24. THERMAL PROPERTIES OF TAG HYDROTHERMAL PRECIPITATES, MID-ATLANTIC RIDGE, AND COMPARISON WITH MIDDLE VALLEY, JUAN DE FUCA RIDGE1

Peter A. Rona,2 Earl E. Davis,3 and Rainer J. Ludwig4

ABSTRACT

To the few thermal conductivity measurements on sulfides/sulfates reported in the literature, we add 35 new values determined by two different methods for samples cored by Ocean Drilling Program (ODP) Leg 158 from the volcanic-hosted active sulfide mound in the TAG hydrothermal field, located in the rift valley of the Mid-Atlantic Ridge near 26░N, 45░W. Thermal conductivity measurements are essential to calculate heat flow and to model sulfide-hosted hydrothermal flow. Fifteen measurements were made on the ship by the half-space method, using a needle probe on seawater-saturated half-rounds of cores of heterogeneous mixtures of sulfide (predominantly pyrite), quartz, and anhydrite breccias. Values range between 6.1 and 10.4 W/(mĚK); one measurement on anhydrite produced a value of 5.4 W/(mĚK). At the Pacific Geoscience Centre, the divided bar method was used to measure twenty values on minicores extracted from other half-rounds of cores with similar mixed compositions and saturated with distilled water. These values range between 5.0 and 14.9 W/(mĚK).

Despite scatter in the data, thermal conductivity values measured by the half-space method are systematically lower than those measured by the divided bar method. Measurements of sediment-hosted sulfides cored by ODP Leg 139 at the Bent Hill site, located at Middle Valley of the Endeavor segment of the northern Juan de Fuca Ridge, exhibited a similar discrepancy of values. Davis and Seeman (1994) and Gr÷schel-Becker et al. (1994) consider values produced by the half-space method to be systematically low for material with high conductivity because of a lack of calibration standards and the presence of nonlinear system behavior at high thermal conductivities. The TAG sulfides generally exhibit higher conductivities and lower porosities than the Middle Valley sulfides; this is attributable to higher pyrite content and more pore-filling quartz and/or anhydrite in the TAG location. Values of thermal conductivity measured in sulfides at both TAG and Middle Valley are high compared with values of seafloor sediment and basalt, which typically range between 1 and 2 W/(mĚK). This marked contrast indicates that seafloor sulfide bodies may act as foci of conductive heat flow in addition to the spectacular convective thermal transfer by black smoker and diffuse venting.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Peter A. Rona, Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901-0521, U.S.A. rona@ahab.rutgers.edu
3Earl E. Davis, Geological Survey of Canada, Pacific Geoscience Centre, P.O. Box 6000, Sidney, British Columbia V8L 4B2, Canada.
4Rainer J. Ludwig, SOEST, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, Hawaii 96822, U.S.A.