CHARACTER OF ACTIVE HYDROTHERMAL MOUNDS AND NEARBY ALTERED HEMIPELAGIC SEDIMENTS IN THE HYDROTHERMAL AREAS OF MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA-RIDGE - DATA ON SHALLOW CORES

The Area of Active Venting hydrothermal field (AAV) occurs within the Middle Valley sediment-filled rift on northern Juan de Fuca Ridge. The AAV is a rhomb-shaped zone of hydrothermally altered sediment 800 m b y 350 m with moderate temperature venting (<276-degrees-C), associated with anhydrite chimneys on hydrothermal mounds, and biota-rich areas associated with diffuse venting. Ten push-core samples (< 30 cm depth) were collected in and near the AAV by the submersible ALVIN during 19 90 to examine the surficial nature of altered hemipelagic sediment and hydrothermal mounds. Within the AAV but away from active vent areas, weakly altered sediment is grey colored and indurated due to dissoluti on of calcareous microfossils and an increase in authigenic calcite an d pyrite. Proximal to vent areas, intensely altered sediment is green colored, with an increased abundance of Mg-smectite, iron-bearing illi te-smectite, pyrite, organic carbon and barite. Altered sediments cont ain elevated contents of MgO, total Fe2O3, MnO, S, Ba, Zn, Cu, Pb, As, Sb and Se, and are depleted in CaO and CO2 relative to unaltered sedi ment. Element enrichments and depletions increase toward active vent a reas on hydrothermal mounds. Tilted hydrothermal crusts, and horizonta l Mg-silicate veins seen in the cores, suggest that hydrothermal mound s grow by inflation, whereas chimney growth and collapse contribute a veneer of hydrothermal sediment. Sediment adjacent to an active chimne y has layers of anhydrite, sulfide and nodular Mg-silicate. The sedime nt contains a high content of MgO and reduced S, and is composed of se rpentine, saponite, talc, pyrrhotite, pyrite, gypsum, anhydrite, marca site, sphalerite, chalcopyrite, isocubanite and galena. The upper laye r reflects recent chimney collapse, whereas the sulfide layer represen ts a residual accumulation of past collapse events. The abundance of M g-silicates and limited biota suggest that recharge occurs over much o f the mound surface, whereas discharge is focused at chimneys. Hydroth ermal sediments in areas of diffuse venting are characterized by stron g H2S odor, abundant quartz, amorphous silica, barite, smectite, organ ic matter and biota. They differ from mound sediments by higher conten ts of Ba and SiO2, and tower contents of S and most chalcophile elemen ts. These areas may represent an incipient stage of hydrothermal mound growth.