Stability of Na–Be Minerals in Late-Magmatic Fluids of the Ilímaussaq Alkaline Complex, South Greenland

Stability of Na–Be minerals in late-magmatic fluids of the Ilímaussaq alkaline complex, South Greenland

Gregor Markl

Various Na-bearing Be silicates occur in late-stage veins and in alkaline rocks metasomatised by late-magmatic fluids of the Ilímaussaq alkaline complex in South Greenland. First, chkalovite crystallised with sodalite around 600°C at 1 kbar. Late-magmatic assemblages formed between 400 and

200°C and replaced chkalovite or grew in later veins from an H2O-rich fluid. This fluid is also recorded in secondary fluid inclusions in most Ilímaussaq nepheline syenites. The late assemblages comprise chkalovite + ussingite, tugtupite + analcime ± albite, epididymite + albite, bertrandite ± beryllite + analcime, and sphaerobertrandite + albite or analcime(?). Quantitative phase diagrams involving minerals of the Na–Al–Si–O–H–Cl system and various Be minerals show that tugtupite co-exists at 400°C only with very Na-rich or very alkalic fluids [log 2 (a /a ) > 6–8; log (a 2+/(a ) ) > –3]. The abundance of Na-rich minerals and of the NaOH-bear- Na+ H+ Be H+ ing silicate ussingite indicates the importance of both of these parameters. Water activity and silica

activity in these fluids were in the range 0.7–1 and 0.05–0.3, and XNaCl in a binary hydrous fluid was below 0.2 at 400°C. As bertrandite is only stable at < 220°C at 1 kbar, the rare formation of epididymite, eudidymite, bertrandite and sphaerobertrandite by chkalovite-consuming reactions occurred at still lower temperatures and possibly involved fluids of higher silica activity.

Institut für Mineralogie, Petrologie und Geochemie, Eberhard-Karls-Universität, Wilhelmstrasse 56, D-72074 Tübingen, Germany. E-mail: [email protected]

Keywords: agpaite, Be-minerals, Greenland, Ilímaussaq, stability relations

Beryllium minerals occur in small quantities in many silicate assemblages exist in K-rich, peralkaline mag- magmatic rocks of broadly granitoid composition and matic rocks. The physico-chemical conditions under in larger quantities in some granitic pegmatites. The which the Na-bearing Be silicates crystallise and the earliest Be mineral in most granitic rocks is beryl; later compositional parameters which favour the Na-bear- minerals, in many cases replacing beryl, include ing or the Na-free Be mineral assemblages in the Ilí- euclase, bertrandite, phenakite (e.g. Mårtensson 1960; maussaq alkaline complex in South Greenland are the „erný 1963, 1968; Roering & Heckroodt 1966; Burt topic of this contribution. Of special interest is the 1975; Hemingway et al. 1986; Markl & Schumacher unique occurrence of large masses of pinkish red tug- 1997), herderite (Burt 1975) and many more. The great- tupite which is used as a semiprecious gemstone in est variety and abundance of Be minerals is invariably the Ilímaussaq area. connected to pegmatitic liquids and late- to postmag- The Ilímaussaq alkaline complex (Fig. 1) was em- matic fluids. Table 1 provides the chemical formulae placed in the late Gardar period at about 1.16 Ga (un- of Be minerals mentioned and discussed in the pres- published data by the author, see also Sørensen 2001, ent paper. this volume) into the Mesoproterozoic Julianehåb gran- In contrast to these ‘normal’ granitoid Be minerals, ite and earlier Gardar sandstones and basalts. The Na-rich peralkaline rocks are typically devoid of the depth of intrusion is estimated as about 3–4 km, corre- Be minerals mentioned above, but contain Na–Be sili- sponding to about 1 kbar pressure (Konnerup-Mad- cates such as chkalovite, tugtupite, epididymite, eudi- sen & Rose-Hansen 1982, 1984). The intrusive complex dymite or sorensenite (Semenov et al. 1965; Semenov was formed by three distinct magma batches which & Sørensen 1966; Andersen 1967; Mandarino & Harris are believed to represent successively more fractionated 1969; Sobolev et al. 1970; Engell et al. 1971 and refer- liquids from an alkali basaltic magma chamber at depth ences therein). Interestingly, no corresponding K-Be (Larsen & Sørensen 1987; Sørensen & Larsen 1987). It

Geology of Greenland Survey Bulletin 190, 145–158 (2001) © GEUS 2001 145 consists of a variety of rock types, ranging from early, just silica-saturated or slightly under-saturated augite syenite through agpaitic nepheline syenites (sodalite foyaite, naujaite and kakortokite) to later, extremely silica-undersaturated nepheline syenites (lujavrites) and finally to hyper-agpaitic assemblages which prob-

ably crystallised from relatively SiO2-rich fluids rather

than from SiO2-undersaturated melts. In addition to this fractionation line towards lower silica activities which comprises two out of three different intrusive events at Ilímaussaq (phases 1 and 3), the second intrusive phase is represented by a small sheet of alkali granite. This rock type is interpreted as an augite

syenite contaminated with SiO2-rich crustal rocks or melts, e.g. with Gardar quartzitic sandstones (Stevenson et al. 1997), although fluid inclusion studies (Konnerup-Madsen & Rose-Hansen 1984) do not support this hypothesis. In the course of the ‘undisturbed’ fractionation proc- ess towards lower silica activities, temperature de- creased from about 950°C to below 450°C, silica activity dropped from about 0.8 to below 0.2 (Larsen & Sø- rensen 1987; Markl et al. in press; Marks & Markl in press) and water activity continuously increased until saturation with a water-rich fluid was reached at the late lujavrite stage. This fluid was most probably connected to the formation of abundant aegirine-, albite- Kvanefjeld ★ and analcime-bearing veins cutting through all other Ilímaussaq rocks, which locally contain Be silicates ★ such as chkalovite and tugtupite. Hence, it appears ★ that Be mineralisation is closely connected to late- ★ stage peralkaline magmatic fluids or liquids as was described in great detail by Engell et al. (1971).

★ ★ ★ Quaternary Be mineral assemblages and textures Augite syenite Alkali granite in the Ilímaussaq complex Sodalite foyaite Be minerals known to occur at Ilímaussaq include chka- Foyaite and pulaskite ★ lovite, tugtupite, sorensenite, leucophanite, sphaerober- Naujaite trandite(?), helvite, genthelvite, beryllite, eudidymite, Kakortokite ★ Lujavrite epididymite, bavenite, barylite, bertrandite and Be-bear- Various host rocks ing leifite (Semenov et al. 1965; Semenov & Sørensen (including Eriksfjord Formation and 1966; Sobolev et al. 1970; Engell et al. 1971; Sørensen et Julianehåb granite) al. 1971, 1981; Petersen et al. 1991, 1994, 1995; Bohse et ★ Be mineral locality 0 1 2 3 km al. 2001, this volume; chemical formulae in Table 1). As far as it can be assessed from the literature and from my Fig. 1. Geological map of the Ilímaussaq alkaline complex based own field observations, the first three of these minerals on Ferguson (1964). The stars denoting the localities where Be minerals have been found are after Engell et al. (1971). Note are the only ones occurring in large quantities (tugtupite that Be minerals additionally occur at various places in the al- is even being ‘mined’ as an important local gemstone); the kali granite. others appear to be restricted to specific local chemical

146 Fig. 2. Photomicrographs of tugtupite replacing chkalovite in a sample from the a Kvanefjeld area. a: Tugtupite (Tug), albite (Ab) and analcime (Anl) enclosing relics of chkalovite (Chk). Crossed nicols. b: Tugtupite (Tug) replacing coarse chkalovite (Chk) in a vein-like manner. Finer grained chkalovite may be a second gen- Chk eration or may be recrystallised during late-magmatic deformation. Author’s sam- Ab ple GM 1401. Anl Tug

1 mm

Chk

Tug Chk

1 mm

environments. Epididymite and eudidymite are poly- assemblage chkalovite + sodalite is replaced by a va- morphs and they have been assumed to have the same riety of assemblages during infiltration with later hy- stability fields in the qualitative and semi-quantitative dia- drous fluids, the most important of which is the grams in this contribution. Helvite, genthelvite and soren- assemblage tugtupite + analcime + albite. This mode senite contain additional components (Zn, S or Sn) that of occurrence is identical to that reported from the complicate evaluation of their stability fields, and there- Kola peninsula (Lovozero complex; Semenov & fore they will not be discussed further in this contribution. Bykova 1960). Typical textures of tugtupite in the late- The most comprehensive accounts of Ilímaussaq stage veins from Kvanefjeld in the northern part of Be minerals and their occurrences are given by Engell the Ilímaussaq complex are shown in Fig. 2 and are et al. (1971) and Sørensen et al. (1971). According to more extensively documented in Sørensen et al. (1971). them, chkalovite is the earliest Be mineral in Ilímaus- Tugtupite forms in evident equilibrium with analcime saq. It occurs together with ussingite or sodalite and and most probably albite (although corroded grains appears to have formed due to reaction of late-mag- of albite associated with tugtupite have been reported matic fluids with nepheline-bearing wall rocks. The in some samples) during metasomatic recrystallisation

147 of the complete host rock. As far as one can tell from the outcrops on Kvanefjeld, the most intensely red coloured varieties come from veins cutting through altered augite syenite, whereas veins cutting through altered naujaite or lujavrite appear to show lighter, pinkish or pinkish red colours. All assemblages from Ilímaussaq reported in the literature so far are collec- ted in Table 2. Figure 1 shows the areas of Be mineralisation in the Ilímaussaq intrusion after Engell et al. (1971) and own observations. In the alkali granite, epididymite was reported to form stable, probably rather late assemblages with quartz and albite (Semenov & Sørensen 1966) which are partly altered to bertrandite. In similar textures, epididymite has also been reported from the Puklen intrusion on Nunarsuit in the Gardar province (Pul- vertaft 1961). In some Ilímaussaq nepheline syenites and in late-stage analcime-bearing veins, epididymite and eudidymite form by decomposition of chkalovite and are themselves replaced by still later tugtupite. All in all, the reported and observed textures support the interpretation that chkalovite is the earliest Be mineral in the undersaturated rocks of Ilímaussaq (maybe contemporaneous with epididymite/eudidymite in the alkali granite) and most later Be minerals formed by decomposition of this early chkalovite during infiltration of later hydrous fluids (see also Sørensen et al. 1971). The temperature and the most important chemical and physico-chemical characteris- tics of these fluids may be estimated using various types of phase diagrams.

800 800 A. acmite + albite + analcime B. ussingite + acmite + albite + relics of nepheline + analcime + relics of nepheline 2 600 600

C) Ab C) ° °

Ne + 2 SiO 500 500 Ab

Ne + 2 SiO

400 2 400

Temperature ( Temperature Jd+ ( Temperature P = 1 kbar Ne + SiO SiO2 P = 1 kbar a = 0.8 2 Jd Ne aNe = 0.8 a = 0.15 Jd+ 300 Jd 300 aJd = 0.15 a = 1 Ne + SiOSiO a = 1 Ab Jd 2 Ab –1.4 –1 – 0.6 – 0.2 –1.4 –1 – 0.6 – 0.2 log a log a SiO2 SiO2 Fig. 3. Temperature–silica activity diagrams for lujavrites from Ilímaussaq (from Sommer 1999). A: Ussingite-free, analcime-bearing assemblages, B: Ussingite + analcime-bearing assemblages. Nepheline and jadeite activities are calculated from measured mineral compositions reported in Sommer (1999). Ab: albite, Jd: jadeite, Ne: nepheline.

148 and for the beryllium minerals beryl, chrysoberyl, Phase diagrams for estimation of Be euclase, bertrandite and phenakite from Hemingway mineral stabilities et al. (1986) and Barton (1986). Although thereby The phase diagrams of this section were calculated loosing its internal consistency, this method provides using the computer program GEOCALC of Lieberman the only means of approximately quantifying the & Petrakakis (1991) with the thermodynamic database chemical parameters responsible for the formation of of Berman (1988) for the anhydrous albite–jadeite– the Na–Be silicates. The results appear reasonable in- nepheline phase diagrams (Figs 3, 4), and the pro- sofar as they agree with theoretical (Schreinemakers) gram SUPCRT of Johnson et al. (1992) with its ther- predictions and observed phase assemblages, but the modynamic database for the rest of the diagrams (Figs actual values may be slightly incorrect. Incorrect val- 5–7). The latter internally consistent database was aug- ues may be the reason for the obviously incorrect fea- mented with data for sodalite from Sharp et al. (1989) ture in Fig. 5, that phenakite saturation occurs at too

149 Fig. 4. XNaCl–silica activity diagram for min- 1 kbar, aJd = 0.1, aNe = 0.5 erals in the Na–Al–Si–O–H–Cl system at 0.5 350°C various temperatures and 1 kbar. Possi- ° + Sod 400 C 2 ble fluid evolution paths during cooling ° 600 C are schematically represented by the thick 6 SiO 6 Jd + 2 NaCl arrows. Anl: analcime, Ab: albite, Cpx: O) 2 0.4 Cpx Ab clinopyroxene, Jd: jadeite, Ne: nepheline, Jd + SiO Sod: sodalite. Ab 2 0.3 Sod = NaCl/(NaCl + H fluid evolution NaCl 0.2 X Sod Jd SiO + Na 6 Ne + 2 NaCl 2 Ab 0.1 Ne Anl Anl Ne –2.5 –2 –1.5 –1 –0.5 log a SiO2 low values to permit stable occurrence of beryl or to constrain as many of the variables as possible, the euclase at 600 and 400°C, respectively. It is assumed problem was divided into several steps as follows. that the actual saturation value for phenakite in terms of the Be2+/(H+)2 activity ratio is about 2 orders of 1. Temperature and a constraints were estimated SiO2 magnitude higher than the calculated value which as a first step using anhydrous albite–nepheline– would be consistent with the rest of the topology on jadeite equilibria (Fig. 3) and results from fluid in- Fig. 5, on which such a possible value is plotted as clusion studies on lujavrites (Sommer 1999; unpub- ‘true phenakite saturation?’. The calculated value for lished data). The mineral compositional data for phenakite saturation is also plotted on Fig. 5 and the these calculations are reported in Sommer (1999); part of the diagram above phenakite and above nepheline and jadeite activities were calculated from chkalovite saturation would correctly have to be the respective solid solutions using the models of termed metastable. The interpretation below, however, Ghiorso (http://melts.geology.washington.edu) and assumes that phenakite saturation occurs at values Holland (1990). These phase equilibria showed that permitting beryl and euclase to be stable in Fig. 5A, very late-stage ussingite-bearing lujavrite (which still B, respectively (‘true phenakite saturation?’). contains sodic clinopyroxene, albite and relics of It was chosen to fix the pressure for all phase dia- nepheline) formed at temperatures around 350°C grams at 1 kbar which is the depth of intrusion of and at silica activities around 0.1–0.2 (referring to Ilímaussaq based on fluid inclusion studies (Konne- unit silica activity for pure quartz at the applicable rup-Madsen & Rose-Hansen 1984). Variations in the P and T). In comparison, fluid inclusion studies in- few hundred bars range do not have a major influ- dicate homogenisation temperatures between 300 ence on the results of this study. Hence, fixing the and 400°C for primary and some secondary inclu- pressure and considering the reactions of interest sions in various lujavrite minerals and in tugtupite (Table 3), the following major physico-chemical pa- (Sommer 1999; Markl et al. in press). Sobolev et al. rameters remain to be investigated: temperature, a , (1970) measured temperatures of 400–460°C for pri- SiO2 2 2+ a /(a +) , a +/a +, a and XNaCl. The latter two mary inclusions in tugtupite and sorensenite; sec- Be H Na H H2O are closely connected as it was chosen to treat the ondary inclusions record temperatures between 100 fluid, as a first approximation, as a binary H2O–NaCl and 350°C. Based on their textures, the chkalovite– mixture with mixing and solution properties calcu- sodalite assemblages are believed to have formed lated according to Aranovich & Newton (1996). In order at some higher temperatures and a temperature of

150 Fig. 5. Activity–activity diagrams involving –1 minerals from the Be–Na–Al–Si–O–H–Cl 1 kbar, 600 °C, X = 0.05, a = 0.1 A NaCl SiO2 system at 600°C (A) and 400°C (B). Field Chkalovite saturation boundaries and saturation lines which were Beryl calculated from thermodynamic data (see –2 text) are plotted as thick lines; short dashed ) true phenakite saturation? 2 lines represent interpolated lines with cor- ) + rect (stoichiometric) slope; and widely H a

( –3 Tugtupite dashed lines have the correct slope but are /

+ (if stable at 600°C) not fitted to any independently constrained 2 Chrysoberyl invariant point. Note that tugtupite may not Be be stable at 600°C and that epididymite and a –4 Sodalite eudidymite may not be stable at SiO2 activities around 0.1. Note also that calculated log ( calculated phenakite saturation occurs at unrealistically interpolated with Phenakite saturation 2+ + 2 low Be /(H ) activity ratios and that the correct slope –5 part above phenakite and chkalovite satu- correct slope but ration in A and B is actually metastable as arbitrary position plotted here. ‘True phenakite saturation?’ shows a possible value for phenakite satu- 678910 ration consistent with the rest of the Be log (a Na+ / aH+) mineral topology. See text for discussion.

1 kbar, 400 °C, X = 0.05, a = 0.1 B NaCl SiO2 –1 Beryl Epididymite/Eudidymite saturation (probably not stable at these low a

true phenakite saturation?

Chkalovite saturation –2

) Tugtupite

2 Euclase ) + H

a ( /

+ –3 2 Chrysoberyl Be Phenakite sauration a

–4 log (

calculated interpolated with Analcime correct slope SiO 2 –5 correct slope but values!) arbitrary position

678910 log (a Na+ / a H+)

600°C was arbitrarily chosen for this stage. Studies of a fluid in equilibrium with the observed phases of Sobolev et al. (1970) indicate still higher tem- of the Na–Al–Si–O–H–Cl system (e.g., analcime, so- peratures in the 860–980°C range, but these results dalite, clinopyroxene, albite etc.) were calculated are impossible to reconcile with all other estimates (Fig. 4). Both variables are tied together when con-

in the Ilímaussaq rocks based on melting experi- sidering a binary H2O–NaCl fluid only. The activity– ments and phase equilibria (Sood & Edgar 1970; composition relationships were derived from the Markl et al. in press; Marks & Markl in press). formulation of Aranovich & Newton (1996) and activities were referenced to unit activities for pure

2. Based on this framework, limits on a and XNaCl H O and pure crystalline halite at appropriate P and H2O 2

151 T. These calculations showed that sodalite-bearing 600°C depends critically on the position of the

assemblages at 600°C require an XNaCl greater than chkalovite saturation line for which no real con- about 0.39 (Fig. 4), while analcime-bearing assem- straints exist at this temperature. At 400°C, phenakite blages at 400°C or below require hydrous fluids should be associated with tugtupite (see Fig. 5B)

with an XNaCl lower than about 0.15 corresponding but probably, in the presence of significant amounts

to an H2O activity of 0.7 or higher (Fig. 4). These of Zn and S in the fluid, genthelvite is stable with values are limits for the formation of the two most respect to phenakite (Burt 1988). However, gen- important Be minerals at Ilímaussaq, chkalovite and thelvite has only rarely been observed at Ilímaussaq. tugtupite. The early chkalovite–sodalite assemblage 2+ obviously equilibrated with a fluid (or melt?) of rela- 4. The combined importance of both SiO2 and Be / tively high salinity, while later tugtupite, although also (H+)2 activity ratio at fixed Na+/H+ activity ratio and Cl-bearing, formed from fluids of lower salinity. fixed fluid composition for the stability of both chkalovite and tugtupite is investigated in Fig. 6 for a 3. These data were then used to place constraints on temperature of 400°C. This diagram again combines the fluid chemistry in terms of Be2+/(H+)2 and Na+/ quantitatively calculated stability field boundaries H+ activity ratios by considering the typical granite and saturation lines with ones not calculated but association of the Na-free Be-minerals beryl, phen- fixed at an arbitrary, but reasonable, position and akite, euclase and chrysoberyl and their stability with correct slope. The salinity of the fluid appears relations with respect to tugtupite and sodalite. In to be of surprisingly little importance for the stabil- an Al-balanced activity–activity diagram as shown ity of the NaCl-bearing mineral tugtupite. Silica on Fig. 5, phenakite, chkalovite and epididymite/ activity, in contrast, is of much greater importance, eudidymite do not have their own stability fields as is the Be2+/(H+)2 activity ratio. This diagram indi- but are only shown by saturation lines because they cates that the assemblage of tugtupite and albite contain no Al. The stability fields and saturation should also contain phenakite as a stable Be phase line for beryl, euclase, chrysoberyl, sodalite and at silica activities above 10–0.4. phenakite were calculated quantitatively, while The diagrams of Fig. 7 show the qualitative phase those for tugtupite, chkalovite and epididymite/ relations among the Al-free Be silicates. Here, no eudidymite – because of the lack of thermodynamic quantitative constraint can be put on the site of the data – were inserted with correct slopes, but at ar- invariant point, where phenakite, chkalovite and bitrary positions constrained by calculated invari- epididymite or eudidymite are stable together. How- ant points and by the observed mineral assemblages ever, the slopes of the corresponding univariant re- reported above. action curves can be readily calculated from reaction Figure 5 shows that at 600°C and 1 kbar, euclase stoichiometry (see Table 3). Phenakite is stable at is not stable with respect to chrysoberyl and beryl, the highest Be2+/(H+)2 activity ratios, chkalovite at while at 400°C, all three Be–Al silicates have stabil- the highest Na+/H+ activity ratios and epididymite

ity fields on such a diagram. Whether tugtupite is or eudidymite at the highest SiO2 activities. stable with respect to chkalovite and sodalite at 5. Bertrandite stability was calculated according to the 600°C is not clear – it may well be that at these reaction: temperatures, tugtupite is only stable at extremely bertrandite = 2 phenakite + H O (unrealistically?) high Be2+/(H+)2 and/or Na+/H+ 2 activity ratios, or that it is not stable at all. At 400°C, at 1 kbar. At this pressure, this reaction lies at about the best temperature estimate for tugtupite forma- 220°C (see also Markl & Schumacher 1997). This tion, the Be–Al silicates are replaced by sodalite or means that the occurrence of bertrandite is clear tugtupite when the Na+/H+ activity ratio exceeds proof of the circulation of low-temperature hydrous about 108, while the Be concentration in the fluid fluids in the Ilímaussaq rocks. It is assumed that appears to be of only marginal importance for this the formation of beryllite and sphaerobertrandite reaction. The diagrams of Fig. 5 also clearly show (Semenov & Sørensen 1966; Andersen 1967) is also that phenakite is expected to be a stable phase in related to these low-temperature hydrous fluids. the Ilímaussaq Be mineral assemblages both at 600°C and at 400°C – with the exception of extremely high 6. The compatibility of the various Be mineral assem- Na+/H+ activity ratios. However, this evaluation at blages observed in various granitoid rock types can

152 Fig. 6. Activity–activity diagram at 1 kbar –3.5 = and 400°C constraining the stability of 1 kbar, 400ºC, a = 0.1 XNaCl 0.2 Jd (below 220 C bertrandite) various Be minerals with regard to vari- fixed aNa+/aH+ ratio, reaction curves Phenakite saturation X = 0.05 involving Be minerals fixed arbitrarily NaCl ous Be-free minerals in equilibrium with (with the exception of phenakite saturation) –4 fluids of variable salinity. Note that only ° phenakite saturation and the vertical field Chkalovite saturation boundaries among the Na–Al silicates are thermodynamically calculated, the other –4.5

lines are fixed at arbitrarily chosen in- ) 2 variant points. The grey arrow represents ) tugtupite + + analcime + a possible fluid evolution path at con- H sodalite + a albite ( stant temperature which would lead to / aegirine + –5 tugtupite formation. Note that everything 2

Be Tugtupite above the saturation lines are only meta- a stable extensions.

log ( –5.5

Sodalite (XNaCl = 0.2) Aegirine (XNaCl = 0.2) Albite –6

Nepheline Analcime (XNaCl = 0.05) (XNaCl = 0.05)

–6.5 –2.5 –2–1.5 –1 –0.5

log a SiO 2

fixed P,T,a H O, fixed P,T,a H O, 2 22 T > 220°C at 1 kbar T > 220°C at 1 kbar

Phenakite Chkalovite ) 2 ) + +) H

a H

a (a ( / / + 2+ Na

Be a a Epididymite/

log ( Eudidymite

log ( Chkalovite

Epididymite/ Fig. 7. Qualitative activity–activity diagrams Eudidymite Phenakite representing the stability relations among various Al-free Be silicates. Note that the slopes are correctly calculated from reac- log (a +/a +) log a tion stoichiometry. Na H SiO2

153 also be illustrated in another way. Figure 8 shows Discussion two SiO2–Na2O–BeO triangles with the inferred Be mineral associations at ‘higher’ (e.g. 600°C) and Stability of Na-bearing versus Na-free ‘lower’ (e.g. 200°C) temperature. These diagrams Be silicates

are projections from albite and an H2O–NaCl fluid, The very pronounced differences in Be mineral as- and in this projection euclase and chrysoberyl can- semblages in peralkaline versus calc-alkaline granitoid not be plotted. At higher temperatures, chkalovite rocks can be explained in terms of fluid chemistry is stable with sodalite and, possibly, with phenakite. which is also an approximate monitor of the melt If stable at these high temperatures, the assemblage chemistry. Na-bearing Be silicates only appear in per- epididymite or eudidymite with phenakite would alkaline or alkaline rocks (Fig. 8), whereas calc-alka- effectively render the co-existence of chkalovite with line intrusives are characterised by Be- or Be–Al beryl impossible. Beryl, however, should be stable silicates without Na. Chkalovite and tugtupite appear with phenakite and with epididymite/eudidymite. to be restricted to peralkaline rocks (i.e. fluids with Tugtupite appears not to be stable at these high extremely high Na+/H+ activity ratios, Fig. 5), whereas temperatures. All these constraints appear to agree epididymite and eudidymite are indicative of mode- with natural occurrences. At lower temperatures, rately alkaline rocks or fluids forming a link to beryl- phenakite is replaced by bertrandite, and beryl and bearing assemblages. Epididymite and eudidymite are chkalovite may become unstable depending on the especially indicative of higher silica activities than those exact details of the chemical environment. Tugtupite observed in agpaitic rocks (Fig. 7). Phenakite at high as well as epididymite/eudidymite may form by vari- and bertrandite at low temperatures are stable in both ous reactions involving chkalovite, phenakite and alkaline and calc-alkaline environments, unless helvite- sodalite. Under these conditions, tugtupite may form group minerals replace them due to the presence of a stable assemblage with epididymite/eudidymite additional components such as Mn, Zn or S (e.g. Burt and bertrandite (or, at temperatures above 220°C, 1988). Chrysoberyl is principally expected to be sta-

with phenakite). Epididymite/eudidymite may be in ble at low SiO2 activities and in high Al environments equilibrium with beryl and bertrandite (or phenakite). but obviously requires additionally either very high

high temperature low temperature (Chk + Sod stable) (Tug + Anl stable)

+Ab Na O Reactions: Na O +Ab 2 1. Chk + Sod = Tug + Anl 2 +H2O – NaCl-Fluid +H2O – NaCl-Fluid 0 2. Phen = Bert 0 1 3. Chk + Phen = Tug + Epi/Eu 1

0.2 0.2 0.8 0.8

0.4 0.4 0.6 0.6 Sod, Neph Anl 0.6 0.6 0.4 0.4 Chk (stable?) agpaitic Chk Tug systems 0.8 0.8 0.2 0.2 alkali Epi/Eu granitic 1 1 systems (stable?) Phen Epi/Eu Bert 0 0 calc-alkaline 0.2 BeO SiO 0.2 BeO granitic 1 0.8 0.6 0.4 0 2 1 0.8 0.6 0.4 0 systems SiO 2 Beryl Beryl (stable?)

Fig. 8. Phase relations in the Be–Na–Al–Si–O–H–Cl system projected from albite (Ab) and a binary NaCl–H2O fluid (variable salinity has no effect on depicted phase relations). At high temperatures (e.g. 600°C, left diagram), chkalovite (Chk) + phenakite (Phen) + sodalite (Sod) or nepheline (Neph) form a stable assemblage. Chkalovite + beryl is not stable if epididymite/eudidymite (Epi/Eu) is a stable phase at these temperatures. At lower temperature (e.g. 200°C, right diagram), chkalovite and beryl may no longer be stable, bertrandite (Bert) replaces phenakite, and tugtupite (Tug) forms by various reactions. Tugtupite should be stable with both epididymite/eudidymite and bertrandite as well as analcime (Anl).

154 Fig. 9. Reaction textures from an Ilímaus- saq sodalite foyaite in which a primary grain boundary between sodalite (up- Sodalite per left) and nepheline (lower right) is replaced by analcime and an analcime + sodalite symplectite. See text for discussion. Scale bar 200 µm. Author’s sample GM 1346.

Analcime Sodalite + Analcime

Nepheline

temperatures or very low Na+/H+ activity ratios, or both, lent in primary fluid inclusions in tugtupite, while pri- to be stable. Natural occurrences appear to be con- mary as well as secondary hydrous fluid inclusions in fined to high-grade metamorphic rocks or metamor- various minerals from various lujavrite samples record phosed pegmatites of low SiO2 and low Na content very low salinities in the 3–5 wt% NaCl equivalent (e.g. Franz & Morteani 1984). range (Sommer 1999; Markl et al. in press). Sobolev et

al. (1970) also report CO2 as an important species in

the gas phase of these inclusions, but CO2 has never been reported by anyone else in any fluid inclusion Stability of tugtupite versus chkalovite from the entire Ilímaussaq alkaline complex, where The above considerations indicate four possible causes methane is generally the most important species in for the conversion from chkalovite- to tugtupite-bear- any gaseous inclusion. Hence, the data of Sobolev et ing assemblages: decrease of temperature, increase of al. (1970) are considered with some caution – per- 2 2+ a , increase of a /(a +) , and increase of a (= haps the fluid inclusions in tugtupite leaked H2O and SiO2 Be H H2O decrease of XNaCl). were oxidised during later cooling, which would ex-

All available field observations, fluid inclusion stud- plain both their increased salinity and the CO2. ies and phase equilibrium considerations detailed However, consideration of the tugtupite-forming above indicate decreasing temperature during forma- reaction above and comparison with a reaction tex- tion of the various Be–silicate assemblages and during ture from a different Ilímaussaq rock, a sodalite foyaite conversion from chkalovite to tugtupite-bearing as- (Schwinn 1999), offers a possible explanation compati- semblages. Furthermore, fluid inclusions provide evi- ble with the observation of fluid inclusions of both dence for the existence of very water-rich fluids at low and high salinity in the late magmatic stages of temperatures in agreement with tugtupite formation the Ilímaussaq complex. This texture in the sodalite estimates. The net reaction connecting the chkalovite foyaite is shown in Fig. 9. There, sodalite and nephe- and the tugtupite assemblages is: line crystals co-existed at high temperatures. At lower temperatures, an obviously H O-rich fluid infiltrated 2 chkalovite + 2 sodalite + 4 H O + 6 SiO = 2 2 2 the rock along its grain boundaries and converted both 1 tugtupite + 4 analcime. sodalite and nepheline to analcime. The reaction zone At Ilímaussaq this reaction appears to occur around consists of a zone of pure analcime close to sodalite,

400°C at SiO2 activities around 0.1 (see Figs 3, 4) and and a zone of symplectic intergrowth of analcime and with fluids of high H2O activity. Sobolev et al. (1970) sodalite close to nepheline. This texture is interpreted reported high salinities of about 21 wt% NaCl equiva- to reflect changes in salinity during analcime growth:

155 while the starting fluid was H2O-rich and therefore ously have an anomalously high pH, as indicated by formed only analcime, it became progressively more the stability of ussingite (NaAlSi3O8 NaOH) and as indi- + + 2+ + 2 saline due to consumption of H2O and liberation of cated by the extremely high Na /H and Be /(H ) NaCl from dissolving sodalite, until the salinity was activity ratios calculated for the stability of the ob- so high that both analcime and a second generation served assemblages. Interestingly, experimental mix- of sodalite were precipitated in a symplectic tures of andalusite, paragonite and H2O develop intergrowth. This texture in the sodalite foyaite very extremely high pH at temperatures in the 400°C range likely formed during a late-stage auto-metasomatic (L. Baumgartner, personal communication 2000) which event similar to the events that led to the formation of is probably caused by the interplay between the analcime–tugtupite assemblages in the late veins, aluminosilicate dissociation (which consumes H+ ions) and hence a similar mechanism may have operated and formation of strong Na-silicate complexes (which there. In the late veins, sodalite was dissolved along replace NaOH-complexes more stable at higher tem- with chkalovite, rendering the fluid successively Be peratures). These features, however, may only be de- and NaCl rich, while analcime precipitated. At a cer- veloped in hydrous fluids with a high Na/(Na+K) ratio tain level of Be and NaCl enrichment, tugtupite began and with very low CO2 contents, such as in agpaitic to crystallise and enclosed some samples of the more fluids. The high pH in these fluids creates, at mode- saline fluid which Sobolev et al. (1970) found. While rate Be and Na ion activities, the extremely high pH- this does not explain their report of CO2, this mecha- normalised activities which, in turn, give rise to the nism shows how the Cl-rich mineral tugtupite could formation of the observed Be silicates. Neither the grow during influx of a hydrous, low-salinity fluid absolute Be nor the absolute Na contents in agpaitic into sodalite-bearing rocks. The pre-existence of fluids have to be very high, if the pH is as high as sodalite may even be a prerequisite for tugtupite for- indicated. In conclusion, the unique mineralisation of mation – the similarity of their crystal structures and agpaitic magmatic rocks and the formation of exotic chemical formulae (tugtupite = sodalite + Be2Si2Al–4) hyper-agpaitic, water-soluble minerals (e.g. Sørensen may also facilitate the chemical conversion of one into & Larsen 2001, this volume) may be an effect of the the other (see also Burt 1991). low oxidation state, low water activity and high Na/ (Na+K) ratio of the parental magma which renders

formation of a hydrous, CO2-poor, K-depleted, high- pH fluid phase possible in the very late stages of mag- Nature of the fluids in late-magmatic, matic evolution. agpaitic systems One type of primary fluid inclusions in lujavrites and most secondary fluid inclusions in lujavrite and all other rock types contain a hydrous fluid with about 3 Conclusions wt% NaCl equivalent (Schwinn 1999; Sommer 1999; Na-bearing Be minerals in the peralkaline Ilímaussaq Markl et al. in press). In rare examples, hydrous fluid alkaline complex, South Greenland, formed during inclusions of higher salinity have been found in these cooling and chemical evolution of agpaitic, late-mag- rock types and also in tugtupite and chkalovite matic liquids and fluids in the temperature range 600– (Sobolev et al. 1970; Konnerup-Madsen & Rose-Hansen 200°C. The earliest mineral to form was chkalovite 1982), and these may be fluids whose compositions which was successively replaced by tugtupite (in large were modified during interaction with sodalite-bear- quantities) and some rare Be silicates in small quanti- ing rocks as detailed above. The possible existence of ties. Qualitative and quantitative estimates of condi- late-stage immiscible hydrocarbon-bearing fluids tions and chemical parameters during formation indi- (Konnerup-Madsen 2001, this volume) does not alter cate that the early assemblages crystallised from liq- the results of the present contribution because the uids (or fluids) of high salinity, while the replacement aqueous fluid would still approximately behave as a occurred later during influx of a hydrous fluid of low binary H2O–NaCl system. salinity. Neither high Be nor high Na contents are nec- If one considers the low-salinity hydrous fluids to essary to explain the observed mineral assemblages, be the typical late-stage fluids of agpaitic rocks (which but the high pH of late agpaitic fluids, as demonstrated at least for Ilímaussaq appears to be reasonable), these by the occurrence of the NaOH-bearing silicate ussin- fluids have one very distinctive feature: they obvi- gite, appears to be responsible for high metal ion/H+

156 activity ratios in the late stage fluids. High pH and und Produkte. Neues Jahrbuch für Mineralogie Abhandlun- high Na/(Na+K) ratios appear to be responsible for gen 108, 166–180. the distinct differences in Be mineral assemblages be- Engell, J., Hansen, J., Jensen, M., Kunzendorf, H. & Løvborg, L. 1971: Beryllium mineralization in the Ilímaussaq intrusion, tween peralkaline, alkaline and calc-alkaline granitoid South Greenland, with description of a field beryllometer rocks. and chemical methods. Rapport Grønlands Geologiske Un- dersøgelse 33, 40 pp. Ferguson, J. 1964: Geology of the Ilímaussaq intrusion, South Greenland. Description of map and structure. Bulletin Grøn- lands Geologiske Undersøgelse 39, 82 pp. (also Meddelel- Acknowledgements ser om Grønland 172(4)). I thank Gunnar Raade for providing missing literature Franz, G. & Morteani, G. 1984: The formation of chrysoberyl in metamorphosed pegmatites. Journal of Petrology 25, 27–52. and some detailed information on Ilímaussaq Be min- Hemingway, B.S., Barton, M.D., Robie, R.A. & Haselton, H.T. eral stabilities. D. Burt, J. Konnerup-Madsen, W.S. Watt, 1986: Heat capacities and thermodynamic functions for beryl, L.M. Larsen, P.S. Neuhoff and H. Sørensen are thanked Be3Al2Si6O18, phenakite, Be2SiO4, euclase, BeAlSiO4(OH), for critical comments on an earlier version of the manu- bertrandite, Be4Si2O7(OH)2, and chrysoberyl, BeAl2O4. Ameri- script. This work was funded by the Deutsche Forsch- can Mineralogist 71, 557–568. ungsgemeinschaft as part of the project Ma2135/1-1. Holland, T.J.B. 1990: Activities of components in omphacitic solid solutions. Contributions to Mineralogy and Petrology 105, 446–453. Johnson, J.W., Oelkers, E.H. & Helgeson, H.C. 1992: SUPCRT92: References A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, Andersen, S. 1967: On beryllite and bertrandite from the Ilí- and reactions from 1 to 5000 bars and 0 to 1000°C. Comput- maussaq alkaline intrusion, South Greenland. Bulletin Grøn- ers & Geosciences 18, 899–947. 2 lands Geologiske Undersøgelse 68 , 11–27 (also Meddelel- Konnerup-Madsen, J. 2001: A review of the composition and II ser om Grønland 181(4 )). evolution of hydrocarbon gases during solidification of the Aranovich, L. & Newton, R. 1996: H2O activity in concentrated Ilímaussaq alkaline complex, South Greenland. In: Søren- NaCl solutions at high pressures and temperatures meas- sen, H. (ed.): The Ilímaussaq alkaline complex, South Green- ured with the brucite-periclase equilibrium. Contributions land: status of mineralogical research with new results. Ge- to Mineralogy and Petrology 125, 200–213. ology of Greenland Survey Bulletin 190, 159–166 (this vol- Barton, M. 1986: Phase equilibria and thermodynamic proper- ume). ties of minerals in the BeO–Al2O3–SiO2–H2O (BASH) system, Konnerup-Madsen, J. & Rose-Hansen, J. 1982: Volatiles associ- with petrologic applications. American Mineralogist 71, 277– ated with alkaline igneous rift activity: fluid inclusions in the 300. Ilímaussaq intrusion and the Gardar granitic complexes (South Berman, R.G. 1988: Internally-consistent thermodynamic data Greenland). Chemical Geology 37, 79–93. for minerals in the system Na O–K O–CaO–MgO–FeO–Fe O – 2 2 2 3 Konnerup-Madsen, J. & Rose-Hansen, J. 1984: Composition and Al O –SiO –TiO –H O–CO . Journal of Petrology 29, 445–522. 2 3 2 2 2 2 significance of fluid inclusions in the Ilímaussaq peralkaline Bohse, H., Petersen, O.V. & Niedermayr, G. 2001: Notes on granite, South Greenland. Bulletin de Minéralogie 107, 317– leucophanite from the Ilímaussaq alkaline complex, South 326. Greenland. In: Sørensen, H. (ed.): The Ilímaussaq alkaline Larsen, L.M. & Sørensen, H. 1987: The Ilímaussaq intrusion: complex, South Greenland: status of mineralogical research progressive crystallization and formation of layering in an with new results. Geology of Greenland Survey Bulletin 190, agpaitic magma: In: Fitton, J.G. & Upton, B.G.J. (eds): Alka- 119–121 (this volume). line igneous rocks. Geological Society Special Publication Burt, D.M. 1975: Beryllium mineral stabilities in the model sys- (London) 30, 473–488. tem CaO-BeO-SiO2-P2O5-F2O–1 and the breakdown of beryl. Lieberman, J. & Petrakakis, K. 1991: TWEEQU thermobarometry; Economic Geology 70, 1279–1292. analysis of uncertainties and applications to granulites from Burt, D.M. 1988: Stability of genthelvite, Zn (BeSiO ) S: an ex- 4 4 3 western Alaska and Austria. Canadian Mineralogist 29, 857– ercise in chalcophilicity using exchange operators. Ameri- 887. can Mineralogist 73, 1384–1394. Mandarino, J.A. & Harris, D.C. 1969: Epididymite from Mont St. Burt, D.M. 1991: Vectors, components, and minerals. American Hilaire, Quebec. Canadian Mineralogist 9, 706–709. Mineralogist 76, 1033–1037. Markl, G. & Schumacher, J. 1997: Beryl stability in local hydro- „ erný, P. 1963: Epididymite and milarite – alteration products thermal and chemical environments in a mineralized gran- •ñ of beryl from V ná, Czechoslovakia. Mineralogical Maga- ite. American Mineralogist 82, 195–203. zine 33, 450–457. Markl, G., Marks, M., Schwinn, G. & Sommer, H. in press: Phase „ erný, P. 1968: Beryllumwandlungen in Pegmatiten – Verlauf equilibrium constraints on intensive crystallization param-

157 eters of the Ilímaussaq intrusion, South Greenland. Journal stability of sodalite in the system NaAlSiO4–NaCl. Geochim- of Petrology. ica et Cosmochimica Acta 53, 1943–1954. Marks, M. & Markl, G. in press: Fractionation and assimilation Sobolev, V.S., Bazarova, T.Y., Shugurova, N.A., Bazarov, L.Sh., processes in the alkaline augite syenite unit of the Ilimaussaq Dolgov, Yu.A. & Sørensen, H. 1970: A preliminary examina- intrusion, South Greenland, as deduced from phase equilibria. tion of fluid inclusions in nepheline, sorensenite, tugtupite Journal of Petrology. and chkalovite from the Ilímaussaq alkaline intrusion, South Mårtensson, C. 1960: Euklas und Bertrandit aus dem Feldspat- Greenland. Bulletin Grønlands Geologiske Undersøgelse 81, pegmatit von Kolsva in Schweden. Neues Jahrbuch für Mi- 32 pp. (also Meddelelser om Grønland 181(11)). neralogie Abhandlungen 94, 1248–1252. Sommer, H. 1999: Kristallisationsgeschichte und Fluidentwick- Petersen, O.V., Randløv, J., Leonardsen, E.S. & Rønsbo, J.G. lung in Lujavriten der Ilímaussaq-Intrusion in Grönland, 143 1991: Barylite from the Ilímaussaq alkaline complex and as- pp. Unpublished M.Sc. thesis, University of Freiburg, Germany. sociated fenites, South Greenland. Neues Jahrbuch für Min- Sood, M.K. & Edgar, A.D. 1970: Melting relations of undersatu- eralogie Monatshefte 1991, 212–216. rated alkaline rocks from the Ilímaussaq intrusion and Grøn- Petersen, O.V., Rønsbo, J.G., Leonardsen, E.S., Johnsen, O., nedal-Íka complex South Greenland, with water vapour and Bollingberg, H. & Rose-Hansen, J. 1994: Leifite from the Ilí- controlled partial oxygen pressure. Meddelelser om Grøn- maussaq alkaline complex, South Greenland. Neues Jahr- land 181(12), 41 pp. buch für Mineralogie Monatshefte 1994, 83–90. Sørensen, H. 2001: Brief introduction to the geology of the Ilí- Petersen, O.V., Micheelsen, H.I. & Leonardsen, E.S. 1995: maussaq alkaline complex, South Greenland, and its explo-

Bavenite, Ca4Be3Al[Si9O25(OH)3], from the Ilímaussaq alka- ration history. In: Sørensen, H. (ed.): The Ilímaussaq alkaline complex, South Greenland. Neues Jahrbuch für Miner- line complex, South Greenland: status of mineralogical re- alogie Monatshefte 1995, 321–335. search with new results. Geology of Greenland Survey Bul- Pulvertaft, T.C.R. 1961: The Puklen intrusion, Nunarssuit, South letin 190, 7–23 (this volume). Greenland. Bulletin Grønlands Geologiske Undersøgelse 29, Sørensen, H. & Larsen, L.M. 1987: Layering in the Ilímaussaq 33–50 (also Meddelelser om Grønland 123(6)). alkaline intrusion, South Greenland. In: Parsons, I. (ed.): Roering, C. & Heckroodt, R.O. 1966: The alteration of beryl in Origins of igneous layering. NATO Advanced Science Insti- the Dernburg pegmatite, Karibib, South West Africa. Annals tutes, Series C. Mathematical and Physical Sciences 196, 1– of the Geological Survey, Republic of South Africa 3, 133– 28. Dordrecht: D. Reidel Publishing Co. 137. Sørensen, H. & Larsen, L.M. 2001: The hyper-agpaitic stage in Schwinn, G. 1999: Kristallisationsgeschichte und Fluidentwick- the evolution of the Ilíaussaq alkaline complex, South Green- lung in agpaitischen Gesteinen der Ilímaussaq-Intrusion in land. In: Sørensen, H. (ed.): The Ilímaussaq alkaline com- Grönland, 132 pp. Unpublished M.Sc. thesis, University of plex, South Greenland: status of mineralogical research with Freiburg, Germany. new results. Geology of Greenland Survey Bulletin 190, 83– Semenov, E.I. & Bykova, A.V. 1960: Beryllosodalite. Doklady 94 (this volume). Akademii Nauk SSSR 133(5), 1191–1193 (in Russian). Sørensen, H., Danø, M. & Petersen, O.V. 1971: On the mineral-

Semenov, E.I. & Sørensen, H. 1966: Eudidymite and epididymite ogy and paragenesis of tugtupite Na8Al2Be2Si8O24(Cl,S)2 from from the Ilímaussaq alkaline intrusion, South Greenland. the Ilímaussaq alkaline intrusion, South Greenland. Bulletin Bulletin Grønlands Geologiske Undersøgelse 63, 21 pp. (also Grønlands Geologiske Undersøgelse 95I, 38 pp. (also Med- Meddelelser om Grønland 181(2)). delelser om Grønland 181(13)). Semenov, E.I., Gerassimovsky, V.I., Maksimova, N.V., Ander- Sørensen, H., Rose-Hansen, J. & Petersen, O.V. 1981: The min- sen, S. & Petersen, O.V. 1965: Sorensenite, a new sodium- eralogy of the Ilimaussaq intrusion. In: Bailey, J.C., Larsen, beryllium-tin-silicate from the Ilímaussaq intrusion, South L.M. & Sørensen, H. (eds): The Ilímaussaq intrusion, South Greenland. Bulletin Grønlands Geologiske Undersøgelse 61, Greenland. A progress report on geology, mineralogy, geo- 19 pp. (also Meddelelser om Grønland 181(1)). chemistry and economic geology. Rapport Grønlands Geo- Sharp, Z., Helffrich, S.R., Bohlen, S.R. & Essene, E.J. 1989: The logiske Undersøgelse 103, 19–24. Stevenson, R., Upton, B.G.J. & Steenfelt, A. 1997: Crust–mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies. Lithos 40, 189–202.

158