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 lowpressure terrain. The compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the CaTiSiO 5 endmember titanite. In most rocks, these vectors are Al 1 F 1 Ti -1 O -1 , Ti -0.25 M 0.25 O -1 OH 1 , and OH 1 F -1 ; in one sample, the Ti -0.25 ⃞ 0.25 O -1 OH 1 vector is replaced by a Si -0.25 ⃞ 0.25 O -1 OH 1 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 H 2 O/HF fugacity ratio, and on host rock composition in terms of Al 2 O 3 /TiO 2 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 Al 1 F 1 Ti -1 O -1 and Al 1 OH 1 Ti -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.