Factors controlling fluxes of fluid, heat, and solutes from sedimented ridge-flank hydrothermal systems

Hydrothermal circulation in midocean ridge-flank crust creates enormous fluxes of heat, fluid, and solutes. Three studies investigate factors that control hydrothermal fluxes from sedimented ridge-flanks. A site of hydrothermal fluid seepage on the eastern flank of the Juan de Fuca Ridge provided the basis for analysis of the effect of sediment permeability and thickness on fluid flux, and of the effect of reaction within the sediment on solute fluxes. Consolidation and permeability measurements indicate that sediments at the seepage site (primarily hemipelagic) are 10 times more permeable than sediments from nearby sites that are not experiencing seepage (primarily fine-grained turbidites). The measured sediment properties combined with other data indicate that a few tens of meters of fine-grained terrigenous, hemipelagic, or calcareous sediment effectively seal the basement aquifer. Much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow. At this site the basement hydrothermal system is a source of NH₄, SiO_2(aq), and Ca, and a sink of SO₄, PO₄, and alkalinity. Reactive transport modeling of pore water geochemistry indicates that reaction within the sediment section increases hydrothermal fluxes of NH₄, SiO_2(aq), SO₄, and PO₄, decreases the hydrothermal flux of Ca, and has a variable effect on the flux of alkalinity. Sediment thickness and fluid flow rate influence solute fluxes.; Lateral fluid flow associated with outcrops of high-permeability basalt may be an important means of ventilating partially sedimented ridge-flank crust. Numerical simulations of hydrothermal circulation through a basalt outcrop and adjacent valley were used to assess the influence of basement permeability distribution, sediment properties, and initial conditions on fluxes of fluid and heat. Development of lateral flow required a barrier to vertical flow between the flow layer and the ocean. Anisotropic permeability or a basement structure in which high permeability is confined to thin layers resulted in larger advective heat loss than isotropic permeability. Net discharge of fluid through the basalt outcrop occurred only in simulations of channelized fluid flow. Sediment properties significantly influenced circulation in homogeneous, isotropic basement only.