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POSIVA Report 1998-4

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Name:

An Isotopic and Fluid Inclusion Study of Fracture Calcite from Borehole OL -KR1 at the Olkiluoto Site, Finland

Writer:

Alexander Blyth; Shaun Frape; Runar Blomqvist; Pasi Nissinen; Robert McNutt

Language:

English

Page count:

55

ISBN:

951-652-042-1; 1239-3096

Summary:

Working report: POSIVA-raportti POSIVA 98-04, 56 sivua
ISBN 951-652-042-1


AN ISOTOPIC AND FLUID INCLUSION STUDY OF FRACTURE
CALCITE FROM BOREHOLE OL-KR1 AT THE OLKILUOTO SITE,
FINLAND


ABSTRACT

The study of calcite fracture fillings provides an opportunity to understand the thermal
and fluid history of a rock formation. Stable isotope, petrographic, and fluid inclusion
investigations are used to determine the temperature and chemical nature of the fluids
that precipitated these calcites.

Based on fluid inclusion analysis of fracture calcite from borehole OL-KR1 at the
Olkiluoto site, Finland, three generations of calcite fillings can be recorded, each
associated with a different fluid. The earliest calcites formed from a high temperature,
low salinity Na-Cl fluid that was probably the result of water/rock interaction involving
meteoric water or seawater during a hydrothermal event. Another possibility is a
meteoric water recharged some time in the past when the Fennoscandian Shield was
located close to the Equator. The next calcite type appears to be the result of a relatively
low salinity fluid, probably of a Na-Cl composition and possibly of magmatic or
basinal brine origin. The last group of calcites records two distinct fluids: a low-salinity,
probably a Na-Cl fluid, and a high-salinity, probably a Ca-Cl dominant fluid. These
fluids may be the result of partial dissolution of previous calcites by a magmatic water
or basinal brine and seawater/ meteoric water mixture.

Each calcite group has a specific stable isotope range, mineral morphology, and
formation temperature range. None of the calcites are in equilibrium with current
groundwaters found in the borehole OL-KR1 based on fluid inclusion analysis and
calculated d18O values. The waters that precipitated the calcites of the observed calcite
groups mostly had d18O signatures from -1.2 to +12.1 ‰ (SMOW), non-typical for
present-day meteoric waters of the Fennoscandian Shield. Calculated d18O signatures
close to present-day meteoric waters were only recorded from the group of platy, grey
calcites (with negative values down to -6.1 ‰, SMOW).


Keywords: Calcite, fluid inclusion, fracture infilling, crystallization temperature, salinity, paleo-fluid,
d13C, d18O, groundwater


POSIVA-raportti POSIVA 98-04, 56 sivua
ISBN 951-652-042-1


AN ISOTOPIC AND FLUID INCLUSION STUDY OF FRACTURE
CALCITE FROM BOREHOLE OL-KR1 AT THE OLKILUOTO SITE,
FINLAND


ABSTRACT

The study of calcite fracture fillings provides an opportunity to understand the thermal
and fluid history of a rock formation. Stable isotope, petrographic, and fluid inclusion
investigations are used to determine the temperature and chemical nature of the fluids
that precipitated these calcites.

Based on fluid inclusion analysis of fracture calcite from borehole OL-KR1 at the
Olkiluoto site, Finland, three generations of calcite fillings can be recorded, each
associated with a different fluid. The earliest calcites formed from a high temperature,
low salinity Na-Cl fluid that was probably the result of water/rock interaction involving
meteoric water or seawater during a hydrothermal event. Another possibility is a
meteoric water recharged some time in the past when the Fennoscandian Shield was
located close to the Equator. The next calcite type appears to be the result of a relatively
low salinity fluid, probably of a Na-Cl composition and possibly of magmatic or
basinal brine origin. The last group of calcites records two distinct fluids: a low-salinity,
probably a Na-Cl fluid, and a high-salinity, probably a Ca-Cl dominant fluid. These
fluids may be the result of partial dissolution of previous calcites by a magmatic water
or basinal brine and seawater/ meteoric water mixture.

Each calcite group has a specific stable isotope range, mineral morphology, and
formation temperature range. None of the calcites are in equilibrium with current
groundwaters found in the borehole OL-KR1 based on fluid inclusion analysis and
calculated d18O values. The waters that precipitated the calcites of the observed calcite
groups mostly had d18O signatures from -1.2 to +12.1 ‰ (SMOW), non-typical for
present-day meteoric waters of the Fennoscandian Shield. Calculated d18O signatures
close to present-day meteoric waters were only recorded from the group of platy, grey
calcites (with negative values down to -6.1 ‰, SMOW).


Keywords: Calcite, fluid inclusion, fracture infilling, crystallization temperature, salinity, paleo-fluid,
d13C, d18O, groundwater


Keywords:

Calcite; fluid inclusion; fracture infilling; crystallization temperature; salinity, paleo-fluid; ()13C; ()18O; groundwater

File(s):

An Isotopic and Fluid Inclusion Study of Fracture Calcite from Borehole OL -KR1 at the Olkiluoto Site, Finland (pdf) (1.8 MB)


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