[P-PS08] Formation and evolution of planetary materials in the Solar System

[PPS08-14] Determination of hydrogen self-diffusion coefficients of fluorapatite using a flow-type steam diffusion reactor

*Chikashi Yoshimoto¹, Isao Sakaguchi², Shoichi Itoh¹ (1.Kyoto University, 2.NIMS)

Keywords:apatite, hydrogen, diffusion, Secondary Ion Mass Spectrometry

Hydrogen isotopic compositions of hydrous minerals have been used as an indicator to estimate the origin and the evolution of water in the solar system bodies such as the Earth. Apatite contains hydrogen as a hydroxy group in its structure and is a mineral found not only on the Earth but also on the Moon, in Martian meteorites, achondrites, and chondrites. It has also been observed in Ryugu, which was explored by Hayabusa2. Thus, apatite is universally present in various solar system bodies and is more resistant to metamorphism and alteration than other hydrous minerals, making it a useful target for hydrogen isotopic measurements. However, the hydrogen isotopic compositions of apatites have often been measured under the assumption that diffusion has not occurred, even though the hydrogen isotopic compositions of apatite can change over time due to diffusion phenomena. Therefore, in order to understand the origin and evolution of water in solar-system bodies, it is necessary to determine diffusion coefficients and evaluate the changes in hydrogen isotopic compositions in apatite through experiments. Several studies have investigated hydrogen-related diffusion in apatite at temperatures exceeding 800°C. However, at those temperatures, the F-Cl-OH exchange reaction becomes dominant, making it difficult to assess the change in hydrogen isotopic composition after crystallization without estimating the diffusion coefficient in the lower temperature range. In a previous study by Higashi et al. (2017), hydrogen diffusion experiments were conducted on fluorapatite parallel to the c-axis under ²H₂O/O₂ vapor flow at 500-700 °C. While a diffusion profile due to isotope exchange reaction was obtained, the fitting by Fick's second law only worked near the surface, suggesting the existence of a fast diffusion pathway due to polishing damage, and the lattice diffusion coefficient may not have been estimated. This study involved hydrogen diffusion experiments of fluorapatite in the c-axis parallel direction (at temperatures of 550-700 °C) and in the c-axis normal direction (at temperatures of 600 and 700 °C) under ²H₂O/O₂ vapor flow. Single crystal fluorapatite sections from Durango were used, and polishing damage was minimized by vibrational polishing with colloidal silica. Depth profiles of ²H concentration were obtained by secondary ion mass spectrometry (SIMS), and the diffusion coefficients were calculated by fitting the data with Fick's second law.
The fitting to the diffusion profiles parallel to the c-axis was fitted to lower concentrations, indicating reductions in the effect of fast diffusion. The calculated diffusion coefficients were lower than those obtained in the previous study at each temperature. The activation energy of diffusion was determined to be 204.9 ± 11.3 kJ/mol, which is higher than the value of 80.5 ± 3.3 kJ/mol reported in the previous study. These results support that the effect of fast diffusion was reduced, as fast diffusion typically has a higher diffusion coefficient and a lower activation energy than lattice diffusion. The activation energy for hydrogen diffusion in fluorapatite was found to be the same as that for oxygen diffusion under wet conditions (Farver and Giletti, 1989), suggesting a diffusion mechanism that involves oxygen vacancies. Meanwhile, the diffusion coefficients for fluorapatite normal to the c-axis at 600 and 700 °C were similar to those parallel to the c-axis, in that order. In this presentation, we will discuss the differences in the diffusion mechanisms between these two crystallographic directions, including the results obtained for the vertical c-axis direction at lower temperatures.