KINETICS OF FLUORINE, CHLORINE AND HYDROXYL EXCHANGE IN FLUORAPATITE

A series of experiments were performed to obtain diffusion coefficients for the exchange of fluorine, chlorine and hydroxyl in natural fluorapatite at both 1 atm and 1 GPa. Gradients in chemical potential were generated in 1-atm experiments by juxtaposing powdered chlorapatite and the natural face (flux parallel to to a-axis) or a polished basal section (flux parallel to to c-axis) of gem-quality fluorapatite single crystals. Compositional contrast was generated in hydrothermal experiments at 1 GPa by encapsulating a fluorapatite crystal + H2O +/- a crimped, but unwelded, inner capsule containing chlorapatite. Both configurations produced compositional profiles in fluorapatite arising from the loss of fluorine and the uptake or loss of chlorine +/- hydroxyl. Electron microprobe and FT-IR analytical traverses were used to obtain composition vs. distance data in the fluorapatite and Boltzmann-Matano analysis and error-function inversion were used to calculate diffusion coefficients. In accordance with the requirement of charge neutrality, diffusion coefficients calculated using either chlorine or fluorine concentration profiles were found to be essentially identical and profiles for hydroxyl are in qualitative agreement with the latter D-values. Diffusion coefficients from 1-atm experiments showed a positive correlation with chlorine concentration, whereas values from experiments at 1 GPa exhibited no such variation. In 1-atm experiments, a decrease in the diffusion coefficient could be correlated with a drop in the bulk OH content of apatite as measured in the cores of run-products. Diffusion coefficients measured in 1-GPa experiments showed no such variability and the OH contents of apatites, as well as measured diffusivities, were uniformly higher. Diffusion coefficients determined in 1-GPa experiments of different duration yielded similar values as did those measured in 1-atm experiments in which the bulk OH content of run-products were the same. Diffusion coefficients measured in this study define the following Arrhenius relations: [GRAPHICS] Results of this study indicate that, arising from changes in P, T or fluid/melt chemistry, apatite may rapidly develop measurable F-Cl-OH zoning profiles via diffusion parallel to to c with little or no gradients expected parallel to to the a-axis. Characterization of compositional gradients in apatite sections along the c-axis may therefore offer the best means of utilizing apatite to evaluate the most recent changes in the halogen chemistry of coexisting fluids or magmas. In addition, calculations using the diffusion data from this study suggest that only under conditions of rapid cooling, low initial temperatures or chemical isolation, will the halogen chemistry of apatite remain pristine. Caution is therefore suggested when interpreting apatite halogen chemistry unless other evidence for halogen retention are available.