ntroduction

• Lattice preferred orientation (LPO) in mantle peridotite
• • caused by dislocation creep with a dominant slip system
  • indication of dynamic flow in the mantle
  • causing seismic anisotropy
  • • What does the decrease in seismic anisotropy with depth indicate?
    • • transition in LPO pattern with pressure?
      • transition in LPO pattern with temperature?
      • decrease in strain rate with depth?
• LPO types of peridotite
• • A-Type: olivine a-axis parallel to lineation, b-axis normal to the foliation
  • • Slip in the [100] direction on the (010) plane: a-slip
  • B-Type: olivine c-axis parallel to lineation, b-axis normal to the foliation
  • • Slip in the [001] direction on the (010) plane: c-slip
  • AG-Type: a- and c-axes on the foliation, , b-axis normal to the foliation
  • • What kind of slip?

• What conditions are these olivine fabrics reflect?
• • If pressure, the mobility of dislocations produced by a- and c-slips has different temperature dependence.
• Dislocation recovery experiment
• • Dislocations are annihilated by coalescence of dislocations with opposite sign
  • Measurement of annihilation rate by comparing the initial and final dislocation density after annealing under quasi-hydrostatic conditions.
  • Dislocation annihilation rate ∝ dislocation mobility

Experimental procedure

• Production of dislocations in a- and c-slip systems
• • Deformation of olivine single crystals in a shear geometry of the [100] or [001] direction on the (010) plane
  • at 3 GPa and 1600 K
  • Assembly: see T-dependence page

• Annealing for dislocation annihilation
• • Pressures of  2, 7 and 12 GPa at a temperatures of 1650 K
  • 5 min to 24 h
  • fO2 at the Ni-NiO buffer, coexisting with enstatite
    

• Dislocation density measurement
• • oxidation decoration
  • SEM

a) a-slip sample before annealing

b) a-slip sample after annealing

c) c-slip sample before annealing

d) c-slip sample after annealing

scale bar: 2 μm

Results

Identical activation volumes:

a-dislocation: 2.7 ± 0.2 cm³/mol

c-dislocation: 2.5 ± 0.9 cm³/mol

Conclusion

No fabric transition between A-and B-types with pressure!

Wang, L., S. Blaha, T. Kawazoe, N. Miyajima & T. Katsura, Identical activation volumes of dislocation mobility in the [100](010) and [001](010) slip systems in natural olivine, Geophys. Res. Lett. 44, 2687-2692, doi:10.1002/2017GL073070, 2017.