Geochemistry of Hydrothermal Vent Fluids from the Northern Juan de Fuca Ridge

Anna Cruse, Ph.D., 2003
Jeffrey Seewald, Advisor

To rigorously constrain the role of organic compounds in crustal alteration processes at mid-ocean ridges, quantitative information on the abundances and distributions of organic compounds in hydrothermal vent fluids is required. This thesis was undertaken to provide this information for several low-molecular weight organic compounds (C1-C4 alkanes, C2-C3 alkenes, benzene and toluene) in fluids collected in July, 2000, at three sites on the northern Juan de Fuca Ridge: the Dead Dog and ODP Mound fields, which are located at Middle Valley, and the Main Endeavour Field (MEF), located on the Endeavour segment. The relative abundances of alkane-alkene pairs were used to constrain subsurface reaction zone temperatures. Isotopic systematics of CO2, C1-C4 alkanes, and benzene provided information on the residence times of fluids in the subsurface and the reaction mechanisms that control the distributions of aqueous organic compounds. The presence of organic compounds in these fluids also controlled the distributions of inorganic compounds—most notably, H2. Integration of the data on both organic and inorganic species in these fluids provides a powerful tool to better constraining models of subsurface conditions in hydrothermal vent environments.