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The Effect of Reduced Activity of Anorthite on the Reaction Grossular

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The Effect of Reduced Activity of Anorthite on the Reaction Grossular AmericanMineralogist, Volume61, pages 889-896,1976 The effectof reducedactivity of anorthiteon thereaction grossular * quartz : anorthite* wollastonite:a modelfor plagioclasein the earth'slower crust anh upper mantle K. E. WrNnoMr AND A. L. BoerrcHrnl Department of Geosciences,The Pennsyluania State (lniuersity Uniuersity Park, Pennsyluania 16802 Abstract The reaction CasAlrSiaor, + SiO, : CaAlzSiros + 2 CaSiOg (erossular) (quartz) (anortbite) (wollastoDite) wasinvestigated at l10ooC,using intermediate-composition plagioclase to determinethe shift of the equilibriumpressure as a functionof compositionat pressureand temperaturecondi- tions representativeof the earth'slower crustand uppermantle. For compositionswithin the rangeAnro-,00, activity of the anorthitecomponent is equalto its molefraction. For composi- tions more albitic than about Anro,the activity of the anorthitecomponent equals its mole fractionmultiplied by a constantfactor of 1.2.This latter behavioris interpretedas a resultof anorthite-likedomains being in an energeticallyunfavorable environment for thesecomposi- tions. Applicationof the data obtainedfrom subsolidusexperiments to meltingbehavior indicates that plagioclaseliquids do not obey Raoult'sLaw, but havean activitycoefficient similar to crystallineplagioclase. This hypothesisis supportedby high-pressuremelting experiments for both anhydrousand hydroussystems. In parts of the lower crustcomposed of basicrock (e.g.gabbro), the calcicplagioclase will be consumedat relativelyshallow levels and over a narrow (2-3 km) depthinterval. On the other hand,plagioclase in regionsconsisting of intermediate(e.g. andesitic) compositions will persistto greaterdepths, and the reactionswill occurover relatively wide depth intervals. This contentionis supportedby the nonuniformity of someseismic discontinurties in the lower crust. Introduction are ever to fully understandthe nature of the lower Plagioclasecomprises the largestvolume of any crust and upper mantle,we must determinethe be- mineral group in the crust of the earth (Ronov and havior of plagioclaseat pressureand temperature Yaroshevsky, 1969)and occursas a primary phasein conditionsrepresentative of this region. a wide variety of igneousand metamorphicrocks. Numerousreactions involving the end-membersal- Among theseare the granulites,amphibolites, ande- bite (NaAlSi3Or) and anorthite (CaAlrSirO,) have sites,anorthosites, and gabbros,all of whichare pos- beeninvestigated (e.g., Newton, 1966;Kushiro and siblecandidates for the lowercrust. Howeier, plagio- Yoder, 1966;Newton and Smith,1967;Hays, 1967; claseis unstableat high pressures,reacting with other Hariya and Kennedy,1968; Boettcher and Wyllie, mineralsto form a higher-densityaluminous phasei 1968,Boettcher, 1970; Akella and Kennedy, l97l ). In (garnet,spinel, kyanite, etc.) plus other phasessuch addition, selectedintermediate compositions have as quartzand clinopyroxene(e.g., Kushiro and Yo- beenstudied (e.g., Green and Hibberson, 1970). Vari- der, 19661'Green and Hibberson, 1970;Akella and ous models for activity-compositionrelations of Kennedy,l97l; Boettcher,1970, l97l). Thus,if we plagioclaseat pressuresand temperaturescorre- spondingto the uppercrust have also been proposed I Presentaddress: Institute ofGeophysics and Planetary physics, (Orville,1972; Saxena and Ribbe,1972; Kerrick and University of California, Los Angeles. California 90024. Darken,1975). However, there exists no comprehen- 889 890 K. E. WINDOM AND A. L, BOETTCHER sivestudy for a wide rangeof plagioclasecomposi- : dP o, tions at pressuresand temperaturesrepresentative of dac" (u-u?),., + (effi)",, the lowercrust and uppermantle. The purpose of the presentstudy is to provide * r(H)" e). experimentaldata that will permit us to formulatea .,.*,...dxn model that can be used to predict the behavior of Since plagioclasein both syntheticand naturalassemblages at high pressuresand temperatures.To obtain this : av.i (+F)",,: -a.s'; information,the reaction eu?),,, ^'(%?) CaaAleSi3Or,+ SiO, G"E),r xi ,, *, .dXn (drossular) (quart,z) : +,?.91"?.t*o,.'",(t) then, "a*.1:',f,,'.?' dAG" : AV, dP - A.t" df was investigatedusing intermediate plagioclase com- + >RZ(q++l ax,' (3). \ dAi /P.r.xi.,. positions.This reactionwas chosen primarily because no solid-solutiotrsother than plagioclaseoccur with For conditionsof constant7 and unit activitiesof all theaddition of Na2Ocomponent, thus permitting the components(i.e., pure solid phases), isolationof plagioclasebehavior. dAG": AVdT (4). Therefore,assuming AZ, constantbetween F andP, Abbreviationsand svmbols used in text : - P: pressure (AG")p,ro,",o (AG,0)po,ro,"to * AV,(P F) (5). : I temperature(Kelvin unlessotherwise stated) For conditionsof constant7 andunit activitiesof all Z: volume componentsexcept i, S : entropy G : Gibbs' free energy dAG": AV,dP + ar(9Jvt) o"n (6) R : gasconstant \ uAi /P,T,Xi... a: activity Therefore, X : mole fraction subscriptr : quantity for a reaction (AG,) p.7o,*, : (AG,o) po,7".*,u : superscript0 standardstate conditions + AV,(P - P') + RTln(at/aio) (7) A : changein a parameter = : T : activity coefficient wherear activity of componenti at X1 znd sto : a : a phase activityof componenti at Xto l. i : componentin a phase The present study is designedto determinethe pressure by a changein Qz : quartz changein equilibrium caused Gr : grossular the plagioclasecomposition. Therefore, Wo: wollastonite (AG,).,r.,xt: o An = anorthite Ab : albite and Pl : plagioclase (AG,o)"",2",7,": o. Alm : almandine Fs : ferrosilite Consequently,equation (7) reducesto Experimentalapproach (P- Po): (-4I!-g4r) (8). The quantitativedetermination of the effectof in- dependentlychanging any of the parametersof state, '1Equation 3 is a generalform for the treatmentofany equilibria P, T, or X, on AG" for any reaction in a multi- asa functionof P, T, or X.It is implicitlyassumed that, where data componentsystem is given by are available,LV, and AS, includeappropriate mixing terms. REACTION GROSSULAR+ QUARTZ: ANORTHITE + WOLLASTONITE 891 Temperolure = constont Grossulor+ Quortz (+ Plogioclose) + o tt o o Plogioclose+ (L Wol loston ite dOG.= 4Y. 6P * nf tn oir" oii =f(x!L) xil + Ftc. 1. Schernaticpressure-composition diagram illustrating the effectof changes in plagioclasecomposition on the equilibriumpressure for reaction(1). Activity-compositionrelations for solutions are out of equilibrium by an absoluteamount of 1000 given by the equation calories.Thus, a changeof pressure(AP) is required for this assemblageto equilibrate. ato = Xr'"yro (9). : For solutionsthat obeyRaoult's law, 7 l. Solu- Experimentalmethods tions in which 7 ) I are said to have a positive deviation from Raoult's law and show a tendency Gels of plagioclasecompositions ranging from mole percent incrementswere towardunmixing, whereas those in which7 ( I have Ans, to Anrooin l0 a negativedeviation and tend to form stoichiometric prepared following a procedufesimilar to that of intermediatecompounds. Luth and Ingamells(1965). These gels were hydro- Substitutionof equation9 into equation8 yields thermally crystallizedin cold-sealpressure vessels -nr (Tuttle, 1949)at approximately800"C and 20,000 (p - po)- EL{iiYi'l) (10). p.s.i. for 14 days.The structuralstate of thesefeld- sparswas determined by G. A. Furst (The Pennsylva- It is now possibleto determineyij, and thus aftl, by nia StateUniversity, personal communication, 1974) experimentallydetermining lD (equilibriumpressure to be approximately80 percentdisordered for struc- for Anroocomposition) and the valueof P for dif- tures crystallizingin a 7A cell. In addition to the ferent plagioclasecompositions under isothermal syntheticplagioclase, a natural sample(Lake County conditions. plagioclase,Anez; Stewart et al., 1966)was usedin The argumentspresented above are schematically someexperiments. illustratedin Figure l Contoursof AG" are shown Startingmixes were prepared from a givenplagio- with the contour labeled AG, : 0 representingthe clase composition together with natural grossular : : equilibriumcurve. An assemblagecontaining grossu- (GrruAlmu;SiO, 39.2, AlzOs 21.8,total Fe as : : = lar, quartz, wollastonite,and Anroois in equilibrium FeO 1.91,MnO 0.63,CaO 35.5,in weight only at pressureIn for the temperaturerepresented percents),quartz, and wollastonite(0.75 percent Fe here.A shift in the plagioclasecomposition causes a asFezOs; Boettcher, 1970, p. 341),then homogenized reductionin the activity of the anorthitecomponent for one minute in a Spexmixer. and resultsin disequilibriumof the assemblage.For 3 K. Jensen,analyst, Argonne National Laboratory. Samplb example,a compositionalchange corresponding to kindly suppliedby Dr. E. Olsen,Field Museumof Natural His- AXl,.l of Figure I results in the assemblagebeing tory, Chicago,Illinois. 892 K. E. WINDOM AND A. L, BOETTCHER A sample of the dried starting material was weighed into a platinum capsule that had been D AO Grossulor +Quortz grew weldedclosed at one end.The capsuleplus sample I O Anorthite +Wollosionite grew O No chonge wasthen heatedfor approximatelytwo minutesin a Bunsen-burnerflame and the otherend welded imme- diately after removal from the flame. One to three capsuleswere loaded into a25.4-mmfurnace assem- o 1? bly (Fig. 2). Boron nitride, fired for 24 hours at Y 800oC,immediately surrounded the capsule(s).APy- o Grossulor+ Ouortz l t0 rex cylinder separatedthe talc from the graphite to o o Anorthite + Wollostonite maintainanhydrous conditions and preventincipient o I melting of the charge.Without theseprecautions, 0_ traceamounts
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