Helium, heat, and the generation of hydrothermal event plumes at mid-oceanridges

Hydrothermal event plumes are unique water-column features observed over mi d-ocean ridges, presumably generated by the sudden release of large volumes of hot, buoyant fluid. Although the specifics of event plume generation ar e unknown, event plumes have been attributed to the rapid emptying of a hyd rothermal reservoir or to rapid heat extraction from a recently emplaced di ke or seafloor lava flows. The chemical and thermal signatures of event plu mes as compared to the underlying steady-state plumes offer important clues to the generation of event plumes. Event plumes have low He-3/heat ratios of similar to 0.4 x 10(-17) mol J(-1), similar to vent fluids from mature h ydrothermal systems. In contrast, the steady-state plumes found beneath the event plumes have elevated and variable He-3/heat ratios of 2 to 5 x 10(-1 7) mol J(-1) Fluids collected directly over fresh lava flows have even high er He-3/heat ratios of 2 to 8 x 10(-17) mol J(-1), up to 30 times the event plume values. These disparate He-3/heat ratios place strong constraints on models of event plume generation, especially models which rely on heat ext raction from seafloor eruptions. Published by Elsevier Science B.V.