Si and Mg self-diffusion coefficient in MgSiO3 perovskite (bridgimanite)

Purpose

  • Use of single crystal MgSiO3 perovskite (bridgmanite)
    • Technique of single crystal growth will be explained after.
  • Simultaneous measurement of Mg and Si self diffusion coefficient

High pressure cell

High-pressure cell of MgSiO3 perovskite (bridgmanite) diffusion annealing experiment

Water content

FT-IR spectra of MgSiO3 perovskite (bridgmanite) single crystals before and after diffusion annealing

An example of diffusion profile

Mg and Si diffusion profile of MgSiO3 perovskite (bridgmanite)

Si and Mg self-diffusion coefficients

Si and Mg self-diffusion coefficients of MgSiO3 perovskite (bridgmanite)

Comparison with previous studies

Comparison of MgSiO3 perovskite (bridgmanite) Mg and Fe diffusion coefficients with previous studies
  • The present DSi agree to the previous studies (Yamazaki et al., 2000; Dobson et al., 2008)
  • Low DFe-Mg similar to DSi (Holzapfel et al.,2005)
  • The present DMg shows clearer equivalency with DSi

Comparison of Mg diffusion

Comparison of Mg diffusion of bridgmanite with upper mantle minerals
Much smaller than the upper-mantle minerals

Explanation of low Mg diffusion

Explanation of low Mg diffusion coefficient of MgSiO3 perovskite (bridgmanite)
  • A-site ion replaces closed packed O: A-site ion is fully surrounded by O
  • No available unoccupied equivalent position for A-site ions
    • 2/3 of 6-coordinated interstitials for B-site ions are unoccupied

Cluster diffusion

MgSiO3 perovskite (bridgmanite) cluster diffusion
  • Defect cluster (De Souza & Martin, 2004)
    • Mg is surrounded by O
      • Without O vacancy, Mg cannot move
    • Identical DSi and DMg
      • Si and Mg move together

Comparison with stishovite

Comparison of Si diffusion coefficients among MgSiO3 perovskite (bridgmanite), SiO2 stishovite and SiO2 quartz

DSi in Pv > DSi in St

Why DSi in Pv >> DSi in St ?

  • Oxygen defects are easier to form in Pv than St
    • Anion vacancies are necessary for cations to hop.
      • Cation : surrounded by Anion
    • Oxygen packing in Pv is more distorted than that in St
    • One O is bonded by 3 Si in St, whereas it is by 2 Si in Pv
    • Mg-O bonding is weak in Pv due to too large distance
      • orthorhombic distortion

DSi in cubic Pv << DSi in ortho Pv???

 

Comparison between Mg-Pv and Ca-Pv

Comparison of crystal structures of Mg-perovskite and Ca-perovskite

CaPv is expected to have low DSi because of its non-distorted oxygen packing.
If so, subducted slab slab must be much harder than the surrounding mantle

Similar activation energy of DSi between Mg-perovskite and stishovite

Explanation of low activation energy of Si diffusion of SiO2 stishovite and MgSiO3 perovskite (bridgmanite)
  • SiO6 octahedra in St is more distorted than those in Mg-Pv
    • St : 3%
    • Mg-Pv : 1 %
  • Ea of St and Mg-Pv are similar
    • about 300 kJ/mol
  •  Small distortion of SiO6 octahedra does not affect on DSi.

 

Summary

  • Low DMg in Pv should be because Mg ions are fully surrounded by O ions in Pv structure.
  • The equality of  DSi and DMg in Pv implies the clustering of vacancies, and both cations move together.
  • The Ea of DSi in Pv and St are similar.
  • Small distortion of SiO6 octahedron does not affect on the local chemical environment of Si.
  • Much higher DSi in Pv than St can be because of distortion of O packing in Pv due to Mg ions.
  • Oxygen packing has large effects on Si diffusion, but the bonding environment has limited effects.
  • The upper mantle viscosity should be almost constant.

Xu, J. S., D. Yamazaki, T. Katsura, X. P. Wu, P. Remmert, H. Yurimoto, and S. Chakraborty, Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite, J. Geophys. Res.-Solid Earth 116, doi:B12205, 10.1029/2011jb008444, 2011.

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