Diffusion models predict that peak metamorphic temperatures are best
recorded by the oxygen isotope fractionation between minerals in a
bi-mineralic rock in which a refractory accessory mineral with slow
oxygen diffusion rate is modally minor to a mineral with a faster
diffusion rate. This premise is demonstrated for high-grade metamorphism
on the island of Naxos, Greece. where quartz-kyanite oxygen isotope
thermometry from veins in high-grade metamorphic pelites gives
temperatures of 635-690 degreesC. These temperatures are in excellent
agreement with independent thermometry for the regional M2 peak
metamorphic conditions and show that the vein minerals isotopically
equilibrated at the peak of metamorphism. Quartz-sillimanite
fractionations in the same veins give similar temperatures (680 +/- 35
degreesC) and suggest that the veins grew near to the
kyanite-sillimanite boundary. corresponding to pressured of 6.5 to 7.5
kbar for temperatures of 635-685 degreesC. By contrast. quartz-kyanite
and quartz-biotite pairs in the host rocks yield lower temperature
estimates than the veins (590-600 and 350-550 degreesC, respectively).
These lower apparent temperatures are also predicted from calculations
of diffusional resetting in the polyphase host-rock system. The data
demonstrate that bimineralic vein assemblages can be used as accurate
thermometers in high-temperature rocks whereas retrograde exchange
remains a major problem in many polymineralic rocks.