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Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients

Hongzhan Fei1, Michael Wiedenbeck2, Daisuke Yamazaki3, Tomoo Katsura1

1Bayerisches Geoinstitut, University of Bayreuth, D95440, Bayreuth, Germany
2Helmholtz Centre Potsdam, D14473, Potsdam, Germany
3Institute for Study of the Earth’s Interior, Okayama University, 682-0193, Misasa, Tottori, Japan

Citation: Fei, H., et al. (2013), Nature 498, 213-215. full text

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Summary:

Water has been considered to significantly affect the mantle dynamics. In particular, experimental deformation studies suggested that even several wt. ppm water in olivine could enhance the creep by orders of magnitude. However, the previous deformation studies have some limitations, e.g., considering only a limited range of water concentrations, applying very high stresses, and using polycrystalline samples with oversaturated water, which might affect the results.

Rock deformation can also be understood by silicon self-diffusion coefficients, because the creep rates of minerals at temperatures as those in the Earth's interior are limited by self-diffusion of the slowest species which is silicon in the case of olivine. In this study, we experimentally measured the silicon self-diffusion coefficient, DSi in forsterite at 8 GPa, 1600 - 1800 K, as a function of water content CH2O from less than 1 to about 800 wt. ppm, showing that DSi is proportional to (CH2O)0.32 (Fig. 1). This CH2O exponent (0.32) is strikingly lower than that obtained by deformation experiments (Fig. 2). In other words, if the water content increases from 1 to 1000 ppm, the creep rate would increase by factor of 4000 based on previous deformation results, but in reality only increases by a factor of 10. Thus, the effect of water on the upper mantle rheology is quite small. The large effect of water on creep in deformation experiments is probably an experimental artifact caused by grain boundary sliding as a result of the free fluid phase in the water over-saturated samples used in the experiments.

Fig. 1. DSivs CH2O at 8 GPa, 1600 and 1800 K. CH2O in samples from dry experiments are below the detection limitation of FT-IR. These data points are plotted at 1 wt. ppm with smaller symbols.

Fig. 2. A comparison of dislocation creep rate calculated from DSi with that measured in deforamtion experiments. The creep rate calculated from DSi is consistent with that measured in single crystal deformation experiments, but much lower than that in polycrystalline deformation experiments. Since the polycrystalline deformation experiments were performed with saturated water, the creep rate was probably enhanced by free water on grain boundaries, rather than grain interior dislocation creep (PC: polycrystalline. SC: single crystal).

Since the effect of water on upper mantle rheology is small, many issues in geodynamics must be reconsidered. The smooth motion of the Earth's tectonic plates cannot be caused by mineral hydration in the asthenosphere. Also, water cannot cause the viscosity minimum zone in the upper mantle. And, the dominant mechanism responsible for hotspot immobility cannot be water content differences between their source and surrounding regions.

Summary:

板块漂移学说告诉我们,刚性的岩石圈板块在塑性的软流圈上漂移。这一运动模式意味着岩石圈(lithosphere)比较硬,而软流圈(asthenosphere)相对应该比较软。在过去,科学家们普遍认为水对岩石强度的影响很大,即使几十个ppm(1 ppm为一百万分之一)的微量水也可以导致橄榄石(上地幔的主要成分为橄榄石)的强度降低几个数量级。因此人们认为水是导致软流圈变软从而使岩石圈能够在软流圈上面漂移的主要原因。

但是我们发现,水对橄榄石强度影响很大这一结论来自于一系列橄榄石变形实验的研究,在那些变形实验中,人们使用了橄榄石多晶样品,以及过饱和的水。在这样的实验条件下,自由水很容易进入晶粒边界,导致晶粒之间的滑移,而非晶体颗粒本身的变形。因为上地幔大部分区域处于水不饱和的状态,自由水不大可能存在。
因为晶体的变形主要取决于晶体内扩散最慢的元素,岩石强度同样可由测硅的扩散速率来研究。我们测了橄榄石中硅扩散系数与含水量的关系,发现水对硅扩散系数的影响很小,即使含水量增加1000倍,硅的扩散速率只增加了10倍,也就是说变形速率也只增加了10倍。而如果按照变形实验的结果,1000倍含水量的变化可以导致橄榄石变形速率增加4000倍。因此根据我们的结果,水对上地幔流变性质的影响很小,岩石圈与软流圈的含水差异不能导致其强度变化很多,也就是说水不是岩石圈能够在软流圈上面漂移的主导因素。