Wed 21st October @ 12 pm, Rm 312.222

The treatment of metamorphism as a sequence of near-equilibrium reactions responding to progressive changes in temperature and pressure (T,P) forms the basis of our understanding of crustal evolution. In fluid-producing reactions during progressive burial of a sediment, there may be an argument to support this, since mineral reactions in the presence if fluid are fast compared to expected rates of T,P increase.

However, much of the Earth’s lower and middle crust and a significant fraction of the upper mantle do not contain free fluids. These parts of the lithosphere exist in a metastable state, both in terms of the temperature as well as the state of stress, and are mechanically strong. When subjected to changing temperature and pressure conditions at plate boundaries or elsewhere, these rocks do not react until exposed to externally derived fluids. Metamorphism of such rocks may consume  fluids through hydration reactions as well as causing recrystallization and deformation, and takes place far from equilibrium through a complex coupling between fluid infiltration, chemical reactions, deformation and mass transfer. Disequilibrium metamorphism is characterized by fast reaction rates, dissipation of large amounts of energy as heat and work, generation of a range of dissipative structures which often controls transport properties and thus further reaction progress, and a strong coupling to far-field tectonic stress. Fluid consuming metamorphism almost invariably leads to mechanical weakening, and strain localization in the lower crust is mainly controlled by the availability of fluids.

A critical issue is the (age-old) question of whether rock volume is preserved during hydration reactions. Hydration reactions that involve an increase in molar volume (e.g. serpentinization) may lead to rock fracture, but I will present a (compelling) example where the stress produced by hydration is coupled to mass transfer in an open system with no deformation involved. The implications are as profound as they are disturbing and suggest much higher fluid fluxes than are generally currently accepted.  You are invited to pick holes in the argument.