Hydration of Corundum-Bearing Xenoliths in the Q0rqut Granite

American Mineralogist, Volume 72, pages29-38, 1987

Hydration of corundum-bearingxenoliths in the Q0rqut Granite Complex, Godthibsfjord, West Greenland

MrNrx T. RosrNc,* DENNts K. Bno Department of Geology, Stanford University, Stanford, California 94305, U.S.A. Rosnnr F. DYlvrBr Departmentof Earthand Planetary Sciences, *"rht*t"r t1;lT;tirl;rLMcDonnell Centerfor the SpaceSciences, St. Louis,

ABSTRACT Thermodynamic analysis of phase relations in a high-gradegneissic xenolith from the anatectic region of the late Archean Q0rqut Granite Complex indicates that fluids, probably derived from the granite complex, causedpartial replacementof primary plagioclase, corundum, and biotite in the xenolith by epidote, margarite, and muscovite. Composi- tional relations within the metamorphic mineral assemblage,together with logarithmic activity and fugacityphase diagrams in the systemNarO-KrO-CaO-FeO-MgO-FerOr-AlrOr- SiOr-HrO, define local equilibrium constraints on temperature, oxygen fugacity, activity of aqueous silica, and cation activity ratios in the fluid phase during hydration of the xenolith. The inferred lithostatic presswe at the time of intrusion of the granite complex is -5 kbar. At this pressureand -580"C, plagioclaseand corundum react with water to form margarite. Equilibrium among plagioclase,corundum, margarite, biotite, epidote, muscovite, and an aqueoussolution in the xenolith at this temperature and pressurerequires an oxygen fugacity of - l0-'7 bars. At temperaturesless than 580"C, the assemblagebiotite + muscovite + margarite + epidote is stableover a wide rangeof cation to H* activity ratios and activity ofsilica in the fluid phase,but definesa narrow range ofoxygen fugacities at any given temperature.The calculated oxygen fugacitiesfor the xenolith are close to the maximum values estimated from Fe-Ti oxide phaserelations in some granites and pegmatites reported in the literature. Hydration of plagioclaseand corundum and the oxidation of biotite in the xenolith to form epidote, margarite, and muscovite require an influx of HrO and possibly SiO, from the surrounding granite.

INrnooucrroN possible range of oxygen fugacities (for), cation activity The Qdrqut Granite Complex (QGC, Fig. l) in south- ratios, and the activity of aqueous silica [c.,or,"o,]in the ern West Greenland intruded high-gradepolyphase xenoliths during the formation of the QGC. grreissesduring the late Archean (Burwell and Friend, Characreization of fluids associatedwith crustal ana- 1979; Beech and Chadwick, 1980). A plagioclase-rich texis and emplacementof deep-seatedgranite bodies may gneissicxenolith within the QGC, probably derived from help in evaluating rnetasomatic redistribution of ele- the Malene supracrustalsequence, contains corundum and ments in the lower crust. In this case,we addressa tec- biotite that are partially replaced by margarite, epidote, tono-metamorphic event ar 2600-2500 Ma characterized and muscovite(Dymek, 1983).Dymek proposedthat the by the formation of anatecticgranite bodies and by pos- textural and mineral-chemical features in the xenolith sible redistribution of alkalis and alkaline-earth elements could be explained by oxidation ofbiotite and hydration near the late Archean amphibolite-granulite facies of plagioclaseand corundum to form epidote, muscovite, boundary in the craton of West Greenland (Moorbath et and margarite. The objective of this paper is to evaluate al., l98l). the thermodynamic constraintson the metamorphic and/ or igneousfluid phaseassociated with this process.Ther- Trrn Q6nour GuNrrn Covrpr,Bx modynamic analysis of phase relations in the system The Q6rqut Granite Complex (Fig. l) is a suite of late NarO-KrO-CaO-FeO-MgO-Fe,Or-Al,O,-SiOr-HrOis Archean granites(2530 Ma) that were formed by at least used here to define local equilibrium constraints on the three distinct intrusive phasesincluding a late pegmatitic phase (McGregor, 1973; Pankhurst et aI., 1973; Burwell * Presentaddress: Geologic Museum, Oster Voldgade 10, DK- and Friend, 1979; Brown and Friend, 1980a, 1980b; I 350, Copenhagen,Denmark. Brown et a1.,l98l). Thesegranites intruded the Archean

0003-o04x/87/0102-0029$02.00 29 30 ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS

Fig. 2. Photomicrographshowing a reactionrim of margarite, muscovite,and epidotebetween a largecorundum por- Fig. 1. Schematicgeologic map of the Godthabstordregion phyroblastand the plagioclaseand biotite matrix. of southernWest Greenland. The circledstar denotes the location of the corundum-bearingxenoliths in the QGC.ISB representsthe location of theIsua supracrustal belt. The mapis mod- photomicrograph in Figure 2, it consistsof large crystals ifiedafter Baadsgaard et al. (1984). ofcorundum (bright red; up to - I cm across),which are everywheresurrounded and replaced by mats of marga- gneiss complex of West Greenland (Bridgwater et al., rite (silvery in hand specimen; very pale brown in thin 1976), which consists of tonalitic-granitic orthogneisses section).These corundum-margarite intergrowths are im- with inclusions of meta-anorthositic and supracrustal mersedin a matrix of finer-grainedmargarite, muscovite, lithologies. The currently exposed crustal level experi- biotite, and epidote (Xcazre:srrorz(orD: 0. l8-0.24). Textures enced granulite-facies metamorphism in the early Ar- in the matrix indicate that the biotite is a relic phasethat chean(3600 Ma, Grifrn et al., 1980)and widespreadam- is replacedby the muscovite.Plagioclase (X*rr,ro, : 0.60- phibolite-faciesand local granulite-faciesmetamorphism 0.80) is the most abundantphase in the xenolith and is in the late Archean. Proterozoic metamorphism reached partially replacedby epidote and/or margarite. upper greenschist-loweramphibolite grade and involved Representativecompositions of the layer silicatesfrom considerablehydrothermal alteration in fault zones(Smith the xenolith are given in Table l. The biotite has an ex- and Dymek, 1983;Rosing, 1984). ceptionally high Al content, indicating about 400/oof the The age of the QOrqut Granite Complex is bracketed eastoniteendmember, which is consistentwith its former by two metamorphic events: a 2800 Ma amphibolite- equilibrium coexistencewith corundum in the xenolith. facies event and a less-pronouncedupper greenschist- The muscovite contains only a small amount of parago- lower amphibolite facies event at ca. 2500 Ma (Baads- nite component, but has a fairly high celadonite compo- gaard, 1983).Lithostatic pressureat the time of intrusion nent, as revealed by low Na and modest (Fe,Mg) con- of the granite is inferred to be -5 kbar based on the tents, respectively. Two analysesof margarite are listed crystallization sequenceand fractionation trend of the in Table l. The first correspondsto the coarser variety QGC in the system SiOr-NaAlSirOr-KAlSirOr-Ca- that immediately surrounds the corundum and is note- AlrSirO* (Brown et al., l98l) and on the reconstructed worthy for its high Na content, indicating about 200/opa- metamorphic history of the GodthAbsford area (Fig. 1), ragonitecomponent. The secondmargarite analysisgiven which reveals that kyanite formed during penecontem- in Table I representsa fine-grained matrix margarite, poraneous regional retrograde metamorphism (Dymek, which has a considerably lower paragonite content 1984).The xenolith studied here is from the lower por- (- 100/0).A final feature worth noting is the exceedingly tion of the QGC in which quartzofeldpathicgneisses with low F and Cl contents of the muscovite and margarite, well-preservedbanded structures are permeated by gra- as compared to biotite that contains up to -0.3 wtoloF nitic bodies on a variety of scalesranging down to cen- and -0.2 wto/oCl. This suggests,but does not prove, that timeter size,as detailed by Brown et al. (1981).It is likely, the fluids responsiblefor the retrogressionwere probably therefore, that the retrograde assemblagein the xenolith poor in halogens. was strongly influenced ifnot controlled entirely by fluids Phaserelations presentedbelow were calculated using associatedwith formation and subsequentcrystallization layer-silicatecompositions reported in Table I (analyses ofthe granite. I through 3) and the averagecompositions ofplagioclase and epidote within the xenolith, together with equations MrNnru.locrc PHASERELATIoNS AND and data describing the thermodynamic properties of THERMODYNAMIC CONVENTIONS minerals, water, ions, and gasesreported by Helgesonet The petrology and mineral chemistry of this xenolith al. (1978),Helgeson and Kirkham (1974a,1974b,1976), have been reportedby Dymek (1983).As shown by the Helgesonet al. (1981), Walther and Helgeson(1977), ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS 3l

Table 1. Mica compositions 60

sio, 34.24 45.41 30.25 30.22 Alro3 19.35 34.60 50.86 50.52 CrrO" 0.05 0.03 0.04 0.05 Tio, 1.21 0.20 0.05 0.02 Mgo 10.00 0.83 o.21 0.09 FeO- 19.49 3.03 0.46 U.Jb ANORTHITE ZnO 0.08 0.00 0.00 0.00 + MnO 0.23 0.00 0.01 0.00 Y CORUNDUM 40 CaO 0.24 0.22 11.86 12.38 f BaO 0.09 0.10 0.03 0.01 kv/"itr HP Naro 0.13 0.42 1.34 0.79 K"O 952 10.50 0.05 0.20 d F o.22 0.01 0.00 0.00 ,/"no\.

cl 014 0.00 0.02 0.00 /\ EXPERIMENTAL BEVERSALS Total'- 9487 95.35 o( ln 94.64 [l 30 / [ ] vELoE (t9z !) o: 11 @ oHATTERJEE (1974) Si 2.643 3.056 2.013 202'l >< SToRRE E NTTSCH (1974) Alrv 1.357 0.944 1.987 1 979 Alvl 0.403 1.801 2.002 2.005 20 0.003 0.001 0.002 0.003 4oo tll""r*o-rruroo 7oo Ti 0.071 0.010 0.003 0.001 Mg 1.151 0.083 0.021 0.009 ". Fe 1.258 0.171 0.026 0.020 Zn 0.004 0.000 0.000 0.000 Fig. 3. Experimentalreversals and univariantequilibrium Mn 0.015 0.000 0.001 0.000 curvefor Reactionl. SymbolsA, B, and C denotethe temper- U 0.020 0.016 0.846 0.887 aturesand pressures ofthe logarithmicactiwity-activity diagrams Ba 0.003 0.003 0.001 0.000 NA 0.019 0.055 0.173 0.102 givenin Figs.44, 4B,and 4C, respectively. Dashed lines denote K 0 937 0.901 0.004 0.017 phaserelations among polymorphs of AlrSiO'(and: andalusite, ky : kyanite,and sill : sillimanite).The curve is computed Note: Columns are (1) biotite, (2) muscovite,(3) margarite(rim on corundum),and (4) margarite(matrix). from equationsand data reported by Helgesonet al. (1978)and 'All Fe as FeO. Helgesonand Kirkham (1974a). .* Lessoxygen for F and Cl.

were computed at the estimated pressure of 5 kbar for the QGC and represent the limiting condition where and Bird and Helgeson (1980). In this study, the ar,o: l. Although it is probable that high concentrations thermodynamic activity of the CaAlrSirO, component in of electrolytesmay have been present in the fluid phase plagioclasesolid solutions was approximated by the ac- associatedwith the retrograde alteration ofthe xenolith, tivity-composition relations reported by Orville (1972). dH2ois commonly closeto unity in thesesolutions if they The activity of AlrO, in corundum was assumedto be containlittle or no dissolvedCO, (Helgeson,1967,1969, unity. Equations and data reported by Bird and Helgeson 1980). Becausethe stability of zoisite and margarite in (1980) were used to evaluate the activities of the the systemCaO-Al'Or-SiOr-HrO-CO, is restrictedto fluids Ca.AlrSi.O,r(OH)and CarFeAl,SirO,r(OH)components whereX.o, < 0.25 at 5 kbar (Chatterjee,1976; Allen and of epidote solid solutions. This mixing model explicitly Fawcett, 1982), it is highly likely that the fluids respon- accountsfor the temperaturedependence of substitution- sible for the formation of epidote and margarite assem- al order-disorder of Al3* and Fe3+in the M, and M3 oc- blagesin the xenolith were water-rich. tahedral sites of epidote. Activities of margarite, biotite, Logarithmic ratios of the activities of cations(Na*, Ca2*, and muscovite were calculated from equations and data Mg2*, Al3*, and K) to H* are used here as descriptive reported by Aagaard and Helgeson (1983) and from as- variables in the phasediagrams (cf. Bowers et al., 1984). sumed random mixing and equal interaction of atoms on Thesedissociated cations are not necessarilythe predom- energeticallyequivalent sites. The standard statesadopt- inant speciesin the aqueous solution. In fact, evidence ed for the thermodynamic components KMgr(AlSir- summarized by Quist and Marshall (1968), Helgeson O,J(OH),, KFer(AlSirO,oXOH),,and KAl,(AlSi3O,oXOH), (1969, 1980),Helgeson and Kirkham (1976),and Frantz require that tetrahedral Al is ordered into the T,O sites and Marshall (1982) indicates that chloride complexesof in biotite and muscovite (cf. Helgesonet al., 1978; Bird the alkali and alkaline-earthcations will be the dominant and Norton, 198l) and that tetrahedralAl in margarite is aqueousspecies at 5 kbar and > 500t. At constant 4H2o, ordered in the two T, sites. Total analyzed Fe in biotite the cation to H' activity ratios used here correspond to was consideredto be FeO. degreesof freedom in the Gibbs phase rule (Helgeson, Activity and fugacity phase diagrams presentedbelow 1970) and, togetherwith equations and data reported by define the mineralogic constraints err"fo, esiozt*),and cat- Helgesonand Kirkham (1976) and Helgesonet al. (1978, ion to H* activity ratios in the fluid phaseduring oxida- I 98 I ), permit quantitative evaluation of solution-mineral tion and hydration of the xenolith. Thesephase diagrams equilibria at high pressuresand temperatures. 5Z ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS

80 B (or, a^\. (4SvX- 7A vis 70 + \cl dI Vcr w NI NI o o o + MARGARTTE-NNoRrHrrr ro^oo"'r.-X%,I ,q 60 o 60 o o o I I CORUNDUM 0 CffiUiDUM )o o) 9 50 50 ruz(oq) )rvz (oq)

Fig. 4. Phaserelations among stoichiometric minerals in the systemCaO-AlrOr-SiO.-HrO in the presenceof an aqueoussolution in which an,o : 1 as a function of log a.^z-/a^- and log cr;o,in the fluid phaseat 550, 580, and 600'C and at 5 kbar (seeFig. 3). The dashedlines denote saturation in the fluid phasewith either wollastonite or quartz.

Prra.sB RELATToNSAMoNG srorcHroMETRrc enthalpiesofthe reactionsdescribing these phase bound- MINERALS aries(<-36 kJ/mol at 500"Cand 5kbar),the logarithm The reaction of corundum, anorthite, and water to form of ar^r"/a'r-and a'or,uo,change only slightly with decreas- margarite is well characterized experimentally (Velde, ing temperaturebetween 587t and 450t. The maxi- l97l; Chatterjee,1974;Storre and Nitsch, 1974).Exper- mum valuesof a.^r./aL- andthe minimum valuesof 4.,or,*, imental reversalsfor this reaction between 2 and 6 kbar in the fluid phase in equilibrium with margarite + co- phase are shown in Figure 3 together with the calculated uni- rundum are definedby the clinozoisite + corundum variant curve for the reaction boundary. The activity of SiOr(aq) in the aqueous solution in equilibrium with the assemblagemargarite + cli- CaAl,Si,O, *.f** + H,O nozoisite * corundum decreasesdramatically with decreasingtemperature owing to the large positive standard : CaAlr(AlrSi.OroXOH)r.(l) molal enthalpy of the reaction characterizingthis assemblage(104 kJ/mol at 500'C and 5 kbar). Valuesof Q.^,*/ Although this reaction can be used to evaluate the max- afr. in equilibrium with this assemblageincrease with de- imum temperature for formation of margarite in the xe- creasingtemperature as a result ofthe largenegative stan- noliths, little information is revealed about the nature of dard molal enthalpy of the reaction describing the mar- the aqueoussolutions associatedwith hydration and ox- garite-clinozoisite phase boundary on the corundum idation of the high-grade metamorphic mineral assem- saturationsurface (- 146 kJimol at 500'C and 5 kbar). blages. To illustrate the stability relations of margarite, phase Sor,urroN-vTINERAL EeUILIBRIA rN THE relationsin the systemCaO-AlrO,-SiOr-HrO are given in QGC xrNor-IrH Figure 4 as a function of log (a.^z-/afr-) and log 4r,or,"o,in Phase diagrams presented above for stoichiometric an aqueoussolution at threetemperatures (550, 580, and minerals and unit activity of HrO (4".o) permit evalua- 600'C) and Prro : P,o,ur: 5 kbar. At 580'C, which is tion of local equilibrium constraintsof the metasomatic slightly below the equilibrium temperature of Reaction I phaserelations in the QGC xenolith. Of particular inter- (Fig. aB), margarite is stable in a narrow field between est are equilibrium constraints imposed by the mineral corundum and anorthite that is characterizedby a wide compositionsin the xenolith and the activity-composi- range in values of aq^2,/a2*and a.,or,uo,in the aqueous tion relations adopted above for plagioclase"",margarite"", solution. With decreasingtemperature (Fig. 4A), the sta- biotite"",muscovite.", and epidote"".The subscript"ss" is bility field of margarite and clinozoisite expands, stabi- used here to denote the mineral compositions in the xe- lizing both minerals with a broader range of activities of nolith used in the calculations presented below which theseaqueous species. conespond to analysesI through 3 for micas in Table I Values of log a.^t-/afi- and log d'or,uo,in a fluid phase and to the averagecompositions of plagioclaseand epi- in equilibrium with corundum and other minerals in the dote. systemCaO-AlrOr-SiOr-HrO are shown in Figure 5 as a The maximum temperatureof hydration of plagioclase function of temperature. Equilibrium between the fluid and corundum to form margarite can be computed from phaseand either sillimanite or kyanite definesthe mini- the law of mass action for Reaction I and the activities mum values of ar^t-/at , and the maximum 4.,or,"o,in the of the components CaAlrSirO, in plagioclase and of fluid phasein equilibrium with margarite and corundum CaAl,(AlrSirO,J(OH),in margaritefrom the xenolith.At (Fig. 5). As a consequenceof the small standard molal 5 kbar and unit activity of water, the logarithm of the ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS JJ

'- PHro-Proro.=5KB H -O TOTAL

GROSSULAR+ CORUNDUM GRANDTEss + CORUNDU[4

CL N0Z0S TE + CORUNDUM t- 'tt. oNI EPDOTEss + CORUNDUM ul o (PLAGIOCLASE ol I MARGARITE+ CORUNDUM (ANORTH TE o 9 + CORUNDUM) [.4ARGARTEss + CORUNDUM

KYANITE+ CORUNDUM KYANITE+ CORUNDUM

500 550 500 550 TEN4PERATURE, TEMPERATURE,"C B o5 o5 OUAPII - - - G -- OUARTZ- --:' -l o _ _qAy'! -!! ulANllL - E E N.4ARGARITE+ CORUNDUM coRUNDL )o" 3-r MARGARITEss+ i + cORLJNDUI.4) ? .-ru-ZU --/ ,-/' --t"'' o // ct rNozorsrL * coRUNDUM o ,/-'-/ FPtDorEss+ coRr,NDUM o o 0 -z^- a c 0- aa GROSSULAR+ CORUNDUM ) GRANDlTEGARNET.. + CORUNDUM -30 3 450 45o too.rrrr*orrrr,'?to 600 TEIVPERATURE,'C Fig.5. Phaserelations among stoichiometric minerals in the Fig.6. Phaserelations among nonstoichiometric minerals in systemCaO-AlrO3-SiOr-HrO in the presenceof corundumand thesystem Na.O-CaO-FerO3-AlrOr-SiOr-HrO in thepresence of an aqueoussolution in whicha^ro : I asa functionof temper- corundumand an aqueoussolution (see Fig. 5 caption).Mineral atureand log a.^z-/ar* and log d.,or,uo, in the fluid phaseat 5 kbar. stabilitiesdenote phase relations for the averagedcompositions The dashedcurves denote saturation in the fluid phasewith of metamorphicminerals within the plagioclase-richgneissic eitherkyanite, sillimanite, or quartz.Corundum is stablewith xenolithof the QGC (seetext). the fluid phasein diagramB only at valuesoflog 4r,o",,o,below kyaniteor sillimanitesaturation. margarite""in the QGC xenolith are given in Figure 7 at 550"C and 5 kbar. Equilibrium among epidote".,marga- activity product of Reaction I is -0.03, corresponding rite.., and an aqueoussolution requires log 4r,or,.o,values to an equilibrium temperatureonly - l0 deg lower than in the fluid phasebetween - 1.0 and - 1.5 and log ac^2-/ that for endmembercompositions given in Figure 3. ai- values between5.95 and 6.5. Local equilibrium of In Figure 6, phaserelations in the systemNarO-CaO- this assemblagewith corundum (point I, Fig. 7A) deter- FerO,-AlrO.-SiOr-HrOare given at 5 kbar for corundum mines the minimum concentrationof SiO, and the max- saturation of the fluid phase. The stability field of epi- imum valuesof ac^r-/azH,and aop,/a3".in the aqueousso- dote."expands dramatically at the expenseof plagioclase., lution. The plagioclase..* margaritess* epidote." and margarite.. with introduction of the pistacite assemblageshown by point 2 in Figure 7A representsthe [Ca.Fe.SirO,,(OH)]component (cf. Figs. 5 and 6). The oppositeextremes for the samefluid species. expanded epidote".field causesthe possible range of ac- Phase relations in the system NarO-KrO-CaO-MgO- tivities of aqueousspecies in equilibrium with the assem- FeO-FerOr-AlrO3-SiOr-HrOare given in diagramsB and blage plagioclase,.* margarile""+ corundum to decrease C of Figure 7 for the limiting caseof corundum saturation relative to the stoichiometric mineral equilibria repre- (point l, Fig. 7A). Points 3 and 4 in thesediagrams de- sentedby Figures3, 4, and 5. Substitutionof Fe3*for Al note valuesof log ar"z-/a2n-,log a-"t-/afi- and log aK*/aH. to form epidote""requires an increasedconcentration of in an aqueoussolution in equilibrium with biotite"", epi- SiO.(aq) but a decreasedvalue of a.^r-/a'r, in the fluid dote"",margarite"", muscovite.,, and corundum at 550"C phaserelative to the stoichiometric phaserelations given and 5 kbar. The diagrams indicate that the activity of in Figure 5. Ca2*in the coexistingaqueous solution exceedsthe activ- Logarithmic activity-activity phase diagrams repre- ity of Mg'z*by severalorders of magnitude. The calculat- senting mineral stabilities for the compositions of epi- ed ratio of log ar^z-/a*e2*varies from 2.35 for the limiting dote"",biotite.., muscovite"",plagioclase"., corundum, and condition ofcorundum saturation(point l, Fig.7A and 34 ROSING ET AL.: HYDRATION OF CORUNDUM.BEARING XENOLITHS

T = 55O'C,ProrAL= PH2o=5 KBAR

cRANDT;s?"^ cHLORITE ss GARNET".- \'C's.^ \ BroTtTEss .l 'olrl r -l +- -qi. > l'-' 4 - "" _ss ol" , ol- 0 o 4 o a ) CORUNDUNiI o 9 ) J 1 EPIDOTE<"--l+ | NIARGARITESS I lvlUSCOVlTLss

r PLAGIOCLAS EI

-30 -20 -1 0 0.0 3 4 s 6 4n LOGO^ ^ LOG;: rruz(oq ) LUU- n Fig. 7. Phaserelations in the system NarO-KrO-CaO-FerOr-A1rO3-SiOrOHrOwhere a',o : l. Diagrams B and C give phase relations in the presenceof corundum. Mineral stabilities denote phase relations correspondingto compositions of metamorphic minerals within the plagioclase-richxenolith of the QGC (seetext).

point 3, Fig. 7C) to l.93 for the plagioclase""-stableassem- 4KFer(AlSirO,oXOH)'* l2Ca,AlrSirOrlOH) + 3^q, annite clinozoisite fluid blage representedby point 2 in Figure 7A. The work by : Frantz and Marshall (1982) indicates that the dissocia- 4KAl,(AlSiO,oXOH), (3) tion constants for CaCl, and MgCl, are similar. If the + l2Ca,FeA,l,Si3O,,(OH)* stoichiometric individual ion activity coemcientsfor Ca2* ?*1,"9_, and Mg2+ are also approximately equal, then the total definesthefr-temperature relations given by curve A-B concentration of Ca2+in the fluid is approximately two in Figure 8 at 5 kbar. This curve describesthe maximum orders of magnitude greaterthan Mg'z*for the conditions values for epidote.", biotite"", muscovite"., and an representedby the diagrams in Figure 7. ,6, aqueoussolution (seebelow). As noted above, substitu- Local equilibrium constraints on the oxidation state of tion of Fer* in biotite"",is not accountedfor in this study, the system can be establishedfrom compositional rela- but will increaseslightly the predicted/o, representedby tions among biotite.. and epidote,".The molar ratios of the curves given in Figure 8. The minimum fo, at tem- Al and Fe3*in epidote""and Fe2+and Mg in biotite., define peratures>520"C at 5 kbar is constrainedby the assem- of the system as a function of pressure,temperature, /", blage epidote.. + plagioclase""* margarite."+ biotite"" + and heterogeneousequilibria constraints on the chemical muscovite""+ aqueoussolution. This equilibrium can be potentials of AlrO, and MgO. In the xenolith, biotite is expressedas partially replacedby fine-grainedmuscovite and epidote. Local equilibrium of this assemblagewith an aqueous 4KFer(AlSirO,oXOH),+ l2CarAl.SirO'lOH) solution can be representedby the reaction + 2CaAlrSi,Os+ 2HrO^+ 30, anonhne tluid 4KFe,(AlSirO,o)(OH), : 4KAlr(AlSi3O'oXOH),* I 2CarFeAlrSi3O'r(OH) + l2CazAlSip,dOH) + 30, + l2}It muscovite eprdote + 2CaAlr(Al rsiro IoXOH)r, (4) : 4KAl,(AlSijOroXOH),+ l2CarFeAlrSi3O,r(OH) maFrite muscovile epiaore + 4Al3*+ 6HrO. (2) which defines the log/or-temperature relations given by fluid curve A-C in Figure 8. Point A denotesthe temperature where Reactions 3 and 4 intersect at equilibrium defined For constant mineral compositions, temperature, and by Reaction I (seeFig. 6). At 550"C and 5 kbar the co- pressure,the law of mass action for Reaction 2 requires rundum-stable (Reaction 3) and the plagioclase""-stable logforto be a linear function of log a^t-/alr- in the aqueous (Reaction 4) assemblagesare representedby points I and solution. Fixing aortt/ail- by saturation of the fluid phase 2 in Figure 7A, respectively.Point C in Figure 8 denotes with corundum, the equilibrium representedby the minimum equilibrium temperature for Reaction 4 ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS 35

'P -P'FLU|D =5Lh HzO

ASSEMELAGE. EPIOOTE, BIOTITE, MUSCOVITE, MARGARITE 5AJ "C ----- PLAGIOCLASE / _- conuruouM

o Gsl', 0 co, J 5/A -20 "C EPIDOTE+ BIoTITE + MARGARITE + MUscoVITE/f 475 "c Y ----\" o -23 1-' ^,rlf -20 -r5 -lo -o5 LoG osio.{oq) Fig.9. Phaserelations in the systemNarO-KrO-CaO-FerO3- AlrOj-SiOr-HrO-O,as a functionof Iogforandlog 4.,or,.n,in the fluid phaseat 5 kbar.The dashedlines represent constant-tem- peratureequilibria involving epidote,biotite, muscovite,and 450 margarite(Reaction 6). The solidlines denote equilibria of this assemblagewith corundum (curveA-B), plagioclase(curve A- TEMPERATURE,'C C), or quartz(curve C-D). PointsI and 2 representphase rela- Fig. 8. Phaserelations in the systemNarO-KrO-CaO-FerOr- tionsdesignated by points I and2 in Fig. 7A. SeeFig. 8. AlrO3-SiOr-HrO-O, as a function of logfo, and temperature at 5 kbar. CurvesA-B{-D outline the areawhere epidote,biotite, with point muscovite, and margarite of the compositions from the eGC corundum at I and with plagioclase""at point xenolith are stable at 5 kbar. This assemblageis in equilibrium 2 (compare with points I and 2 in Fig. 7A). Note that with corundum along curve A-B, plagioclase along curve A-C, over the wide range of values of 4s,oz(.q)characterized by and quartz along curve C-D. Seetext. equilibrium epidote".+ margarite""+ muscovite""+ biotite"" (betweenpoints I and 2 in Fig. 9) that log/o, varies by only about 0.1. Also given in Figure defined by the hydration ofplagioclase". to form epidote.., 9 are the rela- margaritess, and qtJartz by the reaction tionships of fo, and log a.,or,"o,for phaseboundaries representedby curves A-B (Reaction 3) and A-C (Reaction 5CaAl,Si,O, + 2H,O : 2Ca,Al.Si.O,,(OH) 4) in Figure 8. Local equilibrium for the plagioclase""- anonlire--nuia-cti"o,,ii.ip-' bearing assemblage(curve A-C in Figs. 8 and 9) requires + caAl,(Alrsiro,o)(oH), 4sio2(udin the fluid phaseto increasewith decreasingtem- marynle peratnres until quartz saturation is reachedat 5 I 8"C (point * 2fii9,. (5) C, Fig. 9, and Reaction 5). Phaseboundary C-D denotes equilibrium of Reaction 6 for saturation of the fluid phase with quartz. Local equilibrium constraints of margarites"on the /o, The rangeof oxygen fugacitiescomputed above for the conditions of reactions involving epidote.",biotite"", and assemblageepidote"" * margarite""* muscovite""+ bio- muscovite". (i.e., phase boundary l-2, Fig. 7A) can be tite"" is given in Figure l0 as function written as a of temperature at 5 kbar. Also shown in Figure l0 are predicted oxygen phase Srre:(elfl*p,oXOH),+ I 6Ca,AlSi,O,,(OH) fugacitiesestimated from Fe-Ti oxide relations for a large number of granitesand pegmatitesreported in the literature. It is apparent from the phase relations given + 3.750, + Sior("q)+ l.5HrO in Figure l0 that,f", in the xenolith was slightly higher : 5KAl,(AlSirO,oXOH), + I 5CarFeAlrSi3Orr(OH) than values characteristicofmany granites.

+ 2CaAlrAlrSirOro(OH)r. (6) maryante DrscussroN The mineral assemblagesfound in the xenolith are For constantmineral compositions the law of massaction products of a seriesof eventsthat led to the formation of for Reaction 6 requires log fo, to be a linear function of the anatecticQOrqut granite. Observedphase relations in log 4s;ozr,owith a slope of -0.27. This relationship for the xenolith do not allow for explicit definition of the epidote.",biotite"", muscovite.",and margarite""equilibria changesin chemical potentials of thermodynamic com- in the xenolith at 550'C and 5 kbar is given by the dashed ponents associatedwith the oxidation and hydration of line (l-2) in Figure 9. This assemblageis in equilibrium biotite, plagioclase,and corundum. However, the meta- 36 ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS

Oxidation of biotite (Reaction 6) and hydration of corundum (Reaction 1) to form epidote"",margarite"", and muscovite..in the presenceof plagioclase""require trans- fer of HrO and SiOr(aq) from the HrO-rich, quartz-sat- urated granitic melt into the corundum-bearingxenolith. It is apparentthat the massfluxes of SiO' and HrO between the quartz-saturatedgranite and the xenolith were important processesduring the formation of the xenolith's retrograde metamorphic assemblages.Although measurementshave not been made, detailed field obser- N oSr'A 4'+r' vations of compositional gradients within and near the € -ra xenolith in the together with the phase diagrams o, QGC, o presentedabove, will provide a basis for description and J interpretation of local metasomatic-element migration -20 € vY F :::'"':;;lli,'^Jt^.,,.during anatexisin the lower crust. ,h7/ The mineral assemblagebiotite"" * epidote." + mus- / h-,'/ + margarite""is not a buffer sensu stricto since E /, covite.. 'f .,i' Fe-Ti oxades the phasesare solid solutions that change composition /F-tt oLipmanls/l with /or. However, at constant 4rro the assemblagede- /r as shown in Figures 8, /H A Buddingtonand Lindslev 1964 fines a narro* isobaric/o, range, 9, and 10. Like the oxygen-bufferingcapacity ofthe as- ffi soo 6m 7@ 800 garnet,muscovite, magnetite, and quartz .C semblagebiotite, TEMPERATURE proposedby Zen (1985)for some peraluminousgranites Fig. 10. Predictedlog/o, andtemperature conditions for the and metamorphicrocks, thesetypes of assemblagescan formationof retrogrademineral assemblages in plagioclase-rich be used to constrain intensive thermodynamic variables xenolithswithin theQGC (area defined by curvesA-B{-D; see in certain geologicsystems. Fig. 8) and of Fe-Ti oxidesin graniticintrusions reported by Uncertainties associatedwith the thermodynamic cal- Buddingtonand Lindsley (1964) and Lipman (1971). The solid culations given above are complex and largely ambiguous curvesrepresent equilibrium in the systemFeO-FerOr-SiOr and functions of the errors in experimental observationsand Ni-NiO at 5 kbar. thermodynamic models for standard- and non-standard- stateproperties ofthe componentsand phasesofthe systems. Nevertheless,such calculations prove to be useful do allow for theoretical evaluation morphic assemblages tools in the analysisof even complexgeologic systems. of severallimiting casesfor the condition that atro = l. A-B, A-C, and C-D in Three cases,defined by curves AcrNowr,BocMENTS Figure 9, representthe extremesof fo, a,o.ruor,ac^z-/a!,, G. Liou, C. E. Manning, and aop-/a3n-in the fluid phase in equilibrium with the Criticalreviews by MoonsupCho, J' M. Rice,N. M. Rose,and J. V. Waltherhave greatly improved + + muscQ- J. assemblageepidote"" + margarite"" biotite". thecontent of this paper.Debbie Montgomery drafted a number vite.,. These limiting casescorrespond to local equilibri- of the figures.Bobbie Bishop, Halldora Hreggvidsdottir,and um of this assemblagewith corundum, plagioclase"",or ChuckKarish are thankedfor supportiveand stimulatingdis- quartz at temperaturesless than the hydration of plagio- cussrons. from the CarlsbergFoundation and the Danish Nat- clase.,and corundum to form margarite..at -580'C and Support ural SciencesResearch Counsel, to M. Rosing,while a Visiting 5 kbar. Although epidote,.,margarite,., biotite"", and mus- Scholarat Stanfordis acknowledged.This researchwas sup- covite""can be in equilibrium over a wide rangeof a.,or,"o,, portedin partby grantNSF-EAR 84-18129 to D'K.B. this assemblageis stable only for a very narrow range in fo, at any given temperature (Figs. 8, 9, and 10). RnnnnnNcBs In the plagioclase-richxenoliths of the QGC, it is likely Aagaard,P., and Helgeson,H.C. (1983)Activity/composition that the metamorphic and/or igneousfluids were close to rilationsamong silicates and aqueoussolutions: II. Chemical equilibrium with plagioclase..(An7o) at temperatures of and thermodynamicconsequences of ideal mixing of atoms illites,and mixed- > 518'C. This would require 4sio:("q)to increasewith de- on homologicalsites in montmorillonites, layerclays. Clays and Clay Minerals,3l, 207-217. (curve with creasingtemperature A-C, Fig. 9), together Alle;, J.IVI.,and Fawcett,J.J. (1982)Zoisite-anorthite-calcite an increase in the chemical affinity between conrndum stabilityrelations in H'O-CO' fluidsat 5000bars: An exper- and the aqueoussolution resulting in the replacementof imentaland snv study.Journal of Petrology,23, pt. 2' 215- corundum by margarite"".At temperaturesof < 518"C, 239. H. (1983)U-Pb isotopesyslematics on minerals plagioclase..in the xenolith will react with the aqueous Baadsgaard. from the gneisscomplex at Isuakasia,West Greenland. Rap- solution along phase boundary C-D in Figure 9 to pro- port GronlandGeologiske Undersogelse, l 12, 35-42. duce epidote and margarite, and possibly a more albite- Baadsgaard,H., Nutman, A.P., Bridgwater,D., Rosing,M., rich plagioclase(Bird and Helgeson,l98l). McGregor,V.R., and Allaart,J.H. (1984)The zircongeo- ROSING ET AL.: HYDRATION OF CORUNDUM-BEARING XENOLITHS )l

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