Proton Conduction and Hydrogen Diffusion in Olivine: Reconciling Laboratory and Field Observations and Implications for Average Grain Size in the Lithospheric Mantle

Abstract:

Proton conduction is directly related to the diffusion of hydrogen (most likely as free protons) through the Nernst-Einstein equation. Prior attempts to use this relationship have always invoked additional terms to try to reconcile proton conduction and hydrogen diffusion data. However experimental data on hydrogen diffusion through the mineral lattice only constrain the rate of proton migration coupled with defects (such as vacancies) and not the diffusion of free protons. New diffusion experiments on olivine demonstrate that lattice diffusion is indeed highly dependent on the defect site where hydrogen is bounded but in any case is not fast enough to explain the observed laboratory proton conduction experiments. Effective diffusion, which combines lattice diffusion with the far faster grain boundary diffusion, explains both the laboratory results and also field observations, and yields an estimate of average grain size of 0.5-2 cm at 100 km below the Jagersfontein kimberlite field, consistent with petrological observations on xenolith material.