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Insights into the evolution of an oceanic hydrothermal system and a method for constraining estimates of the vigor of hydrothermal convection Alt-Epping, Peter

Abstract

The permeability of the oceanic crust is the primary hydrologic parameter that controls the geometry, the vigor, and the duration of hydrothermal fluid flow at mid-ocean ridges. The coupling of fluid flow, temperature, and chemistry and the effect of the permeability on these coupled processes are assessed to determine whether geochemical data can be used to constrain estimates of basement permeabilities and the vigor of convection. The coupling of flow, temperature, and chemistry is investigated for an open, sedimented ridge setting from the onset of fluid motion after an initial state of a conductive temperature distribution and fluid stagnation, to a near steady-state convective system. Fluid residence times, physical water/rock ratios, temperature conditions, recharge and discharge rates, flow geometries, and the degree of fluid mixing are calculated for the evolving hydrothermal system and the influence of these parameters on mineral alteration and aqueous phase concentrations are discussed. The chemical evolution of the system suggests that despite differences in alteration patterns and the intensity of alteration for different basement permeabilities, the nature of the alteration reactions is unlikely to be a useful parameter for constraining the vigor of convection. Estimates of fluid velocity can be obtained using a formulation that relates the mass transfer between the solid and the fluid phase along the fluid's flowpath to the average velocity along the flowpath. Different regions in the hydrothermal system and a range of chemical species are examined to assess their usefulness in constraining estimates of average flow velocities. The results of these calculations suggest that the mass transfer of aqueous silica may be useful for estimating fluid flow velocities in hydrothermal systems, in particular in those regions of the system at or near quartz equilibrium so that aqueous silica concentration is buffered by quartz and correlated with the temperature distribution.

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