Si self-diffusion in stishovite
Why stishovite?
- Stishovite is one of the major component in the subducted slab
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- its rheology should be significant for the fate of the subdected slab in the lower mantle
- Silica mineral with 6 coordinated Si
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- 6 coordinated Si : Mg-perovskite, Ca-perovskite, MgSiO3 ilmenite, MgSiO3 majorite
- The lowest stability field for 6 coordinated Si
- Undestanding the nature of 6 coordinated Si
Experimental procedure: Single crystal growth of stishovite
Experimental Procedure Annealing experiment
Crystal Orientation by precession camera: (110) or (001)
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Cutting by diamond saw
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Mechanical polishing by diamond paste
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Chemical polishing by colloidal silica
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Coating with 29SiO2 (Thickness: 5-40 nm)
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Annealing at HT and 14 GPa
Conditions of Diffusion Experiment
Isotope analysis
- SIMS Cameca 7f
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- Ion species: O-
- Acceleration voltage: 12.5 kV
- Beam current: 5 nm
- Beam size 5~10 μm
- Mass resolution = 1/3500
- Spattered area: 50×50 or 100×100 μm
Analysis 1. – depth resolution
Analysis 2. -diffusion
Results
Si selfi-diffusion coefficient of stishovite
- [110] : 40 % larger than [001]
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- very small anisotropy
- Relatively low activation energy (330 kJ/mol.)
- EDSi in St ≒ EDSi in Pv
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- Even HT, St and Pv do not become so soft
- DSi in St << DSi in Pv
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- St must be much harder than Pv
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- The fragment of subducted slab should be much harder than the surrounding mantle
Why DSi[110]>DSi[001] ?
- Large corridor in the [100] direction
- Tight in the [001] direction?
- Anyway anisotropy is quite small.
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- high temperature diffusion must be random walk