The influence of plate movement on the evolution of hydrothermal convection cells in the oceanic crust

Abstract

Two types of hydrothermal circulation can be distinguished in the oceanic crust, a very intense convection at spreading centers caused by the intrusion processes at mid-ocean ridges, and a much less vigorous, but more common convection in older, sediment covered crust driven by heat coming from the underlying, cooling plate. We investigated the relation between these two types of convection under normal spreading conditions, i.e. oceanic plate moving away from a stationary spreading center. In particular, we studied the stability of convection cells in a moving plate, the transition between on- and off-axis convection and the temperature distribution in these convection cells. The study is based on numerical calculations using the theory of flow through porous media. Our results show that convection cells associated with the intrusion processes in the accretion zone of a spreading center are stationary with respect to the ridge axis. Convective heat transport in the stationary, on-axis cells is sufficient to remove all the heat released at the ridge axis. The other convection cells, which are not immediately associated with these intrusion processes, are not stationary. Most of them, especially once they are more than 30 km away from the ridge axis, move with the moving plate. This movement occurs regardless of the permeability distribution in the crust. As a consequence, individual segments of the crust are exposed either to continuous upwelling or to continuous downwelling flow once they have left the vicinity of the ridge axis; high water-to-rock ratios can be reached in these long-lived cells in spite of the relatively slow flow velocities. Temperatures in the off-axis cells, even in close proximity to the ridge axis, are low, in general below 100°C. The low-temperature alteration found commonly in the oceanic crust is evidence for the widespread occurrence of these off-axis cells. More specifically, the distinct differences in degree of alteration observed in some closely spaced DSDP cores are in good agreement with the concept of convection cells attached to the moving plate. Because of the movement of the off-axis cells away from the stationary axial cells, a transition zone where new convection cells are formed exists at distances between 5 and 25 km from the spreading center. The formation of a new convection cell is accompanied by a maximum in low-temperature upwelling flow. When the heat transport by the plate and that by fluid convection is of similar magnitude, off-axis cells shift position with respect to the ridge axis and the moving plate. During this stage two cells can merge, which event causes a short (ca. 5000 years) episode of intense upwelling. These episodes, which are not directly related to any intrusive activity, can occur several times in or near a particular segment of the crust. Only during these short upwelling episodes are temperatures up to 200°C reached in the off-axis cells. Chains of hydrothermal mounds near active spreading centers as well as low-temperature hydrothermal deposits such as the Mn lenses commonly associated with ophiolite sequences may be related to the formation of these off-axis convection cells.