HYDROTHERMAL PLUMES ALONG THE EAST PACIFIC RISE, 8-DEGREES-40' TO 11-DEGREES-50'N - PARTICLE DISTRIBUTION AND COMPOSITION

The discovery of a volcanic eruption at the 9-degrees-45.52'N site on the East Pacific Rise (EPR) led us to investigate how the composition of suspended matter in hydrothermal plumes may be used to provide a be tter understanding of the relative age and vent fluid characteristics of a ridge crest system during surface ship surveys. During the TROUGH S (Tropical Ridge Observations of Underwater Geochemical Hydrothermal Signals) expedition we collected hydrothermal plume particles along a 350 km section, extending from approximately 8-degrees-40'N to 11-degr ees-50'N, of the EPR north and south of the Clipperton Transform Fault . Over the segment south of the Clipperton Transform Fault, the S/Fe r atio in the particles ranged from about 0.9 to 10, with the highest va lues immediately over the new vent field at the 9-degrees-45.52'N site . These enrichments exactly coincide with the elevated He-3/heat and C H4/Mn ratios in the plumes over the same vent field. The S-rich partic les are primarily organic and may be related to the 'bacteria blizzard ' associated with the eruption. In addition, very high Cu/Fe and Zn/Fe ratios were observed in the hydrothermal plumes south of the Clippert on Transform Fault. These high ratios are indicative of the predominan ce of high-temperature, H2S-rich vents between 9-degrees-35'N and 9-de grees-50'N. In contrast, the samples from the segment north of Clipper ton Transform Fault had S/Fe ratios which ranged from 0.04 to 0.82, ty pical of mature, Fe-rich hydrothermal plumes. These results suggest th at the gas-rich vent fluids at the 9-degrees-45.52'N site produced the sulfur-enriched particles in the water column. Gas-rich plumes and su lfur-enriched plume particles were also observed underneath the 1986 M egaplume on the Juan de Fuca Ridge (JdFR), suggesting a similar mode o f formation. North of the Clipperton Transform Fault, the hydrothermal particles are dominated by Fe-rich oxyhydroxide particles, characteri stic of mature high-temperature vent fields. The plume maximum is cent ered at 11-degrees-08'N. The Fe oxyhydroxide particles scavenge Ca, Si , P, V, Cr and As from seawater. For P and V the mean element/Fe ratio in the EPR hydrothermal particles are intermediate between JdFR and T AG hydrothermal systems. For As the mean element/Fe ratios of all thre e systems are about the same. New evidence for Si scavenging by hydrot hermal Fe oxyhydroxide particles indicates that this sink for dissolve d Si in the oceans is small.