Hydrothermal circulation at slow-spreading mid-ocean ridges: The role of along-axis variations in axial lithospheric thickness

At several ridge segments along the slow-spreading Mid-Atlantic Ridge, the lithosphere appears to be cooled by centrally located, isolated hydrothermal fields, hundreds of meters wide, extracting as much as 1000 MW from the lithosphere and hosting very large (>10(6) m(3)) sulfide edifices. These fields are possibly fueled by subseafloor hydrothermal cells cooling and leaching the lithosphere up to a few tens of kilometers along axis. However, the detailed mechanisms by which such hydrothermal heat extraction takes place are not well constrained. It is postulated that melt focusing and preferred cooling near transforms result in a thinner lithosphere at the center of slow-spreading ridge segments. In this configuration, and with a depth of penetration controlled by brittle lithospheric thickness, the base of the hydrothermal system is not at constant depth. Here we present models of along-axis hydrothermal circulation showing that pressure gradients generated along this basal slope influence flow dynamics. We show that the size of hydrothermal cells increases with the basal slope alpha. For alpha < 15 degrees-20 degrees, the circulation is unsteady and composed of mulltiple cells producing several discharge zones, some of which can migrate and merge along axis. When alpha > 15 degrees-20 degrees, the circulation reaches steady state and is composed of a single cell with a broad recharge and a focused discharge. Although our models make several simplifying assumptions, we propose that along-axis variations in lithosphere thickness associated with the magmatic and tectonic segmentation of slow-spreading ridges should favor the formation of large and centrally located vent fields, mining heat on several kilometers along axis. We also predict that more short-lived and weaker vent fields may develop away from the segment center.