Blueschist dislocation creep and glide in subduction systems: constraints from glaucophane experiments

Blueschists are a defining component of subducting oceanic crust and influence slab and interface rheology across thermal regimes. Glaucophane, the dominant sodic amphibole in blueschist, is a key rheology-controlling phase, yet its dislocation-based strength is poorly constrained. We conducted load-stepping experiments in a Griggs apparatus at 600-700°C and 1.0 GPa over shear-strain rates from ~1.2 x 10-8 s-1 to ~1.2 x 10-3 s-1, using a starting grain size of < 63 μm. Mechanical data show a stress-exponent transition from 2.8–3.0 at relatively low stresses (dislocation creep) to 13-19 at high stresses (dislocation glide). Undulose extinction, subgrain development, and recrystallized grains corroborate dislocation activity. We derive flow laws for dislocation creep n = 3, Q = 450 ± 15 kJ/mol, A =2.32x1010 MPa-n s-1 and dislocation glide A = 1038.02, σp = 22 GPa, H = 800 kJ/mol, p = 1, and q = 2. Extrapolation to geologic conditions suggests that dislocation glide will not operate under steady-state subduction-interface conditions, and is likely only relevant during transient high stress perturbations. Dislocation creep dominates at temperatures above 450°C at grain sizes of ~1 mm or larger. Comparison with published quartz/metasediment and eclogite flow laws indicates that blueschist strengths are intermediate between these endmembers, implying a meaningful contribution to long-term interface strength in mafic-dominated (sediment-poor) margins. Together, these results refine glaucophane-controlled blueschist rheology and its implications for subduction zone mechanics.