The mechanisms that control the style of high-temperature hydrothermal discharge at ocean ridge crests are not well understood. Sites of vigorous high-temperature focused venting are often accompanied by nearby regions of low-temperature diffuse flow. Moreover, at some sites, high-temperature fluids are not focused at all and emerge as low-temperature diffuse flow as a result of conductive cooling and/or mixing with cooler seawater, whereas at others discharge has evolved from low-temperature diffuse flow to high-temperature focused flow over a number of years. To better understand the mechanisms that control these discharge styles, we construct a two-branch single-pass hydrothermal circulation model to investigate the role of anhydrite precipitation in mixing between deep-seated, high-temperature, sulfate-free upflow and cooler, sulfate-rich seawater circulating through permeable pillow basalts. We also estimate the width of the impermeable anhydrite barrier between focused and diffuse discharge. The model results suggest that focusing can occur in a matter of years provided the permeability in the deep part of the discharge zone is greater than or equal to that of the mixing zone and the permeability of the mixing zone is initially high (~10-11--10-12 m2). If one of the previous conditions is not met, only diffuse flow venting will occur. The width of the impermeable barrier resulting from the precipitation of anhydrite ranges between 1 and 100 m, thus permitting diffuse flow to occur in close proximity to focused discharge.