TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 1), pp. 115-122, 2017

Hydrogen diffusion in the apatite-water system: Fluorapatite parallel to the c-axis

Yoshinori Higashi,1 Shoichi Itoh,1* Minako Hashiguchi,2 Shuhei Sakata,1 Takafumi Hirata,1 Ken Watanabe3 and Isao Sakaguchi3

1Division of Earth and Planetary Science, Kyoto University, Oiwakecho, Sakyo-ku, Kyoto-shi, Kyoto 606-8502, Japan
2Reserch Center for Planetary Organic Compounds, Kyushu University W1-A-622, 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan
3National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

(Received October 12, 2016; Accepted December 6, 2016)

Abstract: 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.
Key words: apatite, hydrogen diffusion, H-D exchange, H2O, SIMS

*Corresponding author E-mail: sitoh@kueps.kyoto-u.ac.jp

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