Metamorphic Reactivity Influences Amphibole Creep Behaviour

ABSTRACT

The rheological behaviour of solid‐solution minerals may play an important role in determining how metamorphic crust accommodates deformation. Whether or not a mineral's deformation is diffusion or dislocation‐mediated depends on a range of extrinsic variables such as stress, strain rate and temperature, as well as intrinsic variables such as diffusivity and water activity. However, it remains unknown if the local gradients in chemical potential that develop between metamorphic phases with evolving pressures and temperatures can also impact the deformation mechanics of metamorphic rocks. Here, we pair electron‐backscatter diffraction (EBSD) data with phase equilibria modelling of high‐variance, low‐variance and compositionally layered blueschist mineral assemblages to demonstrate that the local chemical driving force may directly influence the relative prevalence of dislocation creep and diffusion creep in amphibole. In high‐variance assemblages with glaucophanite domains ( F  = 7), glaucophane displays subgrain boundaries with a consistent misorientation axes distribution indicative of dislocation creep. In domains with rare (meta stable) omphacite, fewer subgrains are observed in glaucophane and the distribution of subgrain boundary misorientation axes is more diffuse. Whereas in neighbouring rocks with low‐variance assemblages ( F  = 1–3), glaucophane subgrain boundaries are rare and the distribution of low‐angle misorientation axes is random. The amphibole microstructures in the low‐variance assemblages are consistent with diffusion creep having been the dominant deformation mechanism. The increased reactivity associated with low‐variance assemblages enhances the chemical driving force for intracrystalline diffusion, thereby accelerating mass transfer and favouring diffusion‐mediated deformation. In contrast, higher variance assemblages exhibit a reduced contribution from chemically driven diffusion and accommodate strain primarily by dislocation creep.