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109
Mineralogical and thermodynamic constraints on
Palaeogene palaeotemperature conditions during low-grade metamorphism of basaltic lavas recovered from the Lopra-1/1A deep hole, Faroe Islands
William E. Glassley
The sequene of secondary minerals that are reported for the Lopra-1/1A well records progressive
zeolite facies to prehnite-pumpellyite-facies mineral progressions consistent with those of other well- studied hydrothermally altered rock sequences. Detailed comparison of the calc-silicate (zeolites and prehnite) mineral distributions of the Lopra-1/1A sequence with those from other regions indicates that this sequence exhibits consistently longer down-hole intervals for secondary mineral species than reported elsewhere. When compared to measured down-hole temperatures reported in other hydro- thermally altered regions, the results suggest that the Lopra-1/1A mineral progression formed under conditions typical of low temperature hydrothermal systems that form shortly after eruption of thick basaltic piles. Maximum temperatures achieved at the 3500 m level of the well were at or below 200°C. The implied geothermal gradient was less than 50°C/km. An analysis of prehnite - fluid composition relationships was also conducted in order to determine if results compatible with the paragenetic sequence study could be obtained from thermodynamic constraints. In this case, the limiting temperature for prehnite formation in equilibrium with albite-quartz-calcite-laumontite (the mineral assemblage at the bottom of the hole) was determined for a range of fluid compositions. The resulting calculations suggest temperatures of formation of prehnite in the range of 140°C to 205°C, a conclusion which is broadly consistent with those reached from study of the paragenetic relationships. Comparison of these results with other studies of palaeogeothermal gradients of the North Atlantic margins suggests a consistent pattern in which relatively low geothermal gradients persisted in the Palaeogene rift basin.
Keywords
: North Atlantic Volcanic Province, thermal history, geothermal gradients, low temperature metamor-
phism, fluid-rock interaction, reactive transport, zeolites, prehnite-pumpellyite
_______________________________________________________________________________________________
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
. E-mail:
glassley1@llnl.gov
Minerals
that
crystallise
from
basaltic
lavas
are
unstable
with
respect
to
a
wide
range
of
hydrous
silicates
and
carbonates
when
subjected
to
low
temperature
conditions(< 300°C)
in the presence of H2O- and CO2-bearing fluids. Recry-
stallisation of basaltic rocks under these physical and chem-
ical conditions results in the development of minerals that characterise the zeolite, prehnite-pumpellyite and green- schist facies. It has been well-documented that the basalts of the East Greenland - Faroe Islands province record ex-
tensive development of minerals characteristic of the zeo-
lite and lower prehnite-pumpellyite facies (Jørgensen 1984,
1997; Neuhoff
et al.
1997; Larsen
et al.
1999). What re-
mains unclear is the temperature history recorded by these mineral assemblages.
Generally,
under
the
lowest
temperature
conditions,
clays, zeolites and hydrous Fe-Mg silicates form, giving
way to less hydrated minerals at higher temperatures. Often this progression is recorded by the presence of a
© GEUS, 2006.
Geological Survey of Denmark and Greenland Bulletin 9, 109-118.
Available at:
www.geus.dk/publications/bull
GEUS Bulletin no 9 - 7 juli.pmd
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109
110
complex sequence of zeolite minerals that have increas-
ingly smaller amounts of molecular water bound in their structures (Bird et al. 1984; Neuhoff & Bird 2001). In principle, therefore, zeolitic and related minerals can be sen-
sitive indicators of temperature conditions.
This temperature sensitivity is complicated by the equal-
ly important sensitivity of the zeolites to the composition
of coexisting fluids. The thermodynamic properties of the zeolites are affected by substitution between the alkali metals,
particularly
Na,
K and Ca, and Al-Si exchange (e.g.
Neuhoff
et al.
1997, 2002, 2003, 2004). The stability fields
of the zeolites are also sensitive to the ratio of calcium activity to hydrogen ion activity (i.e. [Ca++]/[H+]2) in
the coexisting fluid phase (e.g. Surdam 1973; Bird
et al.
1984). Hence, fluid chemistry has a strong influence on both the mineral compositions that develop and the spe- cific mineral phases that form during low temperature recrystallisation.
The purpose of this paper is to define likely bounds for
bottom-hole temperatures and the likely geothermal gra-
dient active at the time of mineral development, based on paragenetic relationships and thermodynamic constraints, taking into account the effects of fluid chemistry. Detailed descriptions of the locations, mineralogies and geological settings for the Lopra-1/1A and Vestmanna-1 boreholes are presented in other chapters in this book and are only summarised here.
Geology
The basalts of the Faroe Islands were erupted subaerially
onto continental crust during opening of the northern North Atlantic. The basalts have been divided informally into an upper, a middle and a lower formation. The lower basaltic sequence is more than 3000 m thick (established on the basis of field exposure and the Lopra-1/1A drilling programme), and ranges in age from c. 58.8 to 56.5 Ma (Waagstein et al. 2002). The overlying basalts and sedi- ments (some of the sediments are coal-bearing) are more than 2000 m thick and were erupted between c. 56 and 55.5 Ma (Larsen et al. 1999). Recrystallisation of the la- vas took place during subsequent burial, leading to the development of a wide range of zeolites and associated calc-silicate minerals (Jørgensen 1984, 1997). The argu- ment that the secondary mineral development results from burial metamorphism, rather than significant tectonic stacking or folding, is based on the relatively flat-lying nature of the basaltic flows and the absence of any kine- matic fabric.
Methods
Compiled published data
Published data from active hydrothermal systems where
temperatures and mineral associations are recorded, pro- vide the most direct evidence of the conditions under which specific mineral assemblages occur. For this reason, published data from a variety of drilled hydrothermal sys- tems with depths less than 4000 m were analysed to iden- tify temperature constraints that would apply to the min-
eral
associations
reported
for
samples
from
the
Lopra-1/1A
drilling
programme(Jørgensen
1984,
1997).
The
reported
Lopra-1/1A assemblages were confirmed by the author
during independent examination of thin sections.
The best available data that correlate downhole tem-
peratures, depth and mineral occurrences are from geo-
thermal systems in Iceland (Kristmannsdóttir & Tomassón 1976), Japan (Seki et al. 1969; Boles 1981), Cerro Prieto (Bird et al. 1984), Wairakei (Steiner 1977) and Toa Baja (Cho 1991). The reports from Iceland and Japan discuss secondary mineral development related to alteration of basaltic rocks, which most closely correspond to the Lopra-
1/1A sequence. The Cerro Prieto locality consists of sedi-
mentary rocks (sandstones, siltstones and mudstones) that are predominately composed of quartz and feldspars. The Wairakei and Toa Baja localities consist of volcanic and volcanoclastic rocks and their associated clastic derivatives. The Wairakei rocks are primarily rhyolitic and the Toa Baja rocks primarily andesitic.
This suite of rock types spans the entire range from
basalts through andesites to rhyolites, thus encompassing
silica-poor to silica-rich compositions with varying abun- dances of alkali metals. On a whole-rock basis, then, the compositional range from these reported systems bounds that of the Faroe Island basalts considered here.
The different tectonic settings represented by these sys-
tems include both rift and convergent margin environ-
ments. Since these different settings evolved through dif- ferent thermal histories, it is likely that the possible ther- mal conditions that may have affected the Faroe Island basalts, will be represented by at least some of the data recorded in the published studies.
The range of fluid compositions at the various sites is
broad. The Cerro Prieto fluids were concentrated solu-
tions with high total dissolved solids and salinities, while many of the solutions reported from the New Zealand region, particularly within the Broadlands-Ohaki (Heden- quist 1990) and Wairakei areas, included CO2-rich and
neutral-pH chloride waters and CO2-poorer fluids oc-
curred within the Iceland system. Thus, the published re-
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111
ports examined include a range of solutions that are likely
to encompass those that may have been present during alteration of the Faroe Island basalts.
Clear differences exist between sites with regard to the
depth and extent of secondary mineral development, re-
flecting the effects of these combined intensive and exten- sive variables (i.e. T, bulk composition, fluid composition etc.). By considering this broad range of systems, it is pos- sible to develop some insight into the extent to which dif- fering geothermal and chemical conditions influenced the development of the mineral associations and how that influence is expressed at the Lopra-1/1A site. Compari- son of the Lopra-1/1A suite with these reported mineral parageneses should provide a strong bound to the ther- mal gradient inferred from these data.
In this study, attention is focused on the calc-silicate
mineral suite, which is comprised of the components
CaO-Na2O-Al2O3-SiO2-H2O-CO2. Although potassi-
um may play an important role in some of these mineral
phases, particularly in zeolites where it may substitute for Na and Ca, it was not considered in this study because it is
generally
low
in abundance in minerals that are character-
istically part of the calc-silicate series in basaltic systems.
The minerals of interest in the calc-silicate system for
the purposes of this study are the zeolites, prehnite, cal-
cite and zoisite-clinozoisite (which are proxies in this study for epidote). This system was selected for detailed consid- eration because it is the most thoroughly characterised for low-grade mineral development. These minerals pos- sess well-characterised structures and compositions. In addition, there has been a long history of research in the geochemical community to derive thermodynamic data for phases in this system (Liou 1971; Glassley 1974; Frey et al. 1991; Neuhoff et al. 1997, 2002; Fridriksson et al. 2001; Neuhoff & Bird 2001). Although of immense im- portance in determining relative conditions in shallow (< 3000 m), low temperature (< 150°C) systems, the clay minerals and chlorites exhibit such structural and com- positional complexity that the thermodynamic data avail- able for modelling their behaviour remain inadequate. For that reason, they are not considered further in this report, although work continues on them.
Consideration of the calc-silicate system also eliminates
complexities that arise due to the effects of variable oxy-
gen partial pressures, which can dramatically influence the stability of iron-bearing mineral phases. Hence, chlo- rites, smectites, Fe-oxy/hydroxides and related phases are not considered here. Two exceptions are considered in this paper. Pumpellyite, which is noted in several other studies
and documented as a mineral phase of limited distribu-
tion at Lopra-1/1A, is considered here as part of the para-
genetic assemblage, but does not play an important role
in establishing the conclusions presented later. Prehnite is also considered here and does possess limited solid solu- tion with an Fe3+ end member. Measured mole fractions
in a limited suite of analysed prehnites (unpublished data
1999, R. Waagstein) average 0.08, with a range from 0.00 to 0.20 for 18 samples. Rose & Bird (1987) have shown that solid solution of as little as 10% of the Fe end mem- ber in Al-rich prehnite can significantly affect prehnite stability. Although the majority of prehnites analysed in the Lopra-1/1A rocks fall below this value, the impact of this effect must be borne in mind and is discussed later in this paper.
Although the stability fields of many of these minerals
are reasonably well established for their ideal composi-
tional end-members, each of these minerals belongs to a solid solution series. Generally, there are very little or no quantitative data available regarding the actual composi- tions of mineral phases in the low-grade rocks described in the referenced reports. In addition, thermodynamic mix-
ing properties of the solid solutions are generally not avail-
able. Hence, when comparing stability relationships from one locality to another, it must be borne in mind that uncertainties of unknown magnitude are inherent in the comparison due to possible differences in the composi- tions of the minerals.
Thermodynamic calculations
Once mineral assemblages and distributions were com-
piled, the sensitivity of mineral development to thermal conditions and composition of coexisting fluids was mod- elled. This effort was undertaken because textural and compositional properties of these secondary minerals at- test to the importance of mass transport involving car- bonate-bicarbonate-bearing aqueous fluids. The thermo- dynamic properties of such solutions influence strongly the stability fields of the minerals and can thus be an ad- ditional means of placing limits on the physical condi- tions at the time of mineral growth.
The calculations employed the aqueous speciation/re-
action progress software EQ3/6 (Wolery & Daveler 1992),
using the .com database. The modelling was accomplished by performing speciation calculations over a range of tem- peratures and compiling the affinities of the possible solid phases that may develop in this system. Affinity here is defined as:
A = 2.303RT log(Q/K)
where A is the affinity (in calories), R is the universal gas
GEUS Bulletin no 9 - 7 juli.pmd
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112
constant (1987 calories/mole-degree Kelvin), T is tem-
perature (Kelvin), Q is the activity product for the rele- vant species in the applicable hydrolysis reaction and K is the equilibrium constant for that same reaction. Affini- ties greater than zero identify mineral phases that are su- persaturated in the water at the specified conditions and affinities less than zero identify mineral phases that are undersaturated for those same conditions. Positive affini- ties thus correlate with minerals that would be expected to precipitate from solution or form from mineral reac- tions in the rock, whereas negative affinities indicate that the respective mineral phase will dissolve, if present.
Particular attention was given to the development of
prehnite since its compositional variability is less than that
of the zeolites and its thermodynamic properties are bet- ter constrained. The affinities were calculated assuming in all cases that the system was saturated in quartz, lau- montite and albite, since these phases coexist with preh- nite (see below). These solids were used to constrain the activities of aqueous SiO2, Al3+ and Na+, respectively. The
same simulations were repeated assuming that calcite was
present as a control for Ca++ activity to determine the sen-
sitivity of the results to this change in the system con-
straints. At the beginning of all of the simulations, it was
assumed that the hydrogen ion activity was near neutral
at the temperature considered. The initial fluid composi- tion (a dilute, neutral-pH water at the temperature con- sidered) was not in equilibrium with the constraining mineral phases but, for each simulation, was allowed to evolve toward equilibrium with the constraining mineral phases. The equilibrium fluid composition that evolved thus represented the composition of an aqueous fluid in equilibrium with the constraining phases and was the be- ginning point for further simulations that considered the effects of temperature and other compositional variables.
The sensitivity of the results to variations in total Cl-
and HCO3- was also considered. In this case, the simula-
tions were conducted for Cl- concentrations between 14
mg/l and 14.410 mg/l, and HCO3- concentrations be-
tween 10 mg/l and 1000 mg/l. This range of values was
selected because it encompasses the vast majority of water compositions from hydrothermal systems around the world (see compilations and discussions in Roedder 1972; Ellis & Mahon 1977; Arnorsson et al. 1983; Fournier 1985).
Results
The depth intervals over which individual minerals occur
at the Lopra-1/1A site are summarised in Fig. 1. Note- worthy in this compilation is that the progression with depth of the zeolite sequence is consistent with that from other localities (see summaries below under 'Compiled data'), and that epidote does not occur, even at the deep- est levels. Also of significance is that most of the minerals persist over depth intervals that exceed significantly any other reported occurrence for that mineral.
Compiled data
The published temperature-depth data compiled from
Iceland (Kristmannsdóttir & Tomassón 1976), Japan (Seki et al. 1969; Boles 1981), Cerro Prieto (Bird et al. 1984), Wairakei (Steiner 1977) and Toa Baja (Cho 1991) are shown in Figs 2-4. For each location, the depth interval over which a mineral occurs is indicated by connected symbols that link the high and low temperature and depth points that define the extent of the mineral phase.
Figures 2-4 also show the depth intervals over which
mesolite, stilbite, heulandite, laumontite and prehnite
occur in the Lopra-1/1A samples (Jørgensen 1984, 1997). The Lopra-1/1A depth-temperature relationships were constrained to be consistent with the following criteria:
0
1000
2000
3000
4000
Pr
eh
n
ite
Heula
n
dite
Mor
de
n
ite
Scolecite
Mesolite
Stilbite
A
n
alci
m
e
Tho
m
so
n
ite
W
airakite
Lau
m
o
n
tite
Pu
m
pell
yite
Depth
(
m
etr
es belo
w surface)
Bottom of hole
Fig. 1. Summary of depth distributions for minerals reported in
the Lopra-1/1A samples (compiled from Jørgensen 1984, fig. 4; 1997, fig. 1). Zero depth corresponds to the ground surface at the drill site. The bottom of the well is indicated. Minerals are ar- ranged along the horizontal axis in a sequence of increasing depth to the right. The depth intervals correspond to the reported oc- currences where the individual minerals are most abundant. In some instances, spot occurrences of minerals occur outside the indicated intervals. Such occurrences can result from local varia- tions in rock or fluid chemical conditions, or the consequences of locally controlled reaction kinetics, and are not plotted here.
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1. Coexistence of analcime and albite is constrained by
Cho (1991) to temperatures less than
c.
120°C. Since
albite is ubiquitous in the Lopra-1/1A volcanics, the maximum depth occurrence for analcime ( c. 1850 m) is assumed to mark the c. 120°C isotherm.
2. Laumontite coexisting with prehnite is constrained to
temperatures less than 160°C (Varna 1989). Since lau-
montite and prehnite occur together over a depth of more than 1000 m and extend to the bottom of the Lopra-1/1A hole, this constraint would place the base of the studied sequence at temperatures less than 160°C.
3. Epidote is considered to require minimum tempera-
tures for development of 200°C (Bird
et al.
1984). The
exception to this would be systems rich in Fe3+ (Varna
1989), which the Lopra-1/1A basalts are not. Epidote
is not reported within the Lopra-1/1A rocks, hence the bottom-hole temperature must be less than 200°C.
4. Pumpellyite requires temperatures in excess of 125°C
for stable growth (Evarts & Schiffman 1983; Bevins
Fig. 2. Temperature-depth distributions reported from active ther-
mal systems for the zeolites chabazite, scolecite-mesolite, mor- denite, stilbite and heulandite. Lines between points indicate the temperature-depth intervals over which the minerals are reported to occur. Data sources are: Kristmannsdóttir & Tomassón 1976 for Iceland; Seki et al. 1969 and Boles 1981 for Japan; Bird et al. 1984 for Cerro Prieto, Baja California; Steiner 1977 for Wairakei, New Zealand; Cho 1991 for Toa Baja, Puerto Rico. The solid line labelled LOPRA is the geothermal gradient derived in Fig. 2, with the depth intervals for Lopra mesolite, stilbite and heulandite in- dicated. Thor. , Thorlakshofn, Iceland; Reyk. , Reykjavik, Iceland; Nesj. , Nesjavellir, Iceland.
Fig. 3. Temperature-depth distribution for laumontite and preh-
nite. Laumontite occurrences are from Iceland, Japan and Toa Ba- ja, and prehnite from Iceland, Toa Baja and Cerro Prieto (see Fig. 3 for references and abbreviations). Also shown for comparison is the inferred temperature-depth distribution for the same Lopra minerals along the derived geothermal gradient (Fig. 2).
Prehnite
Epidote
Depth
(metres below surface)
Te
m
peratur
e
(°C)
Reyk.
Toa Baja
Nesj.
Cerro Prieto
Krafla
Thor.
Wairakei
4000
200
100
400
300
0
LOPRA
0
1000
3000
2000
0
50
100
150
200
250
0
1000
2000
3000
4000
Depth
(metres below surface)
Te
m
peratur
e
(°C)
Low T limit of
epidote
High T limit of
analcime + albite
Low T limit of
pumpellyite
et al.
1991). The first appearance of pumpellyite is at a
depth of c. 2300 m, thus constraining the 125°C iso- therm to be near this depth.
These observations were used to construct a palaeogeo-
therm (Fig. 3). In developing this palaeogeotherm, points 1 (the constraint on analcime and albite coexistence) and 4 (the minimum temperature for pumpellyite develop- ment) were accepted without qualification. It was also as-
sumed that the mean annual surface temperature was 10°C
and that the bottom-hole temperature was c. 200°C. The 200°C bottom-hole temperature, which exceeds the 160°C constraint inferred from coexistence of prehnite and lau- montite (point 2), was used to assure a conservative esti- mate of maximum thermal conditions and represents a compromise between points 2 and 3. In other words, the temperature gradient developed by this approach will over- estimate maximum likely thermal conditions.
The
resulting geothermal gradient is linear. Least squares
regression of the data points gives a correlation of fit of
0.9949 and a gradient of 0.05°C/m, or 50°C/km.
Using this geothermal gradient, the depth intervals for
mesolite, stilbite and heulandite were plotted to be con-
sistent with the permissible measured distance over which these minerals occur. Laumontite and prehnite were placed to be consistent with the implied thermal gradient and temperature constraints, as described above.
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114
250
200
150
100
50
0
Depth
(metres below surface)
Chabazite
Mordenite
Stilbite
Heulandite
Scolecite-mesolite
Reyk.
Toa Baja
Thor.
Nesj.
Krafla
Thor.
LOP
RA
Te
m
peratur
e
(°C)
0
1000
2000
3000
4000
This reconstruction provides a conservative estimate of
the temperature gradient only if the extent of surface ero-
sion since mineral development is small and if there has been minimal tectonic rotation of the volcanic sequence. The consequence of these points is elaborated on below. The following observations are significant for reconstruct- ing conditions recorded in the Lopra-1/1A samples.
1. The zeolite group of minerals is stable at temperatures
throughout the range 40°C to 210°C (Figs 2-4). The
only reported occurrence of zeolites at higher tempera- tures is from the Wairakei, New Zealand, geothermal field, where wairakite is stable at temperatures of 240°C to 250°C, where it coexists with epidote. This is not an assemblage reported from Lopra-1/1A. The corre- sponding geothermal gradients for Wairakei range from a high of > 400°C/km to a low of c. 40°C/km. The highest temperature gradients require active volcanic/ magma systems and are not typical of most environ- ments. Nevertheless, the stability relationships for min- erals from these systems provide useful information for defining thermal stability limits for the minerals be-
ing considered. It should be noted, too, that the high-
er temperature conditions likely reflect convective hy- drothermal environments with highly non-linear geo- thermal gradients. Inevitably, lower geothermal gradi- ents result in a particular mineral being observed over a much longer interval. This then implies that, for a
given combination of rock- and fluid-compositional
characteristics, the lower the temperature gradient, the greater will be the depth range of a borehole over which a particular mineral will occur.
2. Although local conditions (such as rock composition,
coexisting fluid chemistry, local gas chemistry) at each
site determine the exact zeolite sequence, the sequence of minerals generally follows one in which zeolites with high contents of molecular water (e.g. chabazite, scol- ecite, mesolite) are progressively replaced by zeolites with lower contents of molecular water (e.g. heulan- dite and laumontite) at higher temperatures.
3. In all cases considered, the assemblage prehnite-lau-
montite formed near the upper stability field of the
zeolites and prior to the appearance of epidote. The temperature range for stable laumontite is in the range 70°C to 200°C. As noted by Surdam (1973) and Bird et al. (1984), prehnite-laumontite relationships are sensitive to the activity ratio [Ca++]/[H+]2 in the fluid
Fig. 5. Temperature constraints for the indicated mineral associa-
tions or occurrences. See text for sources and assumptions. The
straight line is a least squares fit to the data points. The uncertain-
ty bars for the analcime + albite 'out' and the pumpellyite 'in' data points span 25°C, and are presented only as an inferred, reason-
able uncertainty envelope, in the absence of any available analyti-
cal data. The bar associated with the epidote lower T limit indicates
the range
of
possible
bottom
hole
metamorphic temperatures, based
on
the
alternative
constraint
that
the
maximum
temperature
for
lau-
montite
coexisting
with
prehnite
is
160°C.
See
text
for
further
de-
tails.
Fig. 4. Temperature-depth distribution for prehnite and epidote.
Epidote occurrences are from Iceland, Cerro Prieto, Toa Baja and Wairakei (see Fig. 3 for references). Also shown is the inferred temperature-depth distribution for Lopra prehnite along the de- rived geothermal gradient (Fig. 2).
Prehnite
Laumontite
Reyk.
Toa Baja
Cerro Prieto
Nesj.
Krafla
Thor.
Wairakei
Japan
LOPRA
Depth
(metres below surface)
Te
m
peratur
e
(°C)
4000
200
100
400
300
0
0
1000
3000
2000
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phase. Variation in fluid chemistry is thus the likely
cause for the broad temperature interval observed for laumontite stability.
4. In all cases, prehnite first forms at lower temperatures
than epidote. However, both occur within the higher
temperature range of the zeolites and are stable be- yond the zeolite field (Fig. 4). Prehnite, for example, is reported to be stable in the temperature range 125°C to 340°C in the reports referenced in this study. This temperature interval is the same as reported for the stable presence of epidote, although the lower tempe- rature occurrences of epidote are in systems that have high Ca++ and Fe3+ activity.
The mineral sequence recorded in the Lopra-1/1A well
(Fig. 1) is typical of that reported in other geothermal systems. The zeolite sequence follows the pattern of gen- erally decreasing molecular water content with increasing depth, reflecting the impact of elevated temperatures at deeper levels in the borehole. This observation is general- ly consistent with the view that the thermal history expe- rienced by these basalts was relatively simple.
The highest temperature mineral assemblage that has
developed is the prehnite-laumontite assemblage that is
reported from the depth interval 2100 m to 3500 m. This assemblage clearly must extend beyond the bottom of the hole to an unknown depth. Nevertheless, the 1400 m length of this assemblage is one of the longest such inter- vals reported anywhere in the world. By comparison, the Toa
Baja
prehnite-laumontite
zone,
the
longest interval
report-
ed
for
these
minerals,
has
a
total
length
of
about 850 m,
and a geothermal gradient of between 50°C/km and 70°C/
km. The inferred temperature interval over which the preh- nite-laumontite association formed at Lopra-1/1A is in- ferred to be approximately 120°C to 200°C.
Epidote does not occur in any of the samples from the
Lopra-1/1A suite. Figure 4 shows that this would require
the bottom hole temperature not to exceed c. 250°C to 350°C, which appears to be the temperature interval over which epidote is consistently observed, although lower temperature occurrences have been reported, for example at
Thorlakshofn and Reykjavik in Iceland and at Toa Baja.
As noted above, it is inferred that epidote will not form at
temperatures less than c. 200°C under conditions of low to moderate Fe3+ and Ca++ activity. It is thus assumed that
the Iceland and Toa Baja occurrences reflect chemical en-
vironments that satisfy these conditions.
The Vestmanna-1 hole, which was also part of the drill-
ing programme (Jørgensen 1984, 1997) contains mineral
assemblages typical of the shallowest levels of hydrother-
mal systems and overlap those of the Lopra-1/1A sequence.
If these mineral assemblages developed simultaneously, the computed geothermal gradient for the Lopra-1/1A se- quence would have to be considered a maximum. How- ever, uncertainty exists regarding whether these mineral sequences for these two drill holes are coeval.
Thermodynamic calculations
A suite of thermodynamic calculations, using the code
EQ3/6, was completed to determine the chemical condi- tions in the fluid phase that would constrain development of the mineral assemblage prehnite-laumontite-quartz- albite-calcite found in the wells. In these calculations, it was assumed that sodium, aluminium, calcium and silica aqueous concentrations are constrained by equilibrium with
albite,
laumontite,
calcite
and
quartz,
respectively.
The
calculated saturation state of the solution with respect to
prehnite was monitored, as temperature and bicarbonate and chloride concentrations were changed. CO2 partial
pressure was allowed to evolve in response to the equili-
brium conditions and monitored to assure that it remained within 'real world' bounds. By noting the temperature
Fig. 6. Calculated lower thermal stability limit of prehnite coex-
isting with albite-calcite-quartz-laumontite, as a function of HCO3- and Cl- concentrations in the coexisting aqueous phase.
Contours on the stability limit surface are labelled in degrees cen-
tigrade. The mineral assemblage albite-calcite-quartz-laumont- ite was used in the calculations because it represents the highest temperature mineral assemblage observed in the bottom of the Lopra-1/1A hole.
120°C
100°C
160°C
140°C
180°C
200°C
220°C
Cl-(
mg/l)
0
1.0
0.8
0.6
0.4
0.2
0
HCO
-
(
mg
/l)
3
3000
15 000
12 000
9000
6000
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and bicarbonate and chloride concentrations at which the
solution became saturated in prehnite, it is possible to delineate those conditions that bound the stability field for the prehnite-bearing mineral assemblage.
The results of the calculations are presented in Fig. 6,
which shows the contoured temperature surface for the
stability of prehnite coexisting with a bicarbonate-chlo- ride solution in equilibrium with laumontite-calcite- quartz-albite. The contours map the minimum tempera- ture required for prehnite stability in this system. It must be emphasised that the exact location of these contours is somewhat imprecisely known, due to uncertainty in the thermodynamic data. The uncertainty in the bicarbonate values is approximately ± 50 mg/l, based on interpola- tions between simulations. These results show that preh- nite stability is only slightly sensitive to the solution salin- ity (as indicated by the effect of variation in the chloride ion, Cl-, but is very sensitive to the solution carbonate/
bicarbonate
concentration.
This
behaviour
reflects
the
strong coupling between these variables and Ca specia-
tion and pH. The more concentrated the solution in terms of carbonate/bicarbonate, the higher the temperature nec- essary to achieve prehnite stability. These calculations sug- gest that the wide range of prehnite thermal stability ob- served in natural systems (Figs 2, 5) is due, at least in part, to differences in fluid composition from one location to another. This probably is true for other minerals in this calc-silicate suite as well. As documented by Rose & Bird (1987), the redox state and iron content of the fluid will also be an important variable in controlling prehnite sta- bility, due to the effect of Fe3+ substitution for Al in the
prehnite structure.
Salinities determined from a preliminary fluid inclu-
sion study of the Lopra-1/1A samples (Konnerup-Mad-
sen 1998) gave Cl- concentrations of between 0.167 and
1.49 equivalent weight per cent NaCl, which is approxi-
mately 1000 to 9000 mg/l Cl-. The analytical bicarbo-
nate ion concentrations with this salinity in natural solu-
tions in hydrothermal systems and at these temperatures and pressures are usually in the range of 200 to 800 mg/l (see compilations and discussions in Roedder 1972; Ellis & Mahon 1977; Arnorsson et al. 1983; Fournier 1985) although the actual HCO3-concentrations in the reser-
voirs will be lower than this value and be controlled by
CO2 fugacity. This implies (Fig. 6) that the mineral asso-
ciation prehnite-laumontite-calcite-quartz formed at
temperatures within the range of approximately 140°C to 205°C. This temperature interval is contained within the range of prehnite stability noted in other hydrother- mal systems (see Figs 2, 5) and is thus consistent with natural occurrences of this assemblage. It is also broadly
consistent with the inference from phase relationships
described above, in which it is suggested that this assem- blage spans the temperature interval of approximately 120°C to 200°C.
Discussion and conclusions
Secondary mineral assemblages documented for the ba-
salts recovered from the Lopra-1/1A well are similar to those reported from other hydrothermal systems. Both the specific mineral occurrences and the relative sequence of mineral stabilities define a systematic distribution that records increasing temperature with depth. The absolute length of individual mineral zones, however, is greater than at other well-documented sites, and suggests that the geo- thermal gradient at the time of mineral development was low. The mineral associations, complemented by thermo- dynamic calculations of fluid-rock equilibrium relation- ships, suggest that the temperature at the bottom of the well did not exceed 200°C, implying a maximum thermal gradient of 50°C/km (assuming a surface temperature in the range of 10 to 25°C). This gradient was constructed based on the assumption that the mineral zones are ap- proximately horizontal. There is currently no structural data available to suggest this assumption is far from accu- rate, but it remains to be established conclusively. Fur- thermore, it is also assumed that the total stratigraphic thickness at the time of mineral development did not great- ly exceed that exposed and inferred today. This assump- tion is reasonable, based on the correlations established by Larsen et al. (1999) between the East Greenland volcan-
ic
complex
and
the
Faroe
Islands. The correlations indicate
that
the
current
thickness of basalts in the Faroe Islands is
probably close to that which was originally erupted.
It has previously been suggested that mineral develop-
ment may have occurred in several discrete episodes (Jør-
gensen 1984, 1997). Such an interpretation makes more complex the sequence and timing of mineral growth and may change the absolute depth intervals over which spe- cific mineral associations formed within a given time pe- riod. This, in turn, would require reconsideration of the temperature history since such an observation could re- sult only in shorter absolute depth intervals for each min- eral development period. In this scenario, the currently observed distribution of minerals would represent the sum of the depth intervals over which an individual mineral formed at different time periods, assuming that no single episode of mineral development obliterated evidence of previous distributions of secondary mineral development. Nevertheless, the conclusion that the bottom hole tem-
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perature did not exceed 200°C would still be valid, since
that is based on the mineral association calcite-laumont- ite-prehnite-quartz, the temperature limit of which is constrained by laumontite and prehnite thermal stability and fluid composition effects.
Comparison of the derived geothermal gradient in the
Faroes with those reported for the Atlantic margin region
north of the United Kingdom and in East Greenland dem- onstrates a striking consistency that constrains evolution of the geothermal history in this region. Green et al. (1999) used fission track data from apatites as well as vitrinite reflectance data from a series of wells in the eastern North Atlantic
Province
to determine palaeogeothermal gradients.
They reported geothermal gradients of between 35°C/km
and 90°C/km, with the vast majority of the region falling within the lower portion of the range. Neuhoff et al. (1997) concluded that the zeolite facies metamorphism that af- fected East Greenland flood basalts during initial opening of the northern North Atlantic resulted from recrystalli- sation associated with a geothermal gradient of 40 ± 5°C/
km. The regional heat flow they derived from this con-
clusion is consistent with that reported from a study of metamorphic recrystallisation (Manning et al. 1993). All of these values effectively bracket the inferred geothermal gradient in the Faroe Islands and argue for early develop- ment of relatively low geothermal gradients that persisted for some time in these regions. These results, and those of Larsen et al. (1999), provide conceptual constraints on models of the thermal evolution of this part of the north- ern North Atlantic province during early continental sep- aration and basin development and argue for regions of low geothermal gradients that were not overprinted by later high heat-flow periods.
As a word of caution, it should be noted that these
conclusions are based on the simplifying assumption that
linear geothermal gradients existed during mineral growth in this region. There is substantial evidence in geothermal systems, however, that complex geothermal gradients com- monly develop, such that temperature reversals or near isothermal conditions may develop in response to the lo- cal thermal-hydrological regime, particularly in environ- ments dominated by convection-driven fluid flow. Al- though such features usually develop in regions of high heat flow and are not characteristic of environments such as the Faroe Islands region where heat flow is inferred to be low, evidence is currently inadequate to rule out this possibility conclusively. To evaluate the extent to which such behaviour occurred in the Faroe Islands volcanic prov- ince, a more detailed examination of mineral composi- tion characteristics and distributions would be required, coupled with a more detailed modelling effort.
Acknowledgements
Regin Waagstein kindly provided timely access to thin sec-
tions, mineral composition data and mineral distribution data, as well as informative discussions. His assistance greatly aided this effort. Extensive comments from Den- nis Bird and Bruce Christenson led to significant improve- ments in earlier versions of the manuscript, and are grate- fully acknowledged. The editorial wisdom of James A. Chalmers significantly improved the presentation and style of this paper.
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