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RESEARCH

Hornblende's mechanisms of texture formation

Earth’s mantle includes large regions where mantle rocks deform in a creep-like viscous fashion - mantle flow. The flow is the manifestation of plastic deformation accommodated through the movement of crystallographic defects. To reveal deformation and flow in the mantle, seismologists probe its anisotropic elastic properties (i.e., seismic anisotropy). I am investigating the effect that deformation and deformation history have on the mantle seismic signature using both experimental and numerical tools.

The experimental aspect was accomplished using samples cored from natural dunite (Åheim, Norway) on a Griggs-type rig apparatus under conditions of 1 GPa confining pressure and temperature of 1200° C. The numerical part was done using the ViscoPlastic Self Consistent (VPSC) and D-Rex numerical models. In these experimental and numerical tests, the effect of an initially existing deformed texture was evaluated. Results revealed a transient stage of textural re-alignment where the elastic anisotropy does not represent the mantle kinematics (as people often assume). As such, modeling and interpreting mantle flow must consider this transient stage in which the seismic anisotropy does not reflect mantle flow.

In the second project the preservation of an existing texture under hydro-static conditions was evaluated. It was found that through an oriented grain-growth process the texture may indeed be modified and that the assumed preservation of a 'frozen' texture in the lithosphere mantle may by inaccurate.

Topaz_2023_Image_Twitter.png

Hornblende grains cut by a calcite vein in blue (EBSD map; sample from Aya Varvara, Cyprus).

Fig. Topaz_RX.jpg

Hornbelnde grain is surrounded by small grains with similar orientations (left) and intra-grain misorientations at one of its limbs (right). Rotation suggests the activity of plastic deformation mechanism.

Fig. Topaz_lower crust seismic anisotropy.jpg

Implications for seismic anisotropy in the lower crust. Here two end-members of flow in th lower cryst, lateral flow and radial flow, and the expected seismic signature.  

Deformation of pre-textured amphibolite

Earth’s mantle includes large regions where mantle rocks deform in a creep-like viscous fashion - mantle flow. The flow is the manifestation of plastic deformation accommodated through the movement of crystallographic defects. To reveal deformation and flow in the mantle, seismologists probe its anisotropic elastic properties (i.e., seismic anisotropy). I am investigating the effect that deformation and deformation history have on the mantle seismic signature using both experimental and numerical tools.

The experimental aspect was accomplished using samples cored from natural dunite (Åheim, Norway) on a Griggs-type rig apparatus under conditions of 1 GPa confining pressure and temperature of 1200° C. The numerical part was done using the ViscoPlastic Self Consistent (VPSC) and D-Rex numerical models. In these experimental and numerical tests, the effect of an initially existing deformed texture was evaluated. Results revealed a transient stage of textural re-alignment where the elastic anisotropy does not represent the mantle kinematics (as people often assume). As such, modeling and interpreting mantle flow must consider this transient stage in which the seismic anisotropy does not reflect mantle flow.

In the second project the preservation of an existing texture under hydro-static conditions was evaluated. It was found that through an oriented grain-growth process the texture may indeed be modified and that the assumed preservation of a 'frozen' texture in the lithosphere mantle may by inaccurate.

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Optical image of kink in an experimentaly deformed sample. (image length ~ 3mm)

Amph_Bochum.jpg

Optical image of kink in an experimentaly deformed sample. (image length ~ 3mm)

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