American Mineralogist, Volume 75, pages 859-873, 1990

Fluorian garnets from the host rocks of the Skaergaard intrusion:Implications for metamorphic fluid composition

Curc E. M.lNNrNGr* DnNNIS K. BnoDepartment of Geology, Stanford University, Stanford, California 94305, U.S.A.

AesrRAcr

Zoned, silica-deficient, calcic gamets containing up to 5 molo/o F substitution for Oformed during contact metamorphism of basalts by the Skaergaard intrusion in EastGreenland. Fluorian calcic garnets occur as a retrograde alteration ofprograde wollastoniteand clinopyroxene that fills vesicles and vugs in lavas 30-70 m from the intrusion. Para-genetically equivalent phases include fluorite, wollastonite, calcic clinopyroxene, prehnite,quarlz, and calcite. Electron microprobe analysis shows that the garnets are >93 mo10/ogrossular-andradite (grandite) solid solutions and that the F content does not fully com-pensate for the silica deficiency, suggesting the presence of a hydrous component in thegarnets. The garnets display discontinuous zoning with respect to Al and Fe, and increasesin F, calculated OH, and the Si deficiency with increasing Al concentration are observed.The garnets formed at temperatures between 200 and 420 "C based on the coexistingmineral assemblage, and stratigraphic reconstructions indicate pressures of - I kbar.

Assessment of isobaric, isothermal phase relations in the system CaO-AlrOr-SiOr-HrO-HF allows estimation of fluid characteristics in equilibrium with fluorian grandite-bearingassemblages. The presence of fluorite, wollastonite, quartz, and calcite with these garnetscombined with fluid inclusion data require that the activity (a) of H'O was - I and thateH,aF,was l0-ro5 to l0-Io'o in the coexisting hydrothermal solutions at200-420 "C and Ikbar. IfpH was neutral at these pressure and temperature conditions (5.3 + 0.1), the valueofa.-associatedwithfluoriangarnetformationwas l0-a?to l0 52. TheFcontentofgarnetis extremely sensitive to minor changes in fluid composition. The calculations show thata decrease in pH or an increase in log ao of 0.3 at constant pressure and temperature willdecrease the F concentration in garnet from 5 to 0 molo/0. The results of this study showthat fluorian hydrous grandites provide a mineralogical record ofthe activities ofF speciesin coexisting metamorphic and hydrothermal fluids.

IurnonucrroN

The concentrations of F and OH in minerals such asmicas and apatites have been used to infer activities of Fspecies in coexisting fluids (e.g., Eugster and Wones, 1962;Munoz and Ludington, 1974, 1977;Korzhinskii, 1981;Munoz, 1984). In grossular-andradite (grandite) solid so-lutions, the substitution of OH is well documented (e.g.,Pabst, 1942; Peters, 1965; Cohen-Addad et al., 1967;Lager et al., 1987, 1989), and the recent recognition ofFsubstitution suggests that this phase can also be used toconstrain activities ofF species in geologic fluids. Gran-dite garnets with up to 3.6 wt0/o F have been reportedfrom three geologic environments. These environmentsinclude metamorphosed and metasomatically altered car-bonates (Dobson, 1982; Yun and Einaudi, 1982; Valleyet al., 1983), metasomatically altered alkaline intrusiverocks (Nash and Wilkinson, 1970; Flohr and Ross, 1989),and contact-metamorphosed basalts (van Marcke de

*Present address: U.S. Geological Survey, M.S. 910, 345Middlefield Rd., Menlo Park, California 94025, U.S.A.

0003-o04x/90/070848 59$02.00

Lummen, 1986). Even higher F concentrations (up to 5.6wtolo) have been observed in subcalcic Mn-rich garnetsfrom the porphyry Mo deposit at Henderson, Colorado(Smyth etal.,1990; Seedod 1987).

Cell-edge refinements of natural fluorian garnets indi-cate that F replaces O in the garnet structure (Valley etal., 1983; Smyth et al., 1990). To account for the ob-served F enrichment in grossular-rich fluorian garnets,Valley et al. (1983) suggested that F may enter the garnetlattice via two coupled substitutions:

[tor!(OH,F)o]o : [rorsion]o- (l)

and

[rurMr*(OH,D]+: [6]M3+Ol+, (2)

where the superscripts [4] and [6] denote tetrahedrallyand octahedrally coordinated sites, E represents a vacan-cy in the designated site, and M denotes a cation with thesuperscripted charge. In grandite garnets with low t6lM2+

contents, most F and OH will be accommodated byEquation l. Because Equation I involves exchange ofSi,

859

860 MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION

O, F, and H, the extent of substitution and zoning in F-and OH-bearing grandites may be used to evaluate rela-tive changes in the chemical potentials of the thermo-dynamic components HrO, HF and SiO, in coexistingmetamorphic and hydrothermal fluids.

In this paper we report new analyses ofF-bearing gar-nets from the contact-metamorphosed basaltic countryrocks of the Skaergaard intrusion, East Greenland. Weuse natural phase equilibria to derive a method for usingthe F content of grandite garnets to evaluate the chemicalpotential of the HF in the coexisting fluid phase. Thesemethods are used to compare the East Greenland garnetsto F-bearing grandites from the Adirondacks to evaluatedifferences in metamorphic fluid compositions betweenthe environments.

Occunnnxcn

Heat transfer associated with the emplacement andcooling of the gabbroic Skaergaard intrusion (Fig. l)caused hydrothermal fluid circulation and metamor-phism in its host rocks (Wager and Deer, 1939; Taylorand Forester,19791, Norton and Taylor, 1979; Norton eta l . , 1984; Bi rd et a l . , 1985, 1986, 1988a, 1988b; Man-ning, 1989; Manning and Bird, 1986, in preparation). In-terbedded basaltic aa and vesicular lavas (Fig. 2) weremetamorphosed during crystallization and cooling of thegabbro. The map area in Figure 2 can be divided intothree metamorphic mineral zones: the actinolite * chlo-rite zone (>210 m from the contact), the pyroxene zone(10-210 m), and the olivine zone (0-10 m). Diagnosticminerals include metamorphic olivine in the olivine zone,metamorphic clinopyroxene and orthopyroxene in the py-roxene zone, and actinolite and chlorite replacing mag-matic clinopyroxene in the actinolite + chlorite zone.Peak metamorphic temperatures ranged from - 1000 'C

at the contact to -300'C at 500 m. Lithostatic pressurewas - I kbar as estimated from a burial depth of 4 + Ikm (Manning, 1989).

After the development of prograde metamorphic min-eral assemblages, the contact aureole was crosscut by aseries of vertical veins associated with mafic dike em-placement and the cooling and contraction of the solidi-fied gabbro and its host rocks. Based on fracture-fillingmineral assemblages and textures, the veins can be divid-ed into two types: (l) granophyres with granophyric in-tergrowths of quartz and potassium feldspar that proba-bly reflect crystallization from a silicate liquid, and (2)veins filled by hydrothermal mineral assemblages, in-cluding actinolite + chlorite, epidote + qvartz, and quartz.The granophyres formed shortly after gabbro crystalli-zation and occur within 100 m from the intrusive con-tact. Hydrothermal veins cross cut the granophyres andare found throughout the area shown in Figure 2 (Man-ning, 1989).

Fluorine-bearing garnets are associated with complexcalc-silicate mineral assemblages in paleo-pore structures(amygdules) < 5 cm wide that include breccia matricesand vesicles. Amygdules containing fluorian grandites oc-

cur in a restricted area ofthe pyroxene zone between 30and 70 m from the contact (Fig. 2). Petrographic exami-nation of six garnet-bearing samples from this zone (Fig.2) allows division of the pore-filling minerals into earlyand late assemblages based on overgrowth and replace-ment textures. The early assemblage at the margins of theamygdules consists of ferroan diopside to magnesian hed-enbergite, with minor apatite and titanite, whereas theearly assemblage in amygdule centers consists of pris-matic wollastonite and magnesian hedenbergite.

Fluorine-bearing garnets formed as part ofa later, ret-rograde assemblage altering prismatic wollastonite andpyroxene in amygdule centers. Other retrograde mineralsin amygdule centers are fluorite, calcite, quarlz, acicularwollastonite, and magnesian hedenberyite. Near the wallsof amygdules, prehnite and locally calcite overgrow earlypyroxene, apatite, and titanite. Prehnite and calcite areparagenetically equivalent to the late garnet-bearing as-semblage found in amygdule centers.

The garnets are up to 0.5 mm in the longest dimension,range from subhedral to anhedral crystal forms, and arezoned (Fig. 3). Figure 3 shows that garnet cores have highbackscattered electron intensities, and qualitative energydispersive analysis indicates that these zones are Fe-rich.All garnets display discontinuous outward zoning to moreAl-rich compositions with lower intensities of backscat-tered electrons (e.g., point labeled "b" in Fig. 3A). Zoningin the garnets is also marked by abrupt reversals in com-position; for example, the point labeled "c" in Figure 3,A'is more Fe-rich than earlier formed compositions.

The hydrothermal fluids associated with mineralogicand isotopic alteration of the Skaergaard's basaltic hostrocks were dilute meteoric waters with -1.5 wto/o NaClequivalent (Taylor and Forester, 1979; Bird et al., 1988b;Manning, 1989) and probably did not contain substantialF. However, the later alteration assemblage containingF-bearing minerals such as fluorite and fluorian garnetoccurs only in the portion of the aureole <100 m fromthe contact where granophyre veins are found (Manningand Bird, in preparation). Bird et al. (1986) report fluidinclusions with -8 wto/o NaCl equivalent in quartz fromthese granophyres. It therefore seems likely that the flu-orine-bearing minerals formed as a consequence of localmixing of dilute aqueous pore fluids and fluids exsolvedfrom the granophyres.

ANLr-vrrc.ll, METHoDS

Garnet compositions were determined using a JEOL7334, electron microprobe with five wavelength-disper-sive spectrometers. Operating conditions included l5 kVaccelerating potential, 15 nA current on Faraday cup, adefocused 5-10 pm beam diameter, and counting timesof 40 s for F and 30 s for all other elements. Weightfractions of oxides were calculated using a Bence and Al-bee (1968) matrix correction scheme (Chambers, 1985).Compositional uncertainties given in this study reflectrandom errors in X-ray counts and were calculated fromthe product of the k-ratio, the correction factor used to

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION 8 6 1

Fig. l. Geologic map showing the Skaergaard intrusion and its host rocks (modified after Bird et al., 1986).

862 MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION

CF THE SKAERGAARD EAST CONTACT

7v, rur,"trno* EFI :ffi:aard intrusion

t l r j g a u u t u

- : , . , - - . . - . . - ' . . - . - - . \ ' , " ' r

I i \ ' , i - a - 7 t M a f i c d i k e w f t h t l E - - J D i a b a s e s i r l14, i nca margnar Dreccra t ' . " ' - , , 1

FIIII Vandtaldsdalen l-.-:ll Massive to

lYt I nca marginal breccia [i.-l]r': ' l utaoase strt

ffiffi VanOtaldsdalen [;l Massive to vesicularI . ' . : . : i . : . j macrodike gabbro I . " ' l basal t f low

lffi"-] na basalt flow with, H..":! local massive center

liB E---- --l

le wlr l Volcanic last ic breccia

T. ' l i , , ' l l\d'iiWu"r" , Y Normar raurt: ^ ' , i " i ' .F , i ' \ ->' : ' " .- : : . f f ; ; '"u "U-#X^,16*'&L, ^J , l

7

/.'t:Yl""s k g

r- - . {>' ,r - - . \> '

/-,_-Y,/. :l

rf, 7)\ .1.

" ' r ' r r ' Z

,"-, " o li,

m e t e r s

>2 0

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION 863

TABLE 1. Comparison of analyses of a single garnet zone usingdifferent F standards

AverageAverage analysis

analysis using using fluorianfluorian topaz as phlogopite as

F standard F standard(5 analyses) (4 analyses) Uncertainty'

Weight percent

0 . 1 70 0 20 1 30.070.040.010.230.32

0.29

2.s980.402

<0.0011.8590.1 400.0190.0s3

<0.0052.9280.6360.972

' Uncertainties apply to both analyses and reflect random errors in X-raycounts (see text).

t. All Fe as Fe,O3.f HrO weight fraction converted from OH calculated by Equation 3 (see

text).f Calculated by Equation 3 (see text).

convert the k-ratio to weight percent, and one standarddeviation in the k-ratio (Chambers, 1985).

Fluorine was analyzed using a TAP crystal with astretched 100 A polypropylene spectrometer windowmounted on Ni mesh. The F minimum detection limitwas 0. l0 wto/o under the operating conditions given aboveand the maximum observed F peak-to-background ratiowas 9.8. Fluorian topaz (Barton et al., 1982) was used asthe F standard for all analyses reported here; however,Solberg (1982) and Valley (personal communication,1989) have observed Ihattopaz F Ka peak shapes differfrom those for micas, amphiboles, apatites, and fluorides,and these workers suggest that this can lead to errors of-20o/o in F concentration. To evaluate possible system-atic errors owing to the choice of standard, we reanalyzeda single F-rich portion of a garnet from sample I 187 us-ing as F standards both fluorian topaz (20.3 wto/o F) and

Fig. 3. Backscattered electron images of zoned F-bearinggrandite garnets from the east contact of the Skaergaard intru-sion. High and low intensity signatures correspond to iron- andaluminum-rich zones, respectively. (A) Anhedral garnet (gt) co-existing with clinopyroxene (cpx) and fluorite (fl) in sample 8440.The earliest garnet grains (a) are Fe-rich, the intermediate por-tions (b) show discontinuous, oscillatory zoning alternating be-tween Al- and Fe-rich compositions, and the latest garnet (c) isFe-rich. Field ofview is 120 pm wide. (B) Subhedral, discontin-uously zoned garnet coexisting with wollastonite (wo) and fluoritein 8440. Note well-developed crystal faces and sharp composi-tional breaks at (a). Field ofview is 400 pm wide.

sio,Tio,AIr03FerO"*'MnOMgoCaOF

Sum-F=O

H,Of

S ir41!TiAIFe3t

Fe.'

MnMg

FoH+

2.5960.404

<0.0011.8640.1 360.0190.055

<0.0052.9260 6131.003

0.0060.0060.0010.0080.0040 0040.0030.0050.0080.0680.074

34.29 34.09<0.02 <0.0220.89 20 692.72 2.780.86 0.82

<0.04 <0.0436 07 35.862.s6 2.64

97 39 95.77- 1 . 0 8 - l 1 196.31 95.773.97 3.82

100.28 99.59Atoms per 5 (X + Y) atoms

AL

Fig. 2 . Geology of the Skaergaard east contact. The elevation contours are in meters above sea level, and the labels uv I -uv I 0denote individual lava flows in the map area. Localities at which fluorian grandites have been found are shown with the filled circlesand samples 8440 and I 187 are discussed in the text.

864 MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION

TABLE 2. Representative garnet analyses

c8 c7int. rim

c1 dcore int'

h2 i4nm nm

h1core

d3 d14 d lcore int' rim

Sampleanaly-

sts

Weight percent

34.870.050.35

30.641.570.07

31.55<0.10<0.02

0.45

33.76<0.02

9.4817.970.190 . 1 6

35.340.91

<0.024.03

34.22<0.02

0.1630.560.070.44

32.670.33

<0.021 . 1 4

34.21<o.02<0.0531.03<0.04

0.1432.94<0.10<0.02

1.30

36.240.32

17.297.03o.37

<0.0436.391.47

<0.022.09

31.200.33

15.628.820.29

<0.0435.791.56

<0.027.08

33.69<0.02<0.0531.550.570.14

32.20<0.10<0.02

2.01

35.000.622.17

27.570 . 1 10.07

33.220.38

<o.020.53

35.41 33.891.57 0.039.70 20.93

16.05 2.800.15 0.970.09 <0.04

34.69 35.940.19 2.70

<0.02 <0.022.06 4.39

sio,Tio,Al,o3FerO.**MnOMgoCaOF

H,Ot

Sumf

5 lt4lE

TiAIFe3*Fe,*$MnMgCa

101 .46 99.45 99.62 100.58 100.04Atoms per 5 (X + Y) atoms

100 .1 6 99.51 99.82 100.51

2.968 2.6800.032 0.3200.003 <0.0010.035 0.8871.950 1.0740.013 0.0000.1 13 0 .0130.009 0.0192.877 3.006

2.906 2.7770.094 0.223

< 0.001 0.018<0.005 1.562

1.983 0.3880.000 0.017

<0.003 0.0240.018 < 0.0052.998 2.988

2.9040.096

<0.0010.0161.9520.0000.0050.0562.97'l

2.4410.5590.0191.44' l0.5000.0190.019

<0.0053.001

2.8580.142

<0.001<0.005

1.9810.0330.0410.0182.926

2.9380.0620.0390.2151.7070.0350 0080.0092.988

2.850 2.5620.150 0.4380.095 0.0020.920 1.8650.888 0.1300.084 0.0290.010 0.0620.011 <0.0052.992 2.911

SumFcloHl l

8.000<0.o27<0.003

o.127

7.999 8.0000.228 0.089

<0.003 <0.0031.066 0.324

7.999 7.999 7.999<0.027 0.356 0-386<0.003 <0.003 <0.003

0.368 0.534 1.848

7.999 8.001<0.027 0.101<0.003 <0.003

0.570 0.148

8.000 7.9990.048 0.645

<0.003 <0.0030.552 1. ' t07

. Intermediate between core and rim.-'All Fe as FerO3.t HrO weight fraction converted from OH calculated by Equation 3 (see text).+Ad jus ted fo rO=F .$ Calculated by charge balance.ll Calculated by Equation 3 (see text).

synthetic fluorian phlogopite (9.01 wt0/o F, synthesized byD. Wones and provided by G. Czamanske). Table I givesanalyses of the same garnet zone using the different Fstandards and shows that concentrations of all analyzedelements, including F, are the same within the stated un-certainties.

Calculation of formulae of F- and OH-bearing garnetsfrom electron microprobe analyses is problematic owingto the presence oftetrahedral vacancies, the possibility oftetrahedral coordination of Al, Fe3*, and Ti (Meagher,1980; Deer et al., 1982, and references therein), and un-certainties in the relative amounts of Fe2* and Fe3*. Val-ley et al. (1983) normalized garnet formulae to 5 dodeca-hedral (X) and octahedral (Y) cations and assumed notetrahedrally coordinated Al, Fe3*, or Ti. Flohr and Ross(1989) calculated formulae based on l2 anions, allowingfor tetrahedral coordination of Al. Fe3*. and Ti but as-suming that HrO concentration is equal to 100 minus theoxide sum corrected for other anions. Although substi-tution of Al, Fe3t, and Ti for Si is common in Ti-richgarnets (e.g., Huggins et al., 1977), the amount of tetra-hedral coordination of these cations is small in low-tem-perature OH-bearing garnets in the presence of quartz(e.g., Huckenholz and Fehr, 1982). Because these condi-

tions obtained during the formation of the garnets dis-cussed here, formulae were normalized to 5 (X + Y) cat-ions as described by Valley et al. (1983). This methodyields a minimum estimate of OH concentration throughthe equation

non: 4n6ot1 - lxo I nr i - n{6t4d (3)

where n is the number of atoms of the subscripted ele-ment or vacancy per 5 (X + Y) atoms and IalE representstetrahedral vacancies, which are equal to the differencebetween lz., and 3.0. The assumption that OH balancescharge is supported by infrared absorption spectroscopy,nuclear magnetic resonance spectroscopy, and neutrondiffraction studies on natural and synthetic hydrogran-dites (Cohen-Addad et al., 19 67 ;Lager et al., I 987, I 989).Although additional OH in excess of that calculated fromEquation 3 is unlikely (Lager et al., 1989), the lack ofadirect determination of FerO, and FeO requires that thecalculated OH concentration be considered a minimum.

The greatest uncertainty in garnet compositions is theOH content. Inspection of Equation 3 shows that the un-certainty in computed OH can be calculated as the sumof random errors in Mg, Ti, F, and tetrahedral vacancies,where the latter are equivalent to errors in Si concentra-

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SI({ERGI'ARD INTRUSION 865

tion. The relatively large errors in F and Si (Table l) thuslead to uncertainties of 7.5o/o in the minimum calculatedOH per 5 (X + Y) cations. Such large uncertainties, com-bined with the possibility of additional OH throughEquation 2, indicate that caution should be used in in-terpreting OH concentrations computed by charge bal-ance.

G.LnNnr coMposrrloNs

Representative compositions of fluorian garnets fromsamples 8440 and I187 (Fig. 2) are in Table 2. Garnetsfrom these samples are >93o/o grossular-andradite solidsolutions and range in mole fraction of aluminum [Xo,,where Xo, : no,/(nn + rze.r)] from 0.00 to 0.93. Maxi-mum observed pyralspite components are 0.80/oMgrAlrSi.O, r, 3. 7olo MnrAl2Si3O,r, and I . I o/o FerAlrSirO, r.Garnets are melanitic between cores and rims in sampleI I 87 and contain between 0.039 and 0. 168 Ti atoms per5 (X + Y) atoms (2.0-8.4o/o Ti in Y). All orher garnetscontain <0.5 molo/o Ti. Chlorine is below detection limit(0.02 wt0/0) in all garnets analyzed,.

In Figure 4A the F contents ofcalcic garnets are shownas a function of X^,, and it can be seen that F concentra-tion increases with increasing grossular component. Thepositive correlation between F and X^ suggests that thesubstitution of F into garnet will be favored by bulk com-positions poor in Fer*. Figure 48 illustrates that X., (X.,: n",/3) decreases with increasing grossular content. Allgarnet compositions in this study have X., < 1.0 and aretherefore silica-deficient. Iron-rich zones are closest tofull Si occupancy (nr, : 2.80-2.96), and the maximumsilica deficiency in Al-rich zones is 2lo/o.Ttre Si deficiencyis greater than 4n, in all analyzed garnets, suggesting thepresence of OH (Eq. 3). Minimum OH concentrationscalculated from Equation 3 are 0.1 to 0.9 + 0.1 at Xer <0.1 and 0.5 to 2.2 + 0.1 at Xa > 0.8. The relationshipbetween F and calculated OH, as expressed by nr/(n, +no"), is given in Figure 4C. Although there is a wide scatterin the data, Figure 4C shows that n /(no -l no") < 0.5 inall analyzed garnets. Note that since computed OH is aminimum value, nr/(no I nor) shown in Figure 4C is amaxlmum.

TrnrpnnlruRE oF GARNET FoRMATToN

The temperature of garnet formation can be estimatedfrom the mineral assemblage coexisting with F-bearinggarnets. The formation of prehnite during retrograde al-teration constrains maximum temperatures of the garnet-bearing assemblage to 420 "C at the l-kbar pressure ofcontact metamorphism, based on univariant equilibriumamong stoichiometric prehnite, grossular, zoisite, quaftz,and H,O (Liou, 1971; Connolly and Kerrick, 1985) usingthe data of Helgeson et al. (1978). This is a maximumestimate because substitution of Fe3* and OH in garnetand Fe3* in prehnite and epidote solid solutions will de-crease the temperature of this equilibrium at constantpressure (Bird and Helgeson, 1980; Rose and Bird, 1987).

0 . 8

o . 7

0 . 6

0 . 5

0 . 4

0 . 3

o.20 . 1

0

1

0 . 9 5

0 . 9

0 . 8 5

0 . 8

o . 7 5o.2 0 . 4 0 . 6 0 . 8

x l r

0 . 6

0 . 5

0 . 4

0 . 3

o.2

0 . 1

00 0 . 2 0 . 4 0 . 6 0 . 8 1

xat

Fig. 4. Compositional parameters as a function of mole frac-tion of octahedral aluminum [Xo,, where Xo, : nor/(n^ + no.r-)]in calcic garnets: (A) F atoms per 5 (X + Y) atoms, (B) molefraction of Si (X.,, where X' : n",/3), and (C) n /(n, I non).Uncertainties are calculated from random errors in X-ray counts(see text) and are shown only when larger than the symbols used.

oEoag

+xro

U'troa!lt

o.2 0 . 4 0 . 6 0 . 8X l r

6x

-oc+|rc

lttr

. 8 4 4 0 TI 1 1 8 7 f ,?t--L

tI

IfJJ?_L-

iq

A

T

q to t I

a - . t . . .

" i'i

rT

i{

866

The absence of xonotlite in the alteration assemblage pro-vides an approximate lower temperature limit of 200-275"C, as deduced from the unreversed l-kbar dehydra-tion of xonotlite to wollastonite (Gustafson, 1974). Thusthe garnets probably formed between 200 and 420 "C.

The unreversed experimental data of Huckenholz andFehr (1982) suggest that there is a compositional gap inhydrous grandite solid solutions with a critical tempera-ture of 450-500 'C and a critical composition of X"' -

0.55. Inspection ofFigure 4 shows that no analyses occurbetween X^ : 0.50 and Xo' : 0.75, implying that thenatural samples exhibit a compositional gap that is con-sistent with the experimental observations of Huckenholzand Fehr (1982). Ifit exists, a solvus in hydrous granditewould be an excellent thermometer for low-grade meta-basalts and calc-silicates; but estimation of temperaturesfrom the available experimental data is highly uncertain.

Corvrp,qlrsoN wrrH orHER FLUoRIANCALCIC GARNETS

Fluorine-bearing calcic garnets have been reported inmetamorphosed impure carbonates, metasomatically al-tered alkaline igneous rocks, and contact-metamor-phosed basalts. The occurrence, phase relations, andcompositional characteristics of fluorian grandite in theseenvironments are compared below.

Occurrence of fluorian grandite

Metacarbonates. In impure carbonates, fluorian calcicgarnets are associated with skarn assemblages that de-veloped during prograde contact metamorphism at LostRiver, Alaska (Dobson, 1982, 1984) and at Ulchin andYeonhwa, South Korea (Yun, 1979; Yun and Einaudi,1982). At Lost River, F-bearing grandite occurs in mono-mineralic zones that are crosscut by veins containingfluorite, vesuvianite, and magnetite. Fluid inclusion ho-mogenization temperatures suggest conditions of garnetformation were >400 "C (Dobson, 1984). Associatedminerals in the South Korean skarns include variableamounts of pyroxene, Mn-pyroxenoid, fluorite, quartz,calcite, biotite, chlorite, hematite, and sulfides (Yun,1979); however, no temperature estimates for the for-mation of this assemblage are reported. Fluorine-bearinggrandites also formed during retrograde alteration of am-phibolite facies marbles in the Adirondack mountains ofeastern New York (Valley et al., 1983). Valley et al. (1983)reported fluorian grandites coexisting with quartz, pum-pellyite, prehnite, and calcite. The maximum thermal sta-bility of prehnite limits the temperature of Adirondackgarnet formation. The univariant equilibrium amongprehnite, zoisite, grossular, quartz, and HrO has a steepnegative slope, occurringat 420 "C at I kbar and 380 "Cat 5 kbar, and maximum temperatures inferred from thepresence ofprehnite are therefore relatively insensitive topressure. Thus, although Valley et al. (1983) did not givea pressure estimate for the retrograde alteration, the pres-ence of prehnite in the alteration assemblage requires thatthe maximum temperature of fluorian-garnet formation

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION

in the Adirondacks was 400 ! 20 "C. All F-bearing gar-

nets from metacarbonate lithologies are grossular-an-

dradite solid solutions, with calculated Mn, Mg, and Fe2*concentrations in dodecahedral sites exceeding l0 mo10/oin only a few of the report analyses. The calcic garnets

from Lost River contain 0.7-9.7 molo/o Sn (Dobson, 1982,1984), and Sn content decreases with increasing Aland F.

Alkaline igneous rocks. F-bearing grandite garnets oc-cur as part of metasomatic alteration assemblages in al-kaline igneous rocks that include nepheline syenites inthe Shonkin Sag laccolith, Montana (Nash and Wilkin-son, 1970) and garnet-ijolite xenoliths in nepheline sy-enite at Magnet Cove, Arkansas (Flohr and Ross, 1989).Fluorian grandites are associated with aegerine augite,biotite, zeolites, and nepheline at both localities. AtShonkin Sag, sanidine and arfvedsonite are part of theprimary igneous assemblage, and carbonate occurs withzeolites and garnet cs alteration of magmatic nepheline'At Magnet Cove, fluorian garnets occur as secondaryovergrowths on mag,matic melanite garnets. In addition,apatite, titanite, and magnetite are both primary and sec-ondary phases, and cancrinite occurs with zeolite as al-teration ofnepheline. All F-bearing garnets from alteredalkaline rocks contain >93 molo/o Ca in dodecahedral po-

sitions, but contain substantial concentrations of octa-hedral Ti (5.2-10.7 molo/o). The F content of garnets re-ported by Flohr and Ross (1989) decreases with increasingTi concentration. These garnets also contain minor quan-

tities of Zr, V, Nb, and Na.Metabasalts. In addition to the occurrence in the meta-

basalt host rocks of the Skaergaard intrusion described inthis paper, F-bearing grandites also occur in basalts thatwere metamorphosed during emplacement of the Land'sEnd granite in Cornwall, England (van Marcke de Lum-men, 1986). At the latter locality, garnets occur in quatrz-

rich nodules and are interpreted to have formed at -600

"C (van Marcke de Lummen and Verkaeren, 1985). Co-existing minerals include magnetite, qvartz, potassiumfeldspar, epidote, and apatite. These garnets contain upto 0.27 wto/o KrO, 0.20 wto/oZnO and 0'08 wto/o Cl.

Comparison of fluorian grandite compositions

Compositional characteristics of F-bearing granditesfrom all three environments are in Figure 5. Figure 5Ashows that fluorian grandites from metacarbonates andmetabasalts display a wide range of Al-Fe3* composi-tions. Although the five analyses given by Flohr and Ross(1939) suggest a more restricted range in Xo' in garnetsfrom alkaline rocks (Fig. 5A), examination of their Figure7 indicates that there is a continuous range in Xo' com-positions in Magnet Cove F-bearing grandites from -0

to 0.64. It can be seen in Figure 5,{ that in metabasaltsand alkaline igneous rocks, there is a decrease in X' withincreasing Xo,, indicating greater silica deficiency in moreAl-rich garnets. In contrast, X.' in metacarbonates is ap-parently independent of &', and most fluorian granditesfrom these lithologies have high tetrahedral site occupan-

cy by Si. It is important to note, however, that manygarnet compositions reported by Yun (1979) and Yunand Einaudi (1982) have nr, > 3.02, suggesting possiblesystematic errors in their analyses. The degree of F en-richment with increasing Xo, in metacarbonates is lowrelative to that observed in metabasalts and alkaline rocks(Fig. 5B), which is consistent with the higher Si contentsof garnets in metacarbonates. Figure 5C demonstrates thatno/(no + nor) > 0.55 in grandites from metacarbonate.Although there is evidence to suggest that Yun's analysesmay be in error, the data in Figure 5C show Ihat nF >no, in all available analyses of fluorian grandite frommetacarbonates. The two Adirondack garnet analysesgiven by Valley et al. (1983) suggest a wide range in n,/(n, 1 nor) from this locality; however, nr/(n, -l non)from the two skarn localities is near 1.0. In contrast tothe garnets from metacarbonates, no/(n, * no") in thosefrom alkaline rocks and metabasalts is <0.50, suggestinghigher OH contents of garnets formed in these lithologies.If it is not an artifact of the charge-balance calculation,the difference between calculated OH in metacarbonategarnets and that in garnets from the other localities maysuggest that garnet compositions record contrasts in fluidcomposition between the environments. For example, thelower OH garnets from metacarbonates may reflect lowerHrO activities in fluids coexisting with metacarbonates.However, direct determination of HrO content using IRspectroscopy combined with detailed petrology is re-quired to resolve this relationship.

Fr,uonrll cALCrc cARNETS ANDMETAMORPHIC FLUID COMPOSITION

Compositional relations between fluorian grandite anda coexisting fluid may be evaluated by consideration ofthe constraints imposed by heterogeneous equilibrium.Two linearly independent reactions in the system CaO-AlrO3-SiOr-HrO-HF describe the exchange of F, OH, andSi between real or hypothetical stoichiometric garnet end-members:

Ca.AlrSirO,, + l2}f* + l2F-grossular

: carAlrnrFr2 + 3SiO2("o) + 6HrOF-grossular

and

ca.Alrtr.(oH)n+ l2H* + l2F-hydrogrossultr

: CarAlr!.Fn + l2H2O.F-grossular

The mass-action relations for these equations are

log K o^: 3 log 4.,o,," * 6 log an,o

867

Ca lc - s i l i ca tesI Dobsona Yuno Vsl ley et a l .

Alkal inr Rocks l re labasal is

A Flohr & Ross o Yan Marcke

^ Nash & Wi lk inson . This Study

1

0 . 9 5

0 . 9

0 . 8 5

0 . 8

0 . 7 5

0 . 7

t

X A t

(4a)

(4b)

x l t

Fig. 5. Fluorian garnet compositions from alkaline igneousrocks, metacarbonates (mc) and metabasalts in terms of (A) molefraction of Si, (B) F atoms per 5 (X * Y) atoms, and (C) n'/(n,t non), all vs. mole fraction of Al. Solid lines represent com-

(5a) positional limits of garnets from each environment.

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SI.{ERGI'ARD INTRUSION

c,x

0 . 8

a

c 0 6

xb

; 0 . 4EoF* 0 . 2

0 . 8

Io

c 0 . 6+I

r o . l4

0 80 . 60 . 40 . 2

- l2 los. a- , q. + 1on 4carnr:nrFr:

4 ( a l A l ) S r l U t 2

B

a l k a l i n c

/)',

o a

metabasa l l s

a

l ^ A6 J . , / . ;

. - / :J.

. . al l n e a

868

and

log Koo : 12 log en,o - 12 log ar*a,

+ log jsalsrr2-4Ca:el:o:(oH)r: t

(5b)

where a is activity and K is the equilibrium constant ofthe subscripted reaction. Standard-state conventionsadopted in this study include unit activity ofpure solidsand HrO at any pressure and temperature, unit activityof aqueous species other than HrO in a hypothetical, Imolal solution referenced to infinite dilution at any pres-sure and temperature, and unit fugacity ofgases at I barand any temperature.

Assuming local equilibrium between garnet and fluid,variations in the activities of end-members in the garnetsolid solution require systematic changes in the activitiesof H*, F-, Sio2(uq), and HrO in the fluid phase. The rela-tive magnitudes of these changes can be illustrated bydifferentiating Equations 5a and 5b with respect to thereaction progress variable ({; Helgeson, 1979) at constanttemperature and pressure:

(6a)

P T

Equation 6b states that changes in log(a^-a, / anro) in thefluid phase are proportional to changes in log(a.",o,ror.,r/acat*tzat.os\n) by a factor of 12. Similarly, Equation 6ashows that changes in log(4.".o,ro3Fn/aca3*t2sbotz) are relat-ed to changes in a.*a, , asio26r1, and enro, but in this casethe proportionality is more complex due to the differentstoichiometries of the aqueous species in Reaction 4a.Equation 6 also demonstrates that the activity ratios ofgarnet end-members are extremely sensitive to minorchanges in fluid composition. For example, under con-ditions of constant asio26o1, ar'd anro, Equations 6a and 6breduce to

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION

indicating that garnet compositions depend on F concen-tration and pH in the fluid to the extent that a change oflog an a, of 0. I leads to a change in the activity ratio forgarnet end-members of 1.2 log units.

The chemical potential (p) of the thermodynamic com-ponent HF can be expressed as

ttse : ll"sr * RI ln 4".. (8)

In Equations 4-7 we use the aqueous species H* and Fto define pr. because pH is a useful geochemical variablefor descrjbing metasomatic processes and mineral hy-drolysis. It is important to note, however, that alternatespecies such as HFluo;, HF1*e, or Hr,*., and F2(8"") could alsobe used. This can be seen by writing the following reac-tions between the thermodynamic component HF andthese species:

H F : H ' * F ,HF: HF,"o,,HF: HF(p,),

and

2HF: H2Gu") + F2(F"). (9d)

Differentiating Equation 8 and the mass-action expres-

sions for Equations 9a{ with respect to { yields

(Helgeson, 1970), where /denotes fugacity of the sub-scripted gas. These relations demonstrate that changes inthe activity or fugacity ofeach species can be equated tochanges in p". in precisely the same way.

Finally, Equations 7 and l0 can be combined to give

d log( l r )

Equation I I shows that changes in the composition offluorian hydrous grandite explicitly define changes in trr"r.Similar relations may be written to illustrate that varia-tions in garnet compositions also define changes in thechemical potentials of the thermodynamic componentsSiO, and H.O.

(9a)(eb)(9c)

and

QCztAlza:tr p

d CatAlzEt(OH)rz

0Eoo

d log

' 194-l2.3RrL oE 1, ,

la bg a,-ao I [a log a"o..,l:L dt 1, , : l u ) , ,(6b) :fq-Egnr,*l :![atoe/',*,,r.,*.,'l

, L d t ) , , 2L d t ) , . , ( 10 )

s.2lw1RrL d{ lP r at

A CatAlzntF rz

4CarAl:Si:oru

U 1on Oca:AucrFr:

A Ca jAl2Dl(oH) t2 dt

d log

, . [d log a^-ar , l" l dE ] , , -

(7)

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION 869

Mineral-fluid equilibria involvingEast Greenland fluorian garnet

The value of eH,eF in the fluid in local equilibriumwith fluorian grandites from East Greenland can be de-termined using phase relations and fluid inclusion obser-vations. This value can then be combined with Equations5 and 6 to examine how variations in 4r.ao in the fluidare reflected in measured changes in the F content ofgarnet. Phase relations are presented below using equa-tions and data of Helgeson et al. (1978, 1981).

Fluorian calcic garnets in the East Greenland metaba-salts coexist with wollastonite (>99 mo10/o CaSiOr) andstoichiometric fluorite and quartz. Equilibrium amongthese minerals and an aqueous solution can be repre-sented by the reaction

CaSiO, + 2}tt + 2F : CaF, + SiO, + HrO (lZ)wollastonite fluorite quartz

for which

P "lt

0.5 kb

1 k b

3 k b5 k b

2 6 - 7 . 7

-5.7

-3.7

-1 .7

oGIq)

'n

o!t+t l(rlao

nr 22

YcDo

1 8

1 4

( 1 3 )

The isobaric temperature dependence of log K,, is givenat various pressures in Figure 6. At <500 bars the largevariations in the electrostatic properties and density ofHrO lead to a change from small negative to large posi-tive values of the standard molal enthalpy of Reaction l2(AHir) and result in extreme pressure and temperaturedependence of log K,r. In contrast, the relatively smallchange in Ai1!, with temperature at > I kbar results inonly slight variations in log K,, over the temperature ofgarnet formation. The change in log K,, illustrated in Fig-ure 6, which is larger with isothermal pressure variationthan with isobaric temperature change at > I kbar, indi-cates that accurate knowledge of the pressure is importantfor evaluation of phase relations involving Reaction 12.As noted above, stratigraphic reconstructions indicate alithostatic pressure of I kbar for garnet formation (Man-ning, 1989). At this pressure, a minimum in log K,, occursat 3ll "C, reflecting the fact that AIl?, : 0 at this tem-perature and the relatively small temperature dependenceof log K,, at this pressure (Fig. 6) is a consequence of thesmall magnitude of the change in the standard-state heatcapacity of Reaction 12 at I kbar. In the calculationspresented below we assumed the temperature of garnetformation was 350'C. For 350 "C, I kbar, and pure solids,Equation 13 reduces to

logar*ao- - r /zlog arro: -10.06, (14 )

where -10.06 is -0.5 log K,r. Although we have fixedtemperature at 350 "C, it should be emphasized that theright side of Equation 14 varies by only -0.5 log unitsover the possible temperature range of garnet formation;for example, this quantity is - 10.42 at 200 "C, - 10.00at 31 I 'C, and - 10.50 at 420 "C (Fig. 6).

Equation l4 indicates that determination of log ar.aoin a fluid in equilibrium with fluorian garnet, wollaston-

2 0 0 2 5 0 3 0 0 3 5 0 4 0 0Temperature ('C)

Fig. 6. Logarithm ofthe equilibrium constant for Reaction12 (log K,r; see text) as a function of temperature. The isoplethsrepresent isobaric values of log K,, at the designated pressure,with the exception of the P*, isopleth, along which pressurescorrespond to liquid-vapor equilibrium for pure HrO. Values forlog K,, were calculated using the data of Helgeson et al. (1978,1981). The right vertical axis shows values oflog ao- at pH :5.3 (see text).

ite, fluorite, and quartz requires knowledged ofthe activ-ity of HrO. The value of erro associated with garnet for-mation can be constrained using salinities of quartz fluidinclusions in a granophyre vein near sample 8440 (Fig.2), which are -8 wto/o NaCl equivalent, 1.5 molal (Birdet al., 1986). If fluorian garnet formation were associatedwith mixing of these fluids with dilute pore solutions aswe suggested above, the total salinities would be lowerthan 8 wto/0. Nevertheless, the lowest possible HrO activitycan be determined by assuming this value. At <400 "Cand I kbar, the presence of calcite in the retrograde al-teration assemblage requires this minimum value of ar,oto have been 0.97 as constrained bv the reaction

caSio, + co,: caco. + sio,wolla$tonite calcite qvrlz

( l 5 )

using the equations and data of Bowers and Helgeson(1983). If 8 wto/o NaCl equivalent were assumed, the valueof log ar-ao in the fluid coexisting with fluorian granditegarnets would have been -10.07 at 350'C and I kbar(Eq. la); but if arro were unity, log a*a, would havebeen - 10.06. This demonstrates Ihar arro has little effecton the present calculations, and we have assumed thatarro: l '0 '

Ar enro: I, log a.- associated with garnet formationis linearly dependent on pH (Eq. la). The right verticalaxis in Figure 6 gives values of log a, for pH : 5.3, whichcorresponds to neutral pH (+0.1) of pure HrO at I kbarand 200420 "C. Figure 6 thus shows that if pH wereneutral, log ao- associated with fluorian garnet formationnear the Skaergaard intrusion would have been between-4.7 and -5.2.

Figure 7A shows stoichiometric mineral stabilities as a

log K,, : rcgls*' a-w-' !

"'- ar^tto,a!r, a2,

870 MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION

+N I

(5

+8(Eo

(!

CD-9

7 .8-10 .4 -10 -9 .6

l og a6+ a p -

Fig.7 . (A) Stoichiometric phase relations in the system CaO-AlrO3-SiOr-H,O-HF at 350 'C and I kbar in equilibrium withquartz and pure HrO in terms of log a.,*/a2r- and log aoar-. (B)Phase relations involwing silica-deficient, hydrous, fluorian gros-sular at 350'C and I kbar in equilibrium with quartz and pureHrO in terms of loga-r./a2n" and log ar*a, .Ttle dashed verticallines denote the number of F atoms per 5 (X + Y) atoms re-quired by Reaction 4a (see text).

function of log a.^z-/alr- and log eH,aF il the fluid phasein equilibrium with quartz and pure HrO at 350'C andI kbar in the system CaO-AlrO.-SiOr-HrO-HF. The wol-lastonite and fluorite phase boundaries represent fluidsaturation limits in this projection and define maximumvalues of log a.^z-/al- and log aH-aF-that may be attainedin the fluid without supersaturation of either phase. Thephase relations illustrated in Figure 7A indicate that atprogressively higher log aq"z,/a2n, in the coexisting fluid,the successively stable phases are clinozoisite, prehnite,

and grossular, and that the maximum log a.uz*/a?n, occursin equilibrium with grossular and wollastonite. The min-imum value of log an,ao- for fluorite saturation is - I 0.06,corresponding to equilibrium among wollastonite, fluo-rite, and quaftz. The negative slope ofthe fluorite satu-ration surface indicates that, with decreasing log a"^z-/a'zn,, higher values of log qH*aF are required to maintainequilibrium between fluorite and the fluid phase. Figure7A also shows that the conditions inferred for the for-mation of the East Greenland garnets in amygdule centerscorrespond to the intersection of the wollastonite andfluorite saturation surfaces log a.^z-/az* < 8.13, indicat-ing a gradient in log a.uz*/a?n, from amygdule margins toamygdule centers of at least 0.28 log units over distancesof several millimeters.

The variation inlog a"-ao in the fluid as a function ofthe extent of F and OH substitution in garnet can bepredicted using Equation 5 with measured East Green-land garnet compositions. In the absence of experimentalconstraints on the effects of F and OH substitution ingrandite, we have adopted an ideal ionic mixing approx-imation such that cc.3er2si3o,2 : X'orX\, eCasN2t3Fp: X2"r(no/l2)3, and 4Ca3Ar2tr3(oH)12 : X'z"r(n""/12)3. Because there isevidence for nonideal mixing along the grossular-andra-dite join (e.g., Engi and Wersin, 1987), we evaluated phaserelations for Al-rich garnets to minimize the effect of pos-sible deviations from ideality.

Taking eH2o as unity and substituting quartz for SiO2("q)in Reactions 4a and 4b as required by the presence ofthis phase in the alteration assemblage, Equations 5a and5b can be written as

log Ko^: _ 12log a^*ao + I rceff ( l6a)

and

logKoo : _ 12logan,a,- + 3 logL. ( l6b)' - llon

Figure 4B shows that X' varies widely in the grossular-rich (X", > 0.75) garnets of this study. Taking the averagevalues of Xr, : 0.87 and Xo, : 0.93 for the most Al-richgroup of analyses from sample I187 (Fig. 4B), the valueof 4ca3ar2si3o12 in the garnets is 0.57. Similarly, the mostAl-rich garnets have z.: 0.6 (Fig.4A) and no' : 1.0,yielding values of 4.ur^2r3r12 and ec4At2'3(o.)n equal to l.lx l0-o and 5.0 x l0-0, respectively. When combinedwith Equations 14 and 16, these activities yield valuesfor log &" equal to 117.0 and log &o equal to 120. I inthe presence of quartz, wollastonite, fluorite, and pureHrO. Equations l6a and l6b may now be rewritten as

8 .5

7 .5

-9-11log

-10a6+ap-

8 .6

. 8 .46 l *

(!

+ 8.2(oo

(E

crt- 9 8

tog a,*ar : o.2s "r@#

- s.75,

and

l o g a n ' a r : 0 . 2 5 b e : L - 1 0 . 0 1 '

(r7 a)

350 oC, 1 kbar+ quartz

cl inozois i te

( l 7b)

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SKAERGAARD INTRUSION 8 7 1

showing that in the absence of mineral assemblages suchas wollastonite + fluorite + quafiz, fluorian granditecompositions can be used to determine a"*ao_ in aqueoussolutions. In principle, either Equation l7a or l7b willgive the same value of log ar.ao; however in practice,the large uncertainties in calculated OH from electronmicroprobe analyses lead to large uncertainties in solu-tions to Equation l7b. Thus Equation l7a should be usedto determine a"*eo in the fluid in the absence of a directOH determination.

The isothermal, isobaric relations between log an*a,and garnet composition as computed from Equation 17aare in Figure 78 in terms of F atoms per 5 (X * y) atoms.Figure 78 illustrates that the F content ofgrossular solidsolution increases with increasingaH*aF in the coexistingfluid phase. For values oflog ar-ao less than - 10.40 inthe fluid phase, F in the coexisting garnet will be belowelectron microprobe detection limit [<0.027 atoms per 5(X + Y) atomsl using the analytical conditions employedin this study. In the presence of fluorite and quartz, theF concentration in garnet increases with decreasing loga..z-/a2r.. Figure 7B shows that the maximum F contentrn garnet at 350 "C and I kbar (n. -2-5) will be attainedin the presence ofprehnite, fluorite, and quartz. The ex-treme sensitivity ofgarnet chemistry to fluid compositioncan be seen in Figure 7B by noting that increasing thevalue ofn. in garnet from <0.05 to 0.60 requires a changein either pH or log a. of only 0.3. The relative topologyof phase relations and, n, isopleths in Figure 78 are thesame independent of the temperature range assumed forgarnet formation. However, the values of Ioga.^z-/a2r- and,log ar*ao for phase boundaries and n. isopleths will changewith temperature: if the garnets formed at 200 "C, theintersection ofthe wollastonite and fluorite saturation sur-faces and the n, : 0.6 isopleth will occur ar log qc^2-/aL- : | 1.05 and log an*ao : - 10.42, whereas at 400 .Cthese values are, respectively, T.6S and -10.32.

Mineral-fluid equilibria in Adirondack calc-silicatesThe amount of F and values of n./(no -l nor) vary

among different geologic environments (Fig. 5), suggest-ing that the garnets record diferences in metamorphicfluid composition at each locality. Unfortunately, thepressures and temperatures of formation of most fluoriangarnets are either highly uncertain or are not reported. Inview of the possible effects of pressure and temperatureon such comparisons (Fig. 6), a detailed understanding ofhow variations in garnet composition depend on temper-ature and pressure will only be possible when moreF-bearing garnet occurrences are described. However, theutility of using fluorian garnet phase relations to comparemetamorphic fluid compositions can be illustrated by as-suming isothermal and isobaric conditions among differ-ent localities. As an example, we compare fluid compo-sitions in the East Greenland gamet occurrence with thoseassociated with fluorian garnets in the Adirondacks, forwhich the associated mineral assemblage is well docu-mented.

Aluminum-rich, fluorian calcic garnets associated withretrograde alteration of Adirondack calc-silicates havelower F concentrations and higher no/(n, -t ro") relativeto garnets of the metabasalts into which the Skaergaardintrudes (Fig. 5C). Simultaneous consideration of Equa-tions 5a and 5b suggests lower values of both eH2o andar*a, in the fluid associated with the Adirondack garnetsrelative to those near the Skaergaard intrusion. The val-ues of log a*ao in Adirondack calc-silicates can be es-timated with garnet compositions by first constrainingarro from phase relations among coexisting minerals,which include prehnite, calcite, pumpellyite, and quartzbut not fluorite (Valley et al., 1983). The most Al-richgamet reported by Valley et al. (1983) has n, : 0. 18, Xsi: 0.97, and X^t: 0.96. These values result in activitiesof CarAlrSi.O,, and CarAlrtrrF,, of 0.84 and 0.34 x l0-5,respectively, using the above approximations of activity.If the fluid were a mixture of H,O and CO,. the intersec-tion of the equilibrium

CarAlrSirO,o(OH), + CaCO,prehnirc calcite

: Ca.AlrSirOr2 + H2O + CO,grossular

( l 8 )

with the reaction defining the maximum stability ofprehnite,

5CarAlrSi.O,o(OHLprehnite

:2CarAlrSi.Or, + 3SiO,grossulr quaflz

* 2CarAl.Si.O,r(OH) + 4h,O,zoisite

(1 e)

would require a maximum Xco, of 0.01 and a minimumanro of 0.99 at 350 "C and I kbar, if one uses the equa-tions and data of Helgeson et al. (1978) and Bowers andHelgeson (1983). This leads tolog a,-a, : - 10.21 fromsolution ofEquation 5A at quartz saturation. IncreasingaH2o to 1.0 results in log an-a, : - 10.20, showing thatthis calculation is also relatively insensitive to the wateractivity adopted. Thus, if the Adirondack garnets were tohave formed at 350 oC and I kbar, values of Iog an*ao0.14-0. l5 log units more negative than those associatedwith the East Greenland garnets would indicate a lowerprro (Ee. l0) in the coexisting fluid phase in the formerlocality.

CoNcr,usroNs

The F content of hydrous grandite garnets in the ba-saltic host rocks of the Skaergaard intrusion increases withincreasing Xo, and calculated OH and decreasing X.,.Similar trends are exhibited by F-bearing garnets inmetasomatically altered alkaline igneous rocks and meta-morphosed carbonates. Values of nr/(n, I non) also in-crease with increasing X^,. Compositions of fluorian gar-nets from East Greenland indicate that F substitutes withOH to compensate silica deficiency.

872

The distribution of garnet compositions in the Skaer-gaard host basalts is consistent with formation at 200-420 "C, as constrained by the coexisting mineral assem-blage. Phase relations in the Skaergaard host basalts showthat log an*a, in the coexisting fluid phase was between-10.0 and -10.5 at I kbar and 200420 'C. If pH wereneutral, log a, associated with the formation of theF-bearing garnets of this study would have been -4.7 to-5.2. The calculations presented here predict that fluo-rian gamet compositions are extremely sensitive tochanges in pH or the activity ofF in the fluid phase. Ifpressures and temperatures were similar, phase equilib-rium constraints indicate that lower n, andhigfier nr/(n,* non) in garnets from Adirondack calc-silicates relativeto those from East Greenland reflect lower log ar-a, of-10.21. These results show that F content of garnet as-sociated with low-temperature alteration can provide anexcellent basis for evaluating activities of F species incoexisting metamorphic fluids.

AcxNowr-nncMENTS

This paper represents a portion ofthe senior author's Ph.D. dissertationat Stanford University. We thank Jim Munoz and John Valley, whoseinsightful reviews improved the manuscript. The comments of Nick Rose,J.G. Liou, Eric Seedorfl and l-ou Caruso on early versions of this workare gratefully acknowledged. Financial support for this study was provid-

ed by the Geological Society of America, a Corning Glass Science Fellow-ship, and NSF grants 84-18129,86-06256, and 88-03754 to D.K. Birdand 86-15714 to S.R. Bohlen. Field research was aided by the logisticalsupport ofPlatinova Resources, Ltd., and the Greenland Geological Sur-vey. We also thank Minik Rosing Kent Brooks, and Troels Nielsen fortheir advice and help.

RrrrcnnNcns crrnD

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MeNuscnrm RECETvED Jvw 7, 1989Merquscnrsr AccEprED Apn:r 16, 1990

MANNING AND BIRD: FLUORIAN GARNETS FROM THE SIC{ERGA{RD INTRUSION