Carbon, oxygen and strontium isotope ratios of late-stage ... - Posiva

Working Report 99-39

Carbon, oxygen and strontium isotope ratios of late-stage fracture calcites from the Olkiluoto and Romuvaara research sites Juha Karhu

May 1999

POSIVA OY Mikonkatu 15 A, FIN-001 00 HELSINKI, FINLAND Tel. +358-9-2280 30 Fax +358-9-2280 3719


Carbon, oxygen and strontium isotope ratios of late-stage fracture calcites from the Olkiluoto and Romuvaara research sites Juha Karhu

May 1999

L

LAUS UNTO

Olen tarkastanut ja hyvaksynyt julkaistavaksi erikoistutkija Juha Karhun laatiman, Posiva Oy:n toimeksiantoon 9798/98/MVS liittyvan tyoraportin:

"Carbon, oxygen and strontium isotope ratios of late-stage fracture calcites from the Olkiluoto and Romuvaara research sites"

Espoossa 29.04.1999

Tutkimusprofessori GTK, Tutkimus ja kehitysyksikko Kallioperaja malmitutkimuslinja

Erikoistutkija GTK, Isotooppigeologian laboratorio

~JD~ Pekka Nurmi

JuhaKarhu

03.05.1999 Posiva OY Margit Snellman Mikonkatu 15A 001 00 Helsinki

Viite: Tilaus 9798/98/MVS Ohessa tilaukseen 9798/98/MVS liittyva raportti:

CARBON, OXYGEN AND STRONTIUM ISOTOPE RATIOS OF LATE-STAGE FRACTURE CALCITES FROM THE OLKILUOTO AND ROMUVAARA RESEARCH SITES Sopimuksen mukaan raportista toimitetaan originaali ja yksi kopio.

Lisaksi mukana disketti sisaltaen Abstakti/tiivistelma Kansilehden tiedot Analyysitulokset ExcelS tiedostona

Kunnioittavasti

Erikoistutkija Juha Karhu GTK, Isotooppigeologian laboratorio


Carbon, oxygen and strontium isotope ratios of late-stage fracture calcites from the Olkiluoto and Romuvaara research sites Juha Karhu Geological Survey of Finland

May 1999

Working Reports contain information on work in progress or pending completion.

The conclusions and viewpoints presented in the report are those of author(s} and do not necessarily coincide with those of Posiva.

CARBON, OXYGEN AND STRONTIUM ISOTOPE RATIOS OF LATE-STAGE FRACTURE CALCITES FROM THE OLKILUOTO AND ROMUVAARA RESEARCH SITES

ABSTRACT Two late-stage fracture calcite samples from the Olkiluoto investigation site at Eurajoki and three from the Romuvaara site at Kuhmo were selected for the determination of the isotope ratios of carbon, oxygen and strontium. Samples were selected at Kivitieto Oy, Oulu. Various geochemical and petrographical methods were used in order to find relative homogenous fracture calcite coatings, possibly representing the latest generations of calcite growth in bedrock fractures. Isotope measurements were done at the Geological Survey of Finland, Espoo Oxygen isotope ratios indicate that all these fracture calcites, with one exception, could have been precipitated in equilibrium with present-day groundwaters. Strontium and carbon isotope ratios of these calcites are in the same range as the isotope ratios reported in the literature for other fracture calcites from the Fennoscandian and Canadian shields. A relatively radiogenic Sr-87/Sr-86 value of about 0. 757 for one of the Olkiluoto samples suggests precipitation in an isolated pocket, where Sr is derived preferentially from the alteration of K and Rb bearing minerals such as muscovite or biotite. The late-stage fracture calcites from the both investigation sites are systematically enriched in C-13 relative to the published data on the composition of dissolved inorganic carbon in groundwater samples from boreholes. The difference suggests carbon isotope disequilibrium between fracture calcites and groundwaters. The late-stage fracture calcites analysed in this study apparently represent an earlier stage, not related to the present-day groundwater system. This study is part of the Project EQUIP (contract number FI4 W -CT96-0031) which is eofunded by the European Commission.

Key words: Carbon isotopes, oxygen isotopes, strontium isotopes, fracture calcite, Olkiluoto, Romuvaara, Project EQUIP, European Commission

HIILEN, HAPEN JA STRONTIUMIN ISOTOOPPISUHTEET OLKILUODON JA ROMUVAARAN TUTKIMUSKOHTEIDEN NUORIMPIEN VAIHEIDEN RAKOKALSIITEISSA

TIIVISTELMA Hiilen, hapen ja strontiumin isotooppimaarityksia varten Eurajoen Olkiluodon tutkimusalueelta valittiin kaksi ja Kuhmon Romuvaaran alueelta kolme rakokalsiittikiteytymien myohaisimpia vaiheita edustavaa naytetta. Naytteiden valinta tehtiin Kivitieto Oy:n toimesta Oulussa. V alinnassa kaytettiin hyvaksi erilaisia geokemiallisia ja petrografisia menetelmia, joiden avulla pyrittiin tunnistamaan suhteellisen homogeenisia, viimeisimpien generaatioiden rakokalsiittikiteytymia. Isotooppianalyysit tehtiin Geologian tutkimuskeskuksen laitteistoilla Espoossa. Happi-isotooppikoostumus osoittaa, etta tutkitut rakokalsiittinaytteet voivat yhta poikkeusta lukuunottamatta edustaa tasapainoista kiteytymista tutkimusalueiden nykyisista pohjavesista. Rakokalsiittinaytteiden strontium- ja hiili-isotooppisuhteet osuvat samalle alueelle, milia sijaitsevat kiijallisuudessajulkaistut Fennoskandianja Kanadan kilpialueiden rakokalsiittien vastaavat analyysit. Toinen Olkiluodon tutkimuskohteen tutkituista rakokalsiiteista antoi Sr-87/Sr-86 suhteeksi verrattain radiogeenisen arvon noin 0.757. Tama puoltaa tulkintaa, etta kyseinen rakokalsiitti on kiteytynyt erillisessa taskussa, missa Sr on paaasiassa peraisin K- ja Rb-pitoisten mineraalien, kuten muskoviitin tai biotiitin muuttumisesta. Molempien tutkimuskohteiden nuorimpien vaiheiden rakokalsiitit ovat systemaattisesti rikastuneet C-13 isotoopin suhteen verrattuna julkaistuihin analyyseihin alueiden kairanreikien pohjavesiin liuenneesta epaorgaanisesta karbonaatista. Isotooppiero viittaa hiiliisotooppikoostumuksen epatasapainoon rakokalsiittien ja pohjavesien valilla. On todennakoista, etta tassa tyossa tutkitut myohaisen vaiheen kalsiitit edustavat jotain varhaisempaa tilannetta, mika ei liity nykyiseen syvien pohjavesien systeemiin. Tama tutkimus kuuluu EQUIP-projektiin (sopimus numero FI4WT-CT96-0031),joka on osittain Euroopan komission rahoittama.

Avainsanat: Hiili-isotoopit, happi-isotoopit, strontiumisotoopit, rakokalsiitti, Olkiluoto, Romuvaara, EQUIP-projekti, Euroopan komissio

PREFACE

I thank Margit Snellman, Posiva Oy, and Petteri Pitkanen, Technical Research Centre of Finland, for comments and suggestions and Paula Ruotsalainen, Fintact Oy, for her comprehensive review and suggestions.

TABLE OF CONTENTS

ABSTRACT TIIVISTELMA PREFACE TABLE OF CONTENTS ............................................................................ 1 1 INTRODUCTION.................................................................................... 2 2 SAMPLES ................................................................................................. 2

3 ANALYTICAL METHODS ................................................................... 3 4 RESULTS ................................................................................................. 4

5 DISCUSSION ........................................................................................... 8

5.1 Strontium isotopes ............................................................................. 8 5.2 Oxygen isotopes ................................................................................. 9 5.3 Carbon isotopes ................................................................................ 10 6 SUMMARY AND CONCLUSIONS .................................................... 12 7 REFERENCES ....................................................................................... 13

2

1

INTRODUCTION

While present-day deep groundwaters can be sampled, our knowledge of past water chemistries is based on indirect information. Isotope records in fracture minerals have turned out to be a valuable source of information about past groundwaters. Calcite is the most common fracture mineral studied, and of particular interest are the isotope ratios of carbon, oxygen and strontium. Often several fracture mineral generations are present, and then, the latest of these can be expected to provide information on the most recent evolution of the groundwater system.

This work reports carbon, oxygen and strontium isotope data on selected late-stage fracture calcite samples from the Olkiluoto and Romuvaara investigation sites, in Eurajoki and Kuhmo, respectively. Carbon, oxygen and strontium isotope data on fracture calcites from one of the drill cores (OL-KR1) at Olkiluoto have been published in an extensive study by Blomqvist et al. (1992). At Romuvaara no measurements on the isotope systematics of fracture calcites have been made before this study, but carbon, oxygen and strontium isotope signatures of present-day groundwaters have been interpreted and described by Pitkanen et al. (1996).

This study is part of the Project EQUIP (contract number FI4W-CT96-0031) which is eofunded by the European Commission.

2

SAMPLES

Fracture calcite samples were selected with the purpose of getting representative samples from the youngest calcite generations. Various petrographic and geochemical methods were applied in order to locate the latest phases of calcite crystallization at Kivitieto Oy, Oulu. Two fracture calcite samples from the Olkiluoto research site and three from the Romuvaara site were selected by Aulis Karki ofKivitieto Oy.

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The latest calcite generations at Olkiluoto and Romuvaara seem to be present as very thin calcite films, only a few J.Lm in thickness. Due to technical and analytical problems calcites were for this study separated from relatively homogeneous, thicker fracture coatings, varying from 100 to 500 J.Lm in thickness. From these coatings it was possible to obtain several milligrams of calcite for isotopic analyses. Although these fracture coatings may predate the formation of thin calcite films, they are, in any case, interpreted to represent latest generations of calcite growth and, accordingly, they are in this report referred to as late-stage calcite.

The chosen drill core samples were submitted to the the Geological Survey of Finland, where fracture calcite was separated using a micro-drill technique. The total amount of sample powder obtained varied from 5 to 20 mg of fracture material.

3

ANALYTICAL METHODS

An 1-2 mg aliquot of sample powder was reacted with phosphoric acid at 25°C for more

than 16 hours following the procedures described by Karhu (1993). The resulting C02 gas was purified cryogenically, and carbon and oxygen isotope ratios were determined on a Finnigan MAT 251 mass spectrometer at the Geological Survey of Finland. Oxygen isotope compositions were corrected using a phosphoric acid fractionation factor of 1.01025. Isotope ratios are reported in the 8-notation as permil differences with respect to the PDB standard, defined as 8 = (RsampteiRPnB- 1)x 1000. For carbon isotopes 8 = 8C-13 and R = C-13/C-12 and for oxygen 8 = 80-18 and R = 0-18/0-16. The standard used for

the calibration of the PDB scales for carbon and oxygen is NBS-19 calcite. Oxygen isotope compositions have also been given relative to the SMOW standard using the transforming equation given by Coplen et al. (1983). The 8C-13 and 80-18 values are reproducible to better than ±0.1 %o.

As the concentrations of Sr in fracture calcites from Olkiluoto and Romuvaara were poorly known, relatively large quantities (5-20 mg) of sample powder were used for

4

strontium isotope work. Samples were leached with 0.5M HCl to dissolve the calcite fraction from the insoluble residue. The residue was separated by centrifuging, weighed and the remaining solution was spiked with a Rb-Sr spike solution enriched in Rb-87 and Sr-84. Rubidium and strontium were purified using a cation exchange procedure. Isotopic measurements were carried out using a VG SECTOR 54 mass spectrometer and a dynamic mode of measurement except for rubidium, which was measured on a single collector Nier-type mass spectrometer built at the Geological Survey of Finland. Strontium isotope data were normalized to Sr-86/Sr-88 = 0.1194. Total procedural blanks for Rb and Sr were approximately 0.15 ng. The accuracy of the concentration determinations was about 3% for Sr and 15% for Rb. For Sr isotope composition determinations the analytical uncertainties are given separately in Table 1. Eleven measurements of the SRM987 standard have yielded a Sr-87/Sr-86 ratio of 0.710254±1.7E-05 (1STD).

4

RESULTS

The analytical data are shown in Table 1. Strontium isotope ratios are illustrated in Figures 1 and 2 for Olkiluoto and Romuvaara sites, respectively. Carbon isotope data are presented in Figures 3 and 4, and oxygen isotope data in Figures 5 and 6. In the figures the new analytical results are compared to published fracture calcite analyses from the drill core OL-KR1 at Olkiluoto (Blomqvist et al., 1992) and to analyses of dissolved components in borehole groundwaters from Romuvaara (Pitkanen et al., 1996) and from Olkiluoto (Ruotsalainen & Snellman, 1996; Helenius et al., 1998a; Pitkanen et al., 1998). Rubidium concentrations in the analysed fracture calcites are generally low, varying from 1 to 3 ppm, except for sample OL-EQ33 containing 11.2 ppm ofRb. Strontium contents are significantly higher ranging from 23 to 29 ppm at Olkiluoto and from 80 to 231 ppm at Romuvaara.

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Table 1. Carbon, oxygen and strontium isotope ratios of fracture calcites from Olkiluoto and Romuvaara. Sample#

Borehole Depth m

Olkiluoto OL-EQ3 OL-EQ33 Romuvaara RO-EQ11 RO-EQ19 RO-EQ20

o13c

0180

0180

%o PDB

%o PDB

%o SMOW

Lab. code

Sr-871 Sr-86

:i:2a

.Jo-5

Rb Sr ppm ppm

OL-KR2 236.15 C-509-01 OL-KR9 281.48 C-509-02

-7.91 -9.85

-10.39 -14.72

20.20 15.73

0.72515 0.75659

2.9 3.8

2.8 11.2

23 29

RO-KR9 58.00 C-509-04 RO-KR9 283.15 C-509-05 RO-KR9 229.15 C-509-06

-4.38 -10.52 -7.33

-12.90 -11.56 -11.50

17.61 19.00 19.05

0.71375 0.71801 0.71613

5.2 2.7 1.2

1.3 1.3 0.9

231 137 80

Fracture calcite samples have Sr-87/Sr-86 ratios ranging from 0.713 to 0.725, with the exception of sample OL-EQ33 having a more radiogenic ratio of 0. 757 (Figs. 1 and 2). These strontium isotope ratios are roughly similar to those measured for fracture calcites and deep groundwaters in the Canadian and Fennoscandian shields (McNutt et al., 1990; Blomqvist et al., 1992; Pitkanen et al., 1998).

2

Late-stage calcite

0

8 6

Groundwater

4

Pitkanen et al., 1998

2

4

0

en

iU

OL-KRl

6

Blomqvist et al., 1992

4

Late-stage calcite

2

l

en

c...;

Fracture calcite

0

r-.

2

iU

0 4

Groundwater Pitkanen et al., 1996

2

..0

0 0.7

0.71

0.72

0.73

0.74

0.75

0.76

ze

0 0.7

0.71

0.73

0.72 87

Figure 1. Histogram of Sr-87/Sr-86 ratios of latestage fracture calcites from the Olkiluoto investigation site compared to fracture calcite data from drill core OL-KR1 ofBlomqvist et al. (1992) and groundwater data ofPitkanen et al. (1998).

0.74

0.75

0.76

Sri 6Sr

Figure 2. Histogram of Sr-87/Sr-86 ratios of latestage fracture calcites from the Romuvaara investigation site compared to those of dissolved Sr from Pitkanen et al. (1996).

6

2

Late-stage calcite

10

en

-a d)

ea

8

DIC

6

Pitkanen et al., 1998

4 2

en

~

0

~

10

OL-KR1

,.c

s:::s z

8

Calcite

6

Blomqvist et al., 1992

4 2 0 -20

-10

0

10

Figure 3. Histogram of oC-13 values of late-stage calcite from the 0 lkiluoto investigation site compared to fracture calcite data from the drill core OL-KR1 of Blomqvist et al. (1992) and to groundwater data ofPitkanen et al. (1998). In the latter data set two extreme datapoints with oC-13 = -36.3 and+ 16.8 fall outside the range of the diagram.

-~ r:n

d)

Late-stage calcite

2

~

0

r:n

~

0

DIC

8

;.... Q)

~

§

z

6

4 Pitkanen et al., 1996

2 0

-30

-20

-10

Figure 4. Histogram of oC-13 values of late-stage calcite from the Romuvaara investigation site compared to oC-13 data of dissolved inorganic carbon (DIC) from the report ofPitkanen et al. (1996).

0

7

At the Olkiluoto investigation site both the carbon (Fig. 3) and oxygen (Fig. 5) isotope compositions of the late-stage fracture calcites are in the same range as those reported for fracture calcites by Blomqvist et al. (1992). Nevertheless, the two late-stage calcite samples have a rather large difference in the 80-18 values, suggesting variations in either groundwater compositions or precipitation temperatures. In carbon isotope ratios there appears to be a systematical difference between fracture calcites and dissolved inorganic carbon (Fig. 3). A majority of the fracture calcites are significantly enriched in C-13 relative to dissolved inorganic carbon.

Also at the Romuvaara investigation site the carbon isotope compositions of the late stage calcite samples are distinct from those obtained directly from present-day groundwaters by Pitkanen et al. (1996). As was the case at Olkiluoto, fracture calcites are enriched in C-13 in comparison to dissolved inorganic carbon (Fig. 4 ). Equilibrium with presentday groundwaters

2

0 OL-KR1 Blomqvist et al., 1992

0 0

5

·10

o18 0 C/oo,

15

20

25

SMOW)

Figure 5. Histogram of 80-18 values of late-stage fracture calcite from the Olkiluoto investigation site compared to data on fracture calcites from drill core OL-KR1 analysed by Blomqvist et al. ( 1992). The 80-18 field for calcite in equilibrium with present-day groundwaters has been calculated using 80-18 values of groundwaters reported by Pitkanen et al. (1998) and the fractionation calibration ofO'Neil et al. (1969).

8

Equilibrium with

1precent-day ground waters 1 I

~

f.

Late-stage calcite

0

' 1

5

10

I

.I

I

15

20

25

Figure 6. Histogram of 80-18 values of late-stage fracture calcite from the Romuvaara investigation site. The 80-18 field for calcite in equilibrium with present-day groundwaters has been calculated using the 80-18 data for groundwaters in Pitkanen et al. (1996) and the fractionation calibration ofO'Neil et al. (1969).

5

DISCUSSION

5.1

Strontium isotopes

Most of the fracture calcite samples analysed in this study as well as those reported by Blomqvist et al. (1992) have Sr-87/Sr-86 ratios ranging from 0.71 to 0.73 (Figs. 1 and 2). In groundwaters from boreholes at Olkiluoto the Sr-87/Sr-86 ratios range from 0.718 to 0.720 and at Romuvaarafrom 0.719 to 0.751 (Pitkanen et al., 1996; Ruotsalainen & Snellman, 1996; Helenius et al., 1998a,b; Pitkanen et al., 1998). These values are similar to those obtained for fracture calcites and deep groundwaters in the Canadian Shield (McNutt et al., 1990). The relatively unradiogenic Sr-87/Sr-86 ratios are compatible with plagioclase being a main source of Sr for the deep groundwaters (McNutt et al., 1990).

One of the fracture calcite samples (OL-EQ33) has a high Sr-87/Sr-86 ratio of0.757, which is distinctively more radiogenic than the other samples analysed in this study or those published by Blomqvist et al. (1992). These characteristics suggest a more unusual environment of precipitation for this fracture calcite. Possibly it was formed in an isolated

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pocket, where Sr was derived preferentially from the alteration of K and Rb bearing minerals, such as muscovite and biotite (see e.g. McNutt et al., 1990).

5.2

Oxygen isotopes

The fracture calcite generations selected for analysis from Olkiluoto and Romuvaara are considered to represent a relatively young phase of calcite precipitation. An important question is the possible equilibrium between calcite and present-day groundwaters.

The 80-18 values of present-day groundwaters in borehole samples vary from -12.8 to8.7 permil in the Olkiluoto area (Ruotsalainen & Snellman, 1996; Helenius et al., 1998a; Pitkanen et al., 1998) and from -13.8 to -12.2 permil in the Romuvaara area (Pitkanen et al., 1996; Ruotsalainen & Snellman, 1996; Helenius et al., 1998b). Oxygen isotope ratios of fracture calcite formed in equilibrium with these waters can be estimated applying the fractionation calibration of O'Neil et al. (1969), as modified in Friedman and O'Neil (1977). Accordingly, fracture calcites formed at surface temperatures between 5 and 15°C can be expected to have 80-18 values varying from about 17.9 to 24.6 %o (SMOW) at Olkiluoto and from about 16.8 to 21.0 %o at Romuvaara. These compositional ranges are also shown in Figures 5 and 6 for Olkiluoto and Romuvaara, respectively.

It appears that for the Olkiluoto site only the highest 80-18 values measured by

Blomqvist et al. (1992) and only one (OL-EQ3) of the two, new calcite samples could represent equilibrium precipitation from present-day groundwaters. In contrast, at Romuvaara all three fracture calcite samples could have been precipitated in equilibrium with the present-day groundwaters (Fig. 6).

Lower than expected 80-18 values in most Olkiluoto fracture calcite samples (Fig. 5) could be related to two alternative conditions. Firstly, precipitation temperatures may have been higher, which is possible for old hydrothermal calcites. Secondly, the 80-18 values of groundwater could have been lower, e.g. due to infiltration of glacial waters.

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The former possibility was preferred by Blomqvist et al. (1992), who suggested hydrothermal conditions of formation for many fracture calcite samples on the basis of mineral assemblages.

5.3

Carbon isotopes

While oxygen isotope ratios are suggestive of equilibrium between present-day groundwaters and the late-stage fracture calcites, carbon isotope compositions provide a very different view. At the Romuvaara investigation site the carbon isotope values of fracture calcites are considerably higher than the carbon isotope values reported for groundwaters (Fig 4) by Pitkanen et al. (1996). The 8C-13 values of the late-stage fracture calcites range from -4 to -11 %o suggesting derivation of carbon predominantly from other than organic sources. In contrast, the more negative carbon isotope signatures in groundwaters (Fig. 4) are indicative of organic carbon as a major component in present-day groundwaters.

Carbon isotope systematics in carbonate and groundwater samples from the Olkiluoto investigation site seem to confirm the pattern observed at the Romuvaara site. A majority of the present-day groundwaters are depleted in C-13 relative to the isotopic composition of the fracture calcites reported by Blomqvist et al. (1992) and the data on late-stage fracture calcites given in this work (Fig. 3). A similar relationship has also been observed between fracture calcites and present-day groundwaters in the Canadian Shield (Bottomley and Veizer, 1992).

One explanation for the relatively low 8C-13 values in groundwaters relates to analytical problems. Deep groundwaters are not in equilibrium with atmospheric C02 and contain negligible amounts of dissolved carbon. Therefore, they are very sensitive to atmospheric contamination (Bishop, 1990). If groundwater samples become contaminated, the measured 8C-13 values will decrease giving an impression of biogenic origin for carbon. This possibility can not be completely excluded for the groundwater samples collected

11

before 1993. However, more recent samples have been filtered and bottled in a N 2 gas (grade 6.0) shielded glove box in a field laboratory in order to avoid contamination (Ruotsalainen et al., 1994, 1998).

Another possibility is that fracture calcite generations analysed in this study and groundwaters are not in carbon isotope equilibrium with each other. Accordingly, calcites and groundwaters would represent different fracture systems, separated either geographically or temporally.

The apparent disequilibrium could result from a bias in sampling procedures. Groundwater samples can only be collected from fractures which have a relatively high hydraulic conductivity, while many fracture calcites could actually represent fractures with a low hydraulic conductivity. At low hydraulic conductivities, groundwaters may be open with respect to the isotopic composition of oxygen, but semiclosed with respect to that of carbon. This is possible, because groundwaters contain several orders of magnitude more oxygen than carbon (see e.g. Bottomlay and Veizer, 1992). Under semiclosed conditions the isotopic composition of dissolved inorganic carbon would be buffered by dissolving carbonate bearing phases and it could be distinct from compositions in fractures with higher hydraulic conductivity.

Hydraulic conductivities in the boreholes at Olkiluoto and Romuvaara have been measured by Rouhiainen (1996) and PolHinen and Rouhiainen (1997). In some cases the measurements suggest significant groundwater flow at the sampling depths of the fracture calcites analysed in this study. Relatively high hydraulic conductivities were measured for OL-EQ3 (K=l.8E-6 m/s) and RO-EQ11 (K=2E-7 m/s). The K-value for RO-EQ20 (4.1E-10 m/s) suggests much lower flow and those for samples OL-EQ33 and RO-EQ19 are at the detection limit of the flowmeter. The relatively high conductivity values for OL-EQ3 and RO-EQ11 indicate that these fractures can not be regarded as being semiclosed with respect to carbon. It appears that the calcites from fractures with relatively high flow values and possibly also the others represent an earlier phase of groundwater flow, not related to the present day groundwater system.

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6

SUMMARY AND CONCLUSIONS

Two late-stage fracture calcite samples from the Olkiluoto investigation site and three samples from the Romuvaara site were selected at Kivitieto Oy, Oulu, for carbon, oxygen and strontium isotope analyses to be made at the Geological Survey of Finland, Espoo. Various geochemical and petrographical methods were used in order to find homogeneous calcite precipitates, possibly representing latest generations of calcite growth in bedrock fractures.

Oxygen isotope ratios indicate that all these fracture calcites, with one exception, could have been precipitated in equilibrium with present-day meteoric waters. Strontium and carbon isotope ratios of these calcites are generally in the same range as the isotope ratios reported in the literature for other fracture calcites from the Fennoscandian and Canadian shields. A relatively radiogenic Sr-87/Sr-86 value of about 0.757 for one ofthe Olkiluoto samples suggests precipitation in an isolated pocket, where Sr is derived preferentially from the alteration of K and Rh bearing minerals like muscovite or biotite.

The carbon isotope ratios of the late-stage fracture calcites from the Olkiluoto and Romuvaara investigation sites are enriched in C-13 relative to the published values for dissolved inorganic carbon in groundwater samples collected from boreholes. The apparent disequilibrium suggests that these fracture calcites do not represent those fracture systems that have been sampled for dissolved inorganic carbon. It is possible that groundwater samples have been derived preferentially from fracture systems with relatively high hydraulic conductivity, while fracture calcite samples could include fractures with low hydraulic conductivity. As the reported hydraulic conductivities suggest significant groundwater flow at the sampling depths of the fracture calcites analysed in this study, the present-day conditions are not semi-closed with respect to carbon. These fracture calcites seem to represent an earlier stage of groundwater flow, not related to the present-day groundwater system.

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REFERENCES

BISHOP, P.K. (1990) Precipitation of dissolved carbonate species from natural waters for 8 13 C analysis- A critical appraisal. Chemical Geology (Isotope Geoscience Section) 80, 251-259. BLOMQVIST, R., NISSINEN, P., FRAPE, S. (1992) Dating of fracture minerals from Olkiluoto in Eurajoki (in Finnish with an English abstract), Teollisuuden Voima Oy/Site Investigations. Technical Report 92-27. BOTTOMLEY, D.J., VEIZER, J. (1992) The nature of groundwater flow in fractured rock: Evidence from the isotopic and chemical evolution of recrystallized fracture calcites from the Canadian Precambrian Shield. Geochimica et Cosmochimica Acta 56, 369-388. COPLEN, T.B., KENDALL, C., HOPPLE, J. (1983) Comparison of stable isotope reference samples. Nature 302, 236-238. FRIEDMAN, 1., O'NEIL, J.R. (1977) Compilation of stable isotope fractionation factors of geological interest. In FLEISCHER M. (ed.) Data of Geochemistry, 61h ed., Geological Survey Professional Paper 440-KK. U.S. Government Printing Office, Washington. HELENIUS, J., KARTTUNEN, V., HATANPM, E., MAKINEN, R. (1998a). Groundwater sampling from deep boreholes OL-KR2, OL-KR3, OL-KR4, OL-KR5, OLKR8, OL-KR9 and OL-KR10 at Olkiluoto, Eurajoki in 1997. Posiva Work Report 98-23 (in Finnish with English abstract). HELENIUS, J., KARTTUNEN, V., HATANPM, E., MAKINEN, R. (1998b). Groundwater sampling from deep boreholes KI-KR1, KI-KR5 and KI-KRlO at Kivetty, Aanekoski and deep boreholes RO-KR3, RO-KR4, RO-KR8 and RO-KR10 at Romuvaara, Kuhmo in 1997. Posiva Work Report 98-45 (in Finnish with English abstract). KARHU, J.A. (1993) Paleoproterozoic evolution of the carbon isotope ratios of sedimentary carbonates in the Fennoscandian Shield. Geological Survey of Finland, Bulletin 3 71, 87p. McNUTT, R.H., FRAPE, S.K., FRITZ, P., JONES, M.G., MaCDONALD, I.M. (1990) The 87 Srl6Sr values of Canadian Shield brines and fracture minerals with applications to groundwater mixing, fracture history, and geochemistry. Geochimica et Cosmochimica Acta 54, 205-215. O'NEIL, J.R. CLAYTON, R.N., MA YEDA, T.K. (1969) Oxygen isotope fractionation in divalent metal carbonates. Jour. Chemical Physics 51, 5547-5558.

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PITKANEN, P., SNELLMAN, M., VUORINEN, U., LEINO-FORSMAN, H. (1996) Geochemical modelling study on the age and evolution of the groundwater at the Romuvaara site. Posiva Report 96-06. PITKANEN, P., LUUKKONEN, A., RUOTSALAINEN, P., LEINO-FORSMAN, H., VUORINEN, U. (1998). Geochemical modelling study on the age and evolution of the groundwaters at the Olkiluoto site. Posiva Report 98-10 (to be published). POLLANEN, J., ROUHIAINEN, P. (1997). Groundwater flow measurements at the Olkiluoto site in Eurajoki, boreholes KR1-KR4 and KR7-KR9. Posiva Oy, Work Report-97-27e. ROUHIAINEN, P. (1996). Groundwater flow measurements at the Romuvaara site in Kuhmo, boreholes KR1-KR4 and KR7-KR9. Posiva Oy, Work Report PATU-95-38e. RUOTSALAINEN, P., SNELLMAN, M. (1996). Hydrogeochemical baseline characterisation at Romuvaara, Kivetty and Olkiluoto, Finland. Posiva Work report PATU-96-91e. RUOTSALAINEN, P. (ed.), SNELLMAN, M., HELENIUS, J., KEINONEN, M., VAAHTERA, V., KUUSELA, H., OKSA, M. (1994). Field manual for the water sampling ofTVO. TVO/Site investigations. Work report PATU-94-28. RUOTSALAINEN, P. (ed.), SALONEN, 0. (ed.), HATANPAA, E., HELENIUS, J., KARTTUNEN, V., KEINONEN, M., LAAKSO, T., RANTANEN, M., SELLGE, R. (1998). Field manual for the water sampling ofPosiva, Updated version 1998. Posiva. Work report 98-54, rev. 2.