Quartz‐cemented breccias from the Mid‐Atlantic Ridge: Samples of a high‐salinity hydrothermal upflow zone

Textural, mineralogical, chemical, and fluid inclusion studies on a suite of quartz‐rich, sulfide‐bearing greenstone breccias indicate that high‐salinity, high fluid flow systems were recently active on the Mid‐Atlantic Ridge south of the Kane Fracture Zone. These systems involved fluids with temperatures close to 300°C and salinities almost 3 times that of seawater. Rounded, pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m/s. Homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and NaCl equivalent salinities from 290°C and 10 wt % to 200°C and 4.7 wt % during alteration and cementation of the breccias. Highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias. A continuous supply of the high‐salinity fluids may be generated either by supercritical two‐phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system. Either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415°C and the fluid to have been cooled by conduction or by mixing with lower‐temperature seawater prior to entrapment within the inclusions.