Anhydrite precipitation strongly affects the circulation of hydrothermal systems by changing the permeability structure of the oceanic crust. For example, a hydrothermal reservoir capped by anhydrite precipitates was found beneath the caldera floor of Suiyo Seamount of the Izu-Bonin Arc. We interpret the precipitation to be due to alternating layers of lavas and tephras, typical of arc volcanos, because the layering gives rise to the mixing of high-temperature hydrothermal fluids with low-temperature seawater at layer boundaries. We thus perform two-dimensional numerical simulations of subseafloor fluid flow in a two-layer porous medium to model the shallowest part of the discharge zone near the summit of an arc volcano. The upper layer is set to be more permeable than the lower one. High-temperature Ca2+-rich hydrothermal fluid flows in from the bottom, and low-temperature SO 42--rich seawater flows in from the seafloor. The two fluids meet at the layer boundary, precipitating anhydrite, which serves as the ceiling of a hydrothermal reservoir. Anhydrite precipitates form also in the upper layer, serving as conduit walls for upwelling hydrothermal fluid. When the permeability of the upper layer is in the range 10-10-10 -12 m2, which is typical for unconsolidated volcanic pumice, the pattern of the circulation stabilizes in about 0.5-40 years. The circulation structure and the timescale of its formation are similar to those of the hydrothermal system on Suiyo Seamount.
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