Numerical Model of a Fossil Hydrothermal System in the Southern East Pacific Rise Exposed at Pito Deep

Abstract: The Mid Ocean Ridge system with its volcanism and related hydrothermal activity has been a subject for many studies since the discovery of high temperature hydrothermal vents at the ridge surfaces in the 1970´s. This thesis focuses on deep sea hydrothermal activity on a superfast spreading ridge, the SouthernEast Pacific Rise (SEPR).The ridge is located in the South Pacific, off the coast of South America, and separates the Nazca Plate and the Pacific Plate. A fossil high temperature hydrothermal zone hosted by a fault was sampled 80 m below the lava/dike transition zone in the Pito Deep (a tectonic window intothe SEPR). Geochemical data from the fault zone indicates that cold (<150°C)and hot (<390°) fluids coexisted at the same time whilst the hydrothermal system was active. A numerical model (HYDROTHERM) developed by the USGS was used to recreate the geological settings in the SEPR in order to try to model the hydrothermal activity and fluid flow. The model solves two governingpartial differential equations numerically, the water component flow equation(Darcy law for flow in porous media) and the thermal energy transport equation(conservation of enthalpy for the water component and the porous media). The result of the modeling indicates that cold seawater can penetrate from the relatively permeable volcanic material into a highly permeable fault zone in the sheeted dike unit. The cooler seawater fluid flows down the fault zone,reheats and flows up again in a narrow upflow zone at the edge of the fracture/sheeted dike boundary. The result is a horizontal temperature gradient created in the fractured zone supporting the theory that hot and cold fluids can coexist in a fault hosted hydrothermal zone.