Hydrogen diffusion in apatite parallel to the c-axis under vapor flow

Abstract

Apatite is ubiquitous in various planetary bodies in the Solar System. Because its structure contains a hydroxy group, researchers have performed extensive investigations to determine the hydrogen isotopic composition of apatite to reveal the origins and evolution of water in planetary bodies. The hydrogen isotopic composition in apatite can change owing to diffusion. Although there have been several diffusion studies related to hydrogen in apatite, they vary in experimental conditions, such as the diffusion source and pressure, making it difficult to estimate the change in hydrogen isotopic composition. Herein, we report hydrogen diffusion experiments under a ²H₂O/O₂ vapor flow at 550°C–700°C using apatite slices with better surface conditions by adapting new polishing methods. ²H concentration depth profiles were obtained via secondary ion mass spectrometry, and the diffusion coefficients were obtained by fitting the profiles using Fick’s second law. The temperature dependence of the diffusion coefficients at 550°C–700°C is represented as the following expression:

This indicates a reduction in the influence of fast diffusion paths, reported by Higashi et al. (2017). These diffusion coefficients would be the hydrogen self-diffusion coefficients in apatite by lattice diffusion and should be employed instead. In addition, the activation energy of diffusion was consistent with oxygen diffusion under wet conditions, thus suggesting that hydrogen diffuses via a mechanism similar to that of oxygen in apatite.