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 XAl [= Al/(Al + Ti)] of 0.53. Previously, such high values of XAl 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 CaTiSiO5 endmember titanite. In most rocks, these vectors are Al1F1Ti-1O-1, Ti-0.25M0.25O-1OH1, and OH1F-1; in one sample, the Ti-0.25⃞0.25O-1OH1 vector is replaced by a Si-0.25⃞
0.25O-1OH1 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-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 titanite as a monitor of fluid composition.
© 2015 by Walter de Gruyter Berlin/Boston