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
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
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
Working Report 99-39
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.
3
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.
5
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
9
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.
10
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.
12
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.
13
7
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.
14
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.