case, the opposite is observed.

Crystallinity is not simply related to the age of the samples. While Pal. and I.P. are crystalline in the 0.1 and 2.2 My old samples ( 86 / 10 and 86 / 6 1) , they are amorphous in sample 86175, 0.5 My old.

As attested by their morphology (SEM), their diffraction characteristics ( XRD), and their compositions in major elements, the zeolites in samples 86/ 10 (0.1 My old) and 86/ 75 (0.5 My old) are chabazites and those in sample 86/61

Table 2 : mean mayx element compositions (weight 5%) of the plagioclase and the olivine of sample 86415 and bulk rock composition of the same sample. < : below detection limit, n : not measured


A j SAMPLE &3/21 lel SAMPLE 86/10

15 20 I 15 20 15 20 I 13 15 20

FIG. 2. X-Ray diffraction patterns obtained on oriented specimens of the palagonite and the intergranular products. The horizontal axis scale is expressed in A. (a) sample 86 /2 I: palagonite and I.P. are X- Ray amorphous; (b) sample 86/ 10: the patterns are compatible with a smectitic structure (reflections at 13 or 14 A); the I.P. are more crystalline than the palagonite.

(2.2 My old) are erionites. Zeolites are observed in the oldest samples.

CHEMICAL COMPOSITION OF THE ROCK-FORMING COMPONENTS

Major Elements

The glasses belong to the tholeiitic basaltic series as shown by their contents in major elements (Table 1). The water content of the glasses of samples 86/23 and 86/26 is somewhat high: one can calculate that, with less than 5% contam- inating palagonite (containing more than 10% water), the water content of the glass increases from a standard value of 0.3-0.8%. A contamination by 7% of palagonite is necessary to account for the I .2% water of sample 86/6 I; the proportion of palagonite estimated via microscopic observations and X- ray diffraction (see Analytical Methods) is smaller. One can note that the effect of contamination is restricted to water. Indeed, sodium, which shows a strong depletion in palagonite and is thus a good indicator of the freshness of the glass, does not seem to be affected.

g SiO2 o A1203 l MgO x TX02 A Fe203 O CaO

1

0.

0 10 20 30 40 50 60

Coscentmlioo in Pal. (weight 96) La Nd Sm Eu Gd w J3 Yb

0.20 : I : ! : : : : ! : ! A

FIG. 3. Major element concentrations in I.P. vs. major element concentrations in palagonites (WV%). The straight line equation is y

FIG. 4. (a) REE concentrations in glasses normalized to primitive mantle following Hoffman ( 1988). Glass normalised REEs patterns

= x. of intergranular products (b) and palagonite (c )

The major element compositions of the intergranular products and of the corresponding palagonites are similar (Table 1). The points representing I.P. concentrations vs. Pal concentrations fall, indeed, near the straight line representing an identical composition (Fig. 3). In most cases, the water content of a palagonite is inferior to the water content of the corresponding intergranular products.

Trace Elements

The primitive mantle-normal&d REEs spectrums of glasses, exhibit a light REEs enrichment (normalising constants from HOFMANN, 1988; Fig. 4a). Basalts from the Reykjanes peninsula frequently exhibit such REE patterns (SCHILLING, 1973; O’NIONS et al., 1976; ZINDLER et al., 1979).

The shape of the glass-normahsed REE patterns of palagonite and intergranular products is almost flat or jagged (Fig. 4b, c) . There is, thus, no evidence of a systematic fractionation among the suite of REEs during alteration. The patterns are set between 0.8 and I for samples 86/25,26, IO, 75, and 86/ 61 and I and 1.4 for samples 86/83,21,22,23, and 86/15.

60.00

61 (4

1.80 j I

1.60 t

t

__-_--_--------26 -- _----

0.40

0.20’ 1

4046 v. Dau\ et al.

~- - .-)TRza 1

lnereaaing TRi! ____*

Increaalng macuon progress

Range of the studied Samples

FIG. 5. Schematic representation of the successive events expected while the dissolution of basaltic glass by meteoric solution proceeds (closed-system). Thermodynamic modelling performed by CROWSIER et al. ( 1992) with DISSOL.sol:solution. step 0: initial solution + glass, step a: the glass IS dissolving, the solution is concentrating, step b: Iron hydroxide and kaolinite reach saturation. step c: Iron hydroxide and kaolinite become unstable and dissolve as TOT clay minerals form. TRZ = (SiO~/MgO)~,,/(SiOZ/MgO),,,,, is low. step c to n: the composition of the TOT clay is reequilibrating with the solution as the reaction progress is increasing. TRZ is increasing. step n t 1: zeolites reach saturation. TRZ = I. The TRZ values of the studied samples range from 0.16 to I.

In other words, in the former case the alteration products are REE depleted relative to the primary glass; in the latter they

are REE enriched. Except in sample 86 / 15 and 86/ 6 1, the total REE content

(BREEs = sum of the concentrations of La, Nd. Sm, ELI,

Gd, Dy, Er, Y b. and Y l of the palagonites is greater than the total content of the I.P. (Table 3 ). Palagonites X6/ IO and

X6/26 arc two times as rich as the more crystalline corresponding I.P. Palagonite 01’ X6/61 is REE-poorer than the less crystalline I.P.

The thorium content of the glasses ranges Itorn 0.86X- I.328 (Table 4 ), which is in the range 0.026-I .X4 ppm mea-

sured by HFMONI) et al. ( 1988) for Icelandic tholeiites. Com-

parison of the Th content of alteration products and glasses

leads to results similar to those obtained for REEs. That is the relative enrichment of secondary phases in samples X6/

IS, 86/21, and 86/23. and the relative depletion in samples 86125 and 86126.

Isotopic Ratios

The H7Sr/X’rSr ratios of glasses range from 0.703 l24-

0.703204. They are similar to the values reported for various

Icelandic tholeiites (WOOD et al., 1979; ZINDLER et al., 1979;

HEM~ND et al., 1988). The ratios of the I.P. are higher than

the ones of the glasses (Table 4). This point is discussed in the following section.

GEOCHEMICAL BUDGET

The aim of this section is to quantify the successive geo-

chemical changes undergone by the rock (initially basaltic glass) while submitted to meteoric alteration. This implies ( I ) to assess a parameter of alteration advancement to refer to (reaction progress), and (2) to perform mass balance calculations for the samples sorted according to this parameter.

Table 3 : REE and Y concentrations of the glasses. palagonttes and intergranular products (m ppm) ZREE = (La) + (Nd) + (Sm) + (Eu) + (Gd) + (Dy) + (Er) + (Yb) f(Y).

La ) Nd 1 Sm 1 ELI 1 Gd 1 Dy 1 Er I Yb 1 Y 1 =E 1


1 -’

0.8 -

1 0.6 -

2 . 0.4 -

.I

0-v: : : ! : : : : : : 1 0.00 0.20 0.40 0.60 0.80 1.00

TRz I.P.

FIG. 6. TRZm vs. TRZr.r.. TRZPal(r.r. rap,) = ( SiOz/MgO)ti/( SiOJ MgO)PtiuP. rap,r. The regression line is y = x ( u2 = 0.89), what shows that the geochemical maturity of palagonites and intergranular products are equivalent. The vertical dotted line figures sample 86/24 containing palagonite but no I.P. and the horizontal one sample 86/ 83 containing I.P. but no palagonite. The age is indicated by the figures next to the symbols.

Estimation of the Reaction Progress

The reaction progress of dissolution (5)) is defined here as the mass of dissolved primary phase per kg of solution. It depends on permeability of the rock and time (see discussion ) .

TRZ ratios as indicators of .$

The dissolution of basaltic glass in aqueous solution can be modelized as a function of [ on the basis of thermodynamic data. CROVISIER et al. ( 1992), using the thermodynamic computer code DISSOL (FRITZ, 1975; TARDY and FRITZ, 198 1; MADE et al., 1990), did simulate the evolution of the closed-system basaltic glass + solution and predicted the following sequence of events: ( 1) the first mineral phases reach- ing saturation are an iron hydroxide and kaolinite (step b, Fig. 5); (2) with further reaction progress, these minerals become unstable and dissolve as TOT clay minerals form (step c, Fig. 5); (3) the composition of the TOT clay, con- tinuously reequilibrating with the solution, evolves as the dissolution proceeds. In particular, its (SiO,/MgO) ratio increases with increasing [ (step c to n + 1, Fig. 5); and (4) zeolites (mainly chabazites) precipitate at higher reaction progress (step n + 1, Fig. 5 ) .

Referring to the dependence of the ( SiOz /MgO) ratio in the clays on the reaction progress, CROVISIER et al. ( 1992) demonstrated that the parameter

TRZ = AgrasslAalteration prtiuctr, (1)

where A = ( SiOz/MgO), can be used as an indicator of the reaction progress. When TRZ = 1, the solution is theoretically at equilibrium with a magnesium rich clay and with zeolites.

As shown in the previous part, the samples contain two macroscopically distinct alteration phases (Pal. and I.P.). The major element compositions of these phases being close, one is expected to be as mature as the other (from a geochemical point of view). As shown in Fig. 6, the TRZ ratios calculated for palagomtes ( TRZpar = Aglass IA palagonite) are indeed related to those calculated for intergranular products (TRZ1.r. = Agld

Aintergmnular prcx~uctn ). The regression line is y = x with a cor- relation coefficient (a’) of 0.89 (9 datas). Such a relation indicates that Pal. and I.P. correspond to the same reaction progress: their geochemical maturities are equivalent and there is not any strong chemical gradient in the solution. The TRZ ratios calculated for our samples range from 0.2-l. Samples 86/10, 86/61, and 86/75 with ratios close to 1, actually contain zeolites (namely chabazites or erionites).

Later on, the results of the mass balance calculation are expressed as functions of TRZr.r. (TRZr,,, for sample 86/83 which contains only palagonite). Expressing them as functions of TRZpal would, of course, lead to the same conclusions. TRZ is used to put the samples in a comparative order of geochemical maturities. The highest values of TRZ correspond to the oldest samples (Fig. 6). However, in 86/21- 86/26 samples, 90000 years old, TRZ ranges from 0.16- 0.55. TRZ is, thus, not a simple function oftime. This matter was discussed in detail in CROVISIER et al. ( 1992).

The absolute value of the reaction progress, l, cannot be inferred directly from TRZ. In the following part, a method is proposed to evaluate this parameter.

Evaluating the reaction progress (E) using 87Sr/86Sr ratios

The secondary products are mixtures of clays and amorphous particles precipitated from the solution after basaltic glass dissolution. As CLAUER et al. ( 1975) demonstrated, the strontium isotopic composition of clay minerals is continu- ously equilibrating with the solution. From this, we can infer the isotopic homogeneity of palagonite, intergranular products, and solution at the time of sampling. The *‘Sr/*%r ratio of the solution is intermediate between the one of the glass and the one of the initial water. If the *‘Sr/*%r ratio and the Sr content (Sr) of the initial water is constant through time, the “Sr/*%r ratio of the secondary products is a func-

Table 4: ThmdSrconcenuations (ppm) andWr/%rmtios. Pal. :palagonite.I.P.intcrgranularproducts

Table 5 Reaction progress (in g/l) calculated after equation (2)

SZUtlple [ 8605 1 86D.1 I 86123 1 86125 I 86126 5 = M&l, (84) 1 0.04 1 0.02 1 0.06 1 0.03 1 0.07

tion of the strontium isotopic ratio and strontium concen-

tration of glass and initial water and of {, the mass of glass

dissolved per Kg of solution (the derivation is given in Ap-

pendix 1). It follows that 6 can be evaluated by

t = Mo/Mw = [Srw.(Zw - I,,)-(I, + 9.375)]/

[Sro.(I,r - Io).(& + 9.375)], (2)

where Mo and Mw are the masses of glass and water involved in the reaction, Sro and Srw the strontium content of glass

and initial water, Io, Z,, and Zip are the *‘Sr/*?Sr ratios of the glass, initial water, and intergranular products, respectively.

Evaluation of .$ is made for five samples (86/ 15, 86/21, 86/23,86/25, and 86/26) from the following data: measured

*‘Sr/*%r ratios of glasses and intergranular products (Table 4); measured Sr for glasses (Table 4); estimated Sr and *‘Sr/ *%r for the initial water. We assume that Sr and *‘Sr/*%r of initial water, that is of Icelandic precipitations, were constant

through time over the last 100,000 years (age of sample 86 / 15. the oldest of the five samples, Table 1) and equal to 3 ppb and 0.709, respectively (see Settling and Alteration Con- ditions).

The 6 = M,/M, calculated after Eqn. 2 are reported in Table 5. They range between 0.02 and 0.07 g/L. Taking into account the uncertainties on the Sr concentration and isotopic composition of the initial solution and the inaccuracies in-

herent to chemical analyses, these values must be seen as orders of magnitude.

G~SLASON and EUGSTER ( 1987a,b) calculated the reaction progress for the extremely permeable lava flows of NE Iceland to be less than 0.00 1 g/L, based on the sodium concentration

of rain, spring waters, and rocks. The permeability of these basalt flows ( l-10m3 m3/m2/s) are few orders of magnitude greater than the one of glassy hyaloclastites ( 10-3-10 -’ m3/ m’/s) ( SIGURDSSON and INGIMASSON, 1990). The waters in

glassy hyaloclastites have had more time to reaction with rocks than the waters in the young basaltic flows in NE Ice- land. Thus, it makes sense that the reaction progress calculated in this study is 0.02-0.07 g/L, but the one calculated

for the lava flow to be less than 0.001 g/L.

The Relative Mobility of the Elements

The elements leached during the glass dissolution are either transported away by the solution or trapped in the structure of secondary phases. The geochemical budget evaluates the fraction of each “i” element remaining in the alteration products. In the present study, the calculation is complicated by the fact that the alteration products are made of two phases

Table 6: Density mrswmcnti.

394x V. Daux et al.

( Pal. and I.P.). The first stage of the mass balance calculation is to establish the mass fractions of these secondary phases.

Gytimution of’thc ma.s.s,/~uction ofthe secondur), mineral.\

The fraction of the various phases that constitute the hy-

aloclastites are estimated by point counting under optical

microscope From these surface proportions, volumetric, then

weight proportions are calculated (A method) using the following density data: density measurements for glasses and

palagonites (Table 6 ) ; density data from BERRY et al. ( 1983 ) for magmatic minerals and zeolites; and density of 2.0 for

intergranular products.

As the limits of the secondary phases are diffuse, these proportions are approximate and indicate orders of magnitude. In order to refine this first estimation, the fractions of

the various phases are inferred from the equations stating conservation of mass of oxides ( B method, see Appendix 2 ). The final results of the computation, listed in Table 7 cor-

respond to the best fit between A and B. The calculation of the mass transfer should take into ac-

count the various alteration phases. Nevertheless, we have not taken zeolites into consideration because they represent

a small part of the alteration products (see Table 7). Moreover, we calculate the geochemical budget of the al-

teration by using a global alteration phase (SP: secondary

products) made up of Pal. and I.P. in proportion to their relative importance (Table 7, row 7). The chemical composition of that global alteration phase is presented in

Table 8.

Culculution of‘ the muss trun&s

To calculate a mass transfer one needs to refer to an in- variant parameter. Some authors assume the palagonitisation process to be isovolumetric ( MUELENBACH and CLAYTON,

1972; NOACK, 198 1). Whether it is founded or not, the budget

calculation cannot rely on such a statement as far as the dissolved glass is transformed here not only to palagonite but also to intergranular products. Following HONNOREZ ( 1972)

and FURNES (1978), Fe is used as a normalising constant. This element is certainly not absolutely immobile. Neverthe- less, from comparing the ratios of the concentration in the

global alteration phase to the concentration in glass (Ci,/C&,,) of the various “i” elements, it can be assessed that Fe, with the highest ratio, is the least mobile. Normalising to Fe, may thus lead to underestimate slightly the losses undergone by the rock.

Stating mass conservation for Fe is equivalent to writing

MG 1 Fee = Msp. Fesp, (3)

where Msp is the mass of secondary products formed for a mass, MG, of glass dissolved and Fee and Fesp the Fe concentration in glass and in global alteration phase, respectively.

Equation 3 leads to

&I&P = Fe&h = Q, (4)

Geochemistry of basalt weathering

Table 7 : Pqmtions of the various phases of the hyaloclastites. Comcted results (to the nearest whole numkr) of the calcttlation. Min.: primary minerals; I.P.: intergranular Products; Pal: plagonite.

86/26 60 20 5 0 I5 0.25

86J6 I 404510 2 3 0.22

86l7S 45 40 6 4 5 0.15

86033 45 50 0 0 5 I

where Q is the mass of alteration products formed by unit mass of dissolved glass. For any mobile “i” element

4i’tMSP’ iSP) = MC’ ic, (5)

where qi is the mass of “i” trapped in the alteration phase by unit mass of dissolved “i” and iG and isp element “7 concentrations in glass and secondary products, respectively.

According to Eqns. 4-5

qi = (MG/MSP)‘(kP’ iG) = Q.(~sP. iG). (6)

In order to assign positive values for gains and negative values for losses (gains and losses undergone by the rock), we use in the accompanying figures: V = 100. (Q - 1 ), the relative variation of the mass of matter and ui = 100. ( qi - 1 ), the relative variation of the mass of element “7, expressed as percentages. For instance, Y = -60% means that 0.06 g of matter are transported away by the solution for 1 g of dissolved glass (0.04 g are precipitated as secondary minerals), nAl = -10% means that 0.01 g A&O3 are transported away for 1 g of Al203 dissolved (0.09 g are trapped in the alteration phase).

Results

The losses of matter, expressed as percentages, are plotted vs. TRZ, the indicator of the reaction progress, in Figs. 7-9.

-80 d “ap _*os -&-_.Q-t-.---- .-_._ __,‘_....

,I 0.3 0.5 0.7 0.9 TRZ

FIG. 7. Variation of the total amount of matter (% ) vs. TRZ, the index of the reaction advancement. The losses undergone by the rock

FIG. 8. Losses of SiOr , MgO, CaO and NarO ( % ) undergone while glass is transformed in alteration products versus TRZ, the index of

during the alteration decrease with increasing reaction progress. Errors ondataare:AV= lOOXAQ=2/1OOX V+2andATRZ=4/

the reaction advancement. Errors are: Au, = 100 X Aqi = 4/ 100 X Vi + 4 and ATRZ = 41100 X TRZ. The error bars are smaller

100 X TRZ. The error bars are smaller than the symbols. than the symbols.

4949

The mass balance calculations reveal that the transformation of the pristine glass to alteration products implies for the rock: ( 1) a global loss of matter decreasing with increasing reaction progress. When TRZ is less than 0.3 (i.e., when [ is low), the losses are about 30-40%, while they are about 0% when TRZ is close to 1 (Fig. 7); (2) consistent losses of NarO (up to 90% losses) whatever the degree of advancement (Fig. 8); ( 3) relative losses of CaO, decreasing with increasing reaction progress (-90% < uca < -50%) (Fig. 8); (4) losses of SiOz and MgO also decreasing with increasing reaction progress. When TRZ is equal to 1, SiOz , and MgO are totally preserved (Usi = ~p++~ a 0% ) . The rate of preservation of these elements is clearly related to the reaction progress (Fig. 8); ( 5 ) few losses of A1203 and TiOr (0 > u, 1 -20% for seven out of eleven samples and -20% > Ui > -30% for 3 samples); (6) variable losses of rare earth elements (from -40% to 0%) the extent of which is not related to the advancement of the reaction (Fig. 9); and (7) losses of Th that are very close to the losses of REEs (Fig. 9).

DISCUSSION

Physicochemical Evolution of the Rocks

The following proposal is advanced to describe how the rock + water system operates.

4950 V. Daux et al

-%.I 0.3 0.5 0.7 0.9 I.1 TRZ

FIG. 9. Variation of the total amount of REEs ( ZREE) and of Th retained in the hyaloclastites during the alteration VS. TRZ, the index of the reaction advancement. The error bars correspond to Au, = 8/ 100 X t', t 8 and ATRZ = 4/ 100 x TRZ.

In the hyaloclastites of high permeability, the feeding rate of the solution is high and thus the residence time of the

solution is short. The reaction progress ofdissolution increases slightly through time until a dynamic equilibrium is estab-

lished between the feeding rate of the solution and the rate

of release of the elements from the glass. A compositional steady state is achieved, to which corresponds a low reaction progress.

If the permeability of the hyaloclastite is lower, the feeding rate of the solution is lower too and the time of residence is greater than in the previous case. The reaction progress can increase to greater values. As it is increasing, the solution is logically concentrating. Secondary phases having reached their solubility limit can precipitate. The volume of the hy-

drated secondary phases being regularly higher than the volume of dissolved glass, the porosity decreases as the secondary minerals precipitate. As a consequence, the residence time of the solution increases leading to increasing 5 and increasing concentrations of the dissolved ions that are not buffered by

the secondary minerals. The nature and composition of the mineral assemblages change to fit the solution. Theoretically, if the rate of release of the elements from the glass, the rate of consumption of elements (by the precipitating secondary products), and the rate of renewing of the solution tend to be balanced, the composition of the solution can reach a steady state. Change of the permeability, as a consequence of secondary minerals precipitation, does not allow such an equilibrium to last over long periods of time.

The reaction progress governs the chemistry of the secondary phases. Through their influence on the permeability of the rock, the secondary phases control the residence time

of the solution and thus contribute to regulate .$.

In a closed-system containing basaltic glass and water, the increase of l is not limited by solution renewing. The composition of the solution changes gradually as a function of [. The compositions of the solutions in the natural renewed systems can be seen as instantaneous values of the composition of the solution evolving in a closed-system. In other words, each studied sample corresponds to a particular step of the chemical evolution of a closed basaltic glass + solution system.

The theoretical evolution, through 5, of the losses of matter recorded by the rock in the transformation of glass to secondary products, in a closed-system, is traced in Fig. 10 (de- tails of the modelling can be found in CROVISIER et al., 1992 ) . It can be compared to the setting out of the losses undergone

by the hyaloclastites vs. TRZ ( Figs. 8-9). The Same tenden- cies indeed are observed: the mass budget shows a decreasing

deficit while [ increases, the losses of Na are signilicant whatever the reaction progress can be, the losses of Fe arc very

low to nonexistent, etc. The range of variation ofi calculated

from the strontium isotopic composition of samples 86/ 15, /21,/23,/25,and86/26isreportedinFig. lO(verticaldotted

lines). In this part of the diagram, the losses of NazO are

about 90%, the losses of SiOz range approximately between

-25 and -15%. the losses of MgO range between m-50 and

-25%, the losses of CaO range between -50 and -~-45%, and

the R ratio is close to 0,7. These budgets show a slightly smaller deficit than those

calculated for samples 86/ 15 and 86/21 and arc very close

to those calculated for samples 86/23. 86/25. and 86/26.

Generally speaking the agreement between natural and modelized cases is good. The values off calculated from the isotopic composition of Sr (via Eqn. 2) are realistic.

Fate of REEs and Thorium

We showed that the losses of REEs and Th undergone by the system seem to be independent of the reaction progress. The losses of these trace elements are poorly (or not at all) related to the reaction progress because they do not participate in the mineral structure. The traces under consideration are adsorbed on clay mineral surfaces (see for instance BYRNE and KIM, 1990). The observed increase in the proportion of preserved matter does not necessarily result in an improve- ment of the retention of the trace elements: a greater amount of matter does not inevitably mean more available adsorption sites and greater adsorption capacity. Indeed, the adsorption capacity of a clay assemblage not only depends on the absolute amount of material but also on the size and the crystallinity of the clay particles which make up the phase. In other words, the trace element (REEs, Th) concentrations of the clay phases depend on the crystalline state of the adsorbing min-

Samples 86/15,/21,/23, I25 and 86126

20 f-)

I / 1

IE-04 IE-02 l,OE+OO IE-03 IE-01

Reaction progress (g/Kg) I E+OI

FIG. 10. Theoretical budget of the transformation of a basaltic glass to secondary minerals in a closed-system, according to a modelling performed with the geochemical computer code DISSOL, after CROVISIER et al. ( 1992). The transformation consists in the aqueous dissolution of the glass followed by the precipitation of secondary phases, TOT clays primarily. VW: losses undergone by the rock. Si: SiOl, Mg:MgO, AI:A1203, Fe:Fe203, Ca:CaO, Na:NaO. The values of reaction progress calculated from 87Sr/ 86Sr ratios in samples 86 / 15. 86/21, 86/23, 86/25 and 86/26 are between the dotted lines.

~~~h~rnist~ of basalt weathering

Table 8 : Major ekmcnts (WI %), mm earth, Y and Th concentmtions (ppm) of the global alteration phases (SP). LREE = (Laf + @+I) + fsmt + tE@ + fGd) +foUf + iE+ + tYb> + Cn

86/83 86/21 86/22 W-23 86i24 86i25 1 86i26 86’10 WI5 ( @if75 1 86161

4951

eral. We observed that palagonites contain, on average, a greater amount of REEs than I.P.: it is not only explained by an algebraic counterbalance for water (palagonites are less hydrated than I.P.), but it is also related to the fact that palagonites are generally less crystallised than I.P. (see Rock Forming Components, and DAUX et al., 199 1, for additional information). The influence of the crystalline state on the adsorption capacity is illustrated in samples 86110, 86/26, and 86/61; in the former, the crystalline I.P. are far richer in REEs than the amorphous palagonites, and in the latter, the palagonite, which is the only one to be more crystallised than the corresponding I.P. is also the only one to be poorer in REEs than the I.P. (Table 3).

Geochemically evolved clayey material (high 6) can be immature from a crystallographic point of view (amorphous alteration products of sample 86/75 for instance). The crystalline maturity of clays is neither a function of { nor of time: some of the oldest samples contain amorphous alteration products (86/75), some ofthe youngest are highly crystallised (86110) and crystallised I.P. (t~oc~edr~ smectites) can coexist with amorphous palagonite in the same sample (86/ 26). Whilst we can confidently predict the behaviour of the structural elements of minerals, we will not be able to depict the likely fate of adsorbed elements as long as the parameters controlling the crystalline evolution of clays are not identified.

According to our budget calculation, REEs and Th can be lost during the meteoric alteration of basaltic glasses (up to 40% losses). Some studies ( LUDDEN and THOMPSON, 1979; NESBITT, 1979; ALDERTON et al., 1980; STAUDICEL and HART, 1983; WARD, 1986; SMEDLEY, 1991; NESBITT and WILSON, 1992) with which the present one is in agreement, indicate that REEs can be significantly mobilised during weathering: the aqueous phase involved in dissolution and transport contains these elements in solution as free ions, complexes, or sorbed onto colloidal material (see for instance

HASIUN et al., 1966; VARSHAL et al., 1975). In such solutions as these that leached the hyaloclastites, that is in near-neutral to alkaline solutions, the importance of complexing with car- bonate or bicarbonate anions has been underlined (GOLD STEIN and JACOBSEN, 1988). However, the lack of compre- hensive stability constants for the complex species restricts the determination of the actual speciation of REEs and Th.

One can calculate that the dissolution of 0,Ol g/L to 0,l g/L of basaltic glass, the composition of which is typical of the studied range, supplies about low8 to 10M6 mole of each rare earth element per liter of solution (-0.6 to 70 fig/L) depending on the element and the reaction progress. A part of the leached REEs is trapped by secondary phases (60- 100%), the rest ( low3 to 30 pg/L) is transported off. Data on the REEs concentrations of natural aqueous solutions are scarce. SMEDLEY ( 1991) reported a value of 230 pg/L for the total REEs concentration of a slightly acid (0.45 pm fil- tered) groundwater underlining that the abundance of REEs in aqueous systems is generally lower, especially in alkaline solutions. The range of REEs concentrations estimated for the solutions draining the hyaloclastites is compatible with the total REEs concentration reported by SMEDLEY ( 199 1) . It has been suggested that most REEs occur as colloidal matter in river loads, in estuarine waters, and in groundwater (MARTIN et al., 1976; HOYLE et al., 1984; SMEDLEY, 1991). A colloidal transport could also be an important way for REEs to leave the hyaloclastites.

ImpIicatjons for Nuclear Waste Disposal

As underlined by KRAUSKOPF ( 1988) and PETIT ( 1990), it must be kept in mind that there is no hope to find any natural situation exactly equivalent to a nuclear waste repository, so that conclusions drawn from such studies cannot be directly extrapolated to the case of the long-term safety

4Yj2 V. Daux et al.

of nuclear waste disposal. In other words, one cannot pretend to predict the long-term behaviour of Am, Cm, Pu, and Np

in a repository from the study of Th and REEs during the alteration of a basaltic glass. Nevertheless, the inferences drawn from the study of the natural case guide our reasoning. From that point of view, some results are relevant to the questions raised by nuclear disposal. One can thus underline that clay minerals can adsorb REEs and Th. The trapped trace elements can be held back effectively, over long periods of time, insofar as there is no relation between the age of the samples and the contents in trace elements of their clayey secondary phases. In addition, a part of the leached REEs and Th can be lost during the alteration of a pristine glass, although these elements form somewhat insoluble com- pounds. This can be the result ofparticular or colloi’dal transport.

/lc~noM?led~ment.s-This work was funded by the Commissariat B I’Energie Atomique, SCD, SESD/LECALT, CEN Fontenay-aux- Roses. We thank H. Fumes, S. Gistason, and H. Staudigel for con- structive reviews.

Editorial hurdling: G. Faure

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APPENDIX 1

Estimation of 2: Using “Sr/?% Ratios

The basaltic glass alteration can be seen as a two steps process. The first step is the dissolution, the second the ureciuitation of sec- . . ondary phases. Equations stating conservation of Sr mass can then be written:

Step I-dissolution:

Sr, = Srw + Sro. (AI)

Step Z-precipitation:

Srz = Sr, Srs,,. IA?)

where Srr and Sr, are the masses of Sr in the solution after steps I and 2. respectively, Srw the mass of Sr in the initial solution. and Srsr and Sro the masses of Sr dissolved from the glass and trapped in the secondary products.

Equations A I and A2 can be written for “‘Sr and *%r isotopes:

“Sr, = “Srw i- “‘Sro. (A3)

s7Srz = “Sr, - “Srsr. (A4)

VSr, = *%w + “Src;. (A5)

s6Srz = %r, - %rsp, (A6)

Stating that secondary products have the same isotopic ratio as the solution from which they precipitate is equivalent to:

s’Sr,/s%, = 87Srsp/86Srs, = “Srz / *‘Sr2 (A7)

From (A3), (A5), and (A7):

“‘Srsp/s6Srsp = (“Srw + s’Sro)/(%rw + s%o). (A8)

Equation A8 can be written

Isp = (MC. “Sro + M, .s7SrW)/(Mo. ‘?Go + Mw .s”SrW), (A9)

where Isp is the *‘Sr/%r ratio of the secondary products, Mo and Mw are the masses of glass and water involved in the reaction of dissolution, *‘Sro, *‘Srw, 86Sro, and *%w are respectively the concentrations of s’Sr and s6Sr in the glass and initial water.

The Sr content of a material is the sum of the concentrations of the various isotopes in this material:

Sr 1 *‘Sr + %r + 88Sr iz *‘Sr ( I + 9.375/I), (AlO)

where I = *‘Sr/%r. Using Eqns. A9 and A 10 leads to

Isp = [Mo.Sro+Io/(Io + 9.375) f Mw~Srw~Iw/(lw + 9.375)]/

[&.%,/(I, + 9.375) + Mw.Srw/(Iw + 9.375)]. (All)

where lo and 1, are the “Sr/%r ratios of the glass and the initial water and Srw and Sro their Sr concentrations.

After Eqn. A I 1, the reaction progress can be expressed as a function of Sr isotopic ratios and Sr contents:

[ = MG/Mw = [Srw.(lw - Isr).(Io + 9.375)]/

[Sro.(Isr - Io)*(I, + 9.375)]. (A12)

As palagonites and intergranular products are isotopically ho- mogenous, the *‘Sr/s6Sr ratios of the I.P. reflect the isotopic composition of the secondary products as a whole. Thus Isr = Ii.r., and

l = MG/Mw = [Sr,.(Iw - Ii.r).(lo + 9.375)]/

]Sro.(Iir ~ Io).(Iw + 9.375)]. (A13)

Validation of’ method applicahilitv

In the case of sample 86/2 I (taken as an example), according to Eqn. A 13, [ is related to Ii r = Is, as follows:

((g/L) = 1.3. lo-’ X (0.709 - I,.p)/(I,.r - 0.703124).

As illustrated in Fig. A I, the very low concentration of Sr in the initial meteoric water implies that isotopic equilibrium (Ii.p. = Io = 0.703 124) is established between rock and water when the amount of dissolved glass exceeds 0.3 - 0.4 g per liter of water (the curve is quite tangent to the Y-axis). In that portion of the curve, a small error on the measurement of Irp means definitely inaccurate calcu-

” ‘2 ‘_

o__LY+y- 0.703 0.705 0.707

1 LP.

FIG. A I. ( = MG/Mw, the reaction progress, as a function of It, = 87Sr/86Sr in the clayey secondary products, after Eqn. A 12. Example of sample 86/21.

lation of MG/Mw. In the portion where In is lower than 0.705 (approximately) and higher than about 0.7035 (0.03 <A&/M, < 0.3). the absolute value of the slope decreases: in that range, a significant variation of I,, implies a significant variation of Mo/Mw and the reverse. When I,, range between 0.705 - 0.706 and 0.709 the slope of the curve is very small: in that range, great variations of In are induced by small variations of M,/Mw.

The same calculation was performed using the compositions of the other samples: the values of Io and Sro of the various samples being quite close, similar results were obtained and the curves were identical. The method described below can thus be used to estimate the reaction progress when the value of the “Sr/%r ratio of the secondary products is higher than 0.7035 (approx.). The 87Sr/s6Sr ratios of the intergranular products are actually greater than this value (Table 4).

APPENDIX 2

Estimation of the Mass Fraction of Secondary Minerals

B Method

The studied hyaloclastites are made of basaltic glass, palagonite, intergranular products, and eventually zeolites and primary minerals.

For each “i” oxyde, one can write the equation of conservation of mass:

X,. i, + . . .X,.i, t . * .X,.i, = iBR, (A14)

where i, is the concentration of “i” in “j” phase, iBR is the concentration of “i” in the bulk rock, and X, is the mass fraction of phase j in the bulk rock.

For instance, in a hyaloclastite containing glass, plagioclase as a primary mineral, palagonite, intergranular products, and zeolites. Eqn. A 14 is written for sodium:

+ XI.p.. Nair, + Xzeol.Nazeol = NaBR.

where XC, Xplwo, X,., , Xrp., and X,,, are the mass fraction of glass, plagioclase, palagonite, intergranular products, and zeolite. respectively, in the bulk hyaloclastite.

For a given hyaloclastite, Eqn. A14 is written for SiOl, A&03, FeZOx, MgO. CaO, Na20, and TiOz The mass fraction of the various phases can be inferred from this seven equation system. The accuracy of major element analysis is 1%. The computation was performed taking into account this -t I o/o error on each concentration. The computation thus leads to several possible sets of Xo, Xi+,, Xi,, , &,,I, and Xptimaw mlneralS for each hyaloclastite. Among these sets a choice is made, which corresponds to the best fit with the results obtained by A method.

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