Effects Of Stress on Metamorphic Reaction

We have known for more than a century that stress and metamorphic processes interact, and there are commonly positive feedbacks between deformation and metamorphism. At the heart of these interactions lies a link between stress and chemical potential. Perhaps because these quantities are not directly visible or measurable on the grain scale, controversy remains as to how they relate. Here I present examples of experiments at a variety of conditions which support the key role of a particular equation. (chemical potential) = (Helmholtz free energy) + (normal stress at interface) × (molar volume). Put simply this equation states that at high stress interfaces minerals are more reactive/soluble/unstable than at lower stress interfaces. Some experiments reviewed are on polymorphic phase transformations, others on pressure solution and diffusion creep. The experimental systems are generally simpler than natural rocks, but the equation can be used to make predictions in more complex systems. These are yet to be tested but simple calculations show the effects of realistic stress levels are likely to be significant for conditions of metamorphic reaction in the crust. Even without further theoretical development, some experiments have significance which needs highlighting. In the mantle, the seismic discontinuity at around 410 km marks a polymorphic phase transformation (olivine to wadsleyite). It is usually assumed this is related to depth though modulated by temperature. However, Vaughan et al. (1984) show that the maximum principal stress is what plays the role of pressure in a related phase transformation. Therefore, in regions of high stress in the mantle we need a better understanding of stress levels for more robust explanations of the depths of mantle discontinuities. In both crust and mantle, the metamorphic kitchen sink of analytic techniques needs to be supplemented by new numerical models to incorporate the effects of stress. Vaughan, P. J., Green, H. W. & Coe, R. S. 1984. Anisotropic growth in the olivine spinel transformation of Mg2GeO4 under nonhydrostatic stress. Tectonophysics 108(3-4), 299-322. Wheeler, J. 2020. A unifying basis for the interplay of stress and chemical processes in the Earth: support from diverse experiments. Contributions To Mineralogy And Petrology 175, Art. 116.