Fast anisotropic Mg and H diffusion in wet forsterite

Abstract

Adding hydrogen to forsterite strongly increases the diffusion rate of Mg, but the reason for this is unclear. As Mg diffusion in forsterite can influence its electrical conductivity, understanding this process is important. In this study we use density functional theory to predict the diffusivity of H-bearing Mg vacancies and we find that they are around 1000 times slower than H-free Mg vacancies. H-bearing Mg vacancies are many orders of magnitude more concentrated than H-free Mg vacancies, however, and diffusion is a combination of diffusivity and defect concentration. Overall, the presence of hydrated Mg vacancies is predicted to cause a large (multiple orders of magnitude) increase in both diffusion rate and diffusional anisotropy with a strong preference for diffusion in the [001] direction predicted. In models of experimental data, the effect of water concentration on diffusion is often described by a constant best-fitting exponent. Our results suggest that this exponent will vary between 0.5 and 1.6 across common experimental conditions with pressure decreasing and temperature increasing this exponent. These results suggest that Mg diffusion in forsterite could vary considerably throughout upper mantle conditions in ways that cannot be captured with a simple single-exponent model. Comparisons to measures of hydrogen diffusivity suggest that the diffusion of hydrated Mg vacancies also controls the diffusion of hydrogen in (iron-free) forsterite and that our conclusions above also apply to hydrogen diffusion rates and anisotropy. We also find that cation diffusivity likely cannot explain experimental measurements of the effect of water on electrical conductivity in olivine.

Plain Language Summary

Olivine is an important component of the upper mantle and its properties largely control the properties of the upper mantle. These properties can be strongly affected by the presence of water even in small quantities. In this work we look at the effect of water on one such property—the diffusion of Mg atoms. We find that water can increase both the rate of diffusion and the directional preference of this diffusion by multiple orders of magnitude. Importantly the effect of water is highly dependent upon the prevailing conditions such as pressure, temperature, the buffer for silica activity and the presence or absence of Ti. This means that the effect of water on this property can vary throughout mantle conditions. Our results are also found to apply to the diffusion of hydrogen in forsterite which appears to diffuse through the same mechanism. We also used a simple model of olivine conductivity to predict the effect of water on conductivity. Water has previously been measured to increase conductivity in olivine but our model fails to replicate this effect in multiple ways. This suggests the conductivity increase induced by water in olivine is not due to an increase in ionic diffusion but is due to another unknown mechanism.