Monticellite Marble at Cascade Mountain. Adirondack Mountains. New York

AmericanMineralogist, Volume 63, pages 991-999, 1978

Monticellitemarble at CascadeMountain. Adirondack Mountains. New York

Rosrnr J. Tnecyl Departmentof GeologicalSciences, Haruard Uniuersity Camb ridge, M assachuset ts 02 I 38

Howeno W. JerneeNo Pernn RosrNsoN Departmentof Geology,Uniuersity of Massachusetts A mherst,M assachuset ts 0I 003

Abstract

An Adirondackmarble contains the assemblagecalcite-diopside-forsterite-monticellite, with traceamounts of zincianspinel, titanian andradite, idocrase, and sphalerite. Microprobe analysesindicate that the monticelliteand forsterite both haveMg/(Mg+ Fe)greater than 0.9, and that monticelliteis more iron-richthan coexistingforsterite. Monticellite host grains containrare microscopic exsolution lamellae of forsterite,a featurenoted in only oneother terrestrialmonticellite occurrence. Diopside is stronglyzoned and containsup to 6 weight percentAlrOr and substantial ferric iron, indicating a fassaiticcharacter typical ofcalc-silicate clinopyroxenes. Distributionof Feand Mg amongthe major silicate minerals indicates that the monticellite- producingreaction must be an Fe-Mg continuousreaction combining the end-member reactlon, MgrSiOn* CaMgSirO.* 2 CaCO, : 3 CaMgSiO4+ 2CO2 with two Fe-Mg exchangeequilibria: 2 CaMgSiOn* Fe2SiO.: 2 CaFeSiO,* Mg,SiOn CaMgSirO.* CaFeSiO.: CaFeSirOu* CaMgSiO. Calculationssuggest that Fe doesnot lower the reactiontemperature significantly. The occurrenceof monticellite-freemarble in the sameoutcrops and variablemineral compositions in the calcite-diopside-forsterite-monticelliteassemblage implies that theserocks formed in an aCO, gradientwith aCO, probablydecreasing from centerto edgeof the marblelayer. P-T-XCO, relationsfor the iron-freereaction have been calculated, and suggestthat formation of monticellitein theabove reaction, under presumed conditions of Adirondackregional metamorphismof 8-10kbar and about700'-800oC, requires either great dilution of CO, by HzOin metamorphicfluid, or low aCO,under fluid-absent conditions. A possiblealternative explanationsuggests earlier contact metamorphismat low pressureslollowed by a later Grenville-agehigh-pressure regional metamorphismin which the pre-existinghigh-grade mineralassemblages were not substantiallyaffected.

Introduction documentedoccurrence in metamorphicrocks in the Marbleon CascadeMountain in the Adirondacks easternUnited Statesof monticellite2.which is usu- nearKeene, New York, containsthe assemblagecal- ally restrictedto contactmetamorphic environments cite-diopside-forsterite-monticellite.This is the only 'zChoquette(1960, Table 2) listsmonticellite in themctdes of two quartz-bearingmarbles of the CockeysvilleFormation in Mary- I Presentaddress: Department of Geologyand Geophysics,Yale land.G W. Fisher(personal communication , 1974) has examined University,New Haven,Connecticut 06520. thesethin sectionsand believesthe mineralis misidentified.

0003-004x/78/09I 0-099l$02.00 99I 992 TRACY ET AL.: MONTICELLITE MARBLE as at Skye (Tilley, l95l), Crestmore (Burnham, lessin thin section.It occursin roundedisolated 1959),and West Texas (Joesten,1976). Monticellite, grainsup to 2 or 3 mm. In handspecimen, forsterite Ca(Mg,Fe)SiOn,is an olivine which typically forms in is colorlessto pale greenwhere fresh, and dark gray marbles only in the highest range of metamorphic where partially serpentinized.Monticellite is pale temperatures,above about 900'C at low pressures, brown. Both are colorlessin thin sectionand havea and in rare alkalic and ultramaficrocks and carbona- similar size and isolatedoccurrence as diopside. tites (Bowen, 1922; Eckermann, 1948;Sahama and Thereare rare smallgrains of pale greenspinel and Hytdnen, 1957). The occurrence of monticellite at sphalerite.Both idocraseand andraditic garnet occur CascadeMountain is even more striking in that the as sporadicthin rims on diopsideor monticellite mineral might have been produced during Pre- grainsagainst calcite. Diopside contains inclusions of cambrian regional metamorphismof the Adirondack spinel,and monticellitecontains inclusions of both massif. A further interesting feature of this occur- diopsideand spinel.Forsterite appears to be freeof renceis the presenceof forsteriteexsolution lamellae inclusions. in monticellite hosts.Though monticellite and forste- All phasesin threeselected samples were analyzed rite (MgrSiOa) are known to have a limited mutual by electronmicroprobe. Analyses were made on the miscibility (Warner and Luth, 1973),exsolution fea- MAC microprobeat M.I.T., with operatingcondi- tures have been previously reported for only one tionsof 15kV acceleratingpotential and samplecur- terrestrial occurrence of this mineral pair, in calc- rent of about .025microamps. Data werecorrected silicate blocks included in norite of the Bushveld usingthe methodof Benceand Albee (1968) and the Complex (Willemse and Bensch,1964). correctionfactors of Albeeand Ray (1970). Chemical The monticellite-bearingmarble crops out on the datafor monticellite,forsterite, and diopsideare pre- north flank of CascadeMountain in the Mt. Marcy sentedin Table l. Thesephases and othersare dis- quadrangle, which is being remapped by Jaffe (Jaffe cussedbelow. and Jaffe, in preparation; Jaffe et al., 1977). It is Forsteriteand monticellite involved in an isoclinal recumbentfold with adjacent layers of syenitic gneiss and bordering anorthositic Magnesianolivine rangesin compositionfrom gneiss.It is not clear whether or not the marble was Fo89to Fo94.The mostiron-rich olivine was found intruded by the adjacentgneisses, but it appearsthat in the monticellite-freesample, Ca-22F. There is also all these rock units have been folded and metamor- a significantamount of iron in themonticellite (Table phosed in the Precambrian Grenville (1100 m.y.) 1) andthe ratio Fel(Fe+Mg) is higherin monticellite event that affected the Adirondacks and adjacent than in coexistingforsterite. Terrestrial occurrences rocks to the north and west. Some details of the of monticellite+ forsteriteare very rare-less than a geology of the outcrop area of the marble may be dozenhave been found in the literature,of whichthe found in Baillieul (1976). majorityare igneous.No dataexist on compositions In this paper we describesome of the mineral as- of thesepairs. However, the Fe-Mg fractionationin semblagesand texturesin samplesof the marble, and the CascadeMountain occurrenceis similarto that presentmicroprobe analysesof the silicateand oxide found for coexistingfayalitic olivine and magnesian minerals. The petrology of the monticellite-produc- kirschsteinitein Angra dos Reismeteorite (Prinz et ing reaction is discussed,and the implications it has al.,1977)and in the Sharpschondrite (Dodd, l97l). for the composition and behavior of the metamor- Thus, the preferentialpartitioning of iron into the phic fluid are considered.Finally, we note the con- high-calciumphase is the reverseof fractionation straints that this and other Adirondack rocks place found in most pairs of coexistinglow- and high- on pressure,temperature, and activity of fluid species calciumpyroxenes. during metamorphism. One explanation for this monticellite-forsterite Fe-Mg fractionationmay be found in the distribu- Petrography and mineral chemistry tion of cationsbetween M(l) and M(2) sitesin the Mineral assemblagesidentified in different speci- two olivines.Since monticellite M(2) is essentially mens of the marble include: calcite-forsterite-diop- filled by Ca, all Fe and Mg must be in M(l). On the side-magnetite + spinel; calcite-forsterite-mon- otherhand, in forsteritethere is abundantMg in both ticellite-diopside-spinel-andradite; and calcite- M(l) and M(2). Even if forsteriteand monticellite forsterite-m onticellite-diopside-spinel-idocrase. containedthe samenumber of atomsof Fe per for- Diopside is pale greenin hand specimenand color- mula unit, the FelMB ratio of monticellitewould be TRACY ET AL,,' MONTICELLITE MARBLE 993 higher. But there also appearsto be an absolutepref- larger and more distorted than those in forsterite. erenceof Fe for monticellite, since it contains more While M(2) in monticellite is much larger and more Fe atoms per 4 oxygenformula (Table l). Brown and distorted than M(2) in forsterite, monticellite M( I ) is Prewitt (1973) and Smyth (1975) argue that Fe-Mg larger but only marginally more distorted Ihan M(l) ordering in olivinesis due to the greaterdistortion of in forsterite.Whether this relative distortion of M(l) theM(l) site(Robinson et al., l97l) which causesFe sites is enough to be effective is arguable, but the to prefer M(l). By extension of this argument, Fe increasedsize of monticellite M(l) alone might be might be preferentially partitioned into monticellite enough to causeFe to prefer it. M(l), becausethe monticellite octahedral sites are An important feature of the monticellite is the presenceof rare microscopic exsolution lamellae of forsterite (Fig. l), which are up to about l0 microns Table l. Microprobe analysesof minerals thick and are apparently oriented along the (010)

ea-22t Ca-22 CL-3a planesof the monticellitehost. The forsteritecompo- Fo Fo Mo Fo Mo sition of these lamellae has been established by sio2 40.79 42.t| 37.93 42.42 36.83 electron probe analysis of several larger lamellae, TiO2 0.00 0.00 0.00 0.00 0, 0r which have the approximate composition orzo3 0.00 0.00 0.00 0.00 0,04 Cao.orMgr.rr Feo.orM no.orSir.ooOo. This is the secondre- Cr 203 0.00 0.00 0.00 0.00 0.06 FeO 5. 09 2.85 3.34 3.49 4.47 ported occurrenceof exsolution in terrestrialolivine MnO 0.58 0.05 0.19 0. 1r 0. 50 pairs. Willemse and Bensch(1964) report exsolution M90 52.36 34.2) ZZ.OZ 54.43 22.76 lamellae, presumably of forsterite, in monticellite 15 34.99 CaO o.20 0.16 3s.00 0. grains in calc-silicatexenoliths in the BushveldCom- TotaI qo I ? 99.42 99.08 100.50 plex. Dodd (197l) suggeststhat very small inclusions Cations per 4 oxygens of ferroan monticellite in olivine hosts in the Sharps Si .999 1.005 r. 011 r.003 .985 (Section are recrystallizedexsolution AI .000 .000 .000 .000 .000 chondrite 853) Ti ,000 .000 ,000 .000 .001 lamellae.Though Prinz et al. (1977) do not stateit, a Cr .000 .000 .000 .000 .00I Fe . r04 .057 .o74 .069 .100 similar interpretation is possible for the magnesian Mn .0r4 .001 .004 .002 .011 Mg 1,876 r.929 nqq r. 918 ,908 kirschsteiniteinclusions in olivine from Angra dos 1.0 2 .994 2 Reis. There is no evidencein the Adirondack marble of any monticellite exsolution from forsterite,nor is Fe . o52 ,03s ,099 Fe+M9 any reported by Willemse and Bensch.This discrep- ancy in host and lamella phase may be due to the Diopsides differencein Fe/Mg ratio betweenterrestrial and me- rrm core teoritic specimens.ln other words, the shapeof the sio2 50.'12 s2.08 50.73 51.18 49.50 51.19 solvus between forsterite-fayalite and monticellite- "lrO2 0.24 0.I2 0.71 0.61 0.58 0,23 oI2O 4.07 2.44 4.68 4.).2 )-41 J,ro kirschsteiniteolivines may change significantlywith ' ct203 0.03 0.08 0.07 0.07 0.06 0.00 changing FelMg ratio. FeO 2.A7 2.rB 2.20 1.99 2.61 2.L2 MnO 0.02 0.00 0.05 0.08 o.I7 0.05 Mso 16.05 15.98 15, 57 15. 34 15.95 16,88 CaO 25.I2 25.50 25.03 25,62 25.46 26.43 Na2O 0,00 0.04 0.01 0.00 0.02 0.00 Total 99.r2 99.82 100.05 98.94 99.80 I00.07

Cations per 6 oxyqens

1.875 r.905 L,822 L .8'77 A1 .t25 .095

AI ,052 .027 .057 .065 .058 .014 Tr .006 .003 .019 .016 .015 .006 Cr .000 .oo2 .001 .002 -002 .000 !e .088 .056 .066 .060 .080 .065 Mn .000 .000 .00r .002 .00s .002 M9 .993 .988 r.020 r.or2 Na Fig. l. Photomicrograph of monticellite host (Sample CL-3a) with exsolution lamellae of forsterite probably parallel to (010) of the host. Lensth of scalebar is 100microns. TRACY ET AL.: MONTICELLITE MARBLE

A numberof investigatorshave studied the extent ceedingmore extensivelyalong cleavagetraces in the of crystallinesolution on the join CaMgSiOo- diopside.The garnet is typicallyhoney-yellow and MgrSiO.as a functionof Z and P. The most recent hasa veryhigh indexof refraction.Preliminary com- paper, by Warner and Luth (1973),contains a sum- positionsobtained by microprobeshow some varia- mary of the earlierwork. According to Warner and tion from grain to grain, but all are rather high in Luth, thereis very limitedmiscibility between mon- titanium(about I weightpercent). A typicalcompo- ticelliteand forsteritebelow about 1000'C.Limited sitionis 65 percentandradite and 35 percentgrossu- experimentalevidence on the effectof iron upon the lar with negligibleamount of othergarnet molecules. phase relationsin this systemsuggests that iron ldocrase,like garnet,occurs in rims adjacentto causesthe solvusto narrow,similar to its effectupon diopsidein the samplesexamined, though it does the miscibilitygap in Ca-Fe-Mg pyroxenes.The data occurin discretegrains at otherplaces in the marble of Bowenet al. (1933)indicate complete solid solu- (Baillieul,1976). A probeanalysis of idocrasefrom tion at subsolidusconditions down to 700'Cbetween specimenCL-3a yielded this composition,calculated FerSiOrand CaFeSiOn.Assuming that there is no on an anhydrous basis: (Car.rrMgr.12Fe35rFe3.+rr) significanteffect on the phaserelations caused by the (Als.roFe3.+eoTio.,oXSiE..?Alo.rr)Oru.The ferric iron was smallamounts of iron in the CascadeMountain for- estimatedassuming a stoichiometricformula of 25 steriteand monticellite,the dataof Warner and Luth cations and 36 oxygens.Probe analysisfor fluo- (1973,Fig. a) suggestthat theseminerals crystallized rine and chlorine in idocrasesuggests that OH/ and exsolvedbelow about800'C. (OH+F+Cl) rangesfrom 0.81 to 0.89 (Baillieul, 1976),assuming 4 volatile anions in the formula Diopside givenabove. Further, the birefringenceof idocraseis The diopsidesin all threeof the analyzedsamples very low and somegrains are nearlyisotropic, sug- arestrongly zoned (lower part of TableI ), both in Al gestinghigh OH content(Deer et al., 1962). contentand in the ratio FelMg. Both Al andFelMg decreaseregularly and markedlyfrom core to rim. Spinel There is similar zoning in Ti, while Ca showsno The pale greenspinel which occurssporadically particulartrend. This zoningis most likely due to is magnesium-and zinc-richand poor in iron. Spinel formationof diopsidein a continuous(multivariant) in sampleCl-3a has the composition(Mgo.r'Fe3.+ot reaction(see Thompson, 1976), in which reactants Mno.oorZno.ro)(Alt.*Fefllz)On,in which Fe3+ has been and productssystematically change composition as calculatedon the basisof a stoichiometricformula thereaction progresses. The zoningis apparentlynot of 3 cationsper 4 oxygens.Most of the spineloc- dueto the monticellite-formingreaction, because the curs as small (0.5 mm or less)isolated grains, al- monticellite-freesample (Ca-22F) shows the same thoughit hasbeen observed as inclusions in diopside zoningas monticellite-bearingsamples. and monticellite.The inclusionsdo not appearto be The diopsideanalyses in Table I showall iron as systematicallydifferent in compositionfrom the iso- Fe2+.From the stoichiometryof theseminerals, how- lated grains.The zinc contentof the spinelis not ever,as seenin the structuralformulas, it is apparent surprisingin viewof its coexistencewith sphalerite. that muchof the iron mustbe in the ferricstate. This meansthat theseclinopyroxenes are solid solutions Petrology betweennearly pure end-memberdiopside and cal- i it e-producing reaction cium Tschermak'scomponents. Of the l0-20 percent M ont cell of Tschermak'ssubstitution, about half is CaALSiOu The simplestway to view the chemographyof the and half is CaFes+AlSiOr.This suggestsrather oxi- major phasesin the marble is in the ternary system dizingconditions during crystallization. CaO-MgO-SiO, with excessCOz, shown in Figure 2a. The arrangementof the phasescalcite, diopside, Andradilic garnet and idocrase forsterite,and monticelliteon this diagramillustrates Both of these mineralsare restrictedto reaction that the monticellite-formingreaction, in the absence rim.sbetween diopside or monticelliteand calcite.In of any otherphases, must be: theyform rimsof uniformthickness par- mostcases, Di * Fo * 2Cc: 3Mo + zCOz (l) tially around diopsides,though severalexamples wereobserved of andraditicgarnet actually replacing However,the morecomplicated compositions of the part of a diopsidecrystal, with replacementpro- actual minerals(Table l) and their compositional TRACY ET AL.: MONTICELLITE MARBLE 995

variabilityfrom sampleto sampleindicate that this simplegraphical analysis is insufficient. The most importantcompositional variable, Fe,/ Mg ratio,may be illustratedin projection.Figure 2a may be expandedinto a tetrahedronby addingan FeO apex,and mineralcompositions may be pro- jected from calciteonto the SiOr-FeO-MgOface, becauseall assemblagescontain calcite which is es- sentiallypure CaCOr.Figure 2b showsthis projection with mineralcompositions illustrated schemat- ically.Relative positions are qualitatively correct but havebeen exaggerated to emphasizethat monticellite is more iron-rich than the diopsideand forsterite (*calcite) from whichit forms.With progressof the monticellite-producingreaction, the three-phasetri- anglein Figure2b mustmove from moreiron-rich to moremagnesian compositions. The reactionforming monticelliteis thereforean Fe-Mg continuousreaction in which Z1a,) 2"". Thus reaction(l) may be consideredas a combinationof theend-member reaction (l) andthe followingtwo Fe-Mg exchangeequi- Fig. 3. Compositions of analyzed diopside, forsterite, and libria: monticellite from Cascade Mountain shown on the calcite projection of Fig. 2b. Note that diopside is plotted as if all iron were Fe2+.which mav not be the case.

2 CaMgSiOn* FqSiOo: 2 CaFeSiOof MgrSiOn Qa) CaMgSizOe* CaFeSiOn: CaFeSizOu+ CaMgSiO^ (2b) Figure3 illustrateswithout exaggerationthe actual compositionsof the analyzedminerals on this calcite projection. The three analyzedsamples were all collectedvery close to one another, and it is thereforelikely that they were metamorphosedat the sametemperature and pressure.The discrepanciesin mineral assem- blageand composition (Fig. 3), which must be due to variationsin intensivevariables P, T, or fluid composition,are therefore probably due to localvariation in activity of CO, (aCOr). The effectof loweredaCO, on reaction(l), a simple decarbonationreaction, Fig. 2a. The ternary system CaO-MgO-SiO, with excessCOz. proceedat lower Compositions of the phasesdiopside (Di), forsterite (Fo), calcite would be to allow the reactionto (Cc), and monticellite (Mo) are shown to illustrate the temperature(see Kerrick, 1974).It is thus possible monticellite-forming reaction. that gradientsin aCOzcould producethe variety of .b. Projection from calcite onto the SiOr-FeO-MgO face in a compositionsand assemblagesobserved. CaO-MgO-FeO-SiO, tetrahedron with excess COr. Ranges of An Fe-Mg continuousreaction such as (2) may be olivine, monticellite, and clinopyroxene compositions are shown portrayed loop on an isobarictemper- along with schematic compositions of an equilibrium three-phase as a reaction assemblage(plus Cc) which is undergoing reaction (l) modified by ature-compositiondiagram such as that shown in (2a) and (2b). Figure4 (for background,see Thompson, 1976). Fig- TRACY ET AL.: MONTICELLITE MARBLE

this aCOz effect dependsupon the temperature difference betweenthe Fe and Mg end-membersof reaction (l) or, in other words, upon the steepnessof the Z-X loops. A small temperature difference, as sug- gested below, means that very small variations in (X66, decreose) aCO, could produce the observedeffects illustrated in Di+Fo+Cc Figure 4b.

Estimation of conditions of metamorphism The most effective method of estimating metamorphic conditions is to apply the results of relevant experimental work to the natural assemblages.Un- fortunately, very little experimental work has been done on monticellite-relatedreactions, and only that of Kushiro and Yoder (1964) at elevatedpressures. Di*Fo*Cc The only experimentson reaction (l) in the system CaO-MgO-SiOr-CO, are those of Walter (1963) at pressuresof lessthan one kbar. In addition, there is no experimental evidence bearing on the effect of Fe in lowering the temperatureof reaction (l). Fig. 4a. Temperature-composition diagram showing the However, the hypothetical equilibrium temper- schematic reaction loop for continuous reaction. Note that part of (l), this loop, especiallythe Fe-rich side, may be metastable relative to ature of the iron analogueof reaction i.e., other reactions. The arrows on the loop show the effect of raising Hedenbergite* Fayalite * 2 Calcite or lowering the concentration of COz in the fluid. FerSiOr CaCO' b. Similar T-X diagram, blown up to show the Mg-rich side, CaFeSizO, with hypothetical reaction loops for the different but presumably : 3 Kirschsteinite + 2CO2 (3) isothermal Cascade Mountain samples. Filled circles indicate CaFeSiOr compositions of Mo and Fo in sample CL-3a; triangles are for the same phases in Ca-22; the square is forsterite composition in Ca- can be calculated from known and estimated ther- 22F; diopside is ignored for simplification. mochemicaldata. The relation

Z"q,rr", = 411f"/A,Sf" (4) ure 4a illustrates the relative effect on this loop of can be used to estimatethe temperatureof reaction increase or decreasein aCO2.Figure 4b shows actual (3) at one bar. Entropiesfor fayalite,calcite, and COz compositionsof monticellite and forsterite on oppo- were taken from Robie and Waldbaum (1968),while site sidesof the loops (diopside is ignored for sim- those for hedenbergite and kirschsteinite had to be plification). On this plot it is assumedthat all samples calculated according to the method described by are isothermal and isobaric.The schematicpositions Cantor (1977);these values enabled ASf" to be calcu- of the reaction loops for each assemblage,functions lated. Ar?r9"was then estimated from the Fe-Mg frac- of aCOz,are shown in the figure. These relative posi- tionation of phasesin the CascadeMountain samples tions suggest Ihat aCOz is lowest for sample Ca-22 (data in Table I ). The temperature estimate for reac- and that the reaction has progressed farther in this tion (3) at one bar is 435"C, as compared to an sample than in CL-3a, which has a slightly higher estimated temperature for reaction (l) of 500'C at aCOz. For sample Ca-22F, the aCO, is high enough one bar (from data of Walter, 1963). Though this that the reaction loop has not yet been intersected temperatureestimate may have considerableuncer- and the stableassemblage remains Dif Fo*Cc. If the tainty, it is considered to provide a reasonableap- minerals in Ca-22F had been mote magnesian than proximation of the effectof iron on reaction(l). Note the other samples,then the absenceof monticellite that the presenceof substantialCaTs component in could have been ascribedto a simple bulk composi- the pyroxene (Table I ) will tend to displace the reac- - tion effect,that is, that the rock was too magnesianto tion to higher temperature, at least in part offsetting have encountered the reaction. But the data support the temperature effect of iron. Based upon the estima- the hypothesis that aCOz is the controlling physical tion that the Fe end-memberreaction is probably no variable. It must also be noted that the magnitude of more than 100"C below the Mg end-memberat one TRACY ET AL.: MONTICELLITE MARBLE 997 bar, the effectof the small amount of iron in the dack massifof about 700-800'Cand 8 + I kbar. CascadeMountain samples will be ignored,and lur- Phaseassemblages in orthoferrosilitemicroperthite ther estimatesof physicalconditions will be based gneissless than 5 kilometersfrom theCascade Moun- upon the simplifyingassumption that the systemis tain locality (Jaffeet al., 1978)suggest metamorphic iron-free. pressuresof at least9 kbar at 800'C or 7 kbar at It is clearfrom Walter's(1963) experimental deter- 600oC,but certainlynot more than 12 kbar, based minationof reaction( I ), fluid-saturatedwith XCO, upon olivine-orthopyroxeneFe-Mg fractionationat = 1.0in the fluid, that the reactioncannot occur at XFe>0,9 (Wood, 1975).Further, the coexistenceof geologicallyreasonable temperatures at pressures orthopyroxenewith K-feldsparand the absenceof aboveone kbar if Pfluid : Ptotaland the fluid is rich biotiteand of migmatiticsegregations in suchrocks in COr.This is why monticelliteis typicallyrestricted of graniticcomposition under these conditions im- to very shallowcontact-metamorphosed rocks, e.g. plies either a very low XHrO in a coexistingfluid Tilley(1951), Burnham (1959), Joesten (1976). There phaseor, more probably,a very low aHrO under are two ways to lower the reaction temperatureat fluid-absentconditions. If the monticelliteassem- elevatedpressures. The first is to assumethat fluid blagewas producedduring regional metamorphism pressureis lessthan solid pressure and that thefluid is underthese conditions, then the considerationsdis- pure or nearlypure CO2.Examination of Walter's cussedabove dictatetwo extremepossibilities: (l) (1963)data suggeststhat, for regionalmetamorphic that the marbleat CascadeMountain was suffused conditionsthought to haveprevailed in the Adiron- with a fluid phasevery rich in HrO as comparedto dack massif(i.e., 700-800oC and about 8 kbar, see that, if any, in the surroundingrocks; or (2) that no below),fluid pressurewould havehad to havebeen fluid phasewas present and that the monticelliteas- lessthan about 10 percentof total solidpressure in semblagewas controlledby a very low aCO, in a this case. diffusionalgradient. The diffusionof CO, through The secondway to lower the temperatureis to the Adirondackterrain has been proposed by Taylor assumethat fluid pressureequals solid pressure but (1968)to explainthe oxygenisotope composition of that CO, is diluted in the fluid by H,O or other Adirondackigneous and metasedimentaryrocks. species.A similarexplanation to explainwollastonite The difficultiesinherent in postulatinga regional in Adirondackmarbles was given by Valleyand Es- metamorphicorigin of monticellitemake it tempting sene(1977). The possibilitythat COz was signifi- to arguethat the marblewas contact metamorphosed cantly dilutedby F or Cl hasbeen discounted,be- at low pressureby syeniticor anorthositemagma causeof the apparenthigh OH/(CI * F) ratio in (Isachsen,1975) and laterremetamorphosed at high idocrase.F can be stronglyfractionated into solid pressure.Berg(1977, p. 400)cites Buddington (1968) phases,as shownby Munoz and Ludington(1974, and others in support of the suggestionthat the 1977),suggesting that the fluid may havebeen much anorthositesof easternNorth America were em- poorerin F than the idocrase. placedat relativelyshallow depths (as little as 12to The alternativethat fluid pressureequalled total l8 kilometers)into low-gradecountry rocks. Berg solidpressure but that CO, wasdiluted in thefluid by speculatesthat thosesouth of the Grenvillefront, HrO can be evaluatedquantitatively. P-T-XCOz re- suchas the Adirondackmassif, were later regionally lations for reaction(l) in the systemCaO-MgO- metamorphosedat significantlygreater depths, wip- SiOr-COr-HzOhave been calculated at pressuresup ing out muchof the evidenceof their earlierhistory. to l0 kbar and temperaturesto 1000'C,as described A major problemin applyingthis interpretationto in the Appendix.It is clearfrom thesecalculations theCascade Mountain occurrence is an apparentlack that,above 5 kbar,XCO, mustbe less than about 0. I of recrystallizationand retrogressionofthe supposed for the monticellite-formingreaction to occur at rea- original contact metamorphic assemblage,which sonabletemperatures. At the presumedconditions of might be expectedto occur during granulite facies Adirondackmetamorphism, XCO2 might havebeen regional metamorphism.Furthermore, anorthosite 0.01or less,and the fluid phasewould havebeen very intrusionat depthsas shallowas l2 to l8 kilometers nearlypure HrO. would still be at pressuresrather high for production Some independentevidence exists regarding the of monticellite,though it would slightlyease the temperatureand pressureof Adirondack meta- problemsof fluid behaviornoted above. morphism.Bohlen and Essene(1977a, b) havegiven The presentgeologic evidence, therefore, indicates estimatesof conditionsin the centerof the Adiron- that the CascadeMountain monticellite occurrence is 998 TRACY ET AL.: MONTICELLITE MARBLE

more likely to be of regionalthan contactmetamor- References phicorigin, even though regional metamorphic origin Albee, A. L. and L. Ray (1970)Correction factors for electron requiresunusual composition or behaviorof meta- probemicroanalysis of silicates,oxides, carbonates, phosphates morphicfluid. On the other hand,an origin in con- and sulfates.Anal. Chem.,42,1408-1414. tact metamorphismand survivalof the assemblageBaillieul, T. A. (1976) The CascadeSlide: A MineralogicalInuesti' through later regional metamorphismcannot be gation of a Calc-Silicate Body on CascadeMountain, Townof Keene,Essex County,New York. M.S. Thesis,University of ruled out. Furtherelucidation of the geologyof the Massachusetts,Amherst. areais clearlynecessary for firmer conclusionsto be Bence,A. E. and A. L. Albee(1968) Empirical correction factors drawn. for the electronmicroanalysis of silicatesand oxides.J. Geol., 76, 382403. Berg,J. H. (1977)Regional geobarometry in the contactaureoles Appendix of the anorthositicNain Complex,Labrador. J. Petrol.,18,399- PreliminaryP-T-XCO, relationshave been calcu- 430. E. J. Essene(1977a) Pyroxene thermometry in latedfor reaction( I in the system Bohlen,S. R. and ) CaO-MgO-SiOr- the Adirondackhighlands (abstr.). Am. Geophys.Union Trans., COr-HrO. The method of calculationinvolved the 58,517. "equilibrium constantapproach," as discussedby - 3nd - (1977b)Errors in applying opx-oliv-qtz ba- Kerrick and Slaughter(1976). Experimental data for rometry(abstr.). Am. Geophys.Union Trans., 58, 1242. reaction(l) (Walter,1963, Table l) for runsat 800'C Bowen, N. L. (1922)Genetic features of alnoitic rocks at Isle l-23. plotted Cadieux,Quebec. Am. J. Sci.,203, and abovewere on a lnK us.l/T diagramto -, J. F. Schairerand E. Posnjak(1933) The systemCaO- deriveAfl$ and A^S!.Because all the run data are at FeO-SiOz.Am. J. Sci.,226, 193-284. pressuresof 500bars or less,correction for non-ideal Brown, G. E. and C. T. Prewitt(1973) High temperaturecrystal mixingin fluid wasmade using the dataof Ryzhenko chemistryof hortonolite.Am. Mineral., 58, 577-587. and Malinin (1971).For extrapolationabove 2000 Buddington,A. F. (1968)Adirondack anorthositic series. In Y. W. Isachsen,Ed., Origin of Anorthositeand Related Rocks, p.215- bars(see below), ideal mixing was assumed and the 231.N.Y. StateMuseum and ScienceService Mem. 18. fugacity coefficientsof Burnham and Wall (unpub- Burnham,C. W. (1959)Contact metamorphism of magnesian lished)were used, as suggestedby Ohmotoand Ker- fimestonesat Crestmore,California. Geol. Soc. Am. Bull., 70, rick (1977,p. 1033)and by Slaughteret al. (1976). 879-920. Calculationswere done usingthe followingequilib- Cantor,S. (1977)Entropy estimates ofgarnets and other silicates. rium relation: Science.198. 206-207. Choquette,P. (1960)Petrology and structureof the Cockeysville 0: AG, : AII1 - fA,Sl + PAn-l RTnCOzln/CO, Formation(pre-Silurian) near Baltimore,Maryland. Geol. Soc. Am. Bull.. 71. 1027-1052. (Al) Deer, W. A., R. A. Howie and J. Zussman(1962) Rock-forming Minerals, Vol. I: Ortho and Ring Silicates.Wiley, New York. When rearranged,this becomes: Dodd,R. T. (1971)Calc-aluminous insets in olivineof theSharps : - chondrite.Mineral. Mag., 38,451-458. T (LH? + PL()/(LS9 RnCO"lnJCO2) (A2) Eckermann,H. von (1948)The alkalinedistrict of Alnd Island. 36. As noted above, AI19 and AS9 were derived from SuerigesGeol. Undersok.,Ser. Ca Isachsen,Y. W. (1975)Anorthosite contact relationshipsin the Walter's (1963) daIa. AA data were taken from Adirondacks and their implications for geological history Robieet al. (1967).Isobaric T-XCO' sectionsand P- (abstr.).Geol. Soc. Am. Abstructswith Programs,7,'18-79. Z locationsof reaction( I ) at variousXCO,( 1.0have Jaffe.H. W.. E. B. Jaffeand L. D. Ashwal(1977) Structural and beencalculated from equation(A2). petrologicrelations in the high peaksregion, northeastern Adi- We areaware of the considerableuncertainty in the rondacks(abstr.). Geol. Soc. Am. Abstractswith Programs,9, 279-280. above calculations,especially in the derivationof -, P. Robinsonand R. J. Tracy (1978)Orthoferrosilite and A.I/9 and AS9 from Walter's experimentsusing the other iron-rich pyroxenesin microperthitegneiss of the Mt. equilibrium constant method (see Kerrick and Marcy area,Adirondack Mountains. Am. Mineral.,in press. Slaughter,1976). To indicatethe magnitude of uncer- Joesten,R. (1976) High-temperaturecontact metamorphismof environment,Christmas tainty,maximum and minimum AHI calculatedfrom carbonaterocks in a shallowcrustal -57.4 -51.3 Mountains,Big BendRegion, Texas.,4 m. Mineral.,61,776-781. Walter'sdata were kcal and kcal, re- Kerrick, D. M. (1974) Review of metamorphicmixed volatile spectively;maximum and minimum AS9 were74.1 (HzO-CO,)equilibria. A m. M ineral.,59, 7 29-7 62. gibbs and 68.6gibbs. It was consideredimportant, - snd J. Slaughtet(1976) Comparison of methodsfor calcu- however,to calculateand extrapolatereaction (l) in lating and extrapolatingequilibria in P-T-XCO" space.Am. J. P-T-XCO2space in order to provideat leastqualita- Sci..276,883-916. Kushiro,L and H. S. Yoder (1964)Breakdown of monticelliteand tive estimatesof the effectsof variablefluid composi- akermaniteat high pressures.Carnegie Inst. Wash.Year Book, tion and pressure. 63. 8l-83. TRACY ET AL.: MONTICELLITE MARBLE 999

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