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Redetermination of the Anorthite Breakdown Reaction And American Mineralogist, Volume 73, pages216-223, 1988 Redeterminationof the anorthite breakdownreaction and improvementof the plagioclase-garnet-AlrSiOr-quartzgeobarometer ANonnl.M. Kozror,,Ronrnr C. Nrwrox Department of the GeophysicalSciences, University of Chicago, 5734 South Ellis Avenue, Chicago,Illinois 60637, U.S.A. Ansrnq.cr The reaction 3 anorthite : grossular + 2 kyanite + quartz has been determined exper- imentally over the ranges 900-1 250"C and 19-28 kbar in a piston-cylinder apparatus using a NaCl pressuremedium. Two to four weight percent LirMoOo flux enabledreversal of the equilibrium 200'C lower than previouslyachieved. A point at 650.C, 13.5 + 0.6 kbar was calculated from published studies of 4 zoisite r quartz: 5 anorthite + gros- sularf 2HrOand2zoisite + kyanite+ quartz:4anorthite + HrO, andtheaddition of this point constrainsthe equilibrium curve to P (in kilobars):22.807 (in.C) - 1093. Use of this equation with the plagioclase-garnet-AlrSiOr-quartzgeobarometer results in reduction of the uncertainty of calculated pressuresby a factor of two over previous calibrations. Calculated paleopressuresare 400 bars higher for the assemblagewith silli- manite than those given by Newton and Haselton (1981). Analysis of the data provides an estimate of the I bar,298 K entropy of anorthite that is 3.3 J/K greaterthan the third- law entropy, implying 4oloAl-Si disorder in anorthite. INtlnooucrtoN phic studies (e.g., Lang and Rice, 1985; Hodges and The reaction Crowley, 1985). We have undertaken a new experimental study of the 3 anorthite: grossular + 2 kyanite + quartz (l) anorthite breakdown reaction that reduces some of these uncertainties. Pressure cells of sodium chloride used in forms the basis of an important geobarometerfor meta- the piston-cylinder apparatus allowed greater accuracy in pelitic rocks. The assemblagegarnet * plagioclase * pressure determination. A nonaqueous molten flux, (kyanite, andalusite, sillimanite) + qtartz is common in LirMoOo, was used to promote equilibration of the ex- medium- to high-grade metamorphic rocks over broad perimental charges, making it feasible to attain tightly rangesoftemperature and pressure.The principal sources bracketed reversals at lower temperatures. In addition, a ofuncertainty in paleopressurescalculated from this ba- bracket of the equilibrium curve at 650 "C, 12.8-14.1 rometer are the P-7" location of the end-member reaction kbar, calculated from two zoisite-forming reactions pre- (i.e., Reaction l) and the activity-compositionrelation- cisely determined in gas-pressure apparatus at lower pres- ships ofthe garnet and plagioclasesolid solutions. sures, helps to constrain the curve in the P-7 range of Several experimental studies have been made of the crustal metamorphism. end-member reaction, starting with Boyd and England (1961),who first identifiedthe productsofthe breakdown ExpnnrlrnNTAl METHoDS of anorthite,and includingHays (1967),Hariya and Ken- All runs were performed in the piston-cylinder apparatus,us- nedy(1968), Schmid er al. (1978),Goldsmith (1980), and inga3/t in. (1.91 cm) diameterassembly with an NaCl pressure Gasparik (1984). Most of these experimentswere con- medium and WRe,-WRe' thermocouples.Temperature uncer- ducted at pressuresand temperatureswell above crustal tainties with these thermocouplesare less than +5 'C (Perkins metamorphic conditions and required long extrapolation et al., 1981).The salt was partially molten in runs at 1250"C for geobarometry.Hodges and McKenna (1987) have ar'd 27.3-28 kbar (Table l) as inferred from large inward-grow- pointed out that extrapolation of the equilibria, coupled ing quench crystals of salt next to the graphite furnace. The in- with wide bracketinguncertainties and limited P-Z ranges tersection of the melting curve of NaCl (Clark, 1959) with the of experiments,contributes a large proportion of the un- anorthite breakdown curve just above 1275'C and 28 kbar lim- certainty in any pressureestimate from this barometer. ited the upper temperaturerange of this study. The NaCl pressure medium with piston-out conditions re- An additional complication is the uncertain pressurecor- quires very small pressure corrections (Johanneset aL, l97I; rectionsin the ditrerent solid-media pressuredevices used Mirwald et al., 1975). However, a comparison of experimental in the studies.These factors contribute to an uncertainty reversalsofthe reaction anorthite + HrO : zoisite + kyanite + of at least 2 or perhaps 3 kbar in the P-?" location of quartz in a gas-pressurevessel and with the 3/+in. ( I .9 I cm) NaCl Reaction I in the temperature range 600-800 "C. Large piston-cylinder device by Jenkins et al. (1985) indicated that a uncertainties have also been cited in various metamor- 400-bar subtractive correction should be applied to piston-out 0003-004x/88/0304-0216S02.00 216 KOZIOL AND NEWTON: PLAGIOCLASE-GARNET-AI,SiO5-QUARTZ 217 TABLE1. List of experimentalrun conditionsand results Run Treatment. f fC) P (kbar) Time(h) Results 64 breathing 1250 28.O I gr growth; no zo oz breathing 1250 27.3 20 strong an growth; no zo 50 breathing 1200 zoJ 46 gr growth; trace zo 58 breathing 1200 260 46 an grew; no zo cc 21" Li"MoOo 1150 zb.5 47 gr growth; zo present 47 <1"k v 2o5 1150 zcJ 47 gr growth; zo present 44 2h Li2MoO" 1150 251 23 an grew ol breathing 1150 25.O 43 an grew; no zo 21 2o/" LirMoOo 1100 z+.c 23 gr grew; tr. unknown 15 4"h Li2MoO, 1100 24.0 22 no reaction;minor unkn. 19 4% Li,MoOo 1100 23.4 22 an grew; trace zo, unkn. 49 2h ti2Mool 1050 23.4 66 gr grew; zo present 57 2o/" Li"MoOo 1050 23.0 116 an grew; zo present 72 4YoLi,MoO4 1000 22.s 229 gr grew ao 4% Li,MoO4 1000 22.O 69 no reaction; minor zo, unkn. 75 4"h Li"MoOo 1000 21.5 zto an grew 10 4"k Li2MoO4 9s0 21.3 47 gr grew; minorzo, unkn. 60 2kLi2MoO4 950 21.0 264 no reaction; minor unkn. 30 4% Li,MoO4 950 20.5 137 no reaction; minor zo, unkn 66 4"k Li2MoO4 950 20.5 261 an grew, minorzo, unkn. 74 4% LirMoO4 900 zu.5 286 gr grew 70 4% Li,MoO4 900 19.0 238 an grew A/ote:Pressures listed are uncorrectedgauge pressures(o: +0.5 kbar). zo: zoisite. unkn.: unknown phase (see text). "Breathing" signifiesbreath-moistened charges. Values under "treatment" are in weight percent. 'C mns at temperaturesnear 600 and pressuresnear 10 kbar. form about 200loof intergranularmelt (Fig. 1). Extra kyanite and The correction is certain to be smallerat the higher temperatures quartz were added to the reversal mixes to ensure that all four of our study, which approach the melting point of NaCl. The phaseswere presentduring the runs. approachused here is to subtract200 bars from the experimental Reaction direction was determined by large (>300/o)changes gaugepressure and to add + 200 bars to the pressureuncertainty. in the ratios of the strengthsof anorthite and grossularX-ray Total uncertaintiesin run pressurefrom friction correction and diffraction peaks of quenchedcharges compared to the starting gaugedrift is +540 bars. material. Starting mixes were X-rayed 5 to 6 times to character- Starting materialswere a natural kyanite from Litchfield, Con- necticut(FeO content:0.16 wto/o),a natural quartz from Lis- bon, Maryland, and synthetic anorthite and grossular.Anorthite Taele 2. Syntheticanorthite unit-cell data was made from an ultrapure oxide mixture by sintering at 1300 'C and 1 bar for 2 d, and then treated at 1000 'C and 16 kbar Anorthite sintered Typical run product in a graphite capsuleto standardizethe material. Table 2 lists at 1300'C,1 bar Anorthite(AK14) (no.34) unit-cell parameters.Grossular (a : 11.850 + 0.002 A; was A. Refinedcell parameters synthesizedfrom stoichiometricglass at 1200'C and 26 kbar. a (A) 8.180+ 0.003 8.179+ 0.013- I 185 + 0.009 + + + Reversal mixes having nearly equal amounts of reactant and b (A) 12.857 0.004 12.873 0.008' 12 866 0.006 c (A) 14.175+ 0.006 14.168+ 0.026. 14.169+ 0.017 product phaseswere homogenizedthoroughly under acetonein a (') 93.333+ 0.026 93.047+ 0.082' 93.152+ 0.078 an agatemortar with small amounts of flux (seebelow), dried at Bf) 115.781+ 0.023 115.895+ 0.106. 115.942+ 0.073 450 .C, and sealedin welded Pt capsules. "r (') 91.101+ 0 022 91.256+ 0.073. 91.145+ 0.059 + + + To reverse a reaction in reasonablelaboratory times in the Y(4") 1338.38 0.94 1338.31 5.14- 1338.13 3.43 CaO-AlrO.-SiO2 system below 1200 'C, an intergranular fluid B. X-ray diffraction data (hkl, dspacings, Ad) compared to flux must be used in experimental runs. Water, an oxylate, and anorthite sintered at 1300 "C, 1 bar 202 4.0393 +0.0009-. -0.0055 PbO have been used in the past (Goldsmith, 1980; Gasparik, 130 3.7773 +0.0028-. +0.0049 1984). However, water or oxylate produces excessiveamounts 130 3.6189 +0.0025-- -0.0014 ofzoisite, which may obscurethe anhydrous reaction (Reaction 114 3.3510 -0.0112* 1), and incorporation ofPb into the feldspar structure may bias 220 3.2610 +0 0012-. +0.0012 the reaction. 220 3.1242 +0.0019" +0.0011 Therefore, two molten oxide fluxes, LirMoO. (Ito, 1975) and 132 3.0425 +0.0002* +0.0035 VrOr, were testedin this system.As little as I wto/oVrO, caused 042 2.9496 +0.0018.- -0.0002.- -0.0015 excessivelylarge amounts of melting, with elimination of some 132 2.8262 -0.0069-- of the reactantsin some runs. This flux was discardedbecause i34 2.6542 242 2.5014 0.0* -0.0011 of difficulty in controlling the amount of melting. Two to four 060 2.1397 -0.0003". +0.0003 weight percent of LiMoOo added yielded to the reversal mix 208 1.7689 +0.0008* +0.0005 better results.Runs without flux were successfulat 1200 and "C .
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