Armalcolite in Crustal Paragneiss Xenoliths, Central Mexico Joorr L

American Mineralogist, Volume 80, pages810-822, 1995

Armalcolite in crustal paragneiss xenoliths, central Mexico

Joorr L. Havor,* Enrc J. EssnNn Department of Geological Sciences,University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

Ansrn-c,cr Aluminous armalcolite has been found in two sillimanite-bearing xenoliths that were recently exhumed from the lower crust of central Mexico. The rangesof compositions are (Fefr.jr_ouuMgo,r_orrAlo,r_o,rV3tu_o,oFefr.fio-oouTif.t_,r.)O,and (Fefr.jo_oilMgo,r_orrAlo16_0re- Vfr.fir-oouFefi.]o-ouTit.lr-,r,)Or.The occurrenceof armalcolite is unusual in crustal para- gneissesbecause most terrestrial armalcolite occursin volcanic rocks that are derived from partial melting in the Earth's mantle. Textures suggestthat armalcolite is a late product formed by the reaction rutile + ilmenite"" : armalcolite., during rapid transport of the xenoliths to the surface. Phase equilibria in the system MgO-FeO-FerO.-TiOr, which indicate that armalcolite is stable in the crust at 900-1200 oC, are consistent with this interpretation. Thermodynamic properties are estimated for oxides in the system MgO-FeO-FerOr- TiO, to constrain activity-composition relations for armalcolite and conditions of formation. Activity coefrcients calculatedfor armalcolite range from 0.27 to 1.36,depending on the ilmenite model used, at temperaturesbetween 1000 and 1300 "C. Depth of formation of armalcolite in the crust is not well constrained.Thermodynamic calculationsat 800-1200 "C for the compositions observedindicate that the armalcolite in one xenolith would have been in equilibrium with rutile at values of /o, between the hematite + magnetite buffer (HM) and the fayalite + magnetite + quarlz (FMQ) buffer, and that armalcolite in the other xenolith would have been in equilibrium with rutile and ilmenite at values of /o, between FMQ and two log units below the FMe buffer.

IxrnonucrroN (1988) definitions of armalcolite and pseudobrookitehave Armalcolite (FeorMg rTirOr) 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 occurrencesof armalcolite in ter- cepted procedurefor 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), Cameroi and Bowles'sdefinitions is that pure Fe2*TirO, can be re- Cameron (1973),Haggerty (1975), Velde (1975),El Go- garded as either pseudobrookiteor armalcolite(Fig. l). resy and Chao (1976[ Tsymbalet al. (1980),pedersen Nonetheless,theauthorswillprovisionallyapplythedef- (1981), and Mets et al. (1985).Armalcolite was discov- initions of Bowlesfor solid solutionsin the pseudobrook- ered in two crustal xenoliths of paragneissfrom central iJe erogg until the IMA reevaluatesthis system. Pseu- Mexico (cf. Hayob et al., 1989).The Mexican occurrence dobrookite will be used as a group name (sensulato) as is unusual becausethe 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 therefore are expectedto form at high temperatures(Navrot- Cnvsr.lr, cHEMrsrRy oF ARMAr,coLrrE sky, 1975). Pseudobrookite and armalcolite have two Armalcolite (MgorFefrjTi,Or) forms a complete solid crystallographicallydistinct octahedral sites, Ml (Wyck- solution seriesbetween an Fe end-member(Fer+Ti,O.) offnotation 4c) and M2 (Wyckoffnotation 8/) (Smyth, and a Mg end-member(MgTirOr) at high te-pe.aturei 1974).Analogous to an inversespinel structure,Fe3+ in (Akimoto et aI., 1957;Bowles,1988). Armalcolite is iso- ordered Pszudobrookite(FerTiO') is distributed equally structural with pseudobrookite(Psb: Fel*TiOr), which is bctween M-l and M2, and,Ti4* is incorporated into M2 widespreadin tenestrial volcanic rocks. WhereasBowles's (Lind and Housley, 1972;Bigatti et al., 1993).In ordered armalcolite, Mg2* and Fe2+ are incorporated into Ml, and Tio* is incorporated into M2 (Lind and Housley, * Presentaddress: Department of EnvironmentalScience and 1972; Smyth, 1974; Wechsler et aI., 1976; Brown and Geology,Mary WashingtonCollege, Fredericksburg, Virginia Navrotsky, 1989;WechslerandvonDreele, 1989).There 224ol,U-S-A- is, however, substantialcation disorder in most natural 0003-004x/95/0708-08I 0$02.00 810 HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS 811

Fe 2TiO s

o Ter6trial Other Xflolith Localities This Study - ETI I This Study - ET42 + Lunar ,{;--//- tt P (kbar) -""""" ,/

MgTi20 s FeTi2O5

0L 900 1200 1300 r (oc) Fig. 2. P-T diagramshowing loci of Rt + Gk : Mg-Arm and Rt + Ilm : Fe-Armcalculated from thermodynamicdata in Tables4 and 5 (solidcurves). Loci of Rt + Gk : Mg-Arm calculatedfrom thermodynamicdata of Chaseet al. (1985,long dashes)and Knacke et al. (1991,short dashes) are also shown. Arrowsdenote experimental reversals of Lindsleyet al. (1974)' andthe solid circle is from experimentsof Haggertyand Lindsley (1969).Armalcolite (MgorFeo5TirO5) breaks down to rutile + ilmenite,,at l 010"C at I bar(not shown;Haggerty and Lindsley, : : : : Tisos 1969).Rt rutile, Gk geikielite,Ilm ilmenite,Fe-Arm FeTirOr,and Mg-Arm: MgTiO'. Fig. 1. Temary composition diagramsin the systemsFe'TiO'- FeTirOr-MgTi'O, and MgTi'O'-FeTirO5-TirO5showing com- positionsofpseudobrookite and armalcolite,excluding those with (Akimoto et al., 1957 Haggerty and Lindsley, 1969; >5 molo/oMnTi,Or, CaTirOr, FeZtrOr, and CrrTiO, and >10 Hartzman and Lindsley,1973 Lindsleyet al., 1974;Friel molo/oAlrTiOr. Terrestrial pseudobrookite:Doss (1892),Traube et al., 1977\ and thermodynamic calculations (Navrot- (1892), Palache(1935), Agrell and Langley (1958), Ottemann sky, 1975;Anovitz et al., 1985)indicate that FeTirO' is and Frenzel(1965), Lulkin (1976),Van Kooten (1980),Lorand stable only at relatively low pressuresand high temper- and Cottin (1987), Stormer and Zhu (1994), and this study atures (Fig. 2). Armalcolite occurs in lunar basalts and (ET42'). (1912), von Knorring Terrestrial armalcolite: Schaller terrestrial volcanic rocks, consistentwith an origin at low (1961),Ottemann and Frenzel(1965), Rice et al. (1971), and Cox pressureand high temperature.Substitution of Al3*, Cr3*, Velde (1975), El Goresy and Chao (1976), Pedersen(1981), Ti3* stabilizes armalcolite to lower temperature Tsymbal et al. (1982),Lorand and Cottin (1987),and this studv and of Zt4+ (ETl l). Other xenolith localities: Cameron and Cameron (1973) (Kesson and Lindsley, 1975), whereas addition and Haggerry(1975, 1983).Lunar armalcolite:Agrell et al. (1970), appears to restrict armalcolite to higfuertemperature (Friel Anderson et al. (1970), Haggertyet at. (l 970), Kushiro and Nak- er aI., 1977). amura(1970), Brett et d. (1973),Haggerty (1973a,1973b,1973c, Figure I is a diagram showing observed compositions 1978),Ef Goresyet al. (1973,1974),Papike et al. (1974),Smyth for the system FerTiOr-MgTirOr-FeTirOr-TirO, com- (1974),Steele (1974), williams and Taylor (1974),Wechsler et piled for natural samples of pseudobrookite and armal- (1980). al. (1976),Cameron (1978), and Staninand Taylor Arm colite, excluding those with >5 molo/oMnTirOr, Ca- : : pseudobrookite. armalcolite, Psb TirOr, FeZrrOr, and CrrTiO, and > l0 mol0/oAlrTiOs (Smith,1965; Levy eIal.,1972;Peckett et al., 1972;Reid and synthetic pseudobrookite and armalcolite (Lind and et al., 1973;Tsymbal et al., 1980;Mets et al., 1985;Var- Housley, 1972; Grey and Ward, 1973; Virgo and Hug- lamov et al., 1993).Experimental results of Friel et al. gins, 1975; Wechsler, 1977; Tiedemann and Miiller- (1977) suggestthat CaO may not be incorporated into Buschbaum, 1982; Wechsler and Navrotsky, 1984; Brown pseudobrookite or armalcolite, and lhaIZrO' has a sat- and Navrotsky, 1989; Wechsler and von Dreele, 1989; uration limit in armalcolite of approximately 4 wto/oat Brigatti et al., 1993). 1200- 1300"C and I atm, so it is uncertainwhether so- called armalcolite with high CaO and ZtO, contents (Smith, 1965;Levy et al., 1972;Peckett et al., 1972;Reid Pnnrrrous sruDrEs AND GEor,ocrc SETTTNG et al., 1973)is really armalcolite.Compositions of lunar Armalcolite was first identified in lunar samples from armalcolite plot near the MgTi'Or-FeTirO' binary or at Apollo ll (Anderson et al., 1970). Experimental data rnore reducing conditions in the TirO, field' Terrestrial 8t2 HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS

TlaLE 1. Representativeelectron microprobe analyses of rutile Greenland(Pederson, l98l). Armalcolite that is associ- associated with armalcolite ated with native iron in trachybasaltsfrom the Ukraine ET11 ETl1- ET42" ET42 wasalso discovered by Tsymbal et al. (1982).Lorand and wt7" oxide Pt. 29 Pt. 30 Pt. 29 Pt.2 Cottin (1987) discoveredarmalcolite in ultrabasic cu- ZtO2t' 1.58 1.84 1.79 1.37 mulates from the Laouni layered intrusion in Algeria; sio, 0.02 0.01 0.00 0.00 their armalcolite analysesare similar to Velde's (1975) Tio, 96.63 96.70 95.66 96.63 Al,03 013 0.12 0.2'l 0.04 analysesof armalcolite from lamproites in Montana. Cr,O" 0.07 0.o2 0.14 0.10 Armalcolite was observed in two paragneissxenoliths V.o. 0.50 0.49 0.49 071 (samplesETI I and ET42 of Hayob et al., 1989)from FerO. 0.34 0.68 1.18 130 a Mgo 0.01 0.00 0.00 0.00 Quaternary cinder cone, El Toro, located in the Central MnO 0.00 001 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 volcanic HrO o.12 0.15 0.25 o.25 several centers in the CMp Total 99.40 100.04 99.77 100.42 that contain xenoliths from the deep crust and upper Formulae normalized to 1 cation mantle (cf. Aranda-G6mez and Ortega-Guti6nez, 1987; Zr 0.010 0.012 o.012 0.009 Hayob et al., 1989).The CMP is an elevatedregion of Si 0.000 0.000 0.000 0.000 high heat flow, bounded to the west by the Sierra Madre Ti4- o 977 0.973 0.967 0.969 AI 0.002 0.002 0.003 0.000 Occidental and to the east by the Sierra Madre Oriental. Cr 0.001 0.000 0.001 0.001 The western mountains are part of an extensive, mid- 0.005 0.005 0.005 0 008 Tertiary ignimbrite province, and the easternmountains Fe3* 0.003 0.007 0.012 0.013 Mg 0.000 0.000 0.000 0.000 are composedof Mesozoic sedimentsthat were deformed Mn 0.000 0.000 0.000 0.000 during the Laramide Orogeny. The younger Quaternary 0 000 0.000 0.000 0.000 volcanism erupted oHt 0.011 0.014 0.o2'l o.022 through the CMP and alluvial cover, - transporting xenoliths from the lower crust and upper Inclusionin garnet -. mantle to the surface. More Similar contents of Zr have been reported in rutile associated with detailed field relations are armalcoliteby Haggerty(1987). given elsewhere(Aranda-G6mez, I 982; Aranda-G6mez : f OH Al + Cr + V + Fe3*(e.9., Vlassopoutos et al , 1993)assuming and Ortega-Guti€nez,1987, Luhr et al., 1989). no substitutionof Fe,* or Ti3'.

Prcrnor,ocy armalcolite typically contains a significantamount of Fe3* The armalcolite-bearing xenoliths from El Toro con- (Fig. l). tain primary gamet + sillimanite + qvarlz + plagioclase Several terrestrial occurrencesof armalcolite and fer- * mesoperthite + rutile + graphite + ilmenite. Data for rous pseudobrookite are known (Fig. l). The substance most of these minerals have been reported previously iserite,reported from Janovskyby Schaller(1912), may (Hayob et al., 1989) and will be discussedonly briefly be armalcolite or FeTirOr, although Janovsky considered here. The mesoperthites consist of regular intergrowths it to be an intergrowth of rutile and Fe (wiistite or mag- of coarse(approximately 20 lrm) alkali feldspar and pla- netite?).Von Knorring and Cox (1961)described armal- gioclaselamellae that are unusually rich in ternary feld- colite-pseudobrookite solid solutions in the Karroo vol- sparcomponents (Hayob et al., I 989, 1990).On the basis canic rocks of southern Rhodesia that contain of reintegrated compositions of the mesoperthites,feld- approximately equal amounts of FerTiOr, FeTirOr, and spar thermometry (Fuhrman and Lindsley, 1988) indi- MgTirOr. Ottemann and Frenzel (1965) analyzedseveral cates that the peak of metamorphism was at T > 1025 pseudobrookite samples and an armalcolite from Ger- "C (ET42) and T > 1075 "C (ETll). Compositionsof many. Rice et al. (1971) reported an analysisof armal- coexisting host and lamellae indicate that the xenoliths colite that containsapproximately 80 molo/oFeTirO, from last equilibratedat about 890 eC(ET42) and 880 "C (ETl l) a lamprophyre dike in New Hampshire. Armalcolite was (Fuhrman and Lindsley, 1988).The garnet + sillimanite also found by Cameron and Cameron (1973) in ultra- + q\artz * plagioclase barometer (GASP, Koziol and mafic nodules from Knippa Quarry, Texas, and by Hag- Newton, I 988)yields pressures of 10.0 + I .0 kbar at 880 -r gerty (1975, 1983) from two South African kimberlite "C (ETl l) and 8.9 1.0kbar at 890 "C (8T42) assuming localities; armalcolite from the Dutoitspan kimberlite the garnet is in equilibrium with the exsolvedplagioclase. contains approximately 15 molo/o Ti.Or. Armalcolite Chemical analysesof coexisting oxides were obtained samplesstudied by El Goresyand Chao (1976)from the with a CamecaCamebax electron microprobe (Tables 1- Ries impact crater in southern Germany are some of the 3). A focused beam with an acceleratingpotential of 15 most Fe2+rich (>80 molo/oFeTirO, component).Terres- kV and a sample current of 0.010 pA were standard op- trial armalcolite with substantial Ti3+ was discovered in erating conditions. Well-characterizednatural and syn- Mn-rich armalcolite (-20 molo/oMnTirOr) associatedwith thetic materialswere usedas standards.Counting times of native iron and graphite from the former Soviet Union 30 s or 40000 total countswere usedfor all major elements (Tysmbal et al., 1980; Mers et al., 1985) and in basalts in standards and unknowns. Analytical data were correct- containing metallic iron and graphite from Disko Island, ed using the Cameca PAP program. Ratios of Fe2+/Fe3+ HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS 813

Taele 2. Representative electron microprobe analyses of il- Tler-e 3. Representative electron microprobe analyses of ar- menite from sample ET11 malcolite 'I ET11A ET11 B' ET1 C ET11 ET11 wt"/" ETllA ET11B. ET11C ET42A ET42B" Er42C Rim Rim Rim Xtl 1 Xtl 2 oxide Pt. 13 P1.24 Pt 33 Pt.9 Pt. 10 Pt. 19 wtTooxide Pt.20 Pt.26 Pt. I Pl 17 Pt. 5 ZrO" 0 76 0.57 0.37 0.36 0.42 0.87 ZrO" 0.01 0.09 0.07 0.11 0.13 sio, 0.06 0 08 0.04 0.03 0.06 0.o2 sio, 0.04 0.05 0.03 0.04 0 02 Tio, 66.73 68.36 66.45 53.43 53.99 61.44 Tio, 5412 52.84 52.43 51.66 50.44 Ti,o3 o.71 0.00 0.00 0.00 0.00 0.00 Tiro3 010 1.95 0.00 0.00 1.83 Al2o3 4.21 4.12 3.72 4.38 3.79 4.11 Al,o3 0.05 0.15 0.17 0.10 0.24 Cr"O" 0.18 0.15 0.19 0.19 0.08 0.05 3.23 1.25 0.75 2.09 Cr,O. 0.08 0.06 0.02 0.04 0.10 V,O. 2.47 2.0O 't.76 u.40 0.57 Fe"O" 0.00 O.92 26.71 29.33 10.66 V.o" 0.50 038 '| '19.17 Fe.O. 000 0.00 1.O7 .15 0.00 FeO 21.09 21.37 10.07 6.65 16.41 FeO 41.54 40.48 41.85 45.18 4445 MgO 3 76 5.26 3.32 3.34 5.29 3.87 Mgo 3.68 3.68 2.56 0.30 0.24 MnO 0.15 O.22 0.11 0.12 0.15 0.18 MnO 0.67 0.61 0.60 0.66 0.53 CaO O.O2 0.02 0.05 0.05 0.03 0 02 CaO 0.02 o.02 0.04 0.03 0.02 Total 100 14 100.87 100.61 99.93 100.54 99.72 Total 100.81 100.31 99.30 99.27 98.57 Formulae normalized to 3 cations Formulaenormalized to 2 cationg Zr 0.014 0.010 0.007 0.007 0.008 0.016 Zr 0.000 0.001 0.001 0.001 0.002 Si 0.002 0.003 0.001 0.001 0.002 0.001 Si 0.001 0.001 0.001 0.001 0.000 Ti4+ 1.843 1.855 1.837 1.500 1.491 1.713 Ti4+ 0.992 0.972 0.982 0.986 0.968 Ti3+ 0.021 0.000 0.000 0.000 0.000 0.000 Ti3* 0.002 0.040 0 000 0.000 0.039 AI 0.182 0.175 0.161 0.193 0.164 0.180 AI 0.001 0.004 0.005 0.003 0.007 Cr 0.00s 0.004 0.006 0.006 0.002 0.002 0.001 0.001 0.000 0.001 0.002 0.073 0.058 0.095 0.037 0.022 0.062 0.010 0.007 0.009 0.o12 Fes+ 0.000 0.025 0.049 0.750 0.811 0.297 Fe3+ 0.000 0.000 0.020 0.022 0.000 he'. 0.648 0.579 0.657 0 314 0.204 0.509 FeP+ 0.845 o.827 0.872 0.959 0.949 Mg 0.206 0.283 0.182 0.186 0.290 0.214 Mg 0 134 0.134 0.095 0.011 0.009 Mn 0.005 0.007 0 003 0.004 0.005 0.005 Mn 0.014 0.013 0.013 0.014 0.011 Ca 0.001 0.001 0.002 0.002 0.001 0.001 Ca 0.000 0.000 0.001 0.001 0.001 XilF",p" 0.206 0.283 0.182 0.186 0.290 0.214 XIB''o" 0.134 0.134 0.095 0.011 0.009 Notej ET11A-C are adjacent to ilmeniteanalyses ET11A-C in Table 2. Nofej all analyses are from different crystals. Rim : rim on rutile. Xtl XWro,: Mg/(Mg + Fe'?*+ Mn + 0.5Fe3*+ 0.5V + 0.5Cr + 0.5A1+ : discrete,homogeneous grain. Xllf'o.: Mg/(Mg + Fe'?*+ Mn + 0'5Fe3* O.51Fi ). Fep* and Ti3* were calculatedfrom charge balance. + 0.5TF. + 0.5V + 0.5A1).Fe3* and Ti3* were calculated from charge " Inclusionin garnet. balance. 'Rim on rutilein garnet. grains may not be in equilibrium with garnet in sample ETll, barometric calculations are rather insensitive to and Ti3+/Ti4+ were calculatedby charge-balancerequire- moderate variations in the composition of ilmenite. Us- = ments with mineral formulae normalized about cations. ing ilmenite compositions(4&,o, 0.95), a pressrueof In sample ETll, ilmenite occurs primarily as partial 10.5 + 1.0 kbar is obtained with the garnet + rutile * rims or overgrowthson rutile (Fig. 3a). One ilmenite crys- ilmenite + plagioclase+ quartz barometer (GRIPS' Boh- tal was observedin the matrix that is not associatedwith len and Liotta, 1986), and 8.0 + 1.0 kbar is obtained rutile or armalcolite, although it appearsto be a fine in- with the garnet + rutile + sillimanite + ilmenite + quartz tergowth (Fig. 3b). On the basis of qualitative energy- barometer(GRAIL, Bohlenet al., 1983)at 880'C. These dispersiveanalysis, light and dark regionsare ilmenite that pressuresare consistent with results obtained from the containsvarying amounts of Fe and Mg. All Fe in ilmenite GASP barometer. Textural relations suggestthat ilmenite is calculatedto be Fe2*and most ilmenite analysesrequire that forms rims on rutile is secondaryGig. 3a). Pressures a small amount of Ti3+ to maintain neutrality. of I l-12 kbar (GRIPS) and 9-10 kbar (GRAIL) are ob- 'C Attempts were made to determine if ilmenite in sample tained at 880 using compositions of ilmenite rims ETI I equilibrated with the primary mineral assemblage (4!i,o, = 0.83-{.88), indicating that the GRIPS and (e.g., garnet) by applying the KD thermometer proposed GRAIL barometersare quite robust with respectto mod- by Powncebyet al. (1987)on the basisofthe partitioning erate changesin ilmenite composition (Table 2). Ilmenite of Fe and Mn betweenilmenite and garnet.However, this is not present in sample ET42,but limits of pressureof thermometer yields unrealistically high temperatures > 8 kbar and >7 kbar, respectively,are obtained with the (>1300 oC, assumingideal solution models),regardless GRIPS and GRAIL barometers at 880 "C assuming an of the type of ilmenite used (crystal or rim on rutile), and afg,o. of 1.0. should not be applied to sampleETI I becausethe amount In sampleETI l, armalcolite is associatedwith ilmenite of Mn is very low in both ilmenite and garnet (- I molo/o that either mantles or is intergrown with rutile (Fig. 3a). in each).In addition, garnetin sampleETI I containsa Textures suggestthat the armalcolite formed by decom- substantial amount of Mg (40 molo/o),the effectsof which pression,as there is no evidenceofinjection ofthe basalt are not accounted for in the thermometer of Pownceby host near the crystals of rutile and ilmenite. Armalcolite et al. (1987). is opaque in transmitted light and has a reflectivity sim- Although Ko thermometry suggeststhat some ilmenite ilar to that of ilmenite. Minor amounts of either Fe3+or 814 HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS

d .*---?

Fig. 3. Back-scatteredelectron (BSE) images of oxide textures. Scale bars are 50 pm. (a) Rutile (dark) partially rimmed by armalcolite (medium) and ilmenite (bright) from sample ETI l (b) Ilmenite crystal from sample ETI I; light and dark regions are ilmenite that contains varying amounts of Fe and Mg. (c) Rutile (dark) rimmed by armalcolite (bright) from sample E't42. (d) Primary graphite (dark) included in garnet from sample ETI l. HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS 815

TABLE4. Thermalexpansion and compressibilitydata for phasesused in this study Ref 10 Rutile -5.684 x 10 ' 2453 x 1O 3 1.573x 10 7 1.247x 10 1 0.4541 0.5698 2 'lO-3 Geikielite -1 123 x 10 1 4.442 x 1.020x 10 6 -1.104 x 10-10 3 0.5917 0 4 llmenite 6.830 x 10 2 2.834 x 10 3 8.168x 10 a $.$lQ x 10 11 5 0.s863 1.3006 5 MgTi,O, -1.253 x 10 1 5.066 x 10 3 -5.125x100 3118x10" 6. 0.5917 0 7 FeTi,Os -8.132 x 10 2 3.458 x 10-g 6.064x 10-6 3.314x 10 e 3* 0.5863 1.3006 I Fe,TiOs 9.331 x 10 'z 4.494 x 10 3 -6.145x106 3.227x10e 3' 0.5089 0.4539 9

Note:Vo,: y&s+(ygsl1oo)(a+bT+cT2+ df3)(f€);Vi:Vo,-Vor(ex 10-3P- f x106P'z)(Pkbars).Referencesareasfollows:1: Skinner(1966);i-:nizeiandFinger(1981);3:thissiudy(seetext);4:Liebermann(1976);5:WechslerandPrewitt(1984);6:Brownand Navrotsky (1989); 7 : set equal to geikielite;I : set equal to ilmenite;I : set equal to hematite (Robinsonet al., 1982)' - Validonly for r: 700-1200 "C.

Tir+ are required in armalcolite analysesto satisfy charge THrnrvronvNAMrc pRopERTrEs oF oxrDEs rN THE balance,whereas ilmenite analysesrequire a small amount sYsrEM MgO-FeO-FerO3-TiO2 of Ti3*. The apparent need for trivalent ions in ilmenite Data are not available for the thermal expansion of may be the result of small systematicerrors in the micro- MgTiO. or for the compressibility of MgTirO'. There- par- probe data, particularly since Ti3* is preferentially fore, the coemcientsof thermal expansion (a, b, c, and A titioned into armalcolite rather than ilmenite (Lindsley for geikielite were estimated from the equation in Table et al., 1974;,Kesson and Lindsley,1975). In sampleET42, 4 and armalcoliteis associatedonly with rutile (Fig. 3c).In this sample,there is also no evidenceof injection of the basalt Z(MgTiO,): Z(FeTiO,) + Z(MgO) host near the crystals of rutile and armalcolite, although - z (Feo). (2) both are associatedwith quenchedisochemical melt that rims garnets.It is possiblethat the armalcolite formed by Thermal expansivities of FeO (stoichiometric) and MgO the reaction of rutile with Fe from the nearby garnet or were obtainedfrom Fei and Saxena(1986). Thermal ex- from ilmenite that is no longer present.Variable but sub- pansion coemcientsfor MgTirO, were derived from high- stantial amounts of Fe3* are needed in this armalcolite temperature in situ diffraction data of Brown and Na- to maintain neutrality (Table 3). The Xi,i!,io, in sample vrotsky (1989) for the temperaturerange 700-1200 "C ETll rangesfrom 0.09 to 0.13 and the Xf,;i,,o, ranges and may not be valid outside of this range. The com- from 0.18 to 0.28 (Tables2 and 3), correspondingto a pressibility of MgTirO, was set equal to that of MgTiO, rangein K" of 2.2-2.6 tK" : (M/Fe'z*)A*(Mg/Fe'z+)nml. becausecompressibility data are not available. In contrast, experimental results of Lindsley et al. (1974) High-temperature,in situ X-ray powder diffraction data at I bar suggestthat the distribution coemcient between of Brown and Navrotsky (1989)indicate that disorderin armalcolite and ilmenite has a value between 3.6 and 4.8 MgTirO, increasescontinuously from 500 to 1200lC (and 'C). for temperaturesof 900-1 140 'C. Their data predict the probably up to 1500 Disorder in MgTirO, above ilmenite with 9-13 molo/oMgTiO, should be in equilib- about 1000 "C, however, is not preserved in quenched rium with armalcolite that contains 3040 mol0/oMg- samples(Wechsler and Navrotsky, 1984;Brown and Na- TirOr. The discrepancyin K" predicted from the experi- vrotsky, 1989;Wechsler and von Dreele, 1989).Entropy ments and that observed for the Mexican samples may coefficients of MgTirO, are derived from high-tempera- result from errors in the experiments, disequilibrium in ture heat content data of MgTirO' (Brown and Navrot- the natural samples, or both. Upper limits of pressure sky, 1989)using transposedtemperature-drop calorime- can nonethelessbe estimated for the formation of armal- try (Table 5). Thus, the entropy coefficientsfor MgTi,O' colite from ln K calculated as a function of pressurefor derived from those enthalpy data should include any con- the breakdown of rutile + geikielite to MgTirO, from the tributions to entropy that result from disorder. The 'S!" reactron of MgTi,O, is from Kelley and King (1961)with an additional 10.5 J/(mol'K) added for the third law entropy TiO, + MgTiOr: MgTi,O, (l) at 0 K (Table 5), which was estimated from cation dis- which has been located for the range T: 950-1400'C tribution data on samplesquenched from 973 K (Brown by Lindsley et al. (1974) (Fie. 2). If the rutile, ilmenite, and Navrotsky, 1989). However, these entropy coeffi- and armalcolite were in equilibrium, pressurescan be es- cients are valid only for the temperature range of the timated from values of ln K for Equilibrium l. Use of enthalpymeasurements (700-1500 "C) and shouldnot be Equilibrium I for barometry of natural samples, how- used outside of this range. Thus, standard free energies ever, requires knowledge of the thermodynamic proper- of formation cannot be estimated at 298.15 K for pseu- ties of pure MgTirO, and activity relations for armalcolite dobrookite from the data in Tables 4 and 5. In contrast, solid solutions. thermochemical data and room-temperature Rietveld re- 816 HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS

TABLE5. Molar volume,entropy, and entropycoefficients for phasesused in this study V2"" S3.. (cm3/mol) lJ(molK)l Rutile 18.82 1 50.29 1 62.852 11.363 4.991 -367.10 1 Geikielite 30.86 1 74 56 2 118.365 13.723 13.661 -693.79 3 llmenite 31.70 4 108.91 4 125.222 13.184 14.886 -734.04 4 MgTi.Ou 54.87 137.65 b 205.200 20.736 49.827 -1231.31 FeTirO5 55.75 172.21 6 212.O57 20.200 s1.053 -1271.60 6' Fe,TiOu 54.53 1 172.38 o 192.589 22.008 15.502 - 1121.27 8 ivotei59-S8".:Alnf+8x10-37+Cx 105f2+D(fK).Referencesareasfollows:1:Robieetal.(1978); 2:KeileyandKing(1961); !: \aylor9ry cook(1946);4: Anovitzetal.(1985);5: Linilsleyetal(1974);6: thisstudy(seetext);7:'Brownand Naviotsky(r9-e9); e: Bonnickson(1954). . Validonly for f : 700-1500 qC.

finements of the structure of geikielite quenched from of FeTirO, were estimated from values and equations in high temperature suggestthat Mg and Ti are completely Table 4 and ordered in the two octahedral sites up to temperaturesof at least 1400'C (Wechslerand von Dreele, Z(FeTi,O,): V(MgTi,O,) + Z(FeO) 1989).Entro- - py data for geikielite in Table 5 were taken directly from z(Mgo). (4) measuredvalues (Naylor and Cook, 1946; Kelley and Thermal expansion and compressibility data for FeO King, l96l) without correctionfor zero-pointentropy. (stoichiometric)and MgO (Fei and Saxena,1986) were The location of Equilibrium I was calculated from refit to the equations given in Table 4. available thermodynamic data with the assumption that Anovitz et al. (1985)estimated the entropyof FeTirO, AG. (l) equals0 at 1200 "C and 15 kbar (Fie.2), on rhe from those of pseudobrookite, hematite, and ilmenite. basisof the experimentalreversal of Lindsley et al. (1974). They assumedthat FerTiO, and FeTirO, were completely Data used in all thermodynamic calculationsare given in ordered (normal pseudobrookitestructure of TiFerOr) and Tables 4 and 5. The calculated locus of Equilibrium I fits did not account for entropy from magnetic effects.Their the experimental brackets between 950 and 1200 "C but analysisyields a value of Sor,*: 156.1J/(mol.K). Be- is located at temperaturesthat are too high by at least 25 causemost crystals of pseudobrookitecontain a substan- "C at 20 kbar (Fig. 2). Although the equilibrium is not tial amount of disorder at T > 700 'C (Brown and Na- tightly reversed, results of the experiments suggest a vrotsky, 1989; Brigatti et al., 1993), a better somewhat flatter slope between .C 900 and 1200 and approximation of S!n, and entropy coefficients(A, B, C, greaterslope between I 200 and 1400 "C (using midpoints and D) of FeTirO, can be estimated from of reversals).Curves calculated from Gibbs free energy data tabulatedin Chaseet al. (1985) and Knacke et al. S0 (FeTirO,) : S0 (MgTirO,) * So (FeTiO.) (1991) with thermal expansivitiesand compressibilities So (MgTiO.) + 2.5Vo (5) from Table 4 are located at pressuresthat are too low by an averageof3 and 5 kbar, respectively,for a given tem- (Fyfe and Verhoogen,1958) and perature, and their slopes are somewhat flatter than the : calculatedcurve (Fig. 2). Evansand Muan (1971)calcu- Z(FeTi,O,) Z(MgTi,O,) + Z(FeTiO,) lated the free energies of formation of MgTiO, and - Z(MgTiO.) (6) MgTirO, from the oxides at I bar and 1400 .C on the basis of CO-CO, ratios in equilibrium with Ni and Mg (Table 5). An estimateof S!r, : 172.4J/(mol.K) is ob- titanates. They obtained a value of AG. for Equilibrium tained for FeTirOr, significantly higher than the value -9.2 I of kJ/mol at I bar and 1400 "C (cf. Navrotsky, estimatedby Anovitz et al. (1985).Inherent in Equation 1975),in good agreementwith a value of -9.6 kJlmol 5 is the assumption that FeTirO, and MgTirO, exhibit calculatedin this study. Chaseet al. (1985) reported a similar degreesof disorder with increasing temperature. -7 value for AG. of Equilibrium I of .9 kJlmol, whereas There are no in situ cation distribution data available for -6.3 Knacke et al. (1991)obtained kJ/mol ar I bar and FeTi.Or, but a comparison of cation distribution data for 1400'C. Thesedata are inconsistentwith experimentson synthetic, quenched samples of FeTirO, and MgTirO, Reaction I and are disregardedin this study (Fig. 2). suggeststhat FeTirO, has similar amounts of cation or- Experimental data at pressuresgreater than I bar are dering (Lind and Housley,19721-Grey and Ward, 1973; not availablefor the reaction Virgo and Huggins, 1975; Brown and Navrotsky, 1989; Wechsler (Ishikawa TiO, + FeTiO, : FeTirO, (3) and von Dreele, 1989).Ilmenite and Akimoto, 1957;Stickler et al., 1967)and pseudobrookite oC located at 1140 and I bar (Haggerty and Lindsley, (Akimoto et al., 1957, Muranaka et al., l97l) are para- 1969).Because molar volume data are availableonly at magneticat room temperatureand becomeantiferromag- STP for FeTirOr, thermal expansionand compressibility netic at low temperature. As a first approximation, con- HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS 817 tributions to the configurational entropy of FeTirO' Tlele 6. Calculated activity coefficientsfor armalcolite the basis of a molecularactivity resulting from magnetic transitions at low temperature (FeouMgo.Ti.Os)on model are accounted for in Relation 5 becausemagnetic effects are included in the entropy data of ilmenite (Anovitz et ABL G ABL G al., 1985). Heat capacity,transposed drop calorimetry, r fc) 7flTi'." )flliro' and in situ cation distribution data at high temperature 1010 0.78 1.02 n.d. n.d. are necessaryfor a more accurate determination of the 1100 0.56 0.81 n.d. n.d. 1.36 propertiesof FeTirOr. 1140 n.d. n.d. |.30 thermodynamic 1200 0.42 0.67 1.28 1.33 Thermodynamic data used for pseudobrookiteare giv- 1300 0.27 0.50 1.21 1.24 Tables4 and 5. The thermal expansionof FerTiO' en in Note: ABL values calculated using a-X relations of ilmenite from An- was estimated from dersenet al. (1991);G valuescalculated using a-X relationsof ilmenite from Ghiorso(1990); n.d : not determined Z(Fe,TiO,): Z(FeTi,O,) + Z(Fe,O,) - Z (FeTiO,) (7) (Table 4). The compressibility of pseudobrookitewas set solutionsusing experimental results for Equilibria l' 3, and (l) equal to that of hematite. Thermodynamic data of Rob- 9 if the following are accuratelyknown: volume data inson et al. (1982) were used for hematite. The S!n' of of all phasesat P and T, (2) a-X relations of ilmenite- : pseudobrookite was estimated from geikielite solutions,and (3) K" (Mg/Fe)"*/(M/Fe)"'as a function of P and Z. Volume data used in this study are : (FerOr) So (Fe,TiO,) So (FeTirO,) * So given in Tables 4 and 5. Mixing relations of Andersen et - (FeTiO.) + 2.5Yo (8) ,So al. (1991)and Ghiorso (1990)were usedfor ilmenite-gei- model was assumed (Fyfe and Verhoogen, 1958). Using entropy coefficients kielite solid solutions, and an ideal as a furtction of temper- estimatedfrom FeTirOr, FerOr, and FeTiOr, values of for rutile. The Ko was estimated al. (1974). Activity co- S%- 59* calculated for pseudobrookiteare slightly lower ature from the data of Lindsley eI calculatedfrom than published values of S9- S!n'. Therefore, entropy efficientsfor FeTirO, and MgTirO, coefficientsfor pseudobrookite were taken directly from rPK : -Rr (10) the publishedvalues ofBonnickson (1954). I tv*dP'- lni Jp ^ Acrrvrrv-couprosrrloN RELI\TroNs oF are given in Table 6. The prime refers to the standard ARMAI]COLITE state, chosen to be either MgTirO, or FeTi'O, for armal- Activity-composition (a-X) relations have been eval- colite at ?nof interest, where P' is defined by the locus of uated for ilmenite-geikielite-hematite solid solutions Equilibrium I or 3, respectively. Assuming a molecular : (Andersenand Lindsley, 1988;Ghiorso, 1990;Andersen activity model (i.e., 4il!t'.o, ?fn!i,,o,'Xilt+',o,),negative et al., 1991),although some discrepancies remain in these deviations from ideality are necessaryfor MgTirO', and values that may be traced to the mixing properties as- positive deviations are required for FeTi'O, for the shift- sumed for olivine. No mixing data are available for or- ed loci of Equilibria I and 3 to be coincident with the thorhombic oxides in the ternary system FeTirO'-Mg- locus of Equilibrium 9 (Table 6). Regardlessof the il- Ti,Or-FerTiO,. The Fe-Mg exchangeequilibrium between menite model used, the calculated activity coefficients stoichiometricarmalcolite (i.e., FeorMgo 5Ti2Os) and il- of MgTirO, and FeTirO, decreasewith increasing menite", temperature. MgTiO. + FeTi,O,: FeTiO: + MgTi,O, (9) BlnonnnrnY was locatedat l0l0 "C at I bar by Lindsleyet al. (1974). Pressureswere estimated from Relation l0 with P' de- The composition of ilmenite in equilibrium with fined by the locus of Equilibrium I or Equilibrium 3 for FeorMgorTirO,is FeorMgorTio3(their Fig. l), althouglt rutile-armalcolite + ilmenite pairs from samples ETll exact compositions or chemical analysesare not given. and ET42, assuming various activity models for armal- They studied the equilibrium distribution of Fe2* and Mg colite and ilmenite (Hayob, 1994). Attempts to use the betweenarmalcolite and ilmenite for a rangeof bulk com- derived thermodynamic values to estimate pressure of positions at I bar and 900-l 140lC. A value ofKo between formation of armalcolite were not successful.It is likely 3.6 and 4.8 is calculatedfrom their data. Friel et al. (1971) that the ilmenite and armalcolite did not equilibrate in determined the location of Equilibrium 9 for stoichiomet- sample ET I I , on the basis of low values obtained for the ric armalcolite for the temperaturerange 1000-1200'C as Ko in comparison with data of Lindsley et al. (1974). a function of pressure,although they did not report the Upper limits of pressure estimated from armalcolite composition of ilmenite in equilibrium with compositions are not useful becauseof the substantial FeorMgorTirOr.On the basisof the Kodala of Lindsley et dilution of the MgTirO, and FeTirO, components,and it al. (1974), FeorMgorTirO, should have equilibrated with is difficult to evaluate the effectof additional components FeorrrMgorr5Tio3at ll00'C and with FeortMg"TiO, at such as Al on the stability of armalcolite in the xenoliths. 1200"C, ifthe Ko is independentofpressure. The depth in the crust at which armalcolite formed in the Mixing relationscan be estimatedfor FeTirOr-MgTirO, xenoliths cannot be well constrained. However, by anal- 818 HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS

P = 10 kbar

_10 ET11 -10 log f o, ET11A log fo2 _15 -15

800 1000 1200 r ("c) -25 800 1000 1200 Fig. 4. Log fo.-T diagram at I bar total pressureand g00- 1200'C showingloci of Equilibria ll and 12 (combined),16, r ("c) and 17. Equilibria 16 and 17 are metastablebelow ll40 .rC, Fig. 5. lng diagram at l0 kbar total pressureand 800- where FeTirO, breaks down to rutile + ilmenite. Lined field f".-T I200 "C. Stippled field showsrange in conditions for the peak shows rangesin /o. calculated from Equitibrium 16 for sample L. of metamorphism. Magnetite + rutile : ilmenite was calculated ETI l. Light stippled field representsranges in /o, calculatedfrom from thermodynamic data referencedin this study and data for Equilibrium l7 for samplesETI I and ET42, and.heavy stippled magnetitefrom Robie et al. (1978).Hematite: magnetite,quartz line is calculatedfrom the Ti3+ content of armalcolite for sample + magnetite : fayalite, ilmenite : rutile + Fe, and ETI lA (Staninand Taylor, 1980:ST). The reactionshematite abbrevia- : tions as in Fig. 4. magnetite * Oz, euartz + magnetite : fayalite * Or, and wiistite : iron + O, were calculated from equilibrium expres- sionsin Frost (1991);ilmenite: rutile + iron + O, was calcu- : lated from thermodynamic data referencedin this study. C : assuming that Po,,o P,", and P",o has negligible contri- graphite, CO : carbon monoxide, CO, : carbon dioxide, Fa : butions to total pressure. At I bar total pressure, the par- : fayalite, Fe metallic iron, Fe-Arm : FeTirOs, Hem : hema- tial pressure of each gaseous species can be determined : : tite, Ilm ilmenite, Mag magnetite, Psb : pseudobrookite, as a function of temperature from Qtz : quartz, Rt : rutile, Wus : wiistite. AGo-: -RZln K. (14) Gibbs free energydata from Robie et al. (1978) were used to calculate the location of Equilibria ll and 12 for the ogy with other armalcolite occurrences and from textural range T: 800-1200 'C at I bar total pressure(Fig. 4), criteria, it seems likely that the Mexican armalcolite assumingan ideal model (P,: formed duri ng decompression. f,). Oxygenfugacities more reducing than those of the iron + wiistite (IW) buffer are necessaryto stabilize graphite at high temperaturesat I Itvrpr,rc,lrroNs FoR REDox coNDrrroNs bar (Fig. 4). Metamorphic peak./o, Pressurehas a large effect on the locations ofEquilibria Primary graphite, occurring as flakes along grain bound- I I and 12 in f",-T space (e.g., Nordstrom and Munoz, aries and as inclusions in other primary minerals, is com- 1986).AtP.,> lbar, mon in both xenoliths (e.g., Fig. 3d). In the presence of AGPr= -RIln K + Al1s8(solids) AP. (15) graphite (u. : l), the partial pressure (and fugacity) of O, in equilibrium with graphite is buffered by the equilibria An iterative method was usedto calculate the fo,of Equi- libria I I and 12 using Assumption I 3 and Expression 15 C + YzOr:69 (l l) at l0 kbar total pressure(Fig. 5). Fugacitieswere calcu- C + O,: CO, (r2) lated from fugacity coefficients for CO (Ryzhenko and and Volkov, I 97 l) and CO, (Shmulovichand Shmonov, I 975) at P and Zup to l0 kbar and 1200'C. The calculations P.o+P.o,:P., (l 3) were relterated until convergencewas achieved between HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS 819 the initial and final values for the partial pressuresofCO perimental results on Kilbourne Hole xenoliths, suggest than and COr. The calculated/o, for Equilibria ll and 12 is that feldspar thermometry has an accuracyof better not extremely sensitive to the values of fugacity coeffi- +50'C. The thermometer of Elkins and Grove (1990) cients for CO and CO, or to the ratio of CO/CO,. For yields temperatures that are similar to the models of oC example,at 900 7CO, : ll.3l-15.02 if P.o,:8-9 Fuhrman and Lindsley(1988) and Lindsleyand Nekvasil kbar (Shmulovichand Shmonov,1975), and 7CO : 1.87- (1989) that were used by Hayob et al. (1989)' Melting 1.31 if Pco: l-2 kbar (Ryzhenkoand Volkov, l97l), experimentsconducted by Beardet al. (1993)on a pelite resultingin a calculatedrange in -log fo,of 12.87-12.79. (their sampleKH-I2) from Kilbourne Hole at 900-1000 Convergenceis obtained at 900 "C and I 0 kbar total pres- "C produced no melt, indicating that pelites may be quite sure for valuesofP.o : 1.57 kbar and P.o, : 8.43 kbar, refractory and stable to temperatures > 1000'C. There is correspondingto -log fo,: 12.83.Figures 4 and 5 show no textural evidence ofreheating ofthe xenoliths during the calculated locations of Equilibria I I and 12 (com- decompression.Rims of quenchedmelt that formed upon bined) at I bar and 10 kbar, respectively,at 800-1200 decompression(Hayob et al., 1989) surround garnet in "C. If other fluid specieswere present,the activities of CO both samplesand a small amount of melt (< loloby vol- and CO, would be reduced and the loci of the combined ume) is present along some grain boundaries. However, equilibria (from I I and 12) would be shifted toward low- zoning is absentin all minerals, and if reheatingoccurred, er values of -log /", in Figures 4 and 5. Thus, the pres- it happened rapidly enough such that the compositions ence of graphite provides an upper limit for the fo, at of the primary minerals were not afected. Thus, it is which the xenoliths equilibrated at approximately l0 kbar reasonableto assume that armalcolite formed at about total pressure(Fig. 5). 900-1000 "C at pressureslower than the peak of meta- The presenceof primary graphite, lack of metallic Fe, morphism (10 kbar). and pressureestimates of l0 kbar for the peak of meta- Incorporation of activity coefficients has a negligible morphism indicate that the xenoliths equilibrated at val- effect (e.g., +0. I log unit) on the calculated values of log ues of -log between ll and 15 during the peak of (*0.t log unit) for ETll andET42 in comparison "f", /o, metamorphismat 1025-1075'C (Fig. 5), the minimum wiitr the effect of chemical heterogeneityin the oxides. temperature for the peak of metamorphism estimated on Therefore, ideal molecular activity models were used for the basis of feldspar thermometry in thesesamples (Hay- all phases(i.e., a!;!''o, : 'Y!3h,o,).Values of log /o, esti- ob er al.. 1989). mated at I bar from Equilibrium 16 for sample ETI l, which contains ilmenite, and Equilibrium 17 for ETl l Arrnalcolite formation /o, andET42 are shown in Figure 4. At 10 kbar, Equilibria Phaseequilibria involving armalcolite can be used to I 6 and I 7 are shifted + I .2 and 0.0 log units' respectively, indicated for constrain /o, conditions if the armalcolite * rutile + il- from the I bar loci. The range of log /o, menite were in equilibrium. In an oxidizing atmosphere each sample representsvariations inlog forresulting from at high temperatureand low total pressure,ilmenite forms chemical heterogeneityin the coexisting oxides (Tables an armalcolite-pseudobrookitesolid solution l-3). In sample ET42, the mole fraction of FeTirO' is diluted sufficiently that armalcolite is stable to tempera- 3FeTiO, I t/zOr: FerTiO, * FeTirO, (16) tures <800 "C (e.g., Fig. 2). Armalcolite from sample and FeTirO, oxidizes to form rutile + pseudobrookite ETl l. however.is not stablebelow 900'C (Fig. 4) on the basis of values of log K for Equilibrium 3. Equilibria l6 2FeTirO, * VzOr: 3TiO, + Fe'TiO' (17) and 17 cannot be applied to armalcolite ETI lA' which (Anovitz et al., 1985).These O' buffersare located within lacks a pseudobrookite component and contains a small one log unit ofeach other at I bar total pressurebetrveen amount of Ti3*. Stanin and Taylor (1980)formulated an the hematite + magnetite (HM) and FMQ buffers (Fig. O, barometer for lunar basaltson the basisof the amount 4). Equilibria I 6 and I 7 are metastablebelow I 140 "C at of Ti3+ in armalcolite and proposed that Tir+-rich ar- I bar becauseFeTirO, is not stable. malcolite typically equilibrates at values of log /", be- It is difficult to estimate the temperature of formation tween IW and 1.5 log units below IW. Their results are of armalcolite in the xenoliths. The high Fe2+content of consistentwith experimentsof Friel et al. (1977) in which armalcolite in sample ETll suggeststhat armalcolite armalcolite reacted to form ilmenite + reduced armal- -10.5 formed at fairly high temperature; however, Ti3* and Al colite at valuesoflog /o, below at 1200'C and I should stabilize armalcolite to lower temperatures(Kes- bar. From the expression son and Lindsley, 1975).The effectof Vr+ on the stability of armalcolite has not been studied experimentally but rosfo,:-tt("*) (l8) (by analogy with other trivalent ions) V should stabilize armalcolite. Two-feldspar thermometry of the exsolved feldsparsindicates that the xenoliths did not cool below (Staninand Taylor, 1980),a value for log /o, ofapprox- 900'C until after eruption (Hayob et al., 1989, 1990). imately 0.0 (Fig. 4) is obtained for armalcolite from Data from Beard et al. (1993),which comparethe feld- ETI lA (heavy shaded curve, Fig. 4), where the /o, is in spar thermometer of Elkins and Grove (1990) with ex- log units relative to the iron + wiistite buffer. In Equation 820 HAYOB AND ESSENE:ARMALCOLITE IN CRUSTAL XENOLITHS

18, Ti3* is the mole fraction of TirO, and Fer+ is the tigations and geologicalapplications of equilibria in the system FeO- mole fraction of FeTirO, in armalcolite. TiOr-Al,O.-SiO:-H,O.American Mineralogist, 68, 1049-1058. Bonnickson, K R. (1954) High temperatureheat contents ofsome titan- Drscussronq ates of aluminum, iron and zinc. Journal of the Amencan Chemical Society,77, 2152-2154. Chemical variation in armalcoliteproduces a largerange Bowles, J.F.W. (1988) Definition and range of composition of naturally in calculated /o, for both samples, which may indicate occurring minerals with the pseudobrookitestructure. American Min- disequilibrium on the scale ofa thin section. The range eralogist,73, 1377-1383. Brett, R., Gooley,R.C, Dowty, E, Prinz, M., and (t973) in /o, is typical, however, for crustal and mantle rocks Keil, K. Oxide minerals in lithic fragments from Luna 20 fines. Geochimica et Cos- and indicates that armalcolite solid solutions involving mochimicaActa. 37. 761-773 Fe2+and Mg are stable in terrestrial rocks. The value of Brigatti, M.F., Contini, S, Capedri, S., and poppi, L. (1993) Crysral .fo, \a?r IW for sample ETI lA is more typical of lunar chemistry and cation ordering in pseudobrookiteand armalcolite from Spanishlamproites. European rocks, but the calculated/o, is sensitive to small amounts Journal of Mineralogy, 5, 7 3-84. Brown, N.E., and Navrotsky, A. (1989) Structural, thermodynamic, of Ti3+.Armalcolite seemsio be relatively and rare,however, kinetic aspectsof disordering in the pseudobrookite-typecompound even in volcanic rocks, and bulk composition may be karrooite, MgTi,O,. American Mineralogist, 74, 902-9 12. more important than P-T-.fo, in controlling its stability Cameron, E.N. (1978) An unusual titanium-rich oxide mineral from the (Anovitz et al., 1985). EasternBushyeld Complex. American Mineralogist, 63, 37-39. 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