Dislocation creep and glide in experimentally deformed glaucophane aggregates

Abstract. Glaucophane is a key rheology-controlling mineral in the oceanic crust of subducting slabs at blueschist facies conditions. Studies of naturally deformed glaucophane suggest dislocation-related deformation mechanisms can be activated at some pressure-temperature-stress conditions in subduction environments; however, the strength of glaucophane deforming via these mechanisms remains unconstrained. To address this, we conducted load stepping experiments using a Griggs apparatus at temperatures of 600–700 °C, 1.0 GPa, and shear strain rates of ∼ 1.2×10−⁸ s^-1 to ∼ 1.2×10−³ s^-1, with a starting grain size of <63 µm. The mechanical data from these experiments show a transition in the stress exponent from 2.8 ± 0.2 at relatively low stresses, indicative of dislocation creep, to 14–19 at relatively high stresses, consistent with dislocation glide. Microstructural analyses show kinking, undulose extinction, and a shift from sharp linear grain boundaries in the hydrostatic samples to more rounded and lobate sutured grain boundaries in the deformed samples. High internal misorientations (subgrains, undulose extinction) in both relict and fine-grained regions of the deformed samples further support the activation of dislocation-related mechanisms. Based on these observations, we develop flow laws for dislocation creep n = 3, Q = 450 ± 15 kJ/mol, A =2.32x10¹⁰ MPa⁻ⁿ s^-1) and dislocation glide (Q = 899 ± 43 kJ/mol, C = 1.83x10³², and α = 0.0123). Extrapolations of our flow laws to geologic conditions suggests that dislocation glide is unlikely to occur at steady state conditions, while dislocation creep dominates at temperatures above 450 °C at relatively large grain sizes of ∼1 mm or larger. These insights refine our understanding of glaucophane rheology and its implications for subduction zone mechanics.