Pre-Drilling Observations of Conductive Heat Flow at the Tag Active Mound Using Alvin

Abstract

During the 1993 Alvin dive series to the TAG hydrothermal field, 50 measurements of conductive heat flow were attempted at the 50-m-high, 200-m-diameter TAG active mound. The 43 successful stations included gradient and thermal conductivity measurements made with the 5-thermistor, 0.6- or 1-m-long A/v/« heat flow probes, which with a few exceptions could be pushed into most locations on and off the sulfide mound. The stations were made in a variety of characteristic environments on and off the mound, and were transponder-navigated to an estimated accuracy of ±5 m relative to the 10-m-diameter central complex of black smokers. The distribution of these stations allows a reasonable mapping of coherent patterns in the conductive heat flux from the mound. As might be expected, conductive heat flow values are extremely variable (0.1-86 W/m ) within a few meters of the black smokers, where the station environments were generally pockets of sulfide debris amid larger sulfide rocks with widespread shimmering water indicative of diffuse hydrothermal flow. On the west side of the sulfide rubble plateau that surrounds the central black smoker peak, a coherent belt of very low heat flow ( 5 W/m2) on the sedimented terraces that form the slope down from the Kremlin area of white smokers, suggesting an extension of the fluid flow processes responsible for the white smokers. Heat flow is also high (0.3-3 W/m2) in the pelagic carbonate sediments on the surrounding seafloor within a few tens of meters of the southwest, northwest, and northeast sides of the mound. The distribution of these areas of high and low heat flow in general supports a possible schematic model of subsurface patterns of heat and fluid flow within the active mound (Tivey et al., 1995), and was used to guide placement of several of the holes drilled during Leg 158.