The Substitution of Al and F in Titanite at High Pressure and Temperature: Experimental Constraints on Phase Relations and Solid Solution Properties

INTRODUCTION Experimental studies were carried out to evaluate phase relations involving titanite–F–Al-titanite solid solution in the system CaSiO3– Geological background Al2SiO5–TiO2–CaF2. The experiments were conducted at Titanite is a common accessory mineral in mafic, pelitic 900–1000°C and 1·1–4·0 GPa. The average F/Al ratio in and granitic rocks from many geological environments titanite solid solution in the experimental run products is 1·01 ± (Higgins & Ribbe, 1976; Ribbe, 1982; Enami et al., 0·06, and XAl ranges from 0·33 ± 0·02 to 0·91 ± 0·05, 1993). Titanite may deviate significantly from its ideal consistent with the substitution [TiO]–1[AlF ]1. Analysis of composition by the substitution Al and F for Ti and O the phase relations indicates that titanite solid solutions coexisting (Hollabaugh, 1980; Franz & Spear, 1985; Bernau et al., with rutile are always low in XAl , whereas the maximum XAl of 1986; Fehr, 1991; Oberti et al., 1991; Carswell et al., titanite solid solution occurs with fluorite and either anorthite or 1996). The Al + OH ⇔ Ti + O substitution leads Al2SiO5. Reaction displacement experiments were performed by to the Al–OH end-member vuagnatite CaAlSiO4(OH), adding fluorite to the assemblage anorthite + rutile = titanite + which has a different structure from titanite (McNear et kyanite. The reaction shifts from 1·60 GPa to 1·15 ± 0·05 al., 1976) and is typical of low-temperature geological GPa at 900°C, from 1·79 GPa to 1·375 ± 0·025 GPa at environments (Enami et al., 1993). By contrast, the F–Al 1000°C, and from 1·98 GPa to 1·575 ± 0·025 GPa at substitution is isostructural and is common at high meta1100°C. The data show that the activity of CaTiSiO4O is very morphic temperatures (>500–600°C) and pressures. For close to the ideal molecular activity model (XTi ) at 1100°C, but example, up to 55 mol % F–Al substitution has been shows a negative deviation at 1000°C and 900°C. The results reported from highand ultra-high-pressure metaconstrain G°f,298·15 of CaAlSiO4F to be −2595 ± 3 kJ/mol morphic rocks (Franz & Spear, 1985; Sobolev & Shatsky, and S°298·15 to be in the range of 105·2–109·6 J/mol K, which 1991; Carswell et al., 1996). The common association of in turn can be used to calculate petrogenetic grids involving titanite F–Al-rich titanites with high-pressure environments has solid solutions in the system CaTiSiO4O–CaAlSiO4F. led to the suggestion that equilibria involving titanite solid solution may be useful for constraining pressure, temperature and/or F2 fugacity during metamorphism (Smith, 1977; Franz & Spear, 1985; Gibert et al., 1990;