Stability of fluorite and titanite in a calc-silicate rock from the Vizianagaram area, Eastern Ghats Belt, India P. SENGUPTA 1 , M. M. RAITH 2 AND A. DATTA 1 1 Department of Geological Sciences, Jadavpur University, Kolkata – 700032, India 2 Mineralogisch-Petrologisches Institut, Universita ¨t Bonn, Poppelsdorfer Schloß, 53115 Bonn, Germany ([email protected]) ABSTRACT In the Vizianagaram area (E 83Ŷ29.442¢; N 18Ŷ5.418¢) of the Eastern Ghats Belt, India, a suite of graphite-bearing calc-silicate granulites, veined by syenitic rocks, developed wollastonite-rich veins at 6–7 kbar and > 850 ŶC. During subsequent near-isobaric cooling wollastonite was replaced by calcite + quartz and a graphic intergrowth of fluorite + quartz ± clinopyroxene. Titanite with variable Al and F contents is present throughout the rock. Combining the compositional variation of titanite and recent experimental data, it is demonstrated that the mineral assemblage, the composition of coexisting fluids and the mobility of Al exert a far greater control on the composition of titanite than pressure, temperature or the whole rock composition. Thermodynamically computed isothermal– isobaric logf O 2 – logf CO 2 and logf F 2 – logf O 2 grids in the systems Ca–Fe–Si–O–F (CISOF; calcite-free) and Ca–Fe–Si–O–F–C–H (CISOFV; calcite-present) demonstrate the influence of bulk rock and fluid compositions on the stability of the fluorite-bearing assemblages in diverse geological environments and resolve the problem of the stability of titanite in fayalite + fluorite-bearing rocks in the Adirondacks. The mineralogy of the studied rocks and the topological constraints tightly fix the logf O 2 , logf F 2 and logf CO 2 at )15.8, )30.6 and 4.1, respectively, at 6.5 kbar and c. 730 ŶC. Because of the similarity in the P–T conditions, the compositions of pore fluids in the fluorite-bearing assemblages of the Adirondacks and the Eastern Ghats Belt have been compared. Key words: Al-titanite; calc-silicate; Eastern Ghats Belt; fluorite; Vizianagaram; wollastonite. INTRODUCTION Fluorite and titanite are the two common fluorine- bearing accessory phases in rocks of diverse bulk compositions and widely ranging P–T–fluid regimes of formation (Bohlen & Essene, 1978; Enami et al., 1993; Markl & Piazolo, 1999). However, the potential of these minerals as petrogenetic sensors has been explored only in a few studies (Bohlen & Essene, 1978; Enami et al., 1993; Markl & Piazolo, 1999; Troitzsch & Ellis, 2002). In their pioneering work, Bohlen & Essene (1978) thermodynamically computed the phase relations in the system CaO–FeO–SiO 2 –O–F (CISOF) and dis- cussed the stability relations of fluorite-bearing assemblages from a range of physical conditions cov- ering the granulite facies to skarn-forming environ- ments. The computed phase diagram predicts instability of the end member titanite (CaTiSiO 5 ) in favour of rutile, fluorite and quartz in fluor- ite + fayalite + hedenbergite-bearing assemblages. The presence of titanite in the fluorite + fayalite- bearing samples of the Adirondacks therefore poses a problem. This was attributed to the uncertainties of the thermodynamic data and the effects of additional components (e.g. Al, F, etc.) in titanite. Recent experimental data (Tropper et al., 2002; Troitzsch & Ellis, 2002) have demonstrated that the effects of these additional components cannot explain the presence of titanite in the Adirondacks assemblages and hence, uncertainties in the thermodynamic data adopted by Bohlen & Essene (1978) are indicated. Furthermore, observations in many fluorite-bearing calcic rocks (including the Adirondacks assemblages) show the presence of calcite in addition to the minerals con- sidered by Bohlen & Essene (1978). Therefore in studying the phase relations of fluorite-bearing assemblages it is necessary to incorporate calcite. Compositions of natural titanite normally shows insignificant amounts of REE, Al, F, OH (Higgins & Ribbe, 1976). However, in certain environments extensive substitution of the type Ti þ O $ðAl þ Fe 3þ ÞþðF þ OHÞ has been noted (Oberti et al., 1991; Enami et al., 1993; Carswell et al., 1996; Markl & Piazolo, 1999; Troitzsch & Ellis, 2002). The physico-chemical conditions con- trolling this substitution are a matter of considerable debate (Enami et al., 1993; Markl & Piazolo, 1999). The earlier workers considered pressure and tem- perature as the dominant factors controlling the Al-substitution in the CaTiSiO 5 structure and argued that the mineral becomes aluminous with increasing pressure (Smith, 1981; Franz & Spear, 1985). The J. metamorphic Geol., 2004, 22, 345–359 doi:10.1111/j.1525-1314.2004.00518.x ȑ 2004 Blackwell Publishing Ltd 345
Transcript
Stability of fluorite and titanite in a calc-silicate rock fromthe Vizianagaram area, Eastern Ghats Belt, India
ABSTRACT In the Vizianagaram area (E 83�29.442¢; N 18�5.418¢) of the Eastern Ghats Belt, India, a suite ofgraphite-bearing calc-silicate granulites, veined by syenitic rocks, developed wollastonite-rich veins at6–7 kbar and > 850 �C. During subsequent near-isobaric cooling wollastonite was replaced bycalcite + quartz and a graphic intergrowth of fluorite + quartz ± clinopyroxene. Titanite withvariable Al and F contents is present throughout the rock. Combining the compositional variation oftitanite and recent experimental data, it is demonstrated that the mineral assemblage, the composition ofcoexisting fluids and the mobility of Al exert a far greater control on the composition of titanite thanpressure, temperature or the whole rock composition. Thermodynamically computed isothermal–isobaric logfO2
– logfCO2and logfF2
– logfO2grids in the systems Ca–Fe–Si–O–F (CISOF; calcite-free) and
Ca–Fe–Si–O–F–C–H (CISOFV; calcite-present) demonstrate the influence of bulk rock and fluidcompositions on the stability of the fluorite-bearing assemblages in diverse geological environments andresolve the problem of the stability of titanite in fayalite + fluorite-bearing rocks in the Adirondacks.The mineralogy of the studied rocks and the topological constraints tightly fix the logfO2
, logfF2and
logfCO2at )15.8, )30.6 and 4.1, respectively, at 6.5 kbar and c. 730 �C. Because of the similarity in the
P–T conditions, the compositions of pore fluids in the fluorite-bearing assemblages of the Adirondacksand the Eastern Ghats Belt have been compared.
Fluorite and titanite are the two common fluorine-bearing accessory phases in rocks of diverse bulkcompositions and widely ranging P–T–fluid regimes offormation (Bohlen & Essene, 1978; Enami et al., 1993;Markl & Piazolo, 1999). However, the potential ofthese minerals as petrogenetic sensors has beenexplored only in a few studies (Bohlen & Essene, 1978;Enami et al., 1993; Markl & Piazolo, 1999; Troitzsch &Ellis, 2002).
In their pioneering work, Bohlen & Essene (1978)thermodynamically computed the phase relations inthe system CaO–FeO–SiO2–O–F (CISOF) and dis-cussed the stability relations of fluorite-bearingassemblages from a range of physical conditions cov-ering the granulite facies to skarn-forming environ-ments. The computed phase diagram predictsinstability of the end member titanite (CaTiSiO5) infavour of rutile, fluorite and quartz in fluor-ite + fayalite + hedenbergite-bearing assemblages.The presence of titanite in the fluorite + fayalite-bearing samples of the Adirondacks therefore poses aproblem. This was attributed to the uncertainties of thethermodynamic data and the effects of additionalcomponents (e.g. Al, F, etc.) in titanite. Recentexperimental data (Tropper et al., 2002; Troitzsch &
Ellis, 2002) have demonstrated that the effects of theseadditional components cannot explain the presence oftitanite in the Adirondacks assemblages and hence,uncertainties in the thermodynamic data adopted byBohlen & Essene (1978) are indicated. Furthermore,observations in many fluorite-bearing calcic rocks(including the Adirondacks assemblages) show thepresence of calcite in addition to the minerals con-sidered by Bohlen & Essene (1978). Therefore instudying the phase relations of fluorite-bearingassemblages it is necessary to incorporate calcite.
Compositions of natural titanite normally showsinsignificant amounts of REE, Al, F, OH (Higgins &Ribbe, 1976). However, in certain environmentsextensive substitution of the type
Ti þ O $ ðAl þ Fe3þÞ þ ðF þ OHÞhas been noted (Oberti et al., 1991; Enami et al., 1993;Carswell et al., 1996; Markl & Piazolo, 1999; Troitzsch& Ellis, 2002). The physico-chemical conditions con-trolling this substitution are a matter of considerabledebate (Enami et al., 1993; Markl & Piazolo, 1999).The earlier workers considered pressure and tem-perature as the dominant factors controlling theAl-substitution in the CaTiSiO5 structure and arguedthat the mineral becomes aluminous with increasingpressure (Smith, 1981; Franz & Spear, 1985). The
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occurrence of highly aluminous titanite with XAl
[¼ Al ⁄ (Al + Ti + Fe3+)] up to 0.54 in eclogite faciesrocks was cited as a support of a pressure-induced,Al-substitution (Franz & Spear, 1985; Enami et al.,1993; Markl & Piazolo, 1999). The molar volume ofCaAlSiO4F is smaller than that of its chemicalequivalent paragenesis anorthite + fluorite – a featurethat also supports a pressure induced Al–F substitu-tion (Troitzsch & Ellis, 2002; Tropper et al., 2002).
Subsequent studies on natural samples have revealedthat titanite with XAl values as high as those of ultra-high pressure rocks are also observed in medium to lowpressure metamorphic rocks including the Salton Seageothermal field (Enami et al., 1993; Markl & Piazolo,1999). However, titanite equilibrated at lower pressuretends to show lower F ⁄OH ratios compared to its highpressure counterpart. Combining the data from nat-ural rocks with their own observations on a suite ofrocks from East Antarctica, Markl & Piazolo (1999)argued that pressure and temperature exerted only apartial control on the Al-substitution of titanite, andthat instead the bulk rock and fluid compositions arethe dominant factors. Although, the findings by theseworkers nicely explains the large compositional varia-tion of titanite in many rocks metamorphosed undernearly constant P–T conditions, features such asstrong Al zoning in titanite grains observed in manynatural rocks warrants involvement of other factors aswell. Recent experimental investigations convincinglydemonstrated the P–T control on the alumina contentof titanite (Troitzsch & Ellis, 2002; Tropper et al.,2002). The experimental data also emphasise the roleof the associated phases in determining the composi-tion of titanite.
In this context, we present the textural rela-tions and phase compositional characteristics of acalc-silicate assemblage containing fluorite and tita-nite from the Vizianagaram area (E 83�29.442¢;N 18�5.418¢), Eastern Ghats Belt, India. Integratingthe mutual relations of the minerals and their com-positions in the studied rocks as well as the data fromother well studied rocks and the recent experimentaldata in the Ca–Ti–Al–Fe–Si–F–O–C system, anattempt has been made to understand the factorscontrolling the stability relations of fluorite and tita-nite-bearing assemblages from a range of geologicalconditions.
BACKGROUND GEOLOGY
The Vizianagaram area in the northern part of theEastern Ghats Belt (henceforth EGB), exposes anensemble of repeatedly deformed and metamorphosedgranulite to amphibolite facies rocks of diverse bulkcompositions (Kanungo & Murthy, 1981; Kamineni &Rao, 1988; Grew et al., 2001; Datta et al., 2001).Khondalite (garnet + sillimanite + quartz + alkalifeldspar ± plagioclase ± graphite) is the dominantrock of this area, containing tectonically detached
lenses of felsic, mafic and calc-silicate rocks of variousdimensions. This rock ensemble was intruded by sev-eral generations of syenite and pegmatitic granitesheets (Kanungo & Murthy, 1981; Kamineni & Rao,1988; Datta et al., 2001). Petrological and geochrono-logical data reveal at least two phases of high-grademetamorphism at c. 1.1 Ga and 1.0–0.95 Ga in thispart of the Eastern Ghats Belt (Dobmeier & Raith,2003; Simmat, 2003). During the later high-grade event(the most dominant structural and metamorphic eventof this part of the EGB) the peak temperature rose to‡ 900 �C at 6–7 kbar, followed by near-isobaric cool-ing of more than 200 �C (accompanied by channelisedhydrous fluid flux) and finally superposed by Pan-African amphibolite facies metamorphism (Grewet al., 2001; Datta et al., 2001; Simmat, 2003). Fieldobservations indicate that the syenitic magmas wereemplaced in the calc-silicate granulites in several pha-ses, during and subsequent to the peak of the secondhigh-grade event. The fluorite-bearing assemblageswere found at the contact of wollastonite-rich veins inclinopyroxene + plagioclase-bearing calc-silicaterocks near thick syenitic veins (E 83�29.442¢; N18�5.418¢). Graphite occurs in variable modal amountsin both the syenitic and the calc-silicate rocks.
THE MUTUAL RELATIONS AMONG THE PHASES
The microphotographs of Fig. 1(a,b) show a largethin section domain where fluorite+quartz-bearingassemblages are developed as a thin seam (up to20 mm thick) at the interface of a wollastonite veinwith its host calc-silicate rock. The wollastonite vein(> 80 vol% wollastonite) contains minor quartz,scapolite and interstitial clinopyroxene. Calc-silicatedomains close to the margins of the vein display avariety of replacement and intergrowth textures. Themost striking feature is the presence of a vermicularintergrowth consisting of nearly equal modal pro-portions of fluorite and quartz that replaces the coarsewollastonite grains along their margins and fractures(Fig. 1b,c,f). The symplectite domains often assume afan-like shape that protrudes into the wollastonitegrains (Fig. 1b–d) with grain size increasing awayfrom the wollastonite. Locally, the fluorite + quartzintergrowth encloses corroded wollastonite relicts(Fig. 1d). In these domains, thin clinopyroxene grainsare found to follow the contacts of the coarse wol-lastonite grains (Fig. 1e). Closer to the fluorite +quartz symplectite, the wollastonite grains are oftenpartially replaced by calcite. In many places, anaggregate of calcite + quartz + fluorite ± clino-pyroxene replaces the wollastonite grains along theirmargins and ⁄ or cracks suggesting simultaneousformation of these phases.
At the contact of the fluorite + quartz symplectitewith the calc-silicate host rock occurs a 2–3 cm thickzone rich in coarse scapolite with calcite and quartz(Fig. 1b). The latter two minerals are preferentially
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Fig. 1. (a) photomicrograph of a thin section showing development of the fluorite + quartz-bearing assemblage as thin screen(up to 20 mm thick) at the interface of a wollastonite vein with its host calc-silicate rock. Width of the photomicrograph ¼ 3 cm (b)microtextures across the contact of the wollastonite (Wo)-rich vein and the host calc-silicate rock. Note the development of a delicatefluorite + quartz symplectite (Flt + Qtz) at the interface and the development of coarse scapolite (Scp) + clinopyroxene (Cpx) withscattered titanite (Ttn) grains in the host rock near the wollastonite veins. Width of the photomicrograph ¼ 3.4 mm (c) Fan-shapedfluorite + quartz intergrowth protruding coarse wollastonite grains. Note that the size of the mineral grains in the symplectite decreasetowards the wollastonite contact. Also note relict wollastonite grains within the symplectite. Width of the photomicrograph ¼ 1.72 mm(d) Replacement of coarse wollastonite by symplectitic fluorite + quartz and granular calcite + quartz (Cal + Qtz) along the margins andfractures. Note the presence of wollastonite in the fluorite + quartz symplectite. Width of the photomicrograph ¼ 1.72 mm (e) Sym-plectic intergrowth of fluorite + quartz + clinopyroxene (Flt + Qtz + Cpx) replacing coarse wollastonite. A thin clinopyroxene seamdeveloped along the boundary of the wollastonite grains. Width of the photomicrograph ¼ 1.72 mm (f) Breakdown of coarse scapoliteinto calcite + plagioclase (Cal + Pl) symplectite near the fluorite + quartz intergrowth. Width of the photomicrograph ¼ 0.43 mm.
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concentrated close to the vein. The contact between thecoarse scapolite and wollastonite is often cuspate withits convex side towards the wollastonite. Occasionally,relics of wollastonite are found in the scapolite zone.Moving further towards the main calc-silicate rock, theassemblage becomes clinopyroxene-rich with plagio-clase and scapolite. In many domains in this zone, themodal proportion of plagioclase exceeds that of scapo-lite and the texture resembles the typical clinopyrox-ene + plagioclase-bearing rocks that predominate inthe locality. In these plagioclase-rich domains, scapoliteis found to be replaced by the former mineral to a dif-ferent extent. The plagioclase- and scapolite-rich do-mains are thoroughly recrystallized and show apolygonal granoblastic fabric. The delicate fluorite +quartz ± clinopyroxene intergrowths therefore musthave formed after the cessation of the intense defor-mation.
The coarse scapolite grains near the fluorite +quartz symplectite commonly are replaced by a fin-gerprint-like intergrowth of calcite and plagioclase(Fig. 1f). The two intergrowths are, by and large,noninterfering. However, in places the fluorite +quartz intergrowths marginally replace the latter. Allthese features collectively indicate that both the inter-growths formed during cooling and that the formationof the fluorite + quartz intergrowth outlasted thebreakdown of scapolite. Titanite grains of various sizeand shape are present throughout the rock. This min-eral is also found to coexist with the fluorite + quartzintergrowths. Graphite is present in small amountsscattered throughout the rock except of the fluorite-bearing domains. Some recrystallized clinopyroxenegrains are partially replaced by randomly oriented lateamphibole and ⁄or biotite grains in the graphite-bearing calc-silicate domains. Some biotite flakes werefound to be interwoven with graphite.
MINERAL COMPOSITIONS
Chemical compositions of minerals were analysed inthe Mineralogisch-Petrologisches Institut, Universitatof Bonn, Germany, using a CAMECA MICRO-BEAM electron microprobe. The analytical proce-dures were reported in Sengupta et al. (1999).Selected analyses of the relevant phases are presentedin the Tables 1–4.
Clinopyroxene within the coarse matrix of the calc-silicate rock has low Al2O3 and TiO2 and is practicallyhomogenous with respect to XMg [Mg ⁄ (Mg +Fetot) ¼ 0.48–0.51]. However, where developed as thinrims around wollastonite and within the fluor-ite + quartz intergrowth its composition is distinctlymore Fe rich (XMg c. 0.41) (Table 1).
Wollastonite has nearly end member composition(Table 1). Rims of some grains close to the thinclinopyroxene rim contain almost up to 5 mol%FeSiO3 component.
Scapolite grains are compositionally homogenous butsignificant intergrain variation has been noted indifferent domains of the calc-silicate rock, where theequivalent anorthite content (EqAn ¼ (Al-3) ⁄ 3) variesfrom 0.70 to 0.82. Scapolite is always less calcic thanthe coexisting plagioclase. The S and Cl contents of allthe grains are low (< 0.5 wt%) (Table 2).
Plagioclase is calcic without any notable zoning. Inmost domains no distinct compositional difference
Table 1. Representative electron microprobe analyses ofclinopyroxene, wollastonite and calcite.
CaO 21.53 21.65 21.65 21.62 21.69 15.78 16.99 17.10
Na2O 1.57 1.48 1.54 1.56 1.91 2.76 2.17 1.98
K2O 0.26 0.26 0.23 0.23 0.26 0.08 0.08 0.06
SO3 0.25 0.00 0.00 0.02 0.00
Cl 0.01 0.00 0.00 0.00 0.00
CO2 calc. 4.55 4.68 4.65 4.65 4.65
Total 100.56 100.02 99.60 99.89 100.07 99.95 100.66 100.49
Oxygen basis 25 25 25 25 25 8 8 8
Si 6.64 6.62 6.61 6.74 6.72 2.23 2.17 2.17
Al 5.37 5.38 5.39 5.26 5.28 1.76 1.82 1.82
Fe 0.05 0.06 0.07 0.07 0.05 0.00 0.00 0.01
Ca 3.59 3.63 3.65 3.64 3.66 0.77 0.83 0.84
Na 0.47 0.45 0.47 0.48 0.58 0.25 0.19 0.18
K 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.00
SO4 0.03 nd nd 0.00 nd
Cl 0.00 nd nd 0.00 nd
CO3 0.97 1.00 1.00 1.00 1.00
EqAn 0.79 0.79 0.80 0.75 0.76 An 75 81 82
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between the symplectic (after scapolite) and the por-phyroblastic plagioclase was noted. However, thismineral is slightly less calcic (XAn c. 0.76) in the
wollastonite and fluorite + quartz domains comparedto the calc-silicate domains where these minerals areabsent (XAn > 0.80) (Table 2).
Biotite exhibits XMg c. 0.49 with low TiO2 contentsand F varying between 1.22 and 1.3 wt%, respectively(Table 3).
Amphibole falls in the field of hornblende in theclassification scheme of Leake et al. (1997) and,depending on the phases it replaces, shows a ratherlarge variation of Al2O3 contents (5.85–8.23 wt%). Inthe presence of scapolite and plagioclase, Al2O3 andNa2O contents are always higher (Table 3). Thissuggests Al enrichment in this mineral throughtschermakite and pargasite substitutions.
Titanite shows a large compositional variationwithin a single grain and from one microdomain toanother, depending upon the nature of the coexistingphases. The structural formulae are normally calcu-lated on the basis of fixed numbers of cations oroxygen atoms (Oberti et al., 1991; Carswell et al.,1996; Markl & Piazolo, 1999). In Table 4, thestructural formulae were normalised to three cations.It is to be noted that the values of Si in thiscalculation lie close to one suggesting that theinterpretation of compositional variation will not beaffected by the choice of the recalculation scheme.Furthermore, Ca has values close to unity in all theanalyses indicating that this element was not signi-ficantly replaced by REE. The data show a welldefined negative correlation between Al + Fe3+ andTi (r2 ¼ 0.9). This together with Si and Ca valuesclose to unity are consistent with the substitutionTi + O « (Al,Fe3+) + (F,OH).
The coarse grains of titanite in both the calc-silicateand fluorite + quartz domains show a significantrimward increase in Al and F in the presence ofan aluminous phase like plagioclase or scapolite. XAl
[¼ Al ⁄ (Al + Ti + Fe3+)] changes from 0.17 toc. 0.27 and F from 0.68 to 2.44 wt% (Table 4). How-ever, the small titanite grains occurring within thefluorite + quartz intergrowths or those shielded fromthe latter phases are distinctly lower in Al and thecompositions resemble the core composition of thelarger grains (Table 4). F ⁄ (OH) ratios also vary con-siderably (Fig. 2). No distinct correlation was observedbetween the XF [¼ F ⁄ (F + OH)] and XAl ratios orbetween the size of the grains and their XF content(Fig. 2). However, the titanite grains in the bio-tite + amphibole-bearing calc-silicate domains showlower fluorine contents compared to those in thefluorite + quartz intergrowth domains at similar XAl
contents (Table 4, Fig. 2).
Calcite grains are almost pure CaCO3. In somedomains close to the fluorite + quartz intergrowth itcontains up to 2 wt% FeO (Table 1).
Fluorite, quartz and graphite are effectively purephases.
Table 4. Representative electron microprobe analyses of titanite.
Table 3. Representative electron microprobe analyses of biotiteand amphibole.
Mineral Biotite Amphibole
Analysis 147–3 147–4 147–3 147–4 147–5
SiO2 36.61 36.61 43.88 45.99 48.23
TiO2 2.85 2.78 0.56 0.33 0.27
Al2O3 13.21 13.05 8.85 6.82 5.98
Cr2O3 0.00 0.00 0.02 0.00 0.03
FeO 20.91 20.96 20.87 18.79 19.60
MnO 0.06 0.11 0.12 0.23 0.32
MgO 11.31 11.43 8.49 9.86 10.87
CaO 0.05 0.08 11.55 11.78 11.88
Na2O 0.01 0.03 0.63 0.73 0.62
K2O 10.12 9.95 1.59 0.94 0.73
F 1.12 1.30 0.63 0.73 nd
Cl 0.11 0.16 0.19 0.05 nd
)F ¼ O 0.47 0.55 0.27 0.31 nd
)Cl ¼ O 0.02 0.04 0.04 0.01 nd
Total 95.86 95.88 97.07 95.93 98.53
Oxygen equivalents 22 22 23 23 23
Si 5.54 5.52 6.80 7.10 7.20
Ti 0.32 0.32 0.07 0.04 0.03
Al 2.35 2.32 1.62 1.24 1.05
Cr 0.00 0.00 0.00 0.00 0.00
Fe 2.64 2.64 2.70 2.42 2.45
Mn 0.01 0.01 0.02 0.03 0.04
Mg 2.55 2.57 1.96 2.27 2.42
Ca 0.01 0.01 1.92 1.95 1.90
Na 0.00 0.01 0.19 0.22 0.18
K 1.95 1.91 0.31 0.19 0.14
F 0.54 0.62 0.30 0.35 nd
Cl 0.02 0.04 0.05 0.01 nd
OH 1.48 1.34 1.65 1.64 2.00
XMg 0.49 0.49 XMg 0.43 0.48 0.50
XF 0.14 0.16 Xmg* 0.46 0.51 0.55
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DEVELOPMENT OF THE MINERAL ASSEMBLAGES
The textural relations and the compositional featuresdescribed in the previous sections, attest to severalmineral reactions. Although the formation of thefluorine-bearing minerals is the central theme of thiscommunication, some important fluorine-absentreactions will be discussed in brief to understand and
constrain the P–T–fluid regime during metamor-phism.
Fluorine-absent reactions
Formation of the scapolite + quartz rich seams(without plagioclase) at the contact of the wollastonitevein with the clinopyroxene + plagioclase-bearingcalc-silicate rock indicates the reaction
CaSiO3 þ 3CaAl2Si2O8 þ CO2
! Ca4Al6Si6O24CO3 þ SiO2: ð1ÞReplacement of wollastonite by calcite + quartzaggregates suggests the reaction
CaSiO3 þ CO2 ! CaCO3 þ SiO2: ð2ÞFormation of calcite + plagioclase symplectite afterscapolite can be explained by the reaction
Ca4Al6Si6O24CO3 ! CaCO3 þ 3CaAl2Si2O8: ð3Þ
The first two reactions proceed to the right duringcooling ⁄ loading with or without an increase in fCO2 inthe coexisting fluid, whereas only cooling is requiredfor the breakdown of scapolite by the second reaction(Harley & Buick, 1992). The compositions of thephases of the second reaction and the internally con-sistent database of Holland & Powell (1998) indicatethat the breakdown of scapolite occurred in a tem-perature range of 750–800 �C. Furthermore, thebreakdown of scapolite by reaction (3) instead ofreaction (1) indicates that cooling was nearly isobaric(Harley & Buick, 1992). Recently, near-isobaric cool-ing (at c. 6–7 kbar) from a temperature ‡ 900 �C, hasbeen documented from the adjoining lithologies (Dattaet al., 2001). The textural and compositional featuresof the fluorite-bearing rocks therefore corroboratethese studies.
Sporadic development of amphibole + calcite +quartz after the clinopyroxene ± plagioclase in someplagioclase-rich domains and stabilisation of biotitesuggest a local increase in PH2O
.
Reactions involving fluorine
Reactions without titanite
The replacement of wollastonite by fluorite + quartzintergrowths indicates the reaction
CaSiO3 þ F2 ! CaF2 þ SiO2 þ 0:5O2: ð4ÞVery similar molar volumes of fluorite and quartz(2.454 & 2.269 J bar)1, respectively, Robie & Hemi-ngway, 1995) and the coefficients of these phases in thereaction are consistent with the observed almost equalmodal proportions of the two symplectite phases.Calcite is ubiquitous in the fluorite-bearing domainsand the mineral replaces wollastonite to different
Fig. 2. Compositional variation of titanite in the studied rocks.(a) Ti v. (Al + Fe3+) plot showing a strong negative correlationbetween these compositional parameters suggesting the domin-ance of the substitution Ti + O « (Al,Fe3+) + (F,OH). Theanalyses with higher Al+Fe3+ are observed in the rim of the bigtitanite grains. (b) (Al ⁄Al + Fe3+ + Ti) v. (F ⁄F + OH) plotrevealing that the variation of the two parameters is not inter-dependent. The analyses with lower XF are observed in titanitewithin calc-silicate domains with F-bearing retrograde amphi-bole and biotite (see text).
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extent. Furthermore, the distribution of the calcite +plagioclase and fluorite + quartz intergrowths alsoindicates that breakdown of scapolite and formation ofthe latter intergrowth occurred almost simultaneously.These features suggest that the reactions (2) (3) and (4)were temporally coeval.
Involvement of Fe-rich clinopyroxene in the fluor-ite + quartz symplectite is intriguing. The texturalfeatures such as restriction of the thin clinopyroxenegrains to the proximity of the fluorite + quartz sym-plectite indicate a genetic link between the symplectiteand the clinopyroxene. The Fe necessary for the for-mation of this symplectite was supplied either by thewollastonite (FeSiO3 component) or through ironmetasomatism,
8CaSiO3 þ 2Feþ2 þ 2CO2 þ 2F2 ! 2CaF2
þ 2CaFeSi2O6 þ 2CaCO3 þ 4SiO2 þ O2: ð5Þ
Relics of andradite garnet were never observed in thefluorite-bearing rocks. Furthermore, this mineral alsodid not stabilise in the adjoining fluorite-absent (butgraphite-bearing) calc-silicate rock. This observationtogether with the rimward increase in Fe content of thereplaced wollastonite seem to support that Fe wasprovided by the intergranular fluid (reaction 5). Evi-dence of iron metasomatism in calc-silicate rocks hasbeen reported from many areas (Cartwright & Buick,1994, 2000).
Reactions involving titanite
A further fluoridation reaction in the rock is mani-fested by the concomitant increase of Al and F in thecoarse titanite grains at their contacts with plagioclaseand ⁄ or scapolite. Several studies emphasised the con-trol of P–T on the Al-F-OH substitution, in additionto the ambient fluid composition (Enami et al., 1993;Carswell et al., 1996; Troitzsch & Ellis, 2002; Tropperet al., 2002). Markl & Piazolo (1999), on the otherhand, proposed that the F-vuagnatite substitutionin titanite can be completely controlled by theaTiO2
⁄ aAl2O3ratio of a given bulk composition and the
aHF ⁄ aH2Oratio of the coexisting fluid.
The combined effects of P–T–fluid compositions onthe Al-F substitution in titanite are evident from theexperimental data of Troitzsch & Ellis (2002). Figure 3shows that in the rutile-absent assemblages, theAl content of titanite can either increase or decreaseduring cooling depending on the ambient pressure.This explains the very high XAl values (c. 0.54) of tita-nite in eclogites of the Tauern Window (P > 20 kbar)as well as in the low pressure (P £ 5 kbar) rocks ofEast Antarctica (Fig. 4). The role of the bufferingassemblages is also predicted by the experimental data.In the calcite-present assemblage, a reduction of aCO2
of the coexisting fluid will lead to an increase in Alcontent of the titanite at given P–T conditions. How-ever, an opposite trend is evident when hydrous phases
buffer the Al content of titanite. The distribution of Fv. (Al + Fe3+) for natural titanite from a range of P–T conditions does not show any distinct correlationwith pressure or temperature (Fig. 4).
In view of the foregoing discussion it can thereforebe suggested that the compositions of titanite aloneshould not be used as a P–T sensor unless the nature ofthe buffer assemblages and the fluid compositionare precisely constrained. Although this observationbroadly corroborates the contention of Markl &Piazolo (1999), the arguments of these authors on thedominant role of aTiO2
and aAl2O3of the rock system in
controlling the Al content of titanite is at variance withmany features of the studied rocks. These include (a)the significant variation of Al and F content in titanitegrains separated by only a few hundred micron and (b)the strong Al-F zoning in large grains. Significantcompositional variation in a single grain has also beennoted by Enami et al. (1993) and Carswell et al. (1996).Incidentally, the titanite grains of sample GM694 ofMarkl & Piazolo (1999) that have the highest Al con-tents are also compositionally zoned.
Enami et al. (1993) showed that the (Al + F)-enrichment of titanite was a consequence of chlori-tization of biotite (the source of Al and F) in thesame domain. The chloritization of biotite was alsoobserved close to Al-rich titanite in sample GM694of Markl & Piazolo (1999, their Fig. 3c). The com-positional significance of the neighbouring phase isalso evident from the compositional variation oftitanite–epidote pairs documented by Enami et al.
900
XCO2 = 1PCO2 = Ptotal
0.2
0.3 0.40.5
0.6
0.7
0.3
0.50.6
Cal+ Ky + Qtz
+ Fl
AlF+ CO 2
AlF
Fl + An
An + CO2Cal + Ky + Qtz
28
24
20
16
12
8
4
500 600 700 800
T [˚C]
P [k
bar]
0.4
Fig. 3. Experimentally determined phase diagram of Troitzsch &Ellis (2002) demonstrating the dependence of Al content of tita-nite on pressure, temperature, fluid composition and mineral as-semblage. In isobarically cooled assemblages, XAl of titanite canincrease or decrease depending upon the ambient pressure and theassociated mineral phases. Note that titanite in low pressure rocks(P < 8 kbar) can be extremely aluminous like those of eclogites ifplagioclase and fluorite are the coexisting phases (see text). Thedirection of displacement of these isopleths due to reduced aCO2
isshown by arrows. Note that a decrease in CO2 activity in the porefluid will cause an increase in Al-content of titanite in the calcite-bearing assemblages at given P–T conditions.
F L U O R I TE A N D T I T A N I T E S T A B I L I T Y 35 1
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(1993). Furthermore both Ti and Al are stronglyimmobile in most metamorphic milieus (Thompson,1975; Glassley, 1983; Tracy & McLelland, 1985;Bhattacharya & Sen, 1986; Buick et al., 1993;Dunkley et al., 1999; Sengupta & Raith, 2002).Hence, at given P–T–fluid conditions, activities ofthese components are expected to be controlled bythe compositions of titanite and the other solidphases (Thompson, 1975; Glassley, 1983; Tracy &McLelland, 1985; Bhattacharya & Sen, 1986).Buffering of aAl2O3
and aTiO2by local mineral reac-
tions is also consistent with the variation of titanitecompositions depending upon the species and com-positions of the coexisting phases.
The most relevant reactions that can be applicable tothe studied rocks and also to other anhydrous rocks are:
CaAl2Si2O8ðin plÞ þ CaF2 $ 2CaAlFSiO4 ð6Þ
3CaSiO3 þ 2CaAl2Si2O8 þ 2CaF2 þ 3CO2
$ 4CaAlFSiO4 þ 3CaCO3 þ 3SiO2 ð7Þ
CaAl2Si2O8 þ 2CaCO3 þ 2F2
$ 2CaAlFSiO4 þ 2CO2 þ O2 ð8Þ
2CaAlFSiO4 þH2Oþ0:5O2
$ 2CaAl(OH)SiO4 þF2: ð9ÞThe vapour-absent reaction (6) has been experiment-ally calibrated by Tropper et al. (1999, 2000) andTroitzsch & Ellis, 2002). The reaction has a positiveP–T slope and proceeds to the right with cooling andloading. The reaction (7) also proceeds to the rightwith cooling and ⁄ or loading but an increase in fCO2
also drives the reaction to the right. The reactions (8)and (9), on the other hand, proceed to the right with achange in fCO2 and fO2 ⁄ fF2 ratio.
The reactions (5–9) explain several petrologicalfeatures involving titanite. In the fluorite + quartzdomains, the rimward increase in Al-F in large titanitegrains together with the replacement of wollastonite bycalcite can be explained by the movement of thereactions (6 & 7) towards the right during near-isobariccooling of the rocks (Kamineni & Rao, 1988; Grewet al., 2001; Datta et al., 2001). Since mobility of Albecomes very limited during cooling, the reactions (6)and (7) could operate only in those titanite grains thatwere in contact with aluminous phases. The strong Alzoning in these grains also supports slow diffusion ofAl in titanite at the estimated physical conditions. Thelower Al content of titanite grains occurring within thefluorite + quartz symplectite or shielded by the low-aluminous phases like clinopyroxene therefore indi-cates kinetic inhibition of the CaAlFSiO4-producingreactions rather than a variation of the fluid or rockcompositions. Restriction of the product aluminousphases (such as garnet, sillimanite) closest to the Al-bearing reactant phases in coronitic reaction texturesare a common feature in anhydrous rocks undergoingcooling (Tracy & McLelland, 1985; Dunkley et al.,1999).
The mineralogy and composition of the phases inthe wollastonite + fluorite free calc-silicate rocks, onthe other hand, suggest operation of reaction (8). Inview of the calculated low aCO2
of the graphite-bearingrock and the development of some hydrous phases(amphibole, biotite) in these domains, it can be statedthat the reaction (8) proceeded to the right during areduction of fCO2
. A reduction of fCO2in the coexisting
fluid also destabilises the assemblage rutile + cal-cite + quartz in favour of titanite (Fig. 5). Thisexplains the abundance of highly aluminous titanite (inplace of rutile + calcite + quartz) in rocks that equili-brated with low aCO2
fluids over a large range of P–Tconditions (Franz & Spear, 1985; Carswell et al., 1996;Markl & Piazolo, 1999; Troitzsch et al., 2002; Tropperet al., 2002).
Enami et al. (1993) argued that titanite at hightemperature should be rich in F compared to OHbecause of the thermally induced decomposition of the
0.2 0.4 0.6 0.8
Al + Fe3+ (pfu.)
F (
pfu.
)0.8
0.6
0.4
0.2
0.5
1.0
E93a
M&P99
M&P99(GM694)
F&S85
C96
E93b
E93c
E93b
this work
Fig. 4. Compositional variation of titanite from diverse geo-logical milieus. The compositional range of the mineral in eacharea or set-up has been demarcated by thin lines with theabbreviated authors names. Note that the bulk of the data falls ator below the 1:1 line suggesting the substitution Al + Fe3+ «F + OH to be most dominant. The composition of titanite withhigher OH content will plot away from the 1:1 line. Composi-tional overlaps of titanite from diverse physicochemical condi-tions are to be noted (see text for its significance). Also note thatonly the sample GM694 of M&P99 shows the largest composi-tional spread for F and Al. Interestingly the compositional dataof titanite from the Salton Sea show an increase of F and Al withdecrease in the grade of metamorphism. E93: Enami et al. (1993);a. Salton Sea (£ 1.1 kbar, 200–400�C), b. titanite in granitoids(3 kbar, c.580 �C), c. titanite in skarns. F&S85: Franz & Spear(1985); eclogite (18–40 kbar, 600–1000�C). C99: Carswell et al.(1996); high pressure granulites and eclogites. M&P99: Markl &Piazolo, 1999); amphibolite (4–5 kbar, 500–650�C). thiswork: compositional spread of titanite in the studied rock.
3 52 P . S E N G U PT A E T A L .
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CaAl(OH)SiO4 component compared to its F-ana-logue. Our titanite data also show that the variation ofthe F ⁄OH ratio is independent of the XAl content(Figs 2 & 4). This is in line with the reported titanitedata that also show that F ⁄OH can vary widely inde-pendent of the Al ⁄Ti ratio (Fig. 2). Furthermore, inthe studied rock, a large variation of F ⁄OH was notedin the thin section scale and hence the variation of P–Tis unlikely to be the causative mechanism for thevariation of this ratio. This is supported by the spreadof XF at given P–T–XAl of titanite from other occur-rences (Fig. 4). The data from Salton Sea indicate thatthe CaAl(OH)SiO4 component actually decreases athigher grade. It is obvious that factors other than theP–T conditions also exert a strong control on theF ⁄OH ratio of titanite over a variety of geologicalconditions. The titanite grains, in the studied rock,always show lower F ⁄OH ratios when coexisting withamphibole and ⁄ or biotite compared to the domainswhere these phases are absent (Table 4). This could beeither related to a decrease in F ⁄OH ratio of thecoexisting fluid in these domains or to a preferentialpartitioning of F into amphibole and ⁄ or biotite.The present database, however, does not allow theevaluation of the relative importance of the twopossibilities.
STABILITY OF FLUORINE-BEARINGASSEMBLAGES IN HIGH-GRADE ROCKS:A GEOMETRICAL APPROACH
Existing information on the composition of naturaltitanite indicates that Al2O3, F and OH are the dom-inant species that can enhance the stability of this
mineral through chemical substitution (Oberti et al.,1991; Markl & Piazolo, 1999; Troitzsch et al., 2002;Tropper et al., 2002). Yet, a CaAlFSiO4 content ashigh as 75 mol% cannot explain the stability of titanitein the Wanakena rocks if the thermodynamic data ofBohlen & Essene (1978) are used (Troitzsch et al.,2002). In view of this we have computed a part of theisothermal-isobaric logfF2
–logfO2diagram of Bohlen &
Essene (1978) using the updated thermodynamic dataset of Robie & Hemingway (1995) for the P–T condi-tions of the studied rocks (6.5 kbar and 730 �C).Although high precision thermodynamic data are alsoavailable for titanite in the literature (cf. Xirouchakiset al., 2001a,b), lack of data for fluorite in these studiesis the greatest hindrance for using this data base.Incidentally the P-T data of the studied rocks are verysimilar to the conditions constrained for the Wanakenaassemblage (6–7 kbar and c.730 �C; Bohlen & Essene,1978). The calculated diagram is presented in Fig. 5.The slopes of the univariant reactions were computedfrom the relation,
log fi= log fj $ �mj=mi ð10Þwhere, vi is the coefficient of the species �i� in thereaction, etc.
It is evident from Fig. 5 that the reaction grid basedon the updated thermodynamic data set of Robie& Hemingway (1995) explains the occurrence ofthe assemblage titanite + fayalite + hedenbergite +fluorite + quartz in the Wanakena rock even for alimited Al content of titanite (as predicted by Troitzschet al., 2002). The shift of the univariant reactions in theFe-bearing system for the phase compositions of thestudied rocks has been computed using the relation,
diagram in parts ofthe system Ca-Fe-Si-O-F (CFSOF) calcula-ted using the thermodynamic database ofRobie & Hemingway (1995). The dashedlines indicate the shift of the CFSOF reac-tions due to compositional effects in thesymplectic clinopyroxene. The stability fieldsof the assemblages with hedenbergite+fluorite have been stippled. Also included inthis diagram are some of the relevant reac-tions limiting the stability of the phasestitanite, plagioclase and calcite.The shift of the reaction Ttn fiQtz + Rt + Flt for the maximum Al-contentobserved in this study as well as for maxi-mum Al-saturation has been computed fromthe experimental data of Troitzsch & Ellis(2002). The activities of the end memberphases have been computed using the a–Xprogram of Holland & Powell (2000)(see text for details).
F L U O R I TE A N D T I T A N I T E S T A B I L I T Y 35 3
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A log fO2=B log fF2
¼ A log f oO2=B log f o
F2
þ logðai:aj::::=ax:ay::::Þwhere f o
O2, f o
F2stand for oxygen and fluorine fugacities
for end member phases at the P, T of interest. A and Bare the stoichiometric coefficients of the two species inthe balanced reaction, respectively. ai, aj are theactivities of the solid product phases whereas ax, ay arethe activities of the solid reactant phases. Activities ofthe solid solution phases are calculated using the a–Xrelations adopted by Holland (2000). Since garnet andolivine were absent in the studied rocks, their compo-sitions are considered to be unity. The composition ofthe symplectic clinopyroxene was used in view of itsstability with fluorite. Several interesting points emergefrom the logfF2
–logfO2diagram (Fig. 5). These are:
(a) Titanite cannot be stable with sillimanite +fluorite + quartz unless the rock is Na-free and thetitanite has the highest Al content (corresponding to75 mol% CaAlFSiO4) as suggested by the recentexperimental data of Troitzsch et al. (2002). Thisexplains the absence of sillimanite and rutile in thetitanite-bearing plagioclase + fluorite assemblage inthe studied rock (and in many granitic rocks) and theabsence of titanite in the sillimanite + fluorite-bearingassemblage in the Adirondacks (Bohlen & Essene,1978).
(b) Although the interpretation of Bohlen & Essene(1978) remains valid even if the updated thermo-dynamic database of Robie & Hemingway (1995) isused, some difference is noteworthy. These workerscomputed a shift of the reaction hedenbergite «fayalite + fluorite + quartz for the phase composi-tions of several Adirondacks rocks and argued that thestability field of the product assemblage should overlapwith the paragenesis sillimanite + fluorite + quartzfor most of the plagioclase compositions (Fig. 3 ofBohlen & Essene, 1978). However, the revised reactiongrid and the a–X relations of the phases used in thiswork do not predict an overlapping stability fieldfor any plagioclase composition (Fig. 5). This is inperfect agreement with the absence of sillimanite inthe Wanakena rocks. The sillimanite + fluorite +quartz-bearing assemblage of the Adirondacks rockstherefore must have been stabilised at a higher fF2
compared to the fayalite + hedenbergite granites ofthe Wanakena area (Fig. 5). Our analysis also dem-onstrates that clinopyroxene can only coexist with sil-limanite + fluorite + quartz if the mineral is highlymagnesian and the plagioclase is calcic. It follows thatrocks of basic compositions are more suitable thangranitic rocks for the stabilisation of the assemblageclinopyroxene + plagioclase + sillimanite + fluorite +quartz provided that fO2
is low and fF2high. In many
clinopyroxene-bearing basic rocks, fibrous sillimanite(fibrolite) is found to develop after the aluminousphases during late fluid ingress (Vernon, 1978). This iscommonly explained by a �base-leaching� process in the
presence of a halogen (F, Cl)-bearing fluid (cf. Kerrick,1991). Careful search in these sites could identify thesillimanite + fluorite association in the fibrolite mat.
(c) The logfF2–logfO2
diagram shows that thefluorite-bearing mineral assemblages in �skarns� andother wollastonite ⁄ andradite-bearing calc-silicaterocks buffers the fF2
at distinctly lower values com-pared to the other carbonate-poor rocks with fayaliteand ⁄ or clinopyroxene. This explains the restriction ofthe fluorite + quartz-symplectite to the margins of thewollastonite veins whereas the scapolite + plagio-clase + clinopyroxene host rock only a few mm awayfrom this vein is completely devoid of fluorite. Thetopological constraints of the logfO2
–logfF2diagram
also suggest that fluorite should be a common phase inmetamorphosed calc-silicate rocks. However, this isnot confirmed for the andradite + wollastonite-bear-ing enclosing calc-silicate rocks and can be explainedby the higher fO2
values in these rocks maintained bythe low fCO2
of the pore fluids (Sengupta et al., 1997;Sengupta & Raith, 2002).
(d) The mineral assemblages and phase composi-tional attributes of the studied rocks can now beinterpreted in the reaction grid to obtain the fO2
–fF2
values at the ambient P–T conditions. The a-X relationof Holland (2000) gives similar values of aHd for theclinopyroxene in the studied rocks (aHd c. 0.63 for thesymplectic clinopyroxene) and the compositions pre-sented by Bohlen & Essene (1978) (aHed c. 0.62–0.67).The shift of the invariant points and the related uni-variant reactions for the two areas therefore is similar(dashed lines in Fig. 5). Assuming the fluid to be pureCO2, our phase diagram predicts logfO2
c. )15.8 andlogfF2
c. )30.6 for the assemblage wollastonite +calcite + quartz + fluorite at the estimated P–T con-ditions. A reduced fCO2
value in the fluid will yield evenlower values of log fF2
. In comparison, the Adirondacksassemblages were stabilised at higher logfF2
c. )29.5 to)30.2 at the calculated logfO2
c. )16.4 ± 0.2. The cal-culated logfF2
values in the Adirondacks rocks almostmatch the values quoted by Bohlen & Essene (1978)(logfF2
)28.9 ± 0.3). Our analysis also corroborates thecontention of these authors that fF2
in the naturalassemblage is very low even in the presence of fluorite.
Influence of CO2 on the stability of fluoriteand the system CISOFV
The geometrical relations in the isothermal-isobariclogfF2
–logfO2diagram developed by Bohlen & Essene
(1978) provide a valuable insight into the control of fO2
on the stability of the fluorite-bearing assemblageswhere the solid phases buffer the ambient oxidationstate.
In the studied rocks and also in many fluorite-bearing regionally metamorphosed rocks (Markl &Piazolo, 1998), skarn deposits (Leonard & Budding-ton, 1964; Burt, 1972; Enami et al., 1993) and one
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association of the Adirondacks (Bohlen & Essene,1978) calcite is a common constituent of the fluorite-bearing assemblages. This necessitates consideration ofthe mineral reactions involving calcite and evaluationof the control of fCO2
on the stability of the fluorite-bearing assemblage. To do this, we have added twomore components viz. CO2 and H2O to the CISOFsystem and have considered the phases andradite,calcite, wollastonite, hedenbergite, quartz, fluorite,fayalite, magnetite and vapour. Following the proce-dure of Bohlen & Essene (1978) the geometrical rela-tions have been developed in the CISOFV systemtreating the effects of �extra components� such as Mg,Al, etc. subsequently. At constant P–T, the topologyof the seven component system with c + 2 phases iscomplex (involves 36 nondegenerate invariant points)and there can be several alternative arrangements(Hensen, 1987). However, in consonance with thenatural fluorite-bearing assemblages, we have consid-ered the geometrical arrangements around the invari-ant points [Mag,Fa], [Wo,Fa] and [Wo,Adr]. Theinvariant point [Wo,Adr] has the phases fayalite andquartz. Hence, this invariant point will be metastableat high pressure (P ‡ 7 kbar). The phase rule indicatesthat along each of the nondegenerate univariant reac-tions, only one of the parameters fCO2
, fO2and fF2
canbe independently varied. We have chosen the planeslogfCO2
–logfO2and logfF2
–logfO2to show the interrela-
tionship among the fugacities of the volatile speciesin the fluorite-bearing rocks of different geologicalenvironments (Fig. 6).
The fCO2, fO2
and fF2values at the isothermal-isobaric
(6.5 kbar & 730 �C) invariant points [Mag,Fa],[Wo,Fa] and [Wo,Adr] in the system CISOFV werecalculated using the thermodynamic data of Robie &Hemingway (1995). Based on these values and thecalculated slopes of the univariant reactions aroundthe invariant points, the topological relations in theplanes logfO2
–logfCO2and logfO2
–logfF2have been
developed following Schreinemaker’s principles. In thepresence of a CO2–H2O vapour phase, the CO2-absentreactions have no significance and are metastable.These reactions become relevant only when the rockis vapour phase-absent and ⁄ or conditions Pfluid <<Plith prevail such as inferred for the Adirondacksoccurrences. However, these reactions are also shownas they can be used as good reference lines to under-stand the displacements of the invariant points due toeffects of �extra components�.
Effect of pressure and temperature
The effects of pressure and temperature on the iso-thermal–isobaric topologies are shown in Fig. 7. Here,the invariant points [Wo,Adr], [Wo,Fa] and [Mag,Fa]and some of the related univariant reactions are cal-culated for two sets of P–T conditions suitable forthe reported fluorite-bearing rocks in regionallymetamorphosed rocks (6.5 kbar, 730 �C) and skarn
deposits (4 kbar, 530 �C). Since the effect of pressureon the topology in the log–log diagram is insignificant,a change in positions of the invariant points can beattributed solely to changing temperature. Figure 7reveals some influence of T (and P) on the stability ofthe assemblage clinopyroxene + calcite + fluorite ±wollastonite. For example, a decrease in the inputtemperature causes a drastic change in the positions ofthe bounding invariant points but the stability fieldof the assemblage hedenbergite + fluorite + calcitedecreases by only a small magnitude. Furthermore, therocks will reach fluorite saturation at lower fF2
valuesat lower temperature and vice versa. This explainsthe preponderance of andradite + fluorite-bearingassemblages in low aCO2
skarns. The assemblagehedenbergite + fluorite + calcite can only formunder these conditions when the effects of temperatureand pressure are compensated by Mg and Mn in thebulk rock. Another important feature of Fig. 7 is thatat lower temperature and pressure, the stability field oftitanite is enlarged compared to those of the faya-lite + fluorite-bearing assemblages and hence rutileshould not be common in skarns, a feature also sup-ported by the natural assemblages (Burt, 1972; Enamiet al., 1993 and the references cited therein).
Effects of non-CISOFV components
The geometric relations together with the thermo-dynamic constraints allow the evaluation of the influ-ence of non-CISOFCV components on the stability ofthe fluorite-bearing assemblages. In natural rocks,these components include Mg, Mn and Al which arepreferentially partitioned into the phases clinopyrox-ene, olivine and garnet. Incorporation of these �extra�components will therefore enlarge the stability fields ofclinopyroxene- and garnet-bearing assemblages, andthe invariant points would move along the (Hd) and(Adr) reactions (shown by arrow heads in Figs 5 & 6).The net displacement of the invariant points willdepend on the extent of the reduced activities of garnetand clinopyroxene, respectively.
Application of the grid to natural occurrencesand the stability of the assemblage clinopyroxene +calcite + fluorite
The geometrical relations in the logfCO2–logfO2
andlogfF2
–logfO2planes indicate that the stability field
of the assemblage hedenbergite + calcite + fluorite(Fig. 6), at lower fCO2
will be replaced by andradite-bearing assemblages whereas at higher fCO2
heden-bergite will be unstable. This typical assemblagetherefore will not develop in rocks fluxed by fluidshaving either CO2 or H2O-rich compositions. Thestability field of this assemblage shrinks towards higherfO2
, finally terminating at the [Wo,Fa] invariant point(Fig. 6). Hence, this assemblage is expected to developover a large range of aCO2
in rocks buffered at low fO2.
F L U O R I TE A N D T I T A N I T E S T A B I L I T Y 35 5
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This is corroborated by the occurrence of this assem-blage in the studied rock, the Adirondacks calc-sili-cates (Bohlen & Essene, 1978) and East Antarctica(sample 651 of Markl & Piazolo, 1998). Very low fO2
atmoderate to high fCO2
, on the other hand, will desta-bilise calcite and graphite + fluorite would form
instead (Fig. 6). The presence of Mg will enlarge thestability field of the three phase assemblage whereas Alhas the opposite effect (Fig. 6).
The topological constraints of the reaction gridsalso indicate that the assemblage andradite-richgarnet + clinopyroxene + fluorite + calcite has an
730 ˚C, 6.5 kbar
Adr Mt Qtz
Hd
Adr Qtz
Hd Wo
Mt Qtz
Fa CO2
C + O2
Wo
Adr
HdFl Cal
Adr Qtz
Mt
Qtz
FlC
al
Adr
Mt
Qtz
FlCal
TR
tC
alQ
t z
[Mt,Fa]
[Adr,Fa]
SKARNSKARN
Hd + Fl + Cal + Qtz
Hd + Fl + Cal + QtzAdr + Hd + Fl + Cal
Wo
Cal
Qt z
Hd Fl Cal
Adr
b
FaQ
tzFl
Cal
Hd
Mt
HdFl
Cal
aHd= 0.63
aHd= 0.50
Hd
PCO2=Pload
aHd= 1
ADIRONDACKSADIRONDACKS
EGBEGB
[Wo,Fa]
730 ˚C, 6.5 kbar
Adr Mt Qtz
Hd
Adr Qtz
Hd Wo
Mt Qtz
Fa CO2
C + O2
Wo
Adr
HdFl Cal
Adr Qtz
Mt
Qtz
FlC
al
Adr
Mt
Qtz
FlCal
Ttn
Rt
Cal
Qt z
[Mt,Fa]
[Adr,Fa]
SKARNSKARN
Hd + Fl + Cal + Qtz
Hd + Fl + Cal + QtzAdr + Hd + Fl + Cal
Wo
Cal
Qt z
Hd Fl Cal
Adr
b
FaQ
tzFl
Cal
Hd
Mt
HdFl
Cal
aHd= 0.63
aHd= 0.50
Hd
PCO2=Pload
aHd= 1
ADIRONDACKSADIRONDACKS
EGBEGB
[Wo,Fa]
-12
-13
-14
-15
-16
-17
3 4 5
logf
O2
logfCO2
Fig. 6. Geometrical relations in the isother-mal-isobaric sections for parts of the systemCa-Fe-Si-C-O-F-H. (a) logfF2
–logfO2plane;
(b) logfO2–logfCO2
plane. The fat dashed linesindicate the shifts of the univariant reactionsand the invariant points for the actualcomposition of the symplectic clinopyroxenein the studied rock. The approximate fluidcompositions of the studied assemblages arestippled. The arrowheads indicate the direc-tion of displacement of the invariant pointsdue to reduced aHd. The vertical line atlogfCO2
c. 4.6 is the maximum carbondioxide fugacity at the estimated P–Tconditions. Beyond this limit (dashed field)Pfluid exceeds Pload.
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even more restricted stability limit. It is therefore evi-dent from this analysis that assemblages with heden-bergite + fluorite + calcite ± andradite can tightlyconstrain the ambient fluid compositions, particularlywhen the fO2
values are independently known. Thecalcite-free assemblage garnet + fluorite + clino-pyroxene with wollastonite or quartz, on the otherhand, is stabilised in the presence of a low-CO2 (orwater-rich) fluid and hence the andradite + fluorite-bearing assemblages are expected to occur in skarndeposits (Leonard & Buddington, 1964; Burt, 1972).This possibly explains the rarity of fluorite-bearingassemblages in skarns or in other calc-silicate rocksmetamorphosed in the presence of an aqueous fluid. Itis therefore evident that high-grade rocks with lowoxidation state equilibrated at fluid-deficient condi-tions can develop fluorite-bearing assemblages such asthose reported from the Adirondacks (Bohlen &Essene, 1978; Valley et al., 1990). The topologicalrelations as presented in Fig. 6 indicate that low fO2
Fe-rich skarns should be suitable for andra-dite + fluorite-bearing assemblages whereas magne-sian bulk compositions would develop the assemblageclinopyroxene + fluorite + calcite.
Bohlen & Essene (1978) showed that logfO2of the
Adirondacks rocks varied within a narrow range of)16.8 to )17. The calculated shift of the CISFOVequilibria using the Lake Pleasant ferrosalite com-position together with the independent fO2
valuesconstrain the logfCO2
below 3.8 for the magnetite-freecarbonate assemblage (Fig. 6). This corresponds toPCO2
< 2000 bar (Holland & Powell, 1998). Bohlen &Essene (1978) calculated the fH2O of these samplesbased on independent equilibria involving biotite andshowed that the fH2O
values were lower than 1000 bar(corresponding to a PH2O
< 1500 bar, Holland &
Powell, 1998). These values therefore support thevapour-deficient regime of metamorphism in the Adir-ondacks (Lamb & Valley, 1984; Valley et al., 1990).
To determine the stability limits of titanite- andrutile-bearing carbonate rocks at P–T conditions sim-ilar to the studied rocks, the reaction Ttn « Rt +Cal + Qtz has also been thermodynamically calibra-ted using the data of Robie & Hemingway (1995)(Fig. 6). The shift of this reaction due to incorporationof Al has been calibrated using the activity model ofTroitzsch et al. (2002). It is evident from Fig. 6(b)that titanite with only moderate Al content(CaAlFSiO4 < 30 mol%) will be the stable Ti-phase ina variety of calc-silicate rocks and skarns over a largerange of aHed and aCO2
. This is nicely corroborated bythe ubiquitous presence of this phase in the fluorite-bearing rocks of the Adirondacks (Bohlen & Essene,1978), East Antarctica (Markl & Piazolo, 1999) and inthe study area. Our study therefore corroborates thecontention of Troitzsch et al. (2002) that rutile +fluorite + clinopyroxene-bearing assemblages wouldbe expected in high-pressure magnesian rocks equili-brated at high PCO2
.
The source and composition of the metamorphic fluidof the studied rock
The geometrical relations developed in Fig. 6 allowcalculation of the fCO2
of the fluid that was in equi-librium with the calcite + clinopyroxene + fluorite-bearing assemblages in the studied rock. To determinethe most favourable stability conditions of thisassemblage, the CISOFV reactions are shifted forthe clinopyroxene compositions (Fig. 6). Coexistenceof the assemblage wollastonite + calcite + quartzconstrains the logfCO2
of the fluid at c. 4.1 which, at the
530 ˚C, 4 kbar
fF2
fCO2
[Adr,Wo]
[Wo,Fa][Mt,Fa]
730 ˚C, 6.5 kbar
fCO2
fF2
Hd + Fl + Cal
-26
-24
-22
-20
-18
-16
-14
-12
-45 -43 -41 -39 -37 -35 -33 -31 -29 -27 -25
5 64321
logfF2
logfCO2
logf
O2
[Wo,Fa]
[Mt,Fa]
530 ˚C, 4 kbar
fF2
fCO2
[Adr,Wo]
[Wo,Fa][Mt,Fa
730 ˚C, 6.5 kbar
fCO2
fF2
Hd + Fl + Cal
F
[Wo,Fa]
[Mt,Fa]
Fig. 7. Combined logfO2vs. logfCO2
,logfF2
diagram showing the effects ofpressure and temperature on the stabilityof the assemblage clinopyroxene +fluorite + calcite. At the given P–T condi-tions, this assemblage is not stable beyondthe indicated logfCO2
limits (light pattern)where Pfluid exceeds Pload.
F L U O R I TE A N D T I T A N I T E S T A B I L I T Y 35 7
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estimated P–T conditions (6.5 kbar & 730 �C), givesaCO2
c. 0.2 (after Holland & Powell, 1998). The sig-nificant progress of the reaction (1) leading to thestabilization of abundant meionitic scapolite afterplagioclase in the studied sample, however, indicatesinfiltration of a carbonic fluid at high temperature(> 850 �C, after Holland & Powell, 1998). Similarfeatures have been documented from the adjoininglithologies (Datta et al., 2001). In view of this, the lowaCO2
can be explained by dissociation of the CO2-richpore fluid and simultaneous formation of graphite atthe ambient low fO2 conditions during cooling of thecomplex (Frost & Chacko, 1989). This is consistentwith the observed intimate association of biotite andgraphite. It is also possible that some fluid also per-meated into the studied rock during the cooling of thecomplex. Nevertheless, fF2
of the pore fluid was lowenough at the ambient fO2
to stabilise calcite instead offluorite + graphite (Fig. 5). This explains the absenceof graphite in the fluorite-rich domains.
Textural relations in the studied rock clearly dem-onstrate that calcite and fluorite were formed con-temporaneously thus suggesting a common source forthese two volatile species. It has been stated earlier thatthe fluorite and graphite-bearing calc-silicate rocksare always present close to the syenitic veins. Themineralogy of the syenite (clinopyroxene + perthite ±plagioclase + quartz + titanite ± ilmenite, with littleprimary amphibole and biotite) is consistent with lowPH2O
during the crystallization of the felsic melt (Frost& Bucher, 1993; Markl & Piazolo, 1998). The extensivedevelopment of the carbonate minerals (calcite andscapolite) in the adjoining calc-silicate rocks togetherwith the presence of abundant graphite with similard13C (c. ) 6.67 per mil) in the two lithologies indicatethat the syenitic melt was the source of CO2 andfluorine. Restriction of graphite in the calc-silicate rockclose to the syenitic veins, however, indicates anextremely channelised fluid flow.
ACKNOWLEDGEMENTS
This study was carried out when P. Sengupta wasvisiting the Mineralogisch-Petrologisches Institut,University of Bonn on a fellowship by the Alexandervon Humboldt Stiftung. We thank N. Datta fordeveloping the geology of the area around Vizianaga-ram. Many stimulating discussions over the years withS. Dasgupta are gratefully acknowledged. Financialassistance of this study was provided by the Depart-ment of Science and Technology (India) through aresearch grant to P. Sengupta and by the DeutscheForschungsgemeinschaft (DFG) through a researchgrant to M. M. Raith. A. Datta acknowledges thefinancial help from the council of Scientific andIndustrial Research (India) and the German AcademicExchange Service (DAAD; through a fellowship).We gratefully acknowledge the incisive reviews byE. J. Essene and G. Markl that have led to improve-
ments in this paper. We also thank D. Robinson forthe competent editorial handling.
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Received 13 November 2003; revision accepted 5 February 2004.
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