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Stability of Biotite: Experiment

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Stability of Biotite: Experiment THE AMERICAN MINERALOGIST, VOL. 50. SEPTEMBER, 1965 STABILITY OF BIOTITE: EXPERIMENT. THEORY, AND APPLICATION' Dlvrn R. WoNes, U. S. GeologicalSuraey, Washington, D. C. AND HaNs P. Eucstnn, Deparlment of Geology,The Johns H opkins U niter sity, Baltimore, M arylond'. AesrRecr Biotites on the join phlogopite-annite react to form a number of assemblages and the reactions are governed by the independent intensive parameters, temperature, fugacity of HrO (fnro), and fugacity of oxygen (f6 r) . The most common of these assemblagesin natural occurrences are biotite-sanidine-hematite; biotite-sanidine-magnetite; and biotite-leucite- olivine-magnetite. The compositions of biotites coexisting with sanidine and magnetite were determined for a variety of conditions of furo, fs, and temperature. The compositions lie in the ternary system KFes2+AlSLOro (OH)r, annite-KMg32+AlSraOro (OII)2, phlogo- pite-KFe3r+AlSigOu (H-J, "oxybiotite." Application of regular solution theory to KFerAlSiaOro(OH)s in ternary solid solution yields the following relationship: 3428- 4212(1 - x'), log fsrg : log xr l/2logfo,+ 8.23 - log aKArsi"or- log ap",e T * I where x1 is the mol fraction of KFeaAlSisOro(OH)z in biotite, ar<,rrsi.o,is the activity of KAISLOs in the various mineral phases, and aFg,e,is the activity of FesOr in the various mineral phases. Neither molecular (a: x) or cationic (a: t') ideal solution theory describes the activities of KFedlSLOro(OH)e in "ternary" biotite solid solutions. The position of the fe.-Tcurvesfor biotite-muscovite-magnetite-corundum/alumino-silicate-quartzassemblages were determined by the intersections of the stability curves of biotite and muscovite. Schematic fer-T curves are given for biotite-sanidine-pyroxene-magnetite-quartz assem- blages. Biotite crystallizing from a magma may follow either a more iron-rich trend or a more magnesium-rich trend, depending upon the fo, conditions during cooling. If fs, is decreas- ing with temperature, the Mg/Fe ratios oI the anhydrous phases and the amount of magne- tite either decrease or change very little. The magnesium-rich trend in the biotite composi- tions is represented by constant or increasing f6, (through loss of hydrogen) and leads to an increase in the amount of magnetite present and in the Mg-Fe ratios of the associated ferro- magnesian phases. The two trends may represent, respectively, undersaturation or satura- tion of the melt in regard to H2O. Extrapolation to natural biotites assumes that Ti, Al, and F substitutions are similar to Mg, Fe+s, and O substitutions. Biotite-K-feldspar-magnetite assemblages in low-grade metamorphic rocks ((500o C.) imply either disequilibrium or low HrO pressures. Assem- blages in the gneissesof the northwest Adirondacks imply HzO fugacities of 0.1-10 bars and COr fugacities of 400-10,000 bars during recrystallization of the interbedded gneisses and marbles. Intersections of biotite fsrs-T curves with the "granite minimum melting curve" sug- gest that most biotite-bearing rhyolite and quartz latite magmas were at temperatures I Publication authorized by the Director, U. S. Geological Survey. 1228 BIOTITE STABILITY 1229 greater than 750o C. and at HzO pressures of less than 1000 bars just before extrusion. Biotite phenocrysts permit an estimate of minimum H2O pressure in "granitic" melts; magnetite or orthopyroxene phenocrysts permit an estimate of maximum If3O pressures. INrnonucrroN As biotites are very common constituents of igneous and meta- morphic rocks, knowledge of biotite stabilities and phase equilibria should aid in the determination of certain boundary conditions (tem- perature, water pressure,oxygen pressure and compositional variations) for magmatic and metamorphic processes. Previous investigators have determined the stability of the end members,phlogopite (Yoder and Eugster, 1954) and annite (Eugster and Wones, 1962), and this study attempts to examine the solid solution of those end members. The iron-magnesium ratio of biotites in igneous (Heinrich, 1946; Nockolds, 1947) and metamorphic (Thompson, 1957) rocks has been correlated with various rock types and grades of meta- morphism, and this study establisheslimitations of the intensive vari- ablesat which a particular biotite may be stable. Cation substitution of Fe and Mg in silicate lattices has been discussed by Ramberg (1952),Mueller (1960),Bradley (1962),Bartholom6 (1962), Ghose (1962) and Kretz (1964). Experimental studies have been limited to olivine and pyroxene equilibria, chiefly at liquidus temperatures and one atm. pressure(Bowen and Schairer,1935; Muan and Osborne,1955) whereas this study presents data on Mg-Fe substitutions in a hydrous silicate at elevated pressures.It may, therefore, serve as a model for other Mg-Fe solid solutions of hydrous silicates. The present study is an attempt to define the biotite-sanidine-magne- tite-gas equilibrium as a function of T, P(=Par*Prro), fo' and phase composition at temperatures between 400" and 900o C. On biotite compositions, at f6, values between the FeaOr-FerOaand Fer-'O-FerO+ buffers, no other assemblageswere encountered at the above conditions. This manuscript presents the experiments, their interpretation, and methods of extrapolation in the first part. In the second part applica- tions to some natural assemblagesare discussed.The biotite assemblages will provide, if properly studied, definite limits on the possible tempera- tures and fugacity of components such as H2O, Oz, and Hz in geologic processes. ExpBnrurNIAL TECrrNreuES The techniques used have been discussed by Eugster and Wones (1962) and Wones (1963b). Cold-seal hydrothermal apparatus supplied temperature and pressure.Pressure measurements were good to t/p and I23O D. R. WONESAND H. P. EUGSTER the temperaturecontrol and measurementwas within 5" C. of the stated temperature.Individual experimentswere quenchedby slow cooling (20 minutes) of a sealedpressure vessel. As reactionrates are very slow, this techniquecreated no problemsrelated to "quench phases." Oxygen fugacitieswere controlled by the buffer techniqueof Eugster (1957).Table 1 lists the valuesof oxygenfugacity for the severalbuffers. Muan's (1963) results on the system Pd-Ag-Fe demonstratethe useful- ness of Pd-Ag alloys in hydrothermal experiments with iron-bearing systems.PdaoAgro alloys were used only in the most recent experiments. Phase equilibria were determined by examination of quenched prod- ucts obtained from experimentsusing biotite or phlogopitefsanidine {magnetite as starting materials.The phlogopite,sanidine and magne- Teela 1.Oxvcar.r Fucecrtms ol Oxvcru BumBns(Eucsrrn aNo Wonrs, 1962) - A _ C(P_1) Logfsr:-i+B+ T Bufier IrezOa-FerOr 24912 1l A1 0.019 FesOa-Fer-*O 32730 13.t2 0.083 Nio-Ni 24709 8.94 0 046 SiO:-FezSiO-Fe;Oa 27300 10.30 0.092 I tite were synthesizedas pure phasesand then mixed mechanically to avoid any ambiguity in results.The critical determinationin all experi- ments is the compositionof the biotite coexistingwith sanidine,magne- tite and gas. The exact location of the equilibrium is dependent upon attaining equilibrium by oxidizing a ferroan biotite to a magnesianbiotite-sani- dine-magnetite assemblage,and by reducing a phlogopite-sanidine- magnetite assemblageto a ferroan biotite-sanidine-magnetiteassem- blage. Completereversibility was establishedfor five points in addition to the data for the annite end member. The remainder of the data were obtained by oxidizing a biotite. Such experimentsshould yield a maxi- mum iron content. All indications are that they approach equilibrium within 48-100 hours, whereasthe reversingequilibrium requiresat least 250 hours. A sequenceof experimentstabulated in Table 3c at 750oC. and 800oC., 2070bars, demonstratesthat the approachto equilibrium proceededat highly variable rates. At 750o C., a 24-hour reducing experiment pro- BIOTITI], STABILITY r231 duced a biotite with Fe/Fe{Mg:0.63 whereas a 195-hour reducing experimentproduced a biotite with a value of Fe/Fe*Mg of 0.56. The value producedby oxidation of a ferroan biotite is 0.78 and is probably nearer the true equilibrium value. Where a large spreadbetween oxida- tion and reduction experimentsexists, the oxidation value is used. Experimentsusing oxide mixes as starting materials were given little credence.They were oniy usedin closeconjunction with experimentsus- ing biotites or phlogopite-sanidine-magnetiteas starting materials. ColrposrrroN ol PHAsES As methods of extrapolation and interpolation of the biotite phase equilibria dependheavily on assumptionsconcerning the purity of sani- dine and magnetite,and on the compositionof the gas phase,as well as the biotite composition,discussion of the compositionalvariations of all the phasesis warranted. Biotites.Biotite compositionswere determined by measuringindices of refraction and the (060,331) r-ray spacing using the data of Wones (1963b).Optical measurementsproved the most useful, as the r-ray re- flections are weak and broadenedby reflectionsfrom the inclusionsof original biotite flakes within the newly crystallized sanidine. The method of Gower (1957) was not used as the (004) biotite reflectionis coincidentwith the (311) of magnetite. A complicatingfactor in this study has been the formation of "oxybio- tite" in the biotite solid solutions.Consequently, the biotites are not a simple binary KMg3AlSi3Oro(OH)z-KFeaAISiBOr6(OH),but rather they are members of the reciprocal system (Ricci, 1951, p.371-389) KMgaAlSirOl0(OH)r-3 KFe2+Fe23+AlSiaOrz-KFes2+AiSi3O10(OH) r-3KMg . Fezs+AISieOrz.If the theoretical(and physically
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