Circulation of seawater through the oceanic crust and the chemical reactions that occur along the towpath are the most important processes controlling the composition of vent fluids and mineral formation within the crust and on the seafloor. We use three variably coupled reactive transport models to explore the evolution and the interaction of fluid flow, heat transport and chemical reactions in the oceanic crust and the implications for the composition of vent fluids, mineral alteration patterns and the formation ofseafloor chimney deposits. Our first simulation shows that the combined precipitation of anhydrite and chlorite is an important process in reducing the permeability of the upper oceanic crust. This may lead to complete clogging of the recharge zone or to formation of thermally conductive, and hence hotter, regions in the basement aquifer. In a second simulation we demonstrate that the compositional evolution of carbonate chimneys on the seafloor in an ultramafic oceanic system can be related to specific water-rock reactions in the underlying crust. In a 2D model we examine the rates at which different processes operate. Whereas the fluid flow field and the fluid composition evolve almost instantaneously, the alteration of the rock shows the time-integrated effects of the coupled physical and chemical processes. Chemically induced porosity and permeability changes are relatively slow and thousands of years may pass before they become hydrologically and thermally effective. With these examples we demonstrate that reactive transport simulations can provide values of parameters that are difficult to measure (e.g. permeability), they can explain observations such as changes in vent-fluid compositions by elucidating hidden water-rock interaction at depth, and they can provide outcomes to long-term scenarios that are far beyond the time-scales of direct field monitoring.
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