Geological, chemical, and biological evidence for recent volcanism at 17.5°S: East Pacific Rise

Abstract

The superfast-spreading portion of the East Pacific Rise at 17.5°S is a magmatically robust section of mid-ocean ridge. Plumes characterized by high volatile/metal ratios, which has been hypothesized to be indicative of recent magmatism, were found between 17°22′S and 17°35′S in November–December 1993. Dives with the French submersible Nautile in December 1993 observed young sheet flows and widespread venting centered on the segment's shoalest point at 17°26′S. Eight submersible dives made one year later with the Japanese submersible Shinkai 6500 in September–November 1994 found glassy, unsedimented lavas, a range of diffuse and high-temperature vents, and several types of biologic communities on the ridge crest beneath the volatile-rich 1993 plumes. The diffuse vents range from small areas in the youngest lavas characterized by very low-temperature flow (<10°C) and a relatively low macrofaunal diversity, to larger areas where the (apparent) older surface was covered with a dense and diverse biotope including anemones, tubeworms, mussels and serpulids. Several inactive vents marked by dead mussel/clam beds are present in the southernmost portion of the site in the older lavas. Submersible mapping indicates 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 km at the northern site. The flow is up to 400 m in width at the southern site and may be as much as 1–2 km in width at the northern site. Comparison of the observed 1993 Fe/S, H 2S/heat, and 3He/heat plume values with time 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 several years prior to 1993. Continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light-scattering intensity and rise height of the overlying plumes recorded during 1994 submersible dives. The type and diversity of the vent biota found on the L0 vents also suggests a young system. The spatial and temporal scales of the accretion event at the 17.5°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°S event has more subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system.