The oxygen isotope fractionation behaviour of kyanite in experiment and nature

The oxygen isotope fractionation between kyanite and calcium carbonate has been investigated experimentally at four temperatures in the range between 625 and 775 degrees C at 13 kbar. Because of low exchange rates, the isotop ic reaction was enhanced by polymorphic transformation of andalusite to kya nite. With this experimental modification a close approach to equilibrium w as reached in all runs. The temperature dependence of the equilibrium fract ionation is described by the equation 1000 ln alpha(ky-cc) = -2.62 x 10(6)/ T-2. Application of the experimental results to natural quartz-kyanite-garn et assemblages indicates the preservation of the oxygen isotope composition of kyanite acquired during its formation, reflecting its extremely low oxy gen diffusivity. This refractory behaviour restricts the use of kyanite for thermometry but opens the possibility to use its O-isotope composition as an indicator for recognition of polymetamorphic rock histories and reconstr uction of the prograde evolution of a metamorphic sequence.