GEOLOGICAL, CHEMICAL, AND BIOLOGICAL EVIDENCE FOR RECENT VOLCANISM AT17.5-DEGREES-S - EAST PACIFIC RISE

The superfast-spreading portion of the East Pacific Rise at 17.5 degre es S is a magmatically robust section of mid-ocean ridge. Plumes chara cterized by high volatile/metal ratios, which has been hypothesized to be indicative of recent magmatism, were found between 17 degrees 22'S and 17 degrees 35'S in November-December 1993. Dives with the French submersible Nautile in December 1993 observed young sheet flows and wi despread venting centered on the segment's shoalest point at 17 degree s 26'S. Eight submersible dives made one year later with the Japanese submersible Shinkai 6500 in September-November 1994 found glassy unsed imented lavas, a range of diffuse and high-temperature vents, and seve ral types of biologic communities on the ridge crest beneath the volat ile-rich 1993 plumes. The diffuse vents range from small areas in the youngest lavas characterized by very low-temperature flow (<10 degrees C) and a relatively low macrofaunal diversity, to larger areas where the (apparent) older surface was covered with a dense and diverse biot ope including anemones, tubeworms, mussels and serpulids. Several inac tive vents marked by dead mussel/clam beds are present in the southern most portion of the site in the older lavas. Submersible mapping indic ates that the youngest lava (L0 unit) erupted for at least 4 km along the strike of the ridge at the southern site, and for 5-10 lan at the northern site. The flow is up to 400 min width at the southern site an d may be as much as 1-2 km in width at the northern site. Comparison o f the observed 1993 Fe/S, H2S/heat, and He-3/heat plume values with ti me series measurements at sites on the northern East Pacific Rise and the Juan de Fuca Ridge, where the onset of the magmatic event is known , implies that the last magmatic event at 17.5'S occurred within sever al years prior to 1993. Continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light-scat tering intensity and rise height of the overlying plumes recorded duri ng 1994 submersible dives. The type and diversity of the vent biota fo und on the LO vents also suggests a young system. The spatial and temp oral scales of the accretion event at the 17.5 degrees S EPR site are similar in scale to eruptions on the intermediate-rate spreading North Cleft segment (Juan de Fuca Ridge) but the 17.5 degrees S event has m ore subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system. (C) 1998 Elsevier Science B.V. All rights reserved.