Instability and equilibration of fluid-mineral systems under stress investigated through molecular dynamics

Stress gradients and non-hydrostatic stresses are to be expected in rocks in the lithosphere, even in the presence of fluids. This complexity challenges the reliability of existing hydrostatic thermodynamic models, and, currently, in the geological lterature there is still no accepted theory for evaluating the thermodynamic effect of non-hydrostatic stress on reactions [e.g. 1, 2].

Large-scale Molecular Dynamics (MD) simulations (i.e., >2e6 atoms) give us the opportunity to investigate reactions in deforming systems by directly bridging the scale between atomic-level processes and continuum deformation. With MD, the a-priori assumption of a specific thermodynamic potential is not required, which makes it a robust approach to test existing thermodynamic theories [3]. With MD simulations the energy of the system, the pressure of the fluid, the stress of the solid, as well as the overall dissolution and precipitation process can be monitored over time until the stressed system attains equilibrium conditions.

Our findings indicate that a solid under non-hydrostatic stress can be equilibrated with its pure fluid. However, for deformations at constant temperature, the non-hydrostatic equilibrium differs from the hydrostatic equilibrium in that the pressure of the fluid must increase to maintain equilibrium with the solid. At low differential stresses, such pressure deviations from the reference hydrostatic equilibrium are small, allowing phase equilibria predictions by considering the fluid pressure as a proxy for equilibration pressure, as suggested by previous experimental investigations.

In the presence of substantial non-hydrostatic stresses, the stressed system becomes unstable, leading ultimately to the precipitation of a quasi-hydrostatically stressed crystalline film on the surfaces of the initial highly stressed crystal. During crystallization, the total stress balance is preserved until the newly formed solid-film-fluid system reaches again a stable equilibrium. At the final equilibrium conditions only the low-stressed solid film is exposed to the fluid, bringing back the equilibrium fluid pressure close to the value expected for the equilibrium at homogeneous hydrostatic conditions. While our results agree qualitatively and quantitatively with previous theories of thermodynamics in deformed systems [4,5] and with experiments [6,7], they cannot be predicted by theories proposed to interpret reactions in deformed geological systems [e.g., 2,8].

References

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7) Koehn, D., Dysthe, D. K., & Jamtveit, B. (2004). Geochimica et Cosmochimica Acta, 68(16), 3317–3325.

8) Paterson, M. S. (1973). Reviews of Geophysics, 11(2).