Submarine hydrothermal systems at mid-ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation. The interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow, especially under conditions leading to aqueous two-phase separation. Within the brittle region, fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor. In the ductile region, fluids may be trapped under near-lithostatic pressure, while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure. In a narrow transition zone between these two extremes, a steep pore fluid pressure gradient must exist. In the brittle region itself, fluid pressure will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits. A hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor.

Fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth. As a result of the large volume increase that accompanies two-phase separation, at two-phase fluid must either accelerate upward or exert an increased pressure on its surroundings, possibly leading to enhanced fracturing. The fracture configuration will also affect the behavior of the mixed fluid after phase separation. A two phase-fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well-mixed suspension of vapor and liquid, whereas a two-phase mixture in a constricted, branching series of channels, moving more slowly in a laminar flow regime, is more likely to undergo physical isolation of the phases, leading to venting of a vapor phase and retention of a saline brine phase at depth. We propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets, progressively increasing in diameter and decreasing in abundance, feeding into a main upflow/stockwork zone in which flow directly to the surface takes place in relatively large, unconstricted conduits. Fluid movement within a feeder zone may be modified by capillary flow, tortuous channelways, disconnected fractures, blocking of cracks by mineral deposition, and opening of new fractures. ¿ American Geophysical Union 1988