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Lattice preferred orientation (LPO) and seismic anisotropy of orthopyroxene (enstatite) in mantle xenoliths from Spitsbergen, Svalbard, near the Arctic, are studied. LPOs of enstatite were determined using electron backscattered diffraction (EBSD). We found four types of LPOs of orthopyroxene and defined them as type-AC, -AB, -BC, and -ABC. Type-AC LPO of orthopyroxene is defined as (100) plane aligned subparallel to foliation and [001] axis aligned subparallel to lineation. Type-AB LPO is defined as (100) plane aligned subparallel to foliation and [010] axis aligned subparallel to linea- tion. Type-BC LPO is defined as (010) plane aligned subparallel to foliation and [001] axis aligned subparallel to lineation. Type-ABC LPO is defined as both (100) and (010) planes aligned subparallel to foliation with a girdle distribution of both [100] and [010] axes normal to lineation and [001] axis alignedsubparallel to lineation. We report for the first time the type-AB, -BC, and -ABC LPO of orthopyroxene. We found that the LPO pattern has a correlation with the content of orthopyroxene in the specimen. Nicolet 6700 FTIR (Fourier transformation infrared) study of enstatite showed that type-AC LPO was observed mostly in the samples of enstatite with low water content. It is found that the strength of the LPO of enstatite decreases with increasing water content and has a correlation with the strength of the LPO of olivine: the stronger the LPO of enstatite, the stronger the LPO of olivine. Seismic anisotropy of enstatite was smaller than that of olivine in the same specimen.

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The eclogite in the Flem Gabbro from Flemsøya Island of the Western Gneiss Region in Norway contains atypical eclogitic minerals, such as olivine and orthopyroxene, and can be texturally divided into weakly deformed massive eclogite (MEC) and strongly deformed foliated eclogite (FEC). Based on phase equilibria modeling, peak metamorphic pressure‐temperature conditions of ~600–750 °C and ~1.0–2.5 GPa and ~700–820 °C and ~2.7–3.7 GPa are recorded in MEC and FEC, respectively. These different pressure‐temperature conditions between MEC (high‐pressure, HP) and FEC (ultrahigh‐pressure, UHP) in the same outcrop reflect deformation‐enhanced eclogitization metamorphism (HP to UHP transition) via the addition of external water during subduction/burial to early exhumation stages and metastable preservation of the HP MEC assemblage at UHP condition due to the local lack of deformation and fluid access at the deep subduction interface or around Moho beneath continental collision zone. Based on the mineral microstructures, nondislocation‐based creep mechanisms—such as diffusion creep, grain and phase boundary sliding, and rigid‐body‐like rotation—play dominant roles in governing the deformation features of FEC and developing the crystal preferred orientations of its major constituent minerals. These deformation mechanisms could considerably affect the interplate coupling at the subduction interface or the rheological strength of Moho beneath continental collision zones. Therefore, the effects of metastability‐based (i.e., preservation of low‐pressure assemblages at HP conditions) and contributions of nondislocation‐based creep mechanisms should be included in the future geodynamic and petrological simulations of subduction and collision processes.

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