Experimentally deformed lawsonite at high pressure and high temperature: Implication for low velocity layers in subduction zones

Lawsonite is considered to be one of the most likely hydrous minerals to explain the persistence of seismic low-velocity layers (LVLs) atop subducted slabs to depths of 100–250 km due mainly to the fact that it can persist to these depths, in contrast to other hydrous minerals such as antigorite. Ho...

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Veröffentlicht in:Physics of the earth and planetary interiors 2019-10, Vol.295, p.106282, Article 106282
Hauptverfasser: Iizuka-Oku, Riko, Soustelle, Vincent, Miyajima, Nobuyoshi, Walte, Nicolas P., Frost, Daniel J., Yagi, Takehiko
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Sprache:eng
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Zusammenfassung:Lawsonite is considered to be one of the most likely hydrous minerals to explain the persistence of seismic low-velocity layers (LVLs) atop subducted slabs to depths of 100–250 km due mainly to the fact that it can persist to these depths, in contrast to other hydrous minerals such as antigorite. However, as it is highly anisotropic and subjected to intense deformation during subduction, further constrains on the development of lawsonite crystal preferred orientation (CPO) at high pressure and temperature are required in order to evaluate its role in slowing seismic waves. We have deformed lawsonite aggregates at 5 GPa and 500–800 °C corresponding to 150 km depth and covering temperatures for both cold and hot subduction zones. In this study, we report a new lawsonite CPO pattern resulting from dislocation creep deformation that has not been previously observed. The [100] axes concentrate into the shear direction and the [010] axes are normal to the shear plane, respectively (we refer to this as “type-1” CPO). Such CPO is consistent with our transmission electron microscope (TEM) observations identifying (010)[100] as a dominant dislocation slip-system. The seismic properties resulting from this CPO show the fast P-wave direction to be parallel to the shear direction, and the slow P-wave direction and maximum S-wave anisotropy normal to the shear plane. We performed calculation of omphacite-lawsonite aggregates using our experimental data and varied both the lawsonite contents and the slab dipping angle. Our results show that a reasonable amount of lawsonite can explain Vs reduction in the LVLs, but cannot explain the Vp reduction. However, the presence of lawsonite would induce a rotation of the fast S-wave polarization toward the trench direction as the S-wave anisotropy increases or decreases according to the lawsonite proportion, its CPO strength and the slab dipping angle. We believe that our calculation can now be used for investigation of lawsonite effect in various subductions zones. •Deformation experiments of lawsonite at high PT (5 GPa, > 500 °C)•A Lawsonite CPO yet unknown was obtained.•Grain size and temperature strongly affect CPOs and deformation mechanism.•The dominant slip-system in deformed lawsonite is (010)[100].•Deformation of lawsonite decreases the predicted seismic velocities in atop subducting slabs.
ISSN:0031-9201
1872-7395
DOI:10.1016/j.pepi.2019.106282