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
Final Report Abstract
In order to estimate pressure, temperature and water-content dependence of Coble diffusion creep in the upper mantle, we measured Si grain-boundary diffusion coefficients in forsterite aggregates as a function of pressure from 1 to 13 GPa, temperature from 1100 to 1300 K, and bulk water content from less than 1 up to 350 wt. ppm using a multi-anvil apparatus and secondary ion mass spectrometer in depth profiling mode. We obtained activation energy, activation volume, and water-content exponent of 220 ± 30 kJ/mol., 4.0 ± 0.7 cm3/mol., and 0.26 ± 0.07, respectively. The water-content dependence of Si grain-boundary diffusion in forsterite is thus very small, and as small as that of lattice diffusion. It is also noted that activation energy of Si grain-boundary diffusion is much smaller than that of lattice diffusion. Together with our previous results, the present results predict that Coble diffusion creep dominates low-temperature regions in the upper mantle, whereas dislocation creep dominates in high-temperature regions. Water has negligible effects on diffusion and dislocation creep in the upper mantle, and does not change dominant creep mechanism. There will be a transition from diffusion creep in the shallow upper mantle to dislocation creep in deeper regions. We also measured Mg lattice diffusion coefficients in forsterite as a function of pressure, temperature and water content using the same samples as measurement of Si grain-boundary diffusion coefficients. The activation energy, water content exponent, and activation volume were found 250 ± 30 kJ/mol., 4.3 ± 0.3 cm3/mol. and 1.2 ± 0.2, respectively. It was thus found that Mg lattice diffusion has much larger water-content dependence than Si diffusion. Since Mg lattice diffusion should control ionic electrical conduction in olivine based on the Nernst-Einstein relations, ionic conductivity will be largely enhanced by water incorporation as Mg lattice diffusion is. Ionic conductivity in olivine exhibits a maximum at the top of high-temperature oceanic asthenosphere due to the negative pressure and positive watercontent dependence. This evaluation well explains the high conductivity anomaly observed at 70 to 120 km depth beneath young oceanic plates. Mg grain-boundary diffusion coefficients in forsterite were also measured, and have activation volume of 3.9 ± 0.7 cm3/mol., activation enthalpy of 360 ± 25 kJ/mol. at 8 GPa. and water content exponent of 1.0 ± 0.1. By comparison with the Mg lattice diffusion data, the bulk Mg diffusion in olivine will be dominated by lattice diffusion if the grain size is larger than 1 mm under upper mantle conditions. On the other hand, Mg grain-boundary diffusion will be comparable with lattice diffusion in olivine when the grain sizes are 1 to 100 μm.
Publications
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2016. New constraints on upper mantle creep mechanism inferred from silicon grainboundary diffusion rates. Earth Planet. Sci. Lett. 433, 350-359
Fei, H.Z., Koizumi, S., Sakamoto, N., Hashiguchi, M., Yurimoto, H., Marquardt, K., Miyajima, N., Yamazaki, D., Katsura, T.,
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2018. Mg lattice diffusion in iron-free olivine and implications to conductivity anomaly in the oceanic asthenosphere. Earth Planet. Sci. Lett. 484, 204-212
Fei, H.Z., Koizumi, S., Sakamoto, N., Hashiguchi, M., Yurimoto, H., Marquardt, K., Miyajima, N., Katsura, T.
