Water has no significant effect on oxygen self-diffusion rate in forsterite

Water is thought to play an essential role in dynamical processes in the Earth's interior. A number of studies have reported a significant influence for water on the physical properties of mantle minerals, e. g., electrical properties, elastic moduli, creep rates, atomic diffusivity, deformation fabric, and so on.

In the case of rheology, it was believed that even several tens wt. ppm of water could enhance creep rate in olivine by orders of magnitude. However, this conception is recently withdrawn by Fei et al. (2013) based on silicon self-diffusion experiments because the creep rate of olivine is believed to be controlled by self-diffusion of silicon, which is the slowest diffusion species in olivine. On the other hand, oxygen is the second slowest diffusion species with similar diffusion rate as silicon, and it may also play an essential role in the creep of olivine. In this study, we systematically measured oxygen self-diffusion coefficient in iron-free olivine, namely, forsterite, at 8 GPa, 1600-1800 K, with water content range from <1 up to ~800 wt. ppm. The result clearly shows that water has no significantly effect on oxygen self-diffusion rate in forsterite (Fig. 1). Together with the small effect of water on silicon self-diffusion, we conclude that the role of water on olivine creep is not essential.

Fig. 1. Oxygen self-diffusion coefficient in forsterite against water content

Fei, H., M. Wiedenbeck, D. Yamazaki, and T. Katsura, No effect of water on oxygen self-diffusion rate in forsterite, Journal of Geophysical Research: Solid Earth, 119(10), 2014JB011141, 2014.