Geochemistry of deep-sea hydrothermal vent fluids from the Mid-Cayman Rise, Caribbean Sea

Abstract

This thesis examines the controls on organic, inorganic, and volatile species distributions in hydrothermal fluids venting at Von Damm and Piccard, two vent fields at the ultraslow spreading Mid-Cayman Rise, Earth’s deepest mid-ocean ridge. In Chapter 2, abiotic organic synthesis is shown to occur via two distinct mechanisms in the serpentinizing Von Damm system. Chapters 3 and 4 present in-depth studies of the chemical and isotopic compositions of aqueous species in vent fluids at Von Damm and Piccard to elucidate the role of reaction temperature, pressure, substrate composition, and water/rock mass ratios during the chemical evolution of hydrothermal fluids. At Von Damm, sequential reaction of gabbroic and peridotite substrates at intermediate temperatures can explain generation of the observed fluids. At Piccard, extremely high temperature subsurface water/rock reaction results in fluids that are richer in dissolved H2 than any previously observed fluids worldwide. In Chapter 5, multiple sulfur isotopes were measured on metal sulfide deposits, S0, and fluid H2S to constrain sulfur sources and the isotopic systematics of precipitation in a wide variety of seafloor hydrothermal vents. Areas studied include the eastern Manus Basin and Lau Basin back-arc spreading centers, the unsedimented basalt-hosted Southern East Pacific Rise, and the sediment-hosted Guaymas Basin mid-ocean ridge spreading centers.