Examining the processes that control diffuse flow in seafloor hydrothermal systems can provide insight into what is happening below the seafloor at mid-ocean ridges. The diffuse flow could be either entrained seafloor fluids mixed with hot, upwelling fluids, or diffuse flow could be driven by sidewall heating across some low permeability barrier. This barrier would separate the diffuse flow fluids from the high temperature fluids, effectively preventing mixing. Three parameters were manipulated through a numerical model based on the single-pass model, and the fluid and heat flow results analyzed and compared to observed hydrothermal systems. These three parameters are depth of the extrusive layer at the top of the system, the permeability of the extrusive layer, and the existence of a low permeability barrier separating the high-temperature output from the induced diffuse flow.

Ever since the discovery of seafloor hydrothermal systems at mid ocean ridges, scientists have been trying to understand the complex dynamic processes by which thermal energy is transported advectively by chemically reactive aqueous fluids from Earth's interior to the surface. Hydrothermal systems are generally assumed to consist of a heat source and a fluid circulation system. Understanding the interconnected physical, chemical, biological, and geological processes at oceanic spreading centers is important because these processes affect the global energy and biogeochemical budgets of the Earth system. Despite two decades of focused study of hydrothermal systems, several key questions remain concerning the behavior and evolution of hydrothermal vent systems. Among these are: (a) the partitioning of heat transport between focused and diffuse flow, and (b) the spatial extent and distribution of hydrothermal recharge. These are the main topics of investigation in this thesis. To address these issues, I first use a single-pass modeling approach using a variety of observational data in a simple parametric scale analysis of a hydrothermal vent field to determine fundamental parameters associated with the circulation and magmatic heat transfer for a number of seafloor hydrothermal systems for which the constraining data are available. To investigate the partitioning of heat flux between focused high temperature and diffuse flow I extend the one-limb single pass model to incorporate two single-pass limbs to represent deep and shallow circulation pathways. As a result, I find that 90% of the heat output is from high temperature fluid circulating in the deep limb even though much of the heat loss appears at the seafloor as low-temperature diffuse flow. Next, I use the parametric description of hydrothermal circulation to investigate hydrothermal recharge at the East Pacific Rise 9°50′ N hydrothermal site. Using a 1-D model of recharge through an area of 10⁵ m² elucidated by microseismicity in the oceanic crust I find that anhydrite precipitation is likely to result in rapid sealing of pore space in the recharge zone. This would lead to rapid decay of hydrothermal venting, which is contrary to observations. Then I consider two-dimensional numerical models of hydrothermal circulation in a porous box heated from below. The preliminary results of these models suggests that the anhydrite precipitation zone will be more diffuse, but additional work is needed to test whether anhydrite precipitation will seal the pore space. / Master of Science