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Diffusion of silicon in olivine and forsterite: water effect, diffusion laws, and implication for upper mantle rheology
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DOI：10.1007/s00410-026-02324-x.png)
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Abstract
En 中文
As the slowest species, the diffusion of silicon controls the deformation of olivine and plays a crucial role in Earth dynamics. The diffusion of silicon in olivine remains controversial as literature diffusivities spread over several orders of magnitude. Furthermore, the extent of enhancement by hydrogen, observed in both olivine and forsterite (iron-free olivine) remains unclear. We performed experiments of volume diffusion of silicon in dry forsterite. We conclude to a significant effect of hydrogen as diffusivities from previous hydrous experiments with up to 1000 wt. ppm H2O are up to 3 log unit higher than our anhydrous experiments. We analyze our results together with literature data for both forsterite and olivine, and provide laws for silicon volume diffusion in olivine (Ol) $$D_{{Si}}^{{Ol}} = \left( {10^{{ - 10.50}} + 10^{{ - 8.78}} C_{{H_{2} O}}^{{0.42}} } \right)e^{{ - \frac{{316000 + 1460~P}}{{8.314~T}}}} ,$$ and in forsterite (Fo) $$D_{{Si}}^{{Fo}} = \left( {10^{{ - 11.64}} + 10^{{ - 9.38}} C_{{H_{2} O}}^{{0.42}} } \right)e^{{ - \frac{{316000 + 1460~P}}{{8.314~T}}}} ,$$ where the diffusivity DSi is in m2/s, temperature T in K, pressure P in GPa and hydrogen concentration $$C_{{H_{2} O}}$$ in wt. ppm H2O. Hydrogen starts enhancing the diffusion of silicon in olivine at very low concentrations. Diffusivity is already increased by 1.7 log unit at 1 wt. ppm H2O. Consequently, the diffusion of silicon virtually always occurs in the hydrous regime in mantle olivine. However, variable mantle hydration is not expected to result in drastic variation in diffusivity. A change in olivine hydrogen concentration by a factor 10 modifies diffusivity by a factor 2.6 only. As the deformation mechanism of olivine that should prevail in the asthenosphere, i.e. dislocation creep, depends linearly on silicon diffusivity, olivine viscosity will primarily relate to temperature rather than hydration.
Keywords:
Silicon diffusion
Olivine
Forsterite
Mantle rheology
Water-weakening
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