5:15 PM - 6:45 PM
[MZZ45-P08] Determination of hydrogen self-diffusion coefficients of fluorapatite using a flow-type steam diffusion reactor
Keywords:apatite, hydrogen, diffusion, Secondary Ion Mass Spectrometry
Hydrogen isotopic composition of hydrous minerals has been used as an indicator to assess the origin and evolution of water in solar-system bodies such as the Earth. Apatite contains hydrogen as a hydroxy group in its structure and is ubiquitous in various planetary bodies in the solar system. Compared to other hydrous minerals, apatite is more resistant to metamorphism and alteration, making it a useful target for hydrogen isotopic composition measurement. However, the hydrogen isotopic composition of apatite has been measured under the assumption that no diffusion has occurred, even though the hydrogen isotopic composition of apatite can change from the composition at the time of formation due to diffusion phenomena. Yoshimoto et al. (2024) evaluated the effect of polishing-derived dislocations on crystal surfaces and showed that vibrational polishing with colloidal silica can reduce the effect of fast diffusion due to dislocations. This result indicates that the correct self-diffusion coefficients cannot be obtained due to the effects of fast diffusion unless diffusion experiments are performed under better surface conditions. The activation energy of diffusion at 550 - 700 °C is equal to that of oxygen diffusion under wet conditions (Farver and Giletti, 1989), suggesting that it is an oxygen-related diffusion mode. Therefore, we concluded that OH is the diffusing species for hydrogen diffusion in apatite. However, since apatite is a hexagonal mineral, it is necessary to obtain diffusion coefficients in the direction perpendicular to the c-axis in addition to the direction parallel to the c-axis in order to discuss the diffusion mechanism in more detail and to apply diffusion coefficients to natural materials. In this study, hydrogen diffusion experiments of fluorapatite in the vertical direction of the c-axis (550 - 700 °C) are reported.
Depth profiles of 2H concentration are obtained using secondary ion mass spectrometry (SIMS), and diffusion coefficients were obtained by fitting a model with constant surface concentration, the simplest experimental setup. The experimental results in that direction showed that the fitting area for polishing under the same conditions as Yoshimoto et al. (2024) was only in the vicinity of the surface. In order to obtain a more accurate diffusion coefficient, we attempted to further remove dislocations by pre-annealing at 700 °C with wet flow. The fitting is good, and the results are in line on an Arrhenius plot with the experimental results without pre-annealing. The activation energies are consistent with those of Yoshimoto et al. (2024) in the c-axis direction within a margin of error, and the diffusion coefficients were approximately three times larger. The difference in diffusion coefficients is due to the difference in geometrical distance in crystal.
Depth profiles of 2H concentration are obtained using secondary ion mass spectrometry (SIMS), and diffusion coefficients were obtained by fitting a model with constant surface concentration, the simplest experimental setup. The experimental results in that direction showed that the fitting area for polishing under the same conditions as Yoshimoto et al. (2024) was only in the vicinity of the surface. In order to obtain a more accurate diffusion coefficient, we attempted to further remove dislocations by pre-annealing at 700 °C with wet flow. The fitting is good, and the results are in line on an Arrhenius plot with the experimental results without pre-annealing. The activation energies are consistent with those of Yoshimoto et al. (2024) in the c-axis direction within a margin of error, and the diffusion coefficients were approximately three times larger. The difference in diffusion coefficients is due to the difference in geometrical distance in crystal.