Small effect of water on upper-mantle rheology based on silicon Lattice diffusion coefficients
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 over-saturated 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.
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.
Fig. 1. DSi vs 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 deformation 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).
Fei, H., M. Wiedenbeck, D. Yamazaki, and T. Katsura (2013), Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients, Nature 498(7453), 213-215 [Full text] [Nature News] [Nature News & Views]
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