J . geol. Soc. London, Vol. 140, 1983, pp. 549-550. Printed in Northern Ireland.
Water-rockinteractioninKraflaand
Reykjanes geothermal systems,
Iceland
An extended abstract
Arny Erla Sveinbjornsdottir
Hydrothermalalteration
in twocontrastinggeothermal systems in Iceland, Krafla and Reykjanes, has
been
studied.
Both
are
high
temperature
areas
(Bodvarsson
1961),
where
subsurface
temperature
exceeds 200°C at relatively
shallow
depth.
The
alteration patterns in these systems are quite similar,
inspite of contrastingfluids.TheReykjaneshydrothermal system contains ahighlysaline(Cl
= 19,260
ppm; Bjornsson et al. 1972) circulating fluid at depth
due to influx of seawater, whereas at Krafla the fluid is
non-saline meteoric water.
Oxygen isotope measurements carried out at I.G.S.,
London,demonstratethatextensiveoxygenisotope
exchangehasoccurredbetweenthehydrothermally
alteredrockandthecirculatingfluidintheKrafla
geothermal field. These exchange reactions have left
therockconsiderablylighter-about
lO%c--but have
of the fluid.
hardly affected the isotopic composition
Consequently, the waterirock ratio in the Krafla area
is very
high
(atomicratio
=10-20). Oxygenand
deuterium measurements on the hydrothermal fluid in
the Krafla geothermal field give
6I8O = -11.9%0 and
6D = -86.8%, indicating that it
is derived from the
local precipitation. This contrasts with the prevailing
view that the hydrothermal fluid in the Krafla reservoir
is derived from Vatnajokull (Iceland's largest icecap),
some 100 km S of the Krafla area (Arnason 1975). The
oxygen isotope measurements of quartz (6I8O = -0.9
= -3.6
to-9.2%~)from
to -7.7%) and
calcite
various
depths
in
the
Krafla
system
(hole
KJ-7)
indicate that both phases are close to equilibrium with
the hydrothermal solution at present field temperature
determinedbyStefansson(1981).Unfortunately,in
only one sample was it possible to analyse both calcite
and quartz. Their calculated equilibrium temperature
was similar to the present field temperature, suggesting isotopic equilibrium.
The hydrothermally-altered rocks analysed from the
Reykjanes area are isotopically lighter than the nearby
fresh surface basalt, due to isotopic reactions with the
hydrothermal fluid. The result is about 2-3700 depletion.However,accordingtoSpooner
et al. (1974)
interaction between basaltic rock and sea-water at low
temperature (S300"C) will result in " 0 enrichment of
thebasalticrock.Theisotopiccomposition
of the
Reykjanes
rock
indicates
therefore
that
at
some
earlier stage the circulating fluid was lighter (i.e. more
meteoric) than at present (6"O = -1.08700,Olafsson
& Riley1978).Hydrothermallygrownquartz
= 4.6 to 7.7%0) fromvariousdepthsatReykjanes
(hole RN-8) appears also to record a lighter fluid than
nowpresentinthesystem.However,calcite
(6'*O
= 6.5 to 12.8%) is found to be in equilibrium with the
presentfluidatthepresenttemperature
given by
Hauksson (1981), but indicates that the hydrothermal
water at Reykjanes is isotopically heterogeneous, i.e.
it gets isotopically heavier with increasing depth due to
more
marine
influences.
These
results
reflect
the
different rate of isotopicequilibration of quartz and
calcite. Calcite has been shown to equilibrate rapidly
isotopicallywithaqueoussolutionattemperatures
downto
200°C(O'Neil
& Mayeda1969),whereas
experimental work on quartz indicates that it
is very
difficult toequilibrateisotopicallyevensmallquartz
grains at temperature c6OO"C (Blattner & Bird 1974).
The isotope data indicate, therefore, that the Krafla
geothermal fieldisisotopicallyinequilibriumatthe
present
condition
of thesystem,whereas
in the
Reykjanes field somerelics of its earlier history are
recorded, although the system has been in the present
statelongenoughto
allowcalcite tore-equilibrate.
Accordingly,theisotoperesultsencouragefurther
thermodynamiccalculations of equilibriumbetween
the aqueous phase and the mineral assemblages.
Thermodynamic calculations on the solubility of the
majoralterationphases
inthesetwoIcelandicgeothermalsystemswerecarriedoutusinggeochemical
data from Helgeson and co-workers (Helgeson
1969;
Helgexon er al. 1978). Activitydiagramswereconstructed and corrections made for the actual compositions of the
alteration
phases
as
determined
by
microprobe analyses.
Interpretation of the activity diagrams constructed
for conditions in the Krafla field suggests that the deep
fluid is in equilibrium with chlorite and actinolite, but
epidote is apparently a metastable phase in the lowest
part of the well but
becomes
stable
higher
up.
However, the hydrothermal mineral assemblage in the
bottom part of the Reykjanes well seems to be in ionic
equilibrium with a slightly more concentrated solution
than the mean fluid from the well.Thisresult
is in
agreement
with
the
isotope
interpretation,
which
suggested a heterogeneous fluid in the Reykjanes well.
ACKNOWLEDGMENTS:
I am indebted to the staff at the I.G.S.
stable isotope laboratory in London, where I carried out the
isotope measurements, especially the assistance of Dr Max
Coleman, Mrs Margaret Cox, and John E. Rouse. The
0016-7649/83/0700-0549$02.00
0 1983 The Geological Society
550
Arny Erla Sveinbjornsdottir
Department of Earth Sciences, University of Cambridgealso
dueto
my supervisor, Dr Bruce Yardley, for his
kindly provided mewithmicroprobefacilities.
Thanks are guidance throughout this project.
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of Environmental Sciences, University
ARNYERLASVEINBJORNSDOTTIR, School
of East Anglia, Norwich NR4 7TJ.