New Hydrogen Self Diffusion Coefficients in Olivine Using NanoSIMS

Abstract:

We have previously reported hydrogen self-diffusion coefficients for olivine in deuterium-hydrogen exchange experiments, but were only able to resolve diffusion profiles reliably in the ‘fast’ [100] orientation due to the limited spatial resolution of the Cameca 6f Secondary Ion Mass Spectrometry (SIMS) instrument. Samples were reanalyzed using the Cameca nanoSIMS, which enabled simultaneous measurements of diffusion coefficients in the ‘fast’ [100] orientation and the additional ‘slow’ [010] and [001] orientations to gain a complete 3D view of hydrogen self-diffusion. Measurement precision of deuterium (¹⁶O²H with a Cs⁺beam) is also improved by higher vacuum, better environmental control, and lower background.

Deuterium-hydrogen exchange experiments were conducted at 2 GPa between 750-900 °C by first saturating olivine with homogenous distribution of hydrogen (¹H) and then exchanging some of this with deuterium (²H) in a subsequent experiment. The diffusion coefficents in the [100] orientation that were measured using the nanoSIMS are in good agreement with the previous measurements on the 6f. Diffusion coefficients in the [010] and [001] orientations are very similar in magnitude to one another, ranging between 10^-13 to 10^-14 m²/s, which is over an order of magnitude lower than the [100] orientation. Hydrogen self-diffusion is highly anisotropic in this temperature range, although the activation enthalpies for diffusion in the [010] and [001] orientations are significantly higher than that of the [100] orientation, such that there will be less anisotropy at higher temperatures relevant to the upper mantle.

Comparisons between chemical and self-diffusion measurements allow us to evaluate the various stoichiometric relationships that have been proposed for accommodating hydrogen into the nominally anhydrous structure of olivine. Based on these relationships, we estimate diffusivities of other point defects, small polarons and metal vacancies, as a function of orientation. Finally, we use these new high-fidelity measurements to further advance our model on the contribution of hydrogen to the electrical conductivity of olivine and the upper mantle.