INTERDIFFUSION OF DEUTERIUM AND HYDROGEN IN OLIVINE AND ENSTATITE: EARLY RESULTS

Knowledge of interdiffusion rates of hydrogen and deuterium in olivine and enstatite will improve our understanding of point defects and electrical conductivity in these important mantle minerals. Experiments were performed on spheres and oriented single crystals of San Carlos olivine, and unoriented grains of enstatite. The samples underwent two annealing steps before the diffusion experiment; the first at 1-bar, 1300C at NNO oxygen fugacity for 16+ hrs to set the point defects, followed by a hydrothermal anneal at 2 GPa, 900C and NNO with enstatite as a silica buffer to saturate the samples with ~75 ppm-wt H2O. These samples were then subjected to a shorter hydrothermal anneal at 2 GPa pressure, at 750-900C, for 1-2 hours in a D2O(90wt%)/H2O(10wt%) bath to interdiffuse deuterium, a traceable species of hydrogen, into the hydrated samples. We used the Cameca 6f SIMS at ASU to measure hydrogen and deuterium across samples from each of these experiments. At 800C, D[100] is 2.0e-12 m^2/s. Using concentration boundary conditions from the [100] diffusion profile, we calculate diffusion coefficients for the other orientations as ~3e-13 m^2/s [001] and ~5e-14 m^2/s [010]; however, diffusion lengths in the [010] and [001] directions were only modestly larger than the spatial resolution of our technique. 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 suggests caution in interpreting such values. Additionally, non-oriented San Carlos 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 about 2 orders of magnitude lower than that of pure, synthetic enstatite at 800 C and closer in value to that of chromium-doped synthetic enstatite (Stalder & Behrens, 2006).