Abstract Fe–Mg interdiffusion rates in polycrystalline wadsleyite aggregates have been determined as a function of water content (up to ∼0.345 wt.% H2O) at 16 GPa and 1373–1773 K in a Kawai-type multi-anvil apparatus. Pre-synthesized water-poor and -rich polycrystalline wadsleyite were used as starting materials. Diffusion profiles were obtained across the interface between Fe-free and -bearing diffusion couples, namely, Mg2SiO4 and (Mg0.9Fe0.1)2SiO4 aggregates by electron microprobe. Fe–Mg interdiffusivities by experiments yield D Fe − Mg ( m 2 / s ) = D 0 X Fe n C H 2 O r exp ⁡ [ − ( E + α X Fe + β C H 2 O ) / R T ] , where D 0 = 1.33 − 0.23 + 0.20 × 10−11 m2/s, n = 0.19 ± 0.04, r = 0.29 ± 0.12, E = 92 ± 2 kJ/mol, α = −45 ± 12, and β = −134 ± 2. Our results indicate that water significantly enhances the rates of Fe–Mg interdiffusion in wadsleyite (a factor of 2.4 for fixed temperature and Fe concentration) compared to that in ringwoodite. Although under hydrous condition the transition zone shows the maximum Fe–Mg mixing efficiency as revealed by diffusivity-depth profile in the mantle, homogenization of existing chemical heterogeneity is still very limited at geological time scale only through solid-state diffusion. Combined with the Nernst–Einstein relation, the results suggest that the contribution of water to the electrical conductivity of wadsleyite or ringwoodite may be overestimated from Fe–Mg interdiffusion data at high water content. Further calculation demonstrates that ∼0.1–0.5 wt.% H2O is sufficient to account for the high conductivity values in the upper part (410–520 km) of the mantle transition zone as observed by electromagnetic induction studies.

中文翻译：

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Wadsleyite 中的 Fe-Mg 相互扩散及其对过渡带含水量的影响

摘要 在 Kawai 型多砧装置中，在 16 GPa 和 1373-1773 K 下，多晶瓦德斯利石聚集体中的 Fe-Mg 相互扩散速率已被确定为水含量（高达 0.345 wt.% H2O）的函数。预合成的贫水和富水多晶瓦兹利石用作起始材料。通过电子微探针在不含 Fe 和含 Fe 的扩散偶（即 Mg2SiO4 和 (Mg0.9Fe0.1)2SiO4 聚集体）之间的界面上获得扩散剖面。通过实验得到的 Fe-Mg 互扩散系数 D Fe − Mg ( m 2 / s ) = D 0 X Fe n CH 2 O r exp ⁡ [ − ( E + α X Fe + β CH 2 O ) / RT ] ，其中 D 0 = 1.33 − 0.23 + 0.20 × 10−11 m2/s，n = 0.19 ± 0.04，r = 0.29 ± 0.12，E = 92 ± 2 kJ/mol，α = -45 ± 12，β = -134 ± 2。我们的结果表明，与菱镁矿相比，水显着提高了瓦兹利矿中 Fe-Mg 相互扩散的速率（固定温度和 Fe 浓度的因子为 2.4）。尽管在含水条件下过渡带显示出最大的 Fe-Mg 混合效率，正如地幔中的扩散深度剖面所揭示的那样，但仅通过固态扩散在地质时间尺度上对现有化学非均质性的均质化仍然非常有限。结合 Nernst-Einstein 关系，结果表明，高含水量下的 Fe-Mg 相互扩散数据可能高估了水对瓦兹利石或菱形伍德石电导率的贡献。进一步的计算表明~0.1-0.5 wt。