Lattice Preferred Orientation and Deformation Microstructures of Glaucophane and Epidote in Experimentally Deformed Epidote Blueschist at High Pressure

Abstract

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 (010) poles of epidote were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain where 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.

Diffusion Creep of Sodic Amphibole‐Bearing Blueschist Limited by Microboudinage

“…Sodic amphibole also deforms by rigid body rotation in our samples, as emphasized by the strong foliation developed at shear strains as low as ∼1.5 (Figure 6). Rigid body rotation has also been interpreted to occur in naturally (Aspiroz et al., 2007; Berger & Stunitz, 1996; Elyaszaheh et al., 2018; Tatham et al., 2008) and experimentally (Kim & Jung, 2019; Park et al., 2020) deformed amphiboles. Sodic‐calcic amphibole deforms by diffusion creep and is the strain accommodating phase along with albite within the C′‐shear bands (Figure 7).…”

“…One of the most abundant minerals in mafic blueschists is sodic amphibole, but its mechanical properties are not well understood. While some studies have focused on seismic anisotropy (Kim et al., 2013, 2015; Ko & Jung, 2015; Park & Jung, 2022; Park et al., 2020) and blueschist dehydration through eclogitization (Incel et al., 2017), few have explored the viscous flow properties of blueschist and sodic amphibole. Microanalytical and experimental work on amphiboles show a wide range of deformation mechanisms are possible from dislocation glide (Skrotzki, 1992), dislocation creep (Elyazadeh et al., 2018; Hacker & Christie, 1990; Reynard et al., 1989), diffusion creep (Getsinger & Hirth, 2014), cataclasis (Hacker & Christie, 1990; Muñoz‐Montecinos et al., 2023; Nyman et al., 1992), semibrittle flow (Okazaki & Hirth, 2020), and reaction‐driven creep (Condit & Mahan, 2018; Lee et al., 2022), but the conditions under which each mechanism should dominate in both nature and the laboratory are unknown.…”

Seismic Anisotropy of Mafic Blueschists: EBSD‐Based Constraints From the Exhumed Rock Record

“…It is notable that we do find a moderately strong CPO in the [001] axis in all of our samples, even when the [100] and [010] axes are weakly oriented. Alignment of the [001] axis generates low‐magnitude seismic anisotropy which is typical in the early stages of ductile deformation of blueschists (Park et al., 2020). The strong increase in AVp with CPO strength in the glaucophane [100] and [010] axes (Figure 8a) suggests that strong alignment of these axes exerts a key control on the resultant anisotropy magnitude (and Vp pattern).…”

Duplex Underplating, Sediment Dehydration and Quartz Vein Mineralization in the Deep Tremor Source Region

“…The presence of fine-grained glaucophane and albite in the epidote strain shadow is also consistent with dissolution-precipitation creep (Figure 6a). The CPO of glaucophane (Figure 8e) is similar to that formed by dislocation creep (Behr et al, 2018;Park et al, 2020), but can also be formed by dissolution-precipitation creep (Giuntoli et al, 2018). In contrast to the blueschist matrix, the greenschist lenses are poorly foliated and are absent in the interconnected glaucophane and dark seams (Figure 6b).…”