--> --> Abstract: Geochemistry of Carbon and Sulfur in Hydrothermal Sediments of the Middle Valley of the Juan de Fuca Ridge, by P. A. Baker, S. L. Cross, S. J. Burns, and R. A. Zierenberg; #90992 (1993).

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BAKER, PAUL A., and SCOTT L. CROSS, Duke University, Durham, NC, STEPHEN J. BURNS, Universitat Bern, Bern, Switzerland, and ROBERT A. ZIERENBERG, U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Geochemistry of Carbon and Sulfur in Hydrothermal Sediments of the Middle Valley of the Juan de Fuca Ridge

On Leg 139 of the Ocean Drilling Program, we extensively sampled the Middle Valley hydrothermal system on the Juan de Fuca Ridge. In this region, thick sections of rapidly deposited, Quaternary turbiditic and hemipelagic sediments overlie a basement complex of interbedded basaltic sills and altered sediments. Basement ages in the Middle Valley are estimated at 250,000 yr.

Samples from three drill sites were analyzed in this study. Site 856 is a very young (but extinct) massive sulfide ore body and its adjacent host sediments. Site 857 is thought to represent a hydrothermal reservoir and the overlying sediments. Site 858 is the region of active hydrothermal venting. Oxygen isotopic compositions of diagenetic carbonates at Site 856 indicate that past downhole temperatures were much higher than those measured today in the drill hole. Isotopic temperatures agree with temperatures measured in situ at hole 857C, about 222 degrees C at 409 m subbottom. In holes 858B and 858D, isotopic and measured temperatures of 171 degrees C were observed at only 16 m subbottom.

Using carbon isotopic compositions of diagenetic carbonate nodules and sulfur isotopic compositions of diagenetic sulfides and sulfates, several important reactions were identified within the sediments and the depths and temperatures of occurrence of these reactions were established. Reactions include microbial and abiogenic sulfate reduction (using either dissolved sulfate or anhydrite as oxidants); methanogenesis and methane oxidation, thermocatalytic decarboxylation; and the precipitation of inorganic products of these reactions (metal sulfides and alkaline earth carbonates). Insights into these processes gained by analyzing modern hydrothermal sediments are useful in understanding the formation of ancient petroleum and ore deposits.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.