Multiple scales of hydrothermal circulation in the oceanic crust: Studies from the Juan de Fuca Ridge and flank

Seafloor heat flow patterns influenced by hydrothermal circulation within the upper oceanic crust are used to constrain fluid flow, crustal permeability, and thermal budgets at Middle Valley, northern Juan de Fuca Ridge and along the eastern flank of the Juan de Fuca Ridge. We made 397 heat flow measurements in two areas of active venting in Middle Valley, a sedimented spreading center. Heat flow patterns suggest that the top of the hydrothermal reservoir at the Dead Dog vent area is located ∼30 m beneath the vent field and is separated from a shallow secondary circulation system by a thin, impermeable, diagenetic cap. This cap is a hydrologic barrier, separating the high-temperature, overpressured system at depth from a low-temperature, underpressured system near the seafloor. Circulation above the cap is influenced by hydrodynamic interactions with active vents. Vent fluid recharges at the unsedimented edges of Middle Valley and is driven toward the vent field by small driving forces documented by borehole observatories. These small driving forces, in combination with estimated fluid flux (150 l/min), imply that crustal permeability must be high (10 −12 to 10−10 m2). We estimate the total heat loss for a 260 km2 region of Middle Valley to be 274 MW. Conductive and advective heat loss are approximately equal, in contrast to bare-rock ridges that lose 90% or more of their heat advectively.; Models of lateral heat and fluid flow across a 26 km transect of the eastern flank of the Juan de Fuca Ridge quantify relations between lateral specific discharge, flow layer thickness and formation permeability. Free flow simulations require non-hydrostatic initial conditions and permeabilities of 10−11 to 10−9 m2, but match heat flow observations with lateral specific discharges of 1.2 to 40 m/yr. These lateral flow rates are 10× to 1000× greater than predicted by apparent radiocarbon ages of the fluids. When apparent radiocarbon ages are corrected to account for diffusion, physical and chemical models of lateral flow rates are reconciled.