AN EXPERIMENTAL CALIBRATION OF THE TEMPERATURE-DEPENDENCE OF OXYGEN-ISOTOPE FRACTIONATION BETWEEN APATITE AND CALCITE AT HIGH-TEMPERATURES (350-800-DEGREES-C)

The fractionation of oxygen isotopes between apatite and calcite has b een measured experimentally using the direct exchange technique in a p iston-cylinder apparatus at high temperature and pressure (350-800 deg rees C, 1.1 GPa). The fractional approach to equilibrium ranged from 2 0% to 80% and varies systematically with temperature and run duration. Isotopic, XRD and SEM analyses of the run products indicate that the exchange occurs dominantly by recrystallization in the early stages of the experiments. The equilibrium fractionation over the temperature r ange of this study ranges from similar to -5.5 to similar to -2 parts per thousand but the data at 350 degrees and 425 degrees C have relati vely large errors. For T greater than or equal to 500 degrees C an err or-weighted least-squares fit yields: 1000 ln alpha(ap-cc) = -0.68-(1. 60+/-0.26). 10(6) T-2 The magnitude of the temperature dependence is i ntermediate to that for felsic and mafic minerals relative to calcite. Mineral pairs involving apatite are potentially useful indicators of temperature, cooling rates and equilibrium/disequilibrium phenomena in igneous and metamorphic rocks.