Abstract
The mechanisms of hydrogen mobility in nominally anhydrous minerals (NAMs) impact both the water storage in Earth’s mantle and its effective modification induced by mantle processes. Hydrogen can be stored in point defects in NAMs, and recent experimental studies in simple systems have shown that the diffusivity varies significantly from point defect to point defect. Here, a natural mantle-derived olivine from a peridotite xenolith (Pali-Aike in Chile) was analyzed by Fourier transform infrared spectroscopy (FTIR) to investigate if natural olivine can hold the signature of such site-specific diffusivity. The deconvolution of the spectra allows identifying 10 OH absorption bands; each band is attributed to a specific type of defect. The relative diffusivities associated with different defects were estimated and lead to the following results: (1) the OH bands from Group-I (3650–3500 cm−1) yield similar hydrogen diffusivities, except one band at 3598 cm−1, which yields hydrogen diffusivity five times slower than the others; (2) the OH bands from Group-II (3500–3100 cm−1) reveal heterogeneous hydrogen distribution, which is not compatible with a simple mechanism of ionic diffusion; (3) hydrogen diffusion along the [100] direction is five times faster than along [001] for all defects. The use of hydrogen concentration in olivine as a proxy for the deep water cycle requires detailed FTIR profiles to properly characterize ionic diffusion and its consequences on the bulk hydrogen concentration.