The fore-arc mantle above a subducting slab is a unique site where complex partial melting, melt/fluid–rock interaction, and deformation of mantle rocks occur. To constrain these geological and geodynamic processes we analyzed the deformation microstructures, crystal preferred orientations (CPOs or fabrics), and water content in natural harzburgites that occur as exhumed massifs in the North Qilian suture zone, NW China. These harzburgites are very fresh, and have mineral assemblages of olivine (∼81–87 vol. %), orthopyroxene (∼11–17 vol. %), clinopyroxene (∼1–2 vol. %), and spinel (∼1 vol. %). Detailed analyses of mineral textures, CPO patterns, and rotation axis distributions suggest that the plastic deformation of olivine and pyroxene was accommodated by activating a series of slip systems of dislocation. The olivine (A-/D-type fabric) shows dominant (010)[100] and/or (001)[100] slip systems, as well as other minor [100]-glide, {0kl}[100], and [001]-glide slip systems. The orthopyroxene shows dominant (100)[001] and subordinate (010)[001] slip systems, with minor (100)[010] and (100)[0vw] slip systems. The water content is extremely low in the orthopyroxene (38–44 ppm by weight), equilibrated olivine (4–7 ppm), and bulk-rock samples (9–14 ppm). Previously published refractory mineral and whole-rock compositions, as well as estimated low-pressure (∼1–2 GPa), high-temperature (∼1100–1300°C), low-stress (∼1–4 MPa) and water-poor conditions of deformation, suggest that these harzburgites represent a remnant of fossil fore-arc lithospheric mantle that was probably both formed (experienced partial melting and high-temperature melt/fluid–rock reaction) and deformed in a young and warm fore-arc mantle setting (i.e. juvenile subduction zone). Based on these results, a refined schematic model of olivine fabric distributions in subduction zones is proposed. In this model, the A-/D-type olivine fabrics are inferred to be prevalent in fore-arc lithospheric mantle. The opposing polarizing directions of A-/D-type olivine fabrics with other underlying anisotropic sources in the mantle wedge (e.g. B-type olivine fabrics and oriented serpentinite layers) may weaken the trench-parallel fast S-wave anisotropy contributed by the deformed fore-arc mantle, and thus provide an alternative explanation for the short or nearly null delay times of local shear-wave splitting (sourced from intra-slab earthquakes) that have been detected in some fore-arc regions. In addition, topotactic antigorite fabrics after the A-/D-type olivine fabric might play a minor role in contributing to the low P- and S-wave velocities, high V_p/V_s ratios, and large seismic anisotropies that are typically observed in fore-arc mantle.

The Lindås Nappe, Bergen Arc, is located in western Norway and displays two high-grade metamorphic structures. A Precambrian granulite facies foliation is transected by Caledonian fluid-induced eclogite-facies shear zones and pseudotachylytes. To understand how a superimposed tectonic event may influence olivine fabric and change seismic anisotropy, two lenses of spinel lherzolite were studied by scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) techniques. The granulite foliation of the surrounding anorthosite complex is displayed in ultramafic lenses as a modal variation in olivine, pyroxenes, and spinel, and the Caledonian eclogite-facies structure in the surrounding anorthosite gabbro is represented by thin (<1 cm) garnet-bearing ultramylonite zones. The olivine fabrics in the spinel bearing assemblage were E-type and B-type and a combination of A- and B-type lattice preferred orientations (LPOs). There was a change in olivine fabric from a combination of A- and B-type LPOs in the spinel bearing assemblage to B- and E-type LPOs in the garnet lherzolite mylonite zones. Fourier transform infrared (FTIR) spectroscopy analyses reveal that the water content of olivine in mylonite is much higher (approximately 600 ppm H/Si) than that in spinel lherzolite (approximately 350 ppm H/Si), indicating that water caused the difference in olivine fabric. Fabric strength of olivine gets weaker as the grain size reduced, and as a result, calculated seismic properties for the two deformation stages reveal that P- and S-velocity anisotropies are significantly weaker in the mylonite. Microtextures and LPO data indicate that the deformation mechanism changed from dominant dislocation creep in spinel lherzolite to dislocation creep accompanied by grain-boundary sliding in mylonite. Shear localization in the mylonite appears to be originated from the grain size reduction through (1) enhanced dynamic recrystallization of olivine in the presence of water and (2) Zener pinning of clinopyroxene or (3) by ultracomminution during the pseudotachylyte stage.

Water is known to change the lattice-preferred orientation (LPO) of olivine, which significantly affects seismic anisotropy in the Earth's upper mantle. Research into the LPO of olivine in the deep interior of the Earth has been limited due to inadequate specimens. We report both the water-induced LPOs of olivine and the presence of large quantities of water inside olivine, enstatite, and garnet in garnet peridotites from the North Qaidam ultrahigh-pressure (UHP) collision belt in NW China. We show that the [001] axis of olivine is aligned subparallel to the lineation and that the [100] axis is strongly aligned subnormal to the foliation. This alignment is a known feature of type-C LPO of olivine formed experimentally under water-rich conditions (≥ 700 ppm H/Si) at high pressure and temperature. Enstatite possessed an LPO with the [001] axis aligned parallel to the lineation and the [100] axis aligned normal to the foliation. FTIR analysis of this specimen revealed that olivine contained concentrations of water up to 1130 ± 50 ppm H/Si in clean areas, whereas olivine, enstatite, and garnet contained considerably more water, i.e., 2600 ± 100 ppm H/Si, 5000 ± 100 ppm H/Si, and 21,000 ± 200 ppm H/Si, respectively, when exsolved inclusions were visible. Confocal micro-Raman spectroscopy of these exsolved inclusions revealed that they were composed of hornblende and amphiboles. Straight dislocations were also commonly observed in olivine and are characteristic of olivine that had been experimentally deformed under hydrous conditions. These observations suggest that the type-C LPO of olivine in the North Qaidam UHP belt formed under water-rich conditions.

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.

To understand the lattice preferred orientation (LPO) and deformation microstructures at the top of a subducting slab in a warm subduction zone, deformation experiments of epidote blueschist were conducted in simple shear under high pressure (0.9–1.5 GPa) and temperature (400–500 °C). At low shear strain (γ ≤ 1), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the (010) poles were subnormally aligned with the shear plane. At high shear strain (γ > 2), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain between 2< γ 4, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were in subparallel alignment with the shear direction. The experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role in the formation of LPOs for glaucophane and epidote.