Collaborative Research: Modeling hydrothermal recharge and outflow in oceanic crust analogs with sharp permeability gradients

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Fluid circulation through the oceanic crust at the axis of mid-ocean ridges is a primary mechanism through which the Earth loses its internal heat. At the seafloor, this circulation releases hot fluids into the deep ocean. These hydrothermal sites typically host ecosystems and life forms found nowhere else on the planet and are thought to be one of the places on Earth where life may have originated. Hydrothermal fluid venting often occurs at or near major fault or fracture zones, suggesting that these breaks in the ocean crust can act as highly permeable conduits for fluids escape. It is unclear, however, to what extent these breaks in Earth's crust enable fluids to enter and move downward into the seafloor where they get heated. This research uses analog experiments, using a 3-D printer, and modeling to explore how fluid circulation at mid-ocean ridges spontaneously organizes itself and transports heat in highly fractured and faulted crust. By allowing exploration of the relation between venting sites and major tectonic features, the research facilitates our understanding of geothermal processes and the search for new hydrothermal sites on the seafloor. Broader impacts of the work include integration of research and education and support of three early career investigators, one from an institution in an EPSCoR state (Idaho). Results have applications ranging from terrestrial groundwater hydrology to geothermal energy, carbon sequestration, and the oil industry.This research employs numerical and analog experiments to describe and quantitatively explain the effect of heterogeneous permeability on subsurface flow geometry and heat extraction. Using a 3-D printer, we will generate plastic analogs of oceanic crust, containing a series of regularly spaced tubes that will act as fluid pathways of defined permeability. Within this permeable matrix, a planar slot of prescribed width, inclination, and greater permeability (achieved through wider tubes) will be created, representing the damage zone that typically surrounds active faults. The printed volume will be placed in a glass-walled tank containing a mixture of glucose and water. The fluid will be heated from below to initiate porous convection. A combination of particle image velocimetry, thermo-chromic liquid crystals, and temperature sensors at the top and bottom of the volume will allow quantification of the locations of fluid recharge and discharge and the heat output of the convective system as the permeability contrast and geometry of the slot is varied. Results will be compared to numerical models of porous convection in heterogeneous media and then extrapolated to natural conditions. The research will focus on predicting the conditions under which high-permeability fault zones can trap and focus hydrothermal convection rolls. The combined experimental and theoretical approach will greatly inform the investigation of targeted hydrothermal sites on slow-spreading mid-ocean ridges that sit next to major fault systems or near major crustal heterogeneities.