Microstructural observation of naturally deformed antigorite serpentinite

Serpentinite is now important rock for understanding the dynamics of surrounding area of descending oceanic plate. Deformation mechanisms of the serpentine minerals composed of serpentinite have not been fully clarified so far. We observed microstructures of deformed serpentinite with optical microscope and transmission microscope (TEM) to study the deformation mechanisms of serpentine. The sample is serpentinite collected from the Sashu Fault in the Saganoseki Peninsula of Ohita Prefecture, Japan. This serpentinite is mainly composed of antigorite and minor magnetite and Cr-spinel. The grain size of most antigorites is small such as ca. 10 um to 50 um. But the larger grains with ca. 300 um in size are also observed, which make porphyroslastic-like texture. These antigorite grains show the undulose extinction and shape preferred orientation defining the foliation and lineation. The “S” and “S-C” fabrics are observed under an optical microscope scale. Moreover, these antigorite grains show the lattice preferred orientation characterized by [010] and [001] density maximums subparallel to lineation and normal to foliation, respectively (Soda and Takagi, 2009). The formation of these microstructures observed under an optical microscope can be explained by dislocation creep with [010](001) slip system. However, we can not observe any dislocation microstructures with TEM. The lattice defects such as (001) twin and wobbling of (100) are well developed in the deformed antigorite grains. Now we continue to do TEM work in order to clarify deformation mechanism developed the microstructures.