Measurement of Si grain-boundary diffusion coefficients of forsterite as a function of water content: Estimation of the degree of creep enhancement with water within different regions of the upper mantle

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Knowledge of the rheological properties of mantle minerals is essential for understanding mantle dynamics. Rock deformation experiments suggest that water incorporation weakens the creep strength of mantle minerals and greatly enhances dynamic flow in the mantle. However, this conclusion may be partly compromised by the technical difficulty of deformation experiments. Hence it is important that the large effect of water on creep claimed by deformation experiments be examined by an independent method. Measurement of Si self-diffusion coefficients is such a method because Si diffusion has the slowest rate in mantle minerals and should be therefore the rate-limiting process.Our previous measurements of the Si grain-interior diffusion of forsterite suggest that neither dislocation creep nor Nabarro-Herring creep should be greatly enhanced by water. It is instead creep mechanisms on grain boundaries that are enhanced by water: Coble creep, pressure solution creep and grain boundary sliding. Coble creep is driven by grain-boundary diffusion, and therefore enhancement of Coble creep by water could occur even under water-undersaturated conditions. The effect of water on mantle dynamics would be significant if Coble creep were greatly enhanced by water. On the other hand, interstitial water plays an essential role in pressure solution creep and grain boundary sliding. Therefore, these two mechanisms should be enhanced only under water-saturated conditions. Because regions of water-saturated conditions are thought to occur only under volcanic fronts in the mantle wedge, the effect of water on mantle dynamics would be limited if Coble creep were not enhanced by water.For these reasons, we will measure the Si grain-boundary diffusion coefficient of forsterite under water-undersaturated conditions in this project. Fine-grained pore-free forsterite aggregates will be synthesized by a vacuum sintering technique, and water up to 25,000 ppm will be doped into their grain boundaries. A thin film of 29Si-enriched forsterite will be made on a sample surface and the samples will be annealed for diffusion under the same conditions as the water-doped experiment. The diffusion profile will be measured using SIMS and the water content on grain boundaries will be measured using FTIR spectroscopy.