AmericanMineralogist, Volume61, pages 889-896,1976

The effectof reducedactivity of anorthiteon thereaction * : anorthite* :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- 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 (,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 of glass(quenched liquid) appearedin all runs. All experimentswere performed in piston-cylinder apparatussimilar to that describedby Boyd and Eng- land (1960).Each run was pressurizedto approxi- mately 6 percentbelow the final pressureand then OL-400 600 800 1000 1200 1400 brought to temperature.Run pressurewas then ob- Temperoture "C tainedby increasingram pressureto the desiredread- ing,the true value of whichwas calculated using a -6 Ftc. 3. Pressure-temperaturediagram comparing the results percentfriction correction. obtainedin this study for Gr * Qz : An,oo* Wo with results A temperatureof I100'C was maintainedfor all of Newton (1966),Hays (1967),and Boettcher(1970). runs becauseit is representativeof temperaturesin the region of interest.In addition,reaction rates are slightlyhigher temperatures for assemblagescontain- too slow at lowertemperatures, and melting occurs at ing more albiticplagioclase. Eachstarting mix was X-rayedusing both a No- relcodiffractometer and Guinier powdercamera, and the peakswere indexed.Each run product was also Pt-Pr90./Rh to Thermocouple X-rayed using both techniques,and the relativein- tensitieswere comparedwith those of the starting Stoinless material.Replicates of certainruns, as well as dupli- Stee I cateruns, weremade to checkfor reproducibility.In Pyrophyl Ii te all instances,reproducibility was attainedwithin ten percentchange for all phases,a value taken as the Bross limit for detectablereaction. Solenholfen Limestone plagioclasefrom various Tolc Attempts to analyzethe Pyrophyllife (fired) runs usingthe electronmicroprobe were unsuccessful becauseof the small grain size(l-l0pm) and the Pyrex Encopsuloted Somple intimate mixture with the other phases.Scanning electronmicroscopy was also tried, to determineif Boron Nitride reaction direction could be detected from Leod Foil morphological changesin the various phases.No Grophite Furnoce conclusiveresults were obtained using this approach.

Pyrophyllite ExperimentalResults Pyrophyllite (f ired) A comparisonof our data for Anroocomposition with previousresults is shownin Figure 3. Basedon : _j thesedata, a valueof P0 14.8+ 0.3kbar is takenas F_25.4mm the equilibriumpressure at I l00oC.The resultsof the FIc. 2. Cross-sectionalview of solid-media furnace assemblvused presentstudy are shownin TableI and Figure4. rn exneriments. Uncertaintiesin our data mav arisefrom the fol- REACTION GROSSULAR+ QUARTZ = ANORTHITE + WOLLASTONITE 893 lowing sources:(l) Uncertaintiesin experimental Uncertaintiesin determining reaction direction technique-basedon duplicateruns and past experi- may also arisebecause of a bufferingeffect on the encewith the apparatusused in theseexperiments plagioclasecomposition in equilibriumwith grossu- (seeJohannes et al., l97l ), experimentaluncertainties lar, quartz,and wollastonite.The equilibriumcondi' are believedto be within +10'C and *300 bars.(2) tion for An,oois isothermallyinvariant, and complete Reactionkinetics-it wasfound that the reactionrate reactionto eitherproducts or reactantsis possibleas is not symmetricalabout the equilibriumcurve. Start- a result of a changein pressure.This is not the case ing mixes of 50 weight percentproducts, 50 weight for plagioclasethat is not pure anorthite,because the percentreactants for Anroocomposition resulted in addition of NazO increasesthe variancyby l, creat- essentiallycomplete reaction for runs at pressures ing an isothermally univariant equilibrium curve' lower than the equilibrium curve, whereasruns of Thus, for a P-Xl,] condition not at equilibrium, equal duration at pressuresa comparabledistance reactionwill occurcausing the plagioclaseto become above the equilibriumcurve did not go to com- either more albitic (AG' > 0), or more anorthitic pletion, although enough reaction occurredto in- (AG" < 0), or until one of the other phasesis con- dicatethe senseof reaction.It is probablethat these sumed.This meansthat for initial conditionsnear relativereaction rates occurred for the other plagio- equilibriumwhere the absolutevalue of AG"is small, clasecompositions as well. (3) The equilibriumP-X only a minor amountof reactionwill occurbefore the curveis not strictlyunivariant because of impurities plagioclasecomposition is adjustedto nearthe equi- in the grossularand wollastonite.These impurities librium value and perceptiblereaction ceases.For are minor, and they probably do not significantly this reasonit is difficult to unequivocallydetermine affectequilibrium. the senseof reaction of runs near the equilibrium curvebecause ofthe insensitivenessofX-ray patterns (< - l0 percent)in the proportions TABLEl. Resultsof expcrimentsat I l00oC to smallchanges of phases. DI r- Pte6su!e Run no. Tine Redulta The solid curve shown in Figure 4 is our inter- (uole percent) (kbar) (hour6) pretation of the best fit for thesedata' From this 100 13.0 722 24 ploducts grev curve, values of Ti,l can be calculated,assuming 100 tJ.) 723 24 ploducta grew 100 14.0 724 48 Producto 8re9 the value for AV, is known. We have attemptedto t00 725 24 products grev 100 15.0 135 55 reactenEg Sfew correct this volume term for the effect of thermal 100 l).) 135 72 r@ctanta Srev expansionand compressibilityusing availabledata 90 15.0 r38 72 Products grs and extrapolatingwhere necessary. Thermal expan- 90 r).) 139 72 r4ct6ntg Itev sion data for plagioclase(AneuAb5) and B-quartzup 80 15.0 141 72 pioducts grew 80 142 72 products grew to l000oC,and for grossularup to 800oC,were taken 80 16.0 143 72 grev reactaots from Skinner(1966) and extrapolated to I l00oC.No 70 131 65 product6 grew 70 !32 72 no change data for wollastonitewere found in the literature. 70 15.0 133 72 r€ctants gr4 However,it is possibleto model wollastoniteusing 14.0 54 44 products glq with similar structures.Thermal expansion 15.0 58 24 products grd tJ. ) 56 24 no change for wollastonitewas obtained by extrapolatingvalues 57 24 reactanta 8rs for Woru-Fsuuand Wo.s-Fss1(Skinner, 1966)from 72 products grew 50 15.0 t45 no change 600"C to I l00oC, then extrapolatingthese values to 50 16.5 t47 72 no chaoge data for wol- 50 17,0 148 72 feacEents Stew Wo,oocomposition. Compressibility quartz from Vaidya et al. (1973). 50 15.5 124 24 products grew lastoniteand are )U 15.0 l)b product6 grd Compressibilitydata for grossularand labradorite 50 ro.) 157 no change 50 17.0 158 72 reactants grew (Anur)are from Birch (1966).Table 2lists the values 50 r7,5 72 reecEants greu usedfor thermal expansionand compressibilityand 40 to.) r50 productg Srew 40 r7,0 151 77 products Sreu the molar volumesof eachphase at ll0O"C and l5 40 r52 12 produeto grew to 20 40 18.0 153 72 no change kbar. Isothermalcompressibilities from l5 kbar 40 16.J L54 no ctEnge kbar are small and are neglected.Volume changes 40 r9.0 r)) 72 r4ctaots 814 resultingfrom changesin the compositionof plagio- 30 79 24 products grew 30 18.0 t62 72 products greu claseare also neglected(see Figure 7-41 of Smith, 30 18.5 163 72 no change : 30 19,0 t64 72 no change 1974).The data in Table 2 yield a value AV, 30 r9.i 125 24 reactanta grew 0.70534cal/bar at ll00oC and 15 kbar. Using this K. E. WINDOM AND A L, BOETTCHER

as the structuresbecome more ordered,as evidenced lT.-p*t,* =ttoi-tel by the occurrencesof various solvi below about Grossulor + Quortz 500'C (for a review, see Smith, 19'74).At higher ( + Plogioclose) temperatures,the structuresbecome more disordered and the solvi close.Smith and Ribbe (1969)have proposedthe existenceof unit-cellintergrowths in the Y o plagioclasestructure at temperaturesabove the crests l Fl:,4-Fr{ O O of the various solvi, but below the solidus.In these an-'-\^Anil o O O structures,there is continuity of the aluminosilicate 0_ Plnainalnco + frameworkwith both anorthite-likeand high albite- Wollostonite like domainsoccurring within this frameworkon the scaleof a few unit cells.Compositions in the range Anrooto about Anro-soare ordered on tetrahedral sitesand occur as structureswith the c axis = 14A.

X!f. tmorepercenl) Compositionsmore albitic than about AnTnoccur in structures'resemblinghigh albitewith c = 7A. Ftc. 4. Pressure-compositiondiagram showing experimentally We interpret the resultsobtained in the present determined points and our interpretation of the best fit for these study as a reflectionof this domainassemblage in the data (solid curve). The dashed curve was calculated using plagioclasestructure. For compositionsin the range ail" = xx]". Anrooto about Anro,the plagioclaseunit-cell is inter- pretedas havinga l4A c axiswith the anorthite-like valueand solvingequation l0 for the curvein Figure domainsbeing dominant.For thesecompositions, 4 yieldsvalues of f[l, : 1.0 for the composition 7Ii : 1.00. For compositionsmore albitic than rangeAn8o-100 and Til : 1.2for the rangeAnro-ro. about Anro, the structureis not energeticallyfavor- The dashedcurve in Figure4 was calculatedfor ai,] able for the large l4A anorthite-like.domains, and : XXi,;it doesnot satisfythe experimentaldata for thusthey tendtoward unmixing, resulting in a posi- compositionsmore albitic than Anro.Hence, compo- tive deviationfrom Raoult's law. sitionsin the rangeAnro-ro show a positivedeviation Orville (1972)obtained similar resultsfor plagio- from Raoult'slaw. This composition range probably clase at 700'C and 2000 bars. He interpretedhis extendsto purealbite, based on argumentsdiscussed results as "ideal solution" (riL : l 00) between in the next section. "real" and "fictive" end-membercompositions. We feel that the useof fictive end-membersis not war- Discussionand conclusions ranted, especiallyin the light of the domain theory The positivedeviation from Raoult'slaw of y[l, for plagioclasesolutions. valuesfor plagioclasemore albitic than aboutAnro at The positivesign of dP/dX betweenAnro and An6o the conditions of our experimentsindicates a ten- is speculative.It implies the coexistenceof two dencyto separateinto two structuralentities. A sepa- plagioclasephases, having different compositions but ration of this naturedoes occur at lowertemperatures the sameactivity at a giventemperature and pressure.

TAsI-e 2. Thermal expansion and compressibility data used in calculations

Phase Thernal expansion CoEpressibj.lity Molar volwe at at 1100oC at 15 kbar 1100'C and 15 kbar (percent V increase) (vo - V)/vo (cal/bar)**

Anorrhite 1, 63 0.0223 2.398s wollasronite 1.55* 0.0214 0.94869 Grossular 2.7O 0.00947 3.0464 Quartz 4,18 0.03 68 0.54414

Data obtained by ertrapolating data for Wo-Fs solid-solutions.

Detemi,ned by appLying themaL apattsion and conpressibili.ty data to 1 atn, 25oC noTot uolwne data giuen in Robie and lla,dbam (1968). REACTION GROSSULAR+ QUARTZ = ANORTHITE + WOLLASTONITE 895

The hypotheticalone-atmosphere phase diagram of and pressuresabove about 17 kbar in the hydrous Smith(1974, p.6) showsboth a solvusand a hanging system(Boettcher and Wyllie, 1968). binary loop, each coveringthe compositionrange Information regarding plagioclasebehavior ob- Anro-.'at temperaturesnear I100"C. This phase dia- tainedfrom the studyof reactionI canbe appliedto gram doesnot constituteproof of a two-phaseregion the more complex,natural assemblagesthat contain at 15 kbar pressure,but does lend support to this plagioclase. possibility. As a result of the inflectionsat compositionsAnro The resultsobtained in subsolidusstudies have and Anro (Figure 4), the beginningof reaction for implicationsregarding melting phenomena of plagio- plagioclasecompositions in the rangeAn.o to Anrois clase.The similaritybetween our resultsand thoseof within a pressureinterval of only about 1000bars. Orville (1972)indicates a largepressure-temperature The actualpressure interval is differentfor different region over which plagioclaseexhibits a positivede- reactionsand is a functionof AV,. The intervalis also viation from Raoult's Law. Basedon one-atmo- temperature-dependent,probably wideningwith in- sphereexperiments, Bowen (1913) concluded that the creasingtemperature for a givenreaction. Thus, com- meltingbehavior of plagioclaseobeys Raoult's Law; parablemineralogical assemblages containing plagio- his conclusionhas been confirmedby C. Wayne clasesthat rangefrom calcic-labradoriteto anorthite Burnham (The PennsylvaniaState University, per- will all begin to react within a zoneon the order of sonalcommunication, 1975). This differencein be- 2-3 km thick. Conversely,assemblages containing havior cannot be solelyattributed to either a change more sodicplagioclase (oligoclase, andesine) will per- in temperatureor in pressurebecause the melting sistto higherpressures and will beginto reactover a curvesfor plagioclase.| HrO at 5 kbar (Yoder et al. rather wide pressureinterval becauseof the steepen- 1956)and at l0 kbar (Furst and Boettcher,1972), ing of the equilibrium curve for the beginningof determined at temperaturesand pressuresbelow reaction. thoseof our experimentsyet abovethose of Orville Based on these observations,a possible ex- (1972),appear to obey Raoult's Law. Further, the planationis givenfor the existenceof seismicdiscon- proposedphase diagram of Smith (1974)shows a tinuities (e.g., Ihe Conrad discontinuity)in certain field for a nonquenchablemonoclinic structure (mon- parts of the lower crust,whereas other partsshow no albite)for albite-richcompositions at solidustemper- evidenceof sucha discontinuity.If largeareas of the atures.Consequently, increased temperature does not lower crust are composedof basicrock (e.g.gabbro) result in a continuousisomorphous solid solution containing calcic plagioclase,these rocks will un- series,but rather the occurrenceof three separate dergo plagioclase-consumingreactions at relatively structures,i.e. , a 7A monoclinicstructure for compo- shallowlevels. These reactions would all occurwithin sitionsrich in albite,a 7A triclinic stucturefor com- a zoneof approximately2-3 km thick and may be positionsbetween about Anruand An6o,and a 14A seismicallydetectable if the region is large. Con- triclinic structurefor anorthite-richcompositions. versely,areas composed of rocks(e.9., andesite) con- One reasonable explanation for the apparent taining plagioclasewithin the rangeoligoclase to an- Raoult'sLaw behaviorof meltingphenomena is that desinewill not be involvedin plagioclase-consuming the activity coefficientsof the liquid are similar to reactionsuntil much higher pressures,and the pres- those of the solid, thus cancellingeach other. This sureinterval over which thesereactions begin will be impliesshort-range order in the liquid and a tendency muchgreater. Thus, no sharpbreak will occurand no for the liquid to separateinto two phases(although seismicallydetectable layer will be formed. possi- not necessarilytwo liquids). Interestingly,this Acknowledgments was by Bowen(1913, p. 594). bility alludedto Discussionswith D. M. Kerrick andC. WayneBurnham regard- Someexperimental data supportthe hypothesisof ing crystallinesolutions and with D. K. Smith regardingplagio- positivedeviation from Raoult's Law for plagioclase clasestructure contributed to the manuscript.R. Z Smallfabri- liquids. Anorthite-rich compositions have been catedfurnace parts and maintained the laboratory equipment. This shownto meltincongruently to corundum* liquidat researchwas supportedby the Earth SciencesSection, National DES73-00220A0l. pressuresabove about 7.5 kbar for both anhydrous ScienceFoundation, NSF Grant (Lindsley,1967) and hydrous(Boettcher, 1970) sys- References tems.Albite-rich compositionsmelt incongruentlyto AKELLA,J. lNn G. C. KsNNnoY (1971) Studies on anorthite * jadeite* liquid at pressuresabove about 32 kbar in diopsiderr-hedenbergiteuoat high pressures and temperatures. the anhydroussystem (Bell and Roseboom,1969) Am J. 9ci.270. 155-165 896 K, E, WINDOM AND A, L, BOETTCHER

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