OXYGEN-ISOTOPE FRACTIONATION BETWEEN FLUORPHLOGOPITE AND CALCITE - ANEXPERIMENTAL INVESTIGATION OF TEMPERATURE-DEPENDENCE AND F- OH- EFFECTS/

The fractionation of oxygen isotopes between fluorphlogopite and calci te has been measured experimentally at high temperature and pressure ( 400-800-degrees-C, 11 kbar) by the direct exchange technique, using a piston-cylinder apparatus. The fractionation is described by the follo wing equation: 1000 In alpha(Fphl - Cc) = 0.43 - (1.84 x 10(6))/T2. Th is equation, when combined with a published relationship for quartz-ca lcite, yields calculated temperatures that are, on average, 30 % lower than those obtained from an empirical quartz-biotite calibration whic h has commonly been used in the past. Correlations between oxygen isot ope fractionation and mean oxygen electrostatic site potentials demons trate that both the magnitude and the temperature dependence of the fr actionation measured in this study are consistent with the data for si x other minerals calibrated using the same experimental technique. Exp erimental uncertainties preclude a definitive statement with regard to the magnitude of the fractionation between oxygens in the hydroxyl si tes and those in the rest of the mineral structure. A comparison of th e experimental data with model values, however, suggests that the diff erences are qualitatively consistent with those determined using a par tial fluorination analytical technique.