Crystal preferred orientation of lawsonite and glaucophane in lawsonite blueschist deformed in general shear

Abstract

The crystal preferred orientations (CPOs) of lawsonite and glaucophane in lawsonite blueschist reflect deformation processes in the cold subducting oceanic crust and are crucial for understanding subduction zone dynamics. However, the dominant CPOs of lawsonite and glaucophane and their formation mechanisms in cold subduction zones remain uncertain. We conducted deformation experiments on lawsonite blueschist in general shear at high pressures (1.0–2.5 GPa) and low temperatures (230–400 °C), with varying shear strains (γ = 1.1–4.0). Two types of lawsonite CPOs were found with increasing shear strain: Type-A, where the [010] axis is subparallel to the shear direction, and Type-B, where the [100] axis is subparallel to the shear direction, with the [001] axes subnormal to the shear plane for both types. Additionally, a single type of glaucophane CPO was observed regardless of the magnitude of shear strain, characterized by the [001] axes aligned subparallel to the shear direction and the [100] axes aligned subnormal to the shear plane. This pattern is consistent with those most commonly reported in previous studies. Cataclastic flow was observed in the lawsonite blueschist samples, as indicated by intragranular and shear fractures. In addition, the observation of weak intracrystalline deformation of lawsonite and glaucophane with a minor dislocation suggests that rigid-body rotation with a minor dislocation activity may be a mechanism underlying the CPO formation in these minerals. Our findings highlight that the magnitude of shear strain significantly influences the development of lawsonite CPOs in cold subduction zones.