Abstract
The Galapagos mounds occur in two fields south of the Galapagos spreading center over crust which is between 500,000 and 900,000 years old. Detailed investigations of composition and chronology of these mounds (e.g., Honnorez et al., 1981; Lalou et al., 1984) have shown that their deposition started in 300,000-year-old sediments with the formation of hydrothermal sediments and ended with the recent formation of manganiferous crust. On the basis of numerical models the flow distribution in hydrothermal convection cells near active spreading centers is studied here as a possible formation mechanism for these and similar deposits.The flow at an active spreading center is dominated by a strong convection cell associated with the intrusion process at the ridge axis. This cell is stationary relative to the ride axis. All the other convection cells which form off-axis because of the heat input from the cooling plate are not stationary but move in general with the moving plate away from the ridge axis. As a consequence of this movement, a transition zone exists between the on-axis and off-axis convection where new off-axis cells are formed once the distance between stationary and moving cells has become too large. The beginning of low-temperature convection occurs thus at some distance from the ridge axis (ca. 10 km) in older crust which has in general a thin sediment cover. This convection can be related to the formation of the hydrothermal sediments in the Galapagos mounds.When the spreading rate is relatively low and the rate of fluid flow in the off-axis cells is high, merging of convection cells in this transition zone can occur. The merging episodes cause strong pulses of low-temperature hydrothermal convection, which are not related to off-axis intrusive events. As a segment of the crust moves through the transition zone, it can be exposed to several of these merging episodes. The evolution of off-axis convection cells near an active spreading center provides thus a mechanism which can cause one or more pulses of short-lived (ca. 10,000 years), low-temperature ( approximately 200 degrees C) convection in a segment of the crust. This activity has the characteristics necessary for the deposition of manganese crust or lenses of massive manganese ore associated with marine sediments. Calculated intervals between onset of convection and merging episodes are in good agreement with observations from the Galapagos mounds. Flow rates associated with the merging episodes are sufficient to explain the formation of sizable Mn lenses such as those found associated with the Appenine ophiolites and the Nicoya ophiolite.
