[SMP37-P13] Crystal Preferred Orientation of Secondary Olivine Formed after Orthopyroxene in Mantle Wedge during Serpentinization from the Khantaishir Ophiolite, Western Mongolia
Keywords:Chemical reaction effect, Secondary olivine CPO, Topotactic growth
The metaharzburgite contains both primary olivine (P-Ol) and S-Ol, with Mg# of 0.92-0.93 and 0.96-0.98, respectively. The S-Ol commonly exists as fine-grained aggregates with size of ca. 5 mm, and the aggregate has aligned fractures filled with Atg grains. The microstructural and chemical features of the S-Ol aggregate indicate that the S-Ol was formed after Opx by silica-releasing reaction, which is coupled with silica consuming reaction of P-Ol to produce Atg. P-Ol grains show the prominent CPO which c-axis sub-parallel to the mineral lineation defined by the grain shape of Atg in the matrix and the b-axis sub-perpendicular to the foliation defined mainly by the grain shape of minerals in the matrix. This P-Ol CPO corresponds to an apparent C-type Ol CPO e.g., 3 in the reference frame defined by the present shapes of Atg grains. Atg in matrix shows a strong CPO, which exhibits the c-axis is sub-normal to the foliation and girdle distribution of the a- and b-axis on the foliation plane suggesting the topotactic growth after P-Ol and this relationship corresponds to the type-1 topotactic growth of Atg after Ol 2. Each S-Ol aggregate shows a strong CPO indicating its origin to be a single Opx grain. In contrast, the CPO patterns of individual S-Ol aggregates are different in a thin section. This indicates that Ol grains were not likely to be significantly re-oriented by dislocation creep of S-Ol and that preexisting Opx might not be oriented strongly. The CPO calculated using all of these S-Ol aggregates in the same thin section shows the significantly weaker concentration for each axis than that calculated from each aggregate. Therefore, the S-Ol grains have little contribution to the bulk-rock anisotropy. The Atg bands in S-Ol are developed parallel to each other and might be directly reflected to the cleavage of original Opx. Therefore, the topotactic growth of S-Ol after Opx may be estimated using the orientation of the S-Ol aggregate and the cleavage of Opx determined by the orientation of Atg in the fracture with the (001) plane of Atg sub-normal to the fracture. The CPO of S-Ol in the aggregate has a prominent CPO, which shows mainly c-axis sub-parallel to the fractures (the cleavage of the original Opx) and the b-axis sub-normal to the fractures (type-I topotaxy).
In conclusion, the harzburgite in the Naran massif was originally composed of P-Ol with CPO and non-orientated Opx. During hydration and S-Ol formation within the mantle wedge, topotactic S-Ol after Opx with weak CPO and type-I topotactic Atg after P-Ol were developed.
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