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 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 07-07-2006, 14:19109 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- GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19110 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 07-07-2006, 14:19111 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 ni te H eu la nd it e M o rd en it e Sc o le ci te M es o lit e St ilb it e A na lc im e T ho m so ni te W ai ra ki te La um o nt it e Pu m pe lly it e D ep th ( m et re s be lo w s ur fa ce ) 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. GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19112 113 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 pe ra tu re ( °C ) Reyk. Toa Baja Nesj. Cerro Prieto Krafla Thor. Wairakei 4000 200 100 400 300 0 LO PR A 0 1000 30002000 0 50 100 150 200 250 0 1000 2000 3000 4000 Depth (metres below surface) Te m pe ra tu re ( °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. GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19113 114 250 200 150 100 50 0 Depth (metres below surface) Chabazite Mordenite Stilbite Heulandite Scolecite-mesoliteReyk. Toa Baja Thor. Nesj. Krafla Thor. LO PR A Te m pe ra tu re ( °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 LaumontiteReyk. Toa Baja Cerro Prieto Nesj. Krafla Thor. Wairakei Japan LO PR A Depth (metres below surface) Te m pe ra tu re ( °C ) 4000 200 100 400 300 0 0 1000 30002000 GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19114 115 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°C100°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 H C O – (m g/ l) 3 3000 15 00012 00090006000 GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19115 116 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- GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19116 117 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. References Arnorsson, S., Gunnlaugsson, E. & Svavarsson, H. 1983: The chem- istry of geothermal waters in Iceland. II. Mineral equilibria and independent variables controlling water compositions. Geochim- ica et Cosmochimica Acta 47, 547–566. Bevins, R.E., Rowbotham, G. & Robinson D. 1991: Zeolite to prehnite–pumpellyite facies metamorphism of the late Protero- zoic Zig-Zag Dal basalt formation, eastern North Greenland. Lithos 27, 155–165. Bird, D., Schiffman, P., Elders, W.A., Williams, A.E. & McDow- ell, S.D. 1984: Calc–silicate mineralization in active geothermal systems. Economic Geology 79, 671–695. Boles, J.R. 1981: Zeolites in low grade metamorphic rocks. In: Mumpton, F.A. (ed.): Mineralogy and geology of zeolites. Min- eralogical Society of America Reviews in Mineralogy 4, 103– 135. Cho, M. 1991: Zeolite to prehnite–pumpellyite facies metamor- phism in the Toa Baja drill hole, Puerto Rico. Geophysical Re- search Letters 18, 525–528. Ellis, A.J. & Mahon, W.A.J. 1977: Chemistry and geothermal sys- tems, 392 pp. New York: Academic Press. Evarts, R.C. & Schiffman, P. 1983: Submarine hydrothermal meta- morphism of the Del Puerto ophiolite, California. American Journal of Science 283, 289–340. Fournier, R.O. 1985: Continental scientific drilling to investigate brine evolution and fluid circulation in active hydrothermal sys- tems. In: Raleigh, C.B. (ed.): Observation of the continental crust through drilling I, 98–122. Berlin: Springer-Verlag. Frey, M., de Capitani, C. & Liou, J.G. 1991: A new petrogenetic grid for low-grade metabasites. Journal of Metamorphic Geolo- gy 9, 497–509. Fridriksson T., Neuhoof, P.S., Arnorsson, S. & Bird, D.K. 2001: Geological constraints on the thermodynamic properties of the stilbite–stellerite solid solution in low-grade metabasalts. Geo- chimica et Cosmochimica Acta 65, 3993–4008. Glassley, W. 1974: A model for phase equilibria in the prehnite– pumpellyite facies. Contributions to Mineralogy and Petrology 43, 317–332. Green, P.F., Duddy, I.R., Hegarty, K.A. & Bray, R.J. 1999: Early GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19117 118 Tertiary heat flow along the UK Atlantic margin and adjacent areas. In: Fleet, A.J. & Boldy, S.A.R. (eds): Petroleum geology of Northwest Europe, Proceedings of the 5th Conference, 349– 357. London: Geological Society. Hedenquist, J.W. 1990: The thermal and geochemical structure of the Broadlands–Ohaaki geothermal system, New Zealand. Geo- thermics 19, 151–185. Jørgensen, O. 1984: Zeolite zones in the basaltic lavas of the Faeroe Islands. In: Berthelsen, O., Noe-Nygaard, A. & Rasmussen, J. (eds): The Deep Drilling Project 1980–1981 in the Faeroe Is- lands. Annales Societatis Scientiarum Faroensis. Supplementum 9, 71–91. Jørgensen, O. 1997: Zeolites and other secondary minerals in cav- ities and veins, Lopra-1/1A well, Faroe Islands, 1996, 8 pp. + plates. Unpublished report, Technical studies prepared for Dansk Olie og Gasproduktion A/S, Copenhagen, Denmark (in archives of Geological Survey of Denmark and Greenland, GEUS Re- port File 26129). Konnerup-Madsen, J. 1998: A preliminary examination of fluid inclusions in vug and fracture-filling quartz and calcite from Lo- pra-1/1a, Faroe Islands, 5 pp. Unpublished report, Geological Survey of Denmark and Greenland, Copenhagen. Kristmannsdóttir, H. & Tomassón, J. 1976: Zeolite zones in geo- thermal areas in Iceland. In: Sand, L.B. & Mumpton, F.A. (eds): Natural zeolites; occurrence, properties, use, 277–284. Oxford: Pergamon Press. Larsen, L.M., Waagstein, R., Pedersen, A.K. & Storey, M. 1999: Trans-Atlantic correlation of the Palaeogene volcanic successions in the Faeroe Islands and East Greenland. Journal of the Geolo- gical Society (London) 156, 1081–1095. Liou, J.G. 1971: Synthesis and stability relations of prehnite, Ca2Al2Si3O10(OH)2. American Mineralogist 56, 507–531. Manning, C.E., Ingebritsen, S.E. & Bird, D.K. 1993: Missing min- eral zones in contact metamorphosed basalts. American Journal of Science 293, 894–938. Neuhoff, P.S. & Bird, D.K. 2001: Partial dehydration of laumon- tite; thermodynamic constraints and petrogenetic implications. Mineralogical Magazine 65, 59–70. Neuhoff, P.S., Watt, W.S., Bird, D.K. & Pedersen, A.K. 1997: Tim- ing and structural relations of regional zeolite zones in basalts of the East Greenland continental margin. Geology 25, 803–806. Neuhoff, P.S., Kroeker, S., Du, L.S., Fridriksson, T. & Stebbins, J.F. 2002: Order/disorder in natrolite group zeolites: a 29Si and 27Al MAS NMR study. American Mineralogist 87, 1307–1320. Neuhoff, P.S., Stebbins, J.F. & Bird, D.K. 2003: Si-Al disorder and solid solutions in analcime, chabazite, and wairakite. American Mineralogist 88, 410–423. Neuhoff, P.S., Hovis, G.L., Balassone, G. & Stebbins, J.F. 2004: Thermodynamic properties of analcime solid solutions. Ameri- can Journal of Science 304, 21–66. Roedder, E. 1972: Composition of fluid inclusions. In: Data of geochemistry. U.S. Geological Survey Professional Paper 400- JJ, 164 pp. Rose, N.M. & Bird, D.K. 1987: Prehnite-epidote phase relations in the Nordre Aputiteq and Kruuse Fjord layered gabbros, East Greenland. Journal of Petrology 28, 1193–1218. Seki, Y., Onuki, H., Okumura, K. & Takashima, I. 1969: Zeolite distribution in the Katayama geothermal area of Japan. Japanese Journal of Geology and Geography 40, 63–79. Steiner, A. 1977: The Wairakei geothermal area, North Island, New Zealand: its subsurface geology and hydrothermal rock altera- tion. New Zealand Geological Survey Bulletin 90, 136 pp. Surdam, R.C. 1973: Low-grade metamorphism of tuffaceous rocks in the Karmutsen Group, Vancouver Island, British Columbia. Geological Society of America Bulletin 84, 1911–1922. Varna, C.L. 1989: Mineral reactions and controls on zeolite-facies alteration in sandstones of the Central Transantarctic Moun- tains, Antarctica. Journal of Sedimentary Petrology 59, 688– 703. Waagstein, R., Guise, P. & Rex, D. 2002: K/Ar and 39Ar/40Ar whole-rock dating of zeolite facies metamorphosed flood basalts: the upper Paleocene basalts of the Faroe Islands. In: Jolley, D.W. & Bell, B.R. (eds): The North Atlantic Igneous Province: stratig- raphy, tectonic, volcanic and magmatic processes. Geological So- ciety Special Publication (London) 197, 219–252. Wolery, T.J. & Daveler, S.A. 1992: EQ6, A computer program for reaction path modeling of aqueous geochemical systems: theo- retical manual, user’s guide, and related documentation. Law- rence Livermore National Laboratory UCRL-MA-110662 Part IV, 338 pp. Manuscipt received 22 December 1999; revision accepted 26 May 2005. GEUS Bulletin no 9 - 7 juli.pmd 07-07-2006, 14:19118