Watching the world sweat: development and utilization of an in-situ conductivity sensor for monitoring chloride dynamics in high temperature hydrothermal fluids at divergent plate boundaries

Thesis (Ph. D.)--University of Washington, 2008.The magmatic upwelling that drives plate tectonic motion at divergent plate boundaries also heats seawater circulating within the Earth's crust. The seawater undergoes physical and chemical changes beneath the surface and the resulting buoyant hydrothermal fluid ascends to the seafloor where it is comes out of structures called hydrothermal vents. One subsurface process of particular interest is phase separation, which is the transformation of a homogenous fluid into two phases, each with properties different from the original fluid. Phase separation is the dominant control on chloride in hydrothermal systems and chloride controls the distribution of all other chemical species. Thus, the measurement of chloride in hydrothermal fluids gives insight into extreme subsurface processes that are inherently difficult to probe. Since these processes evolve with time, measurements must be taken on a continuous basis. The research presented herein discusses the development and utilization of an instrument capable of continuously monitoring the hot salty solutions that flow out of hydrothermal pores in the Earth's crust. Instruments were deployed at two different mid-ocean ridge hydrothermal systems. An array of instruments was deployed on the Juan de Fuca Ridge at the Main Endeavour Field 12-15 months after a magmatic intrusion. Tidal changes and non-tidal changes on timescales of minutes to hours were observed. Chloride data were also used to infer subsurface mixing between two non-seawater fluids at depths below the seafloor between 486 and 695 meters.Another instrument was deployed at Bio 9' vent at 9°50'N on the East Pacific Rise in the immediate vicinity of seismometers monitoring earthquake activity. The hydrothermal response to intense seismicity was observed on two separate occasions. On the basis of these observations, conditions of subsurface phase separation were estimated at pressures between 269 and 288 bars and temperatures between 369.7 and 403.5°C. Recurrent chloride spikes were also observed, with magnitudes up to 720 mmol/kg and durations up to 7 minutes. At both study sites, data indicate the influence of subsurface fluids with chloride concentrations greater than seawater. These observations may help resolve the apparent chloride deficit indicated by venting of chloride-depleted fluids over decadal timescales