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 better understanding of the relative age and vent fluid characteristics of a ridge crest system during surface ship surveys. During the TROUGHS (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-degrees-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 ratio in the particles ranged from about 0.9 to 10, with the highest values 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 CH4/Mn ratios in the plumes over the same vent field. The S-rich particles 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 Clipperton Transform Fault. These high ratios are indicative of the predominance of high-temperature, H2S-rich vents between 9-degrees-35'N and 9-degrees-50'N. In contrast, the samples from the segment north of Clipperton Transform Fault had S/Fe ratios which ranged from 0.04 to 0.82, typical of mature, Fe-rich hydrothermal plumes. These results suggest that 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 sulfur-enriched plume particles were also observed underneath the 1986 Megaplume on the Juan de Fuca Ridge (JdFR), suggesting a similar mode of formation. North of the Clipperton Transform Fault, the hydrothermal particles are dominated by Fe-rich oxyhydroxide particles, characteristic of mature high-temperature vent fields. The plume maximum is centered 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 TAG hydrothermal systems. For As the mean element/Fe ratios of all three systems are about the same. New evidence for Si scavenging by hydrothermal Fe oxyhydroxide particles indicates that this sink for dissolved Si in the oceans is small.