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Armalcolite in Crustal Paragneiss Xenoliths, Central Mexico

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Armalcolite in Crustal Paragneiss Xenoliths, Central Mexico American Mineralogist, Volume 80, pages 810-822, 1995 Armalcolite in crustal paragneiss xenoliths, central Mexico JODIE L. HAYOB, * ERIC J. ESSENE Departmentof GeologicalSciences,Universityof Michigan,Ann Arbor,Michigan48109,U.S.A. ABSTRACT Aluminous armalcolite has been found in two sillimanite-bearing xenoliths that were recently exhumed from the lower crust of central Mexico. The ranges of compositions are (Fe5.1s-0.66Mgo.lS-0.2SAlo.lS-0.lS V6.66-o.lOFe6.6o-o.o6 Titt4-l.s6)Os and (Fefi1o-o.S1 MgO.lS-0.29Alo.16-0.19- V6.62-0.06Fe6.1o-o.s1Ti1.19-1.71)Os.The occurrence of armalcolite is unusual in crustal para- gneisses because most terrestrial armalcolite occurs in volcanic rocks that are derived from partial melting in the Earth's mantle. Textures suggest that armalcolite is a late product formed by the reaction rutile + ilmenitess = armalcolitess during rapid transport of the xenoliths to the surface. Phase equilibria in the system MgO-FeO-Fe203- Ti02, which indicate that armalcolite is stable in the crust at 900-1200 °C, are consistent with this interpretation. Thermodynamic properties are estimated for oxides in the system MgO-FeO-Fe203- Ti02 to constrain activity-composition relations for armalcolite and conditions of for- mation. Activity coefficients calculated for armalcolite range from 0.27 to 1.36, depending on the ilmenite model used, at temperatures between 1000 and 1300 °C. Depth of for- mation of armalcolite in the crust is not well constrained. Thermodynamic calculations at 800-1200°C for the compositions observed indicate that the armalcolite in one xenolith would have been in equilibrium with rutile at values of 102 between the hematite + magnetite buffer (HM) and the fayalite + magnetite + quartz (FMQ) buffer, and that armalcolite in the other xenolith would have been in equilibrium with rutile and ilmenite at values of 102 between FMQ and two log units below the FMQ buffer. INTRODUCTION (1988) definitions of armalcolite and pseudobrookite have Armalcolite (Feo.sM~.s Ti20s) has been observed most been accepted by the International Mineralogical Asso- commonly in lunar rocks that equilibrated under reduc- ciation, the definitions appear to violate the currently ac- ing conditions, although occurrences of armalcolite in ter- cepted procedure for symmetrical subdivisions of ternary restrial rocks have been reported by von Knorring and composition space (Nickel, 1992). A bizarre result of Cox (1961), Ottemann and Frenzel (1965), Cameron and Bowles's definitions is that pure Fe2+Ti20s can be re- Cameron (1973), Haggerty (1975), Velde (1975), El Go- garded as either pseudobrookite or armalcolite (Fig. 1). resy and Chao (1976), Tsymbal et ai. (1980), Pedersen Nonetheless, the authors will provisionally apply the def- (1981), and Mets et ai. (1985). Armalcolite was discov- initions of Bowles for solid solutions in the pseudobrook- ered in two crustal xenoliths of paragneiss from central ite group until the IMA reevaluates this system. Pseu- Mexico (cf. Hayob et aI., 1989). The Mexican occurrence dobrookite will be used as a group name (sensu lato) as is unusual because the majority of terrestrial, armalcolite- well as for a specific compositional range (sensu stricto). type minerals reported thus far have been found in vol- Available crystal-chemical data indicate that pseudo- canic rocks. brookite and armalcolite are entropy stabilized and there- fore are expected to form at high temperatures (Navrot- CRYSTAL CHEMISTRY OF ARMALCOLITE sky, 1975). Pseudo brookite and armalcolite have two crystallographically distinct octahedral sites, M 1 (Wyck- Armalcolite (M~.sFe5.1Ti20s) forms a complete solid solution series between an Fe end-member (Fe2+Ti20s) off notation 4c) and M2 (Wyckoff notation 8f) (Smyth, 1974). Analogous to an inverse spinel structure, Fe3+ in and a Mg end-member (MgTi20s) at high temperatures (Akimoto et aI., 1957; Bowles, 1988). Armalcolite is iso- ordered pseudobrookite (Fe2TiOs) is distributed equally structural with pseudobrookite (Psb: Fe~+TiOs), which is between M 1 and M2, and Ti4+ is incorporated into M2 (Lind and Housley, 1972; Brigatti et aI., 1993). In ordered widespread in terrestrial volcanic rocks. Whereas Bowles's armalcolite, Mg2+ and Fe2+ are incorporated into Ml, and Ti4+ is incorporated into M2 (Lind and Housley, 1972; Smyth, 1974; Wechsler et aI., 1976; Brown and * Present address: Department of Environmental Science and Geology, Mary Washington College, Fredericksburg, Virginia Navrotsky, 1989; Wechsler and von Dreele, 1989). There 22401, U.S.A. is, however, substantial cation disorder in most natural 0003-004X/95/0708-0810$02.00 810 HAYOB AND ESSENE: ARMALCOLITE IN CRUSTAL XENOLITHS 811 25 Fe 2TiO 5 o Terrestrial ... Other Xenolith Localities . This Study - ETll . This Study - ET42 + Lunar o 00 P (kbar) ill °0.° .. 1000 1100 1200 1300 1400 1500 + T (0 C) Fig. 2. P- T diagram showing loci of Rt + Gk = Mg-Arm and Rt + 11m = Fe-Arm calculated from thermodynamic data in Tables 4 and 5 (solid curves). Loci of Rt + Gk = Mg-Arm calculated from thermodynamic data of Chase et al. (1985, long dashes) and Knacke et al. (1991, short dashes) are also shown. Arrows denote experimental reversals of Lindsley et al. (1974), and the solid circle is from experiments of Haggerty and Lindsley (1969). Armalcolite (M&>.sFeo.sTi20s) breaks down to rutile + ilmenitess at 1010 °C at 1 bar (not shown; Haggerty and Lindsley, 1969). Rt = rutile, Gk = geikielite, 11m = ilmenite, Fe-Arm = FeTi20s, and Mg-Arm = MgTi20s. Fig. 1. Ternary composition diagrams in the systems Fe2TiOs- FeTi20s-MgTi20s and MgTi20s-FeTi20s-Ti30s showing com- positions of pseudobrookite and armalcolite, excluding those with (Akimoto et aI., 1957; Haggerty and Lindsley, 1969; >5 mol% MnTi20s, CaTi20s, FeZr20s, and Cr2TiOs and> 10 Hartzman and Lindsley, 1973; Lindsley et aI., 1974; Friel mol% Al2TiOs. Terrestrial pseudobrookite: Doss (1892), Traube et aI., 1977) and thermodynamic calculations (Navrot- (1892), Palache (1935), Agrell and Langley (1958), Ottemann sky, 1975; Anovitz et aI., 1985) indicate that FeTi20s is and Frenzel (1965), Lufkin (1976), Van Kooten (1980), Lorand stable only at relatively low pressures and high temper- and Cottin (1987), Stormer and Zhu (1994), and this study atures (Fig. 2). Armalcolite occurs in lunar basalts and (ET42). Terrestrial armalcolite: Schaller (1912), von Knorring terrestrial volcanic rocks, consistent with an origin at low and Cox (1961), Ottemann and Frenzel (1965), Rice et al. (1971), Velde (1975), El Goresy and Chao (1976), Pedersen (1981), pressure and high temperature. Substitution of AP+, Cr3+, Tsymbal et al. (1982), Lorand and Cottin (1987), and this study and TP+ stabilizes armalcolite to lower temperature (ETll). Other xenolith localities: Cameron and Cameron (1973) (Kesson and Lindsley, 1975), whereas addition of Zr4+ and Haggerty (1975,1983). Lunar armalcolite: Agrell et al. (1970), appears to restrict armalcolite to higher temperature (Friel Anderson et al. (1970), Haggerty et al. (1970), Kushiro and Nak- et aI., 1977). amura (1970), Brett et al. (1973), Haggerty (1973a, 1973b, 1973c, Figure 1 is a diagram showing observed compositions 1978), El Goresy et al. (1973, 1974), Papike et al. (1974), Smyth for the system Fe2TiOs-MgTi20s-FeTi20s- Ti30s com- (1974), Steele (1974), Williams and Taylor (1974), Wechsler et piled for natural samples of pseudobrookite and armal- al. (1976), Cameron (1978), and Stanin and Taylor (1980). Arm colite, excluding those with > 5 molOjo MnTi20s, Ca- armalcolite, Psb pseudobrookite. = = Ti20s, FeZr20s, and Cr2TiOs and > 10 molOjo Al2TiOs (Smith, 1965; Levy et aI., 1972; Peckett et aI., 1972; Reid and synthetic pseudobrookite and armalcolite (Lind and et aI., 1973; Tsymbal et aI., 1980; Mets et aI., 1985; Var- Housley, 1972; Grey and Ward, 1973; Virgo and Hug- lamov et aI., 1993). Experimental results of Friel et ai. gins, 1975; Wechsler, 1977; Tiedemann and Miiller- (1977) suggest that CaO may not be incorporated into Buschbaum, 1982; Wechsler and Navrotsky, 1984; Brown pseudobrookite or armalcolite, and that Zr02 has a sat- and Navrotsky, 1989; Wechsler and yon Dreele, 1989; uration limit in armalcolite of approximately 4 wt°jo at Brigatti et aI., 1993). 1200-1300 °C and 1 atm, so it is uncertain whether so- called armalcolite with high CaO and Zr02 contents (Smith, 1965; Levy et aI., 1972; Peckett et aI., 1972; Reid PREVIOUS STUDIES AND GEOLOGIC SETTING et aI., 1973) is really armalcolite. Compositions of lunar Armalcolite was first identified in lunar samples from armalcolite plot near the MgTi20s- FeTi20s binary or at Apollo 11 (Anderson et aI., 1970). Experimental data more reducing conditions in the Ti30s field. Terrestrial -"'-,,---- -- 812 HAYOB AND ESSENE: ARMALCOLITE IN CRUSTAL XENOLITHS TABLE1. Representative electron microprobe analyses of rutile Greenland (Pederson, 1981). Armalcolite that is associ- associated with armalcolite ated with native iron in trachybasalts from the Ukraine ET11 ET11* ET42* ET42 was also discovered by Tsymbal et aI. (1982). Lorand and wt% oxide Pt. 29 pt. 30 Pt. 29 Pt. 2 Cottin (1987) discovered armalcolite in ultrabasic cu- Zr02 ** 1.58 1.84 1.79 1.37 mulates from the Laouni layered intrusion in Algeria; Si02 0.02 0.01 0.00 0.00 their armalcolite analyses are similar to Velde's (1975) Ti02 96.63 96.70 95.66 96.63 analyses of armalcolite from lamproites in Montana. AI203 0.13 0.12 0.21 0.04 Cr203 0.07 0.02 0.14 0.10 Armalcolite was observed in two paragneiss xenoliths V203 0.50 0.49 0.49 0.71 (samples ETl1 and ET42 of Hayob et aI., 1989) from a Fe203 0.34 0.68 1.18 1.30 MgO 0.01 0.00 0.00 0.00 Quaternary cinder cone, El Toro, located in the Central MnO 0.00 0.01 0.04 0.00 Mexican Plateau (CMP) near the city of San Luis Potosi. CaO 0.00 0.02 0.01 0.02 El Toro is one of several volcanic centers in the CMP H2O 0.12 0.15 0.25 0.25 Total 99.40 100.04 99.77 100.42 that contain xenoliths from the deep crust and upper Formulae normalized to 1 cation mantle (c( Aranda-Gomez and Ortega-Gutierrez, 1987; Zr 0.010 0.012 0.012 0.009 Hayob et aI., 1989).
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