Title of the talk: Fluid-mineral equilibrium under stress: insight from molecular dynamics
The interpretation of phase equilibria and reactions in geological materials is underpinned by standard thermodynamics that assumes that the stress in the system is hydrostatic and homogeneous (i.e., the same for all the phases involved). However, stress gradients and non-hydrostatic stresses are common in rocks, and can be developed even in porous systems with coexisting solid minerals and fluids. Currently there is still not an accepted thermodynamic theory that includes the effect of non-hydrostatic stress on reactions, and the use of several thermodynamic potentials in stressed geological system is still debated. Even experiments have not been conclusive, because the direct effect of the applied non-hydrostatic stress on the thermodynamics of the reactions cannot be separated from the indirect effect caused by local stress concentrations.
I will show how the direct effect of a homogeneous non-hydrostatic stress on the solid-fluid equilibrium can be explored with molecular dynamics simulations. In these simulations, the solid and fluid phases are allowed to coexist directly. Their energy, the pressure of the fluid and the stress of the solid can be monitored until the non-hydrostatically stressed system reaches the equilibrium conditions. Results show that for simple systems at the stress range expected in the lithosphere, the shift of the pressure and temperature of the fluid-solid equilibrium is small for geological applications. It will also be discussed how such approach can guide us in evaluating the assumptions of hydro-mechanical-chemical models currently used to describe multiphase geological systems.
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