Interdiffusion of deuterium and hydrogen in olivine and enstatite: Initial results

Interdiffusion rates of hydrogen and deuterium in olivine and enstatite, the most abundant minerals in the upper mantle, will improve our understanding of point defects, and defect dominated processes such as electrical conductivity. Deuterium is interdiffused into San Carlos olivine and enstatite as a traceable species of hydrogen to simulate the process of hydrogen self diffusion. Experiments were performed on oriented single crystals of olivine, and unoriented grains of olivine and enstatite. The olivine samples underwent two annealing steps before the diffusion experiment. The first anneal was at 1-bar, 1300 °C at NNO oxygen fugacity for 16+ hrs to set the point defects. Olivine samples were then saturated with 75 +/- 15 ppm-wt H2O during a hydrothermal anneal in a H2O(100wt%) bath at temperatures ranging between 750-900 °C, 2 GPa, NNO, with the inclusion of enstatite to buffer silica. Enstatite grains did not undergo a dry 1-atm anneal and were saturated with 300 +/- 100 ppm-wt H2O. Finally, deuterium is interdiffused into the hydrated olivine and enstatite; samples were recovered and brought back to the same P-T conditions as saturation in a D2O(90wt%)/H2O(10wt%) bath. We used the Cameca 6f SIMS at ASU to measure hydrogen and deuterium across samples from each of these experiments, and fit interdiffusion coefficients to the resulting deuterium and hydrogen profiles. In olivine D[100] is 2.0e-12 m^2/s at 800 °C, 2GPa. Diffusion lengths in the [010] and [001] directions were only modestly larger than the spatial resolution of our technique. Using concentration boundary conditions from the better quality data in [100] diffusion profile, we estimate diffusion coefficients for the other orientations as ~3e-13 m^2/s [001] and ~5e-14 m^2/s [010]. These values are roughly 1 ½ -2 orders of magnitude lower than chemical diffusion in San Carlos olivine (Kohlstedt & Mackwell, 1998). Activation energies estimated over the temperature range 750 - 800 °C are systematically higher than those previously determined for chemical diffusion; however the narrow temperature range requires caution in interpreting such values. Additionally, non-oriented enstatite used as a silica buffer in the same experiments allowed us to measure interdiffusion of deuterium and hydrogen at 800 °C to be ~3.8e-14 m^2/s. This interdiffusion coefficient is ~1 order of magnitude lower than that of pure, synthetic enstatite at 800 °C and ~1 order of magnitude higher than chromium-doped synthetic enstatite (Stalder & Behrens, 2006).