TITANIUM ZONING IN BIOTITES - AN EXAMPLE FROM THE STRZEGOM-SOBOTKA GRANITES (SW POLAND)

Biotites occurring in the granitoids of the Strzegom-Sobotka massif (S W Poland) commonly exhibit zoning in titanium that decreases from cent er towards margin. The highest variation in TiO2 observed within an in dividual biotite is ca. 1 wt%, while the Fe2+/(Fe2+ + Mg) ratio shows no variation in a scale of individual plates. This suggests that titan ium diffusion in biotite is too slow to obliterate the content of the element established during crystallization of an igneous mica,,whereas iron and magnesium appear to be highly mobile. Titanium is dissolved in biotite due to heterogeneous reactions involving ilmenite (or other Ti mineral), oxygen, and (in some cases) water or quartz. Once calibr ated experimentally, those reactions would yield information relating titanium content in biotite to temperature, oxygen fugacity, and water fugacity (at constant pressure). Zoned distribution of titanium prese rves the information on the evolution of those variables during biotit e crystallization from a magma.