Type-B Crystallographic Preferred Orientation in Olivine Induced by Dynamic Dehydration of Antigorite in Forearc Regions

The crystallographic preferred orientation (CPO) of olivine, specifically the type-B characterized by c-axes aligned parallel to lineation and b-axes concentrated perpendicular to foliation, is essential for explaining the trench-parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type-B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700-750 degrees C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type-C fabric to a type-B fabric, with increasing grain size and dehydration level. The type-B CPO observed in coarse-grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low-temperature dynamic dehydration mechanism, in contrast to the mechanism of high-temperature plastic flow, for explaining the development of type-B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs. This study focuses on understanding why rocks in the forearc regions near subduction zones, where one tectonic plate slides beneath another, have a specific type of olivine arrangement known as type-B olivine fabric. This arrangement is responsible for explaining why seismic velocities have a certain pattern in those areas. However, there has been disagreement about how this arrangement of olivine comes about. To investigate this, we conducted experiments where we subjected rocks rich in antigorite to high temperatures, causing them to transform into olivine. During these experiments, we observed changes in the size and shape of olivine grains, and their arrangement shifted from type-C to type-B as we adjusted the temperature and duration of the experiments. We found a relationship between the shape of olivine grains and how they were arranged. This change in arrangement can be explained by factors like the grains growing in different directions, rotating, and coming together in a specific way. Our discovery suggests that this particular olivine arrangement can occur during the transformation from antigorite to olivine at relatively low temperatures. This arrangement plays a more significant role for the trench-parallel seismic anisotropy in the subduction forearc regions than we previously understood. Dynamic dehydration of antigorite triggers the evolution of olivine crystallographic preferred orientation (CPO) from type-C to type-B Transitional type-C olivine CPO is induced by topotactic and anisotropic growth of olivine during antigorite dehydration Type-B olivine CPO is the final form of antigorite dehydration generated by anisotropic growth, grain rotation, and oriented coalescence