Stability of high-Al titanite from low-pressure calcsilicates in light of fluid and host-rock composition

Titanite of variable Al and F content was found in granulite- to amphibolit e-facies calcsilicates in Central Dronning Maud Land, Antarctica. The highe st observed Al content corresponds to an X-Al [= Al/(Al + Ti)] of 0.53. Pre viously, such high values of X-Al were reported from high-pressure rocks, b ut the titanite of this study is from a low pressure terrain. The compositi onal variations in titanite can be described for all samples by a set of th ree linearly independent exchange vectors added to the CaTiSiO5 end-member titanite. In most rocks, these vectors are Al1F1Ti-1O-1, Ti(-0.25)square(0. 25)O(-1)Oh(1), and OH1F-1; in one sample, the Ti(-0.25)square(0.25)O(-1)OH( 1) vector is replaced by a Si(-0.25)square(0.25)O(-1)OH(1) vector. The actu al amount of exchange along these vectors and, therefore, the amount of Al in titanite, depends on P and T, on the composition of the coexisting fluid phase in terms of its H2O/HF fugacity ratio, and on host rock composition in terms of Al2O3/TiO2 activity ratio. It is inferred that, in suitable che mical environments, high-Al titanite is stable over a wide P-T range. There fore, the Al content of titanite should not be used in geothermobarometry, even qualitatively. Additionally, because of the coupled substitutions Al1F 1Ti-1O-1 and Al1OH1Ti-1O-1, the concentration of F in titanite is strongly dependent on the host rock chemistry. This rules out the easy use of titani te as a monitor of fluid composition.