The current status of titanite–rutile thermobarometry in ultrahigh-pressuremetamorphic rocks: The influence of titanite activity models on phase equilibriumcalculationsPeter Troppera,b,⁎, Craig E. ManningbaInstitute of Mineralogy and Petrography, Faculty of Geo- and Atmospheric Sciences, University of Innsbruck, Innrain 52, A-6020 Innsbruck , AustriabDepartment of Earth and Space Sciences, Geology Building, University of California at Los Angeles, Los Angeles, CA 90095-1567, USAA B S T R A C TA R T I C L E I N F OArticle history:Accepted 12 March 2008Keywords:TitaniteActivity modelAl+F substitutionUHPThermobarometryTitanite, an important accessory mineral in ultrahigh-pressure (UHP) rocks, commonly deviates significantlyfrom its ideal composition (CaTiSiO4O) by the substitution of Al and (F, OH) for Ti for O. This compositionalvariability of titanite could be used in phase equilibrium calculations, if the activity–composition relations in(Al +F)-bearing titanites were sufficiently known. Experimental investigations by Troitzsch and Ellis([Troitzsch, U. and Ellis, D.J. (2002) Thermodynamic properties and stability of AlF-bearing titaniteCaTiOSiO4–CaAlFSiO4. Contributions to Mineralogy and Petrology, 142, 543–563]) and Tropper et al.([Tropper, P., Manning, C., Essene, E.J. 2002. The substitution of Al and F in Titanite at high pressure andtemperature: experimental constraints on phase relations and solid solution properties. Jour nal of Petrology43, 1787–1814.) derived non-ideal-mixing models for solid solutions along the join CaTiSiO4O–CaAlSiO4F.Tropper et al. [Troitzsch, U. and Ellis, D.J. (2002) Thermodynamic properties and stability of AlF-bearingtitanite CaTiOSiO4–CaAlFSiO4. Contributions to Mineralogy and Petrology, 142, 543–563] derived apreliminary regular model in the T range 900–1100 °C, in which the T-dependent interaction parameter,WG, was negative. In contrast, Troitzsch and Ellis ([Troitzsch, U. and Ellis, D.J. (2002) Thermodynamicproperties and stability of AlF-bearing titanite CaTiOSiO4–CaAlFSiO4. Contributions to Mineralogy andPetrology, 142, 543–563]) favored a regular activity model with positive WG. The different signs of theinteraction parameter strongly influence calculated non-ideal titanite activities. Comparing available simpleideal ionic (coupled-ionic, non-coupled-ionic) activity models with the non-ideal models shows thataCaTiSiO4O, calculated with the ideal ionic models is substantially lower at almost all T. Calculation of twosuitable titanite–rutile-involving equilibria for thermobarometry applied to literature data from rocks fromfour UHP terranes shows that ideal ionic models yield the best convergence with independently establishedP estimates. Although the literature data have to be treated with caution (e.g. retrogression, compositionaldisequilibrium etc.), the calculations nonetheless indicate that in terms of T estimates, high-Al (XAlN 0.5)titanites yield a large variation of up to 300 °C in T and low-Al (XAlb 0.2) titanites yield the best convergencewith independently established T estimates. This study shows that P and T estimates derived from idealmodels and experimentally constrained non-ideal models for titanite activity may show large deviations atUHP conditions, ranging from 0.05 to 3.0 GPa and up to 300 °C. Therefore the current status of titanite–rutile-involving thermobarometry allows it only to be applied to Al-rich (XAlN 0.2) titanites from UHP rocks ifindependent, more robust P–T estimates are available. Until better activity constraints are available, it isrecommended that the user employ either the ideal coupled-ionic model for titanite solid solutions involvingmixing on the Ti and the O1-site and compare the results to an experimentally derived activity model, oradopt a range of different activity models (ionic and regular) to obtain a range of P–T conditions.© 2008 Elsevier B.V. All rights reserved.1. IntroductionAt ultrahigh ultrahigh-pressure (UHP) metamorphic conditions(N 2.5 GPa, Liou et al., 1994; Coleman and Wang, 1995; Liou et al.,1998), most silicate lithologies contain a similar eclogitic mineralassem blage, consisting of garnet, clinopyroxene, quartz/coesite,phengite, rutile±titanite, kyanite and zoisite (e.g. Liou et al., 1998).The low variance of eclogite mineral assemblages has long posedChemical Geology 254 (2008) 123–132⁎ Corresponding author. Institute of Mineralogy and Petrography, Faculty of Geo- andAtmospheric Sciences, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria.E-mail addresses: [email protected] (P. Tropper), [email protected](C.E. Manning).0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2008.03.010Contents lists available at ScienceDirectChemical Geologyj o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c h e m g e oproblems for derivation of intensive parameters attending meta-morphism, such as temperature (T) and pressure (P). One strategy tosolve this problem has been to use compositions of phengite ingeothermobarometry (e.g. Page et al., 2007). However, phengitesolid solutions are problematic because thermodynamic data for Si-rich micas are not well characterized, preservation of peak com-positions can be problematic, and many potentially useful equilibriadepend on H2O activity, which is not known independently. Analternative approach to thermobarometry of UHP rocks is to takeadvantage of coexisting accessory phases such as titanite and rutile.Titani te [Ca(Ti,Al,Fe3+)SiO4(O,F,OH)] is a common accessorymineral in mafic, carbonate, pelitic and granitic rocks from manygeologic environments (Higgins and Ribbe, 1976; Ribbe, 1982; Enamiet al., 1993). Because of its participation in net-transfer equilibria,titanite can be useful for the evaluation of pressure (P), temperature(T), and the fugacities of volatile compo nents associated withmetamorphic and igneous processes (e.g. Mannin g and Bohlen,1991; Ghent and Stout, 1994; Xirouchakis and Lindsley (1988); Frostet al., 2000; Troitzsch and Ellis, 2002; Tropper et al., 2002; Harlov etal., 2006; Page et al., 2007). Al–F-bearing titanites are also common inUHP rocks (Sobolev and Shatsky, 1990, Shatsky et al., 1995; Cong et al.,1995; Rubatto and Hermann, 2001; Ogasawara et al., 2002; Castelli etal., 2007). The association of F–Al-rich titanites with high high-pressure environments has led to the suggestion