Hydrogen diffusion in apatite normal to the c-axis: Estimation of hydrogen diffusing species

Hydrogen isotope retention of apatite is important in discussing hydrogen isotopic compositions in natural apatite. In order to estimate hydrogen isotopic variations associated with thermal processes, experiments on the normal diffusion of fluorapatite along the c-axis using ²H₂O vapor as a diffusion source were performed at 550–700 °C. The diffusion coefficients of fluorapatite were determined by secondary ion mass spectrometry (SIMS). Depth profiles of diffusion were obtained by SIMS, and diffusion coefficients were obtained by model fitting. The Arrhenius relationship for fluorapatite normal to the c-axis yielded a pre-exponential factor (D₀) = 9.77 × 10⁻⁶ [m²/s] and activation energy (E_a) = 208 ± 27 [kJ/mol] over the temperature range of the experiment. The diffusion coefficient normal to the c-axis was approximately three times greater than that parallel to the c-axis and the activation energies from both crystal orientations were consistent within the error margin. The difference in diffusion coefficients can be explained by using the nearest oxygen site and considering the diffusion mechanism with OH⁻ as the diffusing species. Based on the similarity of the diffusion mechanism with crystallographic orientation, a closure temperature was calculated using the results of this study. This updated closure temperature is useful for discussing the retention of hydrogen isotopes and for determining the cooling rate from the hydrogen isotopic compositions in natural water-rock interaction under asteroids, planets, and terrestrial geological setting.