Water activation of the E-slip system of olivine by water

Introduction

  • Lattice preferred orientation (LPO) in mantle peridotite
    • Mainly determined by the LPO of olivine
    • caused by dislocation creep with a dominant slip system
    • indication of dynamic flow in the mantle
    • causing seismic anisotropy
    • The decrease of seismic anisotropy could be caused by the LPO transition in olivine
  • 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
      • Most common LPO of peridotite
    • E-Type: olivine c-axis parallel to lineation, b-axis normal to the foliation
      • Slip in the [100] direction on the (001) plane: e-slip
      • A few samples from collision zones and island arc environments
      • E-type fabric has lower seismic anisotropy
  • Open question: can structurally incorporated water activate e-slip in olivine?

Experimental procedure

  • Hydrate olivine single crystal with different water contents
    • Talc + brucite as water source
  • Simple shear olivine single crystal on the (001) plane in the [100] direction
    • at 2~5 GPa and 1473 K

Assembly for shear deformation of single crystal olivine

  • TEM observation on (001) plane
  • Same procedure for water-undoped sample

Results

Dislocation structure of dry sample

  • No dislocations on the (001) plane
    • (001) is not a slip plane
  • Only [001] elongated dislocation were observed

 

FTIR of wet sample

Dominance of Group I band => VSi" defects

Dislocation structure of wet sample

Dislocation lines are observed on (001) plane => E-slip is activated

Discussion

Possible mechanism of water-activated e-slip system

Replacing Si by H atom destroy Si-O bond in the olivine => Easier to slip on (001) plane

Implications

  • Next step
    • Check the mobility of water sensitive and insensitive dislocations.

 

Wang, L., Miyajima, N., Kawazoe, T., Katsura, T., Activation of 100 (001) slip system by water incorporation in olivine and the cause of seismic anisotropy decrease with depth in the asthenosphere. Am. Miner. 104, 47-52, 2019.

 

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