Hydrothermal processes at mid-ocean ridges: Results from scale analysis and single-pass models
Hydrothermal processes at ocean ridge crests involve a complex interplay between the dynamics of heat supply and the evolution of crustal permeability that results in a broad spectrum of issues to challenge hydrothermal modelers. Among these are: (1) the detailed relationships between heat supply and hydrothermal heat transfer, (2) the mechanisms controlling vent stability and long-term evolution of hydrothermal vents, including the relationship between focused and diffuse flow, (3) temporal variability related to magmatic and tectonic events, (4) boiling, phase separation and the evolution of vent chlorinity, and (5) the linkages among heat transfer, fluid flow, geochemical and biological processes in controlling character and evolution of vent fields. Here we address a number of these issues in the context of scale analysis and single-pass models. Scale analysis shows that plume heat transfer in a homogeneous system with an imposed heat flux from below does not correspond to observed black smoker flow. Single-pass models show that black smoker venting requires a conductive boundary layer no more than a few tens of meter thick above a magma chamber and a discharge zone with permeability between 10 -13 and 10 -11 m 2 . Long-term vent stability requires that the thin conductive boundary be maintained, probably by some combination of magma replenishment and crack propagation, and a broad recharge zone to prevent clogging by anhydrite. Event plumes may be driven by dike emplacement provided the dike locally increases the permeability by two orders of magnitude and provides a significant fraction of event plume heat.