The devil's in the details: Hydrogeology of Middle Valley Active Venting Areas

Abstract

A scientific team aboard the R/V Maurice Ewing conducted a fine‐scale examination of the hydrogeology of two areas of active venting in Middle Valley, northern Juan de Fuca Ridge. Earlier expeditions had identified moderate‐to‐high temperature vent activity at the two sites, as well as by‐products of hydrothermal processes: sediment alteration; massive, disseminated sulfide deposits; and extensive vent faunal communities. Preliminary analyses of new data reflecting fluid, heat, and chemical fluxes suggest that the flows are complex in their distributions, have a significant hydrothermal recharge within the vent areas, and may be more vigorous than previously thought.

Fine-scale heat flow, shallow heat sources, and decoupled circulation systems at two sea-floor hydrothermal sites, Middle Valley, northern Juan de Fuca Ridge

“…The background heat flow represents a balance between large-scale magmatic heat supply and conductive and hydrothermal heat loss. Locally, the heat flow is determined by the thickness of the relatively impermeable sediments overlying a more permeable "hydrologic basement" aquifer, and is 400-600 mW/m 2 within several kilometers surrounding the two sites (Davis and Villinger, 1992;Fisher et al, 1997). Heat-flow values consistently elevated above this mean near the vent areas are the consequence of locally shallow hydrologic basement, a conductive contribution from vents, and secondary convection within shallow sediments.…”

“…At the Bent Hill area, heat-flow values range from 0.23 to 28 W/m 2 , and all values above 10 W/m 2 are located within 75 m of the single active vent. A zone of elevated heat flow also surrounds the massive sulfide deposit (Fisher et al, 1997).…”