Deformation-enhanced reaction in experimentally deformed plagioclase-olivine aggregates

Abstract

The effects of deformation on the kinetics of the net-transfer reaction anorthite + forsterite → cpx + opx + spinel ± gt were studied using static and shear deformation experiments. Experiments were performed on dry anorthite-olivine (An92–Fo93) samples at 900°C and pressures between 1,000 and 1,600 MPa in a Griggs apparatus. Deformed (‘non-hydrostatic’) and undeformed (‘static’) samples are compared in terms of phase petrology, reaction rate and reaction mechanisms. Anorthite + olivine reactions are diffusion-controlled as seen from reaction rim structures. In undeformed samples, delayed reaction onset and low reaction rates demonstrate sluggish nucleation of reaction products and slow rates of diffusion at dry conditions, even at 700–900 MPa confining pressure overstepping. The reaction rate is enhanced in deformed An–Fo samples. The higher rate is mainly attributed to a combination of high stresses and viscous deformation processes of the reactants and products, which cause an increase in the nucleation rate of products. The results imply that viscous deformation processes alone can be responsible for the initiation and localisation of metamorphic reactions in dry rocks in the absence of fluid infiltration.