2017 年 51 巻 1 号 p. 115-122
Many studies in the past decade have sought to explore the origin and evolution of water in planetary bodies based on the hydrogen isotopic compositions of apatite. However, no investigation has studied hydrogen diffusivity in apatite. This work reports hydrogen diffusion experiments using a natural Durango fluorapatite carried out under a saturated 2H2O/O2 vapor flow at temperatures of 500–700°C. Diffusion depth profiles for 1H and 2H were measured using secondary ion mass spectrometry (SIMS), indicating that 2H diffusion occurred by an exchange reaction between the original 1H and 2H during annealing. Hydrogen diffusion coefficients were obtained by the fitting of diffusion profiles of 2H using Fick’s second law; they followed an Arrhenius-type relationship. The temperature dependence of hydrogen diffusion parallel to the c-axis at 500–700°C can be expressed as
Hydrogen diffusion coefficients in apatite are several orders of magnitude greater than those of other elements. Hydrogen diffusion in apatite occurs at relatively low temperatures (below 700°C). This study indicates that the hydrogen isotopic compositions of apatite are readily affected by the presence of water vapor through the 1H-2H exchange reaction without changing the total water content in the crystal.
