Oriented growth due to topotactic replacement of antigorite by olivine as a mechanism for the formation of B-type olivine CPO in convergent margins

B-type olivine (Ol) CPO patterns are characterized by an a-axis concentration parallel to the intermediate principle axis of strain and have been proposed by many workers as the cause of seismic anisotropy in the mantle wedge of subduction zones that shows the fast direction perpendicular to the plate movement direction. Experimental work has shown that B-type Ol CPO can form by dislocation creep at relatively high stresses and in the presence of water. Natural examples of B-type Ol CPO have also been reported, but there are several discrepancies with the experimental results. 1) Some natural B-type CPO formed at relatively high temperatures and low stress outside the ranges predicted by experiments. 2) Natural examples lack evidence for the c-slip expected for the formation of B-type Ol CPO by dislocation creep. 3) The high shear stresses expected along subduction boundaries promote the formation of B-type Ol CPO, but these regions are also expected to be associated with the formation of serpentine minerals and even relatively small amounts prevent strong CPO patterns from forming because of grain-boundary sliding occurring between Ol and serpentine. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of antigorite (Atg) schist. In the Happo-One region of the Hida Marginal belt, Japan, dehydration of foliated Atg produces non-deformed secondary Ol formed in veins or patches and peridotite-hornfels where the conversion is complete.The CPO of non-deformed Ol in veins and in the hornfels shows a strong B-type fabric. The veins show consistent Ol CPO irrespective of the vein orientation, implying the CPO is not related to the vein opening direction. The CPO of Atg bordering the vein Ol shows a strong concentration of c-axes at a high angle to the foliation and a strong alignment of b-axes parallel to the lineation. Numerous recent studies have shown this type of Atg CPO is the most widespread in the forearc wedge mantle. Several types of topotaxial growth relationships are known between Ol and Atg. The observed relationships between the Ol and Atg CPO patterns in this study imply topotaxial relationships between the two minerals where [010]atg is parallel [001]ol and [010]ol is parallel to [001]atg. The CPO of non-deformed Ol in peridotite-hornfels shows a strong B-type fabric that is particularly strongly developed in coarse-grained domains. Calculations show that a relatively thin layer (several km) of mantle with the same Ol CPO as the peridotite-hornfels can account for the observed seismic anisotropy of NE Japan. Atg-bearing mantle is predicted to be a widespread component of forearc mantle. As this material is dragged down by the traction of the downgoing slab, it will become deformed and foliated. When this foliated antigorite schist reaches sufficiently high T and P conditions, it will undergo dehydration. Our results show that when this dehydration occurs, the newly formed Ol is likely to have a B-type CPO. These results offer an alternative explanation of how B-type fabrics form in subduction zone settings and explain how B-type CPO can form in relatively high T low stress parts of the mantle wedge away from the subduction boundary.