Physics
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009pepi..172...28h&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 172, Issue 1-2, p. 28-33.
Physics
3
Scientific paper
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 Mg2SiO4 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 cm3/mol). The expression D(m/s)=1.24×10exp[11.8(0.86-X)]exp-229,000+(P-15)×13.9×10 J/molRT 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 fO2 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)SiO3 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 (FexMg1-x)2SiO4 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.
Chakraborty Subhasish
Frost Dan J.
Holzapfel Christian
Rubie David C.
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