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

Abstract

Titanite of variable Al and F content was found in granulite- to amphibolite-facies calcsilicates in Central Dronning Maud Land, Antarctica. The highest observed Al content corresponds to an X(Al) [= Al/(Al + Ti)] of 0.53. Previously, such high values of X(Al) were reported from high-pressure rocks, but the titanite of this study is from a low-pressure terrain. The compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the CaTiSiO5 end-member titanite. In most rocks, these vectors are Al1F1Ti(-1O-1), Ti(-0.25)[](0.25)O(-1)OH1, and OH1F(-1); in one sample, the Ti(-0.25)[](0.25)O(-1)OH1 vector is replaced by a Si(-0.25)[](0.25)O(-1)OH1 vector. The actual 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 chemical environments, high-Al titanite is stable over a wide P-T range. Therefore, the Al content of titanite should not be used in geothermobarometry, even qualitatively. Additionally, because of the coupled substitutions Al1F1Ti(-1)O(-1) and Al1OH1Ti(-1)O(-1), the concentration of F in titanite is strongly dependent on the host rock chemistry. This rules out the easy use of titanite as a monitor of fluid composition.