Hydrothermal Fluid Effects on Sediment Column in Middle Valley, Juan de Fuca Ridge (Northeast Pacific)

Ihsan S. Al-Aasm, Bertrand Blaise

Core PAR 85-34, located near a high heat flow area in Middle Valley, northern Juan de Fuca Ridge, contains carbonate nodules of various sizes (<= 1-5 cm in length). Sedimentological, geochemical, and isotopic results allow us to understand the origin of these concretions and the effects of hydrothermal activity from nearby sulfide vents on the sediment column.

The mineralogy of the olive-gray surface sediment (0-15 cm) is identical to unaffected hemipelagic sediments in the region except for a concentration of barite crystals (up to 1 cm) at the water-sediment interface. In the clay, mud fraction, and bulk sediment, the FeO, S, Ba, Cu, Ni, Pb, Zn and As are more enriched than in normal hemipelagic sediment in the area due to hydrothermal activity.

Petrographic and SEM analysis of the nodules reveal iron calcite and barite minerals in cracks and on the outside part of the nodules with mineralogical and textural variations downcore. Stable isotope curves of these nodules appear to demonstrate the effects of both bacterial sulfate reduction and microbiological methane generation, with consequent extreme ¹³C-depletion in the precipitated carbonate. The curves also demonstrate that the hydrothermal fluid entering the system may have caused the negative shift in oxygen isotopes downcore, although this effect may have been of cyclic or episodic nature.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.