Thermochemically induced transformations in Al-smectites: A Spanish natural analogue of the bentonite barrier behaviour in a radwaste disposal

Applied Geochemistry Applied Geochemistry 20 (2005) 2252–2282 www.elsevier.com/locate/apgeochem

Thermochemically induced transformations in Al-smectites: A Spanish natural analogue of the bentonite barrier behaviour in a radwaste disposal L. Pe´rez del Villar a,*, A. Delgado b, E. Reyes b, M. Pelayo a, J.M. Ferna´ndez-Soler c, J.S. Co´zar a, M. Tsige d, A.J. Quejido a a

b

CIEMAT/DIAE/CHE/Ed.20, Avda. Complutense 22, 28040 Madrid, Spain Dpto. de Ciencias de la Tierra y Quı´mica Ambiental, Estacio´n Experimental del Zaidı´n (CSIC), Prof. Albareda 1, 18008 Granada, Spain c Dpto. de Mineralogı´a y Petrologı´a, Universidad de Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain d Dpto. de Geodina´mica, Facultad de Ciencias Geolo´gicas, Universidad Complutense, Avda. Complutense s/n, 28040 Madrid, Spain Received 15 October 2004; accepted 25 July 2005 Editorial handling by M. Gascoyne Available online 7 November 2005

Abstract The thermal effect induced by the Morro´n de Mateo volcanic dome (Cabo de Gata volcanic region, Spain) on the adjacent bentonitised tuffaceous beds has been studied as a natural analogue of the thermal behaviour of the bentonite-engineered barrier of a geological radwaste repository. These bentonites consist mainly of Fe-rich smectites and were formed in equilibrium with seawater at temperatures between 75 and 95
C, according to the d18O and dD values. In contrast, bentonites from other localities in the region consist mainly of Al-smectites, formed in equilibrium with meteoric water below 25
C. This investigation is focussed on the detection of the chemical differences between smectites from proximal and distal zones to the dome, as well as to test whether the temperatures calculated based on the O and H isotopic values correspond to their formation or transformation. The initial hypothesis was that the chosen smectites could be formed under marine conditions, being later transformed and isotopically re-equilibrated as a result of the intrusion. To check this hypothesis, a detailed mineralogical, chemical, geochemical and isotopic study has been performed on the smectitised tuffaceous materials and the overlaying biocalcarenites outcropping near and far from the dome. The results show that distal smectites are dioctahedral Al-smectites, similar to those from other deposits in the region, while proximal smectites are Fe- and Mg-rich smectites, showing two evolutionary trends on a Fe–Mg–Al ternary diagram. Similar features are observed when their structural formulae are plotted on the muscovite–celadonite–pyrophylite diagram. Thus, they plot in the smectite domain with interlayer charge less than 1, which is mainly due to octahedral substitution for distal smectites, while for proximal ones it is caused by both octahedral and tetrahedral substitutions. In this ternary diagram, the domains of both proximal and distal smectites are partially overlapped. The coexistence of di- and trioctahedral smectites was only detected in one proximal sample. Further, proximal biocalcarenites are enriched in Fe-rich dolomite in relation to the distal ones. The 87Sr/86Sr and d13C values in carbonates and dD in smectites indicate equilibrium with seawater. In contrast, d18O values of carbonates and smectites indicate that they were transformed and re-equilibrated between 40 and 90
C, and between 55 and 66
C, respectively, independently of their location with respect to the dome. *

Corresponding author. Fax: +34 1 3466542. E-mail address: [email protected] (L.P. del Villar).

0883-2927/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2005.07.014

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These data suggest that the transformation of calcite into Mg–Fe-carbonates and the occurrence of Fe- and Mg-rich smectites near to the dome resulted from a chemically induced process at similar temperatures. The compositional differences among samples suggests that Fe, Mg and minor Mn were supplied by a contaminant plume originated from the dome, migrating through the sediments and becoming more diluted away from the source. The absence of a well-defined thermal gradient in the system could be due to the small size, semi-closed and shallow character of the basin, as well as to its high underlying volcanic activity. Finally, the results are discussed in terms of analogue processes that can be expected in the bentonite barrier of a radwaste geological repository.  2005 Elsevier Ltd. All rights reserved.

1. Introduction The disposal of long-lived radioactive nuclear wastes in a geological formation has been considered by many countries as the most suitable way to provide protection to the biosphere. Most of the national programs plan to isolate the nuclear waste using a system based on multiple barriers, in order to retain or delay the harmful long-lived radionuclides long enough for substantial radioactive decay and/or dilution to occur. The near field multiple barrier system is composed of: (i) the waste itself; (ii) the canister containing the waste; and (iii) the bentonite buffer surrounding and isolating the canister from the host rock, or bentonite-engineered barrier. The far-field natural barrier is the host rock, known as geobarrier. In terms of the performance assessment of a deep geological repository of radwastes (DGRR), one of the most important aspects is the evaluation of the behaviour and durability of the bentoniteengineered barrier. The extrapolation of the shortterm laboratory data over a longer time scale is a relevant aspect for such evaluation. Hence, natural analogues are the only tools that provide suitable data for the long-term evaluation of the different parts of a DGRR, such as the bentonite-engineered barrier. For the selection of a bentonite formation as natural analogue of the bentonite-engineered barrier, the following criteria must be accounted (Yusa et al., 1991): (i) analogies between natural exposure and candidate clays; (ii) analogies between environmental conditions and those expected under disposal conditions after burial; (iii) simplicity and availability of environmental conditions; and (iv) availability of chronological data. Based on these criteria, a smectitised volcanosedimentary series intruded by a dacitic volcanic dome, known as the Morro´n de Mateo dome, has

been selected as natural analogue of the thermal effect expected on the bentonite barrier of a DGRR after closure. This natural analogue is located in the Cabo de Gata volcanic region (SE Spain) and close to the Morro´n de Mateo bentonite deposit. This formation fulfils some of the above-mentioned criteria as explained below. Regarding the first criterion, bentonites in the ENRESA conceptual design (Astudillo, 2001) for the engineered barrier come from the El Cortijo de Archidona deposit (La Serrata de Nı´jar zone), which consist mainly of a Wyoming-type montmorillonite (Linares et al., 1993). Therefore, the entire laboratory experiments performed at CIEMAT have been carried out on this bentonite (Huertas et al., 2000; Villar, 2002; Ferna´ndez, 2003). However, bentonites from the Morro´n de Mateo deposit, which is located in contact with the volcanic dome, consist mainly of a Tatalia and Chamber-type montmorillonite (Delgado, 1993), which has the chemical composition of the non-ideal montmorillonites, as defined by Shultz (1969) and Brigatti (1983). The second criterion is closely related to the bentonitisation processes, which produced numerous bentonite deposits in the region, among which those from the Morro´n de Mateo area are highlighted. However, as the region was subjected to hydrothermal, marine and continental conditions from the Upper Tertiary to Quaternary periods, several different models have been proposed to explain the bentonitisation processes at different localities (Reyes, 1977; Linares, 1985, 1987; Caballero et al., 1985a,b). All these models involve hydrothermal solutions derived either from the volcanic activity in the region, or from meteoric water reheated at depth. The interaction between these solutions and the vitrophydic rocks, particularly acid tuffs, generated the bentonite deposits. However, the isotopic studies performed on smectites from El Cortijo de Archidona and Los Trancos deposits (Delgado,

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1993; Delgado and Reyes, 1996) suggested that they were formed by the interaction between meteoric waters and tuffaceous materials at surface temperature (<25
C). These interactions are more pronounced along fracture zones (Delgado, 1993). By contrast, the Morro´n de Mateo deposit was formed by water/rock interaction involving seawater at temperatures ranging between 70 and 90
C, in relation to the Morro´n de Mateo intrusion. The involvement of seawater has been inferred from the presence of marine biocalcarenites interbedded with and covering the tuffaceous materials (Delgado and Reyes, 1993; Delgado, 1993). Concerning the third criterion, the natural system selected is geologically simple, despite the complex geological history of the region. Thus, the outcrops investigated show a normal and gradual sedimentary sequence from partially smectitised tuffaceous beds, with minor marine carbonates, to biocalcarenite beds with some smectitised tuffaceous material. All of these outcrops can be stratigraphically correlated and only the volcanic intrusion and minor faults perturb the exposures located near the dome. In relation to the availability of chronological data, the biocalcarenite beds have been assigned to the Lower Tortonian (Betzler et al., 1997), approximately 11.6 Ma ago (Hilgen et al., 2003), while the Morro´n de Mateo dome has been dated between 11.3 and 10.8 Ma (Di Battistini et al., 1987; Zeck et al., 2000; Zeck and Whitehouse, 2002). That means that the volcanic dome intruded between 3 · 105 and 8 · 105 years after the formation of the volcano-sedimentary series. However, though the lava temperature has been estimated as approximately 820
C, by using the Fe–Ti oxides and plagioclase-hornblende geothermometric pairs (Ferna´ndez-Soler, 2002), the cooling rate of the lava and therefore the interaction period between the volcanic dome and the volcano-sedimentary series is more difficult to evaluate. Nevertheless, the Morro´n de Mateo geological system has been considered as a natural analogue for the bentonite barrier of a DGRR. Thus, the transformation of the smectites as a result of the thermal effect induced by the volcanic intrusion has been studied in order to evaluate the variations in their buffering and physicochemical properties. To achieve this objective a strategic mineralogical, chemical, geochemical and isotopic study of the smectites from the bentonitised layered tuffs, located close and far from the volcanic dome (the Morro´n de Mateo and El Murciano areas, respec-

tively), has been carried out for comparison. The initial hypothesis was that smectites from both areas could be formed from the layered tuffs under marine conditions, and the smectites near the dome were later chemically transformed and isotopically re-equilibrated by the volcanic intrusion. 2. Geological background The Morro´n de Mateo volcanic dome is a massive lava body that gives place to a promontory (197 masl) in the so-called Escullos Depression (Fig. 1(a)). The lava body is composed of biotitehornblende, quartz-rich dacite, with a characteristic red to ochre tint that links this body to the nearby Rodalquilar Caldera Complex (Bordet, 1985; Arribas, 1993; Ferna´ndez-Soler, 1992; Cunningham et al., 1990; Rytuba et al., 1990; Arribas et al., 1995). The most conspicuous structure in the volcanic dome is a wide and well-developed subvertical flow foliation, marked by alternating centimetre bands with slightly different coloration. The flow foliation orientations and their relationships with the present contacts and faults suggest that the dome is really composed of an intricate set of dyke-like subvertical elements, elongated approximately N-S, and rooted in a plug-like feeder-channel around the Morro´n de Mateo peak. Every dyke-like element could have been generated in an individual eruptive pulse, being different pulses probably separated by short intervals of inactivity and cooling (Ferna´ndez-Soler, 1992). The Morro´n de Mateo dacite dome intruded through an older volcano-sedimentary series, which consist mainly, from the bottom to the top, of: (i) a variety of hard, hornblende-rich andesite rocks; (ii) a layer of marine calcarenites-calcirudites; (iii) white layered tuffs; (iv) marine sedimentary rocks; and (v) grey tuffs (Fig. 1(b)). White layered tuffs and marine sedimentary rocks are the most interesting lithologies for the aims of this work. The white layered tuffs are composed of a succession of several types of pyroclastic layers, quite recognisable in spite of the degree of transformation into bentonite. The most common appearance is a regular decimetre-scale succession of laterally very extended and white-coloured soft layers of bentonite-rich tuffs. They were originally composed of white lapilli pumice (pumice-rich layers), alternating with slightly harder tabular layers of a sandy or gritty material (gritty layers). These last layers are brownish to greenish, poorer in bentonite, and

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Fig. 1. (a) Geological map of the studied area, showing the location of sampled profiles (modified after Ferna´ndez-Soler, 2002). The crosssections A–A 0 , B–B 0 and C–C 0 in (a) and Fig. 2 are also indicated. Key: 1: hornblende andesite breccias and lavas. 2: Tortonian marine sedimentary rocks (limestones, calcarenites, conglomerates and sandstones). 3: layered tuffs. 4: grey tuffs. 5: white dacitic pyroclastic tuffs. 6: the Morro´n de Mateo dacite dome. 7: Rodalquilar Complex (rhyo) dacitic rocks. 8: undifferentiated Quaternary sediments. 9: alluvial fans. 10: Pleistocene oo¨litic sandstones (fossil beach). (b) Illustrative cross-sections A–A 0 and B–B 0 through the Morro´n de Mateo sequence, showing the intrusive relation between the Morro´n de Mateo dome and the volcano-sedimentary sequence (Ferna´ndez-Soler, 2002). See location in (a). Key: 1: hornblende andesite breccias and lavas. 2: layered tuffs. 3: Tortonian marine sedimentary rocks (limestones, calcarenites, conglomerates and sandstones). 4: grey tuffs. 5: white dacitic pyroclastic tuffs. 6: the Morro´n de Mateo dome. 7: recent alluvial sediments. The approximate position of sampled profiles 1, 2, 3, and 6 is also indicated.

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composed of a larger proportion of crystals, small lithic fragments, and smaller amounts of lapilli pumice. It is easily recognisable since the bentonite formation has proceeded more extensively in the originally glass-rich white pumice layers than in the crystal-rich gritty layers. The whole sequence of layered tuffs are sub-horizontal or dipping at low angle to the E, but the intrusion of the Morro´n de Mateo dacite dome dragged and folded the layered sequence in its proximity. Data from drillholes and outcrops indicate that this lithological unit may reach a thickness of 60–70 m, although with large lateral variations. This formation shows some sedimentary structures that point to lateral currents as main transport and deposition mechanism. These features suggest that the white layered tuffs may have formed as a result of phreatomagmatic activity occurring by magma–water interaction in a shallow marine environment, probably in a depressed coastal embayment (Ferna´ndez-Soler, 1987, 1992; Ferna´ndez-Soler and Mun˜oz, 1988). The marine sedimentary rocks are placed unconformably on top of the white layered tuffs, giving the most notable lithologies, from the bottom to the top: • A 5–10 m thick layer of poorly consolidated beach sands and sandstones, commonly with bioturbation marks, occurs in some localities near the Morro´n de Mateo dome. • A few meters thick layer of carbonate-cemented conglomerates, composed of decimetre-sized clasts of volcanic rocks and marine fossils. They are on top of the beach sands, or more commonly directly on top of the white layered tuffs. • Bioclastic calcarenite, with massive to layered structure, is the main lithological unit of these marine sediments. It is about 20 m thick, composed of clast-supported, heterometric clasts of bryozoa, echinoderms and bivalves. This association (bryomol) is typical of temperate climate carbonates, which are widespread through the Lower Tortonian basin in the Cabo de Gata region. This unit is laterally very extensive, outcropping in both the Morro´n de Mateo and El Murciano sites. This biocalcarenite unit is deposited on the white-layered tuffs, showing a transitional bed formed by a mixture of tuffaceous materials and marine carbonates (mixed bed). At some places, this mixed bed contains abundant soft-clayey pebbles, which are supposed to be remnants of former pumice pebbles

reworked by sedimentary processes. The biocalcarenite is variably cemented to more dense limestone.

3. Sampling and methods For the purposes of this investigation, 6 crosssections (profiles 1–6) were chosen for sampling. Four of them (1, 2, 3 and 6) are located near the volcanic dome, while the remaining two (4 and 5) far from the dome, in the so-called El Murciano quarry (see Figs. 1(a) and 2). At each cross-section, samples were taken from the upper part of the white layered tuffs, lowermost layer of calcarenite, and mixed layer between them (Fig. 3 and Table 1). Samples were dried in a forced air oven at 30– 40
C for 48 h and then ground and homogenised in an agate ball grinder. Afterwards, they were sieved to a <60 lm grain size, by using a nylon mesh ASTM sieve. The semi-quantitative mineralogical composition of the bulk samples was determined by a powder X-ray diffraction (XRD) method, by using the reflecting powers proposed by Shultz (1964) and Barahona (1974), and the calculation method of Bradley and Grim (1961). A ‘‘X’’ Pert-MPD Philips diffractometer, Cu Ka radiation and a scan speed of 1.5
2h/min were used. The mineralogical composition of white tuff samples has been referred to the crystalline fraction, because they contain significant amounts of amorphous components. Optical polarising and scanning electron microscopy (Zeiss DSM 960), coupled to an energy dispersive X-ray analytical Link eXL system (SEM + EDX), were used in the study of thin polished sections. Some backscattered electron images were taken to illustrate the most relevant mineralogical and textural features of the samples. Clayey fractions (<2 lm) were extracted only from the white tuff samples by the conventional sedimentation method, using de-ionised water. Only sample MTO-11A was treated with a 5% Na hexa-methaphosphate solution for dispersion purposes. Oriented specimens, previously treated with ethylene glycol (EG), dimethyl sulphoxide and heated at 550
C for 1 h were systematically analysed by XRD in the 2–35
2h range for mineral identification. The dioctahedral or trioctahedral character of smectite was deciphered in the 55–65
2h regions from powder XRD patterns of the clayey fractions. XRD patterns of the EG-solvated oriented specimens, the reflecting

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Fig. 2. (a) Cross-section in the western slope of the Morro´n de Mateo hill, with approximate indication of the sampled profile 1. Key: (A) Recent alluvial fan conglomerates. (B) Layered tuffs. (C) Calcarenites and carbonate-matrix conglomerates. (D) Grey massive tuffs. (E) The Morro´n de Mateo dacite dome. (b) Cross-section in the SW slope of the Morro´n de Mateo hill, with approximate indication of the sampled profiles 2 and 3. Key: (A) Hornblende dacite breccias and lavas. (B) Layered tuffs. (C) Beach sands and sandstone, pumice-rich lens. (D) Carbonate-matrix conglomerate and lumaquella. (E) Limestones and calcarenites. (F) Grey tuffs. (G) The Morro´n de Mateo dome. (c) C–C 0 cross-section through the Murciano area, indicating the approximate location of distal sampled profiles 4 and 5 (see location in Fig. 1(a)). Key: (A) Hornblende andesite breccias and lavas. (B) White foraminifera-rich limestones. (C) Layered tuffs. (D) Carbonate-matrix conglomerates and calcarenites. (E) Pleistocene oo¨litic sandstones (fossil beach). (d) Cross-section in the western slope of the Morro´n de Mateo hill, with approximate indication of sampled profile 6 and detailed sketch of the structure in the abandoned quarry in the western slope of the Morro´n de Mateo hill. Key: (A) Hornblende andesite breccias and lavas. (B) Layered tuffs. (C) Beach deposits (clastic sands and sandstones). (D) Calcarenites and carbonate-matrix conglomerates. (E) Grey massive tuffs. (F) White dacitic pyroclastic tuffs. (G) Mn-rich coralline limestones. (H) The Morro´n de Mateo dacite dome.

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Fig. 3. Pictures of the six sampled profiles, showing the sampling points along the Lower Tortonian tuffaceous-biocalcarenite series.

powers proposed by Barahona (1974) for clay minerals and the Bradley and Grim (1961) calculation method were used for their semi-quantitative estimation. Chemical analyses of the smectites were carried out in a JEOL-200Kv JEM 2000 transmission electron microscope coupled to an EDX analytical system (TEM + EDX). The structural formulae were calculated from these EDX-chemical data, considering that total Mg is in octahedral positions. These analyses were carried out in the Electron Microscopy Center ‘‘Luis Bru´’’ from the Universidad Complutense de Madrid. Chemical analysis of the bulk and <2 lm samples (major, minor and trace elements, except SiO2) was

performed in the Chemistry Division of CIEMAT. Three replicates of both samples and certified reference materials (SRM 97b and SRM 98b for clay-rich samples and SRM 88b for carbonate-rich samples) were dissolved using a HF–aqua regia mixture in a closed PTFE vessel and heated overnight. After cooling, HF in excess was removed by addition of HClO4 and the solution was evaporated under an IR lamp. Finally, the residue was dissolved in dilute HNO3. Major, minor and trace elements (except U) were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Jobin Yvon JY48 and JY38 instruments) and flame atomic emission spectroscopy (FAES, PE 5000, Perkin–Elmer). For ICP-AES analysis of major constituents

Table 1 Location, lithology and semi-quantitative mineralogical composition of the bulk samples Samples

Lithology

Location

1

MTO-1 MTO-2 MTO-3

Clayey (1) Mixed (1 + 2) Limestone (2)

Near the dome

2

MTO-4 MTO-5 MTO-6

Clayey (1) Mixed (1 + 2) Limestone (2)

3

MTO-7 MTO-8 MTO-9

4

Q (%)

Plag. (%)

K-Fd. (%)

Amph. (%)

5 2 2

14 4 7

4 2 Tr

0.5 1 1

Near the dome

4 3 3

1 8 6

6 Tr –

Clayey Conglomerate Limestone

Near the dome

5 4 7

15 14 14

MTO-10 MTO-11 A MTO-11 B MTO-12 MTO-13

Clayey (1) Soft pebble Mixed (1 + 2) Limestone (2) Limestone

Far from the dome

5 1 13 12 9

5

MTO-14 MTO-15 MTO-16 MTO-17

Clayey (1) Mixed (1 + 2) Limestone (2) Limestone

Far from the dome

6

MTO-18 MTO-19 MTO-20

Clayey (1) Mixed (1 + 2) Limestone

Near the dome

Zeol. (%)

Crist. (%)

Cc. (%)

Dol. (%)

T.Ph. (%)

1 3 7

12 6 14

0 59 55

0 6 13

64 17 Tr

0.5 1 –

1 0 –

0 Tr –

31 59 82

0 2 2

57 27 7

0 Tr Tr

0 1 –

1 18 5

10 7 6

19 46 53

0 0 6

50 10 8

23 2 7 12 8

0 0 8 Tr –

1 0 1 2 Tr

1 0 3 6 4

6 1 1 3 5

10 8 40 38 60

0 0 0 – –

55 88 26 27 15

2 Tr 2 2

5 8 10 13

3 Tr Tr –

0 2 2 2

1 7 6 22

9 12 16 13

0 46 46 35

0 0 – –

80 24 15 13

1 2 13

12 7 4

0 – –

0 2 –

1 – 1

9 6 10

27 58 70

16 – –

34 25 Tr

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Profiles

Q, quartz; Plag., plagioclases; K-Fd., K-feldspar; Amph., amphiboles; Zeol., zeolites; Crist., cristobalite; Cc., calcite; Dol., dolomite; and T.Ph., total phyllosilicates.

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(Al2O3, Fe2O3, CaO, MgO, MnO, P2O5 and TiO2), the dissolved samples were previously diluted 5 times and 20 mg L
1 of Y (as internal standard) were added, in order to improve instrumental precision. Combined uncertainty (including sample preparation uncertainty, calibration uncertainty and long and short-term reproducibility) of the analytical data are less than 1% for major elements and less than 5% for trace elements. Uranium analyses were performed by laser induced kinetic phosphorimetry (KPA-11, Chemchek Instruments Inc.), by using the sample dissolved as described before. Elemental analyser (Leco CS-244) was used for total C and S analyses. A standard reference soil (Soil #1, EuroVector S.p.A., Milano, Italy) was regularly measured in order to check accuracy of data. Differential thermal and thermogravimetric analyses were carried out using a TG/DTA 6300 system for determining H2O
, H2O+ and inorganic CO2. The instrument was calibrated with a high purity monohydrated calcium oxalate (CaC2O4 Æ H2O, Alfa Aesar, Karlsruhe, Germany). Labile SiO2 in tuff bulk samples and <2 lm fractions was extracted by the Ross and Hendrick (1945) method and determined by UV–Vis spectrophotometry (Beckman DU-7), following the method proposed by the US Environmental Protection Agency (1983). Labile Al2O3 and Fe2O3 were removed by the method proposed by Quejido et al. (1988) and determined by ICP-AES, using matrixmatching calibration. The total exchangeable cations (TEC) of both bulk and <2 lm samples were extracted by successive washing with NH4Cl 1 N at pH 8.2 in a 60% v/v ethanol and distilled water solution. Prior to the determination of the TEC, the soluble salts were removed with a solution containing 70% v/v absolute ethanol and 30% v/v distilled water. The samples were shaken end-over-end for 2 days. After phase separation by centrifuging, the extracted cations were measured by using ICP-AES and FAES. A matrix-matching method was used for calibration of both analytical instruments. Accuracy of the analyses was checked against SAz-1 reference material, which is a Ca-rich montmorillonite, supplied by The Clay Minerals Society. Carbonate fractions from both mixed and biocalcarenite bulk samples were selectively removed by twice treating with Morgan
s solution at pH 4.5, in order to determine the Sr contents and their isotope ratios. Samples for 87Sr/86Sr ratios determination were prepared by passing the result-

ing solutions trough a 10 mL Dowex 50 · 8 cation exchange resin. This isotope ratio was measured in the Geochemistry Laboratories of the University of Granada, by means of a multicollector Finnigan MAT 262 thermal ionisation mass spectrometer (TIMS), by using static detection of the element positive ions. Samples were loaded on a Re filament and mass bias was corrected using 86 Sr/88Sr = 0.1194. Blank values of the procedure are below 0.6 ng. Isotope measurements of O and H in clay minerals were carried out in the Stable Isotope Laboratory of the University of Western Ontario (Canada). Prior to O isotopic analyses, gels and amorphous materials were removed by chemical attack. Furthermore, samples were degassed under vacuum at 150
C for 12 h to remove adsorbed and interlayer water. The O from smectites was extracted by reaction with ClF3. Oxygen was then purified and converted to CO2 using a vacuum line similar to that described by Clayton and Mayeda (1963). For the H measurements, samples were previously degassed under vacuum at 150
C for 12 h to remove adsorbed and interlayer water. Hydrogen was extracted from the remaining sample using the U-reduction method of Bigeleisen et al. (1952), as modified by Keyser and O
Neil (1984) and Vennemann and O
Neil (1993). The isotope ratio measurements were performed using a Prism II mass spectrometer. The isotopic results are reported in the standard delta (d) notation in parts per thousand (&) relative to Vienna Standard Mean Ocean Water (VSMOW) (Coplen, 1994). Reproducibility was better than ±0.2& and ±2& for d18O and dD, respectively. Isotope measurements of C and O in carbonates were carried out at the Stable Isotope Laboratory of the ‘‘Estacio´n Experimental del Zaidı´n’’ (Granada, Spain). Calcite samples were treated with 100% H3PO4 during 12 h in a thermostatic bath at 25
C (McCrea, 1950). Samples containing calcite, dolomite and siderite as major, minor or trace components were treated according to the Al-Aasam et al. (1990) method. CO2 was analysed by means of a Finnigan MAT 251 mass spectrometer, and the experimental error found was lower than ±0.15 for d13C and d18O, using Carrara and EEZ1 internal standards, previously compared with NBS-18 and NBS-19. All the samples were compared with a reference CO2 obtained from a standard calcite prepared simultaneously. Thus, O isotope ratios for carbonates were recalculated tak-

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ing into account the fractionation factor for acid decomposition: 1.01065 for dolomite at 50
C (Rosembaum and Sheppard, 1986) and 1.01044 for calcite at 25
C (Kim and O
Neil, 1997). The specific surface area of both smectitised pyroclastic bulk samples and <2 lm fractions was determined by a BET Flowsorb II 2300 (Micromeritics) equipment. The calculations were carried out by the BET method of the single point. 4. Results and discussion 4.1. Mineralogy The semi-quantitative mineralogical composition of the bulk samples from the smectitised tuffaceous, mixed and biocalcarenite materials is listed in Table 1. In all the samples, plagioclase, K-feldspar, amphibole, undetermined phyllosilicates, cristobalite and undetermined zeolites represent the original mineral assemblage of the tuffaceous fraction and its alteration products. The carbonate fraction in the samples consists mainly of calcite with probably secondary dolomite in some samples, particularly in those from the proximal profiles. The SEM observations and EDX analyses performed on the mixed and biocalcarenite samples allow one to state that the tuffaceous fraction has a complex mineralogy, mainly due to the large number of minor and trace minerals identified. Among them, some hydrothermal and/or diagenetic minerals must be highlighted as indicators of physicochemical environmental conditions under which they were formed. These minerals are zeolites (thomsonite, mordenite, phillipsite, heulandite, clynoptilolite, and probable ferrierite), spherulitic, concentric and fibrous-radiating criptocrystalline silica, framboidal pyrite, sphalerite, Sr-rich barite, celestine, and primary Mn-oxyhydroxides. Other secondary minerals related to meteoric diagenesis, such as gypsum, and As rich Fe-oxyhydroxides from the oxidation of pyrite are also present in all the samples. Unidentified clay minerals, probable smectite, coated by Fe-oxyhydroxide or closely associated with Mn-oxyhydroxides, are present in some samples, as a result of the total alteration of obsidian and/or pumice rounded pebbles. For the description of the carbonate fraction, the samples have been classified in two groups: the first consists of samples located far from the volcanic dome and the second group of samples near the dome.

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Samples from the first group contain two generations of calcite. The first generation consists of rounded-shaped, fine-grained xenomorphic calcite with minor Mn or Mn and Fe (Fig. 4(a)). This generation is mixed with fossil clasts, mainly bryozoa. These features are typical of marine bioclastic calcarenites. The second generation consists of large xenomorphic to idiomorphic plates or crystals of calcite either almost totally pure or with minor Mn or Mn and Fe (Fig. 4(a) and (b)). The latter occasionally replaces other minerals, fills their fissures and replaces bryozoa remains. Both generations act as cement of the partially altered pyroclastic fraction, which, in turn, is later cemented by zeolites and microcrystalline silica. Samples from the second group do not contain detrital primary calcite except in sample MTO-9, in which the original bioclastic texture is preserved. In contrast, large plates of pure calcite and plates of Mn-calcite, showing a close textural relationship between them, predominate in all the samples (Fig. 4(c)). Furthermore, hypidiomorphic to idiomorphic dolomite crystals and dolomite with minor Mn and Fe are relatively frequent in the samples (Fig. 4(d)). In addition, a Fe (Mn) carbonate, with botryoidal texture and a chemical composition similar to that of spherosiderite, is occasionally found in some samples (MTO-3). This Fe (Mn) carbonate is closely associated with ankerite and replaces large plates of pure calcite (Fig. 4(e)). As in samples of the first group, the bryozoa clasts are filled and mineralised by carbonates and colloidal or microcrystalline silica (Fig. 4(f)). The textural and mineralogical differences observed between carbonates from proximal and distal samples seem to indicate the existence of two different geochemical environments, probably developed after sedimentation, as a result of the Morro´n de Mateo intrusion. This intrusion could be the source of Mg, Fe and Mn that metasomatised the original calcite of bioclastic calcarenite. It could operate as the source of a contaminant front migrating through the sediments undergoing dilution away from the source. This may be the reason why neither dolomite nor ankerite and spherosiderite could be formed in the distal samples. Samples from both biocalcarenite and mixed beds contain minor pyrite regardless of their location with respect to the subvolcanic dome. This sulphide is usually included in carbonates and currently almost totally transformed into As-rich Fe-oxyhydroxides, preserving its original framboidal texture. Though

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L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2263

Table 2 Semi-quantitative mineralogical composition of the clayey fractions (<2 lm) from the smectitised tuffaceous samples Profiles

Samples

Location

Smectite (%)

Crist. (%)

Plagioclase (%)

Calcite (%)

Zeolites (%)

Di/trioctahedral

Crystallinityc

1 2 3 4

MTO-1 MTO-4 MTO-7 MTO-10 MTO-11Aa MTO-11Bb

Proximal Proximal Proximal Distal Distal Distal

94 95 97 95 98 86

4 – – – – 6

1 5 – 5 2 8


– 3 – – –

1 – – – – –

Dioctahedral Di- and trioctahedral Dioctahedral Dioctahedral Dioctahedral Dioctahedral

0.91 0.97 0.95 0.92 1.00 0.96

5 6

MTO-14 MTO-18

Distal Proximal

89 76

11 5

– 8

– 11cal + dol

– –

Dioctahedral Dioctahedral

0.94 0.97

NB: % is referred to crystalline fraction. Crist: Cristobalite. 11 cal + dol: calcite (7%) + dolomite (4%). All the samples contain spinel (MgAl2O4), which was detected in the powder XRD patterns. a Soft pebble from the mixed bed. b Clay fraction from bulk sample taken in the mixed bed. c Biscaye (1965) crystallinity index.

the paragenetic position of pyrite is problematic, its textural relationships with carbonates, as well as its textural feature itself, suggest that it could be contemporary with the recrystallisation and metasomatic-diagenetic processes affecting carbonates. The presence of As in the Fe-oxyhydroxides suggests that the original pyrite was an As-rich pyrite. From these data, it can be inferred that reducing conditions prevailed during the aforementioned metasomatic-diagenetic process. Thus, the substitution of Ca by Fe(II) and Mn(II) in carbonates was facilitated. Another diagenetic mineralogenetic event observed in the samples is a silicification process that resulted in the precipitation of colloidal and/or cryp-

b Fig. 4. Backscattered electron images showing: (a) Two generations of calcite with minor Mn (1). Notice the different textural features of primary fine-grained detrital calcite and the large plates of secondary calcite. 3: secondary zeolites and 2: colloidal silica (Distal sample MTO-13). (b) Large plate of carbonate showing the two generations of calcite: in the inner part of the plate, partially re-crystallised primary pure calcite (1) is present, preserving some detrital features. This calcite is coated by the second generation of Mn-bearing calcite (2). 3: colloidal silica (Distal sample MTO-13). (c) Re-crystallised Mn-bearing calcite (1), calcite (4) and neoformed ankerite (3) and dolomite (2). (Proximal sample MTO-20). (d) Re-crystallised calcite (2), idiomorphic dolomite + ankerite (1), Fe–Mn-bearing dolomite (4) and Mg-bearing calcite (5) cemented by colloidal silica in dark grey. 3: partially substituted muscovite plate (Proximal sample MTO-3). (e) Spherosiderite (3) and ankerite (2) substituting secondary calcite (1). 5: void in the rock. 4: neoformed dolomite. 6: probable muscovite. 7: Fe–Mn oxyhydroxides. 8: hornblende. 9: ilmenite. 10: colloidal silica (Proximal sample MTO-3). (f) Bryozoa remains firstly mineralised by colloidal silica (1) and secondly by calcite (2) (Proximal sample MTO-6).

tocrystalline silica from the submarine alteration of obsidian and pumice pebbles into smectites and zeolites. Weathering is the last process affecting the system. It formed As-rich Fe-oxyhydroxides, a second generation of Mn-oxyhydroxides and, locally, gypsum. The semi-quantitative mineralogical composition of the clayey fractions (<2 lm) obtained from the white tuff samples, including representative samples of the soft clayey pebbles (MTO-11A) and their host mixed bed (MTO-11B), is listed in Table 2. Data indicate that samples consist mainly of a high-crystalline dioctahedral smectite, with minor cristobalite, plagioclase, calcite and zeolites. The coexistence of both dioctahedral and trioctahedral smectite has been observed only in one proximal sample. From their structural formulae, two groups of smectites have been distinguished. The first group (Appendix A) contains the smectites from proximal samples (MTO-1, MTO-4 and MTO-18) and the second (Appendix B) by smectites from distal samples (MTO-10, MTO11A and MTO-14). Smectites from sample MTO7 can be considered as intermediate terms (see Appendix A). Smectites from the first group usually show structural formulae similar to Fe–Mg or Mg–Fe rich smectites, while smectites from the second group are similar to montmorillonite. Smectites from this second group usually have approximately 8Si4+ in the tetrahedral sheet and Al3+ > Mg2+ > Fe3+ in the octahedral positions, in an approximate proportion of 4Al3+:2Mg2+:Fe3+. Nevertheless, it is relevant that octahedral occupancy in all the structural formulae significantly exceed 4 cations per O20

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(OH)4 group, particularly in those from the first sample group. They have a chemical composition between beidellite and saponite, showing an octahedral occupancy ranging from 4.22 (sample MTO-1, 8) to 5.25 (sample MTO-18, 4). Smectites of samples from the second group or distal samples also exhibit more than 4 cations in the octahedral sheet, ranging between 4.08 (sample MTO-11A, 5) and 4.42 (sample MTO-10, 4), but with a chemical composition that generally corresponds to an anomalous Mg-rich montmorillonite. This anomalous chemical composition and structural formulae suggest the possibility that different stacking of minor trioctahedral and dominant montmorillonitic layers constitute the analysed particles. Similar anomalous chemical formulae for smectites have been described in previous works (i.e., Paquet et al., 1982, 1987; Duplay, 1984; Kodoma et al., 1988; Mayayo et al., 2000), questioning the assumed compositional gap between di- and trioctahedral smectites (Weaver and Pollard, 1973). However, Grauby et al. (1993) demonstrated that complete solid-solution between di- and trioctahedral smectites does not exist, but rather, on the basis of XRD, TEM and IR analyses, these intermediate compositions seem to correspond to physical mixtures, in different proportions, of di- and trioctahedral smectites stacked together in the same particle. This fact has been interpreted as the result of the alteration of dioctahedral smectites in environments with a high rock/water ratio and a significant solute content (Mayayo et al., 2000). According to this, the coexistence of both di- and trioctahedral smectites in the proximal sample MTO-4, as detected by XRD, can be explained. The octahedral cations have also been represented on an Al–Fe–Mg ternary diagram in order to check the smectite evolution from distal to proximal samples (Fig. 5(a)). This diagram shows that distal smectites, with structural formulae similar to all of the other bentonite deposits in the region (Reyes, 1977; Reyes et al., 1987; Linares et al., 1993; Delgado, 1993), plot close to vertex Al, determining the initial point of two evolutionary trends. The first trend points towards vertex Fe and is formed by smectites from samples MTO-10, MTO-11A, MTO-14 and MTO-1. In this trend, smectites from the distal MTO-10 sample represent the intermediate members, while smectites from the proximal MTO-1 sample represent the Fe-rich end-members. The same initial point and the smectites from proximal MTO-7, MTO-4 and MTO-18 samples determine the second evolu-

tionary trend that points to vertex Mg. Smectites from sample MTO-7 represent the intermediate members, while smectites from sample MTO-4 represent the Mg-rich end-members. All of the intermediate and end-members are represented in sample MTO-18. When the structural formulae are represented on the muscovite–celadonite–pyrophyllite ternary diagram (Fig. 5(b)), all of them plot in the smectite domain, with total interlayer charge below 1. However, net negative charge in distal smectites is mainly due to the octahedral substitutions, while in proximal smectites it is caused by both octahedral and tetrahedral substitutions but in different proportions. Smectites plotted outside of the diagram are those containing more than 4 trivalent cations in octahedral sites (more than 12 positive charges). Though this fact could suggest impurities in the samples, irregularities in the structure or the presence of hydroxide-like interlayers, such as brucite (Mg(OH)2) and/or Fe oxyhydroxides Fe(OH)3 layers (Newman and Brown, 1987), it can be explained as an advanced stage in the transition from di- to trioctahedral smectites, as Mayayo et al. (2000) suggested. In this sense, it is relevant that the proximal smectite domain overlaps that of the distal ones. Consequently, all of these data suggest that a secondary geochemical process affected the system, transforming Al-rich smectites into Ferich and Mg-rich smectites. This process was probably similar to that responsible for the presence of Mg–Fe-rich carbonates in the proximal mixed and biocalcarenite samples, since in both cases the same elements (Fe and Mg) are involved. 4.2. Elemental geochemistry The major and trace elements of bulk samples are listed in Tables 3 and 4. No significant chemical differences exist between proximal and distal samples from the smectitised tuffaceous material, except for MnO contents, which are higher in proximal MTO-1 and MTO-18 samples than in the remaining ones. These Mn-rich samples are also enriched in As, while sample MTO-1 is also enriched in Ba. The differences observed among the CO2 and CaO are related to the carbonate contents in the samples (see Table 1) and they are not related to their location with respect to the dome. Similarly, scarce chemical differences exist among the mixed samples. Nevertheless, variations in the MgO contents, partially associated with dolomite,

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

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Fe 0.1

0.9

0.3

0.7

Legend 0.5

Proximal samples Distal samples

0.5

0.7

0.3

0.9

0.1

Mg a

Al 0.1

0.3

0.5

0.7

3+

K (Al, Fe3+) (Mg, Fe2+)2 Si8

K (Al, Fe )4 Si Al2

Muscovite

b

0.9

Celadonite

Pyrophyllite

3+

(Al, Fe )4 Si8

Fig. 5. (a) Fe–Mg–Al ternary diagram showing the octahedral cations of the smectites. Notice the two evolutionary trends defined by the distal (Al-rich members) and proximal smectites (Mg-rich and Fe-rich end-members). (b) Pyrophyllite–celadonite–muscovite ternary diagram on which smectites are represented according to the octahedral, tetrahedral and interlayer charges. Note that all the smectites plot in the smectite domain and that the proximal smectite area overlaps the distal smectite one.

are related to the location of the samples. Thus, samples near the dome (MTO-2, MTO-5 and MTO-19) contain the highest concentrations of MgO. Among them, sample MTO-2 is also richest in MnO. Finally, all the chemical variations observed in biocalcarenite samples, mainly in the

MgO contents, are also related to the mineralogical composition of the samples (see Table 1), which are dolomite-enriched in the proximal ones. As in mixed samples, some proximal biocalcarenite samples (MTO-3 and MTO-20) are Mn-enriched with respect to the other samples.

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Table 3 Chemical composition of the bulk samples (Major elements) Oxides (%)

Al2O3

Al2O3L

Fe2O3

Na2O

K2O

TiO2

P2O5

H2OT

CO2T

SO2T

Certified Obtained

– –

39.23 ± 28 39.51 ± 63

– –

1.19 ± 1 1.16 ± 3

– –

0.187 ± 3 0.191 ± 5

0.0061 ± 6 0.010 ± 1

0.035 ± 4 0.042 ± 8

0. 066 ± 3 0.069 ± 5

0.066 ± 3 0.063 ± 6

2.39 ± 7 2.34 ± 9

(0.046) 0.049 ± 8

– –

– –

– –

SRM-98b

Certified Obtained

– –

27.03 ± 38 27.21 ± 58

– –

1.69 ± 1 1.68 ± 3

– –

0.59 ± 2 0.61 ± 3

0.0150 ± 6 0.017 ± 2

0.106 ± 5 0.110 ± 8

0.202 ± 9 0.210 ± 12

0.066 ± 3 0.068 ± 5

1.35 ± 2 1.38 ± 6

(0.069) 0.073 ± 9

– –

– –

– –

SRM-88b

Certified Obtained

– –

– –

0.277 ± 2 0.285 ± 9

– –

21.03 ± 7 21.10 ± 9

0.0160 ± 12 0.017 ± 2

29.95 ± 5 30.0 ± 1

0.0290 ± 7 0.030 ± 1

0.1030 ± 24 0.105 ± 6

2.39 ± 7 2.44 ± 9

0.066 ± 3 0.071 ± 7

– –

– –

– –

0.336 ± 13 0.352 ± 26

FeO

Fe2O3L

MgO

MnO

CaO

Samples MTO-1 MTO-4 MTO-7 MTO-10 MTO-11A MTO-14 MTO-18

Smectitised tuffaceous 0.50 11.63 0.38 11.85 0.49 15.46 0.84 17.42 0.48 14.12 0.93 16.70 0.36 10.91

samples 0.27 0.25 0.24 0.28 0.28 0.10 0.29

5.64 4.42 4.12 5.11 6.91 4.53 3.66

0.14 0.30 0.27 0.24 0.17 0.11 0.25

0.70 0.45 0.38 0.52 1.10 0.15 0.46

3.30 4.40 3.90 4.00 5.40 5.20 4.80

1.00 0.07 0.14 0.04 0.04 0.04 0.25

3.20 17.4 12.3 8.60 8.20 2.70 20.3

2.40 1.30 1.90 2.20 1.60 2.00 1.70

1.40 1.90 0.96 1.10 0.60 0.66 0.99

0.55 0.47 0.49 0.69 0.42 0.52 0.41

0.03 0.11 0.13 0.14 0.09 0.14 0.11

9.08 6.39 5.84 6.80 8.98 10.37 5.99

0.62 11.73 6.60 3.34 5.14 0.37 16.13

0.10 0.08 0.06 0.06 0.08 0.06 0.04

Mixed samples MTO-2 MTO-5 MTO-8 MTO-11B MTO-15 MTO-19

n.d. n.d. n.d. n.d. n.d. n.d.

6.30 9.70 10.50 10.90 8.80 7.00

n.d. n.d. n.d. n.d. n.d. n.d.

3.00 3.40 3.00 3.20 4.30 3.40

n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d.

2.60 2.60 1.20 1.50 1.90 2.50

0.82 0.20 0.56 0.42 0.51 0.49

37.7 36.7 29.4 24.6 29.2 36.6

1.20 1.30 2.00 2.20 2.00 2.20

n.d. n.d. n.d. n.d. n.d. n.d.

0.21 0.23 0.52 0.56 0.59 0.26

0.08 0.09 0.13 0.15 0.16 0.08

n.d. n.d. n.d. n.d. n.d. n.d.

28.0 23.0 19.0 17.0 19.0 23.0

n.d. n.d. n.d. n.d. n.d. n.d.

Biocalcarenite samples MTO-3 MTO-6 MTO-9 MTO-12 MTO-13 MTO-16 MTO-17 MTO-20

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

9.60 4.30 12.2 11.8 6.90 10.4 11.5 1.50

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

3.60 2.30 4.60 3.20 3.30 6.00 4.00 1.20

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

2.80 1.30 2.20 1.40 0.93 2.70 1.60 1.50

0.85 0.24 0.35 0.33 0.62 0.40 0.56 1.70

32.4 45.6 29.2 22.8 32.1 26.0 24.0 41.1

2.20 0.30 1.90 2.00 1.20 2.20 2.40 0.40

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

0.36 0.17 0.65 0.63 0.32 0.62 0.49 0.06

0.10 0.07 0.16 0.13 0.14 0.14 0.15 0.06

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

22.0 33.0 19.0 14.0 24.0 17.0 14.0 30.0

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Total SiO2: non-determined; L: labile oxides; n.d.: non-determined. Uncertainties of the reference materials are referred to the last significant digits.

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

SiO2L

SRM-97b

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2267

Table 4 Chemical composition of the bulk samples (Trace elements) Elements (ppm)

As

Ba

Ce

Co

Cr

Cu

La

Ni

Sn

Sr

V

W

Y

Zn

Zr

U

SRM-97b

Certified Obtained

– –

(180) 172 ± 12

– –

(3.8) <5

227 ± 12 230 ± 18

– –

– –

– –

– –

84 ± 2 86 ± 4

– –

– –

– –

(87) 89 ± 6

(500) 468 ± 16

– –

SRM-98b

Certified Obtained

– –

(700) 735 ± 28

– –

(16.3) 19 ± 4

119 ± 5 123 ± 8

– –

– –

– –

– –

189 ± 8 192 ± 10

– –

– –

– –

(110) 119 ± 9

(220) 175 ± 18

– –

SRM-88b

Certified Obtained

– –

– –

(3.8) <10

(1.0) <5

(2.3) <5

– –

– –

– –

– –

64 ± 3 67 ± 5

– –

– –

– –

– –

– –

– –

Samples MTO-1 MTO-4 MTO-7 MTO-10 MTO-11A MTO-14 MTO-18

Smectitised tuffaceous samples 274 535 23 21 <25 128 14 11 <25 108 21 17 <25 84 24 19 <25 155 <10 18 <25 32 36 9.1 239 67 13 9.4

19 23 14 11 <5 6 23

9.6 12 14 12 14 10 13

12 21 22 21 14 23 22

24 12 21 14 17 13 13

<10 44 44 44 44 <10 <10

208 280 268 208 140 178 225

166 94 91 155 158 93 69

<25 <25 <25 <25 <25 <25 <25

<5 6.8 10 16 <5 10 7.4

60 37 46 50 90 54 24

n.d. n.d. n.d. n.d. n.d. n.d. n.d.

3.1 3.0 1.4 3.1 7.8 3.5 3.2

Mixed samples MTO-2 MTO-5 MTO-8 MTO-11B MTO-15 MTO-19

<25 <25 <25 <25 <25 <25

200 80 251 150 122 45

<10 <10 <10 <10 <10 <10

<5 <5 <5 <5 6.3 <5

13 20 26 25 6.6 7.5

9.3 14 12 22 9 9.8

19 18 20 22 23 18

12 7.7 23 26 13 6.2

<10 <10 <10 <10 <10 <10

374 265 422 286 291 266

79 63 121 122 123 49

<25 <25 <25 <25 <25 <25

<5 <5 6.5 12 9.8 <5

12 16 6.2 11 12 9.1

19 22 33 40 47 25

n.d. n.d. n.d. n.d. n.d. n.d.

Biocalcarenite samples MTO-3 MTO-6 MTO-9 MTO-12 MTO-13 MTO-16 MTO-17 MTO-20

<25 <25 <25 <25 <25 <25 <25 <25

286 71 133 126 310 239 192 34

<10 <10 <10 <10 <10 <10 <10 <10

<5 <5 10 6 <5 10 8.4 <5

17 12 38 13 9.5 6.4 <5 <5

13 12 12 11 9.1 8.4 7.4 11

21 17 19 21 18 20 20 22

18 5.5 13 15 13 11 5.1 19

<10 <10 <10 <10 <10 <10 <10 <10

285 325 374 290 312 285 327 151

175 50 131 130 66 114 93 17

<25 <25 <25 <25 <25 <25 <25 <25

6.6 <5 <5 6.5 <5 <5 <5 7.1

12 7.4 17 20 5.4 17 10 <5

22 13 31 51 25 39 41 <5

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Major and trace elements of clayey fractions from the smectitised tuffaceous samples are listed in Table 5. Considering both analytical and mineralogical results (see Table 2), it can be stated that: (i) proximal samples are richer in Fe than distal samples, for a similar smectite content; (ii) some proximal samples (MTO-4, MTO-7 and MTO-18) are also richer in Mg than distal specimens; and (iii) only two proximal samples (MTO-1 and MTO-18) are Mn-enriched with respect to distal ones. Given that the main mineral in these samples is smectite (approximately 93% as average value), the chemical composition of the clayey fractions, particularly major elements, seems to indicate that proximal smectites are richer in ferromagnesian elements than distal ones. Nevertheless, trace elements are not useful to differentiate proximal to distal samples. As a consequence, these geochemical features seem to support the secondary process deduced in the previous section from the mineralogical features of the proximal and distal samples.

4.3. Isotopic geochemistry 4.3.1. 87Sr/86Sr ratio in carbonate fraction from mixed and biocalcarenite samples The 87Sr/86Sr values of carbonates extracted from mixed and biocalcarenite samples (Table 6) range from 0.709219 (mixed sample MTO-5) to 0.709902 (biocalcarenite sample MTO-3). The lower values are close to those for the Lower Tortonian seawater (Veizer et al., 1999), approximately 11.6 Ma ago (Fig. 6(b)). This means that some samples almost totally preserve their original 87 Sr/86Sr ratio, which represents that of marine water at that time. However, the higher values must be explained considering the interaction between seawater and acid volcanic rocks, which exhibit high 87Sr/86Sr ratios. Since the volcanic activity in the region was of approximately 7 Ma (see Fig 6(b)) and the volcanic rocks have 87Sr/86Sr values generally higher than 0.7105 (Fig. 6(a)) (Toscani et al., 1990; Arribas et al., 1995; Benito et al., 1999; Turner et al., 1999; Ferna´ndez-Soler,

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Table 5 Chemical composition of the clayey fractions from the smectitised tuffaceous samples Oxides (%) Al2O3 Al2O3L Fe2O3 FeO Fe2O3L MgO MnO CaO Na2O K2O TiO2 P2O5 H2O
H2O+ CO2 total SO2 SiO2L Total

MTO-1 (Pr) 10.9 0.29 8.10 0.18 0.79 5.5 0.5 0.94 0.44 0.67 0.26 0.03 10.6 5.94 1.32 0.04 n.d. 46.5

Trace elements (ppm) As 98 Ba 229 Ce 15 Co 11 Cr 36 Cu 32 La 12 Ni 16 Sr 99 V 202 Y <5 Zn 110 Zr 119 Li 150 U 2.3

MTO-4 (Pr) 12.7 0.25 6.28 0.28 0.56 8.0 0.03 2.0 0.24 0.58 0.16 0.06 12.7 6.69 0.99 0.02 0.40 51.94 71 51 <10 12 37 36 9.3 15 108 109 <5 128 63 180 1.5

MTO-7 (Pr) 11.3 0.23 5.33 0.24 0.27 7.0 0.03 2.70 0.20 0.37 0.27 0.13 13.3 5.68 1.58 0.04 0.51 49.18 39 28 25 <5 31 40 22 13 88 100 7.9 126 157 290 1.6

MTO-10 (D) 14.7 0.22 4.87 0.20 0.36 6.6 0.03 1.80 0.50 0.80 0.18 0.07 12.2 5.87 1.39 0.04 0.58 50.41 38 52 26 18 21 37 19 13 127 126 7.3 137 124 240 2.3

MTO-11A (D) 13.3 0.23 3.58 0.11 0.65 5.8 0.03 1.60 0.57 0.54 0.15 0.04 9.70 6.12 0.77 0.01 0.56 43.76 38 115 <10 6.8 <5 32 11 10 123 126 <5 81 140 230 2.5

MTO-14 (D) 12.9 0.18 3.05 0.13 0.16 5.7 0.03 0.80 1.3 0.41 0.21 0.06 8.6 5.39 0.95 0.06 0.76 40.69 51 11 33 11 6.4 33 21 8 76 92 9.2 71 183 230 2.8

MTO-18 (Pr) 10.2 0.22 7.89 0.10 0.37 7.1 0.15 7.9 0.75 0.58 0.31 0.40 13.4 7.61 5.87 0.02 0.49 63.34 <25 30 14 11 46 19 18 19 115 99 <5 72 n.d 200 2.4

(Pr): Proximal. (D): distal.

unpublished data), the diagenetic processes involving 87Sr-enriched fluids could cover a period of approximately 4 Ma. That is, until approximately the end of the volcanic activity in the region (see Fig. 6(b)). However, it cannot be stated that there is any relationship between the 87Sr/86Sr values and the location of samples with respect to the volcanic dome. 4.3.2. d18O and d13C in carbonate fractions from mixed and biocalcarenite samples The isotopic signatures of carbonates extracted from mixed and biocalcarenite samples (Table 7) display a relatively high dispersion for both d18O and d13C values, which range from
8.40& to +1.14& (PDB) and from
7.80& to +1.58& (PDB), respectively. It should be pointed out that

those carbonates with d13C values more positive than
2.5& (formed or transformed in equilibrium with marine waters) show a high dispersion in the d18O values (STD = 2.28), which is probably related to a wide temperature range (Fig. 7). However, carbonates with d13C values more negatives than
2.5& (formed or transformed under the influence of meteoric water) present a narrower range of d18O values (STD = 0.45) that is typical of meteoric diagenesis under a narrow interval of temperatures (see Fig. 7). In fact, these last samples approximately plot on the Meteoric Calcite Line (MCL) for the site, which is located on the vertical of the d18O 
5& vs. PDB (Delgado, 1993), though slightly displaced toward more positive d18O values (
4.5& vs. PDB as average value) (Fig. 8). Consequently, only these calcites originated at 15
C in equilibrium with

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282 Table 6 Strontium contents and solution (pH 4.5)

2269

87

Sr/86Sr ratios of carbonate fraction from mixed and biocalcarenite bulk samples removed with Morgan
s

Standard

87

NBS-987 Dolerite WS-E

Sr/86Sr

87

External reproducibility (n)

0.71024 ± 2 0.706583

0.710252 0.706596

0.000004 (106) 0.000014 (39)

Sample

Sr (ppm)

87

Internal reproducibility

Biocalcarenite samples MTO-3 MTO-6 MTO-9 MTO-12 MTO-13 MTO-16 MTO-17 MTO-20

285 325 360 290 315 300 355 155

0.709902 0.709257 0.709385 0.709666 0.709521 0.709634 0.709567 0.709731

0.000014 0.000014 0.000021 0.000043 0.000021 0.000100 0.000014 0.000050

Mixed samples MTO-2 MTO-2R MTO-5 MTO-8 MTO-11B MTO-15 MTO-19

385 265 265 405 285 300 300

0.709308 0.709477 0.709219 0.709465 0.709797 0.709681 0.709241

0.000021 0.000014 0.000014 0.000014 0.000024 0.000021 0.000021

meteoric water, with an d18O value of
4.7& (VSMOW). The high variability of d18O values in carbonates with d13C values more positive than
2.5& is due to the presence of calcite formed under marine conditions, preserving its original d18O signature close to 0& (Anderson and Arthur, 1983), together with other secondary carbonates (calcite, dolomite and siderite) neoformed under different conditions, such as reheated marine waters. Thus, some of the more negative carbonates, such as calcite with d18O =
8.4& (PDB) and dolomite with d18O =
8.3& (PDB), indicate neoformation temperatures close to 60 and 90
C, respectively, if equilibrium with seawater is assumed. Summarising, only two samples preserves their original isotopic signature in equilibrium with cold seawater, while a half of them were transformed in equilibrium with reheated seawater, in a temperature range from approximately 28
C to around 60
C. The remaining samples were neoformed under meteoric conditions. In contrast, dolomite is always a secondary carbonate neoformed at a temperature range from approximately 40
C to around 90
C, also in equilibrium with reheated seawater. Something similar occurs for siderite, if the fractionation equation for dolomite is extrapolated to siderite, since a specific experimental equa-

Sr/86Sr measured

Sr/86Sr

tion for siderite has not yet been established. However, these temperatures do not show correlation with mineralogy and neither with the location of samples, as mentioned for Sr isotopes (see Fig. 7). Consequently, these isotopic data allow one to state that the carbonates found in the studied biocalcarenites clearly resulted from hydrothermal transformation processes that affected to marinebioclastic materials, originally consisting of calcite and probably aragonite. After emersion, a new generation of meteoric calcite was formed as a result of the meteoric diagenesis. 4.3.3. d18O and d2H in smectites from the smectitised tuffaceous samples Proximal and distal smectites show relatively uniform d18O values, ranging from +18.5& to +20.1& (Table 8), the average value being +19.3& (VSMOW). These isotopic signatures are relatively high and could be assigned to edaphic clays (Savin and Epstein, 1970; Savin and Lee, 1988; Lawrence and Taylor, 1971, 1972). However, carbonates mixed with the smectitised layered tuffs have a marine origin and were later affected by diagenetic processes involving reheated seawater (Delgado and Reyes, 1993). Consequently, in order to estimate the most probable physicochemical conditions un-

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L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

Fig. 6. (a) Frequency diagram showing the commonest 87Sr/86Sr values for the volcanic rocks in the Cabo de Gata region (values from Toscani et al., 1990; Arribas et al., 1995; Benito et al., 1999; Turner et al., 1999; Ferna´ndez-Soler unpublished data). (b) Diagram showing the 87Sr/86Sr values from biocalcarenite samples and volcanic rocks in relation to the secular variation of seawater 87Sr/86Sr of Veizer et al. (1999), indicating the probable span of the water/rock interaction processes affecting carbonates.

der which the formation or transformation of these smectites took place two approaches have been attempted. The first considers that these smectites were formed or transformed in equilibrium with seawater, with d18O and dD values close to 0&. Following this hypothesis, the d18O values of the smectites indicate that they could be formed or transformed at a temperature between 55 and 66
C (Fig. 9). Slightly higher temperatures (between 75 and 95
C) were obtained for the smectites from the Morro´n de Mateo bentonite deposit (Delgado and Reyes, 1993; Delgado, 1993), which is located just in contact with the volcanic dome (see Fig. 9). The second approach envisages that smectites were formed or transformed as a result of weathering processes, in equilibrium with meteoric water,

with an isotopic signature from
5.5& to
2.9& V-SMOW. These values were calculated from the average d18O value of meteoric calcites (
5& vs. PDB) from the site (Delgado, 1993). In this manner, a temperature range from 22
C to approximately 46
C is obtained for the studied smectites (see Fig. 9). Though this range is narrower (between 26 and 41
C) when the curve for the d18O average value (19.3&) is considered (see Fig. 9), these temperatures are higher than those expected for surficial processes and therefore this hypothesis must be rejected. The dD values of the smectites (from
83.6& to
64.2& V-SMOW, see Table 8) show a larger dispersion than the d18O values. This fact has been related to post-formational processes, mainly weathering processes, involving isotopic exchanges

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2271

Table 7 Isotopic composition of carbonates (d & vs. PDB) Extracted CO2
3

Calcite

Dolomite

Siderite

Profiles

Samples

d13C

d18O

d13C

d18O

d13C

d18O

1 1 1

MTO-2 MTO-2R MTO-3


4.28
4.25
7.80


4.26
3.87
4.20


2.60
2.87 –


4.44
4.28 –


1.90
2.33 –


4.93
4.52 –

2 2

MTO-5 MTO-6

0.60 0.70


1.00
4.05

0.69 0.91


1.56
4.25

1.58 1.04

1.14
4.39

3 3

MTO-8 MTO-9


1.80
0.70


4.85
5.90

–
0.30

–
6.75

– –

– –

4 4 4

MTO-11B MTO-12 MTO-13


5.98
3.35
2.10


5.10
5.16
8.40

– –
0.98

– –
8.30

– – –

– – –

5 5 5

MTO-15 MTO-16 MTO-17


1.95
0.85
0.20


2.90
3.30
1.25


1.66
0.10 –


4.15
2.96 –

–
1.21 –

–
6.60 –

6 6 6

MTO-18 MTO-19 MTO-20


5.90
0.71
0.52


4.30
5.03
5.40

–

–
4.50
6.12

– – –

– – –

of H (Delgado and Reyes, 1993; Delgado, 1993). Furthermore, this hypothesis is in good agreement with the theoretical calculations performed by Keyser and Kerrich (1991), who indicated that the clayey minerals can exchange H isotopes at 25
C, though the equilibrium is reached after approximately 2 Ma. Taking into account both d18O and dD values of the smectites, and given that theoretical smectite in equilibrium with seawater would have isotopic signatures (dD/d18O) of (
64/+27); (
59/+25); (
45/+21); (
41/+19) and (
35/+17), at 15, 25, 50, 60 and 75
C, respectively (Fig. 10), the dD values of the analysed smectites are not compatible with seawater (see Fig. 10). These dD values would be either compatible with a genesis in equilibrium with meteoric waters or in equilibrium with seawater, if post-formational isotopic exchanges of only H occurred. In order to test this last hypothesis, the geothermometric equation proposed by Delgado and Reyes (1996) has been applied, taking into account that it is restricted to a temperature range from 0 to 150 and to smectites unaffected by post-formational exchanges for H isotopes 3:54  106 T
2 ¼ d18 Osm
0:125dDsm þ 8:95 ðsm : smectiteÞ. Thus, the d18O values measured on the smectites formed at temperatures ranging from 26 to 41
C

0.00
0.80

plot on the curve corresponding to a meteoric water, with a d18O value of
4& (Fig. 11). However, when dD values are plotted ‘‘versus’’ these temperatures, they fit well to a straight line that intersects almost all of the possible dD curves for meteoric water. Among them, the curve with a dD value of
22&, which corresponds to the d18O value of
4&, is also intersected. Furthermore, this straight line shows two tendencies. The first toward the curve corresponding to seawater, with dD of 0&, at a temperature slightly higher than 60
C, and the second toward curves corresponding to very negative dD values, at temperatures lower than 20
C. These two tendencies can be explained by considering that the smectites were probably formed or transformed in equilibrium with seawater at a temperature around 60
C (55–65
C), as previously deduced only from the d18O values (see Fig. 9). At a later stage, weathering processes affected these smectites involving only H isotopic exchanges. The large dispersion of dD values is explained by the variability in the extent of isotopic exchange between these smectites and meteoric waters. Finally, though the temperature range calculated for the smectites is narrower than that calculated for the carbonates, the former approximately should correspond to the average temperature of the latter.

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L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

Fig. 8. d13C (PDB)/d18O (PDB) diagram showing the composition of carbonates from the mixed and biocalcarenite samples. Note that calcites with d13C values more negatives than
2.5& plot close to the Meteoric Calcite Line (MCL) obtained from regional calcites (Delgado, 1993).

Fig. 7. Isotopic composition of the marine and meteoric carbonates in relation to: (A) mineralogy, (B) distance from the Morro´n de Mateo dome and (C) sampled profiles. Temperatures were calculated using the Anderson and Arthur (1983) equation and marine water (d18O = 0& vs. V-SMOW).

4.4. Physicochemical properties Both total exchangeable cations (TECs) and BET specific surface area have been determined in bulk samples from the smectitised pyroclastic bed, including samples MTO-11 A and B from the mixed

bed (Table 9), as well as in their corresponding clayey fractions (Table 10). Based on the data from bulk samples, it can be stated that the TEC is directly related to the phyllosilicate content in the samples and, in all of them, the order of abundance of the exchangeable cations is Mg2+ P Na+ > Ca2+ > K+  Sr2+. No differences between proximal (MTO-1, 4, 7 and 18) and distal samples (MTO-10, 11A, 11B and 14) can be established. The TEC values of the clayey fractions is closely related to the smectite content in the samples, if we take into account the uncertainties in both the semi-quantitative mineralogical composition and TEC measurements. No differences between proximal and distal samples are observed. However, the order of abundance of exchangeable cations depends on the samples, particularly regarding Ca2+, Mg2+ and Na+. It is noticeable that the amount of K+ in the clayey fractions is significantly lower than in the bulk samples, as well as the high content of Mg as an exchangeable cation in almost all the samples. The first feature could be explained

Table 8 Isotopic signature (d18O V-SMOW) of the <2 lm fractions from the smectitised tuffaceous samples Profiles

Samples

Mineralogy

d18O (V-SMOW)

dD (V-SMOW)

Location

1 2 3 4

MTO-1 MTO-4 MTO-7 MTO-10 MTO-11 A MTO-11 B

94% 95% 99% 95% 98% 86%

smectite smectite smectite smectite smectite smectite

19.6 18.9 19.1 18.8 19.7 20.1


77.2
74.4
83.6
64.2
75.5
82.3

Near the dome Near the dome Near the dome Far from the dome

5 6

MTO-14 MTO-18

89% smectite 90% smectite

18.5 19.8


70.4
87.6

Far from the dome Near the dome

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2273

Fig. 9. Diagram showing temperature and d18O (V-SMOW) values of waters. The curves represent the theoretical temperature of formation of smectites and calcites in equilibrium with marine and meteoric waters. Surficial temperature (15–25
C) and meteoric calcites (d18O =
5& vs. PDB) were used to calculate the isotopic composition of the meteoric waters. The Savin and Lee (1988) and Anderson and Arthur (1983) equations were used for calculation. The dotted line represents the most common isotopic composition of smectites from the Morro´n de Mateo bentonite deposit.

by the absence of zeolites in the clayey fractions, given that these minerals can contain K as an exchangeable cation. Regarding the second characteristic, some of this Mg can be present as brucite either as impurities in the clayey fractions or as hydroxide-like layers in the smectite structure, as explained before. In any case, brucite can be easily dissolved during treatment for cation exchangeable extraction, since it is soluble in ammonium salts. The specific surface area has been measured on bulk samples (see Table 9) and on 3 clayey fractions (see Table 10). As for TEC, the specific surface area seems to be closely related to the phyllosilicate and smectite contents in the samples,

without any apparent difference between proximal and distal ones. As a consequence, it seems that no significant differences in the TEC and specific surface area values are observed in relation to the crystallochemical differences observed between proximal and distal smectites. 5. Conclusions, analogies and implications for the performance assessment of the bentonite-engineered barrier The textural, mineralogical and chemical differences observed between carbonates from proximal

Fig. 10. Theoretical isotopic composition of smectites in equilibrium with seawaters (white squares) and in equilibrium with meteoric waters (black-oblique rights) at different temperatures. Studied smectites are shown in black squares. The Savin and Lee (1988) and Capuano (1992) equations were used for calculation of d18O and dD theoretical values, respectively.

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L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282 32 30 28 26 24

18

22 20 18 16

D

14 12

-12 -10

-8

-6 0

-40 -50 -60 -70 -80 -90 -100 -110 -120 -130 -140

-4

0

-2 -6

-22 -38 -54 -70 -86

0

20

40

60

80

100

120

Temperature(˚C) Fig. 11. Graphic representation of the geothermometric equation proposed by Delgado and Reyes (1996). Fractionation of oxygen (above) and hydrogen (below) in the smectite–water (marine and meteoric) system are represented as a function of temperature. The curves indicate the theoretical isotope values of smectites formed in equilibrium with different meteoric waters, as well as in equilibrium with seawaters. The d18O/dD pairs that lie on the Meteoric Water Line (MWL) are
2/
4;
4/
22;
6/
38;
8/
54;
10/
70 and
12/
86, while the pair 0/0 correspond to seawaters. Note how for temperatures other than 26–41
C the isotopic composition of the original water is not compatible with the MWL. However, temperatures plotted on the lower diagram fit well to a straight line that suggests a post-formational exchange for hydrogen isotopes.

and distal samples, as well as the crystallochemical differences detected between distal and proximal smectites suggest the existence of two different

geochemical environments in the site. These different geochemical environments were probably developed after sedimentation of the volcanosedimentary series, as a result of the intrusion of the Morro´n de Mateo dome. Thus, this volcanic intrusion could be the thermal agent and, particularly, the source of Mg, Fe and Mn for the transformation of calcite into Mg–Fe–Mn-carbonates and of Al-rich smectites into of Fe-rich and Mg-rich smectites. The geothermometric study of carbonates allowed the distinguishing of a generation of secondary carbonates (calcite, dolomite and minor siderite) neoformed and isotopically re-equilibrated with re-heated seawater at a temperature range between 28
C and approximately 90
C. Something similar occurs in smectites, since they were formed or transformed at a temperature range between 55 and 66
C, which approximately agrees with the average temperature for the secondary carbonates. However, no relationship between the temperatures of formation or neoformation for each sample and its location with respect to the volcanic dome is observed. As a consequence, a clear thermal gradient from proximal to distal samples cannot be established. This means that the geological environment where the transformation processes affecting carbonates and smectites reached a relatively similar temperature in the whole studied area. This was likely due to the small size, shallow and semi-closed character of the marine basin, as well as the contemporary and intense geothermal activity in the site, of which the Morro´n de Mateo dome was the main, but not unique, volcanic manifestation. Regarding the ENRESA design for a deep geological repository (Astudillo, 2001), the candidate bentonite to build the bentonite-engineered barrier

Table 9 Total exchangeable cations (meq/100 g) and Bet specific surface area (m2/g) of bulk samples from the tuffaceous and mixed beds
(m2/g) Profiles Samples Location Phy (%) Na+ K+ Ca2+ Mg2+ Sr2+ Total CEC A 1 2 3 4 4a 4a 5 6

MTO-1 MTO-4 MTO-7 MTO-10 MTO-11A MTO-11B MTO-14 MTO-18

Proximal Proximal Proximal Distal Distal Distal Distal Proximal

64 57 50 55 88 26 80 34

22.3 17.5 23.9 19.6 35.4 16.1 46.6 0.1

6.6 5.4 6.0 5.0 6.7 3.6 5.3 2.5

12.1 20.6 7.0 14.2 27.9 5.5 17.7 8.1

28.1 27.4 18.9 21.8 39.2 15.6 38.6 15.4

0.1 0.1 0.1 0.1 0.2 0.0 0.1 0.1

69.2 71 55.9 60.7 109.4 40.8 108.3 26.2

45.08 32.91 40.70 40.79 68.66 29.27 54.82 18.52

a Samples from the mixed bed. Sample MTO-11A was previously treated with Na-hexamethaphosphate for dispersion. Phy: phyllosilicates.

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2275

Table 10 Total exchangeable cations (meq/100 g) and Bet specific surface area (m2/g) of <2 lm fractions from the tuffaceous samples Profiles

Samples

Location

Sm. (%)

Na+

K+

Ca2+

Mg2+

Sr2+

Total CEC


(m2/g) A

1 2 3 4 5 6

MTO-1 MTO-4 MTO-7 MTO-10 MTO-14 MTO-18

Proximal Proximal Proximal Distal Distal Proximal

94 95 97 95 89 76

25.2 39.9 49.6 39.6 30.1 0.3

3.2 3.7 3.8 4.2 2.7 2.0

22.8 45.7 28.5 19.3 24.4 51.1

56.5 33.3 23.5 32.9 54.5 38.0

0.1 0.2 0.1 0.1 0.2 0.2

107.8 122.8 105.5 96.1 111.9 91.6

n.d. n.d. n.d. 98.37 n.d. 90.21

Sm., smectite; n.d., non-determined.

is an Al-smectite similar to that from the distal zone (El Murciano area) of the Morro´n de Mateo volcanic dome. Furthermore, the thermal peak induced by radioactive decay of the nuclear spent fuel after burial, estimated to reach some 100
C, is another important physical variable of the man-made system, since this parameter will have a large influence on the hydrated bentonite barrier. This temperature is also similar to that calculated for the natural system studied. After sealing of a deep geological repository, it is expected that the steel container will be affected by two corrosion phases. The first will occur immediately post-closure, under aerobic conditions, and as a result of the oxidation by the air occluded in the bentonite barrier. These effects will be weak and restrictive in space. In contrast, during the second phase, which will start when the barrier is totally saturated (approximately 103–104 a), the oxidation of the steel container will occur, resulting in the formation of both magnetite (Fe3O4) and Feoxyhydroxides or siderite (FeCO3), depending on the physicochemical conditions in the system, and the production of hydrogen. In any case, the probable coexistence of Fe(II) and Fe(III) indicates that transitional redox conditions can prevail in the man-made system. With respect to the natural system all these physicochemical conditions were met as a result of the volcanic dome intrusion. So, relatively high temperature, Fe(II)–Fe(III) concentration, total hydration of the smectitised tuffaceous bed and transitional redox conditions worked all together close to the Morro´n de Mateo volcanic dome. Consequently, among the main processes expected in the bentonite-engineered barrier, the transformation of Al-smectites into other phyllosilicates is the most relevant, as a result of the interaction between smectite, temperature and geochemical conditions.

Based on these analogies, the neoformation of Fe (Mg)-rich smectites observed in the proximal zones of the natural system would be expected in the bentonite barrier of the repository, in which an enhanced Fe concentration close to the container can be induced as a result of its oxidation under intermediate redox conditions. In this scenario, no variations in the physicochemical properties, i.e., TEC and BET specific surface area, of the bentonite barrier will occur, as this study has demonstrated. However, under a stronger or longer interaction, the neoformation of trioctahedral smectites such as saponite-stevensite, Fe(II)-rich saponite or dioctahedral nontronite (as a function of the Mg and Fe(II)/Fe(III) activities in the environment, respectively), corrensite and Fe–Mg-rich chlorite may occur as recorded in some active geothermal and diagenetic systems (Yamada and Nakasawa, 1993; Beaufort et al., 1995, 2001; Bril et al., 1996; Robinson and Santana de Zamora, 1999; Mayayo et al., 2000; Hover and Ashley, 2003). In these circumstances, significant variations in the physicochemical and physicomechanical properties of the bentonite-engineered barrier of a deep geological repository of radwastes could be expected. Acknowledgements Financial support for this work has been provided by ENRESA (Spain). We are grateful to M.D. Sa´nchez de Ledesma M. Sa´nchez and Dr. M. Ferna´ndez from the Chemistry Division of CIEMAT for the chemical analyses, and Dr. A.M. Ferna´ndez for the cation exchangeable determinations. We also thank to L. Gutie´rrez-Nevot for the XRD diagrams and J.M. Dura´n for the BET specific surface area measurements. Dr. R. Pusch and an anonymous reviewer are also thanked for their useful criticisms, comments and suggestions that improved the manuscript.

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L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

Appendix A Chemical composition and structural formulae of proximal smectites (Sample MTO-1) Oxides (%)

1

2

3

4

5

6

7

8

9

10

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

65.74 13.24 13.57 5.91 0.11 0.79 0.63 100.0

65.58 24.39 4.33 4.16 0.35 0.73 0.45 99.9

65.71 15.04 9.37 6.97 0.83 0.99 1.09 100.0

63.69 16.14 10.40 7.05 0.79 0.88 1.04 99.9

65.44 15.79 9.41 6.93 0.77 0.67 0.98 99.9

65.37 16.18 9.61 7.00 0.35 0.83 0.67 100.0

63.85 14.94 10.96 6.90 0.79 1.05 1.51 100.0

66.11 15.68 8.87 6.72 0.50 0.87 1.25 100.0

65.91 19.41 4.87 6.76 0.83 0.85 1.36 99.9

65.28 14.31 11.34 6.67 0.20 1.10 1.10 100.0

Tetrahedral cations Si Al P Tetrahedral

Numbers of cations on the basis 7.97 7.71 7.93 0.03 0.29 0.07 8.00 8.00 8.00

of O20(OH)4 7.76 7.88 0.24 0.12 8.00 8.00

7.85 0.15 8.00

7.77 0.23 8.00

7.94 0.06 8.00

7.85 0.15 8.00

7.94 0.06 8.00

Octahedral cations Al Fe3+ Mg P Octahedral

1.86 1.32 1.07 4.24

3.09 0.41 0.73 4.23

2.07 0.91 1.25 4.23

2.07 0.95 1.28 4.31

2.13 0.91 1.24 4.28

2.14 0.93 1.25 4.33

1.92 1.07 1.25 4.24

2.16 0.86 1.20 4.22

2.57 0.47 1.20 4.23

1.99 1.04 1.21 4.23

Interlayer cations Ca Na K

0.10 0.15 0.02

0.09 0.10 0.05

0.13 0.25 0.13

0.11 0.25 0.12

0.09 0.23 0.12

0.11 0.16 0.05

0.14 0.36 0.12

0.11 0.29 0.08

0.11 0.31 0.13

0.14 0.26 0.03

Interlayer charge

0.37

0.34

0.64

0.60

0.52

0.42

0.75

0.59

0.66

0.57

Tetrahedral charge (%)

9.25

85.34

11.13

40.39

22.35

34.77

30.10

9.60

23.47

11.30

5

(Sample MTO-4) Oxides (%)

1

2

3

4

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

59.68 18.89 7.76 9.85 0.59 1.6 1.64 100

59.89 17.93 8.95 9.77 0.5 1.51 1.44 99.99

60.86 17.18 7.49 10.25 0.42 2.21 1.58 99.99

62.68 16.57 8.73 9.96 0.24 1.72 0.1 100.0

Tetrahedral cations Si Al P Tetrahedral

Numbers 7.28 0.72 8.00

of cations 7.31 0.69 8.00

Octahedral cations Al Fe3+ Mg P Octahedral

1.99 0.76 1.79 4.54

Interlayer cations Ca Na K Interlayer charge Tetrahedral charge (%)

6

7

8

9

10

11

63.44 9.8 7.4 18.1 0.05 0.97
99.67

61.17 17.67 8.54 10.17 0.22 1.03 1.21 100.0

66.36 19.2 4.35 7.69 0.44 0.76 1.19 99.99

63.95 15.91 10.96 6.64 0.49 0.78 1.27 100.0

63.85 14.94 10.96 6.90 0.79 1.05 1.51 100.0

60.25 17.64 8.35 12.66 0.16 1.92 0.00 100.98

60.5 14.38 10.73 12.0 0.20 1.01 1.27 100.1

on the basis of O20(OH)4 7.42 7.57 7.70 0.58 0.43 0.30 8.00 8.00 8.00

7.44 0.56 8.00

7.88 0.12 8.00

7.79 0.21 8.00

7.77 0.23 8.00

7.25 0.75 8.00

7.41 0.59 8.00

1.89 0.88 1.78 4.55

1.88 0.73 1.86 4.48

1.93 0.85 1.79 4.57

1.11 0.72 3.28 5.10

1.98 0.79 1.84 4.60

2.57 0.39 1.36 4.32

2.07 1.00 1.21 4.28

1.92 1.07 1.25 4.24

1.75 0.81 2.27 4.83

1.48 1.05 2.19 4.73

0.21 0.39 0.09

0.20 0.34 0.08

0.29 0.37 0.07

0.22 0.02 0.04

0.13 0.00 0.01

0.13 0.29 0.03

0.10 0.27 0.07

0.10 0.30 0.08

0.14 0.36 0.12

0.25 0.00 0.02

0.13 0.30 0.03

0.83

0.90

1.02

0.51

0.25

0.59

0.53

0.58

0.75

0.52

0.60

87.27

76.24

57.30

83.84

118.46

94.60

22.77

36.72

30.10

143.78

98.66

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2277

Appendix A (continued) (Sample MTO-7) Oxides (%)

1

2

3

4

5

6

7

8

9

10

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

65.61 15.27 7.79 8.28 0.24 1.64 0.00 98.83

65.58 15.88 7.80 8.14 0.31 1.92 0.37 100.0

66.39 15.45 8.38 7.88 0.24 1.66 0.00 100.0

60.65 18.72 8.45 8.10 0.00 2.23 1.80 99.95

65.52 16.03 6.80 8.29 0.00 1.75 1.33 99.72

64.45 15.51 8.34 9.23 0.00 2.47 0.00 100.0

65.12 14.89 7.26 9.76 2.20 0.00 0.78 100.0

60.65 18.72 8.45 8.13 0.00 2.23 1.82 100.0

64.95 15.14 8.07 9.71 0.00 2.13 0.00 100.0

66.50 15.61 7.53 8.24 0.00 2.12 0.00 100.0

Tetrahedral cations Si Al P Tetrahedral

Numbers of cations on the basis 7.75 7.87 7.94 0.25 0.13 0.06 8.00 8.00 8.00

of O20(OH)4 7.38 7.87 0.62 0.13 8.00 8.00

7.76 0.24 8.00

7.87 0.13 8.00

7.38 0.62 8.00

7.80 0.20 8.00

7.94 0.06 8.00

Octahedral cations Al Fe3+ Mg P Octahedral

1.96 0.75 1.50 4.20

2.11 0.75 1.46 4.32

2.12 0.80 1.41 4.33

2.07 0.83 1.47 4.36

2.14 0.66 1.48 4.28

1.96 0.81 1.66 4.42

1.99 0.70 1.76 4.46

2.06 0.82 1.47 4.36

1.95 0.78 1.74 4.46

2.14 0.72 1.47 4.33

Interlayer cations Ca Na K Interlayer charge

0.31 0.41 0.10 1.13

0.25 0.09 0.05 0.63

0.00 0.21 0.00 0.46

0.29 0.42 0.00 1.06

0.23 0.31 0.00 0.76

0.32 0.00 0.00 0.64

0.00 0.18 0.34 0.52

0.00 0.29 0.43 1.02

0.27 0.00 0.00 0.55

0.27 0.00 0.00 0.54

22.17

21.06

12.16

58.39

16.71

38.00

24.68

60.92

36.07

10.74

Tetrahedral charge (%)

(Sample MTO-18) Oxides (%)

1

2

3

4

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

61.48 14.69 9.46 10.49 0.42 1.97 1.48 99.99

63.02 14.02 8.87 10.74 0.30 3.04 0.00 99.99

64.23 15.71 8.57 8.07 0.13 2.18 1.11 100.00

Tetrahedral cations Si Al P Tetrahedral

Numbers 7.52 0.48 8.00

of cations on the basis 7.66 7.76 0.34 0.24 8.00 8.00

5

6

7

8

9

63.74 14.96 7.95 11.34 0.73 1.28 0.00 100.00

64.86 15.15 8.63 9.11 0.25 1.59 0.41 100.00

63.94 14.97 8.81 8.99 0.90 1.95 0.45 100.01

62.86 14.86 9.28 9.50 0.95 1.94 0.63 100.02

65.45 16.90 6.45 8.13 0.58 2.16 0.34 100.01

of O20(OH)4 6.97 7.69 1.03 0.31 8.00 8.00

7.81 0.19 8.00

7.75 0.25 8.00

7.66 0.34 8.00

7.84 0.16 8.00

1.79 0.91 1.73 4.42

2.23 0.62 1.45 4.30

55.45 10.06 13.09 18.47 0.16 1.78 0.98 99.99

Octahedral cations Al Fe3+ Mg P Octahedral

1.64 0.93 1.91 4.49

1.67 0.87 1.95 4.48

2.00 0.83 1.45 4.28

0.46 1.32 3.46 5.25

1.82 0.77 2.04 4.63

1.96 0.83 1.63 4.43

1.89 0.86 1.62 4.37

Interlayer cations Ca Na K

0.26 0.35 0.07

0.40 0.00 0.05

0.28 0.26 0.02

0.24 0.24 0.03

0.17 0.00 0.11

0.21 0.10 0.04

0.25 0.10 0.14

0.25 0.14 0.15 (continued on

0.28 0.08 0.09 next page)

2278

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

Appendix A (continued) Oxides (%)

1

Interlayer charge Tetrahedral charge (%)

2

3

4

5

6

7

8

9

0.93

0.83

0.84

0.74

0.44

0.54

0.74

0.79

0.72

51.20

40.79

28.47

138.73

68.91

35.42

33.17

43.54

22.27

Appendix B Chemical composition and structural formulae of distal smectites (Sample MTO-10) Oxides (%)

1

2

3

4

5

6

7

8

9

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

66.90 18.70 5.65 7.61 0.85 0.20 0.10 100.0

65.97 19.45 5.39 7.27 0.33 1.46 0.13 100.0

65.15 19.34 5.42 7.99 0.33 1.78 0.00 100.0

64.03 17.78 7.44 8.36 0.75 1.43 0.21 100.0

65.85 18.86 5.97 7.73 0.33 1.18 0.08 100.0

66.26 18.98 6.09 7.30 0.37 1.34 0.00. 100.3

67.00 16.50 5.80 7.61 0.35 2.41 0.33 100.0

67.22 18.00 5.65 7.61 1.04 0.49 0.00 100.0

62.18 19.46 6.76 8.22 0.51 1.17 1.70 100.0

Tetrahedral cations Si Al P Tetrahedral

Numbers of cations on the basis of O20(OH)4 7.92 7.82 7.75 7.69 0.08 0.18 0.25 0.31 8.00 8.00 8.00 8.00

7.82 0.18 8.00

7.85 0.15 8.00

7.98 0.02 8.00

7.97 0.03 8.00

7.50 0.50 8.00

Octahedral cations Al Fe3+ Mg P Octahedral

2.51 0.53 1.33 4.37

2.54 0.51 1.28 4.34

2.46 0.52 1.42 4.39

2.21 0.72 1.50 4.42

2.46 0.57 1.37 4.39

2.51 0.58 1.29 4.38

2.30 0.55 1.35 4.21

2.48 0.54 1.34 4.36

2.27 0.65 1.48 4.40

Interlayer cations Ca Na K

0.05 0.05 0.16

0.19 0.03 0.05

0.23 0.00 0.04

0.18 0.05 0.12

0.15 0.02 0.05

0.13 0.00 0.08

0.31 0.08 0.05

0.06 0.00 0.16

0.15 0.40 0.08

Interlayer charge

0.30

0.45

0.50

0.53

0.37

0.31

0.74

0.28

0.78

27.66

39.60

50.84

58.05

49.19

46.99

2.16

11.40

64.25

Tetrahedral charge (%)

(Sample MTO-11A)a Oxides (%)

1

2

3

4

5

6

7

8

9

10

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

64.50 19.83 4.71 6.88 0.56 0.92 2.50 99.99

64.82 19.44 5.38 6.68 0.54 0.79 2.35 100.0

66.68 17.23 5.73 7.09 0.61 1.08 1.58 100.0

66.76 19.09 4.59 6.65 0.71 1.09 1.11 100.0

67.20 18.25 4.41 6.31 0.55 1.19 2.14 100.0

66.01 18.51 4.88 6.80 0.66 0.98 2.15 99.99

65.78 18.41 4.98 7.25 0.86 0.56 2.15 99.99

66.42 16.65 6.17 7.00 0.45 1.11 2.19 99.99

65.87 18.47 4.87 6.79 0.66 0.98 2.35 99.99

67.13 17.29 5.31 6.86 0.82 1.35 1.23 99.99

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

2279

Appendix B (continued) Oxides (%)

1

2

3

4

Tetrahedral cations Si Al P Tetrahedral

Numbers of cations on the basis 7.72 7.75 7.96 0.28 0.25 0.04 8.00 8.00 8.00

5

6

of O20(OH)4 7.92 7.99 0.08 0.01 8.00 8.00

7

8

9

10

7.88 0.12 8.00

7.86 0.14 8.00

7.95 0.05 8.00

7.87 0.13 8.00

8.00 0.00 8.00

Octahedral cations Al Fe3+ Mg P Octahedral

2.52 0.45 1.23 4.20

2.49 0.52 1.19 4.20

2.38 0.55 1.26 4.19

2.59 0.44 1.18 4.21

2.54 0.42 1.12 4.08

2.48 0.47 1.21 4.16

2.45 0.48 1.29 4.22

2.30 0.59 1.25 4.15

2.47 0.47 1.21 4.15

2.43 0.51 1.22 4.16

Interlayer cations Ca Na K

0.12 0.58 0.09

0.10 0.54 0.08

0.14 0.37 0.09

0.14 0.25 0.11

0.15 0.49 0.08

0.13 0.50 0.10

0.07 0.50 0.13

0.14 0.51 0.07

0.13 0.54 0.10

0.17 0.29 0.13

Interlayer charge

0.90

0.83

0.75

0.64

0.88

0.85

0.77

0.86

0.90

0.75

30.83

29.70

5.72

12.30

1.41

14.31

18.26

5.55

14.42

0.07

Tetrahedral charge (%) a

Sample treated with Na-hexamethaphosphate for dispersion (the amount of exchangeable Na is higher than in smectites of the other samples).

(Sample MTO-14) Oxides (%)

1

2

3

4

5

6

7

8

9

10

SiO2 Al2O3 Fe2O3 MgO K2O CaO Na2O Total

66.77 18.20 4.66 8.08 0.42 0.00 1.87 100.00

67.15 18.90 4.37 6.80 0.35 2.00 0.52 100.09

67.09 17.97 4.81 7.35 0.18 0.74 1.85 99.99

66.21 19.20 4.59 7.75 0.18 0.59 1.47 99.99

65.92 19.96 4.47 7.04 0.48 1.01 1.12 100.0

66.36 18.79 4.36 7.20 0.00 0.95 2.34 100.0

66.83 18.38 4.40 7.17 0.54 0.72 1.96 100.0

67.29 19.06 4.66 7.59 1.16 0.24 0.00 100.0

68.00 18.80 5.04 7.26 0.00 0.46 0.44 100.0

67.75 20.36 4.40 6.98 0.51 0.00 0.00 100.0

Tetrahedral cations Si Al P Tetrahedral

Numbers of cations on the basis 7.92 7.94 7.96 0.08 0.06 0.04 8.00 8.00 8.00

of O20(OH)4 7.85 7.82 0.15 0.18 8.00 8.00

7.88 0.12 8.00

7.94 0.06 8.00

7.95 0.05 8.00

8.00 0.00 8.00

7.95 0.05 8.00

Octahedral cations Al Fe3+ Mg P Octahedral

2.47 0.44 1.43 4.34

2.58 0.41 1.20 4.19

2.47 0.46 1.30 4.23

2.53 0.44 1.37 4.34

2.61 0.43 1.24 4.28

2.51 0.42 1.27 4.20

2.51 0.42 1.27 4.20

2.60 0.44 1.34 4.38

2.60 0.48 1.27 4.35

2.76 0.41 1.22 4.40

Interlayer cations Ca Na K

0.00 0.43 0.06

0.25 0.12 0.05

0.09 0.43 0.03

0.07 0.34 0.03

0.13 0.26 0.07

0.12 0.54 0.00

0.09 0.45 0.08

0.03 0.00 0.17

0.06 0.10 0.00

0.00 0.00 0.08

Interlayer charge

0.49

0.68

0.64

0.51

0.58

0.78

0.72

0.24

0.21

0.07

16.03

8.35

6.06

29.87

31.31

15.04

8.27

20.91

0.90

73.59

Tetrahedral charge (%)

2280

L.P. del Villar et al. / Applied Geochemistry 20 (2005) 2252–2282

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