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Amphibolite-Facies-To-Granulite-Facies Reactions in Experimentally

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Amphibolite-Facies-To-Granulite-Facies Reactions in Experimentally American Mineralogist, Volume 75, pages 1349-1361, 1990 Amphibolite-facies-to-granulite-faciesreactions in experimentally deformedo unpowderedamphibolite Bru.or-ny R. H.q.cKEn* Department of Earth and SpaceSciences, University of California, Los Angeles,California 90024-1567,U.S.A. Ansrucr Reactions in mafic amphibolite marking the transition from amphibolite to granulite facies were investigatedby hydrostatically heating and deforming solid cylinders of An.o plagioclase+ tschermakitic hornblende * minor quartz for up to 795 h at temperatures of 700-1000 .C and mean stressesof 0.7-2.1 GPa. Melting of quartz + plagioclase * amphibole beganat 775"Cinthe presenceofHrO and at 900 "C under HrO-undersaturated conditions. The amphibole crystals underwent Mg-Tschermak substitution and the pla- gioclasecrystals became more anorthitic. At 875-1000 'C, the rock became a partially melted granulite with hornblende, anorthitic plagioclase,augite, and hypersthene.The augite and hypersthenenucleated on amphibole crystalsin liquid films adjacent Io quarlz crystals. During subsequentgrowth (and decreasingsurface/volume ratio) the pyroxene crystals abandoned their coherent interfaces with their host amphibole crystal for ener- getically more favorable and more mobile incoherent interfaces and grew outward into the enveloping liquid that was produced by the breakdown of plagioclase* amphibole * qvartz. There is a measurableeffect of stressand deformation on these reactions. Some phasesgrown in deformed sampleshave slightly, but systematically,different compositions from phasesgrown in hydrostatically heated samples.Unfortunately, the system is too complex to assessthe implications for natural metamorphism. Comparison with equilib- rium petrology experimentsindicates that the reactionswere incomplete and the compo- sitions and proportions of the phasesproduced during the experiments are metastable. This srrggeststhat amphibolite-facies-to-granulite-faciesequilibrium textures and phase compositions cannot be experimentally investigated in unpowdered mafic rock because reaction rates are too slow, unless experiments can be conducted for longer than one month. INrnooucrroN that the liquid may have moved away from its source region and that investigation can be conductedonly after This paper reports the results of deformation and hy- the liquid has crystallized. As a result, many details con- drostatic experiments performed on solid cylinders of cerning reaction mechanisms and the development of amphibolite. The experimentsdescribed here are a subset metamorphic texturesremain obscure.There are two ba- ofa larger group ofexperiments that were conducted to sic approachesto laboratory experimentsthat can be con- obtain mechanical data, which are described in another ducted to overcome theseinherent limitations. The first, paper (Hacker and Christie, 1990). This study investi- a widely used approach, is to study powdered material gated disequilibrium transformations during deforma- with an emphasis on understanding compositions and tion-no attempt was made to reach chemical equilibri- modal proportions of phasesat equilibrium. The disad- um. The experiments shed light on the development of vantage of this approach is that rocks are rather tightly textures during prograde metamorphism of amphibolite, packed polycrystalline aggregates,unlike powder, and the effectsof stressand deformation on metamorphism, consequently some information about reaction textures and the compositions and texturesof liquids formed dur- is lost. The secondapproach is to use solid piecesofrock ing fusion ofbasaltic rock. to investigatereaction kinetics, compositions, and modal High-grade rocks visible on the Earth's surfacedo not proportions ofphasesinvolved in the reactions.Although preservetransient featuresthat were presentduring meta- this approach provides information about reaction tex- morphism. For example, studies of anatexis, including tures in a crystalline rock, it does have severaldisadvan- the compositional and textural relationships among the tages.Rates of reaction are slow becausefluid must dif- liquid and restite phases,are hindered in rocks by the fact fuse along grain boundaries (instead ofbetween powder fragments), and the surfacesof grains are not free. This * Present address:Department of Geology, Stanford Univer- means that equilibrium may not be attained during the sity, Stanford, California 94305-2115, U.S.A. duration ofthe experiment and the fine-grainedreaction 0003-o04x/90/I I I 2-1349$02.00 t349 I 350 HACKER: REACTIONS IN DEFORMED AMPHIBOLITE productsmust often be analyzedby time-consuming,high- resolution techniques such as transmission electron mi- croscopy (Brearley, 1987; Rubie and Brearley, 1987). Moreover, becauseof time limitations, the slow heating that occurs during most natural environments cannot be duplicated in the laboratory. In spite of these disadvan- tages,experiments on solid piecesofrock do provide oth- erwise unavailable information about reactions that is complementary to the equilibrium approach using rock powders. The solid piecesofrock used in this study provide the first opportunity to characterizethe textural and chemical relationshipsamong the phasesinvolved in the transition from amphibolite to granulite facies in experimentally deformed basaltic rocks. These are the first experiments ofthis type at high pressureusing unpowdered rock and the first using unpowdered mafic rock. Melting studiesof basaltic materials have been restrict- Fig. l Optical micrographof the natural amphibolitestart- ed to equilibrium studies conducted with rock powders ing material.Dark crystalsare amphiboleand minor iknenite, plagi6slsss quartz. (Yoder and Tilley, 1962; Green and Ringwood, 1968; and palecrystals are and minor Hill and Boettcher,1970; Holloway and Burnham, 1972; Green, 1972; Egg,ler,1972; Cawthorn et al., 19731,Helz, many similar experiments were also performed on syn- 1975; Spulberand Rutherford, 1983;Ellis and Thomp- thetic rock (Hacker and Christie, 1990).The starting ma- son, 1986; Baker and Eggler,1987; Beard and Lofgren, terial is an amphibolite composed of roughly 530/oam- 1989, 1990).During this study, small melt fractionsat phibole, 430loplagioclase, 2o/o ilmenite, 2o/oquarlz, and quite low temperatureswere produced, leading to an in- trace amounts of rutile, sphene,and apatite (Fig. l). The terestingcomparison with previous studiesand providing peak metamorphic temperature of this rock can be esti- information about the composition and distribution of mated as 600 + 50 "C, usingSpear's (1980) NaSi : CaAl initial disequilibrium liquids in fused mafic rock. exchangethermometer for amphibole and plagioclase.The A fundamental assumption in petrology is that the rock has a uniform granoblastic texture and unzoned mechanisms,rates, and equilibrium conditions of trans- minerals, both of which help identi& experimentally pro- formations are not affectedby deformation or deviatoric duced textural and chemical changes.The amphibole is stress.Equilibrium conditions are normally modeled us- tschermakitic hornblende (classificationof kake, 1978; ing hydrostatic thermodynamics, in which only the mean Table l). No other hydrous phaseswere detectedwith the stressis consideredto affectthe internal energyofa phase; electron microprobe or scanning transmission electron any effect that might be produced by strain or deviatoric microscope;hence an experiment begun under vapor-ab- stress is neglectedin the hope that the effect is small. sent conditions can only become hydrous when the am- Theoretical expansions of Gibbs's (1906) treatment of phibole dehydrates.The plagioclaseis Anro $able 2). thermodynamicsby Kamb (1961),Fletcher (1973), Pat- Samples of the natural rock were right circular cylin- erson(1973), Robin (1974),Green (1980, 1986), Heidug ders, 6.25 mm in diameter and 12.7 mm in length, cored and Lehner (1985),Bayly (1985),and Kirby (1987)have with a HrO-cooled diamond drill. The sampleswere all shown, however, that the deviatoric components of a tested in assemblies designed for deformation experi- nonhydrostatic stressfield affect,and in some caseseven ments so that differencesin assemblydesign do not hin- dominate, the thermodynamic potential of phases.The der comparison of deformed and hydrostatically heated extent to which mechanicalprocesses affect chemical pro- samples. In HrO-absent experiments, the sample was cessesis essentiallyunknown becauseoflack ofinvesti- placed inside a Cu cylinder 2 mm thick below a cylinder gation but could be considerable. In the present study, ofboron nitride (Fig. 2A). In experimentsto which I wto/o some sampleswere hydrostatically heatedand some sam- HrO was added, the sample was placed inside a cylindri- ples were deformed. By comparing the two sets of sam- cal Ag jacket 0.1 mm thick surrounded by talc (Fig. 2B). ples, we can evaluate the changesin phasecompositions Ag is permeable to H, (Chou, 1987), and H, produced produced by stressand deformation. during amphibole and talc dehydration presumably dif- fused through the Ag jacket during the experiments,and ExpnnrurxrAl, PRocEDURE thereby affectedvolatile fugacities. HrO was transferred The experiments were conducted in a piston-cylinder by pipette onto the top of the sample after the sample apparatus for studies of rock deformation (using solid had been inserted into the Ag jacket. The HrO was then pressuremedia) of the type designedby Griggs and co- mechanically sealedwithin the jacket by two ceramic end workers (Griggs, 1967;,Blacic,l97l).
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