FAST, FASTER, FASTEST: PRELIMINARY RESULTS FOR FLUORINE MOVEMENT IN TITANITE, AMPHIBOLE, AND BIOTITE

Titanite, amphibole, and biotite are common minerals that may contain substantial amounts of fluorine, potentially preserving information about the F content of melt volatiles and metamorphic and ore-forming fluids. Data from an experimental investigation of F diffusion suggest that these minerals incorporate and retain F very differently.

Experiments on Brazilian titanite at both atmospheric and 50 MPa pressures reveal that F diffusion is faster than any other component (Nd, Sr, Zr, or O) thus far investigated in sphene. While the diffusivities do not fall along a simple Arrhenius relationship for temperatures from 701-906 ºC, they do outline a range from 6.7 x 10^-21 to 8.5 x 10^-20 m²/s. The scatter suggests that F diffusion is greatly affected by minor oxidation variations in the crystals in these experiments.

Faster diffusivities are noted in the c-perpendicular direction in 100 MPa experiments on Grenville tremolite and fluororichterite crystals (provided by the New York State Museum). The data are few, but they indicate that both amphibole composition and the presence of water affect F diffusivity. “Wet” tremolite diffusion at 800 ºC yields a D = 4.3 x 10^-18 m²/s, generally agreeing with previous work on tremolite F diffusion. "Dry” fluororichterite at the same T yields a similar D = 2.0 x 10^-18 m²/s. In contrast “wet” fluororichterite at 700 ºC yields D = 1.56 x 10^-17 m²/s. Diffusion in the c-parallel direction is apparently slower; these experiments have not produced measurable c-parallel diffusion profiles.

Even faster diffusivities are noted in 400 MPa experiments on Brazilian biotites annealed in the presence of F-rich fluids. The previously reported activation energy (Ea) of 314.5 kJ and an intercept (Do) of 22.7 m²/s, from short-duration experiments from 650-750 ºC, may approximate lattice diffusion kinetics. But new longer-duration experiments reveal the significance of defect planes as a mode of component transport in micas. X-ray maps of grains show enhanced fluorine concentrations on c-perpendicular (cleavage) planes and further deformation-induced enhancement. Whipple assessments suggest actual bulk diffusivities may be appreciably higher ( as high as 3.4 x 10^-12 m²/s at 700 ºC, assuming defect planes with an effective width of 10 nm).