OXYGEN-ISOTOPE FRACTIONATIONS IN MUSCOVITE, PHLOGOPITE, AND RUTILE

Oxygen isotope fractionations in laboratory systems have been determin ed for each of the following minerals relative to calcite: muscovite, phlogopite, fluorophlogopite, and rutile. Statistical mechanical calcu lations following the method of Kieffer (1982) were fit to the experim ental data and then used to extrapolate the experimental results to hi gher and lower temperatures. The calculations are represented by a ser ies of equations which allow the reduced partition function ratios (be ta factors) for each of these minerals to be calculated at T > 400 K. These equations can be combined with corresponding equations for calci te, quartz, albite, anorthite, diopside, forsterite, and magnetite (Cl ayton and Kieffer, 1991) to give a large number of mineral-pair fracti onations for use as isotopic thermometers. It was found that the high- frequency vibrations of OH bonds contribute such a small amount of the fractionation factors that they do not introduce significant nonlinea rity to plots of Delta vs. T-2. The commonly used calibrations of quar tz-muscovite and quartz-biotite fractionations are not in good agreeme nt with the present experimental measurements. This probably reflects disturbance of the rock assemblages on which those calibrations were b ased, as a consequence of the high diffusivity of oxygen in micas. The experimental quartz-rutile fractionations are in good agreement with some earlier hydrothermal experiments and with an empirically determin ed calibration. The calculated rutile partition functions of Kieffer ( 1982) are not consistent with the experimental results, probably due i n part to the neglect of the effect of cation mass on the vibrational energies. The large number of mineral systems with measured fractionat ion factors allows a test of various empirical relationships based on oxygen bond strengths. In general, these relationships are successful for anhydrous silicates, but do not adequately account for the behavio r of hydrous minerals or metal oxides.