Influence of differential stress on the growth of wet enstatite and enstatite-forsterite reaction rims

Abstract

Reaction rim growth experiments provide insight into mass transport phenomena, which are important for metamorphic rock-forming processes and deformation mechanisms. We investigated the formation of enstatite single rims between quartz and forsterite and of enstatite-forsterite double rims between quartz and periclase using porous polycrystalline starting materials. About 3 wt% water was added, acting as a catalyst for reactions. Experiments of mainly 4 and 23 h duration were performed in a Paterson-type deformation apparatus at 1000 °C temperature, 400 MPa confining pressure and differential stresses between 0 and 46 MPa. The resulting reaction rim width varied between <1 μm and ≈ 23 μm, depending on duration and type of reaction product. At isostatic pressure conditions, our data indicate that rim growth is proportional to time, controlled by dissolution-precipitation at interfaces of interconnected fluid-filled pores. In contrast, under non-isostatic stress conditions the reaction rim thickness increases non-linearly with time, implying diffusion-controlled growth. The magnitude of differential stress has no systematic influence on the reaction rate. Microstructural observations suggest that deformation-induced reduction of interconnected porosity causes this change in rate-controlling mechanism. For a natural MgO-SiO2 system, the results infer that fast interface-controlled reaction in the presence of high amounts of water is easily suppressed by concurrent deformation.