The Oxidation Ratio of Iron in Coexisting Biotite And

1423

The Canadian Mineralo gist Vol.37. pp. 1423-1429(1999\

THE OXIDATIONRATIO OF IRON IN COEXISTINGBIOTITE AND HORNBLENDEFROM GRANITICAND METAMORPHICROCKS: THE ROLE OF P, T AND f(O2)

NADINES. BORODINA, GERMAN B. FERSHTATER$erro SERGEI L. VOTYAKOV

Institute of Geology and Geochemistry, RussianAcademy of Sciences,Pochtoty per., 7, Eknterinburg, 620151, Russia

Assrnecr

Previously published and new data on the composition of coexisting biotite and homblende from granitic and metamorphic rocks show that the degree of iron oxidation, R [= Fe3*/(Fe2*+ Fe3*)], is different in these two minerals; the R of hornblende is greater. Granulite-facies minerals have the greatest difference in R, whereas in granitic rocks, those minerals show the least difference The oxidation of biotite and hornblende under high-level conditions is accompaniedby the crystallization of magnetite, and newly formed oxidized mafic minerals have a lower Fe/(Fe + Mg) and R than the original ones. Under mesozonal and catazonalconditions, the increasein pressureprevents the formation of magnetite, and oxidation is accompaniedby a significant increase in R; these changes in the chemical composition of hornblende are supplementedby an increase in Al. Since the Al content of homblende is known to be an indicator of pressure, such a correlation of R and Fer+ content with aluminum content points to an increaseof theseparameters with a rise of pressure

Keywords:biotite, hornblende,oxidation ratio, granitic rocks, metamorphic rocks, epizonal plutons, mesozonalplutons , cat^7'onal plutons.

Sorravarnr,

Les donn6esnouvelles et celles tirde de la litt6rature portant sur 1acomposition de la biotite et celle de la hornblendecoexistante des roches granitiques et m6tamorphiquesmontrent qui le degrd d'oxydation du fer, R [= Fe3*/(Fe2*+ Fe3+)],est diffdrent dans ces deux mindraux. La valeur de R est supdrieuredans la hornblende. Les deux mindraux au facibs granulite font preuve de la plus grande diff6rence en R, tandis que dans les roches granitiques, ces min6raux en montrent moins Le taux d'oxydation de la biotite et de la hornblende dansun milieu 6pizonal est accompagn6d'une cristallisation de la magn6tite, et les mindraux mafiques oxydds ndoformds possbdentun rappoft Fe/(Fe + Mg) et un R plus faibles que les mindraux originaux. Dans un milieu m6sozonal ou catazonal, l'augmentation de la pression enffave la formation de la magndtite, et I'oxydation est accompagnded'une augmentation sensible de R; h ces changementsde la composition chimique de la homblende s'ajoute une augmentation dans sa teneur en Al La corrdlation entre la teneur en Al de la hornblende et la pression dtant bien 6tablie, une telle corr6lation de R et de la teneur en Fe3* avec la teneur en aluminium fait penser qu'il devrait y avoir une augmentation dans ces parambtresavec la pression'

(Traduit par la Rddaction)

Mots-cl6s'. biotite, hornblende, degrd d'oxydation, roches granitiques, roches m6tamorphiques, plutons dpizonaux, plutons mdsozonaux.Dlutons catazonaux.

INrnooucrroN stance,the differencesbetween tholeiitic and calc-alka- line series, as was found long ago, depend on P(HzO) The distribution of Fe3+and Fe2+among minerals of andflOz). Accordingly, the rocks of both seriesare dis- igneous rocks is the key to solving such petrological tinguished by the behavior of iron. The different parti- problems as the redox stateof the Earth's crust and the tioning of Fe among silicates and oxides in granitic upper mantle (e.9., Canrl & O'Neil 1996, Wones & rocks is controlled by f(O) and explains many pecu- Gilbert 1982, Speer 1984), and the characterizationof liarities of I- and S-type granites (Chappell & White magmatic series in terms of their composition and de- 1974), the distinction of granites of the magnetite and gree of evolution (Lalonde & Bemard 1993). For in- ilmenite series(Ishihara 1977), and granitic rocks of the

r E-mail address: serfer@online ural.ru

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magnetite-freeand magnetite-bearingfacies (Fershtater the world have been used in this work. Included are et al. 19'18,Fershtater 1987). minerals from granitic rocks from the following mas- The aim of this paper is to document the main fea- sifs: tures of Fe3+and Fe2* distribution between coexisting (1) High-level (epizonal) massifs: Magnitogorsk, biotite and hornblende in granitic and metamorphic Karabulak, Khabarninsk (the Urals), Guadelupe, Cali- rocks in order to assessthe possibilities for geobarom- fornia (Best & Mercy 1967), Aregos, northern Portugal etry and estimatesof oxygen fugacity. (de Albuquerque 7973, 1974), Ben Nevis, Scotland, U.K. (Haslam 1968), and the early series of New En- Bacrcnolwo INnorurarroN gland batholiths, Australia (Wilkinson et al. 1964). (2) Middle-level (mesozonal) massifs: Stepninsk, In granitic rocks of the magnetite facies, more than Novoburanovsk,Krasninsk, Vladimirsk, Gorodischensk 80Voof the Fe is concentratedin magnetite, whereas in (the Urals), Sierra Nevada (Dodge et al. 1968,1969), the magnetite-free facies, the mafic silicates contain all Southern Califomia (Larsen & Draisin 1950), granite of the Fe. Accordingly, homblende and biotite from bodies near the SanAndreas fault (Dodge & Ross 1971r. granitic rocks of the magnetite facies have a relatively Adamants, British Columbia (Fox 1969), and Kitakami, low Fe/(Fe + Mg) value, which is constantand indepen- Japan (Kanisawa 1972). dent of rock composition. In contrast, in rocks of the (3) Deep-level (catazonal)massifs and surrounding magnetite-free facies, this value in biotite and horn- gneissesand amphibolites: Mursinsk, Verkhisetsk (the blende is higher, and increases with increasing SiO2 Urals), Ukrainian and Aldan shields (Kalinichenko er content. Compositions of mafic minerals of the magnet- al.1988, Siroshtanet al.1965, Kitsul el al. 1983),the ite-free facies thus are similar to primary ones,whereas charnockitic rocks of Kondapalli, India (Leelanandam in rocks of the magnetite facies, theseminerals form as 1970), and amphibolites and ganulites from Westem a result of an oxidation stage of rock evolution, being Australia (Stephenson1977). accompanied by "exsolution" in the magnetite. The In order to get some idea ofhow much Fe2+and Fe3* main reaction that connectsthe mineral assemblagesof is present in coexisting biotite and hornblende, we had magnetite-free and magnetite facies is as follows: to consider results of wet-chemical analysesprovided in the "pre-microprobe" literature. Becauseof this, we Fe-rich mafic mineral * o'2- Mg-rich relied predominantly on older results. The composition mafic mineral + Mgt (1) of the Ural minerals was determined by Mcissbauer spectroscopy and wet-chemical analyses, in cases in in which the mafic mineral is amphibole or biotite. combination with X-ray-fluorescence analysis. The The left-hand side of this reiction represents the amounts and ratio of Fe3*and Fe2* determined by wet- minerals from magnetite-free facies, wherias the right chemical analyses and by Mcissbauerspectroscopy side representsthe associationsof the magnetite facies. (Votyakov et al. 1998) are in good agreement. As is well known, the TiO2 content of magnetite in granites is low (usually l-3%o TiO), and corresponds FBarunes oF Fe3+ Ixconronerrou N BrorrrE to temperaturesof 400'-600oC, according to the mag- eNo HonNsLeNrDEFRoM Gnalrrrc Rocrs: netite-ilmenite geothermometer (Ghiorso & Sack CoNsrnerNrs oN Exrsrnc TnBonprrcar- 1991). Such temperaturescorrespond to a postmagmatic ANDExPERTMENTaT- Deta stage and show that reaction (1) predominantly takes place in the solid state. The biotite under consideration, in terms of the co- The ratio R [Fe3+/(Fe2++Fe3*)] in coexisting biotite ordinates determining its nomenclaturc (Rieder et al. and hornblende from granitic and metamorphic rocks 1998), has a ratio 100Fe/(Fe+ Mg) in the range 20-60, depends on the physical and chemical conditions of and the proportion of IVAI is in the range 1.0-1.35 at- equilibration at the magmatic and postmagmatic stages, oms per formula unit (apfu). The Fe3* ion in this min- as well as on the crystallochemical featuresof the mineral is known to be largely in octahedral coordination, erals. Determining the relative influence of these fac- with rare exceptionswhere Si and Al abundancesin the tors is important to reconstruct the conditions of rock are insufficient to fill the tetrahedralsites in biotite formation of the rocks chosenfor a study of the compo- and thus requir" Ivp"3+ (Lalonde et al. 1996,Dawson & sitions of the coexisting biotite and hornblende. Smith 1977, Delaney et al. 1980\. Fe3+forms by rhe dehydrogenation reaction (Wones & Gilbert 1982, Savpr-Bsexo Mprnoos Guidotti 1984, Tikhomirova et al. 1989):

In addition to our own compositional data for 30 Fe2* + OH = Fe3*+ 02- + 1/zIJ.z Q) pairs of coexisting hornblendeand biotite in granitic and metamorphic rocks from the Uralian Paleozoic orogen According to results of a Mrissbauer study of biotite (Votyakov et al. 1998),previously published results for fro-mthe Ural granitic rocks (Votyakov et al. 1994),the approximately 100 pairs of theseminerals from all over FeJ*content increasespredominantlv owins to the oxi-

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dation of Fe2+ions in cls positions in the mineral struc- vIFe3*),in which casethe excesscharge due to the Fe3* ture. In this case,the excesscharge of Fe3* is compen- ions is compensatedby the incorporation of alkalis and satedby an H* vacancy near the Fer+in accordancewith IvAl. reaction (2). Alternatively, Fe3* ions may follow vIAl ions according to the following "ferritschermakitic" sub- TnB DrsrnrsurroN oF Fe3+AND Fe2+ stitution (Dymek 1983): IN COEXISTD{GBTOITE AND HORNBLENDE

vIFe2++ IvSi - vIFe3++ IVAI (3) The most impoftant geochemical feature of Fe ions in coexisting hornblende and biotite is the higher de- A positive correlation between the Fe3+and IvAl or gree of oxidation of Fe in hornblende than in biotite, IvAyvIAl should be observedin the latter case.There is regardlessof the conditions of formation of the granitic no such correlation in the biotite samplesunder consid- rocks and gneisses(Borodina & Votyakov 1996). The eration, however. which confirms the view of Guidotti (1984) regarding the absenceof a substitution in biotite according to mechanism (3), since there is sufficient vIAl to compensatefor "excess" IVAI. In hornblende, Fer+ occurs in octahedral coordination also. The dehydrogenation reaction (2) results in an oxyamphibole component; it is a well-known and experimentally confirmed mechanism of oxidation in amphibole (Clowe et al. 1988, Phillips et al. 1988). As shown for the hornblende samples from the Ural granitic rocks (Votyakov et al. 1998), Fer+ content rises predominantlyowing to the oxidation of Fe2* ions in Ml positions according to reaction (2); in this case,the excesscharge is compensatedby vacancy at the H site, r.e.,O-for-OH substitution. 0"2 .l Given the diversity of mineral compositions, a num- lo* ber of charge-compensationmechanisms have been pro- posed in the past (Hawthome 1981). In addition to the mechanism mentioned, from the theoretical point of view, substitution involving Al also may occur lClowe et al. 1988\, in one of two variants: (i) a tschermakitic type, in which Fe3* ions, as well as vIAl, replace divalent cations, compensatedby the Al-for-Si substitution in the tetrahedralposition, and (ii) a glaucophanetype, where the replacementof the divalent cation by Fe3* or Al is compensatedby the Na-for-Ca substitution. R.K. Popp and his coworkers (Clowe et al. 7988, Phillips et al. 1988, 1989, Popp et al. 1994) have docu- mented the mechanism of Al incorporation along with the formation of an oxyamphibole component for cal- cic and subcalcic amphiboles in experimentson amphibole oxidation under different buffers. They concluded that the extent to which each of the two mechanisms 0 determinesthe Fe3* content of any natural amphibole is " 08 1.2 1.6 2 unknown, but both processesare involved in natural [. r, 3" 4' tll samples. In our collection of hornblende compositions, the - - IVA1 occulrence of the above-mentioned Al incorporation ftc. 1. Fe3* (Na + K) and Fe3+ diagrams for horn- (1-3) (4, (Clowe et al. 1988) is confirmed by the correlation of blende from granitic and metamorphic rocks 5) l. Rocks of high-level (epizonal) granitic plutons (field I), Fe3+and IvAl, as well as by the correlation of Fe3* and belonging to the magnetite facies. 2. Rocks of middle-level the sum of alkalis in all types of rocks except charnockites (mesozonal)granitic plutons, transitional between magnet- and granulites (Fig. 1). ite and magnetite-free facies (field II). 3. Rocks of deep- Thus, the available data make it possible to suggest level (catazonal) granitic plutons, belonging to magnetite- that dehydrogenationis the main mechanism of oxida- free facies (field III). 4. Migmatites and metamorphic rocks tion of iron in biotite, whereasin amphibole,it is supple- of amphibolite grade (field III). 5. Metamorphic rocks of mented by the heterovalent substitution vIFe2* - granulite grade (field IV).

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relation of Rs1to Rs51is shown in Figure 2; almost all Epizonal granitic rocks mainly belong to the mag- the points lie in the field indicative of the higher degree netite facies. Magnetite appearsas a result of oxidation of oxidation of hornblende.The diagram can be divided of primary hornblende and biotite at 400-600"C in the into three fields. Field 1 consists mainly of minerals solid state.The maf,rcsilicates from such granitic rocks from epizonal and mesozonal granitic plutons, field 2 have lower values of Fe/(Fe + Mg) and Fe3*/(Fe2** includes predominantly minerals from metamorphic Fe3*) in comparison with catazonaland mesozonalgra- rocks of the amphibolite facies and associatedcatazonal nitic plutons formed under conditions of the magnetite- granitic plutons, and field 3 contains mostly metamor- free facies (Figs. 2, 3). Magnetite does not form during phic rocks of the granulite facies. In fields 1 and2, a cooling and oxidation of hornblende and biotite from correlation is observedbetween the degreeofFe oxida- theserocks; the R of the silicates increases,whereas the tion R in coexisting homblende and biotite. The differ- Fe/(Fe + Mg) value remains at the same level, imposed ence between Rssyand Rs, is insignificant in field 1, during magmatic crystallization. much higher in field 2, and reachesa maximum in field 3, where biotite has a minimal degree of oxidation. T-P-/(O2) CoNorrIor,{soF BIorIrE Hornblende samplesfrom different geological units ANDHoRNBLENoe OXIDATTON differ in their Na, K and Al contents, which is of significance for the state of Fe in hornblende. Homblende We have summarized the available experimental in epizonal granitic plutons contains the lowest amount data on biotite and homblende stability, and shown the of alkalis, IVAI, vIAl and Fe3+,and has the lowest de- relationships between coexisting minerals in/(O2) - T gree of Fe oxidation (Figs. 1, 2). Hornblende in coordinates (Fig. 4); not all data are known with the catazonalgranitic plutons, derived from H2O-rich mag- samedegree of certainty. The stability fields of the min- mas, is characterizedby the highest values of thesepa- erals in as a function of Fe/(Fe + Mg) are shown on the rameters,whereas hornblende from mesozonalgranitrc basis of experimental data. The values of hornblende plutons occupies an intermediate position. oxidation are calculated using the data of Popp et al. Hornblende from metamorphic rocks of the am- (1994), whereas the same values for biotite are based phibolite facies has a variable composition; however, it on the trends of "buffered" comoositions of biotite lies mainly in the samefield as hornblende in catazonal trends in the Fe3+- Fe2+- Mg system(Wones & Eugster granitic plutons. Hornblendein granulitesand charnock- 1965),which the authorsthemselves called an "educated ites stands out in having the lowest R values, whereas guess".These assumptionsare not consistentwith some its trend on the NAI - (Na + K) diagram is similar to experimental work (Tikhomirova et al. 1989), possibly that ofthe other hornblende samples (Fig. l). becausethe latter did not take the presenceor absence of magnetite into account, but they are confirmed by

Fe/(Fe+Mg)u,

.:ti: .-l '!rt :i ':'.;t.it ,{ '"

0.25 0.50 Fe/(Fe+Mg) "n ii; t;"'d;';;''5i 0.2 0.5 0.8 f . ,, 3" 4lL_:l Frc. 2. Values of Fe3+/(Fe2*+ Fe3*.;in coexisting homblende and biotite in granitic and metamorphic rocks. Groups 1-5 are the same as in Figure 1. The shadedfields correspond Ftc. 3 Values of Fe/(Fe + Mg) in coexisting homblende and to the minerals of epizonal and mesozonal granitic plutons biotite in granitic and metamorphic rocks. Groups l-5 are (I), catazonalgranitic rocks and related metamorphic rocks the same as in Figure 1. Shadedfields representthe miner- of amphibolite grade (II), and metamorphic rocks ofgranu- als from epizonal (I), mesozonal (II) and catazonal (III) lite grade (III). granitic plutons.

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magnetite, and so Rs51and Rs1as well as the Fe/(Fe + Mg) value in these casesis relatively low. The adjust- ment can be illustrated for the / site ofhornblende by the reaction:

(Fe3*o5Fe2*2 5M E) + 0.2O2= (Fe3*63Fe2*2 aMg2no :) + 0.1(Fe3+2Fe2*o4) (4), kl a)t ?t'20 where n representsa vacancy. 1 As Fe3*, Fe2* and Mg cations occupy an octahedral position in which their sum equals 5, their deficit in the right part of the equation is adjustedaccording to one of the above-mentionedmechanisms of heterovalent sub-

I stitution. As is apparentfrom reaction (4), newly formed j_**-_l--" I * i hornblende coexisting with low-Ti, low-temperature 8@ l@o Toc magnetite is characterizedby a lower degree of oxida- FIc. 4 Plot ofl(Oz) verszs T showing coexisting homblende tion in comparison with primary hornblende (since the and biotite from granitic and metamorphic rocks. Thin lines Fe3'/Fe value of magnetite, 0.7, is always higher than representFe/(Fe + Mg) values (solid lines for Bt, dashed that of amphibole) and by a lower Fe/(Fe + Mg). lines for Hbl); thick lines of the same types correspond to The mesozonal and catazonal granitic rocks belong Fe3n/(Fe2*+ Fe3+)values of Bt and Hbl (Wones & Eugster to the magnetite-free facies. The increase in H2O pres- 1965, Popp et al. 197'7, 1994). The figures near the sym- sure expands the fields of stability of hornblende and bols of minerals representthe Fe/(Fe + Mg) values,whereas biotite in theserocks; in the caseof their oxidation, the Fe3+l(Fe2++ Fe3+)values are shown in italics near the cor- P(HzO) prevents the formation of magnetite responding lines Fe/(Fe + Mg) lines fix the upper limit of rise of hornblende and biotite stability for a given Fe/(Fe + Mg) (Fershtateret al. 1918). value. Note the lower level of hornblende stability in com- The Fe/(Fe + Mg) and R values of hornblende and parison with biotite Hornblende with Fe/(Fe + Mg) = 0.S biotite from theserocks are higher than in the epizonal starts to oxidize at -log/(Oz) between 24 and,23,whereas plutons. These values increase gradually, passing from biotite with the same Fe/(Fe + Mg) value is stable up to low-P(HzO) conditions in the epizonalplutons to higher- -logflO2) = 15. The Fe3*/(Fe2*+ Fe3*; Iines for biotite P(H2O) conditions in the mesozonaland catazonalplu- were estimatedaccording to Wones & Eugster (1965), and tons (Figs. 2,3, 5). In the case of granulite-facies the same lines for hornblende were estimated according to metamorphic rocks, where a small amount of HzO is Popp et al. (1994). Short-dashedlines labeled as MtH and present, the data points of homblende lie off the main FMQ represent the magnetite - hematite and fayalite - the magnetite - quartz equilibria. The shaded area is the field trend (freld IV in Fig. 5). Their position confirms of the magnetite facies, and the pattemed areais the fie1dof important role of P(H2O) conditions in the oxidation of primary crystallization of titanian magnetite. hornblende. The well-known fact that the Al content in hornblende correlates with pressure at the time of its formation is the basis of some effective geobarometers the data on natural biotite (Speer 1984) and the results (Hammarstrom &Zen1986, Fershtater1990). Fer* con- of the present study. In pafticular, as Figure 4 shows, tents and R values in turn are directly proportional to Al - there is a large range of f(O) T values over which the contents, although these parametersvary significantly degree of oxidation of the iron in hornblende must be within every pressurezone (Fig. 5). The rise ofpressure greater than that in biotite. An inverse ratio is possible thus is recorded not only in the Al content but also rn only in the low-temperature region. In spite of the the Fe3+content and redox state of the homblende; all relative temperature and f(O) values inferred from the these parameters may serve as indicators of the pres- diagram, these help to qualitatively evaluate mineral- sure of crystallization and can be used for a semi- equilibrium conditions. quantitative determination of pressure[mostly P(H2O)] The diagram also shows that the initial Fer+/Fe value during the oxidation of hornblende. of the hornblende and biotite must be very low. The compositions of magnetite and the mafic silicates such AcxNowr,epcaunN'ts as biotite and hornblende in granitic rocks are consid- ered to have equilibrated in the solid state after solidifi- We are indebted to Drs. Andrd E. Lalonde, Univer- cation of magmatic rocks. The composition of the newly sity of Ottawa, and Catherine McCammon, University formed silicates depends on the fugacity ofoxygen. The of Bayreuth, for their helpful reviews of the manuscript, epizonal plutons belong to the magnetite facies; the and to Robert F. Martin for his valuable comments and oxidation of both mafic silicates in these plutons at a suggestions.This work was supported by the Russian low pressure is usually accompanied by precipitation of FundamentalScience Foundation (grant 9845-64826).

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