Chemical transport in geothermal systems in Iceland

8/7/2019 Chemical transport in geothermal systems in Iceland

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Chemical transport in geothermal systems in IcelandEvidence from hydrothermal alteration

Hjalti Franzson a,, Robert Zierenberg b, Peter Schiffman b

a Iceland GeoSurvey, 9 Grenssvegur, 108 Reykjavk, Icelandb Department of Geology, University of California, Davis, One Shields Avenue, 95616, CA, USA

a r t i c l e i n f o a b s t r a c t

Article history:

Received 1 July 2007Accepted 28 January 2008Available online 4 March 2008

This study focuses on the chemical changes in basaltic rocks in fossil low- and high-temperaturehydrothermal systems in Iceland. The method used takes into account the amount of dilution caused byvesicle and vein fillings in the rocks. The amount of dilution allows a calculation of the primary concentrationof the immobile element Zr, and by multiplying the composition of the altered rock by the ratio of Zr(protolith)/Zr (altered rock) one can compute the mass addition caused by the dilution of the voidfillings, andalso make a direct comparison with the likely protoliths from the same areas. The samples were divided intothree groups; two from Tertiary fossil high-temperature systems (Hafnarfjall, Geitafell), and the third groupfrom a low temperature, zeolite-altered plateau basalt succession. The results show that hydrothermallyaltered rocks are enriched in Si, Al, Fe, Mg and Mn, and that Na, K and Ca are mobile but show either depletionor enrichment. The elements that are immobile include Zr, Y, Nb and probably Ti. The two high-temperaturesystems show quite similar chemical alteration trends, an observation which may apply to Icelandic freshwater high-temperature systems in general. The geochemical data show that the major changes in the alteredrocksfrom Icelandic geothermal systems may be attributedto addition of elements during deposition of pore-filling alteration minerals. A comparison with seawater-dominated basalt-hosted hydrothermal systemsshows much greater massflux within the seawater systems, eventhough both systems havesimilar alterationassemblages. The secondary mineral assemblages seem to be controlled predominantly by the thermal

stability of the alteration phases and secondarily by the composition of the hydrothermal fluids. 2008 Elsevier B.V. All rights reserved.

Keywords:

Icelandbasaltgeothermal systemshydrothermal alterationchemical transportisocon method

1. Introduction

Thelocationof Iceland as a subaerialpart of theMid-Atlantic Ridge,with a wealth of geothermal activity, makes it a unique area for thestudy of waterrock interaction. Temperatures offluidrock interac-tion rangefromabout 2 to 3 C in thegroundwater systems to probablesupercritical values in the high-temperature geothermal areas (Eldersand Fridleifsson, 2005). The rocks that host the geothermal reservoirsare of igneous origin, with basaltic compositions constituting about90% of the volume with the remainder having more evolved com-positions. The abundant geothermal resources are economically im-portant and widely utilized, and have consequently been extensivelystudied from geochemical, structural, geophysical and geologicalpoints of view (e.g., Arnorsson et al., 1983; Bodvarsson, 1983; Martyet al.,1991; Schiffman and Fridleifsson,1991;Lonker et al.,1993; Riedelet al., 2001). The geothermal resources range from low-temperaturesystems, in which shallow groundwaterhas gained heat in response tothe prevalent regional geothermal gradient, to high-temperature

systems in which the high thermal gradient is due to shallow crustalmagmatic activity. The latter type is mostly confined to active volcaniccentres. However, fossil high temperature geothermal systems areexposed by erosion allowing three dimensional access to the subsur-face portions of the hydrothermal systems (e.g. Fridleifsson, 1983,1984). The majority of the hydrothermal systems in Iceland havewaters derived from local meteoric water, although seawater-domi-nated hydrothermal fluids occur in some near coastal geothermalsystems (Sveinbjornsdottir et al., 1986). The general similarity of bothsource fluids and host rocks throughout much of Iceland allowscomparison among hydrothermal systems.

Rock properties play an important role in the petrophysical para-meters of the individual systems and an increasing emphasis has beenplaced on the study of reservoir characteristics. During the last13 years, Orkustofnun (the National Energy Authority of Iceland), withthe financial support of Orkuveita Reykjavkur (Reykjavik Energy), hasworked on a project with the main aim of defining the reservoircharacteristics of the rocks in the geothermal systems in Iceland.While active geothermal systems have the advantage of allowingdirect comparison of rock alteration to fluid composition, this com-parison only relates to a single stage, i.e., the present, in the evolutionof the system. In contrast, sampling of fossil geothermal fields allows

Journal of Volcanology and Geothermal Research 173 (2008) 217229

Corresponding author. Tel.: +354 528 1500; fax: +354 528 1699.E-mail addresses: [email protected] (H. Franzson).

0377-0273/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.jvolgeores.2008.01.027

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systems, calcite is occasionally superimposed on epidoteactinolitezone mineral assemblages, implying carbonate deposition eitherduring cooling of the high-temperature system by inflow of colderfluids or during the succeeding low-temperature episode. Hence,changes in the chemical composition of rocks relative to their freshcounterparts are believed to have occurred up to the highest tempe-rature stage of alteration that the rock has undergone, with possibleeffects due to precipitation of late-stage calcite.

The samples are all either volcanic rocks which have been gra-

dually buried by accumulation of younger volcanics, or intrusive rocks.The age of the accumulated sequence is progressively older and morealtered as it is more deeply buried. However, the same cannot beinferred about the intrusive rocks, as their minimum age is generallyunconstrained. This may be of importance when considering thetiming of alteration, especially within the most intense alterationzones, as intrusions may occur during any stage of the alteration.Rocks intruded at a later stage of alteration may only show a minoralteration effect compared to those intruded earlier, even though theywere subjected to similar temperatures and pressures. Therefore,

contrasts in the degree of alteration within intrusive rocks and theirsurrounding extrusive host rocks should be expected and may dependon factors other than differences in permeability and porosity.

3. Petrographic data

The rock samples are all of basaltic composition, ranging fromolivine tholeiite to quartz normative tholeiite. They would dominantlybe holocrystalline, but with some glass fraction in the most scoracious

partofthelavas.ThemainprimarycomponentsinbasaltsarerelativelyCa-rich plagioclase, clino-pyroxene (augite) and opaques (magnetiteilmenite), and with subordinate amount of olivine, especially in themoreprimitive basalts. Hydrothermal alteration is essentially made upof two components: (a) replacement of primary components in therocks by alteration minerals, and (b) precipitation of alteration mi-nerals into voids in the rock. Hydrothermal alteration in activeIcelandic geothermal systems is systematically zoned with respect totemperature (Fig. 2). The top zone, which contains relatively freshrocks, includes a sequence that does not show any indication of

Fig. 2. The alteration zones used in Iceland, their dependence on temperature and the main alteration features of the primary basaltic rock components.

Fig. 3. Range in primary porosity in Icelandic rocks deduced from petrographic study of the rock samples ( n=127).

219H. Franzson et al. / Journal of Volcanology and Geothermal Research 173 (2008) 217229


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geothermal interaction and mayat the mostshow minor oxidationdueto groundwater circulation. The temperature dependency, as seen inFig. 2, affects both the sequence of alteration mineralsthatfill voids, aswell as the ones that replace primary minerals. Thus olivine and glassarecompletelyaltered near theupper boundary of themixed layer clayzone. Plagioclase and opaques are moreresistant to alterationand maybe only partially altered to the minerals shown on Fig. 2, especiallywhen taking into account that some of the rock samples are lowporosity intrusions. Pyroxene shows in general similar resistance toalteration as plagioclase. It is mainly seen altering into clays and thenactinolite at deeper levels.

Petrographic examination of the rock samples collected for thisstudy used point counting (200 points) to quantify primary porosity,i.e. the original open space in rock prior to alteration (dominantly

vesicles and minor fractures), and to assess how much of that porosityhad been filled by deposition of alteration minerals. Two hundredpoints were counted on each rock thin section. Fig. 3 shows thedistribution of primary porosity in Icelandic rocks, where porosityranges from zero up to about 70%. As alteration proceeds void space inthe rock is progressively filled with alteration minerals. Fig. 4 showsthe relationship between the degree of pore filling and the extent ofalteration, defined as the percentage of alteration minerals relative toprimary minerals. Rocks with a primary porosity, as petrographicallydetermined, below approximately 15% have variable degrees ofinfilling (Fig. 5). In contrast, rocks that initially exceeded the 15%primary porosity threshold tend to show near complete filling of theprimary pore-space. This implies a non-linear relationship betweenpermeability and porosity in the Icelandic basaltic samples.

Fig. 4. The relation between degree of infilling and the total percentage of alteration minerals (i.e., % alteration minerals divided by the total mode) of a sample. Samples are groupedaccording to their alteration zone (from Franzson et al., 2000).

Fig. 5. The rate of deposition of alteration minerals into vesicle filling in relation to alteration zones and primary rock porosity. Note that there is apparently more rapid (i.e.,

efficient) deposition in rocks withN

15% primary porosity, as determined petrographically (from Franzson et al., 2000).

220 H. Franzson et al. / Journal of Volcanology and Geothermal Research 173 (2008) 217229


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4. Chemical data

The chemical analyses were made by two commercial chemicallaboratories, The Caleb Brett Laboratory in England and McGillUniversity in Canada. Both used standardized XRF techniques. Valuesfor samples analyzed by both laboratories are generally withinanalytical error. The samples were analyzed for major, minor, andseveral trace elements. Loss on ignition (LOI) was measured in all the

samples, and in some samples, CO2 and Stotal were specifi

cally ana-lyzed. The trace elements analyzed included Zr, Y, Zn, Cu, Rb, Sr, Nb,Ga, Ce, V, Pb, U, Th and As. All the analyses presented here have beenrecalculated to 100%, without LOI, to allow direct comparison withunaltered protoliths. The compositional range of relatively unalteredrocks has been acquired from other sources for comparison with thedata on altered rocks presented here. Analyses describing the evolu-tion trend within the Hafnarfjall central volcano and the Hvalfjordurplateau basalt succession are from Franzson (1979), and analyses fromthe Geitafell central volcano are from Thorlacius (1991). Chemicalanalysis from the Krafla central volcano (Karl Gronvold, unpublisheddata) and the ReykjanesLangjokull volcanic zone (Sveinn Jakobsson,unpublished data, 1999) have been used as reference samples forcomparison with the altered samples where relatively unaltered sam-ples are unavailable from the respective areas.

Loss on ignition values range from zero up to about 13%. Fig. 6shows the relationship between the % of remaining primary rockcomponent (i.e.= 100-(rock alteration+ void filling)) in basaltic lavaflows and LOI, where the samples have been grouped into alterationzones. The figure clearly shows a strong correlation between LOI andthe extent of alteration in these samples. It also shows an inversecorrelation between the percentage of primary component and theminimum LOI for samples as a function of alteration grade. Smectitezeolite grade samples with LOI b1% have a much higher percentage(N80%) of primary components than epidoteamphibolite sampleswith similar LOI values. This is probably due to more abundant ofalteration minerals with low-water contents in the higher gradesamples. LOIconsistsmainlyof H2O

+ andCO2. The latterwas measuredseparately in only part of the sample group. The results show that CO2

is typicallyb1% in samples with less than 60% alteration intensitywithhigher CO2 values (b6.5%) only present where alteration is moreintense. A good correlation is between CO2 and calcite in the samples.

5. Methodology for calculating mass transfer

Themethodology used in this study is based upon thepetrographiccharacteristics of Icelandic basalts altered in geothermal systems.

Specifically, the methodology arises from the key observation that

much of the alteration process entails the filling of primary pores bysecondary minerals, effectively diluting the chemical composition ofthe protolith by mass addition (Fig. 7). This mass addition will dilutemost of the components in the original rock and thus whole rockanalyses will show lower concentrations of immobile elements eventhough there has been no mass flux of these elements. The extent ofdilution is known through point-counting as discussed above. Byidentifying components in the rock that have remained immobileduring alteration and taking into account this dilution effect, theprimary composition of the immobile element in the sample may becalculated.

One way of assessing the effect of chemical dilution on the rockcomposition is to plot the elemental concentration against the rockdilution (Fig. 8). In this diagram the concentration from z to x

represents the concentration range measured for element A inunaltered basalt. The tie lines from 100% dilution (point R) to pointsz and x represent the amount of dilution of z and x, respectively,assuming that the dilutant mineral (or minerals) contains no chemicalcomponent A. If element A is immobile and has y concentration in thesample, the dilution will cause the apparent decrease of that com-ponent along the line towards R. One may therefore expect that allimmobile elements within the basalt range will, with increasingdilution, be contained within the field labelled a. A mobile element

Fig. 6. Loss on ignition (LOI)plottedagainst the % primary rockcomponent(i.e.,100-rockalteration and void filling). Also shown are the best fit lines belonging to each of the

alteration groups.

Fig. 7. A sketch showing a simplified model for the dilution of a chemical rock com-ponent with the introduction of vesicle filling. The Y-component in the rock issymbolised in black and becomes relatively reduced when vesicle fillings are added tothe sample.

Fig. 8. Diagram showingeffect of dilution on the concentration of hypothetical chemicalcomponent A during alteration of basaltic rocks. Fields labelled a, b, and c:represent, respectively, the range of diluted compositions following no mass transfer,

addition of component A, and loss of component A. See text for more details.



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line R to y in Fig. 8. Barrett and MacLean (1994) suggested that bycomparing the concentration of immobile elements prior to and afterhydrothermal alteration the mass changes that have taken placeduring the hydrothermal alteration event can be calculated. Thecalculation of mass change removes the misleading effects of closure(constant sum of 100%) on the relations between elements in un-treated samples (Barrett and MacLean, 1994). After determining theprimary concentration of a givenimmobileelement (inthis case Zr) by

the method described above, we can then calculate the reconstructedcomposition for each rock component by multiplying the originalvalue by a factor equal to the concentration of Zr in the precursordivided by the concentration of Zr in the altered sample. In this way,we have now resurrected the volume of each chemical componentto what it was in the primary rock. If dilution represented the onlychange in rock composition, then all elements other than those con-tained in the pore-filling precipitates would reveal their originalvalues by the correction and would plot on the one-to-one line whencompared to an unaltered protolith. This allows us to test the hypo-thesis that mass dilution during pore filling is the dominant chemical

change affecting the altered rocks. It is of coursenot alwayspossibletodetermine the composition of the unaltered protolith. However, sinceZr can be shown to be essentially immobile in these rocks, we canbracket the potential compositional variations in primary compositionif we can determine the fractionation relationship between Zr and theelement of interest in unaltered rocks. By plotting the corrected Zrconcentration against another component and comparing it with thefractionation trend of the respective volcano one can assess whether

enrichment, depletion or no apparent change has occurred duringalteration. The extensive data available on the composition of freshbasaltic rocks from Iceland provides constraints on the fractionationtrends with respect to Zr. The limited range of fractionation, from thespread of Zr concentrations in unaltered basalts, provides constraintson the possible original variations in other elements and allows us torecognize significant mass flux.

Forthisexercise, thesampleshave been subdivided into 3 groups asdescribed above, i.e. the Hafnarfjall and the Geitafell central volcanoesand the Hvalfjordur plateau basalt succession. Although Icelandic vol-canoes follow, in general, a Thingmuli tholeiitic evolutionary trend

Fig. 13. Chemical components plotted against Zr. Xs are least altered rocks from Hafnarfjall and Geitafell central volcanoes (Franzson, 1979; Thorlacius, 1991). Diamonds are rocks of033% intensity alteration, squares are rocks of 3366% alteration and triangles are rocks of 66100% alteration. The line delineates the primary compositional field of the respective

volcanoes.



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to its lower geothermal gradient. These do, however, indicate SiO2,andK2O enrichment and subtle enrichments in Al2O3, FeO,MgO, Na2Oand possibly Sr. Comparison with some of the trace elements isdifficult due to lack of data from fresh rock equivalents.

6. Isocon plots

The data presented above allows us to evaluate the general beha-viour of individual chemicalcomponentswithin a groupof samples foran entire hydrothermal field. Themethod also allows easy comparisonbetween the magnitude of compositional variation related to igneousfractionation trends relative to those that can be attributed to hydro-

thermal alteration. The isocon method of determining mass flux has

the disadvantageof requiring sample by sample comparison to specificprecursor rock compositions. A distinct advantage of the isoconmethod is that it allows direct evaluation of the massflux of elementsin a specific sample, allowing easier recognition of coupled geochem-ical behaviour between elements (e.g., K and Na). Our approach inusing the isoconplots was tofirst correct the altered rock geochemicaldata for the known effects of dilution and to plot these extrapolatedvalues against appropriate precursor compositions to identify massflux variations that are not due to simple rock dilution by alterationminerals.

Theisocon diagrams(Fig.15) compare the relationshipbetweenthevarious components within four representative samples, two chosen

from each of the central volcanos. Two of the samples have a primary

Fig. 14. Chemical components plotted against Zr. + are fresh rocks from the ReykjanesLangjokull volcanic zone (Sveinn Jakobsson, unpublished data, 1999), x are least altered basaltlavas from Hvalfjordur lava succession, squares are lavas from Krafla central volcano (Karl Gronvold, unpublished data), and triangles are the rock samples of this study.



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Zr content of about 150 ppm(H-99, G-62), while theothers have about300 ppm (H-54, G-28) representing relatively primitive and evolvedbasalt compositions, respectively. The error bars attached to each ofthe components represent the range of primary compositional valuesexpected at each Zr concentration value within the individual volcano(Fig. 13). Although error limits on the recalculated compositions aredifficult to quantify, onemay expect that theerror would increasewithincreasing dilution. This is easily observed in Fig.8 where a small error

in petrographic estimation at high dilution could divert the extrapola-tion line considerably down towards the base line. If the samples haveall been properly corrected with respect to mass addition, then themedian line should represent the line of no chemical change. Indeed,the immobile elements (Ce, Zr, Y, Nb) tend to lie on this line. Anexception is Y in theHafnarfjallsamples,which maypossiblybe due toa systematic analytical error in the primary trend in the volcano, asdeduced from the Zr/Y plot (Fig. 13). In contrast, Fig. 15 indicates atendency for slight Al2O3 and FeO enrichment in the altered samples,as is also clearly indicated in Fig. 13. MgO can be either enriched ordepleted, as are Na2O and K2O. One might expect that Na2O and K2Owould generally show similar geochemical behaviour and Fig. 15confirms that the alkalis are either both depleted or both enriched inany given sample. Na2O and K2O are rather easily leached from rocks

during hydrothermal alteration and many rocks are in fact depleted inthem. One reason for enrichment may be due to remobilization fromnearby more felsic volcanic and intrusive rocks. This hypothesis couldbe tested by looking atfield relationsof the altered basalt samples thatshow alkali enrichment. The isocon plots generally confirm theconclusions that have been attained through plots shown in Figs. 914 and support the proposition that changes in chemical compositionof the altered rocks is mainly due to addition of mass to the rock bydeposition of alteration minerals in primary pores.

7. Discussion

Geothermal systems in Iceland havea number of common features.The rocks are dominantly of basaltic composition, with minor

amounts of rocks having more evolved compositions within central

volcanic complexes. Most of the hydrothermal waters have lowsalinity with chloride b200 ppm (Arnorsson and Andresdottir, 1995),consistent with heating of local groundwaters. Exceptions are thehigh-temperature systems on the Reykjanes Peninsula which showsalinity approaching that of seawater (Sveinbjornsdottir et al., 1986).Fluid inclusion studies indicate, however, that much of the alterationin the Reykjanes systems may have occurred during an initialfreshwater stage during the last glacial period (e.g., Franzson et al.,

2002). Studies on the alteration zones in several fields show quitesimilar distributionsof alteration minerals(Fig.2). There also seems, ingeneral, to be a close correlation between alteration zones and theprogressive breakdown of the primary rock and the alterationproducts formed. The alteration is, to a large extent dependent ontemperature, and the maximum temperature at any depth in theexplored systems is constrained by the boiling curve. The commonoccurrence of hyaloclastites in the presently-active systems is notshared by the fossil hydrothermal systems developedin Tertiary rocks,which are dominantly in holocrystalline volcanic rocks erupted in theabsence of a glacial icecap. Volcanic glass is very readily altered duringgeothermal activity, and this will increase the availability of chemicalcomponents (e.g., SiO2) into thermal fluids. Therefore the chemicalflux in the presently-active geothermal systems may be higher than it

was for the Tertiary systems.It must be emphasized that this study describesthe overall changes

that have taken place during the whole lifetime of the hydrothermalsystems. Mineralogical studies of vesicle and vein fillings at Geitafellcentral volcano showdistinct episodes of alteration (Fridleifsson,1983,1984). The same pattern of chronologically distinct hydrothermal epi-sodes has been recognized in many of the explored active high-tem-perature systems (e.g., Lonker et al., 1993). The number of mineralspecies in individual vesicles and veins in the high-temperature sys-tems at Reykjanes, Nesjavellir, Olkelduhals and Svartsengi (Franzson,1983, 2000; Franzson et al., 2002), based on about 2000 observations,is very similar. About 70% show two void-filling minerals, about 25%contain three and about 5% show four or more void-filling minerals.The proportion of mono-mineralic veins and vesicles has not been

checked specifically. It is interesting to note that many minerals seem

Table 1

Chemical changes in samples subjected to hydrothermal alteration from the Hvalfjordur basalt succession and Hafnarfjall and Geitafell central volcanoes

Chemicalcomponent

Hvalfjordur Hafnarfjall Geitafell

Depletion Unchanged Enrichment Depletion Unchanged Enrichment Depletion Unchanged Enrichment

SiO2 X x xTiO2 X (x) (x)Al2O3 (x) (x) (x) x xFeO(tot) X (x) x xMnO X x (x) (x)

MgO X (x) x (x) x xCaO X (x) (x) x x (x) x xNa2O X X (x) x x x xK2O X (x) x (x) (x) x (x)P2O5 X (x) x xCe X x xCu (x) x x (x)V X ? ?Zn x x xGa X ? ?Nb X x xPb ? ? ?Rb X ? ? ? (x)? (x)? (x)?Sr (x) (x) x (x)? ?Th ? ? x? ? ?U ? ? ?Y X x x

Zr X x xAs ? ?S ? ? ?

x-affirmative, (x)-probable, (x)?-possible, ?-uncertain.



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to grow to a certain threshold grain size, after which they are suc-ceeded by another mineral (e.g. clays, epidote, prehnite, wairakite,chalcedony and sometimes quartz). Other minerals like calcite, an-hydrite (the latter in saline systems), and sometimes quartz, are morerarely constrained by size andtendto fill the available space. Whateverthe reason, the limit of mineral size does increase the probability thatone could find at least two minerals in any one vesicle. Calcite is themineral that is most likely to fill available space. However, calcitegenerally precipitates at a late stage in the mineral sequence after thedeposition of the other minerals. The above observations imply thattypical rocks in a given high temperature system retain open porespace for sufficient time to record the effects of more than one geo-thermal episode. Thus the enrichment/depletion trends tend to accu-

mulate over the lifetime of the system. It has also been noted thatchanges in mineral deposition sequences can be correlated regionallywithin individual geothermal fields (e.g. Franzson, 2000), which im-plies that mineral deposition is controlled by large scale hydrologicalchanges in the geothermal system.

It is instructive to compare the low salinity Icelandic geothermalsystems with basalt-hosted seawater-derived hydrothermal fluids (Altand Teagle (2000) and references therein and Butterfield (2000) andreferences therein), as evidenced in ophiolitic rocks and black smokerhydrothermal systems. The overall similarity in the composition of ba-salts from the seafloor, ophiolites and subaerial Iceland facilitates com-parison of these systems. Their physical volcanology does show somedifferences that could lead to differences in fluidrock interaction.Extensional tectonics and rift zone volcanism are common to each of

these settings, but the deep water eruptions, common on mid-ocean

ridges and ophiolites, are different from the subareal eruptions in Ice-land (Batiza and White, 2000).

Seafloor eruptions produce more abundant glassy volcanics andhyaloclastites than subaerial eruptions in the Tertiary, which shouldenhance the alteration potential of seafloor rocks. However, Pleisto-cene eruptions in Iceland often occurred beneath ice cover and resul-ted in extensive formation of hyaloclastite. Seafloor pillow lavas andsheet flows, which characteristically show drain-back features, havevery high intrinsic, large scale porosity and permeability, but thepermeability drops significantly in older, more deeply buried lavasdue to collapse and infilling by later eruptions (Perfit and Chadwick,1998; Fornari et al., 2004). However, submarine lavas are rarely vesi-culated. In contrast, flow top and bottom breccias and highly vesi-

culatedflow tops in subaerial lavas provide laterally continuous zonesof high porosity that results in anisotropic permeability that favorslateral flow and restricts vertical convection (Fornari and Embley,1995; Fornari et al., 2004).

One major compositional difference between subarial and sub-marine lavas is in the concentration of volatile elements, particularlysulfur. Sulfur concentrations of MORBs are typically about 1000 to1500 ppm, in contrast to values of about 100 ppm in subarial lavas thathave degassed during eruption(Wallaceand Anderson,2000).Becausepotentially acid-generating volatiles (e.g. SO2,HCl,CO2) are lost duringeruption, subsequent hydrothermal leaching will not involve thesespecies and the resultant hydrothermal fluids may not attain low pHsthat facilitate rock alteration. This is particularly true for hydrothermalsystems developed in plateau basalts, suchas at Hvalfjordur. Magmatic

degassing during intrusive/eruptive events can result in short lived

Fig.15. Isocon plots for four samples from Hafnarfjall (H-54, H-94) and Geitafell (G-28, G-62) central volcanoes. The samples were selected with about 150 and 300 ppm Zr, and fromthe 66100% alteration group and moderate SiO2 enrichments. See text for further explanation.



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compositional changes in geothermal systems due to acidificationreactions (Armannsson et al., 1982). In hydrothermal systems devel-oped in longer-lived central volcanoes (e.g. Hafnarfjall, Geitafell), latestage intrusive rocks can supply both heatand acid-generating volatilespecies that canchange thepattern of alteration of the older, degassedlava flows. Degassing of sulfur from basalt can affect the mobility ofother elements as well, particularly the base metals. For example,submarinebasalts erupted at higher pressures dueto theweight of the

overlying water column and therefore do not degass sulfur as readily.Copper in submarine basalts is typically concentrated as chalcopyriteand is generally only mobilised by high temperature (N330 C)hydrothermal fluids due to the low solubility of chalcopyrite insulfide-bearing fluids. In contrast, copper in subareallyeruptedbasaltsthat have degassed sulfur, chalcopyrite may not be stable leavingcopper in a form that is easily leached and transported by lowtemperature, oxidized fluids (Lincoln, 1981).

The upper temperature limits in shallow geothermal fields aretypically set by the boiling temperature curve of the fluid. In deeper,higher salinity systems rapid changes in the physical and chemicalproperties of the fluid at temperatures approaching the critical pointprovide similar temperature constraints. The higher pressures andsalinities inherent in submarine geothermal systems therefore favourhigher ultimate temperatures of waterrock reaction in the deeperportions of these system. It is interesting that the general zonation ofalteration minerals in Icelandic geothermal systems (Fig. 2) is similarto those observed in drill holes throughout the oceanic crust (Alt andTeagle, 2000 and references therein; Wilson et al., 2006) andophiolites (Schiffman et al., 1991; Gillis and Banerjee, 2000).

The major difference in subarial geothermal systems recharged bymeteoric water and submarine systems recharged by seawater is thesalinity of the fluid. Chloride complexing is significant for many cat-ions and strongly enhances the solubility of most minerals, especiallyat elevated temperatures. Higher concentrations of major elementcations in seawater-recharged hydrothermal fluids will also affect thestability of alteration minerals making it all the more surprising thatthe zonation of alteration minerals in seafloor hydrothermal systemsis so similar to that in Iceland. Although the general pattern of

alteration mineral zonation is similar, the extent of hydrothermalalteration and mass flux from high temperature seafloor systems issignificantly different from that of the Icelandic geothermal systems.The geochemical data presented in this paper document that themajor changes in the compositions of altered rocks from Icelandicgeothermal systems result from the addition of elements duringdeposition of pore-filling alteration minerals. Isocon plots of alteredsamples, corrected for dilution, show relatively small mass fluxes formost elements, even at high degrees of alteration. This generalobservation is true across the range of alteration zones even in rocksthat show epidoteamphibole alteration. In contrast, basalt/seawaterreaction results in significant metasomatism of the rocks(Seyfried andDing, 1995). As seawater is heated by circulating in the oceanic cruston the recharge limb of hydrothermal convection cells, metasomatic

addition of seawater-derived Mg and Ca into basalts releases hydrogenions lowering the fluid pH (measured at 25 C) into the 34.5 rangethat is typical of black smoker fluids. Calculated in situ pH at hightemperature areabout oneunit lower than neutral (Ding and Seyfried,1992). This low pH fluid has significant capacity for hydrogen meta-somatism in the deep reaction zone and the rising limb of the con-vective circulation. Acid alteration of the basalts releases a significantflux of cations into the hydrothermalfluids resulting in highly cation-leached alteration zones, enriched in aluminum and silica, in focussedupflow zones where integrated waterrock ratios are high (Humphriset al., 1998; Zierenberg et al., 1988, 1995, 1998; Teagle and Alt, 2004).Silicified basalts in hydrothermal upflow zones can have silica con-tents in excess of 80 wt.% (e.g. Zierenberg et al.,1995). Alternatively, Feand Mg metasomatism is hydrothermal upflow zones can result in

pervasive chloritization of basalt resulting in bulk compositions

approaching chlorite with SiO2 as low as 35 wt.% (Humphris et al.,1998) and FeOT+MgON25 wt.% (Zierenberg et al., 1988). Thus themost significant difference between seawater and meteoric waterrecharged basalt-hosted hydrothermal systems is the mass flux fromthe system, not the mineral assemblages which seem to be controlledpredominantly by the thermal stability of the alteration phases andsecondarily by the composition of the hydrothermal fluids.

8. Conclusions

This paper reports a method to compare hydrothermally alteredbasaltic rocks and their protoliths that facilitates correction for massadditions to the rock, during hydrothermal alteration. This methodtakes into account the amount of mineral deposition into the rock, asdetermined by petrography, and recalculates the mass addition usingthe immobile element concentration. The derived recalculated com-position of the altered rocks can then be compared with the leastaltered protoliths from the same areas. The comparison showed thathydrothermal alteration causes pronounced Si-enrichment as well asa considerable Al,Fe, Mg and Mn enrichment. Elements such as Ca, Na,K, are also mobile and can be either depleted or enriched. Immobileelements include Zr, Y, Nb, Ce, and probably Ti. An important con-clusion from this work is that the predominant process leading tochemical flux in these rocks is deposition of alteration minerals inprimary pore space, butmassflux due to recrystallization and replace-ment reactions are of secondary importance. This contrasts withseawater-dominated hydrothermal systems where metasomatic reac-tions result in extensive mass flux. Another important conclusion isthat similar chemical changes were experienced in the two Tertiarycentral volcanoes studied, suggesting that these may represent a ge-neral trend in Icelandic high-temperature systems. A study of asample suite offlood basalts within a low-temperature zeolite alter-ation environmentof Tertiaryage also shows chemical changes akin tothe former, but the changes are more subtle. It is also significant thatthe general zonation of alteration minerals in Icelandic alterationzones is similar to those in seawater-dominated hydrothermalsystems in spite of the differences in the extent of metasomatism.

This implies that temperature, notfluid composition, provides thefirstorder control on mineral assemblages in basalt-hosted hydrothermalsystems.

Acknowledgements

WethankHoward Dayand Shuwen Liufor very helpful discussion onvarious aspects of this work, and the late Valgardur Stefansson forcritically reading the manuscript and his vigorous support. Patrick R. L.Browne and an anonymousreviewerare thanked forcriticallyreviewingthe paper and suggesting a number of improvements. This paper waswritten during the sabbatical leave of the first author at the GeologicalDepartment, University of California, Davis in the USA. The financialsupport of Orkustofnun to complete this work is acknowledged.

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Chemical transport in geothermal systems in Iceland