Fe–Mg interdiffusion in wadsleyite: The role of pressure, temperature and composition and the magnitude of jump in diffusion rates at the 410 km discontinuity

Abstract

The limited stability range of wadsleyite seriously impedes our ability to constrain kinetic parameters (e.g. activation energy, activation volume) using experiments carried out over a wide range of temperature and pressure. We have carried out a new measurement to extend the experimental temperature range of the dataset of Chakraborty et al. [Chakraborty, S., Knoche, R., Schulze, H., Rubie, D.C., Dobson, D., Ross, N.L., Angel, R.J., 1999. Enhancement of cation diffusion rates across the 410-kilometer discontinuity in Earth’s mantle. Science 283, 362–365] to the maximum possible limit for that experimental setup. This result allows us to (i) obtain a better constrained value for activation energy for Fe–Mg diffusion in wadsleyite at 15 GPa (∼230 kJ/mol), and (ii) characterize the compositional dependence of Fe–Mg diffusion in wadsleyite. Evaluation of all data available in the literature [i.e. this study; Chakraborty et al., 1999; Farber, D.L., Williams, Q., Ryerson, F.J., 2000. Divalent cation diffusion in Mg 2SiO 4 spinel (ringwoodite), β-phase (wadsleyite), and olivine: implications for the electrical conductivity of the mantle. J. Geophys. Res. 105, 513–529; Kubo, T., Shimojuko, A., Ohtani, E., 2004. Fe–Mg interdiffusion rates in wadsleyite and the diffusivity jump at the 410 km discontinuity. Phys. Chem. Miner. 31, 456–464] reveals that there is a strong pressure dependence of the diffusion coefficient (activation volume ≈14 cm 3/mol). The expression D ( m 2 /s ) = 1.24 × 10 − 6 exp [ 11.8 ( 0.86 − X Mg ) ] exp − 229,000 + ( P − 15 ) × 13.9 × 10 3 J/mol R T is an excellent description of all experimentally measured diffusion coefficients in wadsleyite and points to consistency between the various studies from different laboratories that used different methods. This expression should provide a robust basis for extrapolation of diffusion data for wadsleyite to conditions removed from the experimental ones, e.g. for modeling processes in the interiors of cold subducting slabs. Moreover, characterization of fO 2 and water contents of wadsleyites in the current study and consideration of the pressure dependence of diffusion rates in olivines confirms that the jump in diffusion rates at the olivine–wadsleyite boundary in the transition zone is at least six orders of magnitude; increased water contents in wadsleyite can cause the actual enhancement [Kubo et al., 2004] to be as much as seven orders of magnitude. Taken together with the observation that diffusive mixing is practically impossible in the lower mantle [Holzapfel, C., Rubie, D.C., Frost, D.J., Langenhorst, F., 2005. Fe–Mg interdiffusion in (Mg,Fe)SiO 3 perovskite and lower mantle reequilibration. Science 309, 1707–1710] has a maximum at the lithosphere–asthenosphere boundary in the olivine-bearing upper mantle [Holzapfel, C., Chakraborty, S., Rubie, D.C., Frost, D.J., 2007. Effect of pressure on Fe–Mg, Ni and Mn diffusion in (Fe x Mg 1− x ) 2SiO 4 olivine. Phys. Earth Planet. Int. 162, 186–198], and decreases with depth within the transition zone (this work), these results establish the base of the lithosphere and the top of the mantle transition zone as the key regions for mixing and erasure of chemical heterogeneities in the mantle.