Oxygen isotope study of quartz-Al2SiO5 pairs from the Mica Creek area, British Columbia: implications for the recovery of peak metamorphic temperatures

Oxygen isotope analyses of quartz-Al₂SiO₅ pairs have been made for samples from the Mica Creek area, British Columbia. We have analysed quartz–kyanite nodules and quartz–kyanite and quartz–sillimanite in multiphase pelitic rocks from the staurolite–kyanite, kyanite, and sillimanite zones. Apparent temperatures calculated from oxygen isotopic fractionation range from 555 °C (staurolite–kyanite zone) to 695 °C (sillimanite zone). Temperatures from the quartz–kyanite nodules range from 630 to 675 °C. Some of the nodules show isotopic disequilibrium. Most of the results confirm predictions that bimineralic rocks will yield an estimate of peak metamorphic temperatures, when the less abundant mineral (an aluminium silicate) is the slower oxygen diffuser. Using cooling rates of 10–100 °C Ma^?1 for the multiphase rocks, measured crystal sizes and modes, the Fast Grain Boundary diffusion model with ‘wet’ diffusion data (P_H2O?1.0 kbar) yields predicted apparent temperatures which are generally lower than the measured apparent temperatures. The agreement is improved if slower diffusion coefficients are used. This suggests that f (H₂O) during cooling was lower than that of the hydrothermal experiments and thus that there was little interaction with aqueous fluids of internal or external origin to modify the isotopic compositions. The measured apparent isotopic temperatures and apparent garnet–biotite Fe–Mg exchange temperatures show very poor agreement for the sillimanite zone samples, with the garnet–biotite Fe–Mg exchange temperatures generally higher than the oxygen isotope temperatures. Compared with the other calibrations that we tested the measured apparent temperatures using the Sharp calibration show the best agreeement with recently published P–T grids, although some variability in agreement is expected due to variable f (H₂O) during cooling.